Journals >Infrared and Laser Engineering
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2025
Volume: 54 Issue 5
35 Article(s)
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Infrared
Infrared pulsed thermal wave optical flow imaging detection of defects in thermal protective materials
Chenghao PAN, Shang GAO, Tao DI, Hao WANG... and Jian JIANG|Show fewer author(s)
ObjectiveThermal protection materials, with their excellent thermal insulation and mechanical properties, have been widely used in the aerospace field. However, during the service life of reusable vehicles, impacts from airborne ice, raindrops, or even debris falling from the vehicle itself can cause damage to the surfObjective Thermal protection materials, with their excellent thermal insulation and mechanical properties, have been widely used in the aerospace field. However, during the service life of reusable vehicles, impacts from airborne ice, raindrops, or even debris falling from the vehicle itself can cause damage to the surface and interior of the thermal protection materials, severely compromising the safety of the vehicle. Therefore, there is an urgent need for a non-destructive testing and evaluation technique to ensure the quality of thermal protection materials. Considering the challenges posed by the porous structure and thermal insulation properties of thermal protection materials, which result in rapid localized heating under non-uniform heating and significant lateral heat diffusion effects, this paper proposes an infrared pulse thermography method based on optical flow imaging. This approach aims to mitigate the interference caused by lateral heat diffusion and enhance the accuracy and reliability of defect detection in thermal protection materials.Methods Transverse heat flow suppression (THFS) technique based on the optical flow method is proposed and applied to infrared pulse thermography, resulting in infrared pulse thermography optical flow imaging (Fig.1 ). This approach aims to mitigate the effects of transverse heat flow and enhance the detection accuracy of thermal protection materials. First, heat flow vector equations along the X and Y directions on the surface of thermal protection materials are derived using the optical flow method, and the inverse heat diffusion equation is formulated. Next, amplitude and phase feature images in the frequency domain corresponding to the surface of the thermal protection materials are obtained using the FFT and THFS coupling algorithm (Fig.2 ). Finally, the defect location and size are quantified using a local threshold segmentation method.Results and Discussions Based on the above method, a three-dimensional simulation model was constructed using COMSOL Multiphysics software (Fig.3 (a)), and non-uniform pulse excitation was applied. Through comparative studies of the simulation results, the differences in normalized FFT amplitude/phase and normalized FFT-THFS amplitude/phase (Fig.5 ) were analyzed. It was found that the proposed method effectively reduces the impact of uneven heating and lateral thermal diffusion, enhancing the normalized contrast between defective and non-defective regions. An infrared pulse thermography testing platform (Fig.8 ) was set up for experiments. Detailed comparisons were conducted on the relationships between the signal-to-noise ratio (SNR) of different feature parameters and defect diameter and depth (Fig.13 ), the relationships between the SNR of different feature parameters and the diameter-to-depth ratio of defects (Fig.14 , Fig.15 ), and the effects of defect local threshold segmentation (Fig.16 ). The proposed method improves the SNR and accuracy of defect detection, enabling the quantified defect size to be closer to the actual size.Conclusions This paper proposes an infrared pulse thermography optical flow imaging method with lateral heat flow suppression based on the optical flow method. First, the fundamental principles of Thermal Heat Flow Suppression (THFS) are elaborated using the optical flow method, along with the introduction of a 3D thermal wave model under non-uniform linear pulse heat flow excitation. To effectively extract defect features, the thermal wave image sequence is processed using Fast Fourier Transform (FFT), and the normalized FFT amplitude/phase is compared with the normalized FFT-THFS amplitude/phase. Simulation results demonstrate that THFS significantly reduces the impact of lateral heat diffusion, enhancing the contrast between defect and non-defect regions. Finally, infrared pulse thermography optical flow imaging was applied for non-destructive testing and evaluation of thermal protection material specimens with artificial flat-bottom holes. Experimental results show that the proposed method can effectively detect defects with a minimum diameter-to-depth ratio of 5 and a minimum diameter of 6 mm. The maximum detection error for defect diameter was reduced from 12.67% to 5.12%, and the defect detection signal-to-noise ratio was improved by up to 17.98%. These results confirm that infrared pulse thermography optical flow imaging effectively mitigates the interference of lateral heat diffusion and improves the accuracy and effectiveness of defect detection in thermal protection materials..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240537 (2025)
Performance optimization design of InAs/InAsSb type-II superlattice mid-wave barrier infrared photodetectors based on simulation
Ning XIE, Lianqing ZHU, Bingfeng LIU, Xiaoping LOU, and Mingli DONG
ObjectiveInfrared detectors play an essential role in the fields of infrared imaging, medical analysis, missile warning, biochemical gas detection and security monitoring. Among them, InAs/InAsSb, a type II superlattice material grown on GaSb substrates, has become a key material for high-performance infrared focal plaObjective Infrared detectors play an essential role in the fields of infrared imaging, medical analysis, missile warning, biochemical gas detection and security monitoring. Among them, InAs/InAsSb, a type II superlattice material grown on GaSb substrates, has become a key material for high-performance infrared focal plane arrays due to its outstanding properties. However, InAs/InAsSb T2 SL infrared detectors suffer from high dark current values that limit their performance. To solve this issue, researchers redesigned the barrier structure of the device and investigated the dark current generation mechanism. The nBn-type barrier structure features a large energy barrier in the conduction band, providing self-passivation and effectively reducing the tunneling current. However, in InAs/InAsSb devices with nBn-type barrier structures, it is still challenging to precisely regulate parameters such as doping levels and thicknesses between different structural layers. Therefore, the device structure is investigated through numerical calculations, focusing on the dominant dark current mechanisms and structural characteristics. The optimal parameter configurations are explored to improve the device performance.Methods Based on the structural characteristics of InAs/InAsSb infrared detectors, the dominant mechanisms of dark current and the associated band structure were systematically analyzed. Numerical simulations incorporating Poisson's equation, continuity equations, and heat equations were employed to precisely optimize key parameters, including the doping concentration of the absorption layer (AL), the doping concentration and thickness of the barrier layer (BL), operating temperature, and material composition. The optimized design establishes a high-energy barrier to effectively block majority carriers and allow minority carrier to migrate. By achieving a near-zero valence band offset, the design significantly reduces the dark current in the device.Results and Discussions Simulation results indicate that when the Sb composition in the AlAs1-xSbx barrier reaches 0.91, the valence band offset approaches zero, facilitating smooth transport of minority carriers across the barrier and significantly reducing the dark current (Fig.3 ). An increase in the barrier thickness results in a gradual rise in the valence band offset (VBO), which becomes more pronounced when the barrier thickness exceeds 100 nm (Fig.4 ). For the absorption layer, higher doping levels increase electron concentration and decrease hole concentration, but the impact of doping concentration variations on dark current reduction is minimal (Fig.5 ). In the barrier layer, higher doping concentrations lead to a higher absolute value of the dark current turn-on voltage. Moreover, excessive doping in the barrier layer results in pronounced band offsets, hindering minority carrier transport (Fig.6 ). The dark current decreases with lower temperatures, aligning with its expected temperature dependence (Fig.7 ).Conclusions Numerical calculations were performed to determine the optimal structural parameters for the nBn barrier-type T2 SL InAs/InAsSb/B-AlAsSb mid-wavelength infrared detector. When the absorption layer doping concentration is precisely controlled at 1×1013 cm-3, the barrier layer doping concentration at 1×1015 cm-3, and the barrier layer thickness set to 80 nm, the detector achieves a low dark current density of 4.5×10-7 A/cm2 under a high-temperature condition of 140 K and a bias voltage of -0.5 V. This performance meets the requirements for applications in infrared imaging, medical diagnostics, missile early warning, and related fields..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240551 (2025)
Review of intelligent technology in infrared imaging tracking systems
Junting YU, Shaoyi LI, and Bangrong XIE
Significance In recent localized wars, infrared imaging-guided missiles and multi-mode composite guidance weapons, such as air-to-air, air-to-ground, and air-to-sea types, have been widely used in actual combat to strike important or high-value military targets such as radar positions, missile weapon depots, military cSignificance In recent localized wars, infrared imaging-guided missiles and multi-mode composite guidance weapons, such as air-to-air, air-to-ground, and air-to-sea types, have been widely used in actual combat to strike important or high-value military targets such as radar positions, missile weapon depots, military command centers, and airports. These systems have played an important role in successfully carrying out tactical and operational objectives, securing air/sea superiority, and supporting ground operations. However, current infrared tracking systems face challenges in actual combat environments, such as complex background environments, strong countermeasures, intelligent countermeasures, and multi-target, multi-task demands, which significantly increase uncertainty in operational effectiveness. Intelligent technology in tracking systems is one of the core aspects of intelligent infrared imaging stabilization and tracking systems, and an important technological approach to improving stable tracking capabilities in increasingly complex battlefield environments. Researching and summarizing the development status and trends of foreign infrared imaging tracking systems' intelligent technology is of significant guiding importance for the development of infrared imaging stabilization and tracking technology in China.Progress Figure 1 and Tab.1 present and summarize several typical infrared imaging-guided missiles in recent years, indicating that infrared imaging-guided weapons have been widely used in air, sea, and ground operations. However, the recent emergence of infrared-guided weapons facing complex countermeasure environments highlights their vulnerability to interception or deception, revealing the inadequacy of current infrared-guided weapons in counter-jamming capabilities. At the same time, the development of the intelligence levels of infrared imaging stabilization and tracking systems in different application scenarios, as shown in Tab.1, indicates that: 1) Introducing artificial intelligence technology into infrared imaging stabilization and tracking platforms to significantly improve the intelligence level of tracking systems is a key development direction. 2) Utilizing intelligent tracking systems with strong capabilities such as automatic target acquisition, automatic target recognition and tracking, autonomous situational awareness and trajectory planning, and anti-jamming has become a current trend in missile intelligent technology development. Conclusions and Prospects The increasing complexity of future battlefield environments and the diversification of tasks will inevitably drive the development of tracking system technology toward intelligent solutions. In a combat environment characterized by strong countermeasures, intelligent technologies, and collaboration, the development of interference and anti-interference technologies will always represent an enduring scientific challenge. Intelligent anti-jamming in complex environments will remain a fundamental problem in the field of infrared imaging stabilization and tracking systems. This study aims to provide a reference for the intelligent development of infrared imaging tracking systems in China. In the future, infrared imaging tracking systems will gradually achieve single-system level cognitive AI tracking technology and group-collaborative brain-like AI tracking technology, which will improve performance in collaborative detection, collaborative penetration, collaborative anti-jamming, and collaborative strike through the integration of multi-platform collaborative sensing and intelligent guidance methods..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240567 (2025)
Laser
Research of 532 nm/1 064 nm dual-band high reflective coatings applied in space-based lidar
Yu YUN, Chunxia WU, Minglei ZHANG, and Yuejian CHEN
ObjectiveSpace-based laser radar (lidar) technology is widely acknowledged as an indispensable detection method in critical fields such as geospatial mapping and national defense. The 532 nm/1064 nm dual-band reflectance coating constitutes a vital component within lidar systems, performing essential functions in opticObjective Space-based laser radar (lidar) technology is widely acknowledged as an indispensable detection method in critical fields such as geospatial mapping and national defense. The 532 nm/1064 nm dual-band reflectance coating constitutes a vital component within lidar systems, performing essential functions in optical path transmission and the regulation of spectral efficiency. Electron beam evaporation (EB) is the predominant technique employed for the fabrication of HfO2/SiO2 dual-band reflective coatings, which are characterized by high damage thresholds. However, due to the inherent limitations of the fabrication process, HfO2/SiO2 thin films typically exhibit a porous and loosely compacted microstructure, rendering them highly susceptible to the adsorption of water vapor under atmospheric conditions. The subsequent desorption of this water vapor under vacuum operating environments leads to spectral shifts within the thin films, which significantly compromises the operational stability and performance reliability of the laser radar system. Therefore, it is necessary to improve the current preparation method to meet the needs of different usage environments. For this purpose, a waterproof vapor laser film is designed in this paper.Methods The HfO2/SiO2 multilayer film is designed and fabricated using electron beam evaporation, followed by the deposition of a 20 nm Al2O3 layer on the top and side walls of the multilayer as a water vapor barrier, using atomic layer deposition (ALD) (Fig.3 ). The performance of the water vapor barrier is evaluated by testing the spectral characteristics in both vacuum and atmospheric conditions (Fig.4 ). The laser damage threshold at 1064 nm and 532 nm is assessed using a Nd:YAG laser to evaluate the laser performance (Fig.5 ).Results and Discussions Upon testing the thin films under both vacuum and atmospheric conditions, it is observed that the films fabricated via the electron beam (EB) process exhibit an average overall drift of approximately 2.5% at the central wavelengths of 1064 nm and 532 nm (Fig.4 (a)). In contrast, when the top and side walls are coated with a 20 nm layer of ALD-Al2O3, only a negligible drift of 0.3% is observed (Fig.4 (b)). This demonstrates excellent vapor barrier performance. Moreover, the bandwidth of the samples remains stable in both atmospheric and vacuum conditions, with no discernible reduction in reflection efficiency following the deposition of the ALD-Al2O3 layer in the vacuum environment. Furthermore, the damage threshold of the multilayer film with the ALD-Al2O3 coating (13.1 J/cm2) is found to be slightly lower than that of the multilayer film without the ALD-Al2O3 coating under 532 nm laser irradiation (15.7 J/cm2) (Fig.5 (a)-5(b)). Under 1 064 nm laser irradiation, the damage threshold of the multilayer film after ALD-Al2O3 coverage (41.5 J/cm2) is observed to be marginally lower than that of the uncoated film (44.5 J/cm2) (Fig.5 (c)-5(d)). The current results are sufficient to fulfill the practical operational requirements.Conclusions A waterproof vapor laser film has been prepared. Due to the dense microstructure of the Al2O3 film prepared by atomic layer deposition, it was covered on the top and sidewalls of the HfO2/SiO2 multilayer film prepared by electron beam evaporation. Testing the spectrum under atmospheric-vacuum conditions showed that the drift caused by water vapor decreased from 2.5% to 0.3%, demonstrating good water vapor barrier performance. Additionally, the laser damage threshold test of the film showed that after covering with the ALD film, the threshold decreased from 15.7 J/cm2 at 532 nm to 13.1 J/cm2, and from 44.5 J/cm2 at 1064 nm to 41.5 J/cm2. This may be due to defect attachment during the transport process. Nevertheless, this waterproof vapor laser film still meets operational requirements and enhances atmospheric-vacuum stability..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240411 (2025)
Thermal property research of large-aperture lunar orbit satellite laser reflector
Xueyu WANG, Kai TANG, Zhien CHENG, Hongjun FANG... and Maojin YUN|Show fewer author(s)
ObjectiveSatellite laser ranging (SLR) is a high-precision satellite orbit measurement method. Laser reflectors, as a key payload for SLR, have been used on many Earth orbit satellites and play an important role in fields such as astronomical geodynamics. In recent years, with the resurgence of the moon exploration craObjective Satellite laser ranging (SLR) is a high-precision satellite orbit measurement method. Laser reflectors, as a key payload for SLR, have been used on many Earth orbit satellites and play an important role in fields such as astronomical geodynamics. In recent years, with the resurgence of the moon exploration craze, many countries have begun to try to carry laser reflectors on satellites in lunar orbit. For high-orbit satellites such as lunar orbit satellites, an array reflector composed of a large number of small-aperture corner reflectorsis usually used to ensure the success rate of ranging. However, due to the limitations of satellite resources, the weight and size of the payload are usually required to be as small as possible, so arrayed laser reflectors are no longer suitable. Single large-aperture laser reflectors are not only lightweight and small in size, but also have the advantage of concentrating reflected laser energy, making it possible for lunar orbit satellites to carry high-performance laser reflectors. However, the thermal environment in lunar orbit is complex and changeable, and large-aperture corner reflectors are significantly affected by temperature effects, which can lead to a decrease or instability in the reflector's reflective ability. Therefore, it is essential to conduct research on the thermal characteristics of large-aperture laser reflectors. Methods A far-field diffraction pattern (FFDP) simulation model of the laser reflector was established. The simulation obtained the FFDP and optical scattering cross (OCS) of the laser reflector with three different apertures of 32, 38, 102 mm. The reflection capabilities of these three laser reflectors of different apertures were analyzed and compared. A thermal simulation model was established for the large-aperture lunar laser reflector in orbit, and the temperature conditions of the large-aperture laser reflector under different operating conditions were obtained. Based on the results of the thermal simulation, the FFDP and OCS of the large-aperture laser reflector under the influence of different temperature effects were experimentally tested, and the optical performance and in-orbit reflectivity of the large-aperture laser reflector affected by temperature effects were further analyzed.Results and Discussions The FFDP of the 102 mm diameter laser reflector is more concentrated, and the peak light intensity is about two orders of magnitude higher than that of the 32 mm and 38 mm diameter laser reflectors (Fig.4 ). Similarly, the OCS value of the large-aperture laser reflector is also two orders of magnitude higher than that of the small-aperture laser reflector (Fig.4 ). According to the results of thermal simulation, the on-orbit temperature of the laser reflector ranges from -34.8 ℃ to 75.0 ℃, and the laser reflector shows the characteristics of low temperature at the bottom and high temperature at the tip, with a certain longitudinal temperature difference. The longitudinal temperature difference increases with the increase of light conditions. It is 0.9 ℃ in the absence of light, and can reach 6.1 ℃ under long-term sunlight irradiation (Fig.7 , Tab.4 ). The FFDP and OCS were tested under different longitudinal temperature differences by heating the laser reflector. When the longitudinal temperature difference is below 1 ℃, the FFDP energy is concentrated and the reflector has good optical performance. After 1 ℃, the FFDP begins to disperse and the optical performance of the reflector gradually begins to decline (Fig.10 ). The OCS value gradually decreases with an increase in the longitudinal temperature difference (Fig.11 ). However, it is worth noting that different heating methods can lead to different rates of decrease in the OCS value.Conclusions The large-aperture lunar laser reflector has an ideal reflective capacity. When the laser reflector is not exposed to sunlight, its longitudinal temperature difference is kept below 1 ℃, which is the best observation window.When the reflector is exposed to sunlight, the longitudinal temperature difference increases by more than 1 ℃, and the reflectivity is seriously affected, so observations are usually avoided during this period. Future research should focus on new materials and structural designs to enhance the stability and reflective capacity of large-aperture reflectors in extreme temperature environments. This will promote the development of deep space exploration and scientific research and help humanity gain a deeper understanding of the moon and its resources..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240500 (2025)
Research on the compensation method for positioning error of laser tracker active aiming target
Zhi CHENG, Qifan QIU, Yang LI, Guoming WANG... and Zili ZHANG|Show fewer author(s)
ObjectiveLaser trackers, known for their high precision in large-scale geometric measurements, play a crucial role in industries such as aerospace and precision manufacturing. These sophisticated instruments are employed to ensure accurate and reliable measurements of large components and assemblies. During the measureObjective Laser trackers, known for their high precision in large-scale geometric measurements, play a crucial role in industries such as aerospace and precision manufacturing. These sophisticated instruments are employed to ensure accurate and reliable measurements of large components and assemblies. During the measurement process, active targets utilize automatic aiming technology to maintain the reflector's orientation towards the laser tracker. This functionality significantly expands the beam reception angle, enhances measurement efficiency, and minimizes the need for manual adjustments. As a result, active targets offer substantial benefits over traditional fixed cooperative targets, which lack these advanced features. The primary objective of this study is to design a laser tracker cooperative target equipped with active aiming capabilities. By addressing and compensating for geometric errors inherent in the target's complex mechanical structure, this design aims to meet the stringent performance requirements of typical precision measurement applications. This innovation promises to improve measurement accuracy and operational efficiency, making it a valuable tool in precision engineering fields.Methods This study involves the design of the overall structure of the active aiming target, including components such as the base, azimuth axis system, pitch axis system, reflecting prism, and PSD (Position Sensitive Detector) detection unit (Fig.1 ). By constructing a three-closed-loop feedback control model comprising current loop, velocity loop, and position loop, and establishing a segmented PID controller with miss distance detected by PSD as feedback input, the target's active aiming function for the measurement laser was realized. By analyzing the sources of geometric errors, the transmission chain of motion parameters, and the structural topology, a kinematic error model incorporating all geometric error parameters and motion parameters was established (Fig.6 ). To accurately identify model parameters, a comprehensive global optimization method was proposed, which combines genetic algorithms and the least squares method. Initially, the genetic algorithm is employed to perform global optimization and determine the initial values for the parameters. Subsequently, the least squares method is utilized to achieve local optimization based on these initial values, thereby refining the parameter estimates for improved accuracy. This hybrid approach ensures both broad exploration and precise fine-tuning in the parameter identification process. Experiments were conducted using measured data for model parameter identification and coordinate measurement compensation, validating the effectiveness of the proposed geometric error compensation method.Results and Discussions A testing system was established to validate the aiming performance and the effectiveness of the geometric error compensation (Fig.7 ). The target, when rotated using a rate turntable (Fig.8 ), maintained aiming at a maximum angular rate of 51.9 (°)/s (Fig.9 ), with a miss distance not exceeding ±27 μm. The introduced optical path variation remains below 7.1×10-5mm, ensuring consistency between dynamic and static measurement results. The error compensation reduced the maximum value of the prism vertex coordinate offset error from 9.753 mm to 0.057 mm (Fig.12 ) and the target displacement error from 0.182 mm to 0.014 mm (Fig.13 ). The results indicate that the designed target demonstrates effective aiming performance. Additionally, the proposed geometric error compensation method enhances the accuracy of coordinate measurements. This ensures the system meets the stringent requirements of standard industrial precision measurement applications. The results underscore the efficacy of the target design and the error compensation technique in achieving high precision within industrial environments.Conclusions An active aiming target for industrial large-scale laser precision measurements was designed. By implementing a three-closed-loop feedback control model (Fig.3 ), the target achieved adaptive aiming of the laser tracker's measurement beam under rapid and random motion conditions. A kinematic error model was established, and a model parameter identification method based on genetic algorithms and the least squares method was proposed. Experimental results indicate that the designed target exhibits excellent active aiming performance with a maximum angular rate of 51.9 (°)/s. The proposed methods significantly reduce the impact of geometric errors on measurement accuracy, decreasing the maximum value and root mean square value of the target displacement error by 81% and 84%, respectively. These methods provide a theoretical basis for the practical application of the target..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240550 (2025)
Close-range pulsed coherent Doppler wind lidar based on single AOM and single-stage fiber amplifier
Hao LUO, Wuyi LI, Zengli XIAO, Dandan JIANG, and Zhi LI
ObjectiveDoppler wind lidar makes use of the principle of optical Doppler shift between signal and reference light to measure radial velocity at different distances. Compared with other traditional wind sensing methods, wind lidar has advantages of remote sensing, relatively higher temporal and spatial resolution, movaObjective Doppler wind lidar makes use of the principle of optical Doppler shift between signal and reference light to measure radial velocity at different distances. Compared with other traditional wind sensing methods, wind lidar has advantages of remote sensing, relatively higher temporal and spatial resolution, movability. Therefore, wind lidar is becoming a more and more important tool for precise wind measurement in wind energy industry including wind resource assessments, power performance measurement, wind turbine yaw control, individual pitch control, etc. However, the cost of the nacelle wind lidar for turbine control limits its mass application. Moreover, as wind turbines are being installed at an increasing rate today from onshore to offshore and complex mountainous terrain, the ground-based wind lidar for site and resource assessment is limited on data availability for power shortage in outdoor environment. Due to challenges in cost, volume, power consumption, the wind lidar needs improvement. Fiber laser is the main source of cost and power consumption for short-range wind lidar. Therefore, it is necessary to optimize the design of fiber laser to meet the requirement of wind energy industry for short-range wind laser.Methods A pulsed coherent doppler short-range wind lidar system based on single-stage fiber amplifier is built in this paper. To solve the problem of pulse envelope modulation caused by the interference between light at different modulated frequencies in optical path of double-pass AOM, we design a double-pass AOM scheme based on the structure of PBS and Faraday mirror (Fig.1 ). Then, we adopt the single-stage double-pass fiber amplifier to replace the traditional two-stage fiber amplifier (Fig.2 ). The 80 MHz AOM and fiber amplifier are arranged in the same double-pass path so that it gets higher saturated output power compared with the separated scheme (Fig.3 ).Results and Discussions The slope efficiency and small signal gain coefficient of the double-pass fiber amplifier in CW mode is 16.7% and 36 dB, respectively (Fig.2 ). The output power of the fiber laser source designed for short-range wind lidar is 42.5 mW at pump of 449 mW. The pulse repetition frequency is 10 kHz and the pulse width is 300 ns. Owing to the time switching role of the AOM, the SMSR of the output pulse reaches 62 dB (Fig.3 ). On August 24th, the test wind lidar system based on sing-stage fiber amplifier (10 kHz, 300 ns, 4.25 µJ) is compared with the calibrated wind lidar(30 kHz, 200 ns, 6 µJ) at Nan Jing (32.16N, 119.02E) at distances from 50 to 200 m. The fitting correlation coefficient of wind velocity is higher than 0.99 at 100, 160, 200 m. The average of the deviation is less than 0.03 m/s and the standard deviation is less than 0.07 m/s. The system measurement results show that the wind velocity data of test lidar and calibrated lidar have high correlation.Conclusions A pulsed coherent wind lidar system with single-stage fiber amplifier is designed. The system is characterized by simple structure of single AOM and single fiber amplifier. The single pulse energy of the fiber laser is 4.25 µJ with pulse repetition of 10 kHz and pulse width of 300 ns. Owing to the polarization separation of different shifted frequencies light in odd and even times reflected by the faraday mirror, the output pulse envelope is smooth. The wind measurement performance of the compact test wind lidar system is compared with the calibrated wind lidar at different distances from 50 to 200 m. At the distance of 200 m, the fitting correlation coefficient of wind velocity data is 0.9973 . The average of the deviation is 0.0218 m/s and the standard deviation is 0.0625 m/s. In the future, we will improve the output pulse energy of the fiber laser source by increasing pump power and suppress SBS effect so that a medium-range wind lidar system can be realized..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240562 (2025)
Analysis of detection performance of Rayleigh scattering wind lidar
Shijun ZHAO, Jianfeng CHEN, Yulong SHAN, Chao YANG, and Shuai HU
ObjectiveThe wind field in the middle and upper atmosphere is pivotal to our daily lives as well as the aerospace field. Rayleigh scattering wind lidars, possessing distinct advantages, play an instrumental role in probing these atmospheric layers. In order to evaluate the efficacy of a Rayleigh scattering Doppler windObjective The wind field in the middle and upper atmosphere is pivotal to our daily lives as well as the aerospace field. Rayleigh scattering wind lidars, possessing distinct advantages, play an instrumental role in probing these atmospheric layers. In order to evaluate the efficacy of a Rayleigh scattering Doppler wind lidar based on dual-edge technology, a comprehensive evaluation was conducted. Considering the extensive altitude range of the lidar, its measurement capabilities were statistically analyzed by comparing its synchronous observations with those of radiosondes and ERA5 reanalysis data. Considering the extensive altitude range of the lidar, its measurement capabilities were statistically analyzed through comparing its synchronous observations with those of radiosondes and ERA5 reanalysis data.Methods This study investigates the measurement performance of Rayleigh scattering wind lidar employing double-edge technology. Statistical analysis is employed to compare the measurement performance of lidar, radiosonde, and ERA5 reanalysis data. The procedure consists of the following steps: Lidar data preprocessing, radiosonde data preprocessing, reanalysis data preprocessing, comparison of radiosonde and reanalysis data, and comparison of lidar, radiosonde, and reanalysis data.Results and Discussions Radiosonde and reanalysis data span the altitude range of 10-31.5 km. At most altitudes, the wind speed deviation is less than 2 m/s, and the wind direction deviation is below 5°, demonstrating good overall consistency. However, above 20 km, the random deviation in wind direction becomes significant in the reanalysis data (Fig.12 ). Therefore, at altitudes where radiosonde data are available, it is preferred as the reference, while at altitudes without radiosonde data, reanalysis data may serve as the comparison reference. Lidar and radiosonde data can be compared within the altitude range of 11.5-32 km. At most altitudes, the wind speed deviation is less than 3 m/s, and the wind direction deviation is below 10°, indicating good overall consistency. The random deviation in wind direction increases significantly above 20 km. Averaging the lidar data over time reduces wind transients, resulting in wind measurements that are consistent with radiosonde data (Fig.13 ). Lidar and reanalysis data span the altitude range of 11.5-32 km, showing good overall consistency. At most altitudes, the wind speed deviation is below 3 m/s. Wind speed deviation increases with altitude. Below 38 km, wind speed deviation is less than 10 m/s, while above 45 km, it exceeds 25 m/s. At most altitudes, the wind direction deviation is below 20°, and at the majority of altitudes, it is less than 10°. Considering the distribution of random deviations in wind speed and direction, the overall trends in both are consistent below 45 km, with significant deviations observed above this altitude (Fig.14 ).Conclusions The middle and upper atmospheric wind fields are closely linked to both human living conditions and aerospace activities. Rayleigh scattering wind lidar offers distinct advantages in detecting these wind fields. To evaluate the detection performance of Rayleigh scattering Doppler laser wind radar utilizing double-edge technology, the measurement performance of lidar is statistically assessed by comparing its synchronous observations with radiosonde data and ERA5 reanalysis data across different altitudes. The results indicate that radiosonde and reanalysis data are consistent within the altitude range of 10-31.5 km. Wind speed deviation is below 2 m/s, and wind direction deviation is under 5°, providing a reference for evaluating the detection performance of lidar. Compared to radiosonde data, within the altitude range of 11.5-32 km, the wind speed deviation is under 3 m/s, and the wind direction deviation is below 10°. The random deviation in wind direction increases significantly above 20 km. Compared to the reanalysis data, lidar demonstrates good overall consistency. The wind speed deviation is below 3 m/s from 11.5-32 km and under 10 m/s from 32-38 km. Wind direction deviation is generally under 10°. The overall trend of lidar is consistent below 45 km and exhibits significant deviation in wind speed above this altitude, with deviations exceeding 25 m/s..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240563 (2025)
Newsletter
Experimental study on inversion of diffusion velocity based on temperature gradient
Bingbing LI, Ran SONG, Lili JIANG, and Qi WU
Objective This study aims to develop a novel method for measuring diffusion flow velocities based on temperature gradient inversion, which is particularly effective in density-driven flow fields. The proposed method addresses the challenges faced by traditional acoustic and optical Doppler velocimeters in low-particle-Objective This study aims to develop a novel method for measuring diffusion flow velocities based on temperature gradient inversion, which is particularly effective in density-driven flow fields. The proposed method addresses the challenges faced by traditional acoustic and optical Doppler velocimeters in low-particle-concentration environments, where achieving accurate measurements of ultra-low-speed diffusion flows is difficult.Methods The experimental setup involves generating a temperature gradient in a controlled water tank and using an optical Doppler velocimeter for flow velocity calibration. Temperature and flow velocity data are simultaneously monitored and collected. The flow velocity is then inverted using a temperature gradient model. The experimental results are analyzed, and a relationship between temperature gradient and flow velocity is established through data fitting.Results and Discussions The experimental results demonstrate that the proposed method achieves a velocity measurement resolution of 0.1 mm/s, with the error between the inverted flow velocity and the actual flow velocity being within 6%. The temperature gradient changes and flow velocity exhibit a highly consistent trend, indicating a proportional relationship. The study also employs COMSOL Multiphysics software for simulation to validate the experimental findings, confirming the accuracy and reliability of the derived fitting formula for flow velocity inversion.Conclusions The study successfully proposes a measurement method that leverages temperature gradient inversion to measure diffusion flow velocities. This method offers a new technical approach for measuring diffusion flow velocities in particle-scarce or clear water environments, complementing existing measurement techniques. It holds promise for applications in environments where traditional velocimetry methods are limited, such as in clear or particle-free waters..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20250070 (2025)
Influence of sodium silicate on the refractive index of seawater using a CMOS module based refractometer
Shiyu CHEN, Cong ZHANG, Xiaoxue BAI, Muzi ZHANG, and Juan SU
ObjectiveSalinity is one of the most important parameters in oceanography. The study of high-precision measurement technology of ocean salinity has far-reaching significance for revealing the influence of distribution and change of ocean salinity on ocean dynamics and global change. Dissolved silicate (DSi) is an imporObjective Salinity is one of the most important parameters in oceanography. The study of high-precision measurement technology of ocean salinity has far-reaching significance for revealing the influence of distribution and change of ocean salinity on ocean dynamics and global change. Dissolved silicate (DSi) is an important nutrient in the hydrosphere and an important component of seawater salinity, and the influence of its content on absolute salinity should not be ignored. However, the contribution of silicate to the conductivity is not significant, and the traditional conductivity sensor can not accurately perceive its content. To obtain high precision ocean salinity data, it is necessary to understand the influence of dissolved silicate concentration on the measurement results of seawater refractive index, electrical conductivity and salinity.Methods Based on the principle of optical refraction, a new type of measurement system for seawater refractive index with V-shaped groove is established. The system uses a high-performance CMOS camera module as a position sensor to measure the refractive index and salinity of seawater by accurately measuring the tiny position changes of the light emitted from the V-shaped groove. The resolution of the system is 2.45×10-7 RIU, and the measurement range is not less than 36 000 points. Based on the measurement of refractive index of Chinese standard seawater with different concentrations of sodium silicate (salinity is (35±0.003) PSU), the effects of silicate on the measurement results of refractive index and salinity were studied, and the effects of dissolved silicate on the measurement results of refractive index, conductivity and salinity of seawater were compared with the traditional measurement results.Results and Discussions The experimental results show that the refractive index contribution of sodium silicate seawater can be effectively reflected by optical refraction method. At 25 ℃, the refractive index of (35.000±0.003) PSU standard seawater linearly increases by 1.62×10-4 RIU with 1 g/kg increase of sodium silicate concentration, and the corresponding practical salinity increases by 0.907 PSU and absolute salinity increases by 0.911 g/kg. However, the contribution of sodium silicate to the conductivity of seawater is not obvious, but due to the polarization effect, with the increase of sodium silicate concentration, the statistical decline of the conductivity measurement results is caused by the increase of sodium silicate concentration of 1 g/kg, resulting in a decrease of 0.054 mS/cm in the conductivity measurement results, and the corresponding practical salinity is 0.054 PSU. The difference between the two methods is 0.961 PSU, and the corresponding absolute salinity is 0.966 g/kg, which is close to the concentration change of sodium silicate.Conclusions When the silicate concentration is higher than 0.001 g/kg, the contribution of silicate to seawater salinity can be effectively reflected by optical refraction method. When the silicate concentration is low, the refractive index resolution of the optical system needs to be further improved. Compared with conductivity measurement, optical refraction method has a wider range of material measurement, and can sense the existence of conductive and non-conductive substances in seawater..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20250078 (2025)
Dynamic performance of seawater density sensor based on spectral interference principle
Xiaoxue BAI, Xin WANG, Muzi ZHANG, Mengzhen WANG... and Qi WU|Show fewer author(s)
ObjectiveSeawater density, as a critical parameter governing oceanic dynamic processes, demands precise measurement for advancing marine environmental studies. While optical sensors offer inherent advantages in density measurement through refractive index detection—including electromagnetic immunity, high sensitivity, Objective Seawater density, as a critical parameter governing oceanic dynamic processes, demands precise measurement for advancing marine environmental studies. While optical sensors offer inherent advantages in density measurement through refractive index detection—including electromagnetic immunity, high sensitivity, and provide response to non-ionic solutes—their measurement stability in dynamic marine environments remains compromised by fluid flow interference. This study addresses this challenge by proposing a novel anti-flow structural design to improve measurement consistency under hydrodynamic disturbances. Traditional conductivity-temperature-depth (CTD) sensors, though widely utilized, suffer from inherent limitations. Their indirect density calculation via TEOS-10 equations exhibits reduced sensitivity to non-ionic components and necessitates energy-intensive pumping systems that introduce mechanical vibrations. Our objective focuses on developing a flow-immune optical sensor that eliminates auxiliary pumps, minimizes turbulence-induced errors, and validates its performance through systematic laboratory and field experiments. By optimizing fluid exchange patterns within the sensing cavity, this design aims to achieve high-precision density measurements under varying flow conditions, thereby enhancing the reliability of optical sensing technology in practical oceanographic applications.Methods This study developed a seawater density sensing system based on spectral interferometry. The sensing probe features two parallel sapphire optical windows spaced 10 mm apart, allowing seawater to flow freely through the measurement cavity. A pump-free anti-current design was implemented through hydrodynamic-optimized channel geometry: V-shaped inlet channels with a 45° inclination angle paired with linear drainage grooves (Fig.2 ) enhance unidirectional flow efficiency. Variations in refractive index were detected via wavelength shifts of interference extrema using a high-resolution spectrometer.Results and Discussions In the seawater flow velocity simulation device, the measurement environment was simulated with seawater flow velocity ranging from 0.01 m/s to 2 m/s. Under these conditions, the interference fringe contrast recorded by the optical density sensor remained above 0.8, and the fluctuation in density measurements was consistently within the order of 10-3 kg/m3. Subsequently, a profiling test conducted at a maximum depth of approximately 3600 meters to evaluate, the consistency between the measurements of optical density sensor and the CTD sensor. The density difference between the two sensors remained within ±4×10-3 kg/m3, confirming the effectiveness of the anti-flow design in complex oceanic environments.Conclusions This study successfully demonstrates the effectiveness of an anti-flow optimized optical sensor for measuring seawater density in dynamic environments. Laboratory tests conducted under controlled flow conditions (≤2 m/s) confirmed exceptional stability, with standard deviations in density measurement maintained below 1×10-3 kg/m3. Field profiling experiments revealed strong consistency between optical and CTD sensor measurements, with a mean absolute density error of 2.4×10-3 kg/m3. The anti-current design eliminates the need for pumping systems while improving fluid exchange efficiency, effectively suppressing turbulence-induced signal fluctuations through hydrodynamically optimized channel geometry. These findings establish the sensor's capability for high-resolution density monitoring in complex marine environments, offering a potential supplement to CTD sensors, and advancing real-time monitoring of fine-scale oceanic processes..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20250106 (2025)
Optical Design and Fabrication
Optimization design of the barrel structure for large-aperture refractive space telescopes
Zhiyu HUANG, Junqing ZHU, Yongxian WANG, Weiqi HUANG, and Yingjun GUAN
ObjectiveAs the most important main support structure of a space telescope, the cylindrical mirror barrel structure accounts for a relatively large proportion of the mass. Due to the larger number of lens groups and larger system aperture, the system is heavier and larger in size, which needs to be structurally lightenObjective As the most important main support structure of a space telescope, the cylindrical mirror barrel structure accounts for a relatively large proportion of the mass. Due to the larger number of lens groups and larger system aperture, the system is heavier and larger in size, which needs to be structurally lightened; In addition, the cylindrical lens barrel structure needs to focus on the impact of the structure on the performance of the entire optical system. In addition to considering the specific structural properties such as strength and stiffness, the cylindrical lens structure also needs to focus on the impact of the structure on the various performance indexes of the optical lens as well as the impact on the performance of the entire optical system. Therefore, in order to design the mirror cylinder structure with reasonable structural form, high lightweighting rate and high stiffness, it is necessary to optimize its design.Methods First, based on an empirical structural design approach, the primary support structure of the large- aperture refractive space telescope was designed, and a finite element model was established. The finite element analysis method was used to perform topology optimization on the cylindrical mirror barrel structure, resulting in a reasonable optimized structural model. Then, dimensional optimization was performed to determine the optimal structural parameters. Afterward, the optimized mirror barrel model was reconstructed. Based on the optimization results, static and dynamic analyses were conducted to evaluate the surface shape indices of the lenses, the safety factors of each structure and lens, and to assess the overall system performance and safety. Finally, experimental testing was carried out on the large-aperture refractive space telescope to verify whether the structural performance of the optimized design met the design requirements.Results and Discussions After the optimization design, under the influence of axial gravity, the maximum displacement of the large-aperture refractive space telescope was 6.719 μm; Under the 5 ℃ temperature rise loading condition, the maximum deformation was 6.697 μm. These deformation values are relatively small, indicating that the large-aperture refractive space telescope can meet the displacement and deformation requirements under static loading conditions. The displacement and deformation parameters of each lens were fitted under static conditions to obtain the surface shape indices of each lens. Taking the surface shape index of the large-aperture lens 10 as an example, under the 5 ℃ temperature rise (with a reference temperature of 20 ℃) and axial gravity coupling condition, the maximum peak-to-valley (PV) value of the large-aperture lens surface was 9.62 nm, and the maximum root mean square (RMS) value was 2.21 nm. These values satisfy the design requirements of PV<λ/10 and RMS<λ/50 (λ=632.8 nm). Under dynamic loading conditions, the first-order fundamental frequency of the mirror barrel structure increased to 2871 Hz, significantly improving its structural stiffness. The overall mass was reduced from the original 3.022 kg to 1.053 kg, achieving a 65.16% reduction in weight, resulting in a more rational lightweight structural form. The fundamental frequency of the entire system was 2074 Hz, which is well above the required system fundamental frequency (not lower than 150 Hz), ensuring that there will be no resonance with the carrier.Conclusions The lens assembly was machined using optical centering processes. The lens group was installed onto a centering lathe with the help of a specialized fixture, and the optical axis of the lens group was aligned with the lathe's main spindle using a centering instrument. Afterward, precision turning was performed on the lens group to meet the design requirements. The mirror barrel was processed using casting, turning, and other methods, followed by blackening or stray light elimination treatments. Finally, the lens assembly was placed into the mirror barrel. After assembly and adjustment, the system was tested using a ZYGO interferometer. The test results showed that the system wavefront aberration of the large-aperture refractive space telescope was 0.124λ, which is less than the design requirement of 0.15λ, indicating good imaging quality and validating the effectiveness of the structural optimization. The optimization design method used in this paper can provide valuable reference and guidance for the optimization design of similar optomechanical structures..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240529 (2025)
Optical Imaging, Display and Information Processing
Laser interference image inpainting with global semantic perception and texture frequency domain constraints
Peiyao ZHAO, Bin FENG, Xinpeng YANG, Xikui MIAO... and Qing YE|Show fewer author(s)
ObjectiveLaser interference is a significant issue in imaging systems, where interference spots can obscure key target information, leading to a considerable degradation in image quality and complicating subsequent image analysis tasks. This is particularly problematic for systems that require high-precision imaging, sObjective Laser interference is a significant issue in imaging systems, where interference spots can obscure key target information, leading to a considerable degradation in image quality and complicating subsequent image analysis tasks. This is particularly problematic for systems that require high-precision imaging, such as surveillance, military reconnaissance, and remote sensing applications. To address this issue, it is essential to develop a method for effectively restoring laser interference images. The goal of this study is to design an advanced image restoration network that can accurately recover images affected by laser interference. This network will utilize a combination of global semantic perception and texture frequency-domain constraints to enhance the quality of the restored images. The proposed model aims to provide a solution for inpaintinging images where laser interference results in significant distortion, ensuring the restoration of both global context and fine texture details. This research is crucial for improving the performance of imaging systems under laser interference conditions, thereby enabling more reliable and accurate analysis for various practical applications, including defense, environmental monitoring, and remote sensing. The development of such a network is essential for meeting the growing demands of high-precision image inpainting in dynamic and challenging environments.Methods A laser interference image restoration network is designed in this study. The model consists of two stages: the first stage uses a hybrid network structure that combines self-attention mechanisms and a hierarchical feature extraction module. The network leverages a sliding window self-attention mechanism to gradually expand the receptive field, allowing the model to capture both local and global context information (Fig.2 ). In the second stage, a contextual attention mechanism is employed to refine the restoration results by analyzing the correlation between the undisturbed and interfered regions of the image (Fig.7 ). To enhance the restoration of fine texture details, a cosine transform loss function is integrated into the model, optimizing the image in the frequency domain for better preservation of high-frequency components (Fig.9 ). The performance of the network is evaluated using various image quality metrics, including SSIM and PSNR, to measure the accuracy of the restored images (Tab.1 ). The network is trained using a large dataset of laser-interfered images and their corresponding ground truth to ensure robust performance under varying interference conditions (Fig.11 ).Results and Discussions The comparison results in Tab.4 reveal significant differences in the structural similarity (SSIM) between different algorithms in both the interfered and non-interfered regions. Our proposed method outperforms the others in both areas, achieving SSIM values of 0.969 for the interfered region and 0.997 for the non-interfered region, demonstrating its exceptional capability in restoring complex texture details in the interfered regions while maintaining the image quality in the non-interfered regions. In contrast, the Pconv method achieves an SSIM of 0.957 in the laser-interfered region, which is slightly inferior to our approach, indicating that it struggles with fine texture restoration. The SSIM for the non-interfered region is 0.992, which is close to the ideal level. The CoordFill and PatchMatch methods show significantly lower performance, with SSIM values of 0.711 and 0.687 for the interfered region, respectively. These results indicate that both methods are less effective in restoring laser interference, especially in terms of recovering detailed textures. In the non-interfered regions, their SSIM values of 0.982 and 0.980 are also lower than those of Pconv and our method, further highlighting their limitations in maintaining overall image quality. These results demonstrate the superior performance of our proposed algorithm in laser interference image inpainting, both in terms of detail preservation and global image consistency.Conclusions A laser interference image restoration network is proposed in this study, which effectively addresses the challenges of restoring images affected by laser interference. The network is composed of a two-stage structure: the first stage utilizes a hybrid module combining self-attention mechanisms and hierarchical feature extraction, while the second stage employs a contextual attention mechanism to refine the restoration results. The network incorporates a cosine transform loss function to enhance texture detail recovery in the interfered regions. Experimental results demonstrate that the proposed method outperforms existing algorithms in terms of both structural similarity and texture detail restoration, achieving superior SSIM values of 0.969 in the interfered region and 0.997 in the non-interfered region. Additionally, the model excels in maintaining overall image quality and effectively restoring complex textures in regions affected by laser interference. This approach offers a promising solution for improving the performance of imaging systems in environments where laser interference is a concern, enabling more reliable image restoration for a wide range of practical applications. The robust performance under varying conditions demonstrates its potential for real-world deployment in applications such as surveillance, military reconnaissance, and remote sensing.
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240569 (2025)
Non-Line-of-Sight imaging based on improved CNN-Transformer
Shuai LIU, Mingjun WANG, and Yiming ZHOU
ObjectiveNon-Line-of-Sight (NLOS) imaging is a technique that calculates the imaging of a target behind an obstacle through an intermediary. The emergence of this technology is driven by the need to obtain data in complex environments where direct visual access is not possible. It is widely used in various fields, inclObjective Non-Line-of-Sight (NLOS) imaging is a technique that calculates the imaging of a target behind an obstacle through an intermediary. The emergence of this technology is driven by the need to obtain data in complex environments where direct visual access is not possible. It is widely used in various fields, including security surveillance, robot navigation, and medical imaging. Unlike traditional imaging technologies, NLOS imaging not only overcomes the limitations of the line of sight but also relies on the scattering of light to obtain information about the target in complex environments. However, after light undergoes multiple reflections and scattering, the signal strength is significantly attenuated, and the quality of the signal received through the intermediary surface is often affected by noise. Therefore, effectively improving target reconstruction accuracy and noise reduction has become a key challenge in NLOS imaging technology.Methods Pulsed laser and Time-of-Flight (ToF) technology is a commonly used method for Non-Line-of-Sight (NLOS) imaging. The system typically consists of a laser source, a time-resolved single-photon avalanche diode (SPAD) detector, a relay wall, and hidden objects (Fig.1 ). Leveraging deep learning techniques, the paper proposes an enhanced CNN-Transformer neural network (Fig.2 ). This network utilizes a lightweight cross-attention mechanism to establish a bidirectional bridging architecture, where the CNN and Transformer operate in parallel, forming a feedback loop (Fig.3 ). This design maximizes the strengths of CNNs (MobileNet) in local feature processing and Transformers in global interaction modeling, enabling deep interaction between local and global features within the network to generate rich deep local and global representations. Specifically, the process begins by extracting shallow features based on physical priors. These shallow features are then concatenated with tokens and passed to the Transformer via the bidirectional bridging architecture, where global features are interactively learned through multi-layer self-attention mechanisms. The global features captured by the Transformer are subsequently fed back into the CNN and fused with the shallow local features, enhancing the CNN’s understanding of local details. Finally, by integrating local and global features, the system achieves more accurate reconstruction of occluded 3D targets.Results and Discussions To evaluate the performance of the proposed CNN-Transformer neural network for NLOS imaging reconstruction, we compared it with existing methods, including physics-based approaches: FBP, LCT, FK, and RSD, as well as the deep learning-based method LFE, all trained on the same simulated dataset as this work. Quantitative results (Tab.1 ) show that the proposed method achieves the best reconstruction performance for both intensity and depth images. For intensity images, the PSNR metric outperforms FK and RSD by 6.39 dB and 5.45 dB, respectively, and improves upon LFE by 1.66 dB. For depth images, the RMSE metric shows a 22% reduction compared to LFE. Additionally, quantitative results on an unseen test set further demonstrate the superior performance of the CNN-Transformer in both intensity and depth reconstructions, highlighting the network's exceptional generalization ability to unseen targets. Qualitative results (Fig.4 ) further validate these findings. Reconstructions from FBP and LCT are blurry, while FK and RSD methods recover major structures but lack detail. The LFE method improves upon traditional physics-based models but still struggles with fine details. In contrast, the proposed method not only accurately recovers the primary structural contours but also excels in restoring intricate details.Furthermore, qualitative results on real-world datasets (Fig.7 ) align with the findings from simulated data. FBP, LCT, and FK methods exhibit significant noise and blurred boundaries, while LFE shows some improvement but still lacks detail. The proposed method, however, achieves the best performance in both detail restoration and noise suppression.Conclusions In order to improve the detail restoration capability of target reconstruction in Non-Line-of-Sight (NLOS) imaging, this paper proposes an improved CNN-Transformer neural network. This network constructs a bidirectional bridging architecture through a lightweight cross-attention mechanism and parallels CNN (MobileNet) with Transformer, fully leveraging the efficiency of CNN in local processing and the advantages of Transformer in global interaction encoding. The dual fusion of local and global features is achieved. Experimental results show that the proposed method outperforms existing physical models and deep learning models on both simulated and real-world data, effectively solving the problem of detail blur and noise interference in traditional Transformers for NLOS imaging. Moreover, the method demonstrates excellent generalization ability for unseen data, validating its robustness and practicality, and provides new insights for the application of NLOS imaging technology in complex scenarios..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240570 (2025)
Special Issue—Hyperspectral Technology and Applications
Differential Raman combined with VGG16 and DCGAN for the inspection of food packaging
Junxia ZHOU, Chunyu LI, Hong JIANG, and Xuejun ZHAO
ObjectivePropose an inspection method for food packaging that integrates differential Raman spectroscopy, feature extraction using the VGG16 model, and cluster analysis, as well as a method for expanding differential Raman spectra with DCGAN. The aim is to address the issues of extensive feature extraction work and limObjective Propose an inspection method for food packaging that integrates differential Raman spectroscopy, feature extraction using the VGG16 model, and cluster analysis, as well as a method for expanding differential Raman spectra with DCGAN. The aim is to address the issues of extensive feature extraction work and limited sample quantities in traditional inspection methods.Methods Researchers initially collected spectral data from 66 different food packaging samples, resulting in a total of 71 spectral data samples. These data were plotted into spectral graphs using python and manually classified (Fig.1 ) with prior knowledge (Tab.1). Subsequently, the VGG16 model was used to extract 512-dimensional feature vectors from the spectral graphs. To verify the effectiveness of VGG16 feature extraction, Principal Component Analysis (PCA) and Sammon mapping were employed for dimensionality reduction (Fig.2 ), followed by K-means and Gaussian Mixture Model (GMM) clustering analysis on the reduced data (Fig.4 ), comparing the clustering results with manual classification outcomes. Additionally, researchers explored the quality of spectral graphs generated by DCGAN under various training strategies (Fig.6 , Fig.9 ) and compared the quality differences between generated and actual spectral graphs using the VGG16-PCA visualization method (Fig.7 , Fig.8 , Fig.10 , Fig.11 ).Results and Discussion The VGG16-PCA-K-means and VGG16-PCA-GMM clustering methods achieved accuracy rates of 91.5% and 88.7%, respectively, confirming the high efficiency and accuracy of VGG16 in extracting differential Raman spectral features. When the proportion of generated spectral graphs increased, visualization results showed significant changes in the distribution of actual spectral graphs, and the distances between generated graphs and their similar counterparts also increased (Fig.10 ). Moreover, as the number of DCGAN iterations increased, the distance between generated and actual spectral graphs in the visualization diagram became closer, indicating higher quality of the generated graphs (Fig.11 ).Conclusions This study successfully integrated VGG16 and DCGAN into the differential Raman spectroscopy inspection of food packaging. The VGG16 model effectively extracted spectral graph features and achieved high classification accuracy rates through PCA dimensionality reduction and K-means and GMM clustering algorithms. The training strategy of DCGAN and the quality of generated spectral graphs significantly impacted the results of VGG16 feature extraction; by adjusting the training strategy, higher quality spectral graphs can be generated, thereby enhancing the accuracy of feature extraction clustering. Although this study did not establish a classification model, the work of generating spectral graphs laid the foundation for building more accurate and efficient classification models in the future. Future work will focus on exploring strategies to improve the quality of generated samples to further enhance the accuracy and efficiency of the inspection..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240473 (2025)
Evaluation method for lake eutrophication using aerial hyperspectral image
Yan HUANG, Yanqi FANG, Mingzuan XU, Jianlong SHI... and Yan JI|Show fewer author(s)
ObjectiveWith the development and utilization of lakes by people, the problem of eutrophication in lakes has also intensified. By evaluating the nutritional status of lakes, people can understand the water quality of lakes, analyze the causes of eutrophication, grasp the laws of lake environment, and provide basic dataObjective With the development and utilization of lakes by people, the problem of eutrophication in lakes has also intensified. By evaluating the nutritional status of lakes, people can understand the water quality of lakes, analyze the causes of eutrophication, grasp the laws of lake environment, and provide basic data for the rational development and utilization of lake resources. Traditional sampling and analysis methods are time-consuming, costly, and easily affected by environmental interference. Hyperspectral technology can quickly and comprehensively obtain water quality parameters of lake, and is widely used for evaluation of lake eutrophication. In previous studies, the data sources were mostly ground data or low spatial resolution hyperspectral data, which could not reflect the detailed changes in water quality parameters. The relationship between water quality parameters and spectra is complex and variable, and the simple linear model is difficult to accurately quantify the content of water quality parameters. The single indicator is not suitable for evaluation of lake eutrophication, and the comprehensive evaluation indicator needs to be established. For this purpose, the evaluation method for lake eutrophication using aerial hyperspectral image is built.Methods The evaluation of lake eutrophicationn had been carried out using aerial hyperspectral image with a spatial resolution of 0.5 m and combined with ground measurement data. Firstly, the collected data was preprocessed to obtain reflectance data and five water quality parameters including Chlorophyll-a, Total Phosphorus, Total Nitrogen, Secchi Depth, and Permanganate Index. The correlation analysis was conducted on the preprocessed data and water quality parameters, and the first-order derivatives with higher correlations were selected for relevant features using Competitive Adaptive Reweighted Sampling (CARS). Then, the water quality parameter inversion model has been constructed using the Least Squares Support Vector Regression (LSSVR) algorithm based on Quantum Particle Swarm Optimization(QPSO). The model accuracy has been evaluated using the coefficient of determination(R2) and Root Mean Squard Error(RMSE), and the comparative analysis was conducted. Finally, the comprehensive nutritional status index has been calculated and compared with the measured values, and the eutrophication of lake in the research area has been evaluated using aerial hyperspectral image.Results and Discussions Five different inversion models were compared, and the water quality parameter inversion model constructed based on first-order differential transformation has the highest accuracy with R2 > 0.8(Tab.3), and inversion values are close to the measured values(Fig.4 ). The results of the comprehensive nutrient index are accurate, with MRE = 0.91% and RMSE = 0.50 when compared to measured values(Fig.4 ). The evaluation results for lake eutrophication in the study area have a high spatial resolution, accurately and detailed reflecting the distribution of nutrient status in the lake (Fig.5 ). The research area is in a state of mild eutrophication, which is consistent with the content of the water environment quality report of Yangzhou City. The method achieves high-precision and rapid evaluation of lake eutrophication. Due to limitations in flight conditions and project support, data collection was conducted only within the lake area of Gaoyou City and did not fully cover the entire Gaoyou Lake region. The results cannot represent the eutrophication level of the entire Gaoyou Lake.Conclusions An evaluation method for lake eutrophication based on aerial hyperspectral data is proposed. This method uses aerial hyperspectral data with a spatial resolution of 0.5 m, which details the variations in water quality parameters and accurately depicts the spatial distribution of eutrophication in lake. An optimized machine learning algorithm was employed to construct the water quality parameter inversion model, fully extracting relevant information to improve the accuracy of the inversion results. This method can quickly and accurately evaluate of lake eutrophication, but is subject to limitations such as sample collection and spatiotemporal distribution, and its applicability to other complex lake remains to be further studied. The data collection process may be affected by environmental factors such as weather and lighting, leading to poor data quality and reducing the accuracy of the inversion results. Comparative experiments under different climate and lighting conditions can be further conducted to verify its accuracy..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240475 (2025)
Multi-spectral fusion dehazing method based on polarization spectral images(invited)
Ying ZHANG, Xing WANG, Siwei ZHANG, Songran DOU... and Xihai XU|Show fewer author(s)
ObjectiveHaze degradation of near-earth remote sensing images due to atmospheric aerosol scattering is a significant challenge in remote sensing applications. It not only reduces image quality but also impairs the accuracy of downstream tasks such as target detection, environmental monitoring, and disaster response. ThObjective Haze degradation of near-earth remote sensing images due to atmospheric aerosol scattering is a significant challenge in remote sensing applications. It not only reduces image quality but also impairs the accuracy of downstream tasks such as target detection, environmental monitoring, and disaster response. The growing demand for high-quality remote sensing data has led to an increasing need for effective haze removal methods. Traditional approaches often struggle with complex atmospheric conditions and rely heavily on single visible-light images. With the advent of multispectral and polarization-based imaging technologies, new opportunities have emerged for more precise and effective haze removal by utilizing the distinct polarimetric characteristics of light. This enables better penetration of light through haze particles, improving the clarity and accuracy of images, especially in challenging conditions. The proposed dehazing method, which integrates visible and near-infrared (NIR) polarization spectral images, addresses these challenges by accurately modeling airlight radiance and polarization degree.Methods The dehazing method proposed in this study, based on the fusion of visible and NIR polarization multispectral images, consists of two main steps. In the first step, the atmospheric light polarization state distribution model is utilized to estimate the atmospheric light intensity and polarization degree at an infinite distance. By using the relationship between the Stokes vector and the Mueller matrix, the polarization spectral images corresponding to the maximum and minimum light intensities are obtained. These images are then processed to generate the visible polarization spectral dehazed image. In the second step, the dehazed visible image is fused with the NIR image to further improve the image quality, resulting in a final dehazed image with rich details (Fig.1 ).Results and Discussions In order to objectively evaluate the performance of the proposed image fusion dehazing method, several commonly used image quality assessment metrics were employed for quantitative analysis. Table 1 presents the comparative results of the dehazing methods, including SCHAUL, HE, FFA-Net, and FREDERIKE, based on different metrics. The results demonstrate that the proposed method excels in all evaluated metrics, particularly in Information Entropy (IE) and Contrast (IC), both showing significant improvements. Specifically, the information entropy improved by approximately 3.6% relative to the original image, while the contrast increased by about 3.26 times, indicating higher image information retention and better dehazing performance. Compared to other methods, the proposed method preserved more image details during the dehazing process, particularly in the handling of vegetation and distant scene areas. The method demonstrated superior dehazing effects, with a notable recovery of fine details in both the visible and near-infrared spectral bands. This reflects the method's ability to achieve photorealistic results that enhance visual clarity and retain structural details across different layers of the image, particularly in challenging conditions like haze. Furthermore, the adaptive fusion approach used in the proposed method played a key role in the effective combination of the visible and NIR polarization spectral images, leading to a significant improvement in both the detail recovery and overall image clarity. Despite the introduction of some granularity noise in certain high-frequency edge regions due to slight inconsistencies in pixel values during the fusion process, the method's ability to maintain the global image quality remains outstanding, with noise levels effectively reduced after post-processing. The results, therefore, not only validate the effectiveness of the method but also highlight its superiority over existing dehazing techniques.Conclusions The proposed dehazing method, based on the fusion of visible and NIR polarization spectral images, demonstrates significant improvements in image quality. By leveraging both the visible and NIR spectral characteristics, the method effectively mitigates the haze and enhances the fine details of the images. The fusion of these two spectral sources allows for better retention of information and improved contrast. The results indicate that the method significantly outperforms conventional dehazing techniques in terms of both subjective image quality and objective evaluation metrics. The proposed approach not only restores image details but also preserves the essential information of the scene, making it a promising solution for remote sensing applications, particularly in hazy environments where traditional methods fail to produce satisfactory results. The effectiveness of this method is validated through both qualitative and quantitative assessments, confirming its potential for practical use in various imaging and remote sensing tasks..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240523 (2025)
Degradation-aware transformer for blind hyperspectral and multispectral image fusion(back cover paper·invited)
Xuheng CAO, Xiaopeng HAO, Yusheng LIAN, Xuquan WANG, and Xinbin CHENG
ObjectiveHyperspectral and multispectral image fusion is a widely used technique to generate high-resolution hyperspectral images (HR-HSI) by combining the spatial details of multispectral images (MSI) with the spectral richness of hyperspectral images (HSI). Existing methods, however, often rely on predefined degradatObjective Hyperspectral and multispectral image fusion is a widely used technique to generate high-resolution hyperspectral images (HR-HSI) by combining the spatial details of multispectral images (MSI) with the spectral richness of hyperspectral images (HSI). Existing methods, however, often rely on predefined degradation priors and fail to fully capture the complex spatial-spectral interactions, which limits their performance in practical scenarios. Therefore, this paper proposes an adaptive degradation-aware feature fusion framework to address these limitations.Methods The proposed framework consists of three components: a feature fusion network and two degradation-aware networks for spatial and spectral domains (SpaDNet and SpeDNet). Firstly, a spatio-spectral cross-attention mechanism is introduced, which explores the hierarchical correlations between spatial and spectral features to enhance their interaction in the reconstructed images. Secondly, leveraging the physical characteristics of the degradation process, SpaDNet and SpeDNet adaptively learn degradation priors from the input images, enabling efficient blind fusion without predefined degradation information. Lastly, a subspace loss function is designed to decouple spatial domain interference during spectral degradation modeling, enhancing the precision of degradation-aware learning.Results and Discussions The proposed framework is validated on benchmark datasets, including CAVE and Harvard, achieving improvements in peak signal-to-noise ratio (PSNR) by 0.81 dB and 0.76 dB compared to existing methods. In experiments conducted on a hybrid-resolution imaging system, the reconstructed spectral band images demonstrated superior structural texture and signal strength accuracy. These results underscore the proposed method's ability to produce HR-HSI with enhanced spatial-spectral consistency, even in real-world scenarios where degradation priors are unknown.Conclusions The proposed adaptive degradation-aware framework introduces a novel approach to blind HSI-MSI fusion, addressing the limitations of existing methods. The integration of spatio-spectral cross-attention and adaptive degradation modeling ensures robust and accurate fusion results. Experimental results on both synthetic and real-world datasets demonstrate the method's effectiveness, offering a significant improvement over state-of-the-art methods..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240592 (2025)
A rapid classification study of black marker handwriting based on differential Raman spectroscopy (invited)
Hong JIANG
ObjectiveThe purpose of this study is to develop a simple, rapid, accurate, and non-destructive classification method for black ballpoint pen ink markings. This method is designed to address the need for a reliable and efficient technique capable of distinguishing ink signatures, which is critical for applications suchObjective The purpose of this study is to develop a simple, rapid, accurate, and non-destructive classification method for black ballpoint pen ink markings. This method is designed to address the need for a reliable and efficient technique capable of distinguishing ink signatures, which is critical for applications such as document authentication, forensic investigations, and trace evidence analysis. By using advanced spectroscopic techniques, the goal is to create a practical solution that allows for precise ink classification while preserving the integrity of the samples, making it suitable for use in real-world scenarios that require fast and reliable results. Methods A portable differential Raman spectrometer was employed to examine 102 black ballpoint pen ink samples. The spectrometer was set to dual-frequency output (Δλ ≤ 1 nm), with a single-frequency laser power of 250 mW, linewidth ≤ 0.06 nm, and a wavelength of 785 nm. The spectral range covered 180-2 800 cm-1, and the scanning time for each sample was 3 seconds. The raw spectra were preprocessed using the Z-score standardization method, and a spectral clustering model was applied for sample classification. Additionally, a two-layer feedforward neural network was used to train the data.Results and Discussions The 102 black ink samples were classified into six distinct categories. Further analysis showed that most samples contained titanium dioxide and titanium blue pigments. Oxalates and talcum powder exhibited weak correlations in the majority of the samples, while phenol and polystyrene showed significant correlations in some samples, though this relationship was less apparent in others. Spectral clustering further divided the samples into five categories. The accuracy of the differential Raman spectroscopy method was 97.1% for both the training and testing sets in the feedforward neural network. ROC curve analysis demonstrated high accuracy, indicating excellent performance in the classification task.Conclusions This method provides a simple, fast, and non-destructive approach to classifying black ballpoint pen ink markings, offering a new tool for ink classification and identification. With its high accuracy and ease of use, this method shows great potential for applications in document authentication, forensic analysis, and other areas where ink classification is crucial. The ability to perform accurate classification without damaging the samples makes it an ideal solution for various practical uses..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240606 (2025)
Two-dimensional infrared light field manipulation based on a digital micromirror device(invited)
Xiaoshuai MA, Dongxu ZHU, Zhuoren WAN, and Ming YAN
ObjectivePicosecond pulsed lasers with a GHz repetition rate are commonly used in biophotonics, laser industrial processing, and high-speed precision measurement due to their high repetition rate and short pulse duration. To control these systems, it is necessary to modulate the light field of the pulses. A 4F system iObjective Picosecond pulsed lasers with a GHz repetition rate are commonly used in biophotonics, laser industrial processing, and high-speed precision measurement due to their high repetition rate and short pulse duration. To control these systems, it is necessary to modulate the light field of the pulses. A 4F system is typically employed to spatially separate the frequency components of the laser. Conventional systems rely on diffraction gratings, which are limited by groove density and size, restricting spectral resolution to the nanometer scale. Higher spectral resolution can be achieved with echelon gratings that use higher diffraction orders, but these systems are often complex, sensitive to noise, and expensive. To address these challenges, this paper proposes combining a Virtually Imaged Phased Array (VIPA) with a diffraction grating to achieve two-dimensional spatial dispersion. This method enhances the generation and modulation of picosecond pulses with GHz repetition rates while improving anti-interference capabilities and overall spectral resolution.Methods This paper presents a two-dimensional light field modulation device based on a Digital Micromirror Device (DMD) (Fig.1 ). The system utilizes a VIPA to disperse the frequency of the input pulsed laser in the longitudinal direction. Additionally, a diffraction grating is employed to separate the frequency components in the lateral direction. The DMD is positioned as a mask in the Fourier plane of the system to achieve spatial modulation of the light frequency. The modulated light is then directed to a spectrometer or autocorrelator to evaluate the modulation effect.Results and Discussions The infrared camera was utilized to examine the spatial dispersion of optical frequencies in the Fourier plane. In this plane, the optical frequencies are distinctly separated, forming a clear linear array (Fig.2 ). A target was positioned in the Fourier plane for imaging, allowing us to investigate the relationship between two-dimensional space and optical frequency. The results of the spectral imaging correspond closely with the digits on the target (Fig.3 ). A DMD was placed at the focal plane to manipulate the two-dimensional optical field, achieving a signal-to-noise ratio of 46.02 dB (Fig.4 ). The system produced more than 52 pulses, with a repetition frequency of 16.67 ps (Fig.5 ), and allowed for relative delay control of high-repetition pulses at intervals of 250 ps and 500 ps (Fig.6 ).Conclusions To address the need for generating and controlling high-repetition-rate picosecond pulse light, a two-dimensional optical field control system based on a DMD is proposed. This system employs a virtual phase array and a diffraction grating to disperse the frequency of the input pulsed laser in two-dimensional space. A DMD is positioned at the Fourier plane of the system to facilitate optical field control. The physical model of two-dimensional optical frequency dispersion is analyzed to inform the selection of the key components of the system. The system's capability for two-dimensional optical frequency dispersion is validated through experimental results, which demonstrate that the optical field in the Fourier plane forms a linear array, with its spatial position corresponding to the optical frequency. To confirm the feasibility of the proposed system, a DMD is utilized to control the spatially dispersed optical frequencies, successfully generating more than 52 pulses with a repetition frequency of 16.67 ps. Additionally, relative delay control of 250 ps and 500 ps is achieved. The system showcases a signal-to-noise ratio of 46.02 dB. The proposed two-dimensional optical field control system demonstrates high spectral resolution and stability, features a high modulation speed, and provides an effective and practical approach for precisely controlling high-repetition-rate picosecond pulses..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240615 (2025)
Research on single-cavity dual-comb integrated mid-infrared light source(invited)
Caixin CHEN, Sixuan LI, Tingting LIU, and Ming YAN
ObjectiveThe mid-infrared spectral region of 2-20 µm, containing the molecular fingerprint region and atmospheric window, is considered one of the most promising spectral regions in the area of spectroscopic research. Mid-infrared dual-comb spectroscopy has significantly broadened the practical application domainsObjective The mid-infrared spectral region of 2-20 µm, containing the molecular fingerprint region and atmospheric window, is considered one of the most promising spectral regions in the area of spectroscopic research. Mid-infrared dual-comb spectroscopy has significantly broadened the practical application domains of optical frequency combs and emerged as a powerful tool for the detection of greenhouse gases and the analysis of trace gas owing to its high signal-to-noise ratio, high resolution, high acquisition speed, and wide spectral bandwidth. While Mid-infrared dual-comb systems based on electro-optic combs or micro-resonators combs offer simple experimental setups, they have limited spectral bandwidth and number of comb teeth. The mid-infrared systems based on mode-locked lasers have significant advantages over frequency precision and resolution but require expensive servo systems to stabilize the repetition rate and carrier envelope offset rate. The two combs output by a single-cavity dual-wavelength laser have good relative frequency stability without complex phase-locking systems. Therefore, mid-infrared dual-comb systems based on single-cavity dual-wavelength lasers have excellent characteristics including low cost, simple structure, high integration, and high coherence. However, a problem of great importance that needs addressing is the slow drift of repetition rate difference in free-running single-cavity dual-wavelength systems. Furthermore, there is still room for further optimization of the integration of mid-infrared dual-comb sources.Methods An all-polarization-maintaining single-cavity dual-wavelength erbium-doped fiber laser is built as the near-infrared light source, mode locked by semiconductor saturable absorber mirrors. The repetition rate difference (Δfr) of two pulses output by the near-infrared fiber laser is precisely stabilized by a phase-locking system (Fig.1 ). The detection of multi-heterodyne beat notes is performed to demonstrate the coherence between two pulses (Fig.4 ). The supercontinuum in Si3N4 waveguide is seeded by pulses after amplification. The two combs multiplex a single silicon nitride waveguide to optimize the integration of mid-infrared system (Fig.1 ).Results and Discussions The two pulses output by the single-cavity dual-wavelength erbium-doped fiber laser achieved highly overlapped spectrums with center wavelengths at 1560 nm and 1561 nm, and 3 dB spectral widths of 4.74 and 4.83 nm, respectively (Fig.2 ). The spectral widths of two pulses after amplification exhibited a significant increase, spanning from 1520 nm to 1580 nm, while maintaining a high degree of spectral overlap (Fig.2 ). Both pulses are amplified with an erbium-doped fiber amplifier so that they provide femto-second and high-intensity laser pulses (duration <100 fs, peak power >15 kW) to seed supercontinuum generation in Si3N4 waveguide (Fig.2 ). The near-infrared system demonstrated the capability to adjust Δfr across a wide frequency range, spanning from 0 to 41.9 kHz (Fig.2 ). Furthermore, the frequency jitter of Δfr was reduced from Hz to MHz order of magnitude using the phase-locking system (Fig.3 ). The radio frequency domain multi-heterodyne beat notes exhibited clear comb teeth, indicating high coherence between two combs (Fig.4 ). The broadband mid-infrared spectrums based on supercontinuum generation in the silicon nitride waveguide were successfully accomplished, covering the important spectral range of 3.2-3.6 µm (Fig.5 ).Conclusions The experimental results demonstrated the generation of a highly integrated and coherent mid-infrared dual-comb source based on supercontinuum generation in the silicon nitride waveguide seeded by a single-cavity dual-wavelength erbium-doped fiber laser. The frequency jitter of Δfr was effectively stabilized by a phase-locking system, resulting in a highly coherent dual-comb source with standard deviation of frequency jitter of Δfr reduced to the sub-mHz order of magnitude. The peak powers of both pulses after amplification were 15 and 17 kW, respectively. The high-intensity pulses multiplexed the same silicon nitride waveguide for supercontinuum generation, generating highly overlapping mid-infrared spectrums with spectrum widths of 2 µm and significantly improving the integration of the mid-infrared system. The broadband mid-infrared spectrums both cover the important spectral range of 3.2-3.6 µm, which is applicable to the detection of hydrocarbon molecules. The integrated mid-infrared system combines the advantages of high integration, high coherence, and broad bandwidth, providing a reliable light source for subsequent mid-infrared dual-comb spectroscopy applications..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20250064 (2025)
Highly diffraction-efficient prism gratings for the satellite-borne methane imaging spectrometer(cover paper·invited)
Quan LIU, Nenghua ZHOU, Kexin WANG, Zhiwei CHEN... and Xinhua CHEN|Show fewer author(s)
ObjectiveMethane (CH4) is one of the major greenhouse gases in the atmosphere, second only to carbon dioxide (CO2) in atmospheric content, but its contribution to the greenhouse effect per unit concentration is about 25 times that of CO2. With the development of human civilization, the total amount of CH4 has been incrObjective Methane (CH4) is one of the major greenhouse gases in the atmosphere, second only to carbon dioxide (CO2) in atmospheric content, but its contribution to the greenhouse effect per unit concentration is about 25 times that of CO2. With the development of human civilization, the total amount of CH4 has been increasing, which has an important impact on the Earth's climate change. By monitoring the concentration of CH4, the trend of earth's climate change can be more accurately assessed, and by identifying the main sources of CH4 emissions, corresponding measures can be taken to reduce emissions. Imaging spectrometers, which uniquely combine the advantages of imaging and spectral detection, have been widely used for remote sensing monitoring of the atmosphere and environment. The prism grating is an important dispersive element of the satellite-borne methane imaging spectrometer. Unlike the traditional grating, the prism grating structure is submerged in the high refractive index medium, which improves the dispersion and resolution of the grating by a factor of n, where n is the refractive index of the high refractive index medium. Therefore, compared with the traditional grating, the prism grating with the same spectral resolution can reduce the size of the grating to achieve more compact optomechanical structures. Therefore, the development of the prism grating is of great significance.Methods According to the needs of methane detection, and in order to further improve the compactness of the system, a high refractive index material, TiO2 dielectric film was introduced into the quartz prism grating. The diffraction characteristics of the prism grating were analyzed by the Finite-Difference Time-Domain (FDTD). In order to achieve a the first-order diffraction efficiency greater than 70% in the 2.275-2.325 μm band, for the rectangular groove shape, the duty cycle is in the range of 0.3-0.45, TiO2 film thickness between 165-170 nm, and slot depth between 800-950 nm. Experimentally, holographic lithography-ion beam etching combined with atomic layer deposition technique was used to fabricate prism gratings. Firstly, a layer of photoresist of appropriate thickness was uniformly coated onto the prism substrate. Second, holographic exposure and development were performed with a krypton ion laser (wavelength of 413.1 nm) to form a photoresist grating mask, and the slot shape of the photoresist grating mask was effectively controlled by adjusting the exposure and development time. Then, the prism substrate was etched by ion beam etching under the mask of photoresist, and the photoresist grating pattern was transferred to the prism substrate. The TiO2 film layer was then deposited using Atomic Layer Deposition (ALD) technique. Finally, vacuum coating was applied to create a reflective silver film.Results and Discussions Experimentally, a prism grating with a period of 1020 nm and an effective grating area larger than 110 mm×275 mm was fabricated using holographic lithography-ion beam etching combined with atomic layer deposition technology (Fig.5 ). The grating groove shape is trapezoidal, the groove depth is 872 nm, the central duty cycle is 0.42, and the thickness of the TiO2 film layer is 166 nm (Fig.4 ). The prism grating diffraction efficiency measurement adopts a dual optical path test method to eliminate the influence of the fluctuation of the light source and ensure the accuracy of the test. Figure 6 shows the experimental setup for measuring the diffraction efficiency. In the test, the beam splitter divides the light beam into two paths, one to monitor the stability of the light source, and the other for the measurement of the optical path. The experimental test shows that the first-order diffraction efficiency is greater than 70% in the 2.275-2.325 μm band.Conclusions The diffraction characteristics of prism gratings for methane imaging spectrometers are analyzed in detail. In order to further improve the compactness of the system, a high refractive index material TiO2 dielectric film has been introduced into the quartz prism gratings, and for the rectangular groove shape, when the duty cycle is in the range of 0.3-0.45, when the thickness of the TiO2 film layer is between 165-170 nm, and when the groove depth is between 800-950 nm, the grating's diffraction efficiency is higher than 70%. The diffraction efficiency is higher than 80% when the TiO2 film layer thickness is 165 nm and the slot depth is between 870-930 nm. High-quality photoresist grating masks were fabricated using holographic lithography in the experiments. The combination of Ar ion-beam etching and CHF3 reactive ion-beam etching was optimized during the etching process, and a quartz prism grating with a duty cycle of 0.42 in the middle, a slot depth of 872 nm, and trapezoidal bottom angles of 85.5° and 82° on both sides was fabricated, and then a 166 nm TiO2 film layer was deposited by using the Atomic Layer Deposition (ALD) technique, and finally a reflective silver film was vacuum-plated. In order to eliminate the influence of light source fluctuation, the dual-optical path test method was adopted for the diffraction efficiency measurement, and the first-order diffraction efficiency was greater than 70% in the band of 2.275-2.325 μm, which was successfully applied to the satellite-mounted methane imaging spectrometer..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20250081 (2025)
Optical system design of a hyper-spectral imaging sensor in broadband with large field of view for land observation(inner cover paper·invited)
Haisheng FENG, Jian WEN, Su WU, Tao WANG... and Lei YU|Show fewer author(s)
ObjectiveThe design of a high-performance hyperspectral imaging spectrometer with a large field of view and broad spectral range is crucial for advanced environment monitoring and remote sensing. Such instruments are needed on airborne and spaceborne platforms for observing environmental features, disasters, agriculturObjective The design of a high-performance hyperspectral imaging spectrometer with a large field of view and broad spectral range is crucial for advanced environment monitoring and remote sensing. Such instruments are needed on airborne and spaceborne platforms for observing environmental features, disasters, agriculture, and urban areas in detail. However, conventional hyperspectral systems struggle to simultaneously achieve wide spectrum coverage, high spectral resolution, and large field coverage. Traditional designs (e.g., Offner spectrometers) can reach a spectral resolution of 2-5 nm in the ultraviolet to near-infrared range with high optical throughput, but they often suffer from complex alignment, high cost, and limited numerical aperture. To address these challenges, this study aims to develop an optical system for a hyperspectral imager that covers a broad waveband (350-1000 nm) with a wide field of view (30°) and high resolution, providing an effective new method for ground object observation in environmental monitoring.Methods The optical system is divided into two parts: the telescope system and the imaging spectrometer, which are designed separately and then coupled at the slit to achieve the complete system. The telescope system adopts an off-axis two-mirror optical configuration, with the design operating in the ultraviolet-visible-near-infrared spectrum (350 nm to 1000 nm), a field of view of 30°, and an F-number of 2.5, ensuring high optical throughput. The imaging spectrometer improves upon the traditional Dyson configuration by replacing the reflective grating with a combination of a lens and a reflective grating. Specifically, the original reflective grating is replaced with a special combination where the front surface is a transmissive lens, and the rear surface serves as a grating diffraction element. This design overcomes the engineering limitations of the traditional Dyson configuration. The modified structure maintains concentricity while addressing aberrations caused by air gaps. Optical design software was used to optimize both the telescope and spectrometer systems, minimizing the aberrations across the entire field of view and spectrum. Finally, a prototype was built, and key performance tests (spectral resolution and imaging) were conducted to validate the design.Results and Discussions The optical performance of both the telescope and imaging spectrometer systems meets the design requirements. Figure 2 shows the Modulation Transfer Function (MTF) of the telescope system across the full field of view and full spectral range, where the MTF at the Nyquist frequency of 56 lp/mm corresponding to the detector is greater than 0.7, indicating excellent imaging quality. The spectrometer design likewise achieves MTF above 0.7 across the 350-1000 nm band with very small spot sizes due to effective aberration correction. When integrated, the full system still maintains an MTF above 0.5 at the detector’s Nyquist frequency (50 lp/mm) in all fields (Fig.8 ). The corresponding spot diagrams confirm that image spots are confined within 16 μm, validating the combined system’s optical quality. In laboratory tests, the prototype instrument’s spectral resolution was measured to be approximately 2.1 nm across the band (Tab.1 ), in close agreement with the 2.07 nm design prediction. In an outdoor imaging demonstration, the sensor captured clear hyperspectral images with fine detail (Fig.12 ), demonstrating a spatial resolution on the order of 1mrad. These results confirm that the developed hyperspectral system meets its design requirements and can effectively capture both spectral and spatial information of environmental targets.Conclusions This study developed a large field-of-view, broadband hyperspectral imaging sensor optical system that meets the stringent requirements for environmental observation. By combining a wide-aperture off-axis two-mirror telescope and an improved Dyson imaging spectrometer, the design achieves both high spatial resolution and high spectral resolution over the 350-1000 nm waveband. The key innovation lies in the modified Dyson spectrometer with a lens-grating element, which allows a larger numerical aperture and a practical configuration without sacrificing optical performance. The final instrument achieves a spectral resolution of approximately 2.1 nm and a spatial resolution of 1mrad, as validated by both simulation and prototype testing. The extended field and broad spectral capabilities of this hyperspectral imager make it highly suitable for airborne or satellite-based environmental monitoring. The outcomes of this design provide a strong engineering basis and reference for future development of advanced hyperspectral imaging systems in remote sensing..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20250112 (2025)
A survey on hyperspectral remote sensing unmixing techniques based on autoencoders(inner cover paper·invited)
Kai QIN, Yuxi HAO, Yingjun ZHAO, Xin CUI... and Qinglin TIAN|Show fewer author(s)
Significance Hyperspectral remote sensing faces the persistent challenge of mixed pixels, where sub-pixel heterogeneity compromises analytical accuracy. Hyperspectral unmixing addresses this by decomposing mixed pixels into pure materials (endmembers) and their abundances, enabling precise sub-pixel information extractSignificance Hyperspectral remote sensing faces the persistent challenge of mixed pixels, where sub-pixel heterogeneity compromises analytical accuracy. Hyperspectral unmixing addresses this by decomposing mixed pixels into pure materials (endmembers) and their abundances, enabling precise sub-pixel information extraction. Deep learning, particularly autoencoder-based architectures, has emerged as a transformative approach, surpassing traditional physical models and shallow neural networks in modeling complex spectral mixing mechanisms.Progress This survey systematically reviews autoencoder-based hyperspectral unmixing methods. First, the fundamentals of autoencoder networks are introduced, emphasizing their feature extraction, data reconstruction, and unsupervised learning capabilities, which align naturally with unmixing tasks. The evolution of model architectures is analyzed across three phases: 1) Single-network structures using basic autoencoders to validate feasibility; 2) Module-integrated networks incorporating convolutional, recurrent, or Transformer modules to enhance spectral-spatial feature modeling; and 3) Adaptive networks employing neural architecture search (NAS) or plug-and-play mechanisms for dynamic optimization.Conclusions and Prospects Autoencoder-based unmixing has demonstrated remarkable progress, yet challenges persist in handling complex mixing scenarios, computational efficiency, and real-world adaptability. Future directions include: 1) Architectural innovation through fusion with transformers, generative models, or multimodal data; 2) Physics-guided deep learning with tighter embedding of scattering/radiative models; 3) Lightweight deployment for onboard satellite processing; and 4) Cross-domain expansion into medical imaging or environmental monitoring. Advancing "physics-informed, data-adaptive" hybrid intelligence frameworks will be pivotal for achieving robust, interpretable, and scalable hyperspectral unmixing, ultimately enhancing precision in Earth observation and beyond..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20250131 (2025)
[in Chinese]
Photothermal simulation of laser protective/mid-infrared antireflective multilayer on ZnS window
Yuanrui CAO, Lishuan WANG, Huasong LIU, Shiqi YANG... and Xinshang NIU|Show fewer author(s)
Objective Infrared optical system is vital for aircraft in the terminal stage. As a fragile part of optical system, optical windows are under the threat of external environment, which could bring interference and damage to the windows. With the development of laser theory and technology, laser protection becomes more aObjective Infrared optical system is vital for aircraft in the terminal stage. As a fragile part of optical system, optical windows are under the threat of external environment, which could bring interference and damage to the windows. With the development of laser theory and technology, laser protection becomes more and more important for infrared optical windows. Laser protection can be sorted as linear protection, nonlinear protection, phase transition protection and so on. Among those protection methods, linear protection has the characteristic of low-cost and high performance on small angles incident, which is suitable for optical windows in the terminal stage. Linear protection method usually includes multilayers which have high reflectivity during the wave band of laser. Due to the small scale, high reflective films have lower laser damage endurance. Thus the laser induced damage theory of thin film is widely discussed around world. The relevant study about high laser induced damage threshold thin film is usually on single layer and double layers, also the film material is in visible band. It’s important to know the laser damage of laser protective multilayer for infrared optical window. Therefore, the laser thermal effect and optical characteristic of a laser protective/mid infrared anti-reflective multilayer is studied.Method A model of laser damage on laser protective/mid infrared anti-reflective multilayer is establish (Fig.1 ) with the help of finite element analysis method. Using two couples of material HfO2/SiO2 and ZnS/YbF3, two types of multilayers are designed on ZnS optical window substrate. By comparing the optical performance and laser thermal effect(Fig.3 -Fig.4 ), the multilayer structure of HfO2/SiO2 is used for further study. Single variable method is chosen and the effect of thickness, reflective index and extinction coefficient of layer material on multilayer is studied.Results and Discussions By comparing infrared window without protection, infrared window with single layer coating and infrared window with laser protective/mid infrared anti-reflective multilayer coating, the result shows that laser protective/mid infrared antireflective multilayer can reduce the temperature of optical window when laser incident. The analysis of HfO2/SiO2 multilayer concludes three parts: thickness of top three layers, reflective index of HfO2 and SiO2, extinction coefficient of HfO2 and SiO2. All three variables can influence the temperature of multilayer, reflectance at 1.06 μm and average transmittance during 3-5 μm. When the error of thickness of top three layers goes to 10% larger than the ideal value, temperature of multilayer grows to 88% higher, reflectance at 1.06 μm becomes 0.5% lower and average transmittance during 3-5 μm becomes 0.2% lower. When the error of reflective index of HfO2 goes to 10% smaller than the ideal value, temperature of multilayer grows to 55% higher, reflectance at 1.06 μm becomes 14.4% lower and average transmittance during 3-5 μm becomes 1.7% lower. The result shows that film/substrate temperature grows one order of magnitude higher when extinction coefficient kL grows two orders of magnitude higher. When the extinction coefficient of SiO2 grows 1×10-5 larger, the temperature becomes 93 degrees larger, meanwhile the temperature becomes 25 degrees larger for HfO2 at the same condition.Conclusions When preparing optical multilayers, there always have error on structure and material properties. By establishing finite element analysis model about laser protective/mid infrared anti-reflective multilayer on ZnS substrate, thickness, reflective index and extinction coefficient are studied to confirm the relationship with multilayer laser thermal effect and optical properties. At the range of -3%-3%, influence of thickness and reflective index error on temperature and optical properties can be ignored. By comparing two materials, the change of temperature caused by extinction coefficient of SiO2 is much stronger than HfO2, which mean that reducing the extinction coefficient of SiO2 is a good way to reduce laser thermal effect and advance the ability to endure laser attack. The study can provide an idea for designing laser protective and mid infrared antireflective multilayer with high induced laser damage threshold..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240492 (2025)
[in Chinese]
Research on digital control method for 633 nm iodine stabilized wavelength standard
Mingda CHU, Bo TANG, Jianbo WANG, Cong YIN... and Ming KONG|Show fewer author(s)
ObjectiveThe 633 nm iodine-stabilized He-Ne laser, based on saturated absorption frequency stabilization, is globally recognized as a national length standard due to its high stability, long lifespan, good wavelength reproducibility, visible wavelength, and compact size. Its laser frequency is used to reproduce the defObjective The 633 nm iodine-stabilized He-Ne laser, based on saturated absorption frequency stabilization, is globally recognized as a national length standard due to its high stability, long lifespan, good wavelength reproducibility, visible wavelength, and compact size. Its laser frequency is used to reproduce the definition of the meter. To meet the demand for portability and intelligence in the field of metrology and to expand the application scenarios of iodine-stabilized He-Ne lasers, this study investigates a digital stabilization method for the wavelength standard of the iodine-stabilized He-Ne laser based on a microprocessor. Digital algorithms are employed to implement key technologies for laser frequency stabilization, and the control system integrates Bluetooth communication, allowing operators to monitor the laser status in real time via host software and adjust optimization parameters as needed for different lasers.The specific algorithm principles are illustrated in Fig.1 , Fig.2 , Fig.5 and Fig.9 . This method overcomes the cumbersome adjustments, operational difficulties, and limited intelligence of traditional analog technologies, significantly enhancing the portability and intelligence of the 633 nm wavelength standard, laying a solid foundation for industrial applications.Methods This study investigates a digital control method for the iodine-stabilized wavelength standard based on a microcontroller (MCU). Key technologies such as signal modulation, harmonic demodulation, and frequency stabilization are implemented using digital algorithms. By simply adjusting a few parameters, the signal generation frequency can be easily modified, and harmonic signals can be demodulated with a high signal-to-noise ratio, enabling precise identification of absorption peaks and rapid locking of the laser frequency. The reliability of this digital stabilization method is verified through experiments measuring absolute frequency with an optical frequency comb and comparative tests with an analog control system, as shown in Fig.14 and Fig.15 .Results and Discussions Frequency measurements were conducted between a digitally stabilized unsaturated vapor cell iodine-stabilized laser and a digitally stabilized saturated vapor cell iodine-stabilized laser using a femtosecond laser optical frequency comb. The results show that the short-term and long-term frequency stability closely match the Allan variance results from the analog control system. The maximum frequency fluctuation of the unsaturated iodine-stabilized laser was 13 kHz, yielding a reproducibility of 2.74×10-11, while the saturated laser exhibited a maximum frequency fluctuation of 4 kHz, resulting in a reproducibility of 1.4×10-11. Overall, the frequency stability and reproducibility of the saturated laser controlled by the digital method were better compared to those of the unsaturated laser, with both showing performance metrics comparable to lasers using the analog control system. It significantly improves the portability and intelligence of the control system, showing great potential for broader applications.Conclusions This paper introduces a digital stabilization control method for the 633 nm iodine-stabilized wavelength standard, utilizing a self-developed digital control system to implement all functionalities traditionally achieved through analog control. The digital stabilization system can connect the laser control status to a host computer via Bluetooth communication protocol, allowing real-time monitoring of the laser status and supporting on-demand optimization of parameters to accommodate different laser heads. This enhances the system's interactivity and monitoring capabilities, making the entire stabilization control process simpler and further improving portability and intelligence. The stabilization control system is applied to both saturated and unsaturated gas cell 633 nm iodine-stabilized lasers, with their absolute frequencies calibrated using an optical frequency comb. The results show that the stabilization metrics of the digital stabilization laser are comparable to those of the traditional analog circuit stabilization laser. The digital stabilization method developed in this paper effectively controls the iodine-stabilized laser, overcoming the complexities of debugging in analog control systems and the low level of intelligence. It offers a higher level of intelligence and task expansion capability, meeting the new demands of intelligent and digital development in metrology..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240463 (2025)
Method for low-temperature displacement measurement of complex opto-mechanical structure based on symmetrical laser displacement sensors
Xiao TANG, Baoyu YANG, Yinong WU, Chengdong SHAN... and Yuhan LI|Show fewer author(s)
ObjectiveInfrared detection technology plays a crucial role in meteorological observation, ground detection, and astronomical observation. To enhance the sensitivity and reduce background noise of the infrared detector, it is necessary to lower the operating temperature. The radiant cooler has advantages of no vibratioObjective Infrared detection technology plays a crucial role in meteorological observation, ground detection, and astronomical observation. To enhance the sensitivity and reduce background noise of the infrared detector, it is necessary to lower the operating temperature. The radiant cooler has advantages of no vibration and no power consumption. It has been applied in various satellites to cool infrared detectors and optical components. To meet the cooling requirements, the radiant cooler has a complex mechanical structure. It is assembled at room temperature but operates at a low temperature condition. The thermal deformation of this complex structure causes relative displacement of the optical components, a phenomenon known as low temperature displacement, thus degrading imaging quality. The low temperature displacement of the radiant cooler should be obtained by calculation or measurement to serve as the basis for structure assembly. Aiming at the difficulty of measuring displacement within the complex structure using laser displacement sensors, an indirect measurement scheme is proposed.Methods A measurement test system using equipment and laser displacement sensors is established in this paper. The laser displacement sensor measures the surface of the equipment extending from the complex structure of the radiant cooler, converting the measurement of displacement of surfaces inside the radiant cooler into the measurement of displacement of the equipment (Fig.2 ). The measurement system uses multiple symmetrically arranged laser displacement sensors to measure simultaneously displacements of 3 directions (Fig.2 ). Multilayer insulation and heaters were used to keep the temperature of laser displacement sensors between (20±2) °C, and other parts of the measuring system in the vacuum chamber (Fig.3 ). A low temperature displacement measurement test of the mid-wave channel of a W-type radiant cooler was carried out, and the translational components of the low temperature displacement are obtained.Results and Discussions The sensor measurement value changes and low temperature displacement results in the X and Y directions were obtained (Tab.2 ). Sensor measurement values were obtained multiple times for the position of equipment in both normal condition and low temperature condition. Then the average values and standard errors were obtained for further derivation of low temperature displacement. The controlled temperature of sensors leads to a standard error of around ±10 μm. As a result, in the X direction, the low temperature displacement was 51 μm with a propagated error of ±11 μm; In the Y direction, it was -47 μm with a propagated error of ±24 μm. These results align well with the calculated values. The random error in sensors mainly arises from the sensor's linearity and the influence of temperature fluctuations. There are three types of system errors in the measuring system, which are caused by local thermal deformation of the sensors, the thermal deformation of the equipment, and rotation of the equipment (Fig.4 ). By averaging the values from opposing sensors on both sides, system errors can be reduced, thus enhancing the accuracy of the measurement results.Conclusions A measurement method using the equipment and laser displacement sensors was proposed, providing a solution for measuring deformation in complex structures. An equipment extending from the complex structure was designed, low temperature displacement was measured by using opposing sensors and taking the average, and the temperature of sensors were controlled to be more accurate. The low temperature displacement measurement test was carried out on a W-type radiant cooler, with measured low temperature displacements of 53 μm in the X direction and 47 μm in the Y direction, and the measurement uncertainty were around ±11 μm and ±24 μm, respectively. This method significantly reduced system errors caused by sensor thermal deformation and equipment rotation, thereby improving the accuracy. Compared with eddy current displacement sensors, the use of laser displacement sensors eliminated the need for low temperature calibration, further enhancing measurement precision, and the relative error of this method was comparable to that of mentioned cases using photogrammetry in the vacuum environment..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240477 (2025)
Online aberration detection method of the high-altitude deformable mirror in multi-conjugate adaptive optics system
Junjie LI, Xian RAN, Nanfei YAN, Dingkang TONG... and Changhui RAO|Show fewer author(s)
ObjectiveThe aberration of high-altitude deformable mirror (DM) in multi-conjugate adaptive optics (MCAO) system can lead to the decrease of the image contrast of the extended target and the decline of the closed-loop capability of the MCAO system. However, the aberration of the high-altitude DM in MCAO system is diffiObjective The aberration of high-altitude deformable mirror (DM) in multi-conjugate adaptive optics (MCAO) system can lead to the decrease of the image contrast of the extended target and the decline of the closed-loop capability of the MCAO system. However, the aberration of the high-altitude DM in MCAO system is difficult to detect online due to its special conjugate position. The existing MCAO system usually requires regular removal of the high-altitude DM from the system for aberration detection and flattening. Some MCAO systems under construction may integrate interferometers in the optical path of high-altitude DM, but it is very challenging. So far, there is no simple way to detect high-altitude DM aberration online in MCAO system. Therefore, it is necessary to seek an online aberration detection method for high-altitude DM with certain accuracy. For this purpose, a multi-direction wavefront sensor based method is proposed.Methods A multi-direction wavefront sensor can detect aberration information in different sub-area of a high-altitude DM from different line of sights. To retrieve the aberration of the entire high-altitude DM, it is necessary to detect wavefront in multiple directions. By using wavefront stitching method, the wavefronts detected by different line of sight directions should be concatenated in a certain position in order to obtain the complete wavefront information of the high-altitude DM.Results and Discussions Numerical simulation and experimental validation are carried out for the MCAO system parameters of the 1-meter new vacuum solar telescope at Yunnan Observatory. The detection capability of the proposed method for high-altitude DM is given (Fig.3 ). The relationship between the aberration detection accuracy and the conjugate height of deformation mirrors is also given, for the MCAO system of the telescope with 1 m aperture and 1' field of view, the proposed method has a lower error for the detection of deformed mirrors with 2 500 -4 000 m conjugate height (Fig.5 ). Finally, the proposed method is used to online detect the aberration of 313-units DM of NVST MCAO system, and the detection error is 43.6 nm (Fig.9 ).Conclusions This article addresses the problem of difficulty in online detection of static aberrations of high-altitude DM in MCAO systems. An online aberration detection method based on pupil multi-direction wavefront sensor is proposed for a dual-conjugate adaptive optics system. Through theoretical and simulation analysis, it is found that the method has detection ability for deformation mirror aberration modes higher than the pupil detection order. However, there is also wavefront stitching error caused by inaccurate measurement of the pupil's Piston mode. In the experiment, the 7×7 sub aperture multi-direction Shack Hartmann wavefront sensor of the NVST MCAO system was used to detect aberration of a 313-units deformable mirror conjugated to 2.8 km. There is very small difference between the results of high-order self-collimated Shack Hartmann and the results of proposed method, which verified the correctness of the proposed method. Without additional detection devices, the proposed method can effectively reduce the static aberration of high-altitude DM, providing good closed-loop initial conditions for MCAO systems..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240484 (2025)
Research on optical comb remote calibration based on fiber link network
Jing MA, Fei MENG, and Liang CHEN
ObjectiveOptical frequency combs, which are critical for bridging microwaves and optics, represent significant advancements in optical frequency metrology. They are of paramount importance for high-precision spectral analysis, accurate absolute distance measurement, and the high-precision frequency standards establishmObjective Optical frequency combs, which are critical for bridging microwaves and optics, represent significant advancements in optical frequency metrology. They are of paramount importance for high-precision spectral analysis, accurate absolute distance measurement, and the high-precision frequency standards establishment. Several metrology institutions in China have established optical wavelength and frequency standard devices based on optical frequency combs, providing traceability to microwave frequency standards for precise laser sources like frequency-stabilized lasers, ensuring accurate and reliable measurement of related quantities in industrial and research fields. As standard devices, optical frequency comb systems must undergo the same traceability process—comparison and calibration—to ensure accurate and reliable measurement values. However, due to their complex structure and large volume, the optical frequency comb devices used in practice are challenging to move for calibration. Therefore, the authors proposes a method of optical frequency comb comparison based on optical fiber network and optical "common-view method," mainly for the calibration of remote optical frequency combs, thus achieving regular traceability and calibration of optical frequency combs.Methods A method of calibrating remote optical comb using optical fiber networks is proposed. The optical combs including the standard comb and those to be calibrated, and the transmission laser to be measured are located at different nodes and connected to the network through optical fiber interfaces (Fig.1 ) respectively. During the comparison or calibration process, the absolute frequency of the transmission laser is measured simultaneously at different locations using the standard optical comb and other optical combs under test. The measurements obtained are then processed and corrected for errors. The main error sources in optical comb calibration are experimentally evaluated. Initially, the noise of optical fiber link 1 (Fig.3 ) was measured with the self-heterodyne loopback method. Subsequently, it also involves the experimental assessment of synchronization counting errors using manual and program-controlled counting modes respectively. It was also investigated whether there was consistency between the precision and stability of the rubidium clock and the optical frequency measurement results obtained using the optical comb under test, which was referenced to the rubidium clock. Finally, the calibration results of the optical combs are given comprehensively by correcting the above errors and absolute frequency measurement results.Results and Discussions The experiment measured the noise of the optical fiber link and the noise floor of the optical frequency transfer devices. The results were shown in Fig.4 . The transmitted frequency stability was measured to be 5.1×10-15 at 1 s, and 7.0×10-15 at 300 s respectively. Meanwhile, a frequency offset of -0.07 Hz was introduced by the fiber link. Because the rubidium clock utilized as the reference of the optical comb under test, the fiber link noise in the experiment can still meet the requirements of optical comb calibration without active fiber noise-cancelled. The authors also studied different counting methods and their accuracy differences, as shown in Fig.6 and Fig.7 . The relative frequency offset is 6.55×10-15 due to the manual counting method and 2×10-16 due to by the programmed counting method, respectively. Both methods can meet the comparison requirements. Considering the objects and application scenarios of the comparison method, the manual counting method is recommended. The calibration result of the optical comb’s accuracy is shown in Fig.9 , where the triangle represents the result of 6 relative frequency offsets, and the red line is their mean value: 2.08×10-10; measurement uncertainty is 3.16×10-13; The gray area is the relative uncertainty of SRS FS725 rubidium clock frequency value used in the experiment. Therefore, it can be seen from the Fig.9 that the optical absolutely frequency measurement results are within the rubidium clock frequency deviation range. The measurement stability of the comb of under test is investigated also, as shown in Fig.10 , and is consistent with the rubidium clock stability.Conclusions The authors demonstrate remote optical comb calibration and comparison via an optical fiber network, a valuable contribution to meeting the practical requirements. Initially, the authors introduce the principles of remote calibration and comparison with fiber network. Subsequently, by using the internal fiber network and ultra-stable laser, the measurement accuracy and stability of the optical comb under test, referenced to the rubidium clock, were calibrated using a standard optical comb that was referenced to a H maser, and provided the corresponding calibration values. Additionally, the optical frequency calibration results were compared with the performance of referenced rubidium clock, and the validity of the calibration work was verified. Meanwhile, the authors investigate the influence of optical frequency transmission noise due to fiber link, remote synchronous counting method, and frequency drift elimination method of measured laser using "common-view method" on remote calibration and comparison of optical comb, and quantitative evaluation results are given. This technique can be further expanded for application in the precision comparison and calibration of optical comb systems, referencing superior frequency standards such as H maser..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240516 (2025)
New definition and light quantum implementation method for basic units in international system of units
Yapeng PEI, Ya GAO, Junqi ZHANG, and Jun YANG
ObjectiveSince its inception, the definition methods of the International System of Units (SI) have not remained static. With the continuous advancement of science and technology, the definitional benchmarks for SI physical quantities have gradually shifted from material artifacts to quantum physical constants. AgainstObjective Since its inception, the definition methods of the International System of Units (SI) have not remained static. With the continuous advancement of science and technology, the definitional benchmarks for SI physical quantities have gradually shifted from material artifacts to quantum physical constants. Against this backdrop, at the 26th General Conference on Weights and Measures Held on November 16, 2018, the measurement methods for all seven base units of the SI were redefined, and these new definitions officially came into effect on May 20, 2019. Consequently, SI physical quantities are now defined in terms of quantum constants rather than material artifacts, endowing the SI with greater stability and universality. This transformation represents one of the most significant changes since the creation of the SI, marking a milestone in its history and having profound implications for scientific research and industrial processes. As a result, the realization of SI base units based on the new quantum definitions has become a focal point of scientific research worldwide. This paper introduces the new definitions of the seven SI base units and explores the current international approaches to their realization based on photonic quantum principles.Methods In this paper, both introductions on the importance of SI, and new definition methods, especially current light quantum methods, of basic units in SI have been implemented. Then, development of SI has been prospected, for providing research ideas and a way to rapidly understand relative contents for researchers. Based on investigations on abundant domestic and abroad literatures and works, combined with study experiences from authors of this paper, new definition method for seven basic units in SI has been described in detail. Then, the latest domestic and abroad realization method for obtaining basic unit values in SI has been analyzed and concluded.Results and Discussions In recent years, the rapid development of quantum optical measurement methods has led to breakthroughs in various detection technologies, including but not limited to single-photon detectors, single-photon sources, photon-number-resolving detectors, and photon detection surpassing the standard quantum limit. While these advancements have promoted the development of quantum metrology standards related to optical radiation, they have also introduced new challenges in single-photon metrology. In the field of candela quantization research, the United States has been a global pioneer in comprehensively advancing key technologies for photon metrology standards, achieving significant milestones such as high-speed superconducting nanowire single-photon detectors with detection efficiencies exceeding 90% and pW-level absolute cryogenic radiometers monitored by superconducting transition-edge sensors. Since 2008, the European Union has initiated research programs under the banner of the "Quantum Candela", establishing a roadmap framework for developing optical radiation standards based on photonic quantum principles.Conclusions The current SI base units are established by linking the units to defining constants through modern quantum science and technology, representing an innovative approach in the evolution of the SI and a significant milestone in its scientific and technological progress. In the future, any newly discovered physical phenomenon that can establish a clear relationship between a physical quantity and an SI defining constant can be utilized to realize SI units. With the advent of the quantum era of the SI, quantum technology-based metrological standards, particularly those based on photonic quantum standards, are being increasingly refined and matured, continuously pushing the boundaries of precision. A new quantum-based international measurement system will consequently be redefined. Furthermore, the new definitions of SI base units are driving the development of next-generation quantum measurement instruments, enabling more accurate and reliable measurements, shortening and flattening the traceability chain, and significantly impacting and challenging national governance systems, management frameworks, and traditional human perspectives..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240553 (2025)
Method for measuring deformation of large antennas on remote sensing satellites based on image and laser ranging fusion
Ziyi ZHU, Peng SUN, Mingli DONG, Bixi YAN... and Lei WANG|Show fewer author(s)
ObjectiveAfter the deployment of satellite antennas in orbit, they are prone to deformation and vibration due to various factors, which can affect observational performance. Single measurement methods struggle to meet the demands for high-precision in-orbit measurement of the antenna's surface shape in terms of botObjective After the deployment of satellite antennas in orbit, they are prone to deformation and vibration due to various factors, which can affect observational performance. Single measurement methods struggle to meet the demands for high-precision in-orbit measurement of the antenna's surface shape in terms of both performance and environmental adaptability. To address this, this paper proposes a high-precision satellite panel deformation measurement method that combines a camera and a scanning laser rangefinder.Methods First, a mathematical model for the fusion measurement of a camera and scanning laser rangefinder was established, consisting of a camera, a two-dimensional turntable, and a laser rangefinder, and the coordinate system transformation relationships between each module were analyzed. Next, a high-precision pointing method for the laser rangefinder based on BSLO-d bio-inspired optimization was developed. This method uses the BSLO optimization algorithm to globally optimize the two large rotation angles of the turntable, achieving preliminary autonomous alignment of the laser axis, and then uses image plane information to precisely align the laser axis. Subsequently, a 3D coordinate calculation method for the fusion of the camera and scanning laser rangefinder was proposed. This method utilizes range data and image plane coordinates, along with camera extrinsic parameters, the calibrated laser exit point, and the laser axis direction vector to compute the 3D coordinates of target points. Finally, the feasibility of the pointing angle calculation and 3D coordinate measurement methods was verified through experiments.Results and Discussions The proposed method demonstrated high precision in both pointing angle calculation and 3D coordinate measurement. From the BSLO-d pointing angle optimization result (Fig.5 ) and the pointing error experimental results (Fig.6 ), it is evident that the BSLO-d pointing method can quickly and accurately find the optimal rotation angle with minimal pointing error. By calculating the pointing angles for seven target points using BSLO-d and comparing them with the angles obtained through manual alignment, the results showed that the root mean square errors (RMSE) for the horizontal and vertical angles were 0.435 mrad and 0.787 mrad, respectively (Tab.1 ). In terms of 3D coordinate measurement, using the V-STARS high-precision photogrammetric measurement system as the ground truth, the measurement errors for 28 target points in the X, Y, and Z directions were statistically analyzed. The results showed that the RMSE in the X, Y, and Z directions were 0.948 mm, 0.268 mm, and 0.127 mm, respectively (Tab.2 ). From the error charts for the X, Y, and Z coordinates of the 28 points (Fig.7 ), it is clear that this fusion measurement method not only improves measurement precision but also demonstrates high stability. Additionally, the method allows for full-field measurement of the target points. Furthermore, from the experimental setup diagram (Fig.4 ), it can be seen that the measurement system is compact, easy to assemble, and calibrate.Conclusions This study addresses the autonomous measurement of satellite antenna deformation after deployment in orbit by proposing a 3D coordinate system that fuses a camera with a rotating scanning laser rangefinder. The system accurately integrates data from multiple sensors, and the feasibility and accuracy of the method were experimentally validated. The method successfully achieves autonomous alignment of the laser rangefinder through visual guidance via a non-orthogonal two-dimensional turntable, and computes the 3D coordinates of target points using distance and image plane data. Experimental results demonstrate that this method is fast, accurate, and provides high precision in both pointing angle calculation and 3D coordinate measurement. Additionally, the system is compact, easy to assemble, and calibrate. Thus, this method offers a new solution for high-precision in-orbit measurement of satellite antenna surfaces and provides a new technical path for other application scenarios..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240555 (2025)
Fusion metrology of nano-length traceability chain
Yujie ZHANG, Lihua LEI, Yuqing GUAN, Liqin LIU... and Gang LING|Show fewer author(s)
ObjectiveIn terms of micro-nano structure measurement and determination, the fusion measurement of multiple traceability methods and measurement methods has become a trend of future measurement. Taking linewidth measurement as an example, it is difficult for a single metrological measurement method to provide accurate Objective In terms of micro-nano structure measurement and determination, the fusion measurement of multiple traceability methods and measurement methods has become a trend of future measurement. Taking linewidth measurement as an example, it is difficult for a single metrological measurement method to provide accurate structural information, and it must rely on the full combination of multiple measurement methods (such as scanning electron microscopy measurement, light scattering measurement, and small-angle X-ray measurement) and traceability methods (metrological atomic force method and transmission electron microscopy measurement). The comparison level of the traceability chain with line width as the carrier is limited by the line width edge and the probe effect, which leads to the bottleneck of the comparison accuracy. Therefore, it is urgent to seek new alignment carriers as a breakthrough to improve the accuracy of traceability chain alignment. The idea of using a frequency-doubling silicon grating combining atomic lithography and soft X-ray interference lithography as the comparison carrier to carry out nano-length traceability comparison was proposed.Methods A smaller-scale grating reference material with a grating spacing of 1/2 of the chromium atom self-traceable grating is developed based on the combination of atomic lithography and soft X-ray interferometric lithography, which can be directly traced to the wavelength corresponding to the atomic transition frequency and has the characteristics of self-traceability. Based on the characteristics of high precision and small size, it can be used as a comparison carrier, and the fusion measurement of nano-length traceability chain is proposed, which can effectively improve the measurement accuracy, equivalence and comparability of "top-down" and "bottom-up" traceability in China by studying the data fusion law between "top-down" and "bottom-up" traceability chains.Results and Discussions The high-precision small-size grating can effectively avoid the error source of comparison with line width as the carrier (Fig.3 ), so as to improve the comparison accuracy of the two traceability chains. A new method for fusion metrology of nano-length traceability chain is proposed by using the frequency-doubling grating generated by the combination of chromium atom lithography and soft X-ray interferolithography as the carrier (Fig.1 ). The traceability chain comparison with the grating as the carrier satisfies the two necessary conditions of high precision and small size, and the accuracy of fusion measurement can be effectively improved through the study of measurement error transmission and data fusion law.Conclusions On the basis of the grating made based on atomic lithography technology, the soft X-ray interferometric lithography technology is combined with the grating technology to produce a grating comparison carrier that can be used as the fusion measurement of nano-length traceability chain, and a new idea of using a frequency-doubling silicon grating combined with atomic lithography and soft X-ray interferometric lithography as a comparison carrier to carry out the comparison of nano-length traceability chain is proposed, which can effectively improve the measurement accuracy, equivalence and comparability level of "top-down" and "bottom-up" traceability in China. The successful implementation of the fusion measurement of nano length traceability chain mentioned in this paper is of great foundational significance for establishing a measurement system that integrates multiple traceability pathways and measurement methods and improving the level of advanced nano manufacturing in China..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20250074 (2025)
Attitude calibration method of micro nano grating based on laser interferometer
Wenzhe ZOU, Yuqing GUAN, Chuangwei GUO, Yujie ZHANG... and Gang LING|Show fewer author(s)
ObjectiveMicro nano gratings, as subwavelength structural optical benchmarks, are periodic structural optical elements that achieve precise control of light field phase and energy through photolithography, etching, ion implantation, mechanical grinding, polishing, and coating steps. The displacement sensing system consObjective Micro nano gratings, as subwavelength structural optical benchmarks, are periodic structural optical elements that achieve precise control of light field phase and energy through photolithography, etching, ion implantation, mechanical grinding, polishing, and coating steps. The displacement sensing system constructed based on its diffraction interferometry principle, with grating period as the quantized traceable measurement benchmark, breaks through the limitations of traditional laser interferometers that rely solely on a single wavelength. It combines the anti multimode interference characteristics under complex working conditions, miniaturized embedded integration characteristics, and cross scale compatibility, and has become the core technology for sub nanometer displacement sensing in extreme environments. The urgent need for robustness to multi physical field coupling in opto mechatronic integrated systems has made precise decoupling of error transfer links in micro nano gratings a cutting-edge challenge in the field of ultra precision measurement. Research has shown that when a grating is integrated as an embedded sensing unit in an electromechanical coupling system, its multi degree of freedom pose deviation (roll, pitch, yaw) will cause asymmetric distortion of the diffraction field and nonlinear drift of the interference phase, forming a nanoscale error transmission chain. In recent years, by integrating vector diffraction field reconstruction, quantum phase reference feedback, and multi axis collaborative control theory, the academic community has gradually established a grating displacement table cross scale error collaborative suppression system, providing a theoretical foundation for the paradigm evolution of grating metrology from "static calibration" to "dynamic control". This research direction not only promotes the innovation of the grating manufacturing assembly traceability technology chain, but also opens up a new path for autonomous perception calibration of intelligent equipment at the interdisciplinary level.Methods In order to address the common problem of phase distortion in the diffraction field caused by multidimensional installation errors of micro nano gratings, and to reduce the nanoscale errors caused by position calculation, this paper proposes a new method of pose co-calibration based on the phase modulation characteristics of vector diffraction fields. This article analyzes the causes of pose errors in micro nano gratings, describes the assembly state of one-dimensional gratings relative to displacement tables using the roll, pitch, and yaw angles of one-dimensional gratings, and quantitatively analyzes their error values based on vector diffraction theory. Innovatively constructed a traceable calibration chain based on laser interferometer benchmark, and achieved multi degree of freedom collaborative calibration of grating displacement table through a composite control strategy of extreme value tracking and interferometer indication closed-loop feedback.Results and Discussions The grating pose calibration method based on laser interferometer can effectively reduce the pose error of micro nano gratings in the grating displacement measurement system, and is feasible.Through effective calibration during the system construction process, the measurement uncertainty of the calibration method can be effectively controlled, and the introduction of laser interferometers enables traceability of pose calibration for micro nano gratings. The final synthesized uncertainty of this calibration method is 2.17 nm, with a maximum calibration difference of 0.019 μm. The standard deviation of the displacement measurement system after calibration is 0.32 nm. Prove that the method is stable and reliable.Conclusions This paper proposes a calibration method based on feedback data from laser interferometers to address the problem of grating pose errors in nanodisplacement measurement systems. The method involves single axis correction of the three-axis pose (pitch, roll, yaw) of micro nano gratings. The method achieves multi degree of freedom collaborative calibration between gratings and displacement tables through a composite control strategy of extreme value tracking and interferometer indication closed-loop feedback. In the process of system construction, the calibration of each important link was carried out to reduce the overall system uncertainty, and the effectiveness of this method was verified through experiments. Finally, the measurement uncertainty of this calibration method was analyzed..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20250093 (2025)
[in Chinese]
Femtosecond laser fabrication of high-temperature sensor based on fiber Fabry-Perot interferometer
Qianhao TANG, Yixin ZHU, Huaijin ZHENG, Shengjie LI... and Shuangchen RUAN|Show fewer author(s)
ObjectiveWith the development of industry, high temperature sensing is widely used in the industrial field, which not only requires high precision, good stability and fast response of high temperature sensors, but also hopes to reduce the manufacturing cost of sensors. Optic fiber high temperature sensor can well meet Objective With the development of industry, high temperature sensing is widely used in the industrial field, which not only requires high precision, good stability and fast response of high temperature sensors, but also hopes to reduce the manufacturing cost of sensors. Optic fiber high temperature sensor can well meet the above application requirements, and has a good development prospect. At present, the common fiber optic high temperature sensor structure too pursues the improvement of sensitivity at the expense of stability. At the same time, the processing process is complex and difficult to batch preparation.Methods A femtosecond laser micromachining system was employed to precisely ablate the air microcavity located at the end of a single-mode fiber, thereby creating a fiber FPI structure. Three distinct structures of varying dimensions were fabricated, and the reflection spectra of these samples were analyzed using Fourier transform techniques. The temperature sensing characteristics of the samples within the range of 200-800 ℃ were evaluated. Finally, the high temperature stability of the sensor at 800 ℃ was measured, and the detection accuracy was calculated.Results and Discussions The three samples were processed by femtosecond laser, and the air microcavities were all 30 μm, and the distance between them was 20, 40, 80 μm, respectively. The maximum error of sample processing size was not more than 1 μm. The preparation process had good repeatability, the insertion loss was around 11 dB, and the fringe contrast of the samples was greater than 10 dB. The temperature sensing characteristics of the sensor at 200-800 ℃ were measured. The temperature sensitivity at 200-400 ℃ was 8.6 pm/℃, and the high temperature sensitivity at 400-800 ℃ was 11.3 pm/℃. Finally, in the stability test experiment, the maximum deviation of the measured temperature did not exceed ±1.77 ℃, indicating that the structure had good stability.Conclusion A high-temperature optic fiber sensor based on Fabry-Perot interferometer was designed and fabricated by using femtosecond laser microfabrication. The sensor has high accuracy, good stability, compact probe structure, small size, flexibility, and strong adaptability. It has potential application value in high-temperature measurement environments with long distances and narrow spaces. Three different sizes of samples were prepared in the experiment, verifying the feasibility of the processing technology and providing certain reference value for industrial low-cost and mass production..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240541 (2025)
Alignment integration technology of micro-polarizer array sensor
Yuanyu JI, Chuanlong GUAN, and Jinkui CHU
ObjectivePolarization imaging reveals distinct targets that are difficult to distinguish in conventional optical imaging by capturing the polarization or depolarization characteristics of light after it is reflected, refracted, or scattered by different objects. To achieve better polarization imaging, various polarizatObjective Polarization imaging reveals distinct targets that are difficult to distinguish in conventional optical imaging by capturing the polarization or depolarization characteristics of light after it is reflected, refracted, or scattered by different objects. To achieve better polarization imaging, various polarization imaging systems with different structures and principles have been developed to obtain and utilize polarization information, such as polarization angle and degree of polarization. Among these, focal plane polarization imaging systems inscribe four polarization directions into each pixel region, simultaneously acquiring polarization intensities from different directions. Compared with other polarization sensor image acquisition methods, this system is compact in structure and offers strong real-time performance. However, the fabrication of focal plane polarization imaging sensors currently depends on high-precision electron beam lithography equipment. By contrast, nanoimprint lithography (NIL) provides a low-cost and efficient approach to directly integrate micro-nano polarizer arrays onto the surface of image sensors. The integration technology of nanograting arrays onto image sensors mainly consists of two parts: the fabrication of flexible UV nanoimprint composite masks and the alignment imprinting onto image sensors. This paper focuses on the cross-scale alignment issue of micron-scale patterns with millimeter-scale templates during the CMOS image sensor alignment imprinting process.Methods Based on an analysis of the impact of alignment deviations on the polarization performance of micro-polarizers during the fabrication process (Fig.4 -Fig.5 ), a microscopic optical image-based alignment method is proposed to ensure pixel alignment between the micro-polarizers on the composite mask and the image sensors (Fig.8 ). A high-precision alignment device was developed, which can be structurally divided into three components: a microscopic vision module, an alignment adjustment module, and an image processing module (Fig.9 ). During the alignment process, the system does not rely on high-precision alignment marks. The microscopic vision module uses high-magnification microscopic imaging to capture the microstructures of the millimeter-sized nanoimprint template and the sensor surface. The image processing module employs Hough transform to identify the linear features of the contours of the micro-polarizer array and the pixel array, and an outlier iterative elimination algorithm (Fig.11 ) is used to accurately compute the orientation and position information of the target array. Finally, the alignment adjustment module performs alignment adjustments using a precision micro-motion platform.Results and Discussions A testing device is established to evaluate the alignment performance of the integrated micro-polarizer array sensor (Fig.12 ). The sensor's response to specific linearly polarized light is observed, demonstrating consistency with the designed performance of the micro-polarizer array (Fig.13 ). A further quantitative analysis is conducted on the uniformity of the pixel response to linearly polarized light for pixels with the same polarization sensitivity on the integrated sensor. The standard deviation of pixel responses across different regions of the aligned micro-polarizer array sensor ranges within the range of 15 to 40. In contrast, for an unaligned micro-polarizer array sensor, the standard deviation of pixel responses across different regions is around 100 (Fig.16 ). This indicates that the overall polarization response uniformity of the sensor is significantly improved after alignment. Pixel alignment is achieved within a polarized pixel range of 500×500, reducing the effective pixel loss caused by crosstalk. Furthermore, an outdoor experimental setup was set up to detect the distribution of polarized light in the sky under natural conditions (Fig.17 ). The polarization angle diagram calculated based on the light intensity output from the sensor can clearly identify the solar meridian and distinguish buildings at the edges of the image (Fig.18 ). Conclusions Aiming at the cross-scale alignment challenges in the integration technology of micro-polarizer array sensors using nanoimprint lithography, an alignment scheme was designed. This scheme avoids the use of high-precision alignment marks and directly captures the geometric features of the surface microstructures of the alignment object and target using microscopic optical imaging. The probabilistic Hough transform algorithm and the outlier iterative elimination algorithm are employed to ensure the efficient and accurate extraction of geometric features, achieving an overall torsional error of no more than 0.02° and pixel dislocation error within 20% of the pixel size. The final micro-polarizer array sensor demonstrates uniform and highly consistent pixel responses in the same polarization-sensitive direction. Pixel alignment is achieved within a polarized pixel range of 500×500, allowing more effective utilization of all the polarization information obtained by the pixels..
Infrared and Laser Engineering
- Publication Date: May. 25, 2025
- Vol. 54, Issue 5, 20240578 (2025)
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