Journals >Acta Photonica Sinica
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2023
Volume: 52 Issue 4
25 Article(s)
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Design and Development of Absorbance Sensor for Ocean Towing System
Qingsheng XUE, Xijie HAO, and Fupeng WANG
Absorbance is an important parameter to describe the optical properties of sea water, it can directly reflect the transparency of sea water and the degree of light attenuation of sea water, it is one of the inherent optical properties of water. The results of absorbance of sea water have an important guiding effect on Absorbance is an important parameter to describe the optical properties of sea water, it can directly reflect the transparency of sea water and the degree of light attenuation of sea water, it is one of the inherent optical properties of water. The results of absorbance of sea water have an important guiding effect on the field of underwater optical communication and spectral detection. Too little spectral information can be obtained by the existing absorbance measurement equipment, hyperspectral absorbance measurement equipment has problems such as expensive price and large volume, so it is not suitable for large range of in situ absorbance detection. In order to achieve a wide range of long-term measurement of sea water absorbance, an in-situ absorbance sensor suitable for ocean towed observation systems is designed. The sensor can realize the synchronous detection of multi-band absorbance, select LED as a light source of the sensor, and select photodiode as detector respectively of the sensor, the optical path of the sensor absorption cell is 10 mm. The transmission optical path structure design is adopted, the multi band LED is located at the transmitter end of the sensor, and the detector is located at the receiving end of the sensor. And realize the miniaturization and low-power consumption design of the sensor through the multi-channel side-by-side arrangement of the photomechanical structure layout and highly integrated circuit optimization. The sensor selected 8 LED light sources of relatively important feature bands at 340~980 nm for absorbance measurement. The 8-channel LEDs at the transmitting end are modulated with 781.25 Hz sine wave signals, after the beam is collimated, it is attenuated through the absorption cell, and then converged to the photodetector at the receiving end. After receiving the signal, the photodetector demodulates and analyzes the signal using a digital phase-locked algorithm based on the cross-correlation detection principle to achieve absorbance measurement. From the optical perspective, the reference detector was placed near the LED light source to calibrate the influence of light source fluctuation on the absorbance result. The sensor used a narrow-band filter with a high cut-off depth to filter out stray light outside the band at the receiving end. At the same time, it can avoid the influence of scattered light from adjacent channels. From the circuit point of view, the signal received by the detector is realized by a digital phase locking algorithm. The narrow-band optical filter and digital phase-locked processing algorithm at the receiving end can effectively suppress the influence of background stray-light and signal interference noise, and realize in situ detection of seawater absorbance in the open field environment, which is the key to achieve high precision absorbance measurement. The laboratory precision test and the comparison test with the same type of equipment and the South China Sea test prove that the absorbance sensor has the advantages of high precision, small volume, strong anti-environmental interference ability, low power consumption and stability, and the precision of the sensor is better than 0.000 1 AU (Absorbance Unit). In addition, the absorbance sensor uses photodiode as the detector to make the whole system have a fast response speed, which can capture the rapid change of the absorbance of the marine environment, the absorbance sensor can achieve different detection requirements by replacing the light source of other bands and the corresponding filter. During the use of the sensor, temperature and salinity sensors are also used to adjust the temperature and salinity of the absorbance results, so as to obtain more accurate measurement data. The absorbance sensor can also adapt to the complex water environment by designing circulation cell of different path lengths. It can also carry ships in coastal waters to achieve continuous detection by sailing, which has a wide range of application prospects..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0401001 (2023)
Influence of Outer Scale of Ocean Turbulence on Propagation Characteristics of Gaussian Beams
Yi YANG, Huan NIE, Xiaobo WANG, Jianlei ZHANG, Hanyu HE, Xiaofen QIU, and Yunzhou ZHU
Underwater laser communication has the advantages of high information transmission rate, low delay, and high confidentiality, making it gradually become a research hot spot. Turbulence effects are usually studied based on the turbulent refractive index power spectrum model. The power spectrum inversion method for oceanUnderwater laser communication has the advantages of high information transmission rate, low delay, and high confidentiality, making it gradually become a research hot spot. Turbulence effects are usually studied based on the turbulent refractive index power spectrum model. The power spectrum inversion method for oceanic turbulence phase screen simulations lacks a model for the effect of outer scale on optical properties, so that the recently proposed new models for the refractive index power spectrum of ocean turbulence that include outer scale have all been made single-parameter by simplifying the two-parameter exponent. However, the advantage of this simplification is not absolute in the calculation of optical parameters, in particular, the Nikishov power spectrum model in the form of a linear combination of scalar spectra is still used in most studies on the transmission theory in ocean turbulence. Therefore, in this paper, we first analyzed the necessity of the introduction of the outer scale turbulent parameter, and then proposed a modified Nikishov power spectrum model of oceanic turbulence containing outer scale parameters based on the Nikishov power spectrum model, and verified the model on the convergence characteristics of the power spectrum and the normalized spectrum. By comparing with the existing power spectrum models containing outer scale parameters, the proposed modified spectral form is simpler and easier to separate the variables than the existing power spectral models containing outer scale parameter, and can be used in subsequent studies to derive analytic theoretical expressions for optical parameters based on fluctuating optics theory, with the help of hypergeometric functions, to form a complete system description of the exponential spectrum. Considering the joint effects of seawater absorption, scattering and turbulence, based on the proposed power spectrum model, a phase screen simulation model of the composite seawater channel is established, and the influence of the outer scale on the probability density function of the received light intensity is studied under the exponential power spectrum model using the Monte Carlo simulation method based on the phase screen. The effects of outer scale on the optical properties of Gaussian beams and the effects of outer scale on the time-domain expansion of signals are analyzed by fluctuation theory and phase screen simulations. The proposed modified spectral model of the exponential Nikishov spectrum is verified, and a Monte Carlo simulation model of the phase screen of the composite seawater channel is established based on this model. The variation of optical properties with transmission distance and turbulent outer scale is investigated by phase screen simulation, and the correctness of the phase screen method to simulate the effect of outer scale on optical properties is verified by comparison with the defining equation calculation. The variation of turbulent time-domain expansion properties with transmission distance and outer scale is also innovatively investigated. The results show that the effect of turbulence effect on optical properties increases with the increase of transmission distance, which is reflected in the beam drift, beam diffusion and scintillation index properties; the larger the outer scale, the greater the effect of turbulence effect on beam drift, beam diffusion and scintillation index properties, the greatest effect is also found at infinite outer scale. The probability density distribution and the time-domain expansion characteristics of the optical intensity are less affected by the turbulent outer scale and are mainly affected by the transmission distance, and the time-domain expansion value shows an approximate quadratic function with the transmission distance. The results of this paper will contribute to an in-depth understanding of oceanic turbulence and provide a theoretical basis for further analysis of Gaussian beam propagation in seawater and the effect of turbulence on signal transmission characteristics..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0401002 (2023)
Characteristics of Low Coupling Coefficient，Narrow Linewidth，High Order Bragg Grating Based on SiO2 Waveguide
Xiaojie YIN, Jinghui WANG, Zhiyuan ZHENG, and Zeguo SONG
As an important frequency selective component, Bragg grating is widely used in the fields of lasers, sensors and filters. Especially in the field of narrow linewidth lasers, Bragg grating is an important component for narrowing the linewidth of lasers. The linewidth and reflectivity of Bragg grating itself have a decisAs an important frequency selective component, Bragg grating is widely used in the fields of lasers, sensors and filters. Especially in the field of narrow linewidth lasers, Bragg grating is an important component for narrowing the linewidth of lasers. The linewidth and reflectivity of Bragg grating itself have a decisive influence on the performance and reliability of narrow linewidth lasers. The narrower the linewidth of the Bragg grating itself is, the greater the mode competition between the cavity mode of the gain chip and the longitudinal mode of the grating will be improved, and the temperature stability of the wavelength will also be improved accordingly.In this paper, SiO2 waveguide material with low transmission loss was selected. The refractive index of the waveguide cladding was 1.444 7, the refractive index of the core layer was 1.455 6, and the refractive index difference was 0.75%. Using the single-mode condition simulation of the waveguide transmission mode, the cross-sectional size under the single-mode condition of the waveguide was calculated to be 6 μm×6 μm. Under this single mode condition, starting from the wavelength equation of Bragg grating satisfying Bragg reflection condition, the paper mainly analyzed the coupling between TE-TE modes. Through the derivation of the refractive index change formula and the normalization equation, the three-dimensional numerical model of the coupling coefficient of Bragg grating was finally deduced, and the coupling coefficient variation relationship corresponding to the change of the grating structure was simulated when the grating etching depth increases from 1 μm to 6 μm and the duty cycle increases from 0.5 to 0.8. On this basis, the paper also further analyzed the numerical relationship between the etching depth, duty cycle of waveguide Bragg gratings, the reflectivity and FWHM of the gratings, thus establishing a high-precision theoretical model for the design of waveguide Bragg gratings, and designing a series of waveguide Bragg grating devices under this model. The grating waveguide was prepared by contact ultraviolet exposure process with large process tolerance and Inductive Coupled Plasma (ICP) etching, then the waveguide Bragg grating device wafer was prepared by Plasma Enhanced Chemical Vapor Deposition (PECVD) growing the upper cladding with the same refractive index as the lower cladding. Then, the waveguide Bragg grating device designed in this paper was finally prepared by cutting, polishing, and other back-end processes.In the paper, a waveguide Bragg grating test platform was built using a 1 550 nm broadband spectrum light source, a circulator, a spectrum analyzer and a single-mode fiber. The fabricated devices were tested and analyzed in detail. The final test results show that the data relationships between the etching depth, duty cycle of the SiO2 Bragg grating prepared in this paper and the coupling coefficient, reflectivity and FWHM of the device were completely consistent with our theoretical model. Finally, a SiO2 waveguide Bragg grating device with 1 554.053 nm center wavelength, -8.2 dB reflectivity and 89 pm FWHM was designed and fabricated. The low coupling coefficient, narrow linewidth and high-order Bragg grating devices designed and fabricated in this paper were simple in process and low in cost, and have broad application prospects in the fields of filters, sensors and external cavity narrow linewidth lasers..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0405001 (2023)
Image Restoration Method of Synthetic Aperture Optical System Based on Sparse Prior
Shuo ZHONG, Bin FAN, Dun LIU, Haibing SU, Hao ZHANG, Hu YANG, and Artem NIKONOROV
In order to obtain higher imaging resolution, it is usually necessary to increase the aperture of the optical system. However, a single large aperture optical system has problems such as difficult processing and heavy weight. Synthetic aperture technology uses multiple sub-apertures to obtain the resolution equivalent In order to obtain higher imaging resolution, it is usually necessary to increase the aperture of the optical system. However, a single large aperture optical system has problems such as difficult processing and heavy weight. Synthetic aperture technology uses multiple sub-apertures to obtain the resolution equivalent to a single large aperture in space according to a certain arrangement. However, the following problems are imaging blurring, mainly due to the reduction of light transmission area and system common phase error. The reduction of the light transmission area will cause the change of the point spread function of the system, which will lead to the attenuation of the optical transfer function in the middle and low frequency parts.The common phase error is caused by the synthetic aperture system in the process of light path construction and instrument assembly. The common phase error makes the beams of each sub-aperture no longer have the same phase, and the beams passing through each sub-aperture cannot form interference after reaching the image plane, which leads to the blurring of the final image, even in severe cases, it's can not be imaged. All of the above results in the degradation of the imaging quality of the synthetic aperture system. Some large errors can be solved by improving or debugging the optical system, while the inevitable errors can only be restored through the angle of the image. To solve this problem, a sparse prior image restoration method based on norm is proposed. Firstly, the degradation process of the synthetic aperture system is modeled, the influence of different filling factors on the modulation transfer function is analyzed, and the PSF with piston error is calculated. Further, through statistics, it is found that most of the dark channel values of the image degraded by the synthetic aperture system become non-minimum values, which is found to be caused by the convolution process through mathematical verification. Therefore, this paper proposes an image restoration algorithm model based on the dark channel theory. In the objective function, the dark channel and gradient prior are constrained in the form of L0 norm as the regularization term, and the point spread function calculated under the optical system array structure is used as the initial fuzzy kernel. Finally, to solve the L0 norm and nonlinear problems, a semi-quadratic splitting method is proposed to calculate alternately, and obtain the estimated clear image. In the experiment part, the simulation experiment and the actual scene experiment are carried out respectively. Peak Signal-to-Noise Ratio(PSNR), Structure Similarity (SSIM) and Gray Mean Gradient (GMG) are used as evaluation indicators, and are compared with the Wiener Filter method and Richardson-Lucy method. In the simulation experiment, five groups of different piston error parameters are set first, and the image is degraded according to these parameters. The average PSNR, SSIM and GMG of the method in this paper reach 23.13 dB, 0.77 and 20.31. Evaluation and comparison show that the method in this paper is applicable to the degraded image with piston error. After that, seven groups of different scene images are set, and the average PSNR, SSIM and GMG of the method in this paper reach 23.79 dB, 0.80 and 30.28 with small variance, which verifies the stability of the method in different scenes restoration. Finally, in the experiment of the actual scene, a special synthetic aperture camera is used to take remote sensing images and real scene images respectively, and the synthetic aperture array structure parameters are set to be consistent with the simulation. In the remote sensing image experiment, the average PSNR, SSIM and GMG of the method in this paper reach 23.04 dB, 0.65 and 24.58. In the real scene experiment, the average PSNR, SSIM and GMG of the method in this paper reach 24.11 dB, 0.90 and 15.27, which shows the effectiveness of the method in the actual scene..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0410001 (2023)
Improved Faster-RCNN Based on Multi Feature Scale Fusion for Automatic Detection of Microaneurysms in Retina
Weiwei GAO, Yile YANG, Yu FANG, Bo FAN, and Nan SONG
The presence of Microaneurysms (MAs) is the earliest detectable small abnormality of Diabetes Retinopathy (DR), a retinal disease that represents the leading cause of blindness among the middle-aged population globally. So the detection of MAs in fundus images is an important and challenging step for early diagnosis anThe presence of Microaneurysms (MAs) is the earliest detectable small abnormality of Diabetes Retinopathy (DR), a retinal disease that represents the leading cause of blindness among the middle-aged population globally. So the detection of MAs in fundus images is an important and challenging step for early diagnosis and prevention of critical health conditions. In particular, MAs are small vascular lesions consisting of swollen capillaries due to weakened vascular walls. Thus, retinal MAs can be associated with various ophthalmic and cardiovascular conditions. For instance, retinal MAs have been demonstrated as a risk factor for strokes. Therefore, it is crucial to detect the disease at its earliest stages to prevent its progression and consequent potential vision loss. However, MAs are a small target relative to the fundus image. Because the visual conditions are not ideal, MAs may present a low contrast with the background or may be affected by uneven illumination in the image. In addition, MAs may also be confused with other structures in the image, such as microbleeds, pigmentation changes, and dust particles in the fundus camera. Therefore, the automatic detection of MAs in fundus images is a significantly challenging task. The existing MA detection algorithm is difficult to achieve accurate detection of the lesion. Therefore, an improved faster-RCNN (Faster-RCNN-Pro) detection algorithm based on multi feature scale fusion is proposed. Firstly, the structure of feature extraction network and Region Proposal Network (RPN) are improved by using multi feature scale fusion to increase the utilization of micro target features; then, the quantization error introduced in the process of pooling the region of interest is eliminated by homogenizing and pooling the region of interest; finally, by redesigning the smooth L1 loss function in the loss function, a balanced L1 loss function is obtained to realize the optimization of the loss function, so as to effectively reduce the imbalance between large gradient difficult samples and small gradient easy samples, so that the model can be better trained. For the automatic detection of MAs in fundus images, the optimized Faster-RCNN network model is trained and tested on the Kaggle DR dataset, and compared with other methods. Based on the Kaggle DR dataset, the ablation experiment analysis verifies that the proposed improved Faster-RCNN-Pro based on multi-feature scale fusion can effectively improve the automatic detection performance of MAs. Specifically F-score is increased by 9.36% compared to that of Faster-RCNN. In addition, the performance of this method is compared to the automatic detection performance of MAs based on YOLO, CNN, and traditional methods including image processing and classifiers. The results demonstrates that the F-scores of the methods based on deep learning, including YOLO, CNN and the Faster-RCNN-Pro proposed in this study, are superior to the methods based on image processing and classifiers. F-score of traditional methods such as image processing and classifiers is low because traditional algorithms are easily limited by parameters. The lesion candidate regions of MAs extracted from a complex background, such as the fundus images, are more prone to interference and can not be excluded, and will eventually become FP, resulting in a low P value and affecting the F-score. Moreover, the two-stage model of Faster-RCNN presents a significantly better detection performance, such as F-score, due to the existence of RPN; the detection accuracy of the Faster-RCNN-Pro has been significantly improved. However, the deep neural network may also present the overfitting phenomenon, which may lead to some MAs not being detected. Therefore, the R values based on deep neural networks in previous studies are lower than those obtained by using traditional algorithms; however, Faster-RCNN-Pro can overcome this problem. So the proposed Faster-RCNN-Pro can accurately and effectively detect MAs in fundus images, demonstrating a better detection performance..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0410002 (2023)
Visible-to-infrared Image Translation Based on an Improved Conditional Generative Adversarial Nets
Decao MA, Yong XIAN, Juan SU, Shaopeng LI, and Bing LI
Using visible images to obtain corresponding infrared images is an effective approach to address the lack of infrared images in infrared guidance, infrared countermeasures, and infrared object recognition tasks. At present, the infrared radiation properties of the target can be efficiently simulated by current methods Using visible images to obtain corresponding infrared images is an effective approach to address the lack of infrared images in infrared guidance, infrared countermeasures, and infrared object recognition tasks. At present, the infrared radiation properties of the target can be efficiently simulated by current methods that use modeling of infrared properties to obtain infrared simulation images. However, the simulation process of this method requires tedious operations such as the classification and segmentation of target materials, and the infrared images obtained by the simulation lack texture information. The infrared image generation algorithm based on Generative Adversarial Networks (GAN) can effectively alleviate the problems of cumbersome and labor-intensive infrared image generation. However, some current infrared image generation algorithms based on GANs are prone to the problems of lack of image detail information and lack of structural information. This paper proposes a visible-to-infrared image translation algorithm based on an improved Conditional Generative Adversarial Nets (CGAN). Different from the current UNet network and its variants which focus on the utilization of the underlying features of the image, the generative network not only focuses on the utilization of the underlying features of the image, but also strengthens the utilization of the underlying features of the image. In addition, some network tricks of the ConvNext network are incorporated. Techniques such as using fewer normalization layers, layer normalization instead of batch normalization, etc. The adversarial network improves the quality of the generated images by calculating the first-order feature statistics (mean) and second-order feature statistics (standard deviation) of the generated images. The mean value contributes to the generation of grayscale information of infrared images, and the standard deviation contributes to the generation of structural information of infrared images. The research of the adversarial network focusing on the image receptive field features is transformed into the research of the image feature statistical information, which reduces the constraints on the generative network and releases the greater potential of the generative network. In the experiment, three datasets were used, namely the VEDAI dataset, the OSU dataset and the KAIST dataset, and six objective evaluation metrics were used to evaluate the quality of the generated images, including peak signal-to-noise ratio, structural similarity, multi-scale structural similarity, learning perceptual image patch similarity, Fréchet inception distance and normalized cross correlation. Compared with existing typical infrared image generation algorithms, the experimental results show that the proposed method can generate higher quality infrared images and achieve better performance in both subjective visual description and objective metric evaluation. In the matching application experiment, this paper adopts three traditional matching algorithms: SIFT algorithm, SURF algorithm and ORB algorithm, and three matching algorithms based on deep learning: the D2-Net algorithm, SuperGlue algorithm and LoFTR algorithm. The experimental results show that compared with the use of the visible image for matching, the image conversion algorithm is used to match the corresponding infrared image converted by the visible image, which can effectively reduce the matching endpoint error. The experimental results show that matching end point error is not strictly proportional to the six objective evaluation indexes, but there is a positive correlation between matching end point error and objective evaluation indexes. In general, the better the performance of objective evaluation indicators, the smaller the corresponding matching end point error. In summary, this paper proposes an improved conditional generative adversarial network for converting visible images to corresponding infrared images. The proposed method effectively alleviates the problems of the lack of texture detail information and the lack of structural information in the current infrared generation algorithm based on conditional generative adversarial network image generation in the process of image generation. The generated infrared image has good application value in the matching of the visible image and the infrared image. In addition, this paper provides new ideas for other image translation tasks..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0410003 (2023)
Light Field All-in-focus Image Fusion Based on MDLatLRR and KPCA
Zefeng HUANG, Shen YANG, Huiping DENG, and Qingson LI
The imaging of a light field camera can retain the spatial and angular information of light, therefore, different from traditional two-dimensional imaging, the light field camera can capture the light field directly in one shot, but it will sacrifice the spatial resolution and angular resolution of the image, so the quThe imaging of a light field camera can retain the spatial and angular information of light, therefore, different from traditional two-dimensional imaging, the light field camera can capture the light field directly in one shot, but it will sacrifice the spatial resolution and angular resolution of the image, so the quality of the image obtained is lower than that of the image generated by the native image sensor. This problem has prevented the application of light field imaging from gaining popularity. The development of multi-focus image fusion technology and digital refocusing technology provides ideas for improving the resolution of light field imaging. To improve the spatial resolution of light field imaging, we propose a full-focus fusion algorithm of light field image based on multi-scale latent low-rank decomposition and kernel principal component analysis by combining digital refocusing of light field image with multi-focus image fusion. First, by reprojecting the light field, the light is projected from the original focusing plane to the refocusing plane to generate a refocusing image with the focusing region, defocus region and blurred boundary region. After digitally refocusing, the spatial resolution of the light field image located in the focusing area is greatly improved. To extract the focus area accurately, the multi-level image decomposition method based on latent low-rank representation is used to decompose each refocusing image into a base layer and several saliency layers. Then, a two-region image sharpness extraction algorithm is used to calculate the image sharpness of the base layer, and a multi-scale saliency extraction algorithm is used to extract the visual saliency of the saliency layer's gradient domain. Secondly, the feature coefficient matrices of the base layer and each saliency layer are reshaped and concatenated. The kernel principal component analysis is used for dimension reduction fusion to obtain the fusion feature coefficient matrix which retains the feature information of both the base layer and the salient layer. Finally, the initial focusing decision map was generated by comparing the fusion feature coefficient matrix corresponding to each refocusing image. The small structure removal is applied to the initial decision map to eliminate the influence caused by image noise, and guided filtering is used to process the initial focusing decision map to generate the final focusing decision map, which solves the fusion problem at the fuzzy boundary, so that the fused image is smoother at the boundary of the focusing decision map and has a better visual effect. In order to verify the effectiveness of the proposed method, two sets of experiments are carried out: the full-focus fusion experiment and the multi-focus image fusion experiment. In the full-focus fusion experiment, several kinds of light field data, including objects located at different depths and flat backgrounds were selected from the HCI dataset, which can test the full-focus fusion ability of the proposed algorithm for complex textures and multi-objects. In the multi-focus image fusion experiment, six traditional multi-focus image fusion methods are selected for comparison with the proposed method, the experimental data are from the LFSD dataset, and ten objective evaluation indicators are selected for comparison. The experimental results show that the algorithm has better performance in visual effect and edge information richness compared with traditional methods. Subjectively, the full-focus images fused by the proposed method have higher spatial resolution and detailed texture compared with the original light field imaging. The resulting full-focus image is more in line with the visual perception of human eyes, objectively. Compared with the other six multi-focus image fusion methods, the multi-focus image generated by the proposed algorithm has the characteristics of high definition, excellent visual effect and high image contrast..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0410004 (2023)
Single-frame Infrared Dim Target Detection Based on Guided Filter and Segmented Adaptive Thresholds
Dezhen YANG, Jingying HUANG, Songlin YU, Jinjun FENG, Jiangyong LI, and Tong LIU
A single-frame infrared dim and small target detection algorithm is proposed for the remote infrared target detection system based on the mobile platform. The system faces challenges in detecting targets due to the platform's movement and changes in the background, leading to false alarms. To address this problem, A single-frame infrared dim and small target detection algorithm is proposed for the remote infrared target detection system based on the mobile platform. The system faces challenges in detecting targets due to the platform's movement and changes in the background, leading to false alarms. To address this problem, the proposed algorithm combines guided filtering and nine-square-grid filtering for target enhancement, performing block adaptive threshold segmentation through regions of different complexity to maintain a low false alarm rate while detecting targets in different complex scenes. The background of the image is estimated using guided filtering with edge-preserving characteristics to alleviate edge clutter interference. The local grayscale maximum characteristic of dim and small targets is used to calculate the probability of the target using a nine-square filter based on soft threshold non-maximum suppression. Areas that don't satisfy the target characteristics in the background suppression results are eliminated by weighting. A block adaptive threshold segmentation method is proposed to extract the candidate target using the sigmoid function to design the mapping curve of the standard deviation of gray value to parameter k for threshold calculation. The proposed method outperforms classical methods such as Top-Hat, LCM, and Max-Median, with Signal-to-noise Ratio (SNR) and Background Suppression Factor (BSF) indicators maintained at optimal and sub-optimal levels. The recall rates of scenes with different complexity under constant false alarm respectively reached 87.97%, 84.93%, and 86.22%, improving the recall rate of infrared dim and small target detection. The algorithm is adaptable in engineering applications, as demonstrated by the addition of multi-frame target association on the basis of single-frame image detection. The hardware transplant of the infrared dim and small target detection algorithm is implemented using FPGA+DSP signal processing architecture, achieving a processing speed of over 75 frame/s for object detection on 320×256 infrared images. Therefore, the proposed algorithm effectively realizes the real-time detection of dim and small targets with low false alarm rates and scene robustness..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0410005 (2023)
Microshutter Array Design of Field-of-view Gated Imaging Systems for All-time Star Sensor
Liang FANG, Weimin WANG, Qiang WANG, Xin CHENG, Zhenjie FAN, Hui ZHANG, Rujin ZHAO, and Enhai LIU
Traditional all-time star sensors usually have a narrow Field of View (FOV) and adopt single star tracking mode. Because of only one target star in the FOV, the optical system should be installed on a two-dimensional rotation/scanning platform, or multiple telescopes are used to achieve synchronous measurement, which cTraditional all-time star sensors usually have a narrow Field of View (FOV) and adopt single star tracking mode. Because of only one target star in the FOV, the optical system should be installed on a two-dimensional rotation/scanning platform, or multiple telescopes are used to achieve synchronous measurement, which can lead to many shortcomings such as large volume, low reliability and poor autonomy. The optical imaging system based on FOV gated technology can obtain the wide FOV and strong sky background radiation suppression ability at the same time by combining a large total FOV with a narrow gated FOV, which is expected to achieve multi-star detection and star pattern recognition in the daytime. It has the advantages of small volume, light weight and good autonomy. In the FOV gated optical imaging system, it is necessary to use a key device of microshutter array to quickly switch the gated FOV. Microshutter arrays need to have the characteristics of large element size, high duty cycle and high response speed. At present, there is no microshutter array that meets the requirements can be used. In this paper, a short-wave infrared band FOV gated imaging system principal prototype is designed. In this system, two sets of microlens arrays are used, the aperture of the microlens element is 4 mm, the FOV gated imaging channel number is 7×7, and each imaging channel can obtain near diffraction limit imaging quality. The microshutter array is placed behind two sets of microlens arrays, and the size of the microshutter element is also 4 mm. The position of the microshutter element corresponds to that of the microlens element one by one. In order to meet the application requirements of the FOV gated imaging system, the main design parameters of the microshutter element are determined as follows. The area duty cycle is not less than 90%, the switching response time is not less than 25 ms, and the drive voltage is not more than 120 V. Based on the principle of electrostatic parallel plate capacitive drive and the MEMS bulk silicon process, two rectangular silicon thin plates are designed as light shields for microshutter elements. Furthermore, the light shields are also used as upper electrodes, and the lateral sides of silicon substrate are used as lower electrodes. By loading and removing the drive voltage, the switching of the open and closed states of a microshutter element can be realized. Considering the material characteristics, parameters such as electrode width, cantilever beam width and thickness are first determined. Then, according to the mathematical model of the microshutter element, the influence of different number and length of cantilever beam on the drive voltage and response time is analyzed. The calculation results show that the driving voltage increases with the number of cantilevers beams and decreases with its length. In order to minimize the driving voltage, the number of cantilevers beams should be 2, and the beam length should be greater than 200 μm. On the other hand, since the longer the cantilevers beam, the longer the response time of the microshutter element. In order to meet the 25 ms response time requirement, the beam length of the cantilevers beam is determined as 200 μm.The Ansoft Maxwell electromagnetic field simulation software is used to simulate the electrostatic moments of the microshutter at different torsion angles and then the Comsol Multiphysics finite element analysis software is used to calculate the torsional elastic coefficient of the cantilever beam. According to the balance conditions of the electrostatic moment and the elastic recovery moment, the variation of the torsion angle and the drive voltage is presented. The results show that the microshutter element can be opened when the drive voltage is 106.4 V, which is basically consistent with the theoretical calculation result of 114 V and demonstrates the feasibility of the design parameters of the microshutter array. In addition, since only one microshutter element is turned on under normal operating conditions, the light incident to the other unopened microshutter elements can be reflected or absorbed. The reflected light can form stray light within the optical system, which will affect the distribution of sky background radiation on the image surface of the detector. Therefore, according to the structural layout of the designed FOV gated imaging system, the effect of the surface reflectivity of the microshutter element on the stray light in the system is simulated and analyzed by using the advanced stray light analysis software ASAP. The results show that as the surface reflectivity of the microshutter element increases from 3% to 80%, the average illumination of the sky background radiation at the image plane on the surface of the detector increases very little, and the impact on stellar detection is almost negligible. Therefore, in the design of microshutter arrays, there is no special requirement for the reflectivity of the microshutter surface. This study provides a theoretical basis for the processing of microshutter array in FOV gated imaging system..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0411001 (2023)
Rendering and Display of Light Field Images Using Conjugate Perspective Coherence Cameras
Mengtao ZHOU, Yimin LOU, Juanmei HU, Yushun FENG, and Fengmin WU
Light field display is an important true Three-dimensional(3D) display technology. It can provide the observers with rich 3D perception such as binocular parallax, motion parallax, monocular focusing, and so on. Light field display technology has a wide range of applications in various fields such as industrial and agrLight field display is an important true Three-dimensional(3D) display technology. It can provide the observers with rich 3D perception such as binocular parallax, motion parallax, monocular focusing, and so on. Light field display technology has a wide range of applications in various fields such as industrial and agricultural production, daily life, military defense, game and entertainment. It is a platform technology, so it has attracted much attention. High-quality light field display requires a large amount of light field image information. How to quickly acquire light field image information is still an important challenge in this field.The light field information acquisition technique mainly includes two categories one is optical acquisition technique and the other is digital rendering method. Optical acquisition technique can capture light field information of real scenes, but the acquisition system is complex and requires precise calibration. The digital rendering method uses computer technique to generate light field images. It can be combined with computer graphics technology to render special artistic effects flexibly. Model-based Rendering (MBR) and Image-based Rendering (IBR) are two typical digital rendering methods. The MBR method can get high-quality light field images, but the rendering efficiency needs to be improved. The IBR method renders faster, but the image quality needs to be enhanced. Both of these methods suffer from the pseudoscopic problem of 3D images.In order to improve the rendering efficiency and quality of light field images, a rendering algorithm of light field image and display system based on conjugate perspective coherence camera are proposed. The algorithm is called Conjugate Multiple Viewpoint Rendering (CMVR). The depth mapping relationship between a normal perspective camera and a conjugate perspective camera is analyzed. The conjugate perspective cameras are constructed and used to render the light field images without pseudoscopic problem in one step. Therefore, the image encoding process is avoided. At the same time, the perspective coherence between the conjugate cameras is used to reduce the redundant calculation and accelerate the rendering process. Compared with traditional algorithms, the proposed algorithm has strong parallel processing capability and is suitable for existing graphics processing hardware to accelerate the process. The rendering pipeline for light field images is built using a hybrid programming technique of CPU and GPU. The test results show that the CMVR algorithm can achieve good image quality comparable to the rendering pipeline of commercial application program interface OpenGL. The rendering efficiency of CMVR algorithm is higher than traditional single viewpoint rendering algorithm when the viewpoint number is larger. The CMVR algorithm is insensitive to the number of viewpoints, and is especially suitable for the rendering of high-density light field images. And the more viewpoints are rendered, the more obvious the efficiency improvement is. In addition, the algorithm can also be effectively compatible with texture mapping, lighting and other technologies in computer graphics to achieve realistic scene rendering. In order to verify the correctness of the algorithm, a light field display system is built. The display system is mainly composed of a light source, a high-resolution LCD display screen, a lens array and a diffuser screen. The 3D images of virtual scenes with good horizontal and vertical parallax are outputted by using the rendered light field images and display system. Vivid 3D imaging effects are obtained. The proposed algorithm can provide an effective tool for fast rendering of light field images. It can also be used in dynamic 3D display, augmented reality, virtual reality, and other fields..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0411002 (2023)
Erasing of the Free Induction Decay by Incoherent Ionization Injection
Haicheng MEI, Xiang ZHANG, Santiago LÓPEZ, Qi LU, Siyu QIN, Liang XU, Eduardo OLIVA, and Yi LIU
Strong field physics is an important frontier of current physics research. In recent years, the rapid development of ultrafast laser technology has enabled researchers to obtain laser sources with shorter pulse width, higher pulse energy, and wider tunable range. With the advancing of the research on the interaction beStrong field physics is an important frontier of current physics research. In recent years, the rapid development of ultrafast laser technology has enabled researchers to obtain laser sources with shorter pulse width, higher pulse energy, and wider tunable range. With the advancing of the research on the interaction between strong laser and matter from the traditional perturbation regime to the non-perturbative region, rich strong field physical phenomena such as tunneling ionization and high-order harmonics have been observed. When the high-intensity femtosecond laser propagates in in a transparent medium, it can form a light-plasma filament due to nonlinear optical effects including Kerr effect and plasma generation. In particular, the study of “air lasing” radiation produced by interaction of intense femtosecond laser pulses with air molecules has attracted many attentions in recent 10 years. Air lasing has the characteristics of bidirectional emission, high intensity, high coherence and remote generation, and holds potential application in the field of optical remote sensing. Traditional optical remote sensing collects scattered signals or fluorescent signals of lasers on the atmospheric targets. These optical signals do not have directionality, which poses challenges to the sensitivity of ground collection devices and limits detection accuracy and sensitivity. As a new concept light source, air lasing is expected to greatly improve the signal strength of optical remote detection due to its capacity of emitting coherent beams from the remote atmosphere to the ground. In recent years, several important progress have been achieved as to the understanding of the air lasing signal generated by the interaction of strong-field laser and the most important component of air, nitrogen. Regarding the underlying mechanism of the coherent emission of neutral nitrogen molecule, it is now accepted that the emission at 337.4 nm is Amplified Spontaneous Emission (ASE). In contrast, the lasing mechanism of N
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0414001 (2023)
Complex Coupling Model and Morphology Effect of Femtosecond Laser Ablation of Tooth Surface
Xingzu MING, Xuekun LI, Chengji MI, Guoqi HE, Xian ZHOU, Chao LI, and Rui MING
The two-temperature model is commonly used to analyze the femtosecond laser ablation process, simulating laser ablation by analysing the coupling of electrons to the lattice and the temperature change. Due to the extremely short pulse width of the femtosecond laser, a variety of dynamic effects are generated during theThe two-temperature model is commonly used to analyze the femtosecond laser ablation process, simulating laser ablation by analysing the coupling of electrons to the lattice and the temperature change. Due to the extremely short pulse width of the femtosecond laser, a variety of dynamic effects are generated during the ablation process, such as energy accumulation effects, defocusing effects and dynamic material absorption rate effects. Therefore, when studying the femtosecond laser ablation process, it is necessary to consider the effect of dynamic effects on the size of the ablation. The existing ablation models need to be improved to improve the efficiency of femtosecond laser processing and the accuracy of ablation models. In this paper, a quantitative relationship between the variation in the number of pulses and the total laser energy absorbed by the material is established for the energy accumulation effect; the laser energy at different ablation depths is also obtained based on the effect of the variation in ablation depth on the laser focus radius. By coupling the energy accumulation effect and the defocusing effect into the dual-temperature model, the temperature distribution of the electrons and lattice of the laser-ablated surface gear material 18Cr2Ni4WA is obtained by the finite difference method at different pulse widths and energy densities. The temperature at equilibrium is around 3 000 K, which reaches the cavitation temperature of the face gear material 18Cr2Ni4WA, and the material can be ablated, and the depth and radius of the ablation crater are 4.07 μm and 24.23 μm, respectively.Considering that the surface of the material processed by the femtosecond laser is mostly a complex curved surface, the angle between the laser beam and the surface to be processed during the processing of the curved material will change due to the vertical writing of the laser beam. The laser beam is tilted to ablate the surface of the material and only the laser component perpendicular to the material surface acts on the microstructure of the material surface. Therefore, it is necessary to establish quantitative relationships between the refractive index of the laser beam and the tilt angle, as well as between the focus radius and the tilt angle, to analyze the effect of the tilt angle between the laser beam and the machined surface on the size and shape of the machined surface when the laser beam is ablated on curved materials. The simulations show that the effective laser intensity decreases significantly after the tilt angle between the laser beam and the material being processed exceeds 40°. The laser spot is elliptical on the surface of the material. Finally, it was experimentally verified that when the laser energy density was 1.783 J/cm2 and the bottom angle of the machined tooth surface was too large, the ablation process only occurred on the surface of the material; when the energy density was 2.376 J/cm2 and the number of laser pulses was 3 000, the microstructure of the ablation crater was good and fine. The results show that the laser energy decreases with the change in tilt angle when the surface is ablated by the femtosecond laser, while the energy distribution of the laser spot affects the change in the depth of the ablation crater..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0414002 (2023)
Passively Q-switched Dual-wavelength Laser Based on Nd∶GdVO4 Crystal
Chengzhu SHEN, Miao HU, Mengmeng XU, Haozheng LI, and Huan SONG
Dual-wavelength pulse lasers have a wide range of applications in terahertz wave generation, Doppler radar coherence detection, spectral studies and so on, so how to generate dual-wavelength pulse lasers is of great significance. Dual-wavelength pulse lasers can be realized by Q-switched technology, which can be divideDual-wavelength pulse lasers have a wide range of applications in terahertz wave generation, Doppler radar coherence detection, spectral studies and so on, so how to generate dual-wavelength pulse lasers is of great significance. Dual-wavelength pulse lasers can be realized by Q-switched technology, which can be divided into active Q-switching and passive Q-switching. Compared with the active Q-switched lasers, the passive Q-switched lasers have the advantages of simple technology and small size. In order to obtain a stable dual-wavelength pulsed laser, one approach is pumping two crystals or combinatorial crystals into a cavity, another approach is pumping a single crystal into a cavity. Compared with the dual-crystal laser, the single-crystal laser has the advantages of simple structure, convenient operation, and easier miniaturization. But there is gain competition between two wavelengths of single crystal laser, so it is difficult to output two-wavelength pulse laser. In this paper, a 1 063 nm/1 065 nm dual-wavelength pulsed laser using a single Nd∶GdVO4 crystal and a single Cr4+∶YAG crystal is proposed.The Q-switched process of laser can be described by rate equation. By establishing a simulation model based on rate equation, the influence of pump power on the time-domain characteristics of laser output under different reflectivity of output mirror is studied. The theoretical results show that when the reflectivity of the output mirror is changed, the threshold inversion particle number density of the two wavelengths will change accordingly. Therefore, the dual-wavelength pulsed laser can be realized by changing the reflectivity of the output mirror. When the π-polarized output mirror reflectivity is in the range of [0.52,0.71] and σ-polarized output mirror reflectivity is 0.95, the laser can output dual-wavelength passively Q-switched pulse. By increasing the pump power, π-polarized single-wavelength pulse, dual-wavelength multiple-on-one pulse, dual-wavelength one-on-one pulse, dual-wavelength one-on-multiple pulse and σ-polarized single-wavelength pulse can be generated in sequence.In order to verify the influence of pump power on time characteristics of dual-wavelength pulse, a Y-type-cavity dual-wavelength laser is designed. The reflectivity of π-polarized output mirror is set to 0.60, and that of σ-polarized output mirror is set to 0.95. The stimulated emission cross section of Nd∶GdVO4 crystal is kept constant by setting the cooling temperature of the temperature controller at 20 ℃. The output of the Nd∶GdVO4 laser is coupled to two channels of the oscilloscope by two identical photodetectors. The pulse waveform of the Nd∶GdVO4 laser at different pump power is measured by changing the pump source. With the increase of the pump power, the pulse laser with the time mentioned above characteristics is output in turn, which is consistent with the numerical simulation results. When the pump power is 5.51 W, the output of the laser is a one-on-one pulse of two orthogonal polarized wavelengths, of which the π and σ polarized wavelengths are 1 063.23 nm and 1 065.52 nm respectively, the average power is 323 mW and 462 mW respectively, and the peak pulse power is 11.62 W and 20.35 W respectively, the pulse repetition rate is 141 kHz.In this paper, by setting up the rate equation model of two-wavelength quadrature polarization passively Q-switched laser based on Nd∶GdVO4 crystal, the condition of laser output dual-wavelength pulse and the influence of pump power on the time characteristics of laser output under different reflectivity of output mirror are studied. The simulation results show that the dual-wavelength pulse can be realized by adjusting the reflectivity of the output mirror to change the double-wavelength threshold inversion of the particle number density. On the premise of realizing dual-wavelength pulse output, the output pulse waveform can be changed by changing the pump rate. The experimental results are in good agreement with the simulation results under the condition of 20 ℃..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0414003 (2023)
Simulation Test Research of Constant False Alarm Laser Ranging Circuit Based on Nonlinear Dependent Source APD Model
Wei WANG, Qihang ZHAO, Sen YANG, Ke ZHANG, Xingbo LIU, and Tongyu LIU
Avalanche Photodiode (APD), with its advantages of small size, fast speed, and high sensitivity, stands out from a large number of photoelectric detection devices and is widely used in pulsed laser ranging reception. In the field of engineering practice and academic research, reasonable modeling of APD devices and fullAvalanche Photodiode (APD), with its advantages of small size, fast speed, and high sensitivity, stands out from a large number of photoelectric detection devices and is widely used in pulsed laser ranging reception. In the field of engineering practice and academic research, reasonable modeling of APD devices and full closed loop simulation of Constant False Alarm Rate (CFAR)laser ranging receiving circuit using this model can greatly improve system design efficiency, reduce research and development costs, which is of great significance to the performance evaluation of laser ranging receiving system. At present, many institutions at domestic and abroad have established a variety of APD circuit simulation models. Most of these models are based on the carrier rate equation. After proper and reasonable approximation, the APD is equivalent to a three terminal circuit composed entirely of electronic components through mathematical simulation. However, the establishment of these models is based on devices' internal parameters and material properties, involving intellectual property rights, trade secrets and other reasons, which are usually not provided by manufacturers. Therefore, these models are suitable for the research and development process of devices, and not for the full closed loop dynamic simulation of CFAR laser ranging circuit. On the market, APD device manufacturers often only disclose the external characteristics of the device, which makes the existing models difficult to use effectively in the actual scientific research work. Therefore, how to use the limited device parameters to explore new modeling methods, carry out mathematical modeling and circuit packaging for external characteristic parameters, establish the APD circuit simulation model, and realize the full closed loop dynamic simulation of CFAR laser ranging circuit is still a problem to be solved. In this paper, an avalanche tube model based on a nonlinear correlation source is established to solve the above problems, and the full closed loop simulation test of CFAR laser ranging circuit is carried out using this model. Taking the C30950E avalanche tube assembly, a product of EG&G Canada, as an example, the paper first refers to the C30950E data manual, and uses the Curve Fitting toolbox in Matlab to perform curve numerical fitting according to the correlation between avalanche gain, noise (temperature) and bias voltage. Through fitting, the exponential function model of APD input, output and bias voltage control is established; Then, on the basis of this mathematical model, the nonlinear correlation source control in Multisim 14.2 software is used for circuit encapsulation; Finally, the range receiving amplifier circuit with APD constant false alarm bias automatic control and short-range scattering time gain control is designed, and the automatic gain control (AGC) signal is added. The full closed loop circuit of the constant false alarm laser range receiving amplifier circuit is built using the above APD circuit simulation model, and simulation research is carried out for the system detection sensitivity, false alarm rate and other indicators. The simulation results show that the CFAR ranging circuit can automatically capture the optimal avalanche gain operating bias voltage of APD according to the design value of the false alarm rate. Based on the simulation model, the minimum optical power detection of 25nW is realized. The false alarm rate within the measurement range is 0, and the false alarm rate outside the measurement range is about 65 times per hundred milliseconds. The results are close to the actual values. The simulation tests verify the correctness and accuracy of the simulation model. The simulation provides a design optimization approach for the research and development of APD constant false alarm laser ranging receiver circuit, and the overall modeling and simulation ideas are also instructive for the computer-aided analysis and design of other OEIC circuits..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0425001 (2023)
Estimation of Rydberg Atomic State Population Excited by Two-photon Resonance
Qiang AN, Chengjing LIU, Kai YANG, Jiawei YAO, Bo WU, and Yi LIN
Alkali atom vapor cells are the essential component of quantum microwave measurement equipment. By means of laser pumping technology, alkali atoms can be easily excited from the ground to Rydberg states. Alkali atoms is very sensitive to electric filed because of their very large polarizability, huge electric dipole anAlkali atom vapor cells are the essential component of quantum microwave measurement equipment. By means of laser pumping technology, alkali atoms can be easily excited from the ground to Rydberg states. Alkali atoms is very sensitive to electric filed because of their very large polarizability, huge electric dipole and low ionization threshold field and so on. Recently, by virtue of the strong interaction of microwave field and Rydberg atoms, alkali atom vapor cells have been widely applied to detect the amplitude, frequency, phase and polarization of electric field, especially the microwave electric field. Quantum microwave measurement technology has shown significant advantages, such as the break of probe size independent of wavelength, extremely high sensitivity and accuracy, very broad spectrum measurement, and very large dynamic range. In the past decade, the technology has shown great potential for application in the monitoring of ultra wide band electromagnetic spectrum, the metering of microwave electric field, microwave imaging and communication, etc. Thereinto, the state population of Rydberg atoms in vapor cell is one of the decisive factors affecting measurement capability. Up to now, certain properties of vapor cell can be obtained by optical or other measurement technologies, such as the thickness, refractive index and transmittance of glass envelope, atomic ratio and density in vapor cells. However, all of them can not directly reflect the Rydberg atomic states in vapor cell, which could give rise to difficulties to the performance optimization of quantum measuring equipment based on atom vapor cells. In this paper, a theoretical calculation model of Rydberg atomic state population excited by two-photon resonance has been established by using the ideal gas state equation, and the Rydberg blockade effect and gas atomic distribution are comprehensively analyzed. Meanwhile, an estimation method of Rydberg atomic state population, which is acomplished by aid of optimal Electromagnetically Induced Transparency (EIT) signal of Rydberg atoms, has been proposed and demonstrated experimentally under shading condition and room temperature, by means of the cylindrical (~1.0 cm in diametre and length) and cuboid (~1.0 cm in width and height, and ~2.0 cm in length) vapor cell, respectively. The vapor cells are filled with saturated cesium (133Cs) atoms at 300 K, and the intensity of pressure is ~6 666.1 Pa. In order to obtain the theoretical calculated and experimental estimates of the Rydberg state population excited by two-photon resonance, the EIT experimental setup has been put up by ~852 nm and ~1 020 nm semiconductor laser. Both of them are produced by TOPTICA Photonics. The typical spectral linewidths (5 μs integration time) of two semiconductor lasers are ~100 kHz. The ~852 nm laser, which is stabilized on the saturated absorption spectral signal of 133Cs D2-line, is employed as a probe laser to irradiate into the vapor cells to excite the 133Cs atoms from the ground state (
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0426001 (2023)
A Multi-Fano Channel High Sensitivity MIM Waveguide for Sensing
Meina SONG, Yiping HUO, Yunyan WANG, Pengfei CUI, Tong LIU, Chen ZHAO, and Zuxiong LIAO
Due to the existence of diffraction limit, the optical performance of traditional photonic devices is limited greatly. Surface Plasmon Polaritons (SPPS) can overcome the traditional optical diffraction limit and localize light in sub-wavelength range, so it has very important applications in the transmission, processinDue to the existence of diffraction limit, the optical performance of traditional photonic devices is limited greatly. Surface Plasmon Polaritons (SPPS) can overcome the traditional optical diffraction limit and localize light in sub-wavelength range, so it has very important applications in the transmission, processing and control of optical waves in high-density photonic integrated circuits. For the advantages of low ohmic loss, long propagation distance and easy fabrication, MIM waveguide has become one of the most promising waveguides. Square resonators are often used in SPPs-based MIM waveguide systems because of simple structure and easy fabrication. Fano resonance has a sharp and asymmetric line shape and is very sensitive to the refractive index of surrounding environment. Compared with a single Fano system, multiple Fano resonances can realize multi-channel sensing and have the ability of parallel processing, which has attracted great attention. In this paper, a metal-insulator-metal waveguide structure consisting of a square ring resonator with a central rectangular air path and a bus waveguide with a baffle is proposed, which is studied by using COMSOL Multiphysics 5.4 based on the Finite Element Method (FEM). Because of its low power dissipation, silver is chosen as the metal material of MIM waveguide. The thickness of the structure is large enough (much larger than the optical wavelength), and the simulation results of 3D model and 2D model are basically the same, so the 2D model is adopted to reduce calculation quantity. The optical properties of the structure are studied, and the formation mechanism of Fano resonance is discussed according to the transmission spectrum and magnetic field distribution. In addition, the influence of changing structural parameters is discussed. The application of the proposed structure in sensing is summarized. Firstly, the transmission spectra of a single square cavity, a square ring cavity and the proposed structure are given. The square ring resonator with a central rectangular air path can increase the effective cavity length and change the propagation path of SPPS in the resonator by providing more plasma resonance modes. Secondly, in order to understand the formation mechanism of Fano resonance of the proposed structure, the magnetic field distributions at the peak positions of Fano resonance are given. The simulation results show that the proposed structure can excite quadruple Fano resonance, and a filter band is formed in the transmission spectrum. The position and intensity of Fano resonance and the width of the filter band can be adjusted conveniently by changing the structure parameters. Thirdly, the number of Fano resonances can be adjusted by changing the side length of the square ring, and six Fano resonances and two filter bands can be obtained at most. At the same time, the filter width and the center wavelength can be regulated. The bandwidth of the proposed structure is defined as the wavelength range in which the transmittance is less than 1%, and the maximum bandwidth of the proposed structure is 275 nm. Therefore, the structure can be used for making band-stop filter. Finally, the application of MIM waveguide based on SPPs in refractive index sensor is studied. In biomedicine, it is very important to measure glucose concentration in the body, because it is often used to check the level of blood glucose. The proposed structure is very sensitive to the refractive index of the filled medium. The application of the proposed structure in the detection of glucose concentration is studied by investigating the relationship between the resonant wavelength of each Fano and glucose concentration. In order to evaluate the performance of glucose concentration sensor, the maximum sensitivity and Figure of Merit(FOM)of the proposed structure are calculated, which are 3 028 nm/RIU and 157.14 nm/RIU, respectively. The sensitivity and FOM value of the proposed structure are compared with that of MIM waveguide structure proposed in recent years..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0426002 (2023)
Scattered Ray Tracing Based on Rejection Sampling Method with BRDF Discrete Numerical Value
Wenyou QIAO, Zhishan GAO, Qun YUAN, Dan ZHU, Ningyan XU, Xulei LUN, and Xiaoyu CHE
The effect of stray light on system signals in space platform optical machine systems cannot be ignored. The components of an optical machine system are essential sources of stray light, and their surface light scattering characteristics directly affect the distribution of stray light in the system. Ray tracing and strThe effect of stray light on system signals in space platform optical machine systems cannot be ignored. The components of an optical machine system are essential sources of stray light, and their surface light scattering characteristics directly affect the distribution of stray light in the system. Ray tracing and stray light analysis based on the surface scattering characteristics of system components is an important research content in optical design and simulation. The Bidirectional Reflectance Distribution Function (BRDF) is commonly used to accurately characterize the scattering property of surface about optical machine structures in stray light analysis. The Monte Carlo method (MCM) is the primary method of scattered ray tracing in optical machine systems. It is commonly used in scientific research experiments and stray light analysis software. The core of Monte Carlo scattered ray tracing lies in the reasonable selection of the bidirectional scattering distribution function model and the correct construction of the probability model. In practical applications, due to the particularity of the micro-morphology and texture distribution of the surface of the system components, the surface scattering characteristics show complex and diversified features, which are accompanied by the innovation of machining technology and the appearance of new material surfaces. The number of BRDF models in the commercial software is small, and the application scenarios are limited, so the real-time measured discrete data of BRDF on the optical surface are needed. In some instances, the reconstruction of the BRDF function model and the numerical analysis process is complicated. Moreover, there are some problems, such as fitting errors and limitations of application conditions of the model. The inverse transformation method is often used to solve the probability model of ray tracing. Although the inverse transformation method can efficiently generate random samples that obey the specified distribution, the BRDF of most scattering models is modulated by ray coordinate variables. In the design process of complex probabilistic models based on the reconstructed BRDF model for complex PDF, there are problems such as Cumulative Distribution Functions (CDF) without analytical solutions. To simplify the modeling process of surface scattering and enhance the applicability of the scattered ray tracing method, this paper proposes a way to directly trace the scattered rays based on the surface discrete BRDF measurements. Under the condition that the surface is isotropic, the procedures are shown as follows: Firstly, the spatial coordinate transformation of the discrete measurement data of surface BRDF, which discontinuously varies versus the scattering angle, is converted from the scattering angle hemisphere space to the direction cosine space; Then, the BRDF data distributed in the direction cosine space is obtained by equal interval assignment interpolation equivalent method. Then, a new scattering probability model is designed using the advantage of the rejection sampling method unlimited to the CDF-solving process. The BRDF numerical ratio in the cosine space direction represents the probability distribution of discrete rays. The space coordinates of scattered rays are screened out by setting the test conditions to realize the scattering ray tracing. To verify the accuracy and applicability of the proposed method, the same incident angle, the number of tracing rays, and other parameters in the simulation were set. The simulation program is prepared in Matlab according to the proposed method, and the simulation results in Matlab are compared with those in LightTools. Wherein the BRDF model and parameters of Harvey, ABg, and multiple scattering surfaces characterizing the surface scattering characteristics of various optical and mechanical components were set in LightTools. Different visual and mechanical parts are modeled and simulated. BRDF data in the section where incident light and mirror-reflected light are located were obtained using the analytical formula of the BRDF model and its parameters. The above data were taken as the discrete measured data of BRDF in the simulation program of the proposed method in this paper. The general quality index UQI is the comparison evaluation index of the simulation results between commercial LightTools software and the proposed method. The results show that the scattering energy distribution obtained by the proposed ray tracing method is highly consistent with the LightTools simulation. The different optical and mechanical components are modeled and simulated, the UQI values are all above 0.998, and the fluctuation range is small. The calculation results of the ray tracing method in this paper are accurate and have good applicability..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0429001 (2023)
Simultaneous Measurement of Thermal and Mutual Diffusion Coefficients by Dynamic Light Scattering
Peng GAO, Guanjia ZHAO, Jianguo YIN, and Suxia MA
The thermal and mutual diffusion coefficients of fluid working fluids are important transfer properties that characterize their heat and mass transfer in the field of refrigeration, petroleum, chemical and others. Reliable thermal and mutual diffusion coefficient data are normally necessary for design and optimization The thermal and mutual diffusion coefficients of fluid working fluids are important transfer properties that characterize their heat and mass transfer in the field of refrigeration, petroleum, chemical and others. Reliable thermal and mutual diffusion coefficient data are normally necessary for design and optimization of the equipment and process. The thermal and mutual diffusion coefficients can be determined with different equipment. The dynamic light scattering method owns the advantages of measuring properties under equilibrium condition and in the non-contact and absolute way. However, it is still not an easy way to obtain the two properties simultaneously and reliably by the dynamic light scattering method. Therefore, the present study tries to understand the influence of sampling time, incident angle, viscosity, refractive index deviation and Lewis number Le in dynamic light scattering method on the reliability and accuracy of the method. In consideration of the difference of viscosity and refractive index, three binary systems of n-Hexane/n-Decane, n-Hexane/n-Hexadecane and n-Hexane/n-Decane at the defined molar fractions (0.50/0.50, 0.06/0.94 and 0.85/0.15, respectively) are selected as reference fluids. The first and third systems have similar viscosity, but different refractive index difference of the components; the second and third systems have similar refractive index difference, but different viscosity. The measurement is performed in the saturated condition at different sampling times, incident angles and in a wide temperature range. Correlations are established based on the experimental data for both properties. The results show that the thermal diffusion coefficient and Le number decrease as the temperature increases, while the mutual diffusion coefficient increases as the temperature increases. The relaxation times of temperature fluctuations and concentration fluctuations of the binary system of n-Hexane and n-Decane with equal molar fractions satisfy a proportional relationship with the inverse of the square of the wave number, and the temperature and concentration fluctuations corresponding to the heat and mutual diffusion coefficients are verified to be consistent with the hydrodynamic model at the same time. The maximum fitting deviations between the measured thermal diffusivity and the calculated value of the fitting equation are 1.50%, 1.10% and 1.00% respectively, and the average absolute deviations are 4.00%, 0.34% and 1.95% respectively; the maximum deviations of mutual diffusion coefficient are 2.00%, 5.62% and 1.00% respectively, and the average absolute deviations are 1.06%, 2.58% and 0.31% respectively. As the sampling time is 1.5 to 3 times of the relaxation time, the incident angle is between 8° and 12°, the system has lower viscosity and higher refractive index deviation (>4%) and the Le value is 10~80, so it is possible to obtain both the thermal and mutual diffusion coefficients of the binary mixtures with an uncertainty of less than 5%. The dynamic light scattering experimental system developed in this study can be used to obtain the thermal and mutual diffusion coefficients of various complex binary systems simultaneously under the above defined conditions, providing a method for the study of diffusion in complex binary systems..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0429002 (2023)
Polarization Spectrum Measurement Technology Based on Intensity Modulation
Hui LI, Qingsheng XUE, Haoxuan BAI, and Zijian LI
As one of the common methods to measure polarization information, intensity modulated polarization spectrum measurement technology has the advantages of high real-time performance and high time resolution, and is widely used in many fields. In this method, the stokes vector of the target light is modulated to the carriAs one of the common methods to measure polarization information, intensity modulated polarization spectrum measurement technology has the advantages of high real-time performance and high time resolution, and is widely used in many fields. In this method, the stokes vector of the target light is modulated to the carrier signal of different frequencies by the intensity modulation module, and the spectral information of each element is modulated to the channel of different frequencies by the Fourier transform, finally, the polarization spectrum is obtained. Although the technique of intensity modulated polarization spectroscopy is simple and effective, there are still spectral overlap to varying degrees, which makes the Stokes parameters between channels interfere with each other and affects the accuracy of spectral reduction. At the same time, to meet the requirements of wide-band detection, on the basis of a certain spectral resolution of the spectrometer, the thickness of the intensity modulation module delay is rationally designed to reduce the interval between channels in the optical path difference domain, so as to achieve the purpose of wide-band operation range. Secondly, by increasing the thickness of the delayer, the modulation information generated by the polarization module is collected with a high-resolution spectrometer to achieve a wide-band detection range. Based on using high resolution spectrometer to realize the requirement of wide band detection, aiming at the shortage of overlapping between different channels in the optical path difference domain of the current intensity modulated polarization spectrum measurement technology, the generation of aliasing error in optical path difference domain is analyzed. The influence of different toe-cutting functions on the measurement error of intensity-modulated polarization spectrum is studied by using windowed interpolation FFT transformation. Firstly, the theoretical analysis of intensity modulation polarization spectral measurement technology is carried out to analyze the relationship between the design of the retarder thickness in the polarization module and the spectral resolution of the spectrometer. And the design relationship between the two delayers is introduced to complete the design of the intensity modulation module. Secondly, according to the design index of the intensity modulation module, the process of demodulating and recovering the intensity information of the incoming light and the information using different toe-cutting functions is simulated. The distribution of different channels in the optical path difference domain is analyzed under different toe-cutting functions. The polarization spectrum information of different channels is restored, the four Stokes vectors are normalized, and the calculation of polarization degree and error analysis are completed. Finally, the intensity modulated polarization spectrum measurement system is simulated by ZEMAX software, and the polarization module is adjusted and installed according to the simulation model. At the same time, the polarization spectrum measurement device is built with high resolution spectrometer, parallel light tube and other devices, and the polarization spectrum measurement experiment is carried out. During the experiment, before demodulation of the metering light, it is necessary to calibrate the reference light and calculate the demodulation coefficient required in the process of demodulation. Analyze the correctness of the demodulation coefficients according to the principle of polarization spectrum measurement to eliminate the influence of installation and adjustment errors on the demodulation work. At the same time, the source spectrum of the unmodulated halogen lamp was measured and compared with the recovery result as reference light to verify the accuracy of polarization spectrum recovery under different toe-cutting functions. The results show that choosing the appropriate toe-cutting function according to the design index of the polarization spectral measurement system can reduce the spectral leakage effect caused by the truncation of light intensity information, reduce the aliasing information between different channels in the optical path difference domain, and improve the accuracy of polarization spectral recovery. The maximum error of polarization of linear polarized light with different toe-cutting functions is reduced from 0.059 3 to 0.001 4, and the polarization is close to 1. The above research provides an important reference for eliminating the “ring” effect caused by spectral aliasing in the process of intensity-modulated polarization spectrum measurement..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0430001 (2023)
Hyperspectral Image Denoising Based on Fast Tri-factorization and Group Sparsity Regularized
Xiaoyu GAO, Jingyuan BAI, Yangzhi HUANG, and Jifeng NING
Hyperspectral Image (HSI) has rich information, it has been widely used in various fields. Due to the limitations of various factors, such as lighting conditions, transmission conditions and imaging instruments, HSI is polluted by various noises, which not only reduces the visual quality but also brings difficulties toHyperspectral Image (HSI) has rich information, it has been widely used in various fields. Due to the limitations of various factors, such as lighting conditions, transmission conditions and imaging instruments, HSI is polluted by various noises, which not only reduces the visual quality but also brings difficulties to subsequent processing. Many existing traditional denoising models still use nuclear norm minimization to iteratively solve the matrix rank minimization, and each iteration involves singular value decomposition, so these algorithms have a high computational complexity; in addition, total variation item fails to explore shared group sparsity patterns of difference images. In summary, how to express low rank more quickly and express sparsity more accurately is still a difficult problem. Under the framework of combining local low-rank and global group sparsity, this paper proposes the Fast Tri-factorization and Group Sparsity (FTFGS) model. In local modules, FTFGS model partitions the HSI into overlapping 3-D patches and converts patches into a matrix by lexicographical sorting. This operation conforms to the physical characteristics of HSI, avoids the formation of ill-conditioned matrices, and can better protect the details in the local blocks. This patchwise approach can reduce the dependence on the hypothesis that noise in HSIs is independent and identically distributed. When dealing with small-scale matrices, the Fast Tri-factorization (FTF) is used to decompose these matrices into two orthogonal factor matrices and a core matrix, the size of the core matrix and its L2,1 norm minimization are used to more accurately and quickly represent the local low rank. FTF explores the low rank, which has the advantages of lower computational complexity and faster speed than the nuclear norm, furthermore, FTF digs deeper into the low rank because the low rank constraints are transferred to a smaller core matrix. When exploring the sparsity, the existing total variation regularizations do not consider the group sparsity property of HSI and so on, the local area structure is the same for all bands, as is the smoothed structure. This paper proposes a new weighted spatial-spectral group sparse regularization to explore the shared group sparse pattern in each gradient direction of HSI. With this strategy, the local and global modules are executed alternately to express the local low-rank and global group sparsity properties of HSI and remove complex mixed noises. In the comparative experiments, intuitive visual effects, quantitative numerical evaluation and qualitative comparisons are used for evaluation. From the visual effects, the FTFGS model better preserves image details and texture information, and the visual effect is significantly improved. Compared with the five classical denoising methods, the average peak signal-to-noise ratio index is increased by 1.75 dB, the average structural similarity index is increased by 0.003, the average feature similarity index is increased by 0.002, and the denoising accuracy is significantly improved. Moreover, in the qualitative comparison experiment, the spectral curve of our model is closest to the original image. The validation effect on the real dataset further proves its effectiveness. The reason for the good results is that compared to other models, FTFGS not only improves the local low-rank term, but also better explores the sparse prior of the image with the group sparse term. The time complexity analysis of the model verifies the effectiveness of the FTF framework. The model makes full use of the prior information of the HSI, which not only develops more accurate approximate representations for the low-rank and sparse, but also improves the speed while ensuring the optimal solution. The model is robust, fast and effective, and has certain research value for remote sensing and other application fields..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0430002 (2023)
Fiber Optics and Optical Communications
Quasi-distributed Temperature and Salt Sensor Based on Drawing Tower Grating
Yuqi WANG, Zhen PAN, Yadian JI, Dian FAN, and Ciming ZHOU
Seawater temperature and salinity are the most critical and fundamental physical parameters necessary for all oceanographic disciplines, which have important theoretical value and practical significance for studying ocean climate change, monitoring marine ecological environment, exploiting and utilizing marine resourceSeawater temperature and salinity are the most critical and fundamental physical parameters necessary for all oceanographic disciplines, which have important theoretical value and practical significance for studying ocean climate change, monitoring marine ecological environment, exploiting and utilizing marine resources, and ensuring military security, etc. The development of high-performance sensors for seawater parameter measurement has become one of the research hotspots. In recent years, optical fiber sensing methods have provided a new solution for high precision measurement of physical parameters with the advantages of anti-electromagnetic interference, corrosion resistance, small size, and real-time distributed measurement. At present, the widely studied optical fiber temperature and salt sensors mainly include optical fiber interference type sensors and fiber optic grating type sensors. Researchers at home and abroad have realized the design of optical fiber interference type temperature and salt sensors by micromachining the optical fiber, such as taper pulling, reverse taper pulling, side polishing, dislocation welding and core diameter mismatch welding, and achieved some research results. However, there are generally problems such as great fabrication difficulty and poor structural stability, which are difficult to meet the application requirements of marine engineering. In contrast, optical fiber grating type temperature and salinity sensor are mainly designed based on Fiber Bragg Grating (FBG), and a single FBG can form a sensor, which is simpler to manufacture, more stable in structure and more adaptable to the environment. However, the current fiber grating temperature and salinity sensors mostly adopt FBG with high reflectivity, which can only perform discrete point measurements and cannot realize distributed sensing. Besides, previous reports mostly used spectrometer demodulation, which cannot observe the real-time response of temperature and salinity. Therefore, a quasi-distributed temperature and salt sensor based on Drawing Tower Grating (DTG) is proposed in this paper. The sensor uses DTG coated with Polyimide(PI) as the salinity sensing element. The PI coating expands or contracts linearly in volume when in contact with solutions of varying salinity. The expansion or contraction response caused by the change in salinity is converted into an axial strain loaded on the PI-coated DTG, and the salinity can be measured by monitoring the drift of its central wavelength. During the experiment, the central wavelength of the fiber grating is demodulated in real time by the fiber grating array demodulator, and its data is collected and recorded by the computer in real time. In the temperature compensation coefficient measurement experiment, the optical fiber sensor was placed in the circulating temperature field from 25 ℃ to 30 ℃ and then back to 25 ℃. Compared with the electronic temperature sensor used for calibration, it can be found that both PI coated DTG and uncoated DTG can accurately measure the ambient temperature, and have good consistency and repeatability. Their temperature sensitivities are 10.24 pm/℃ and 10.02 pm/℃ respectively. In the experiment of simultaneous temperature and salt measurement, the fiber optic sensor was placed into a high concentration NaCl solution of 5 mol/L to make the PI coating lose water and shrink sufficiently. Then deionized water or low concentration NaCl solution was added to gradually dilute it to 4 mol/L, 3 mol/L, 2 mol/L, 1 mol/L, and 0.6 mol/L to observe its salinity response. In order to simulate the actual working environment of the sensor, the whole system was in a room temperature environment without temperature control operation, and the sensor was still able to measure the temperature of the solution accurately, and the average salinity sensitivity obtained after compensation was -5.58 pm/(mol/L). The experimental results show that the sensor can simultaneously measure the temperature and salinity of seawater in real-time and quasi-distributed, and also has the advantages of wide measurement range, high measurement accuracy and easy fabrication, which has certain prospects for application in marine engineering..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0406001 (2023)
Compressed Sensing Channel Estimation for MIMO-VLC Systems Based on DFT-SAMP Algorithm
Feng ZHANG, Jinwei LING, Yenan LIU, and Li ZHAO
Visible light communication is a new communication method that uses the visible light band as a communication carrier and takes into account lighting and data transmission. It has the advantages of no electromagnetic interference, rich spectrum resources and so on. Combining MIMO with VLC system can improve the communiVisible light communication is a new communication method that uses the visible light band as a communication carrier and takes into account lighting and data transmission. It has the advantages of no electromagnetic interference, rich spectrum resources and so on. Combining MIMO with VLC system can improve the communication capacity and rate of the system. However, MIMO-VLC systems need channel estimation to obtain channel state information to ensure the reliability of system communication. Although the commonly used LS channel estimation algorithm has a low complexity, it requires a lot of pilot overhead, which leads to a reduction of spectrum efficiency. Compressed sensing is applied to channel estimation to reduce pilot overhead and improve channel estimation performance because it can sample signals at a rate lower than Nyquist sampling rate and has a higher reconfiguration progress. The commonly used compressed sensing reconstruction algorithm, OMP algorithm, needs to predict the sparsity of the channel, and the true sparsity of the channel is usually unpredictable, so it has limitations in practical application. The SAMP algorithm can adaptively reconstruct the channel characteristics when the channel sparsity is unknown, which solves the condition of predicting the channel sparsity, but also increases the number of iterations of the algorithm and reduces the efficiency. Aiming at the problem of slow running speed of the SAMP algorithm, this paper proposes a Prediction-sparsity Adaptive Matching Pursuit(SAMP) algorithm based on Discrete Fourier Transform(DFT). Firstly, the sparsity of the channel impulse response is preestimated by the sparsity prediction method of DFT. Taking the estimated sparsity as the initial step of the algorithm can quickly approach the real sparsity and improve the efficiency of the algorithm. Secondly, the SAMP algorithm is used to reconstruct the channel impulse response to improve the accuracy of channel estimation and ensure the reliability of system communication. According to the performance analysis of a MIMO-VLC system with 2 inputs and 2 outputs, the mean square deviation performance of the algorithm in the paper is significantly better than that of the LS algorithm. When the forward error correction code rate threshold (3.8×10-3) is satisfied and the pilot number is 16, the algorithm in this paper improves by 2 dB compared with the LS algorithm, and by 4.5 dB when the pilot number is 32. At the same time, the bit error rate performance of the algorithm in this paper is equivalent to that of the SAMP algorithm as a whole, which shows that the sparsity prediction method based on DFT will not reduce the reliability of system communication while improving the efficiency of the system. The system bit error rate increases with the increase of modulation order M, and the performance gain of the proposed algorithm is more obvious than that of the LS algorithm with the increase of modulation order. When the error rate reaches the FEC threshold and the modulation order is M=8, the performance of the algorithm is improved by 3.5 dB compared to the LS algorithm, and by 10 dB when the modulation order is M=64. This result shows that, when the modulation order is higher, the reduction of bit error rate is more obvious, which is conducive to the improvement of system communication efficiency. For the efficiency of CS algorithm, the DFT based sparsity prediction method significantly improves the running speed of the DFT-SAMP algorithm proposed in the paper. Compared with the SAMP algorithm, the efficiency of the algorithm in the paper increases by about 68% at 16 pilots and 69% at 32 pilots..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0406002 (2023)
Remote Sensing and Sensors
Image Quality Evaluation Method for Optical Remote Sensing Satellite Based on an Array of Point Sources
Yujun ZHENG, Weiwei XU, Xin LI, Xiaolong SI, Baoyun YANG, and Liming ZHANG
Ground pixel resolution and modulation transfer function are two important parameters for image quality evaluation of high spatial resolution optical remote sensing satellites, which are of great significance in target recognition, image interpretation, and information extraction. We present an image quality evaluationGround pixel resolution and modulation transfer function are two important parameters for image quality evaluation of high spatial resolution optical remote sensing satellites, which are of great significance in target recognition, image interpretation, and information extraction. We present an image quality evaluation method for remote sensor using an array of point sources, which takes the light, small, and automated reflected point source array as reference. Two image quality evaluation parameters of remote sensor ground pixel resolution and modulation transfer function can be obtained at the same time. The two-dimensional Gaussian model is used to describe the point spread characteristic of the optical remote sensing satellite imaging system. Selecting the 5×5 pixel values of each reflective point source remote sensing image into the point spread function model and use the least squares method to fit the two-dimensional Gaussian surface to obtain the image point coordinates. According to the ground pixel resolution detection principle and combined with the ground point source position measurement, the ground pixel resolution of the remote sensor is obtained. Based on the image point coordinates, all point source image data are positionally registered, and the image data after data rearrangement is fitted again with the two-dimensional Gaussian surface to obtain oversampled, sub-pixel interpolated optical remote sensing satellite imaging system point spread function, and then obtain the system modulation transfer function. In the image quality evaluation test of ZY-3 satellite based on reflective point sources, a 4×4 reflective point source array with non-integer pixel intervals was concentrated on the calibration test site along the flight direction and the linear array direction. The distance between adjacent point sources is 10.25 pixels, and the distance between two point sources is 20.5 pixels. The point spread function of the optical remote sensing imaging system can be sub-pixel interpolated to 0.25 pixels, which can effectively overcome the sampling effect of the imaging system and suppress the influence of random noise. According to the principle of collinearity test, the sum of the centers of adjacent reflection point sources should be equal to the distance between the centers of phase reflection point sources. The test results show that the error of the collinearity test results between the optical remote sensing satellite detector linear array and the flight direction is less than 0.002 pixels. It shows that the extraction result of array point source image point has high precision and accuracy. The standard deviations of the ground pixel resolution detection results in the linear array direction and flight direction of optical remote sensing satellite detectors are 0.020 6 and 0.021 5, relative deviations are 6.4‰ and 6.1‰, which shows that the point source image pixel extraction accuracy is high and the linear array detection element of the remote sensor has good rigidity and stability in a local area. Comparing the results of the on-orbit modulation transfer function detection of the ZY-3 satellite by the array point sources method and the double-edge method, the difference between the two methods in the flight direction and the linear array direction is 0.000 2 and 0.012 6. Compared with periodic targets, array point sources have the characteristics of light weight, miniaturization, and automation. In the process of quantitative detection of the ground pixel resolution of optical remote sensing satellites, the array point source method is not affected by the subjective factors of image interpreters and can suppress the influence of atmospheric and random noise. The array point source method is a two-dimensional modulation transfer function direct detection method according to the physical definition of modulation transfer function, which can intuitively describe the point spread characteristic of the photoelectric remote sensing imaging system. The array point sources can also be used as a reference target for the radiometric calibration of optical remote sensing satellites. The on-orbit radiometric calibration of remote sensors can be realized by setting up a multi-level point source array on the ground, combined with the measurement of atmospheric optical characteristic parameters. Therefore, the array point sources can comprehensively realize the image quality evaluation and radiometric calibration of optical remote sensing satellites..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0428001 (2023)
Remote Sensing Image Fusion Based on Two-branch U-shaped Transformer
Wensheng FAN, Fan LIU, and Ming LI
Multi-spectral images are key references for earth observation. However, capturing rich spectral information introduces limited spatial resolution in multi-spectral imaging. To overcome the trade-off between spatial resolution and spectral resolution in remote sensing, panchromatic images with high spatial resolution bMulti-spectral images are key references for earth observation. However, capturing rich spectral information introduces limited spatial resolution in multi-spectral imaging. To overcome the trade-off between spatial resolution and spectral resolution in remote sensing, panchromatic images with high spatial resolution but poor spectral information are adopted to complement multi-spectral imagery. As a result, the technique of fusing high-resolution panchromatic images and low-resolution multi-spectral images, namely pan-sharpening, is developed and facilitates various remote sensing application. Existing pan-sharpening methods can be roughly divided into four main categories: component substitution, multi-resolution analysis, variational optimization and deep learning. Each category has its own fusion strategy. Recently, a number of deep-learning-based methods are developed and obtain superior performance on fusion quality. These methods are typically based on convolutional neural networks and even combine the idea of generative adversarial networks. However, the inadequate extraction of global contextual and multi-scale features always leads to a loss of spectral information and spatial details. To solve this problem, a two-branch u-shaped transformer is proposed in this paper. Firstly, the multi-spectral and panchromatic images to be fused are partitioned into non-overlapping patches with fixed patch sizes, and each patch is embedded into a vector. The embedding vectors have the same feature dimension and contain the rich spectral and spatial information of the image patches. Subsequently, the embedding vectors of the multi-spectral and panchromatic images are fed into the two branches of the transformer encoder to extract hierarchical feature representations, respectively. The encoder consists of shifted windowing transformer blocks and patch emerging layers. Therefore, it can fully extract global and multi-scale features. In the encoding process, hierarchical panchromatic feature representations are injected into multi-spectral feature representations to obtain hierarchical fused feature representations. Besides, the high-level features are further fused through a transformer-based bottleneck. The transformer decoder progressively up-samples the high-level fused feature representation via patch expanding layers and suppresses redundant features via feature compression layers. In the decoding process, the hierarchical representations from the encoder are aggregated with the high-level fused feature representation via skip connections to avoid information loss. Finally, the decoder produces a high-resolution fused feature representation. Rearrangement and transposed convolution operations are used to reconstruct the desired high-resolution multi-spectral image from embedded patches. To validate the effectiveness of the proposed method, extensive experiments are conducted on three datasets acquired by Gaofen-2, QuickBird and WorldView-3 satellites. Since the ground-truth high-resolution multi-spectral image is non-existent, the multi-spectral and panchromatic images are spatially degraded according to the Wald's protocol, and the original multi-spectral images can be used as the reference images to supervise the training of the proposed network. The proposed network is trained for 500 epochs by using an AdamW optimizer. The mean absolute error between the reference image and the fusion result is used as a loss function to guide the optimization of the proposed network. To evaluate the fusion results, four full-reference indices are adopted for testing at the reduced resolution. One no-reference index with its spectral distortion and spatial distortion components is used for testing at the full resolution. Since the feature dimension of embedding vectors and the size of partitioned patches are important hyper-parameters that affect the performance and the computational complexity of the proposed method. Several model variants are built to observe the impact of the two hyper-parameters. The variant with an embedding vector dimension of 192 and a patch size of 4 has the best fusion results. However, compared with its huge computational cost, the improvement of fusion results is relatively limited. Therefore, the embedding vector dimension is set to 128 and the patch size is set to 4 in this paper. Subsequently, the proposed method is compared with eight widely used fusion methods to verify its effectiveness. With the original multi-spectral image as the reference image, the methods are firstly compared on images acquired by the three satellites at reduced resolution. Through visual results and residual maps between the fusion results and the reference image, it can be observed that the proposed method obtains the best visual quality and the smallest errors. As for quantitative evaluation via the objective indices, the proposed method also has the best quantitative results in terms of all the indicators on all the three kinds of testing data. Next, all the methods are compared on the original images acquired by the three satellites at full resolution. The visual result of the proposed method shows better preservation of both spectral information and spatial details than those of other methods. As for objective quantitative evaluation, the proposed method obtains the best results in terms of all the metrics on the Gaofen-2 data. On the QuickBird and WorldView-3 data, the proposed method shows better values than other methods on the spatial and overall indices. In conclusion, the reduced-resolution and full-resolution experimental results on the three data sets demonstrate that the proposed method outperforms other methods in terms of both subjective visual effect and quantitative metrics..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0428002 (2023)
Block Image Enhancement Method Based on Global Adaptive Processing
Bin ZUO, Qiang XU, Ran PANG, Jinlong XIE, Yuwei ZHAI, and Fang GAO
Aiming at the problem that the background noise is excessively enhanced by the existing panchromatic remote sensing image enhancement algorithms when the object is enhanced in the image, a block image enhancement method based on global adaptive processing is proposed. Firstly, block processing is carried out and image Aiming at the problem that the background noise is excessively enhanced by the existing panchromatic remote sensing image enhancement algorithms when the object is enhanced in the image, a block image enhancement method based on global adaptive processing is proposed. Firstly, block processing is carried out and image enhancement parameters of each image block in the image are calculated respectively. Remote sensing images contain many types of ground objects, and the difference between different ground objects is great. If unified enhancement parameters are used to enhance the whole image, the enhancement effect of some ground objects is often not ideal. Local enhancement can solve the above problems well. This method mainly applies to a small area near the target object, and can fully use the image's dynamic range to represent the change of image grayscale. Next, global adaptive processing is carried out. Global adaptive enhancement parameters are calculated and used to modify the enhancement parameters of noise blocks. This paper proposes a global adaptive enhancement technology based on a grayscale histogram for global adaptive processing of locally enhanced images, which can obtain the grayscale distribution of background objects in the image. This method helps to determine the brightness of the target relative to the background, and after determining the relative brightness, the enhancement parameters with better enhancement effect can be determined. Then, the adjacent image blocks are merged by building block difference factor, and the image blocks are classified as detail blocks and noise blocks. Local block enhancement will lead to more highlighting noise in some image blocks. In order to accurately identify these image blocks and use global adaptive enhancement parameters based on grayscale histogram to correct these noise blocks, this study calculated the block difference factor between adjacent image blocks. According to this, the information of texture and brightness between adjacent image blocks can be judged whether the abrupt change occurs at the boundary of the block. Finally, the parameters of each pixel are obtained by interpolating the enhancement parameters based on image blocks, and panchromatic remote sensing images are enhanced according to these parameters. The proposed method is used for panchromatic remote sensing images with different scenes, and the effects of various enhancement methods are evaluated based on various indexes. It is found that the proposed enhancement methods have good performance. By comparing the enhancement results of different algorithms for remote sensing images near ports, it can be seen that the proposed algorithm can reduce the dynamic range of images and effectively enhance the details of objects in the image, which can improve the clarity of ship objects in remote sensing images near ports. By comparing the enhancement results of cloud coverage images by different methods, it can be seen that the proposed algorithm can effectively suppress cloud interference on ship targets in the images. Even if some ships are covered by clouds, the contrast of grayscale of the image pixels around the target can be increased as much as possible by this algorithm, which corrects the shortcomings of over-enhancement or under-enhancement in the cloud covered image processing by previous methods. This indicates that the proposed algorithm can effectively solve the problem of over-enhancement of background noise while enhancing the details of target objects in the image by existing panchromatic remote sensing image enhancement algorithms. The average running time of different algorithms is also calculated, and it is found that the average running time of the proposed algorithm is 0.14 s, which is slightly less than the image processing time of the contrast enhancement method. The method can eliminate the residual error of the existing radiometric correction processing to a certain extent and make the same object in different images comparable..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2023
- Vol. 52, Issue 4, 0428003 (2023)
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