Journals >Acta Photonica Sinica
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2023
Volume: 52 Issue 9
24 Article(s)
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High Quality Bessel Beam Array Generation Method Based on Computer Generated Holography
Ruidi ZHANG, Yaxuan DUAN, and Zhengshang DA
Bessel beam array has been widely used in femtosecond laser processing, particle capture, optical microscopy, optical communication, and other fields. Especially in the field of industrial processing, the Bessel beam plays an important role in the process of pore structures with the ratio of height to depth due to its Bessel beam array has been widely used in femtosecond laser processing, particle capture, optical microscopy, optical communication, and other fields. Especially in the field of industrial processing, the Bessel beam plays an important role in the process of pore structures with the ratio of height to depth due to its long focal depth characteristics. For the preparation of large-area periodic pore micro-nano structures, the parallel processing method of the Bessel beam array can significantly improve the machining efficiency. The machining quality of materials is closely related to the quality of the light field of the Bessel beam array, so it is significant to study the generation method of Bessel beam array with high quality. The traditional Bessel beam array generation methods include: multi-axicon phase serial superposition method, Dammann grating, and axicon phase superposition method, which can generate parallel and divergent Bessel beam array, respectively. However, the generated Bessel beam array has problems of poor uniformity and low diffraction efficiency. Therefore, two computational holography methods are proposed in this paper, which can generate high-quality parallel and divergent Bessel beam arrays respectively. Firstly, the computational hologram model of the proposed method is established, and the multi-axicon phase parallel splicing method is proposed, which effectively reduce the background noise of the optical field by improving the“aperture utilization ratio”of the window; The multi-lens and axicon phase superposition method is proposed, the multi-lens phase superposition method is used to generate multiple focus distributions on the observation plane, and then the beams of each focus are modulated into Bessel beams by superimposing axicon phase, thus forming Bessel beam array,the key of this method is the multi-lens phase superposition to generate multi-focus distributions with controllable position. Secondly, holograms of a 3×3 Bessel beam array are generated by the proposed method and the traditional method, and then simulated respectively, the transverse optical field distribution and diffraction pattern of the Bessel beam array in free space are obtained, the uniformity and diffraction efficiency of the Bessel beam array generated by the proposed method and the traditional method are compared and analyzed. The simulation results show that the uniformity and diffraction efficiency of the parallel Bessel beam array generated by the proposed method are 98.94% and 78.12%, respectively; the uniformity and diffraction efficiency of the diverging Bessel beam array generated by the proposed method are 97.95% and 79.23%, respectively. Finally, the images of 120 mm, 130 mm and 140 mm along the transmission direction of Bessel beam array are collected through experiments, which are highly consistent with the simulation results. Compared with traditional methods, the uniformity of parallel and divergent Bessel beam arrays produced by the proposed method is increased by 2.97% and 4.70%, respectively, and the diffraction efficiency is increased by 48.22% and 54.75%. The method proposed in this paper provides a technical approach to the generation of high quality Bessel beam arrays and has certain engineering application value..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0909001 (2023)
High Photon Efficiency Image Reconstruction Algorithm Based on Depth Range Selection for Single Photon Counting LiDAR
Fanxing MENG, Tongyi ZHANG, Yan KANG, Ruikai XUE, Xiaofang WANG, Weiwei LI, and Lifei LI
Photon counting imaging technology is a new type of active imaging technology, which obtains depth information of the target by accumulating histograms of echo photons. It can be combined with Time-correlated Single-photon Counting (TCSPC) to achieve high temporal resolution. Compared with passive imaging systems, it hPhoton counting imaging technology is a new type of active imaging technology, which obtains depth information of the target by accumulating histograms of echo photons. It can be combined with Time-correlated Single-photon Counting (TCSPC) to achieve high temporal resolution. Compared with passive imaging systems, it has stronger robustness and is widely used in fields such as biomedicine, target recognition and remote sensing imaging. But it takes a long time to accumulate thousands of echo photons. In some environments with low Signal-to-background Ratio (SBR) and very few echo photons, such as military reconnaissance and other fields, long-term data collection can not be satisfied, and the ability to reconstruct 3D scenes is affected by noise photons and vacant pixels. This paper proposes a high photon efficiency image reconstruction algorithm based on depth range selection. The algorithm achieves strong resistance to noise and fully improves photon utilization efficiency through two steps: selection of target depth range, adaptive supplementation and TV regularization. Specifically, the selection of the target depth range allows us to gain depth range at the initial stage of the reconstruction process, paving the way for subsequent processing. This process is divided into five steps: merging all data into histogram, peak searching for the histogram, potential signal range determination, signal range review and select signal range. These five processes can ensure that the depth range we obtain is more accurate than setting threshold gating to the histogram. The photon screening process can remove all the noise outside the depth range, thereby reducing the error we introduce when we fill in the vacant pixels. Compared with relying on fixed neighborhood data to supplement vacancies, supplementation using adaptive neighborhood data has a higher photon utilization efficiency and will be more suitable for environments with very few echo photons. Finally, TV regularization is used to smooth the residual noise in the depth range. The simulation and experimental process have verified that even in the case of low SBR and very few echo photons, our algorithm can still effectively reconstruct the 3D image of the scene. We reconstructed the simulation data of different degrees of echo photons when SBR=0.04 and compared it with the high photon efficiency algorithm and the Unmixing method. We also input the data preprocessed by the proposed method into the Unmixing method for processing (Preprocess-unmixing, PP-Unmixing) to verify the contribution of accurately selecting the target depth range. The preprocessing here only includes the selection of target depth range. The results show that our method can distinguish scene edges in any case, and the reconstruction effect and RMSE are better than the other three methods. Our proposed method is also a fast reconstruction method. In addition, a comparison between the PP-Unmixing method and the Unmixing method proves the necessity of accurately selecting the target depth range. In addition to the simulated data, two experimental scenarios further verify the feasibility of the proposed algorithm. In experimental scenario 1, SHIN D's method can not accurately estimate the depth range, resulting in a large deviation in the reconstructed depth map. And as the number of echo photons decreases, the image becomes increasingly blurred and the scene cannot be resolved, even though it has a fast reconstruction speed. The Unmixing method is better than SHIN D's method in terms of reconstruction effect, but it still can not completely reconstruct the scene in the case of fewer echo photons, the filtering for noise is not thorough enough, and its running speed is still the slowest. The method proposed in this paper can clearly distinguish the scene in any case. Even in the extreme environment with SPPP=0.47, the time-consuming and RMSE of results are only 0.032 m and 37.2 s. In experimental scenario 2, our method can retain more detailed information than the other two methods, especially in the extreme case when the SPPP=0.7, and the other two methods can hardly detect the house information. Further, in terms of RMSE and time consumption, SHIN D's method is the fastest, but its RMSE is the largest, and the RMSE of the Unmixing method is comparable to our method, but the reconstruction speed is still the slowest. Therefore, our method has more advantages in comprehensive ability and is more suitable for an environment with few echo photons and low SBR. In summary, our method has a significant reconstruction effect on both simulation data and experimental data, which proves that this method is more suitable for the situation of extremely low SBR and a very small number of echo photons. In terms of computing speed,it is also a fast reconstruction method. In addition, the proposed method has better applicability to the situation where there are multiple depth targets in the scene, and further research and verification will be carried out in the future..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0910001 (2023)
Low-light True Color Image Enhancement Algorithm Based on Adaptive Truncation Simulation Exposure and Unsupervised Fusion
Yongcheng HAN, Wenwen ZHANG, Weiji HE, and Qian CHEN
Low-light true color image enhancement is an important branch in image processing. The images obtained in low-illuminance environments often have low brightness, low contrast, noise, and color distortion. Due to the complexity and diversity of target scenes and imaging equipment, it is difficult to directly obtain satiLow-light true color image enhancement is an important branch in image processing. The images obtained in low-illuminance environments often have low brightness, low contrast, noise, and color distortion. Due to the complexity and diversity of target scenes and imaging equipment, it is difficult to directly obtain satisfactory high-quality images in low-illumination environments. There are many problems in the information content of these low-light true color images, and the image is not good for viewing and is not conducive to advanced image tasks in the later stage. Aim at solving the problems of detail loss in the dark area and excessive enhancement in the bright area, a low-light true color image enhancement algorithm based on adaptive truncation simulation exposure and deep fusion is proposed. The algorithm has the function of brightness suppression. We design an adaptive truncation simulation exposure method and use an unsupervised network model to fuse the exposure sequence to achieve a flexible and efficient fusion of multiple exposure images of fixed size. First, an exposure sequence about the original low-light true color image is generated by simulation, and then the convolutional network is used to learn the weight map corresponding to the exposure sequence. We can obtain the final enhanced results by weighted fusion within the network. Most classic simulated exposure algorithms either map the image linearly or use existing enhancement algorithms such as histogram equalization. The number of simulated exposures is often determined artificially in pursuit of as many exposure sequences as possible covering different brightness levels, which results in many redundant images in the simulated exposure sequence. To address these problems, we propose an adaptive gamma correction method which can effectively avoid this redundancy. The light and dark regions of the image are segmented first, then truncated adaptive gamma correction is carried out. Finally, the appropriate exposure sequence is obtained by guided filter denoising. After obtaining the multi-exposure sequence, an efficient fusion method is necessary. At present, the fusion method for single low light image enhancement is mainly weighted hierarchical fusion, which has large computation and low robustness. It is easy to produce halo and seam phenomenon. To address these problems, we propose an unsupervised network model, including a context aggregation network based on dilated convolution which can achieve low resolution weight map efficiently, and a deep guided filter that can strike a balance between image quality and efficiency. And the final enhanced result is obtained by the weighted average of the simulated exposure sequence and the weight map. To verify the superiority of the algorithm, we collect vast low-light true color image datasets and compare the enhanced results with many state-of-the-art low- light image enhancement algorithms from subjective and objective perspectives. And a laboratory environment dataset is collected using a low-light night vision camera and a three-channel true color camera. Experimental results show that the algorithm we proposed has the best NIQE scores of the public datasets, and the best PSNR and SSIM scores of the laboratory environment dataset, among which NIQE is reduced by 4.49, PSNR is increased by 4.28 and SSIM is increased by 1.94%. In addition, the color reproduction effect of the algorithm is very good, and the color difference of the proposed algorithm is the smallest. At 8.71×10-2 lx illuminance, the color difference of the proposed algorithm is reduced by 14.83% than the suboptimal algorithm, and at 1.02×10-2 lx, it is reduced by 3.05%. The algorithm proposed can significantly improve the brightness and contrast of the image, has good robustness, and will not produce excessive enhancement. It can effectively restore the image details while taking into account the color information and enhance the results to be true and natural..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0910002 (2023)
Quantitative Retrieval of Coherent Transfer Function via Fourier Ptychographic Microscopy
Jiaming CHEN, An PAN, Aiye WANG, Caiwen MA, and Baoli YAO
The performance of an optical imaging system is typically characterized by the intensity Point Spread Function (PSF) or Optical Transfer Function (OTF). But the Coherent Transfer Function (CTF) is better for describing the coherent optical imaging system. Though the CTF characterizes the complex amplitude transfer propThe performance of an optical imaging system is typically characterized by the intensity Point Spread Function (PSF) or Optical Transfer Function (OTF). But the Coherent Transfer Function (CTF) is better for describing the coherent optical imaging system. Though the CTF characterizes the complex amplitude transfer properties of the light field, it is hard to measure compared with PSF. Fourier Ptychographic Microscopy (FPM) is a promising computational technique that can obtain both complex amplitude information of an object and the CTF of coherent imaging system, which provides a way to retrieve the CTF. FPM, combining the concept of aperture synthesis and phase retrieval, is a recently developed imaging technique that allows the reconstruction of high-resolution complex images with an extended field of view. By acquiring a series of low-resolution brightfield and darkfield images under inclined illumination and stitching them together in the Fourier domain, FPM can break through the frequency limit of the employed objective determined by its numerical aperture. Consequently, the space-bandwidth product of the optical imaging system can be effectively increased without precise mechanical scanning. The flexibility with low-cost hardware requirements makes FPM a powerful tool particularly potential for imaging biomedical samples in the field of digital pathology. Although many advanced FPM techniques have been proposed to achieve higher data acquisition efficiency and recovery accuracy in the past few years, little is known about the precision, stability, and requirements of the CTF, especially when there are inevitable system errors. If FPM can retrieve high-precision CTF, it will provide a new means for CTF calibration. Therefore, this thesis mainly studies the acquisition of CTF with high precision, stability and efficiency via FPM. In this paper, we investigate the reconstruction quality of the CTF under different system errors with different targeted algorithms and find that the reconstructions of CTF is more robust than the reconstructions of object. In addition, under the condition of good recovery of object function, different objective algorithms can also recover basically the same CTF. Therefore, the CTF recovered by FPM algorithm can be used to quantitatively characterize coherent optical systems. Based on this, we report a sub-region translation method named ST-FPM, which is used in Fourier ptychographic microscopy imaging. Based on the basic assumption that the aberration of adjacent local fields is basically unchanged, asymmetric spatial information is introduced to eliminate the grid noise caused by periodic illumination, which improves the recovery accuracy of CTF and accelerates the convergence speed of CTF reconstruction in limited images. The recovered CTF is deconvolved with incoherent images. And the contrast is additionally improved compared with the traditional FPM. In addition, this method can realize image refocusing without the prior information of defocus. In addition, we study the spatial and frequency domain data redundancy of Fourier ptychographic microscopy to recover the coherent transfer function, and find that at least about 40% spectral overlap rate is needed to accurately reconstruct the coherent transfer function, which is 10% higher than that without aberration. And at least 25 original low-resolution images are needed for the stability of coherent transfer function. Finally, we discuss the necessary conditions for stable CTF reconstruction, and verify the conclusion in simulation and experiment..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0911001 (2023)
Research on Digital Drive Imaging Quality of Silicon-based OLED Micro Display Based on Super Pixel
Yuan JI, Honggang CHEN, Baoliang CHEN, Yin ZHANG, and Yiqing XU
With the development of new display technology and the continuous warming of the concept of metauniverse, micro-display has been widely used in near-eye display fields such as virtual reality and augmented reality. The silicon-based OLED micro-display has become the best choice for near-eye display devices due to its aWith the development of new display technology and the continuous warming of the concept of metauniverse, micro-display has been widely used in near-eye display fields such as virtual reality and augmented reality. The silicon-based OLED micro-display has become the best choice for near-eye display devices due to its advantages of small area, high brightness, fast response, wide color gamut and high resolution. traditional silicon-based OLED micro-displays mostly adopt analog drive mode, but it is difficult to meet the requirements of high resolution and high refresh rate due to the limitation of the conversion speed and accuracy of digital-to-analog converter. The digital driving mode adopts pulse width modulation method, which makes the human eye perceive different gray levels by changing the light and dark time of pixels. With the advantages of fast switching speed, high stability and low noise, digital drive is more suitable for silicon-based OLED micro-displays with fast photoelectric response. However, with the further improvement of the requirements for the imaging quality of micro-display, the current scanning mode for digitally driven micro-display is difficult to meet the massive data transmission caused by the improvement of resolution and refresh rate. At the same time, digital drives mostly adopt the addressing display period separated sub-field method. The distribution of the luminous sub-field is not continuous within a frame time, and the dynamic false contour phenomenon occurs when the moving picture is displayed, which seriously affects the viewing quality of the human eye.Aiming at the imaging problems such as low resolution, low refresh rate of micro-display and dynamic false contour when displaying moving pictures, this paper proposes a digital driving scanning strategy based on super-pixel technology and digital driving principle. This method uses the integral property and visual persistence property of the human eye to quickly switch between two lower resolution images in the process of display. When the switching frequency is far beyond the critical flicker frequency that the human eye can perceive, the human eye will recognize two lower resolution images as one higher resolution image due to the spatial dislocation of the two images on the displays. A super-pixel model is built using MATLAB and the image quality of the super-pixel processed image is evaluated. Under the condition that the data transmission bandwidth is reduced by 50%, the average PSNR of the super-pixel image is about 34.086 dB, and the average SSIM is about 0.942. According to the integration method, the image quality of the super-pixel image perceived by the human eye is not significantly different from the original image. This scanning strategy provides an effective solution for improving the resolution and refresh rate of micro-display. In order to further prove the effectiveness of the super-pixel scanning strategy, this paper analyzes the mechanism of dynamic false contour and simulates the dynamic false contour of the image under the super-pixel scanning mode. Considering the existence of visual threshold in human eyes, the dynamic false contour of super-pixel is evaluated by combining and extending the just noticeable distortion integral method. Compared with traditional scanning methods, the super-pixel scanning method displays two sub-frame images in one gray period, and the display time of each sub-frame only accounts for half of the gray period. When the integration method is used to simulate the gray level of pixels perceived by the human eye, it is necessary to integrate the subfields of two sub-frame spans. The simulation results indicate that under the proposed scanning strategy, the probability of the error between the maximum perceived brightness of the human eye and the actual brightness of each pixel in the process of image movement is equal to 0 and not more than 8 gray levels is about 93.3% and 99.3%, respectively. Compared with 19 subfields and CGPWM scanning mode, the dynamic false contour phenomenon under the super-pixel scanning strategy has been significantly improved, and the display effect conforms to the human eye observation experience. According to the super-pixel scanning strategy, a digital driving type super-pixel micro-display controller was designed, and a system verification platform based on FPGA was built. The full-color digital driving type silicon-based OLED micro-display with resolution of 2 048×2 048 was successfully driven, which proved the feasibility of the proposed scanning strategy and laid the foundation for the application of the digital driving type silicon-based OLED micro-display based on super-pixel..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0911002 (2023)
Target Localization Technology Based on Biomimetic Curved Compound Eye Camera
Shuaimin ZHU, Wenge GUO, Tao LIU, Yuanjie ZHANG, Huangrong XU, Dengshan WU, Xiaojun ZHOU, and Weixing YU
Using a vision system to locate a target is a necessary step for its three-dimensional detection of the target. The traditional single-aperture imaging system can only obtain the geometric image information of the target. A compound eye vision system has the advantages of large field of view, large depth of field, multUsing a vision system to locate a target is a necessary step for its three-dimensional detection of the target. The traditional single-aperture imaging system can only obtain the geometric image information of the target. A compound eye vision system has the advantages of large field of view, large depth of field, multi-channel imaging, and can obtain the depth information of the target and be sensitive to fast moving targets. At present, a common visual positioning method is to use the binocular vision system to locate the target based on the parallax between two cameras. However, because the binocular vision system has only one set of constraints, and the baseline is fixed, the binocular vision system has low positioning accuracy in the long distance, while the compound vision system has more constraints because of the number of sub-eyes. In the long distance, the positioning accuracy is higher than the binocular vision system. It has aroused a wide attention of researchers. This paper uses the bionic curved compound eye camera developed in the laboratory to carry out the research of 3D positioning and 3D reconstruction. The compound eye vision system consists of a curved compound eye, an optical relay image conversion subsystem and a high-definition image sensor. In this paper, CAL Tag calibration board and MATLAB stereo calibration toolbox is used to calibrate the internal parameter matrix of the compound eye camera and the rotation matrix and translation vector between the sub-eye and the world coordinate system. Based on the principle of binocular vision positioning, a mathematical model for multi eye positioning is established on a compound eye vision system developed in the laboratory, and positioning experiments are conducted. The experimental system includes a laser rangefinder, black cardboard, and a compound eye vision system. The laser spot is used as a positioning target. Because the shape of the sub-eye is circular, the hough circle transformation algorithm is used to detect the sub-eye of the compound eye system, and the sub-eye number is determined according to the center coordinates and radius of the circle. Because this experiment is carried out under dark conditions, the background gray value is low and the spot gray value is high, so the gray centroid method is used to locate the centroid of the spot and obtain the centroid of the spot taken by different sub-eyes. The three-dimensional coordinates of the centroid of the spot are obtained from the coordinates of the centroid of the spot in the camera pixel coordinate system according to the corresponding relationship between the pixel coordinate system and the world coordinate system. The linear equations of several sub-eyes are combined to form the overdetermined equations and the optimal solution is obtained by the least square method. The distance measurement experiment results show that the distance measurement error of the compound eye camera is less than 2% within a range of at least 4 meters. The experimental results show that the bionic curved compound eye camera prepared in the laboratory could carry out more accurate three-dimensional positioning of objects in space. The error caused by the laser jitter and the size change of the light spot with the distance change on the positioning result is analyzed in detail. In the aspect of target 3D reconstruction, the sift algorithm is used to detect and match the feature points of the target images of different sub-eyes, and the RANSAC algorithm is used to remove the wrong matching points, to obtain the accurate feature point matching of the target captured by different sub-eyes. Then, according to the corresponding relationship between the pixel coordinate system obtained by camera calibration and the world coordinate system, the three-dimensional coordinates of the feature point in the world coordinate system are calculated from the coordinates of the sub-eye pixel coordinate system, and the complete reconstructed point cloud of the target is obtained through point cloud stitching. The 3D reconstruction experiment is carried out by the reconstruction algorithm. The experiment takes the cube covered with speckles as the reconstruction target. The cube is photographed at about 0.6 meters from the camera, and a relatively complete 3D reconstruction point cloud is obtained. The research results in this paper show that the bionic curved compound eye camera has great development potential and application prospects in the fields of 3D positioning, 3D reconstruction and optical navigation..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0911003 (2023)
Research Progress of Tunable Mid-infrared Solid State Laser Pumped by Near-infrared Laser（Invited）
Kai CHEN, Degang XU, Yixin HE, Kai ZHONG, Jining LI, Yuye WANG, and Jianquan YAO
Tunable mid-infrared lasers based on nonlinear optical frequency conversion play a vital role in application fields including environment monitoring, remote sensing and biomedical diagnosis. The long-term development of near-infrared laser technology has led to a high degree of commercialization of near-infrared pumpedTunable mid-infrared lasers based on nonlinear optical frequency conversion play a vital role in application fields including environment monitoring, remote sensing and biomedical diagnosis. The long-term development of near-infrared laser technology has led to a high degree of commercialization of near-infrared pumped lasers. The utilized of the commercial near-infrared laser as the pump source is easy to realize miniaturization, high power and high stability operation of the tunable mid-infrared laser. Nonlinear optical crystal, which is the core component of the tunable mid-infrared laser, determines the output characteristics of the mid-infrared laser source. Suffering from multi-phonon absorption, the tunable output band of traditional oxide crystals is limited to below 4 μm. On the other hand, the most commonly used ZnGeP2 has strong two-photon absorption at 1.06 μm. High-quality mid-infrared crystal pumped by near-infrared laser have remained of great interest in recent years. In this paper, we reviewed the application of the newly developed non-oxide crystals in mid-infrared laser generating. BaGa4Se7 and BaGa4S7 have wide transparency range, high laser damage threshold and nonlinear coefficient. Using a low repetition frequency pump source, the tunable output range covers the entire mid-infrared band, and the output energy achieves mJ-level even in the long wave infrared band. Under a pump repetition rate of hundreds of Hz, the average output power in the mid-wave infrared band reaches 1 W. However, due to the low thermal conductivity of these two crystals and the near-infrared absorption, there is no report on the near-infrared laser pumped source with repetition rate of kHz-level and average output power of W-level. Subsequent research mainly focused on the improvement of pump and crystal cooling conditions. LiGaSe2 and LiGaS2 crystals are suitable for near-infrared ultrashort pulse pumping to produce mid-infrared lasers due to their large band gap. In particular, LiGaS2 crystal has high laser damage threshold and thermal conductivity. At present, there have been many reports about the generation of mid-infrared laser with repetition frequency of kHz or even MHz. The femtosecond laser source based on LiGaS2 crystal has been applied to the research of vibration sum-frequency spectrum detection, biomolecular fingerprint spectrum recognition, etc. However, due to the small geometric size of LiGaSe2 and LiGaS2 crystals in the existing reports, their output powers under nanosecond laser pumping are relatively low. In addition, LiGaSe2 and LiGaS2 crystals have an obvious absorption peak near 8 μm. The transmittance above 8 μm decreases rapidly, so it is not suitable for the generation of long-wave infrared lasers. The improvement of crystal synthesis and growth process will help to play the potential of LiGaS2 crystal in broadband tuning and high-power laser generation. LiInS2 and LiInSe2 are newly developed crystal with high band gap. The laser damage threshold of LiInS2 crystal and LiInSe2 crystal is relatively low, so the existing reports mostly based on picosecond/femtosecond laser system. Under nanosecond laser pumping, it is difficult to achieve mJ-level, high energy mid-infrared laser generation. The current research is mainly focused on broadband tunable mid-infrared laser generation. Although the current output average power is low, LiInS2 and LiInSe2 crystals have high thermal conductivity and low thermo-optical coefficient, so these crystals have the potential to be used in the generation of high repetition rate and high average power mid-infrared lasers. At present, the main bottleneck lies in the synthesis and growth process of large size and high-quality crystals. CdSiP2crystals have high thermal conductivity, nonlinear coefficient and band gap, and the cutoff wavelength in the short-wave direction is relatively short. Using a near-infrared laser pump source, the output energy reaches mJ-level under low repetition rate operation. The output average power exceed 100 mW with repetition frequency of several MHzs. High efficiency 6~7 μm generation with 1 064 nm laser pumping can be achieved under non-critical phase matching condition. The output band can be expanded to 2~8 μm by combining pump wavelength tuning and angle tuning. CdSiP2 also has great potential in the on-chip application. However, the laser induced damage threshold of CdSiP2 crystal is low, and the transmittance above at 8 μm decreases rapidly, which limits its application in high power and long-wave infrared laser generation. Quasi-phase-matched crystals represent a new research direction of mid-infrared nonlinear optical crystal materials. Quasi-phase matching technology can utilize the maximum nonlinear coefficient and avoid walk-off effect, so tunable mid-infrared source based on quasi-phase-matched crystals have the advantages of high conversion efficiency and can realize mid-infrared output in the whole transparency band. Orientation-patterned gallium phosphide (OP-GaP) has high nonlinear efficient and thermal conductivity. It has great application potential in high power and high efficiency middle infrared laser generation. However, the synthesis of high-quality single crystals with large aperture and high uniformity are difficult. The improvement of material growth technology will significantly improve the output power of existing mid-infrared lasers based on quasi-phase-matched crystal materials. The further research will focus on: 1) improvement of the crystal quality, especially the size and the uniformity of the crystal; 2) improvement of the output characteristics of the near-infrared pump laser; 3) development of the new nonlinear optical crystals..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0914001 (2023)
Tunable Repetition-rate High Power Picosecond Green Laser
Sizhi XU, Ying XIA, Yubo GAO, Xing LIU, Deqin OUYANG, Yewang CHEN, Xu WU, Fanghua XU, Li GUO, Junqing ZHAO, Chunyu GUO, Qitao LV, and Shuangchen RUAN
In this paper, an 1 064 nm, 148.7 W, 8 ps laser is obtained through a multi-stage end-pumped Nd:YVO4 solid-state laser amplification system. Further a 1mm-diameter spot is coupled to a LBO crystal for frequency doubling, and a 95 W, 6.43 ps laser output is obtained at 532 nm, with a tunable Pulse Repetition Frequency (In this paper, an 1 064 nm, 148.7 W, 8 ps laser is obtained through a multi-stage end-pumped Nd:YVO4 solid-state laser amplification system. Further a 1mm-diameter spot is coupled to a LBO crystal for frequency doubling, and a 95 W, 6.43 ps laser output is obtained at 532 nm, with a tunable Pulse Repetition Frequency (PRF) of 500 kHz~4 MHz. The Nd∶?YVO4 Master Oscillator Power Amplifier (MOPA) consists of a high-power oscillator stage, fiber pre-amplifier and four end-pump amplifier stages. The seed is an all-fiber laser mode-locked by SESAM with a pulse duration of 7.8 ps and PRF of 20 MHz. The Acoustic Optical Modulator (AOM) is used for reducing the PRF to realize a higher peak power. The end-pump power amplifier is a four-stage free-space bulk amplifier based on neodymium-doped yttrium vanadate (Nd∶?YVO4) crystals for high-efficiency frequency conversion. It has a high polarization-dependent gain spectrum due to the birefringence of the uni-axial YVO4 crystal. Each amplifier is comprised of a 0.3% doped a-cut Nd∶?YVO4 crystal having dimensions of 4 mm×4 mm×15 mm and is mounted on a water-cooled heat sink. All amplifiers are end-pumped by a fiber-coupled 878.6 nm (400 μm core, NA=0.22) Laser Diode (LD), instead of conventional 808 nm LD, to reduce the degradation of the beam quality by thermal effects. The first amplifier stage is a double-pass configuration and increases the signal average power to 13 W for a 65 W pump light, while the second amplifier stage is a single-pass and scales up to 56 W of average power for a 115 W pump light. To match the amplified light with the pump light of the third stage, a lens of 80 mm focal length is added between the second stage and the third stage. Finally, 56 W, 2 MHz infrared (1 064 nm) 8.08 ps output with M2<1.5 is obtained. Then, the 56 W infrared picosecond laser is used as the source of the pre-amplification, and after shaping, it is focused to the third and fourth-stage amplifying module. This module uses two 115 W LD pumps with a center wavelength of 878.6 nm to pump two Nd∶YVO4 crystals. The seeding picosecond laser passes through the third crystal and is then deflected by a dichroic mirror (DM3) to the fourth crystal for further power amplification. An output power of 148.7 W is achieved, and the beam quality factor of the amplified pulses is MX2=1.72 and MY2=2.18. along horizontal and vertical directions, respectively.After shaping the output, a 1 mm-diameter spot is coupled to LBO crystal for frequency doubling. The highest output power is 95 W at 2 MHz PRF, corresponding to the best second harmonic conversion efficiency of 65%. The conversion efficiency is expected to be further improved with the fundamental frequency optical power, the beam quality factor is MX2=1.27, MY2=1.42. At the highest average power output, the stability of the system is observed for more than 6 hours, and the power fluctuation RMS is lower than 0.8%. In addition to achieve the highest frequency-doubling conversion efficiency, the effect of the fine regulation of the temperature of the LBO crystal on the optimal critical phase matching condition of the crystal is also studied. The laser system has the advantages of simple optical path, high average power, good beam quality, tunable repetition frequency and high stability. It is an ideal fundamental frequency light source for high-power ultraviolet and deep ultraviolet lasers, and has important applications in the fields of industrial processing and scientific research. It is expected to achieve more efficient and can be utilized for high-quality processing fields such as battery welding, and hard and brittle materials processing..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0914002 (2023)
Single-frequency Ring Laser Oscillation and Power Amplification System
Jingwei SUN, Yang YU, Kaixin WANG, and Chunqing GAO
All-solid-state single-frequency pulse lasers have been widely used in coherent laser detection, laser remote sensing, laser ranging, and other fields due to their advantages of narrow line width, long coherent length, high stability, and compact structure. In recent years, in the field of coherent laser detection, theAll-solid-state single-frequency pulse lasers have been widely used in coherent laser detection, laser remote sensing, laser ranging, and other fields due to their advantages of narrow line width, long coherent length, high stability, and compact structure. In recent years, in the field of coherent laser detection, the demands for wind field measurement, aerosol detection, and coherent imaging are increasing, and single-frequency lasers are the key devices of lidar. Among them, the 1 064 nm single-frequency pulsed laser based on Nd∶YAG crystal can not only be directly used as the light source of lidar for wind field and aerosol detection, but also can generate single-frequency laser output of other wavelengths through nonlinear frequency conversions such as frequency doubling, sum frequency, and optical parametric oscillation. In this paper, an LD-pumped single-frequency Nd∶?YAG master oscillator and power amplifier system with a wavelength of 1 064 nm has been developed. Nd∶YAG crystal has a high absorption at 808 nm. The crystal absorbs a large amount of the pump light, which will induce the thermal lensing effect and decrease the quality and stability of the beam. The thermal effect is more serious when the crystal is pumped continuously. Therefore, it is very important to decrease the thermal effect. In this paper, the steady-state heat transfer model of the Nd∶YAG rod is studied, and three Nd∶?YAG rod models with different parameters are built for comparative analysis. It is shown from the simulated results that the temperature of the Nd∶?YAG rod can be decreased by low doping concentration and end-face bonding. In the experiment, a six-mirror ring cavity is used as the oscillator. A Faraday polarizer, a polarizer, and a half-wave plate are inserted into the cavity to eliminate the spatial hole-burning effect and obtain a unidirectional single-frequency laser output. The laser pulse is obtained by an acousto-optic Q-switch. A pulse output with an energy of 2.18 mJ and a pulse width of 63.2 ns is obtained at the repetition rate of 25 Hz, and its single-frequency characteristic is validated by monitoring the waveform of the output pulse. In order to achieve a higher energy output, a power amplification system is established after the oscillator. The pulsed laser with an energy of 1.85 mJ incidents in the amplifier after being shaped by a lens. An LD side-pumped Nd∶YAG module is used in the power amplification system. After the amplification, a laser output of 15.85 mJ with a pulse width of 62.7 ns is obtained with the gain of about 8.6 times. The single-frequency ring laser oscillation and power amplification system has potential applications in lidar and optical parametric oscillators..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0914003 (2023)
Process Gas Detection of C2H2 Preparation by CH4 Cracking Based on Hollow Core Fiber Enhanced Raman Spectroscopy
Fu WAN, Rui WANG, Weiping KONG, Qiang LIU, Hongcheng SUN, Mingyong WANG, and Weigen CHEN
In the chemical industry, accurate measurement of the components and amounts of gas products in the C2H2 preparation by CH4 cracking is critical for monitoring reaction progress and assuring the quality of C2H2 production. The partial oxidation technique of creating acetylene from natural gas (mostly methane) is essentIn the chemical industry, accurate measurement of the components and amounts of gas products in the C2H2 preparation by CH4 cracking is critical for monitoring reaction progress and assuring the quality of C2H2 production. The partial oxidation technique of creating acetylene from natural gas (mostly methane) is essentially an incomplete combustion of methane involving oxidation, cracking, water-gas shift, and acetylene breakdown processes. H2, CO, CO2, C2H2, C2H6, and C2H4 are the most important intermediate products.Gas chromatography-mass spectrometry, nano-sensor techniques, infrared spectroscopy, and other approaches are commonly used to identify process gases. The detection time of gas chromatography and mass spectrometry is long, and it is difficult to miniaturize the equipment, so there are some limitations when using these method; the nano-sensor method detects a single component, and cross-sensitivity problems will occur between different gas components; infrared spectroscopy can not detect multi-component gas mixtures, and it can not realize the detection of homonuclear diatomic gases. Raman spectroscopy is a technique for directly determining the characteristics and concentrations of substances. Raman spectroscopy directly measures the features and concentrations of substances by using spherical Raman scattered light of various frequencies generated by substances under laser irradiation. Any inert gas with several components can be detected and studied at the same time. Raman spectroscopy detection is limited in gases due to the narrow Raman scattering cross section and moderate Raman scattering impact. This makes identifying the process gases utilized in CH4 cracking to produce C2H2 challenging. The invention of hollow-core fiber has enabled the creation of a novel medium for the interaction of gas and laser. In the hollow-core fiber, both a gas chamber for testing and a light transmission medium can be employed. It can extend the time the laser interacts with the gas and increase the collection efficiency of spherical Raman light, both of which will boost the gas Raman signal.A gas fiber-enhanced Raman spectroscopy detection system based on hollow-core anti-resonant fiber is conceived and built in this article. To considerably filter out the silicon noise created by the hollow-core fiber, the CCD noise reduction and small-aperture cooperative noise reduction methods are developed, which enhanced the signal-to-noise ratio by roughly 6 times. Under the following conditions, the Raman spectra of the principal gases generated during the cracking of CH4 to make C2H2 are detected: laser power of 200 mW, coupling efficiency of 95%, pressure of 1 bar, and integration duration of 60 s. Each gas has many Raman shift spectral peaks, peak attribution, and Raman spectral intensity calculated. In order to obtain higher sensitivity and more accurate analysis, typical Raman spectral peaks for each gas are recognized based on the idea of relatively independent peaks and high peak intensities. H2, CO, CO2, CH4, C2H2, C2H6, and C2H4 have characteristic Raman spectral peaks of 595, 2 142, 1 389, 2 917, 1 974, 2 957, and 1 342 cm-1, respectively. The limit of detection for H2, CO, CO2, CH4, C2H2, C2H6, and C2H4 are calculated to be 6.3, 26.6, 1.2, 2.2, 4.2, 3.9, and 9.1 L/L?bar, respectively. The Raman spectra of each separate gas are analyzed, and a quantitative analytical model between the gas concentration and the intensity of the distinguishing Raman peaks is constructed. The calculated R2 is greater than 0.999.In order to confirm the detection performance of the developed hollow-core fiber-enhanced Raman spectroscopy platform for real samples, the components and contents of the gases in the process of C2H2 preparation by CH4 cracking in a process line of Sinopec Chongqing Svw Chemical Co., Ltd. are examined. The Raman spectral peaks of the seven gases are clearly discernible, and individual Raman peaks of each gas exist fully separately, with no evidence of a cross-over phenomenon. From the intensity of the characteristic Raman spectral peaks of each gas and the obtained linear quantitative analysis curves, the concentrations of H2, CO, CO2, CH4, C2H2, C2H6 and C2H4 in the gas mixture are calculated to be 560 588.51 μL/L, 230 678.21 μL/L, 33 107.65 μL/L, 56 086.77 μL/L, 77 945.56 μL/L, 1 307.19 μL/L and 1 823.55 μL/L, respectively, proving that the process gases meet the quality requirements (H2: 548 993.78 μL/L, CO: 236 006.59 μL/L, CO2: 34 362.33 μL/L, CH4: 58 064.38 μL/L, C2H2: 75 506.62 μL/L, C2H6: 1 372.30 μL/L, C2H4: 1 917.75 μL/L), the concentrations of each gas are in accordance with the calibration values of the chromatograph with an error of about 2.11%~4.95%. The application of this method allows for greater control over the manufacture of C2H2 via CH4 cracking..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0930001 (2023)
Spatiotemporal Evolution of Aluminum-lithium Alloy Plasma Using a Coaxial LIBS System under Vacuum
Zixiao CUI, Huace WU, Ding WU, Jiamin LIU, Xiaohan HU, Ran HAI, Cong LI, and Hongbin DING
As an important technique for in-situ wall diagnostics in tokamaks, Laser-Induced Breakdown Spectroscopy (LIBS) has been demonstrated to effectively detect fuel retention and element distribution on the wall surface of the Experimental Advanced Superconducting Tokamak (EAST). However, despite its potential, the accuratAs an important technique for in-situ wall diagnostics in tokamaks, Laser-Induced Breakdown Spectroscopy (LIBS) has been demonstrated to effectively detect fuel retention and element distribution on the wall surface of the Experimental Advanced Superconducting Tokamak (EAST). However, despite its potential, the accurate quantitative analysis of wall surface elements remains a critical challenge for LIBS technology. One of the primary factors contributing to this challenge is the significant spatiotemporal non-uniformity exhibited by the laser-induced plasma under vacuum conditions. Therefore, investigating the spatiotemporal evolution of the plasma holds great significance for optimizing signal quality and enabling qualitative and quantitative analysis in wall diagnostics using LIBS. In this work, the researchers employed a coaxial optical structure based on a linear array of optical fibers, this setup enabled them to perform spatiotemporal integration and spatiotemporal resolution measurements of aluminum-lithium alloy plasma emission spectra generated by pulsed laser ablation at a wavelength of 1 064 nm, a pulse width of 5 ns, and a power density of 6.3 GW/cm2 under vacuum conditions. The emission time of the plasma was evaluated to be ~1 μs, and the emission area size was ~1 cm. The spatiotemporal evolution behavior of continuous radiation, ionic lines, and atomic lines were analyzed to determine the emission time scale of different radiative mechanisms of laser-ablated plasma. Spatially resolved measurements revealed that the spatial distributions of aluminum (Al) and lithium (Li) atoms and ions were distinct from each other, revealing an element“spatial separation”phenomenon that occurred during the lateral expansion of the laser-ablated aluminum-lithium alloy plasma in a vacuum environment. The results of the spatiotemporal evolution of the species' lateral expansion velocity showed that the ion velocity of the same element was greater than its atomic velocity (Al III>Al II>Al I, Li II>Li I), this discrepancy in velocities can potentially be attributed to the acceleration provided by the formed transient sheath. Additionally, the study discussed the differences in velocity among different elements in the same charge state, attributing it to the“mass separation”effect and ions recombination. In conclusion, this work aimed to tackle the challenges associated with quantitative analysis of wall surface elements using LIBS in tokamaks. By employing a coaxial optical structure and conducting detailed spatiotemporal measurements, the researchers gained valuable insights into the plasma's behavior during laser ablation. The study shed light on the emission time scale of various radiative mechanisms, spatial distributions of elements, and the velocities of ions and atoms. These findings contribute to understanding plasma-wall interactions in tokamak devices, providing crucial information for the development of advanced wall diagnostics techniques in fusion research..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0930002 (2023)
Noninvasive Detection of Skin Moisture Content Based on Spatially Offset Raman Spectroscopy
Rongsui YUAN, Zhiqiang WU, Rongzhi NIE, Wenting ZHONG, and Keqin ZHANG
In this study, the feasibility of detecting skin water content based on spatial displacement Raman spectroscopy and portable fiber optic spectrometer was explored. A device was constructed that can achieve an optimal spatial displacement distance of 300 μm. It is selected to detect signals noninvasively under the surfaIn this study, the feasibility of detecting skin water content based on spatial displacement Raman spectroscopy and portable fiber optic spectrometer was explored. A device was constructed that can achieve an optimal spatial displacement distance of 300 μm. It is selected to detect signals noninvasively under the surface of the skin and reduce the interference of pollutants such as water and oil on the surface of the skin. Instead of using the traditional spatial shift Raman spectroscopy device to achieve the relative shift of excitation point and collection point by mechanical offset or light blocking, a micro-shiftable light source was set up to adjust the spatial shift by using the conjugate relationship between the light source and the excitation point, and at the same time, a microscopic imaging device was introduced to control the adjustment, which finally realized the continuous adjustable spatial shift of less than 1 000 μm.The ratio of the intensity of Raman shift at 3 390 cm-1 (water Raman peak) and 2 935 cm-1 (protein Raman peak) is employed to characterize the skin moisture content, a portable spectrometer is employed to make the device moveable, the probe and the spectrometer are connected by optical fiber to make the probe flexible. Therefore, the design meets the clinical requirements. In total, 70 spectra from different locations were collected (7 people, 10 spectra each) in the experiment, and the spectra were processed by a de-backgrounding algorithm, and then the exact Raman peak intensity was obtained. The intra-group correlation coefficient was used as an index for consistency analysis. The intra-group correlation coefficient of 0.889 for a single measurement showed that the results of the two methods were in good agreement.In this study, four experiments with different spatial displacement amounts were set up, and the spatial displacement distances were 200 μm, 300 μm, 500 μm and 1 000 μm. The skin spectra from the same person at the same location measured under different spatial displacement amounts can be seen to be significantly different only in signal intensity, while there is no significant and regular change in the spectral profile and details, and it is impossible to distinguish which spectrum is better The results of the quantitative analysis can only be judged by the results of the quantitative analysis. The experimental and analytical procedures described above were repeated at four spatial shifts in the study, and it was finally confirmed that the best results could be obtained at a spatial shift of 300 μm.After proving the spatial sensitivity of the device by statistical methods and obtaining the optimal device parameters, we also hope to conclude a more intuitive method to quickly judge the signal superiority by spectral characteristics. As the spatial displacement distance increases, the spectral height decreases in turn, but the consistency shows a trend of first increasing, then decreasing and then increasing with the decreasing spectral height, there is no rule, so we cannot judge the signal superiority from the spectral height. We note that even for the same person at the same location, the relative intensity of the water signal (the ratio of the intensity of the Raman shift at 3 390 cm-1 and 2 935 cm-1) is not equal under the detection conditions of different spatial shift distances, the reason is that not all depths of the skin can correctly reflect the water content. Overall, if the relative intensity of the water signal is less than 0.05, then the detection results will only have low confidence, for example, when the spatial displacement distance is 500 μm in the experiment, the relative intensity of the water signal is only 0.023, and the intra-group correlation coefficient is only 0.316. Under such conditions, it is necessary to adjust the optical path or check whether the skin of the subject has damage, scars and other problems; if the relative intensity of the water signal is close to 0.10, you can judge that the optical path configuration at this time is basically qualified, only need further fine tuning to determine the best parameters, the empirical method can quickly and effectively filter the weak signal, strong background spectrum, improve the efficiency of the study.It is proved that the experimental device can obtain a spectrum with good repeatability. The design of the fiber spectrometer and probe separation also meets the practical requirements, which is convenient for a large number of human body measurement experiments and has the potential to be transformed into medical devices. Indicating that the skin water content measured based on the spatial displacement Raman spectroscopy has high effectiveness and is expected to achieve convenient detection of the skin inner water content..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0930003 (2023)
Structure Components Optimization Design of Ultraviolet Imaging Spectrometer
Jianyu YANG, Xuan YANG, and Jianhua ZHENG
The ultraviolet imaging spectrometer operates within a wavelength range of 340 nm to 390 nm, with its components serving as the core of the entire instrument. This design contributes to a more compact configuration of the device. Given that the initial design of the spectrometer's structural components is somewhat The ultraviolet imaging spectrometer operates within a wavelength range of 340 nm to 390 nm, with its components serving as the core of the entire instrument. This design contributes to a more compact configuration of the device. Given that the initial design of the spectrometer's structural components is somewhat generous in dimensions, there's a pressing need to reduce and optimize the weight of the main structural parts while ensuring sufficient rigidity. The primary module of the spectrometer and the frame of the imaging objective lens bear the main load and possess a relatively intricate design. Furthermore, the collimating mirror module, with its sizable volume and cantilevered structure connected to the main module, necessitates optimization. Consequently, this paper primarily focuses on refining the design of these three components. By using the variable density topology optimization method, the main module and the collimating mirror module are iterated for 80 times, and the imaging objective lens frame module is iterated for 50 times. The objective function gradually converges, and the best material distribution of important parts is obtained. As the result of topology optimization is to remove redundant structural quality by digging holes and arranging reinforcing ribs, there are some unreasonable local features in the obtained structure, so it is necessary to optimize its size parameters in detail on the basis of topology optimization. Using Latin hypercube random sampling method, a multiple linear regression model is established, and the sensitivity of key dimension parameters to system performance is analyzed. The second generation of non-dominated sorting genetic algorithm is used to complete the size optimization design of spectrometer structural components. The optimization results show that although the first-order frequency decreases, it is still higher than 100 Hz, and the lightweight rate of the structure reaches 58.7%, and the quality of the mirror surface is improved. A 1g gravity load is applied to the whole structure of the spectrometer. From the overall effect, the influence of gravity field reduces the performance of the system. However, due to a certain margin in the design, the change result is still acceptable. The lens surface deformation of the spectrometer structural components at 10 ℃ and 30 ℃ is investigated, and the influence on the optical system is analyzed. Although the spatial resolution and spectral resolution of the corresponding system at two temperature conditions decreased to some extent, it is still within the design tolerance of the system. After analysis, the lens that has the greatest influence on the system is the change of the surface shape of the collimating mirror, so it can be determined that the temperature stability of the collimating mirror must be strictly controlled. When analyzing the stray light, it shows that the stray light brightness is about 1×10-6 of the normal light, and the stray light is very small, which can meet the requirements of the optical system, indicating that the stray light of the spectrometer structure has been well suppressed. Through the finite element software, the random vibration simulation analysis and mechanical test verification of the spectrometer structure components are carried out. The results of random vibration test show that the maximum acceleration response magnification of the spectrometer structure component in X axis is 3.2, which is 4.4% different from the analysis result, the maximum acceleration response magnification in Y axis is 3.05, which is 4.72% different from the analysis result, and the maximum acceleration response magnification in Z axis is 3.6, which is 4.5% different from the analysis result. After the mechanical test, the standard spectral line lamp is used to test the spectral characteristics. Compared with the results before the mechanical test, the maximum quantitative change of the spectral characteristics test is 0.4 pixels. The experimental results show that the structural components of the spectrometer meet the requirements of mechanical environment test, and the structural stability is good. The work of this paper provides a reference and application for the structural design of other ultraviolet imaging spectrometers..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0930004 (2023)
Fabrication and Characterization of SiO2/CNC Composite Film with Dual Optical Properties
Congcong CHI, Jiarong XIAN, Panpan QU, Xin XU, Danjie ZHANG, Feifei BAI, Jiangxue REN, Yunan ZHOU, and Xinggen XU
Structural color can be created when the ordered microstructure of an object interacts with incident light. Compared to the traditional pigment colors, it is bright, long-lasting, and environmentally friendly. As a result, it is of great significance for further research and development. Cellulose Nanocrystals (CNC) anStructural color can be created when the ordered microstructure of an object interacts with incident light. Compared to the traditional pigment colors, it is bright, long-lasting, and environmentally friendly. As a result, it is of great significance for further research and development. Cellulose Nanocrystals (CNC) and silica are representatives of organic and inorganic particles that can be self-assembled to form structural color. Cellulose nanocrystals, originating from natural cellulose through sulfuric acid hydrolysis, can be stabilized in water through electrostatic repulsion owing to the sulfonic ester groups on their surface. At certain concentrations, colloidal solutions of these particles will self-assemble into cholesteric structures and retain them in dry films, resulting in structural colors caused by their optical activity. The periodic photonic crystal formed by the self-assembly of silica produces iridescent color attributed to the shielding effect of photonic band gap under incident light. At present, cellulose nanocrystals and silica are mainly combined with other materials respectively to form composites, but there are some shortages, such as the high cost of techniques or the complex appearance of optical properties. In order to solve the above problems, here we propose to fabricate the composite film with dual optical properties by self-assembly of the suspension of cellulose nanocrystals and polydisperse silica. Different optical performance was exhibited under natural light or point light source. In this study, the seed solution method of adding silica source step by step to control the hydrolysis-condensation rate of silica microspheres was used, resulting in the formation of polydisperse silica. The silica diameter ranges from 450 nm to 650 nm, which is distributed symmetrically around the mean diameter of 600 nm. Under a point light source, the photonic crystals formed by vertical self-assembly display iridescent color. Furthermore, the water dispersion of silica was blended with commercial nanocellulose solution, then composite films were fabricated by casting assembly. The effect of Polyvinylpyrrolidone (PVP) and Polyvinyl Alcohol (PVA) on the structural color was investigated. It is found that both PVP and PVA will make the structural color of cellulose nanocrystals red-shift and the silica iridescent color more brilliant. Better film formation is resulted with higher dosage of polyvinyl alcohol, while the film becomes less flexible with an increased dosage of PVP. When the aqueous dispersion of 0.3% (w/w) silica and 2% (w/w) cellulose nanocrystals are mixed in a solution mass ratio of 1∶4, with 1.2% PVP and 20%PVA, the better film-forming property is presented with dual optical properties. The film prepared here has both the chiral nematic structure of cellulose nanocrystals and the photonic crystal structure of silica.The mutual superposition of structural colors will broaden its application in anti-counterfeiting, optical encryption, etc..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0931001 (2023)
Atomic and Molecular Physics
Microwave Electric Fields Measurement with One-dimensional Standing-wave Fields Based on Rydberg Atoms
Ke LI, Jianfei TIAN, Hao ZHANG, Mingyong JING, and Linjie ZHANG
Due to the large distance between the electrons and the nucleus and the large electric dipole moment, the interatomic interaction of the Rydberg atoms are weaker than those of the ground-state atoms. Therefore, the external electric field has a greater influence on the Rydberg atoms. This property leads to the fact thaDue to the large distance between the electrons and the nucleus and the large electric dipole moment, the interatomic interaction of the Rydberg atoms are weaker than those of the ground-state atoms. Therefore, the external electric field has a greater influence on the Rydberg atoms. This property leads to the fact that the Rydberg atoms is extremely sensitive to the external electric field. Therefore, electric field measurement based on Rydberg atoms is a hot spot, especially in microwave electric field measurement. In addition, thanks to the long lifetime of the Rydberg atoms, there are possibilities to achieve higher sensitivity beyond classic electric dipole antenna.Enhancement measurement of microwave electric field is demonstrated based on the Electromagnetically Induced Transparency (EIT) effect of the Rydberg atoms, in which a one-dimensional standing wave of coupling light field is formed. This paper presents comprehensive research for measuring microwave electric field based on the coherent enhancement of a one-dimensional standing wave field of coupling light based on the electromagnetic induction effect of Rydberg atoms. A four-level system of cesium atoms at room temperature is constructed. At first, the cesium atoms in the ground-state (6S1/2) are excited to the immediate state (6P3/2) by a diode laser (probe light, ~852 nm). Secondly, a 510 nm laser (coupling light) exits the immediate state atoms to the Rydberg state. The transmission of the probe light, which is derived from the electromagnetic induced transparency effect, is recorded. In an atom vapor cell with an antireflection film coating at the wavelength of coupling light, a one-dimensional standing wave field of coupling light is achieved using a mode-matching reflection optical path. The influence of the coherent enhancement of the one-dimensional standing wave coupling light field on the electromagnetically induced transparency transmission is observed and analyzed. Then, the cesium atoms are coupled with the nearby Rydberg state by the incoming microwave electric field. The microwave electric field in the four-level atomic system causing a splitting of the transmission spectrum of probe laser which is known as Autler-Townes splitting. The coherent enhancement of the one-dimensional standing wave coupling light field on the EIT-AT splitting spectrum is observed, and the strength of microwave electric field with amplitude modulation is measured using the spectrum analyzer.The experimental results have shown that the amplitude and linewidth of the Rydberg electromagnetically induced transparency spectrum is enhanced significantly, in which the one-dimensional standing wave coupling light field is formed. Compared to increasing the output power of the coupling laser, the measurement of amplitude-modulated microwave electric field is investigated with the one-dimension standing wave in detail. The results show that for the lower-power microwave electric field, the signal-to-noise ratio is improved by about 4 dB and the instantaneous bandwidth is increased by 1.38 times in the one-dimensional standing wave field. A flatter frequency response is obtained for higher-power microwave electric field, while an apparent bimodal frequency response curve is observed without a standing wave field. The coherent power enhancement at the propagating direction of coupling light and the detuning of the probe and coupling light induced by Doppler effect are responsible for improving the signal-to-noise ratio and flattening frequency response curve. The precision measurement of microwave electric field is attributed to a frontier research field that can be applied to developing microwave communication and radar. The coherent enhancement of Rydberg atoms EIT one-dimensional standing wave coupling light field can be adopted as a new method to develop the electric field probe (sensor) with low power consumption, flat frequency response curve and high dynamic range, which will be significant for the development and the application of the corresponding metrological standard..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0902001 (2023)
Fiber Optics and Optical Communications
Absolute Phase of the Radio Frequency Transfer over Optical Fiber with Phase Self-calibration
Jinbo ZHANG, Liang HU, Qi LI, Jianping CHEN, and Guiling WU
As basic physical quantities, time and frequency are the research basis of many applied fields, such as verification of basic physics, clock-based geodetic survey, positioning and navigation. Optical fiber is an ideal medium for high stability time-frequency transmission due to its advantages of low loss, large bandwidAs basic physical quantities, time and frequency are the research basis of many applied fields, such as verification of basic physics, clock-based geodetic survey, positioning and navigation. Optical fiber is an ideal medium for high stability time-frequency transmission due to its advantages of low loss, large bandwidth and strong anti-electromagnetic interference. At present, mainstream frequency transmission schemes can be roughly divided into optical frequency, optical frequency comb and radio frequency transmission. However, most of the existing time-frequency transmission systems can only guarantee a constant phase difference during the operation of the system, and ignore the phase difference change that may occur after relocking when the system is restarted or the system link length is changed. This situation cannot meet the needs of some coherent applications, such as distributed phased array radar, radio telescope arrays. These applications not only require stable phase difference during operation, but also require that the value of phase difference does not change after multiple restarts, which can provide reference signals of the same frequency and phase between different sites to achieve more effective coherent processing.In this paper, an absolute phase transfer scheme for optical fiber radio frequency based on adjustable optical delay line is proposed. The scheme makes full use of the round-trip transmission delay of the time signal to determine the integer period of the phase of frequency signal. Combined with the high precision phase measurement of microwave frequency, a microwave frequency signal with a fixed phase difference between the remote signal and the local signal can be obtained when the system is shut down and restarted for many times. In this scheme, 1PPS signal is not directly used as a time reference signal. First, RC differential circuit and avalanche triple laser are used to convert 1PPS signal into narrow pulse signal, and then surface acoustic wave filter is used to filter the narrow pulse signal to obtain the narrow band time reference signal. By means of frequency division multiplexing, time signal and microwave frequency signal are coupled in the same wavelength channel for transmission so as to avoid the delay difference introduced in different wavelength transmission. The system uses the way of wavelength division multiplexing to transmit. The return time frequency signal is obtained by a loop method and compared with the local reference signal to obtain the round-trip link delay and link cumulative phase. The cumulative phase of the link is used as an error signal to control the optical delay line to stabilize the link, and then the absolute phase transmission is realized by using the calculated link delay to calibrate the integer period of frequency signal. Compared with other time-frequency co-transmission modes, the system of this scheme is simpler. It does not need to adopt multiple modulation modes, nor does it directly modulate 1PPS signal. Narrow-band filter can be used to separate the time signal from the microwave frequency signal so that they do not affect each other.The experiment was carried out on a 60 km optical fiber link. After transmission, the system obtained a frequency stability of better than 4×10-14@1 s, 5×10-17@10 000 s. After the link is stabilized, the measured stability of time transfer (TDEV) is 10 ps@1 s and 0.3 ps@10 000 s. The results show that this scheme has good link compensation effect. When the system is restarted several times, the maximum inconsistency of the average phase difference is 0.008 rad, corresponding to 0.15% of the whole cycle, which can ensure the realization of good phase consistency..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0906001 (2023)
In-situ Monitoring of the Internal Status of Lithium Batteries Based on Fiber Bragg Grating Sensors
Wenhui SHI, Hao WANG, Hui CAO, Yixin LIU, Jianyu LI, Jiajin ZHENG, and Wei WEI
As one of the most important energy storage technologies today, the safety and reliability of lithium batteries have always been of great concern. The thermal stability and pressure stability of lithium batteries are important parameters that affect their safe and reliable operation. The internal electrochemical reactiAs one of the most important energy storage technologies today, the safety and reliability of lithium batteries have always been of great concern. The thermal stability and pressure stability of lithium batteries are important parameters that affect their safe and reliable operation. The internal electrochemical reactions will cause changes in temperature and stress during operation. Abuse of lithium batteries can cause rapid increases in temperature and stress of electrodes, leading to degradation of battery performance and even safety accidents such as combustion or explosion. Therefore, real-time monitoring of internal temperature and stress changes in lithium batteries is crucial for the long-term safe and stable operation of lithium batteries. However, current monitoring methods used for temperature and stress inside lithium batteries just focus on single parameters or external measurements, which have problems such as poor resolution and limited accuracy, making it difficult to monitor the changes inside the battery. In order to improve the healthy level of lithium-ion batteries monitoring, this paper proposes to use fiber Bragg grating sensing technology to monitor the changes. The gratings are implanted to collect real-time temperature and stress changes of the battery anode, realizing an optical channel for in-situ monitoring of the lithium-ion battery anode. Furthermore, combined with the battery test system, the connection between electrical and optical sensing signals is established. In the system, the temperature sensitivity of the FBG sensors is 9.3 pm/℃, and the stress sensitivity is 1.2 pm/με. The FBG sensors are mounted in different ways to achieve accurate measurement of dual parameters. Both ends of FBG1 are fixed for strain measurement. FBG2 fixes single end to monitor temperature and functions as temperature compensation for FBG1 at the same time. FBG3 is outside the battery, which is used to measure the external temperature of working environment. The experimental results show that the FBG sensors can remain good sensing performance at 400 ℃. The implantation has no effect on pouch cell performance, nor does it affect the sensing performance of FBG sensors. During the working cycles of lithium batteries, the detachment and embedding of lithium ions can cause temperature changes, resulting in a sensor wavelength shift of 100 pm, which means temperature increases by 11.1 ℃. The coefficient of thermal expansion of anode is 25.5 με/℃. After temperature compensation, the stress change of anode can be observed, indicating that the change in stress is influenced by current. In other words, the hop of current can cause the anode to contract and the resulting stress will result in a wavelength drift of 21.96 pm at most, which is approximately 18.3 με. According to our research, different charge and discharge rates have different effects. The faster the rate, the greater the variations in temperature and stress. The temperature change is 2.8 times and the stress is 4.4 times higher at 10 mA than at 2.5 mA. If the rate of charge or discharge further increases to 50 mA, the operating temperature will exceed 45 ℃. After 300 cycles at 45 ℃, the volume expansion rate of battery is about 10%, and the battery is likely to malfunction. The implantable grating monitoring system in this paper can not only measure the temperature and stress changes caused by electrochemical reaction with high precision, but also has fast demodulation speed, which is conducive to real-time and accurate monitoring of the thermal runaway and deformation bulge failure of lithium batteries. The research results are conducive to quantifying and evaluating the possible thermal runaway and volume expansion problems in the electrochemical process, which is expected to provide an effective experimental reference for the safe use of lithium batteries..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0906002 (2023)
Instrumentation, Measurement and Metrology
Highly Reflective Surface Shape Measurement Technology Based on Multiple Viewpoints and Fringes
Zhizhuo WANG, and Rongshen LU
Nowadays, three-dimensional measurement of highly reflective surface is an important part of industrial measurement. With the continuous development of automobile assembly, precision polishing, free-form surface processing and other industries, the demand for measurement of large size, large curvature and high reflectiNowadays, three-dimensional measurement of highly reflective surface is an important part of industrial measurement. With the continuous development of automobile assembly, precision polishing, free-form surface processing and other industries, the demand for measurement of large size, large curvature and high reflective industrial parts such as aircraft tail, LCD panel, automobile body shell and windshield is increasingly strong. However, due to the large difference between the optical characteristics of the surface and the diffuse reflection object, it is difficult to achieve the ideal effect by the traditional three-dimensional measurement method.The phase deflection method is a non-contact high reflection surface 3D measurement method developed in recent years. Its high sensitivity and easy correction of system errors make it popular with many researchers. Generally, there are two main methods for the above measurement using deflection method: one is to improve the size of the screen and the field of view of the camera, but it is limited by accuracy and size. The other common method is to install a single-view system on the manipulator, and reconstruct a large size surface during the movement. However, the introduced manipulator system error reduces the accuracy, and the system mostly outputs the relative point cloud coordinates based on the gradient data, so the point cloud needs to be continuously spliced through the point cloud registration algorithm during the scanning process, so the calculation cost is high; however, this paper sets the camera array, and at the same time carries out the reconstruction based on absolute coordinates from multiple angles. It can effectively ensure the accuracy of local measurement, and realize accurate measurement of large size, large curvature and high reflection surface faster through the direct splicing of visual angles.In this paper, the LCD screen is used as the reference plane, and the camera array is used to realize the accurate reconstruction of the highly reflective surface shape based on multi-viewpoints. First, the mirror image of the reference plane target in the standard plane mirror is obtained using each camera, the external parameters of the reference plane and each camera through mirror calibration are obtained, and then the camera array is globally calibrated using the uniqueness of the reference plane. Then, the phase-shifting fringe are projected using the display screen to the measured surface, and the reflected modulated images are collected by each camera to obtain the absolute point cloud coordinates of the surface to be measured relative to each viewpoint. Finally, through global stitching, a larger size and higher curvature highly reflective surface shape measurement can be achieved compared to a single viewpoint system. At the same time, based on the planarity of the reference plane and the standard plane mirror, this paper proposes a coplanar constraint to optimize the geometric parameters and global parameters of the viewpoint and reduce the system error. The validity and accuracy of this method are verified by measuring the standard mirrors with different shapes..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0912001 (2023)
3D Object Detection with Fusion Point Attention Mechanism in LiDAR Point Cloud
Weili LIU, Deli ZHU, Huahao LUO, and Yi LI
With the rapid development of computer vision, object detection has made remarkable achievements in 2D vision tasks, but it still can not solve the problems such as light changes and lack of depth that occur in actual scenes. The 3D data acquired by LiDAR makes up for some defects existing in the 2D vision field, so 3DWith the rapid development of computer vision, object detection has made remarkable achievements in 2D vision tasks, but it still can not solve the problems such as light changes and lack of depth that occur in actual scenes. The 3D data acquired by LiDAR makes up for some defects existing in the 2D vision field, so 3D object detection is widely studied as an important field in 3D scene perception. 3D object detection in the field of autonomous driving is an important part of intelligent transportation, and the 3D object detection algorithm based on LiDAR point cloud provides an important perception means for it. Perception is a key component of autonomous driving, ensuring the intelligence and safety of driving. 3D object detection refers to the detection of physical objects from sensor data, predicting and estimating the category, bounding box, and spatial position of the target. However, due to the unstructured and non-fixed size characteristics of point clouds, they can not be directly processed by 3D object detectors and must be encoded into a more compact structure through some form of expression. There are currently two main types of expressions: point-based and voxel-based methods. Voxel-based methods have higher detection efficiency, but their detection accuracy is lower than that of methods based on raw point clouds. Therefore, how to improve the detection accuracy of voxel-based methods while ensuring detection efficiency has become a research hotspot in recent years.In view of the problems of loss of fine-grained information and insufficient ability to extract point cloud features in the 3D object detection algorithm for Pillar-encoded point clouds, this paper proposes a 3D object detection algorithm based on PointPillars that integrates point-wise spatial attention mechanism and CSPNet. Firstly, the point-wise spatial attention mechanism is integrated into the pillar feature network layer, which can enhance the network's ability to extract local geometric information and retain deep-level information, making the obtained key features more suitable for detection tasks. Point-wise spatial attention follows the basic structure of self-attention, which can effectively avoid the impact of redundant point clouds or noise points on features, strengthen the description of features with less coverage of point clouds, and to a certain extent solve the problem of information loss based on Pillar-encoded point clouds. Secondly, replacing the ordinary convolution in the downsampling module that extracts high-dimensional features from pseudo-images of point clouds with CSPNet can achieve gradient flow segmentation, further enhance the network's learning ability while reducing computational complexity, and improve model detection accuracy.Finally, the algorithm in this paper improves the 3D detection accuracy by 2.23%, 2.25%, and 2.30% in easy, medium, and hard cases, respectively, compared with the benchmark network under the application scenario of highway with car class in KITTI as the detection target. The experimental results show that the algorithm in this paper has significantly improved the detection performance, while the detection speed reaches the real-time detection level, which has some positive significance for the optimization and improvement of autonomous driving technology, and has great potential in the application scenario of highways..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0912002 (2023)
Optical Device
Inverse-designed Non-local Metasurface Differentiator
Haoran QIN, Junlong KOU, Jiarong ZHU, Zixin ZHOU, Junzhuan WANG, Zhaoxian CHEN, and Yanqing LU
Differential computing is of great significance for data analysis and processing, especially in image edge recognition and extraction. Edge enhancement technology is particularly useful in compressing data, checking objects for defects, etc., making it a hot research topic in the past decades. Compared with electric ciDifferential computing is of great significance for data analysis and processing, especially in image edge recognition and extraction. Edge enhancement technology is particularly useful in compressing data, checking objects for defects, etc., making it a hot research topic in the past decades. Compared with electric circuits, differentiators that use light as the transport carrier have great advantages in terms of speed, dissipation and stability. However, conventional optical dielectric lenses are still inferior in terms of accurate and rapid response due to some reasons, such as non-negligible losses during reflection, aberrations caused by spherical surfaces, diffraction limits of light and more importantly, their bulky geometries. Using sub-wavelength optical metastructures, i.e., two-dimensional artificial structures, can overcome these inherent constraints and enable the implementation of efficient, responsive and miniaturized optical devices. By rationally utilizing rather than suppressing the interaction between the pixels, non-local metasurfaces can greatly reduce the thickness of the structure and the contrast of refractive indexes between different materials. This allows better tailoring of light behaviors as it propagates in a specially designed structure under specific conditions, thus achieving more complex optical functions. Notably, previous researches tend to focus on particular application scenario but ignore the generality. To remedy this shortcoming, here we combine the optimization algorithm with the inverse designing of the optical metasurface. Different operation logics can be achieved by modifying the objective function of the optimization algorithm and using the design steps of associating the operation logic to be implemented with an objective function, optimizing the structure, and verifying functionality. We propose a one-dimensional second-order differentiator by inversely designing the material distribution, leading to a non-local metasurface. The transfer function of the designed structure agrees well with our objective function, i.e., the relationship between incident angle and transmittance satisfies the quadratic function, especially when the incident angle is smaller than 10 degrees. We also design a Laplace transformer to prove the generality of the method. Then, we assess the robustness of the design by calculating the objective function or transfer function after making appropriate changes to the obtained optimal solution and comparing the results before and after the change. Finally, we verify their functionality in identifying image edges by illuminating light waves perpendicular to the objects, and then calculating their transmitted waves. In this way, we prove that the transmitted waves can reflect the profile of incident waves in the corresponding polarization direction. The optimization effect of a one-dimensional second-order differentiator is remarkable that the error between the transfer function of the optimal and theoretical solutions is significantly decreased. When optimizing the Laplace transformer, there is still some deviation between the transfer functions of the optimal solution and the theoretical solutions, which may be a locally optimal solution due to the fast convergence of the algorithm itself. Through verification, we find that the structure we design still hold feature of differentiation. Specifically, we use a hollowed-out silver layer as the barrier between the incident plane wave and the structures arranged by two thousand unit cells. The barrier layer only allows waves with a spatial distribution consistent with pattern“N”to pass through, and the transmitted wave reflects the profile of the special pattern. Therefore, we prove that this method has a strong generality and error tolerance. In addition, this method can be extended to the design of other spatial operations, such as integration or spatial filter. Predictably, this method has great potential for designing optical computing units considering its high tolerance for the algorithm and manufacturing process. Furthermore, we can map all the material distributions and their transfer functions calculated in the iteration process to form a database, with which the optimization efficiency can be further improved..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0923001 (2023)
Multifunctional Device Design Based on Composite Gratings and Gradient Supersurfaces
Hai LIU, Siyi ZHAO, Cong CHEN, Peng GAO, Yaowei DAI, Jiaming ZHAO, Yinhui WAN, Xiangyu LU, Xinyan WANG, and Lei LI
Most of the current research can only realize the asymmetric transmission of light singly, in the face of the demand for multifunctional application scenarios, the design of multifunctional integrated devices has become a development trend. Therefore, the combination of grating and super-surface, which can realize diffMost of the current research can only realize the asymmetric transmission of light singly, in the face of the demand for multifunctional application scenarios, the design of multifunctional integrated devices has become a development trend. Therefore, the combination of grating and super-surface, which can realize different functions in different incidence and polarization states, is a multifunctional device combining a metal/medium/metal composite grating and a gradient super-surface structure. The asymmetric transmission of light is realized through the asymmetric structure of the composite grating, and when the modulated wave vector matches the wave vector of the Surface Plasmon Polaritons (SPPs), the SPPs excited by the metal grating produce single-band light transmission, and the anomalous reflection of light is realized through the gradient hypersurface. When an x-polarized light wave is incident, the SPPs are unidirectionally excited at 1 550 nm in the forward-incidence direction, and the forward transmittance at this wavelength is up to 0.9. In the reverse-incidence direction, the SPPs are unidirectionally excited at 1 128 nm, and the backward transmittance at this wavelength is up to 0.86. Due to the different periods of the upper and lower gratings and the difference in permittivity at the incident interfaces, the surface-iso-polarized excitations can be excited in a single-band light transmission. The upper and lower gratings cannot excite SPPs in the same band at different incidence directions, and the reverse transmission is suppressed in the forward excitation band and vice versa. In order to obtain the best performance of the asymmetric transmission characteristics, the parameters of the device are optimized to enhance the interaction between the beam and the grating. The effect of the grating transverse position on the transmission spectrum is investigated. Changes in the grating transverse position alter the strength of the coupling effect, leading to the splitting of the transmission peaks, the emergence of double peaks or even multiple peaks, and the broadening of the transmission spectrum, and the effective refractive index of the double-layered grating and the relative phases of the light passing through the sub-wavelength grooves of the two gratings are also varied, leading to changes in the resonance wavelengths and the transmission spectra. The changes in the transmission spectra of the upper and lower metal gratings and the intermediate SiO2 film under different parameters are also investigated. In addition, in the direction of reverse incidence, the device exhibits zero absorption in the entire band from 1 300 to 1 400 nm, and the reflectivity is greater than 0.9, which can be used as a reflector. In order to verify that the asymmetric transmission phenomenon occurs only when x-polarized light waves are incident, the transmission spectra of y-polarized light waves incident on the device in different incident directions are also simulated, and it is found that the device does not exhibit asymmetric transmission when y-polarized light waves are incident, due to the fact that the SPPs field component reaches its maximum at the metal/dielectric boundary, and decays exponentially in the dielectrics at the two ends of the metal. In visible and infrared light, the real part of the permittivity of most metals is negative, so that the permittivity of the metal is different from that of the surrounding permittivity, and only light waves in the x-polarized state can efficiently excite the SPPs. Given that the asymmetric transmission properties and reflection properties of gratings at the incidence of light waves in the x-polarized state have been extensively studied, the anomalous reflections are achieved by irradiating the underlying phase-gradient hypersurfaces with light from the other wavelength band in order to realize the versatility of the device. When the y-polarized state light wave is incident, in the reverse incidence direction, the light directly irradiates the phase gradient super-surface, and according to the Generalized Snell's Law, in order to form a phase discontinuous super-surface to make the length of the underlying Ag nanostructures of each unit structure in the super-unit is different, which forms the reflective phase delay, and produces the phenomenon of anomalous reflections when the light is irradiated. To solve the problem of asymmetric transmission and single function of anomalous reflection devices, unidirectional excitation of SPPs and the Generalized Snell's Law are combined to realize multifunctionality, which provides a reference for a variety of polarization-related multifunctional devices and integrated optical components..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0923002 (2023)
Design Photonic Crystal All-optical Logic Gates Using Machine Learning
Jianwei CHEN, Ran HAO, Chunlian ZHAN, Shangzhong JIN, Pengju ZHANG, Xingang ZHUANG, and Feng FEI
The all-optical logic gate is the core component of the photonic computer, optical signal processing, and all-optical network. Based on the photonic crystal, the all-optical logic gate has attracted much attention due to its simple structure, low loss, fast operation speed, and small volume. Photonic crystal waveguidesThe all-optical logic gate is the core component of the photonic computer, optical signal processing, and all-optical network. Based on the photonic crystal, the all-optical logic gate has attracted much attention due to its simple structure, low loss, fast operation speed, and small volume. Photonic crystal waveguides can manipulate light for logical operations, which may open up new prospects for photonic computing and optical interconnection networks. However, the design of photonic crystal logic gates is still an iterative process, and the reverse acquisition of geometric structures according to requirements is the key to solving practical engineering problems. To accelerate the performance analysis of photonic crystals and the design of all-optical logic gates, a neural network design of bandgap transmission photonic crystal all-optical logic gates was proposed. In this study, TensorFlow was used as the development framework of the neural network, and a forward performance characterization and inverse structure prediction model of the photonic crystal waveguide was constructed: the forward performance characterization model had 13 fully connected layers, and the total number of parameters trained by the neural network was 197 612, which can realize the timely prediction of the structure of the photonic crystal waveguide to the optical performance; the inverse structure prediction model had 26 fully connected layers, and the total number of parameters trained by the neural network was 155 704, which could reversely design the structure parameters of the photonic crystal waveguide according to the required optical performance, which is helpful to solve practical engineering problems. The Intel Core i9-10940X processor and RTX 3080 Ti graphics card are used for the forward performance characterization and reverse structure prediction network, with training times of 0.2 and 0.36 hours, respectively. The coefficient of determination between the predicted and actual values of the computational neural network was 0.997 for the forward neural network and 0.998 for the inverse network, which shows that the predicted value is very close to the actual value, demonstrating the accuracy of the network. In addition, using the inverse neural network, the structure parameters of the photonic crystal logic gate were successfully predicted according to the required optical properties, such as group index, photonic bandgap, and working wavelength. This logic gate uses gap soliton transmission. When the frequency of the input signal is at the edge of the photonic gap, the output port of the logic gate is nonlinearly disturbed by other input signals. By controlling the frequency and amplitude of the input pulse, the angular frequency displacement caused by the Kerr nonlinearity can be controlled, thus realizing logical operation. The time-domain finite difference method is used to simulate the AND and NOT operations of the all-optical logic gate. The period of the photonic crystal logic gate is 420 nm, and the output port is 70 periods away from the input port. The logic gate performed AND and NOT operations on the Gaussian pulse input signals of“10”and“11”in the time domain, and the output pulse signals of AND and NOT were detected as“10”and“01”, respectively, demonstrating the accuracy of the logic gate. Compared with the input pulse and the output pulse of the AND operation, the pulse width of the input signal was 10 ps, and the output signal was 10.36 ps, with a change of 3.6%. Moreover, when the input pulse intensity was reduced to
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0923003 (2023)
Ultrafast Optics
Generation of Femtosecond Magnetic Pulses by Circularly Polarized Vortex Laser-driven Plasma
Han WEN, Peng XU, Liangwen PI, and Yuxi FU
Research on pulsed magnetic fields dates back to the early 20th century. Nowadays, ultra-short pulsed magnetic fields are being utilized to better understand ultrafast physical microprocesses, such as domain motion and spin-orbit interaction, with time scales ranging from microseconds to femtoseconds. In particular, feResearch on pulsed magnetic fields dates back to the early 20th century. Nowadays, ultra-short pulsed magnetic fields are being utilized to better understand ultrafast physical microprocesses, such as domain motion and spin-orbit interaction, with time scales ranging from microseconds to femtoseconds. In particular, femtosecond magnetic field pulses are of great significance for studying ultrafast magnetization, ultrafast demagnetization, ultrafast magnetic storage, and spin ultrafast dynamics. However, traditional pulsed magnetic fields are limited by the performance of the pulse power supply and the mechanical strength of the coil and cannot achieve higher pulsed magnetic field strengths. Additionally, the pulse length of the magnetic pulse generated by the pulse power supply is at the millisecond level, which makes it unsuitable for studying faster magnetic dynamics processes. Fortunately, recent studies have shown that when ultra-short pulse lasers interact with plasmas, hot electrons are produced on the surface of the plasma target. These hot electrons are then excited and pass through the target material, producing strong charge separation on the back surface of the target material. Under the action of the laser, these excited electrons are accelerated, generating strong electromagnetic radiation. Consequently, using ultra-short pulse lasers to drive electron flows is currently the most promising method for generating femtosecond magnetic field pulses. Thus, the goal of this paper is to use a three-dimensional model to simulate the interaction between the driving optical field and the plasma target. This simulation will help to study the physical processes involved, such as the propagation of the optical field, the movement of free electrons, vortex currents, and pulse magnetic field generation. By optimizing the relevant parameters, this research aims to generate femtosecond magnetic field pulses.In this paper, we employ the Particle-In-Cell (PIC) method as our simulation approach. This method utilizes the Vlasov-Maxwell equation set to accurately describe the self-consistent dynamics in plasma simulation. The electrons in the plasma are subject to the Lorenz force, which generates new current density as they move. This equation effectively corrects the electric and magnetic fields through the charge density and current density. The driving light described is a circularly polarized vortex beam, with a wavelength of 800 nm and an optical field intensity of approximately 1016 to 1021 W/cm2. The pulse width of the beam is roughly 10 fs. The plasma density ranges from 1018 to 1020 cm-3, and is confined within a cubic space with a side length of 30 λ0. During the simulation process, we only consider refractive index changes due to electron density and do not account for non-linear effects. Additionally, we assume that the ions are stationary and that the initial velocity and temperature of the plasma are both 0.During theoretical simulation, a proportionality gradient between momentum potential and the strength of the light field is created due to the lowest intensity of the vortex beam at its center. This gradient then forms a potential well, preventing electrons from escaping outward and producing a structured electron beam with a femtosecond duration. In addition, particles acquire angular momentum in their radial motion within the laser field, generating a vortex current. This in turn produces a pulsed magnetic field based on the current magnetic effect.The simulation results indicate that when circularly polarized vortex beams, with light field intensities of the order of 1016 to 1021 W/cm2, interact with plasma densities ranging from 1018 to 1020 cm-3, they can generate ultra-short magnetic pulses with peak intensities of 0.5~50 tesla and pulse time widths of about 10 fs. The effects of driving laser intensity and plasma density on these magnetic pulses are discussed through a simulated system calculation. The results show that the pulsed magnetic field intensity is proportional to the square root of both laser intensity and plasma density. Increasing electron density and laser intensity may facilitate the generation of ultra-short strong magnetic fields, providing numerical references for the production of femtosecond magnetic pulses in experiments.We expect that the simulation results above will facilitate the introduction of ultra-strong, ultra-short magnetic pulses into the femtosecond ultrafast realm, thereby supporting the advancement of research on ultrafast magnetic and spin dynamics, electronic motion and spin microprocessing control, ultrafast spin-electron magnetic storage applications, and magnetic switching..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0932001 (2023)
Research on Driving Technology of Wide Microstrip Amplitude Division Imaging Based on Pulse Power Synthesis Technology
Shiduo WEI, Yongsheng GOU, Yang YANG, Penghui FENG, Baiyu LIU, Jinshou TIAN, Xu WANG, Hengbo LIU, Hantao XU, and Yihao YANG
When a pulse current with a rise time of about 100 ns and an amplitude of tens of MA is applied to a wire array or jet load, the load will rapidly ionize and form a plasma. Due to the Lorentzian force, these plasms will rapidly implode towards the axis and eventually stagnate in the center, forming a high temperature aWhen a pulse current with a rise time of about 100 ns and an amplitude of tens of MA is applied to a wire array or jet load, the load will rapidly ionize and form a plasma. Due to the Lorentzian force, these plasms will rapidly implode towards the axis and eventually stagnate in the center, forming a high temperature and high density plasma and further emitting strong X-rays, a process known as Z-pinch. Z-pinch has been widely used in High Energy Density (HED) physics research for decades, including radiation source development, radiation actuation science, dynamic material properties, Magneto-inertial Fusion (MIF) and Inertial Confinement Fusion (ICF). In order to explore the structure, properties and motion laws of matter in the ultra-small space and ultra-fast time scale, the research and measurement techniques of ultra-fast phenomena represented by the variometer framing camera technology have become the main tools in use.X-ray framing cameras are widely used for two-dimensional plasma imaging in the Z-pinch process. This type of frame camera requires selective pulses to excite the Microchannel Plate (MCP). Because the width of the pulse is very narrow, only a microstrip region has voltage at a time, and photoelectrons generated by the X-ray image formed through a pinhole in the region at the input surface of the MCP will be gained and be imaged to the screen on the screen. The exposure time of each image is determined by the half-width of the selected pulse and the characteristics of the framing tube. The MCP with different equivalent impedances will realize the framing camera imaging with different frames. The width and length of the transmission microstrip line of the ultra-wide frame traveling-wave selective framing camera are up to 20 mm and 95 mm, and the equivalent impedance is about 6 Ω. To actuate the beamsplitter, gating pulses with electric field peaks of more than 3 kV, pulse durations on the order of nanoseconds or hundreds of picoseconds, and spectral widths of tens to thousands of megahertz is required. In this paper, the power coupling method based on Wilkinson structure power splitter is adopted to synthesize the narrow-band pulse with low amplitude into the high-voltage pulse with the required amplitude. However, limited by the characteristics of the transistor device itself, the pulse source whose amplitude is higher than 5 kV and the front edge is better than 100 ps and the jitter is better than 20 ps is close to the technical limit of electronics. To obtain higher power gate pulse it is necessary to adopt multichannel pulse power synthesis technology.In this paper, a power coupling method based on Wilkinson structure power splitter is adopted to synthesize the narrow-band pulse with low amplitude into the high-voltage pulse with the required amplitude. The large bandwidth of the multi-section impedance converter is used to improve the working bandwidth of the power coupling, so as to meet the pulse coupling of different spectrum. The simulation software is used to design the power coupling circuit with the working frequency band of 300 MHz~3 GHz, and the loss generated in the system is optimized to achieve high efficiency coupling. Combined with the high-voltage narrow pulse output and synchronization control circuit of the preceding stage, the high-voltage pulse with peak voltage exceeding 3.2 kV is synthesized by using eight single-channel pulses with peak voltage of about 1.3 kV and pulse width of about 3.5 ns, pulse leading edge of about 600 ps. The pulse width was within 3 ns and the pulse leading edge was within 600 ps. In the pulse spectrum range of 300 MHz to 3 GHz, the two-channel synthesis efficiency is 83.5%, 88% at a specific frequency, and the eight-channel synthesis efficiency is 58%, up to 68% at a specific frequency.Finally, the coupled high-voltage pulse is input into the 20 mm microstrip amplitude-divider. The transmission line of the microchannel plate inside is 20 mm wide and 95 mm long, and the equivalent impedance is 6 Ω. The output pulse amplitude is 1.433 kV, the pulse width is 3.63 ns, and the pulse front is 747.3 ps, which fully conforms to the design requirement that the output voltage of the tube must exceed 800 V. At present, the coupling technique can generate driving pulses for use. In the future, the coupled pulses can be shaped by adjusting the delay of the eight pulses. At present, the high voltage driven pulse source based on this technology has been applied to I-MCP1.0 framing camera and can be used to explore the high energy density physics research with Z-pinch as the core..
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0932002 (2023)
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