Journals >Acta Photonica Sinica
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2025
Volume: 54 Issue 1
17 Article(s)
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Multi-image Encryption Based on QR Codes and Fresnel Phase-only Holograms
Chuan SHEN, Junqiao PAN, Anlin WANG, Ze SHEN... and Sui WEI|Show fewer author(s)
Image encryption is a widely used method of image information protection, and multiple-image encryption methods are rapidly evolving in the face of the exploding amount of information. Aims to solve the problem that existing optical image encryption techniques frequently experience optical contamination, which degradesImage encryption is a widely used method of image information protection, and multiple-image encryption methods are rapidly evolving in the face of the exploding amount of information. Aims to solve the problem that existing optical image encryption techniques frequently experience optical contamination, which degrades the quality of the decryption image, a multi-image encryption method based on computational holography by combining Quick Response (QR) codes and Fresnel phase-only holograms is proposed. We expect to introduce a data container QR code with an error correction mechanism, where the information is transformed into a Fresnel phase-only hologram in the form of a QR code. It can be cleared even when suffering from optical contamination, within the error correction limits of the data containers, by using their error correction mechanisms and fast information acquisition capabilities. The original information is protected from optical contamination and high-quality, even lossless, results from optical decryption are produced. The encryption process of our method firstly employs a QR code encoder to convert the plaintext image to be encrypted into a corresponding dynamic QR code, and then the QR code is used as the target image to generate a Fresnel phase-only hologram corresponding to the target image using the Gerchberg-Saxton algorithm in the Fresnel domain. These phase-only holograms are transformed into integers through linear mapping; subsequently, with the help of a chaotic system, a noise-like distribution made up of chaotic sequences is generated; multi-image integration is then achieved using the exclusive OR (XOR) superposition operation, and the result of the obtained operation serves as the final ciphertext. The decryption process is the inverse of the encryption process, in which the XOR superposition operation between the ciphertext and the decryption key of the target image extracts the Fresnel phase-only hologram of the image to be decoded. Next, the pixel value of the hologram is inversely mapped to a real number. This real number is then combined with the optical key of the hologram for the purpose of reconstructing the QR code. Finally, the recovered QR code is decoded using a decoder to obtain the plaintext image. In this paper, numerical simulations and optical experiments are used to verify the effectiveness of the proposed method. First, for the numerical simulation of encryption and decryption, various sizes of color, binary, and greyscale images are chosen. Second, a phase-only Space Light Modulator (SLM) based on liquid crystal on silicon (LCoS) is loaded with the generated Fresnel phase-only holograms, and SLM propagates the modulated light waves to the imaging surface. This optical system is constructed to verify the true state of QR codes against optical contamination. The experimental results show that a decoder can extract the original image from the optical reconstruction results. The reconstructed QR code is eventually captured by a camera as the modulated light wave propagates to the image plane. On the one hand, regarding the key sensitivity, since the security of the proposed method depends on the chaotic key and the optical key, first, the chaotic key sensitivity is verified by adjusting the control parameter, initially set value, and the number of initial chaotic sequences discarded by the chaotic system, respectively. The experiments confirmed a tiny change in the three parameters will result in incorrect decryption results. Second, we carried out sensitivity simulation experiments on optical keys by altering only the reconstruction distance and wavelength. Similarly, we discovered that a small parameter change resulted in incorrect decryption results, indicating that the key is sufficiently sensitive. Analyzing the robustness is crucial since, on the other hand, there is always a chance of information loss during the sharing of information. In this study, a series of cropping attack tests have been developed to simulate varying degrees of data loss to thoroughly assess the anti-data loss efficacy of the encryption method in question. We used three different data loss scenarios in our study: 3.5%, 16%, and 25%. The experimental results demonstrate that, despite the various degrees of data loss, the decrypted QR code image is still able to decode the original image. Noise is unavoidable during data transmission, both salt & pepper noise and Gaussian noise are analyzed. The first case, the salt & pepper noise with intensities of 5%, 10%, and 20% is added to the ciphertext, respectively. The second case, the Gaussian noise with intensities of 0.5, 0.8 and 1.0 (means: 0, sigma: 0.05) is added to the ciphertext, respectively. All of reconstructions can be decoded to produce the original image, and the experimental findings demonstrate that they are still within an acceptable range. This shows that the proposed method has robustness in complex environments and its ability to sustain more steady decryption results in the face of numerous unfavorable factors. Therefore, this method offers a highly practical and reliable alternative path for multi-image encryption in the field of optical information security. With the application of this method, which provides multi-image encryption without the use of optical dimension multiplexing techniques, the difficulty of developing encryption systems is greatly reduced. A redundant error correction system is conceived to protect the target image from optical pollution by marrying holograms with QR codes..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0110001 (2025)
Topology-Based Bidirectional Radar-Optical Fusion Algorithm for USV Detection at Sea
Hao TANG, Xuan PENG, Wei XIONG, Yaqi CUI... and Huiyuan XING|Show fewer author(s)
In recent years, there have been significant developments in the field of Unmanned Surface Vehicles (USVs), with a notable increase in the number of applications in both military and civilian contexts. Equipped with radar and optical systems, USVs are designed to markedly enhance detection capabilities. While radars arIn recent years, there have been significant developments in the field of Unmanned Surface Vehicles (USVs), with a notable increase in the number of applications in both military and civilian contexts. Equipped with radar and optical systems, USVs are designed to markedly enhance detection capabilities. While radars are capable of determining target distance and bearing in all weather conditions, they are limited in their ability to classify targets. In contrast, optical systems have strong colour perception and classification abilities, with angular resolution comparable to that of lidar systems. However, their ranging capabilities are limited, and they are susceptible to instability in adverse weather conditions. The fusion of radar and optical systems, by leveraging their complementary advantages, effectively augments the detection capabilities of USVs.Radar-optical fusion methods can be broadly categorised into two primary approaches: the linkage method and the matrix transformation method. The linkage method entails the rotation of the optical servo in accordance with the azimuth and elevation angles of radar-detected targets, thus enabling the optical system to capture target images. Subsequently, the data is transmitted to the data centre, where it is analysed and decisions are made by the relevant personnel. Although this method is particularly focused on angular transformation and is effective in scenarios with fewer, unobstructed targets, it is unable to accurately detect multiple targets simultaneously. In contrast, the matrix transformation method unifies radar and optical information within a single coordinate system through the application of mathematical operations. By focusing on radar-detected points to generate Regions Of Interest (ROIs), this approach employs the Intersection over Union (IoU) algorithm for association. The matrix transformation method is predominantly applied in the autonomous driving sector. It facilitates the fusion of millimetre-wave, lidar, and onboard camera data, enabling multi-target detection and 360° capture via multiple fixed cameras for comprehensive radar association. Despite the high precision of automotive lidar systems, their detection range is typically limited to a few hundred metres. This limitation renders them unsuitable for maritime applications, where longer detection ranges are required, and errors are magnified due to the effects of sea waves and platform instability.In order to address these challenges, this article introduces a novel topological bidirectional fusion algorithm, which is aimed at improving the detection of radar and optical fusion for USVs. The YOLOv7-tiny algorithm for image detection is enhanced, and matrix transformation is employed to project radar points onto image coordinates. This study addresses the issue of vessel sway by applying the PROSAC algorithm to fit and correct the projected radar points. In order to reduce the computational demands of the system, a new coarse correlation gate has been designed which takes into account both sensor system error and position error. In order to advance the association process, topological association metrics are introduced, which serve to supplement the missing angular information that is a consequence of radar projection. This is achieved by integrating polygon angle similarity and the similarity of centreline connections, in addition to triangle similarity. Secondary detection and association are conducted on unassociated radar points in the vicinity of images and unassociated optical detection frames. This is achieved by selecting the nearest neighbouring radar points based on bearing lines.The empirical data analysis reveals a notable enhancement in the modified YOLOv7-tiny algorithm's mAP@0.5, which has increased from 0.883 to 0.93. The proposed topological bidirectional fusion algorithm achieved a remarkable accuracy rate of 92.76%, which surpassed the performance of the traditional IoU method. This enhancement provides a foundation for the potential of long-range radar and optical fusion detection for maritime Unmanned Surface Vehicles (USVs) and serves as a significant reference for research in the field of radar and optical association detection on USVs. The findings highlight the potential of advanced algorithmic approaches in addressing the challenges posed by environmental factors and platform instability, thereby paving the way for more reliable and effective situational awareness in marine environments..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0110002 (2025)
Spatial-spectral Collaborative Unrolling Network for Pansharpening
Jianwei ZHENG, Hongyi XIA, and Honghui XU
To address the limitations inherent in physical device acquisition, pansharpening offers a computational alternative. This process aims to enhance the spatial resolution of Low-Resolution Multispectral Images (LRMS) by integrating textural information from Panchromatic (PAN) images, thereby generating High-Resolution MTo address the limitations inherent in physical device acquisition, pansharpening offers a computational alternative. This process aims to enhance the spatial resolution of Low-Resolution Multispectral Images (LRMS) by integrating textural information from Panchromatic (PAN) images, thereby generating High-Resolution Multispectral images (HRMS). Recently, a growing number of deep learning-based methods, leveraging their enhanced feature extraction capabilities, have been introduced, demonstrating exceptional results in improving fusion quality. However, many of these methods continue to exhibit two notable shortcomings. For one thing, the universally adopted black-box principle limits the model interpretability. For another thing, existing DL-based methods fail to efficiently capture local-and-global dependencies at the same time, inevitably limiting the overall performance. By gathering the merits of nonlinear network architectures and interpretable optimization schemes, Deep Unfolding Network (DUN) has shed new light on pansharpening. However, current DUNs lack a dedicated design for both estimating the degradation matrices and extracting intricate information from the proximal operator. To address the conundrums, we propose a novel Spatial-Spectral Collaborative Unrolling Network (SCUN). An alternating optimization-based Half-Quadratic Splitting (HQS) is practiced to solve the resulting model, giving rise to an elementary iteration mechanism. Under the guidance of iterative optimization theory, this network achieves Adaptive Degradation Matrix Estimation (ADME) and spatial-spectral prior operator learning through multi-scale cascade strategies, point convolution operations, and Transformer technology. During the ADME step, the overall estimation undergoes an end-to-end iterative block, allowing for adaptive modeling of complex spatial and spectral structures. On that basis, we employ customized multiscale convolution and point convolution to simulate the degradation processes of both spatial and spectral degradation matrices. Moreover, the proposed convolution method is reassigned in each unfolding iteration, endowing it with a highly adaptive capability. To address the limitations of prior operators, we propose a collaborative complementary mechanism that enables the approximation of operators and facilitates the joint exploration and acquisition of global-local and spatial-spectral features. This is achieved through a combination of convolutional layers and attention mechanisms. The entire prior module is designed as a U-shaped architecture network, following the process of “embedding-encoder-bottleneck layer-decoder-deembedding” to extract refined feature representations. Initially, the intermediate variables are processed through an embedding layer, which segments them into non-overlapping patch markers. These patch markers are then fed into two Spatial-Spectral Collaborative Modules (SSCMs) and a bottleneck layer consisting of a single SSCM to explore comprehensive properties. Each SSCM is composed of three key components, including Spatial-Spectral Collaborative Attention (SSCA), Scale-Aware Channel Collaboration (SACC), and Mixed-Scale Feed-forward Layer (MSFL). Specifically, the SSCA subassembly includes two Transformer blocks. The first is the Spatial Transformer Block, which primarily transfers high-frequency texture features from PAN images to HRMS. The second is the Spectral Transformer Block, which focuses on transferring spectral features from LRMS to HRMS images. After extracting these two attention features, a multi-head self-attention mechanism is further applied to deeply fuse the spatial and spectral information, thereby achieving enhanced collaboration and complementarity of the target information. Within SACC, we dynamically assimilate and cross-converge characteristics originating from size-varied receptive fields via multiscale convolution, while simultaneously introducing channel attention to model the spectral dependency of MSIs. Similarly, to amplify the nonlinear feature transformation stemming from attention layers, our MSFL incorporates a mixed-scale strategy and subsequently a cross-complementary mechanism is introduced to emphasize the important components of the multiscale convolutions. With all modules organically assembled, the final proposal stands out as the initial attempt to systematically capture local-global and spatial-spectral information during model unfolding, guaranteeing an appealing pansharpening performance. Experimental results on multiple remote sensing datasets demonstrate that the proposed method outperforms comparative methods, achieving a PSNR gain of 0.798 dB on the GF-2 dataset..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0110003 (2025)
Inversion Method of Ship SO2 Emission Rate Based on Machine Vision
Weiwei HE, Xiangyu LIU, Huiliang ZHANG, Qixin TANG... and Mingkun SUN|Show fewer author(s)
Emission rate is a critical parameter for assessing the environmental impact of ships, as it directly relates to the amount of pollutants released into the atmosphere per unit of time. Monitoring and controlling ship emissions has become a pressing global challenge due to the increasing contribution of maritime activitEmission rate is a critical parameter for assessing the environmental impact of ships, as it directly relates to the amount of pollutants released into the atmosphere per unit of time. Monitoring and controlling ship emissions has become a pressing global challenge due to the increasing contribution of maritime activities to air pollution and climate change. However, traditional monitoring techniques, while effective in capturing spatial pollutant concentration distributions, face inherent limitations in their physical principles. These methods rely on localized or single-point measurements, making them unsuitable for accurately capturing dynamic processes like plume transport and dispersion. As a result, they fail to deliver precise real-time emission rate measurements.To address these limitations, this study leverages the unique advantages of UV cameras for two-dimensional pollutant imaging and proposes an innovative method for calculating ship exhaust emission rates using the Farneback optical flow algorithm in machine vision. UV imaging enables spatially resolved detection of pollutant concentrations over a large field of view, offering a significant improvement over traditional point-based methods. The Farneback optical flow algorithm, which has been widely validated for rigid body motion estimation, is adapted to analyze the dynamic behavior of fluid plumes in this study.The SO2 UV camera utilizing the specific absorption characteristics of SO2 molecules in the UV spectrum. By recording the spectral signals generated from SO2 absorption of UV light and converting them into visualized images, the camera provides precise detection and localization of SO2 emission sources. However, SO2 molecules exhibit significant absorption of solar radiation in the 280~320 nm wavelength range, where black carbon particles in the exhaust also introduce absorption interference. To address this, a dual-channel imaging framework is proposed. This framework, based on the Beer-Lambert law and the differential absorption cross-sections of SO2 and black carbon particles, includes both monitoring and correction channels. By calculating the optical thickness difference between these channels, the system effectively eliminates black carbon interference, yielding accurate SO2 optical thickness measurements. Additionally, a spectrometer channel is incorporated to establish calibration curves, correlating SO2 optical thickness with gas concentration for precise retrieval of SO2 concentrations in ship exhaust.Building on these principles, a dual-channel UV remote sensing imaging system was developed to enable quantitative monitoring of ship exhaust emissions. Field experiments were conducted at Yantai Port, targeting a passenger ro-ro ship, to collect image data of exhaust plumes under real-world conditions.The Farneback optical flow algorithm was employed to analyze the SO2 concentration image sequences. By using consecutive concentration images as input, the algorithm calculated pixel-wise motion vectors based on the assumptions of constant local gradient and smooth optical flow. This analysis produced a dense optical flow field, providing detailed information on the flow direction and velocity of the exhaust plume. To calculate the emission rate, the cross-sectional region of the plume was extracted, and the SO2 concentration values for each pixel were combined with the velocity vectors from the optical flow field. A discrete convolution process was applied, allowing the pollutant emission rate to be determined over specific time intervals. The integration of high-resolution UV imaging and machine vision-based optical flow analysis provides a robust and scalable framework for real-time emission rate monitoring.The results of this study validate the reliability and effectiveness of the proposed approach for SO2 concentration measurement and emission rate calculation. The integration of the Farneback optical flow algorithm with UV imaging enables precise, real-time tracking of dynamic emission processes. This method provides critical support for the real-time tracking and quantitative analysis of ship pollutant emissions, enhancing the enforcement of maritime emission regulations. Furthermore, the technical framework and algorithm developed in this study are broadly applicable and can be extended to monitor other atmospheric pollutants, including NO? and particulate matter..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0110004 (2025)
Extended Depth-of-Focus of Cooled Infrared Optical Systems using Wavefront Coding
Xinzhu WANG, Lei YANG, Wenhao DUAN, Tong YANG... and Hongbo XIE|Show fewer author(s)
The depth-of-focus of traditional infrared optical system always has the limited value. Installation errors and environmental temperature changes can cause defocus, degrading the image quality of infrared optical system. Therefore, infrared system imaging with extending depth-of-focus has been a popular research area iThe depth-of-focus of traditional infrared optical system always has the limited value. Installation errors and environmental temperature changes can cause defocus, degrading the image quality of infrared optical system. Therefore, infrared system imaging with extending depth-of-focus has been a popular research area in optical design. Currently, the depth-of focus expansion methods that researchers usually use have certain drawbacks. As wavefront coding technology gradually rises, it provides a new way to effectively expand the depth-of-focus of infrared optical system.Common infrared detectors are mainly divided into two types uncooled and cooled models. Compared to them, cooled infrared detectors can effectively eliminate the influence of stray light and thermal noise, and have many advantages in applications. Cooled infrared optical system refers to an optical system that matches a cooled infrared detector. The characteristic is that the cold stop of the detector is required as the aperture stop of the infrared optical system. Due to the fact that the phase mask in wavefront coding system needs to be placed at the position of the aperture stop, in order to avoid structure contradiction and reduce design complexity, infrared optical system based on wavefront coding generally matches an uncooled detector, and hardly matches a cooled detector.Therefore, this paper proposes an optical system that matches with a cooled infrared detector. The system uses a secondary imaging structure. It uses the cold stop of the detector as the aperture stop, and makes the cold stop efficiency achieve 100%. At the same time, the parallel plate is also similar to the effect of an aperture stop by controlling the direction of the light. This structure solves the structure conflict between the required position of the phase mask and the position of the cold stop. Finally, the parallel plate is changed to the extended polynomial surface type. And the phase mask surface is determined by modifying the polynomial coefficients, successfully introducing the wavefront coding technology into the cooled infrared optical system.In terms of optimizing phase mask parameter, the method of direct iterative calculation by normally combining optical software with evaluation algorithms is not adopted. Instead, the appropriate value is determined by analyzing the different effects of phase mask parameter values. The specific process is to connect the optical design software with Matlab. Firstly, we use optical design software to track the light rays. Secondly, we use Matlab to read the different MTF values corresponding to the cut-off frequency as the phase mask values change, and draw the relationship curve between them. From this, it can be concluded that the phase mask value cannot be too large. Finally, we select four defocus image plane positions within a distance range of 20 times the depth-of-focus of the original system. Taking cosine similarity as the evaluation standard and using Matlab to read and calculate the data in optical software, we can get the similarity degrees between MTF curves with different phase mask values. The relationship curve between the phase mask values and the MTF similarity of the wavefront coding systems at different defocus positions can be drawn. Based on the above conclusions, the critical value that can maintain consistency of MTF is determined as the appropriate phase mask value. After selecting the phase mask value, through comparison, it is found that the MTF of normal infrared optical system gradually decreases with the increase of defocus, and many zero positions appear within the cut-off frequency. After adding a phase mask, the MTF of wavefront coding system keeps good consistency at different defocus positions, which shows that the system is insensitive to defocus.The blurred images of the wavefront coding system are obtained through software simulation experiments. And then PSF is used as a filter in Lucy-Richardson algorithm to restore the images. The lines and characters in the restored images are clearly visible, which verifies that the cooled infrared optical system based on wavefront coding successfully expands the depth-of-focus range to 20 times that of the original system. Considering that the image results are mainly observed directly by human eyes, we not only use the common evaluation method based on root mean square error of image pixels, but also introduce an evaluation index based on human visual perception to comprehensively evaluate the qualities of the restored images. Through the comprehensive analysis of the MSE and MSSIM results, it is concluded that the qualities of the restored images are good. The main factors that cause the slight differences between the original images and the restored images are noise, artifacts, ringing, and match errors between coding and decoding. In addition, the results also reflect that the wavefront coding system can break through the diffraction limit of traditional infrared optical system to some extent, and will have more research space and prospects..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0111001 (2025)
Development of Low Wavefront Distortion Dichroic Mirror for Super-resolution Imaging
Hongyu HUANG, Ruisheng WANG, Xiaomin LIN, Xianpeng LIANG... and Quan YUAN|Show fewer author(s)
In recent years, with the rapid development of fluorescence imaging technology, there has been an increasing demand for detection. Super resolution fluorescence microscopy, by repeatedly activating and quenching fluorescent molecules to break through diffraction limits, combined with fluorescent labeling, can achieve nIn recent years, with the rapid development of fluorescence imaging technology, there has been an increasing demand for detection. Super resolution fluorescence microscopy, by repeatedly activating and quenching fluorescent molecules to break through diffraction limits, combined with fluorescent labeling, can achieve nanoscale imaging and quickly and accurately detect human diseased cells. Therefore, it is widely used in the field of medical detection. Traditional dichroic mirrors mainly play a role in separating spectra in optical systems, while in fluorescence imaging optical systems, the performance of dichroic mirrors not only needs to consider spectral performance, but also the reflection wavefront distortion (also known as surface flatness PV) of the device. When the surface flatness of the device is poor, that is, when the PV value of the reflected wavefront distortion test result is large, phenomena such as “defocusing”, “astigmatism”, focal plane separation, and blurred spot size can occur after passing through the dichroic mirror. This problem causes the optical system to be unable to accurately distinguish cell structures at the nanoscale, thereby limiting the imaging results of the detection system. The reason for the poor surface flatness of the device is that during the deposition of multi-layer optical films, the surface flatness of the optical device can change under the action of film layer stress, resulting in changes in the surface shape of the coated product. Therefore, there is an urgent need to develop low wavefront dichroic filter devices. This article studies the effect of substrate resistance to thin film stress and finds that the stress change of thin films deposited on JGS1 is smaller than that on K9. Therefore, JGS1 is selected as the experimental substrate; by studying the effect of annealing temperature on the stress of Ta2O5, Nb2O5, and SiO2 material film layers, it is found that as the annealing temperature increases, the stress of Ta2O5 material film reverses from compressive stress to tensile stress, while the stress of Nb2O5 and SiO2 materials remains in compressive stress. It was proposed to achieve low wavefront distortion by canceling out the positive and negative stresses of the film layer. Ta2O5 and SiO2 materials were selected as high and low refractive index materials, and the relationship between the film thickness ratio and annealing temperature at the time of stress cancellation of the two materials was inversely fitted. According to the relationship, it can be seen that the film thickness ratio of Ta2O5 and SiO2 materials needs to be at least greater than 2.6∶1. After preparation, annealing can achieve stress reduction. Force offset; based on the fitting results, the film structure was designed and adjusted. The physical film thickness ratio of the two materials was controlled by changing the optical thickness of the high refractive index material while ensuring that the spectrum was met. The physical film thickness ratio of Ta2O5 material and SiO2 material was designed to be 4.3∶1. Finally, the optical thin film was prepared by sputtering deposition, and the reflection wavefront distortion was further optimized by gradually increasing the annealing temperature. Finally, a dual band low wavefront distortion dichroic mirror was prepared. After testing, a 2 mm thick quartz substrate was used, and the spectral test results show thatthe Tavg>97%@504~544 nm & Tavg>97%@588~800 nm, Tabs<2%@450~491 nm & Tabs<2%@556~566 nm, the wavefront distortion (reflected PV value) test result is 0.087 λ, and it has passed the environmental durability test..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0111002 (2025)
Experimental Study of 450 nm Narrow-bandwidth Semiconductor Laser Based on Reflective Volume Bragg Grating
Botao CAO, Yunhao YAN, Congpeng CHAI, Lingwei GUO... and Shilong ZHAO|Show fewer author(s)
To explore the suppression effect of volume Bragg grating on the temperature drift and spectral line broadening caused by thermal effects during laser operation, a volume Bragg grating with a central wavelength of 449.7 nm was used as the external cavity reflector for blue semiconductor lasers. Under the external cavitTo explore the suppression effect of volume Bragg grating on the temperature drift and spectral line broadening caused by thermal effects during laser operation, a volume Bragg grating with a central wavelength of 449.7 nm was used as the external cavity reflector for blue semiconductor lasers. Under the external cavity mode-locking condition, the influence of different environmental variables on the laser output characteristics was studied. In the experiment, an optical system was built by using a blue light diode, a fast-axis collimating lens and a slow-axis collimating lens to achieve stable laser output. The control variable method was adopted to study the influence of a single variable. The water-cooling temperature, the excitation current and the distance between the slow-axis collimator and the volume Bragg grating were changed step by step, and the spectra in the free-running state and those under the external cavity mode-locking were compared and analyzed. Firstly, the wave-locking position of the volume grating was determined, and an adsorption device was used to modulate the position of the volume grating up and down to compare the free-running spectrum and the wave-locked spectrum under external cavity modulation. The experimental results showed that under the condition of constant water cooling at 22 ℃, when a 3 A current was applied to the blue semiconductor chip, the central wavelength of the free-running laser output spectrum was at 447.4 nm, and the central wavelength of the laser output spectrum locked by the volume Bragg grating was at 449.7 nm, and the spectral line width was narrowed from 1.4 nm to 0.3 nm. When the current increased from 1 A to 3 A, significant wavelength drift and spectral line broadening caused by thermal effects were observed in the free-running spectrum. Moreover, as the current increased, the thermal effect of the semiconductor chip became more significant, and the central wavelength shifted towards the long wavelength at an accelerated speed. However, by adding the volume Bragg grating as the external cavity mirror of the semiconductor laser to form external cavity feedback, the feedback light participated in the competition of stimulated emission inside the chip, achieving the wave-locking effect. The central wavelength of the output light remained consistently at 449.7 nm, and no significant wavelength drift was detected. However, as the current increased, the competitiveness of the feedback light inside the chip weakened, and side lobes appeared in the free-running spectrum. Meanwhile, there was a slight widening of the spectral line width. When the current was fixed at 3 A and the temperature of the water-cooled copper heat sink increased from 16 ℃ to 22 ℃, the central wavelength of the free-running spectrum increased from 446.9 nm to 447.4 nm, and the temperature drift coefficient was approximately 0.083 nm/℃. Under the same condition, by adding the volume Bragg grating, the central wavelength consistently stabilized at 449.7 nm, and the full width at half maximum was only 0.3 nm, with no obvious temperature drift phenomenon occurring. By changing the distance from the volume grating to the slow-axis collimating lens and analyzing the output spectrum, when the distance between the volume Bragg grating and the slow-axis collimating lens increased from 1 cm to 6 cm, the central wavelength remained unchanged, with a stable wave-locking effect. When the distance reached 6 cm, a weak side lobe appeared in the spectrum, and the position of the side lobe corresponded to the central wavelength of the free-running spectrum. After wavelength locking, there was still a linear relationship between the laser output power and the input current. This research provides a reference for the development of high-brightness, high-power, and high-color-purity blue semiconductor lasers..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0114001 (2025)
Study on Ozone Retrieval Algorithm for Monitoring Near Space Ultraviolet Radiation
Yuan AN, Xianhua WANG, Hanhan YE, Hailiang SHI... and Erchang SUN|Show fewer author(s)
The ultraviolet radiance is primarily influenced by the ozone concentrations in the near space. Besides, the ozone is a strong oxidant and an important source of OH in the near space. It will change the atmospheric physical and chemical property directly and/or indirectly. So, the research on ozone profile with the higThe ultraviolet radiance is primarily influenced by the ozone concentrations in the near space. Besides, the ozone is a strong oxidant and an important source of OH in the near space. It will change the atmospheric physical and chemical property directly and/or indirectly. So, the research on ozone profile with the high spatial and temporal resolutions, accuracy will more effectively address the requirements of applications and principles of atmospheric photochemical.The satellites become the viable options to obtain the ozone profiles in the middle and upper atmosphere and local coverage. The ozone limb payloads begin to age for providing the available observation data at present and a few new missions are planned in the future. So, the retrieval algorithm of ozone profile using the radiance obtained by nadir mode is necessary. The ozone priori profile is an important role in the retrieval algorithm based on the optimization estimation technology. The higher precision of priori, the higher precision of the inversion results. The ability to simulate high spatial and temporal ozone profiles with the Goddard Earth Observing System-Chemistry (GEOS-Chem) model is used to construct high spatial and temporal resolution ozone priori profile information for retrieval algorithm of ozone profile in near space. The ground-based measurements and ozone profile products from limb sensors are used to validate the inversion results.A variety of inversion experiments are conducted, in order to assess the feasibility and accuracy of the a priori ozone profile from GEOS-Chem. On the one hand, a comparison experiment of the two types of inversion results is carried out based on the a priori ozone profile constructed by GEOS-Chem and the TpO3 climatology. The superiority of the results based on GEOS-Chem is demonstrated by comparing them with the ground-based measurements and OMPS_LP ozone profile products in China region at typical time. The R2 of GEOS-Chem inversion results are larger than TpO3 inversion results. The TROPOMI Level 2 ozone profile products are selected as the other comparison for further inversion experiments in a wider range of scenarios. The GEOS-Chem inversion results and it are validated and compared with the ground-based measurements and ozone profile products from limb sensors. The only validation measurements of ground-based ozonesonde are the Hong Kong King′s Park stations in the China. The relative difference is used to verified the accuracy of GEOS-Chem inversion results and TROPOMI Level 2 products. The comparison and verification show that the inversion results based on GEOS-Chem model are consistent with the high-precision ground-based measurements. The relative difference between the GEOS-Chem inversion results and ozonesonde measurements are from -5.90% to 37.12%. However, the relative difference between the TROPOMI Level 2 products and ozonesonde measurements are from -9.40% to 52.92 %. These lead to a variation of UV radiation from 1.22×10-6 to 5.50×10-6 W·nm-1·m-2 in the wavelength of 280.25 nm. Moreover, the GEOS-Chem inversion results and TROPOMI Level 2 product are compared with the OMPS_LP load ozone product in order to obtain the accuracy of the GEOS-Chem inversion results in different altitude areas. In the Xizang regions, the relative difference between the GEOS-Chem inversion results and OMPS_LP Level 2 products are from -31.08% to 39.76%. However, the relative difference between the TROPOMI Level 2 products and OMPS_LP Level 2 products are from -45.55% to 53.93 %. In the Southeast region, the extreme value of relative difference between the GEOS-Chem inversion results and OMPS_LP Level 2 products is 42.11%, and the corresponding value of TROPOMI Level 2 products is 63.02%. The different precision ozone profile results lead to the ultraviolet radiation to change by a maximum of 50.27% and -41.79% approximately in the two regions. The relative differences show that GEOS-Chem inversion results have more accuracy and stability than the TROPOMI Level 2 products.The relative difference between the inversion results based on the GEOS-Chem ozone priori profile and validation data is much smaller than those based on the TROPOMI Level 2 products. The oscillation of the relative differences is smoother than the later. These results show that the accuracy of the GEOS-Chem inversion results can be improved greatly. As the GEOS-Chem model advances and ongoing updates are made to meteorological field data and chemical mechanisms, the accuracy and reliability of ozone profiles derived from its inversion will see enhancement. This improvement opens up the potential for refining ozone inversion accuracy. The change in ultraviolet radiation can be reflected better in the near space..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0130001 (2025)
Z-shaped Multi-pass Cavity Enhanced Raman Spectroscopy for Detecting Transformer Faulting Gases
Changding WANG, Pinyi WANG, Zijie TANG, and Weigen CHEN
The oil-immersed transformer represents a pivotal core component within the newly devised power system. It is of paramount importance to guarantee the secure and reliable operation of this apparatus, as it serves as the foundation upon which the overall stability and integrity of the power system is contingent. The accThe oil-immersed transformer represents a pivotal core component within the newly devised power system. It is of paramount importance to guarantee the secure and reliable operation of this apparatus, as it serves as the foundation upon which the overall stability and integrity of the power system is contingent. The accurate detection of transformer fault characteristics and the timely detection of potential faults can effectively prevent major safety accidents. During overheating, discharge faults and catalytic/aging processes (under the action of copper, iron and other metals) in the operation of oil-immersed power transformers, the insulating oil and solid insulating materials, including insulating paper, laminates or wood blocks, etc., decompose, producing a variety of gases which are dissolved in the transformer oil. By measuring the concentration of these gases in the oil or gas phase after pumping, it is possible to detect early faults in the transformer and to carry out the necessary maintenance in time.At present, commonly used multi-component gas detection methods mainly include gas chromatography, electrochemical sensing, semiconductor sensing, infrared absorption spectroscopy/photo-acoustic spectroscopy and so on, but all these methods have certain limitations. Raman spectroscopy offers several advantages over traditional gas detection methods, including high selectivity, non-destructive detection, and real-time detection. However, the Raman scattering cross-section of the gas is relatively small, and the Raman scattering intensity of the trace gas is weak, which results in a low sensitivity of Raman spectroscopy for the detection of gases. Therefore, how to enhance the intensity of the gas Raman scattered light signal is a core problem that urgently needs to be solved for Raman spectroscopic gas detection. Based on the cavity enhanced Raman detection technology, the multiple inverse cavities can extend the path length of the laser and the gas, which is one of the commonly used methods for gas Raman signal enhancement. Currently used Herriot multi-pass cavity or near-concentric multi-pass cavity can achieve tens to more than one hundred times the number of reflections, but the number of laser reflections in the cavity is still small, resulting in Raman signal enhancement of low amplitude, the gas detection limit is difficult to meet the requirements of the transformer fault characteristics of gas detection.This paper proposes a Z-shaped folded multi-pass cavity enhanced Raman spectroscopy detection technique. The optimal number of beams in the multi-inverse cavity is determined by establishing a mathematical relationship between the Raman signal intensity and the number of laser reflections in the multi-pass cavity. The design of intracavity parameters enables the realization of 144 intracavity beams, which complete two cycles within the cavity. This greatly extends the interaction length between the laser and the gas, thereby improving the Raman signal intensity of the gas. The constructed Z-shaped folded multi-pass cavity enhanced Raman spectroscopy detection platform was utilized to achieve highly sensitive detection of the primary fault characteristic gases of transformers. Furthermore, a high-fit quantitative analysis model between the gas concentration and the intensity of Raman spectral peaks was established based on the identified characteristic peaks. Notably, the lowest detection limit of each gas surpassed the IEC standard. This method represents a novel and efficacious approach to the highly sensitive detection of transformer fault characteristic gases..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0130002 (2025)
Optimization of Geometric Attenuation Factor in Six-parameter Spectral BRDF Model
Yue YU, Liguo WANG, Biao WANG, Zhiqiang YANG... and Lei GONG|Show fewer author(s)
In studies related to the scattering characteristics of objectives of physical context, the Bidirectional Reflection Distribution Function (BRDF) is the preferred option for describing the anisotropy of objectives. The geometric attenuation factor in BRDF model has a great impact on the accuracy of BRDF model, which isIn studies related to the scattering characteristics of objectives of physical context, the Bidirectional Reflection Distribution Function (BRDF) is the preferred option for describing the anisotropy of objectives. The geometric attenuation factor in BRDF model has a great impact on the accuracy of BRDF model, which is related to the target surface undulation, roughness and refractive index of the material. The current research and optimization of geometric attenuation factor is mostly from the perspective of surface geometric structure, such as target surface masking and shadowing. The wavelength is not included in influencing factors. However, the relative value of wavelength and surface undulation height directly affects the determination of target surface roughness, thus interfering the geometric attenuation factor, which seriously affects the accuracy of BRDF model. It is necessary to take the wavelength as a part of geometric attenuation factor, study their functional relationship and condense their law of influence.This paper establishes an empirical functional model of the geometric attenuation factor by optimizing the existing geometric attenuation factor from experimental data. Based on the six-parameter spectral BRDF model, the measurement data of target plants is used to inverse the six-parameter values at different incident wavelengths, and the influence law of wavelength on the six-parameter is condensed. The numerical relationships among error values, wavelength, and geometric attenuation factor are analyzed to calculate the expression for the geometric attenuation factor as a function of wavelength. The feasibility of the optimization idea in this paper is verified by comparing the Root Mean Square Errors (RMSE) before and after adding the coefficients related to wavelength.The results show that adding a coefficient, which is exponentially correlated to wavelength, to geometric attenuation factor can significantly improve the accuracy of the six-parameter spectral BRDF model at large scattering angles and reduce the modeling error due to the influence of wavelength. The optimized geometric attenuation factor better fits the measured value versus wavelength curve, and the RMSE of target plants is reduced by about 40% compared to pre-optimization. The wavelength has a weak effect on the a, c parameters, while the effect on the b, kb, kd, kr parameters is more pronounced. Wavelength mainly affects the range of numerical changes and the band location of trend turning point. The specific pattern is affected by the material and surface roughness of target plants after preliminary analysis..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0130003 (2025)
Beam Spreading Scaling Law of Gaussian Beam Propagating in Anisotropic Non-Kolmogorov Maritime Atmospheric Turbulence
Hanyin HU, Bowen YANG, Zheqiang ZHONG, and Bin ZHANG
In recent years, the propagation characteristics of Gaussian beams in anisotropic non-Kolmogorov maritime atmospheric turbulence have attracted significant attention. However, beam spreading is inevitable upon propagation, leading to the decrease of beam quality of Gaussian beams in far field. The analysis of beam spreIn recent years, the propagation characteristics of Gaussian beams in anisotropic non-Kolmogorov maritime atmospheric turbulence have attracted significant attention. However, beam spreading is inevitable upon propagation, leading to the decrease of beam quality of Gaussian beams in far field. The analysis of beam spreading of Gaussian beams, using the power spectrum inversion and the multi-layer phase screen method, is quite time-consuming and challenging to efficiently address the rapid and accurate prediction of beam spreading in practical applications. Although many researches have been focused on the propagation characteristics of Gaussian beams in maritime atmospheric turbulence, the scaling law for beam spreading is still lacking. Especially, compared to terrestrial environments, the maritime environment exhibits distinct characteristics, including higher humidity levels and temperature variations. These environmental differences lead to variations in the propagation characteristics of Gaussian beams. Consequently, establishing scaling law is essential for predicting the beam spreading of Gaussian beams in maritime atmospheric turbulence. To effectively predict and evaluate the impact of maritime atmospheric turbulence on the beam spreading, this study investigates the scaling law for beam spreading of Gaussian beams in maritime atmospheric conditions.A comprehensive physical model has been developed to characterize maritime turbulence using the power spectrum inversion method to generate the turbulence phase screen. Subsequently, numerical simulations have been carried out by the use of the multi-layer phase screen method. Based on the Kolmogorov turbulence spectrum and taking into account the atmospheric coherence length for non-Kolmogorov scenarios, the scaling law for beam spreading of Gaussian beams in maritime atmospheric turbulence has been established. The effects of key parameters, including atmospheric anisotropy, spectral power index, initial beam quality, and wavelength, on the beam spreading of Gaussian beams propagating through maritime turbulence have been investigated. Simulation results, obtained under various parameter settings, yield curves that represent beam spreading and calibration factors under different parameters. The least squares method was then employed to fit these results, leading to the derivation of a beam spreading calibration formula. Furthermore, an error analysis has been performed on all numerical calculation data in relation to the derived calibration formula to validate the effectiveness and applicability of the proposed calibration approach.The results indicate that, within the wavelength range from 1 μm to 4 μm, the initial beam quality from 1 to 3, the refractive index structure from 5×10-16 to 5×10-13 m3-α, the anisotropy factor from 1 to 4, and the spectral power index from 3.1 to 3.8, the error between the predicted beam spreading and the numerical calculation results remains within 15%. Upon these results, the calibration formula exhibits a maximum error of approximately 14.4%, with an average error of around 2.82%. Particularly, in cases where β0>1 or ζ>1, the maximum error is reduced to below 10%. This reduction arises from the amplified impact of the initial beam quality on beam spreading as initial quality of the laser beam deteriorates. Furthermore, as anisotropy factor increases, the influence of turbulence on beam spreading decreases, augmenting the relative impact of the initial beam quality and consequently reducing the error of the scaling law.The scaling law was proposed to predict the beam spreading of Gaussian beam in anisotropic non-Kolmogorov maritime atmospheric turbulence rapidly and accurately. Firstly, the variations of beam spreading with laser and turbulence parameters, including atmospheric anisotropy, spectral power index, initial beam quality, and wavelength, were analyzed in detail. Then, the simulation results yield curves that represent beam spreading and calibration factors under different parameters. Subsequently, the least squares method was employed to fit these results, leading to the derivation of the scaling law. Furthermore, the errors between the results predicted by the scaling formula and the simulation results were analyzed. The results show that, within the specified parameter range, the errors are less than 14.4%, with the average error below 2.82%..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0130004 (2025)
Effect of Plasma Modulation on Terahertz Waves Emitted by Gas Filamentation
Huicheng GUO, and Chengpu LIU
As terahertz technology is widely used in spectroscopy, imaging, medicine and other fields, the demand for high-quality terahertz light sources is becoming increasingly apparent. Compared with traditional terahertz generation technology, the technology of femtosecond laser-induced gas plasma to generate terahertz wavesAs terahertz technology is widely used in spectroscopy, imaging, medicine and other fields, the demand for high-quality terahertz light sources is becoming increasingly apparent. Compared with traditional terahertz generation technology, the technology of femtosecond laser-induced gas plasma to generate terahertz waves has attracted widespread attention due to its advantage of being able to generate strong terahertz waves without damage threshold. Among them, the two-color laser filamentation method has the advantage of high conversion efficiency in the infrared to terahertz band, which greatly increases the energy of terahertz waves. However, it took a long time for the physical mechanism behind it to be understood based on the semi-classical theory of transient photocurrent model. According to previous studies, terahertz wave mainly originates from the radiation generated by the asymmetric photocurrent caused by the two-color field excitation plasma process, however, plasma will strongly modulate the terahertz wave. Therefore, in the actual terahertz generation process, both plasma modulation and photocurrent radiation play an important role. At present, many research groups have carried out work related to plasma modulation of terahertz, however, there are relatively few studies on the comparison of the contribution of plasma modulation and photocurrent radiation to terahertz generation.This paper first gives a theoretical model in three-dimensional conditions, including Maxwell's equations describing the propagation effect, the photocurrent equation describing the generation of photocurrent and plasma modulation, the density evolution equation describing the change of free electron density, and the tunneling ionization equation describing the ionization rate, and details the main conditions and related parameters used in this paper.In the simulation part, we first study the dielectric ionization and terahertz generation process at the center point based on the finite difference time domain algorithm, clarify the synchronization of terahertz generation and plasma step ionization, and confirm that terahertz wave will inevitably drive the newly generated free electrons synchronously with the two-color field, thereby modulating the terahertz wave. Subsequently, by studying the forward and backward terahertz generation under near-field conditions, it is found that ignoring the modulation of plasma on photocurrent radiation will cause it to have a maximum at zero frequency, while introducing plasma modulation on photocurrent radiation will eliminate the radiation maximum at zero frequency. This is because the modulation of plasma on photocurrent radiation greatly suppresses the frequency components below the plasma frequency. In addition, the introduction of plasma modulation of photocurrent radiation causes a certain degree of blue shift in the radiation spectrum. This is because the photocurrent radiation accelerates the vibration of free electrons in the plasma, which in turn causes the overall radiation spectrum to move toward the high frequency direction. Further, by studying the forward and backward terahertz generation under far-field conditions, it is found that when the modulation of plasma on photocurrent radiation is not considered, the forward radiation spectrum has a smooth single-peak structure, while the backward radiation spectrum presents a series of equally spaced modulation structures, and this phenomenon is accurately explained by the principle of coherent superposition. When considering the modulation of the radiation wave by plasma, it is found that the components of the forward far-field radiation and the backward far-field radiation below the plasma frequency are significantly suppressed due to the shielding effect of the plasma, while the components above the plasma frequency are enhanced to a certain extent, which is also caused by the photocurrent radiation accelerating the vibration of free electrons in the plasma.Based on the calculation and analysis of terahertz generation under different conditions, this paper confirms the low-frequency suppression and high-frequency blue shift and enhancement of terahertz wave by plasma modulation, points out the incompleteness of the explanation of coherent superposition of single-point radiation, and proposes a three-step model to more comprehensively explain the physical mechanism of terahertz generation, deepen the understanding of terahertz wave generated by laser-induced plasma, and has a good guiding role in experiments..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0130005 (2025)
Fiber Optics and Optical Communications
FBG Wavelength Demodulation Method Based on Support Vector Regression Optimized by Sparrow Search Algorithm
Yinggang LIU, Fei LI, Yubo YUAN, Rui LI... and Xinyi XU|Show fewer author(s)
Fiber Bragg grating (FBG) wavelength demodulation technique based on tunable Fabry-Perot (F-P) filters (FFP-TF) results in a degradation of the accuracy of the demodulation system due to the hysteresis and temperature drift characteristics of piezoelectric ceramics (PZT) inside the FFP-TF. Artificial intelligence machiFiber Bragg grating (FBG) wavelength demodulation technique based on tunable Fabry-Perot (F-P) filters (FFP-TF) results in a degradation of the accuracy of the demodulation system due to the hysteresis and temperature drift characteristics of piezoelectric ceramics (PZT) inside the FFP-TF. Artificial intelligence machine learning algorithms can reduce the demodulation error of the system without increasing the complexity of the system. Therefore, this article proposes an FBG wavelength demodulation method based on Support Vector Regression (SVR) optimized by Sparrow Search Algorithm (SSA). The main purpose is to establish a nonlinear fitting relationship between F-P tuning voltage and transmission wavelength, replacing wavelength reference hardware. This method can save system costs, improve system demodulation accuracy, and contribute to the integrated development of demodulation systems. First, the reference grating, driving voltage, tuning time and F-P surface temperature are taken as the input features of the model, and the F-P transmission wavelength is taken as the output feature, and the F-P transmission wavelength compensation model is established by SVR. The SSA-optimized hyperparameters C and g are then input into the SVR model for training to obtain the target compensation model. Meanwhile, the training method of combining sliding window with SVR is proposed to improve the model's global optimization-seeking ability by updating the sliding window's size and sliding speed, which avoids the problem of the model's deterioration in generalization ability over time. After the model training, the correlation coefficient (R2), Mean Square Error (MSE), and Root Mean Square Error (RMSE) are introduced to evaluate the model and obtain the optimal compensation model. Finally, the FBG wavelength demodulation system based on SSA-SVR is built in Labview and the optimal training model is called. Then the real-time compensation ability of the model is checked by the stability experiment and the cooling experiment. The law of the wavelength compensation error in the frequency range of 0.5~2.5 Hz driving is explored experimentally, and the compensation ability of the SSA-SVR model is compared with PSO-SVR, LSSVR, and KRR models. The experimental results show that in the frequency range of 0.5~2.5 Hz, the error between the target value output and the real value of the SSA-SVR algorithm model decreases with the decrease of the driving frequency, which indicates that the demodulation system is more accurate under the low-frequency driving. The fitting coefficient R2 of the SSA-SVR algorithm reaches 0.999 99 in both training and test data, which is an improvement of 0.001 compared to the KRR algorithm, and the wavelength demodulation error is reduced by more than ten times. At 2.5 Hz driving frequency, the Mean Absolute Error (MAE) of SSA model is 38.689 pm, which is reduced by 28.078 pm compared to the PSO algorithm error, similarly, at 0.5 Hz driving frequency, the SSA model has a MAE of 19.83 pm, which is reduced by 48.8% compared to the PSO optimization algorithm error of 39.68 pm. In addition, the SSA model has the smallest error in all of the different drive frequency tests, showing better generalization performance, while the LSSVR model performs better at high frequencies and shows negative optimization in the low-frequency tests, and the KRR model has the largest error in a wide range of frequencies, indicating that this model has the worst fit. In the stability experiment at 25 ℃, the fluctuation of the demodulation value based on the SSA-SVR model is kept within 15 pm, and the average absolute error between the demodulation value and that of the sm125 demodulator produced by MOI company in the U.S.A. is 7.28 pm, whereas the stability of the traditional polynomial demodulation method of the reference grating is poorer, and the demodulation error of the SSA is reduced by 88.5% compared with it. For the cooling experiments from 40 to 90 ℃, the detuned MAE of the SVR compensated model is 7.44 pm, which is 27.6% lower relative to the 5.39 pm of the polynomial fitting method. The experiment proved through error analysis that this method is superior to the reference grating polynomial demodulation method in terms of real-time demodulation and stability, and the error between it and the sm125 demodulator remains within a small range. Compared with the traditional method, this method models the wavelength drift during the natural temperature change process of FFP-TF, realizes the nonlinear fitting between the F-P driving voltage and the transmitted wavelength over a wide range, and verifies that the fiber grating demodulation system can effectively reduce the grating wavelength demodulation error without the help of hardware reference..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0106001 (2025)
Distributed Fiber Optic Acoustic Sensing System Based on Fading Mask Autoencoder and Application in Water Navigation Security Events Identification
Miao YU, Yutong HE, Tianying CHANG, Hongliang CUI... and Qingguo GAO|Show fewer author(s)
In recent years, intelligent navigation security is a hot spot in the field of water safety protection. Distributed optical fiber acoustic sensing technology based on phase sensitive optical time domain reflectometer can realize distributed monitoring of multi-point disturbance along optical fiber. However, due to the In recent years, intelligent navigation security is a hot spot in the field of water safety protection. Distributed optical fiber acoustic sensing technology based on phase sensitive optical time domain reflectometer can realize distributed monitoring of multi-point disturbance along optical fiber. However, due to the complex water environment and the fading of system signals, it is difficult to identify the disturbance signals stably and effectively. The main noise sources of distributed optical fiber acoustic sensing system are interference fading and polarization fading. Both of these phenomena greatly weaken the signal at the fading point, resulting in signal distortion. Under water, the cable is less coupled with the environment, and is more susceptible to the influence of water waves, currents and other factors. Therefore, timely and effective elimination of fading interference plays an important role in the identification of water security events. Combining fading mask, attention mechanism and self-supervised learning, this paper proposes a distributed optical fiber acoustic sensing system based on fading mask autoencoder for ship security event recognition in waters. This method is aimed at the ship event signal in the water area, and generates a basically noise-free signal by shielding fading noise into the deep learning model, so that the model can learn directly and greatly reduce the influence of fading noise on signal recognition. Mask autoencoder is an extensible self-supervised learner. It combines the attention mechanism in the form of a mask to achieve high-precision training with the simplest coding-decoding model structure, while it can also transfer learning only through the model weights of the encoder. On this basis, the fading mask autoencoder method is more helpful for the distributed optical fiber acoustic sensing system to achieve effective event recognition. Firstly, the amplitude signal of distributed optical fiber acoustic sensing is analyzed to determine the fading position. The model is then pre-trained using the upstream task of the fading mask autoencoder. The basic characteristics of distributed optical fiber acoustic sensing signal are learned by means of random mask. Finally, the downstream task of fading mask autoencoder completes the intelligent event recognition training by fading position mask. In this paper, four kinds of ship security events collected at the water test site are used as classification data, and the fading mask autoencoder is compared with the mask autoencoder and three related models. The results show that the average training accuracy of the fading mask autoencoder is 98.34%, which is 4.9% higher than that of the mask autoencoder. The average training loss was 0.1094, which was 0.095 less than that of the mask autoencoder. The average test accuracy was 93.01%, which was 6.45% higher than that of the mask autoencoder. Compared with the other three models, the fading mask autoencoder has higher training accuracy, lower Loss and faster convergence speed. Its average performance index is about 4.88%-7.62% higher than other models. Therefore, the fading mask autoencoder model based on the improved mask strategy can extract useful information from the signal more efficiently and accurately for training, and has better stability and generalization, which is suitable for the identification of navigation security events in waters..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0106002 (2025)
UAV UV Information Collection Method Based on Deep Reinforcement Learning
Taifei ZHAO, Jiahao GUO, Yu XIN, and Lu WANG
In recent years, Unmanned Aerial Vehicles (UAVs) have been widely used across various fields due to their high mobility, flexibility, and cost-effectiveness. In the civilian fields, UAVs are utilised for activities such as agriculture, environmental monitoring, and search and rescue operations. Conversely, in the militIn recent years, Unmanned Aerial Vehicles (UAVs) have been widely used across various fields due to their high mobility, flexibility, and cost-effectiveness. In the civilian fields, UAVs are utilised for activities such as agriculture, environmental monitoring, and search and rescue operations. Conversely, in the military fields, UAVs are employed for a range of purposes, including surveillance, reconnaissance, precision strikes, and target guidance. Ground-based battlefield reconnaissance sensor systems currently deployed by military forces include battlefield reconnaissance radars, magnetic sensors, infrared sensors, vibration sensors, acoustic sensors, and pressure sensors. UAVs are increasingly playing a crucial role in information collection for these ground sensors. Traditional communication methods for information collection typically rely on radio communication, which can be severely disrupted or rendered unusable in environments with electromagnetic shielding or interference. Solar-blind ultraviolet (UV) light, operating within the 200 nm-280 nm wavelength range, offers virtually no background noise in low-altitude airspace and provides all-weather, non-line-of-sight communication capabilities. This makes it an ideal communication method in electromagnetically challenged environments due to its excellent environmental adaptability, high confidentiality, and strong resistance to electromagnetic interference. Compared to line-of-sight UV communication, non-line-of-sight communication does not require precise alignment between the transmitter and receiver and offers greater flexibility in receiver positioning, making it more suitable for collecting information from ground sensors. However, traditional algorithms often face limitations in handling complex information collection tasks, particularly in terms of computational resources, adaptability, and real-time performance. Deep reinforcement learning (DRL) algorithms, as an emerging intelligent decision-making method, enable UAVs to autonomously complete tasks by learning and experimenting within the environment. This makes DRL an ideal approach for autonomous UAV navigation and data collection tasks.This paper addresses the challenge of UAV information collection in the presence of electromagnetic interference by employing an adaptive elevation angle UV non-line-of-sight communication method and utilizing DRL algorithms to tackle the information collection task. First, a UAV mobility model is established, followed by the proposal of a UV non-line-of-sight air-to-ground communication model with variable transmission and reception angles. Detailed modeling of the UAV's energy consumption is then carried out, considering flight energy consumption, the energy consumption of the electro-optical pod, and communication energy consumption. Subsequently, an information collection model is established. This integrated model balances task execution time, energy consumption, and communication quality during the information collection process. Given that the optimization problem is NP-hard, traditional polynomial optimization algorithms are inadequate for solving it. Therefore, this problem is formulated as a Markov decision process. To enable the UAV to make better decisions regarding flight direction, speed, and UV transmission and reception angles, a reward function tailored to the information collection task is designed. This reward function comprehensively considers time, energy, communication path loss, and the UAV's return to base. The Double Deep Q-Network (DDQN) algorithm, which separates action selection from evaluation, still faces overestimation issues in high-dimensional state and action spaces. This paper proposes that in the information collection scenario, the UAV must consider multiple directions and speeds of movement while adaptively adjusting the UV communication angles during the collection process. Compared to previous discrete environments with smaller action spaces, this scenario requires a larger action space. To better adapt to this scenario, improvements such as dual target networks, prioritized experience replay, and entropy regularization are incorporated into the classical DDQN algorithm, enhancing its adaptability and stability.To verify the effectiveness of the improved DDQN algorithm and explore the impact of different UV parameters, sensor quantities, and UAV flight altitudes on information collection time and energy consumption, comparative simulations with the classical DDQN algorithm are conducted. The proposed adaptive elevation angle DDQN algorithm effectively completes the information collection task, demonstrating at least a 13% improvement in time efficiency and a 14% reduction in energy consumption across multiple scenarios compared to the classical DDQN algorithm..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0106003 (2025)
Integrated Optics
Research on High Efficiency and High Compactness Vertical Silicon-based Grating Couplers
Ning DUAN, Congcong LI, Zhengnan YUAN, Chenxi SHI... and Ge YAN|Show fewer author(s)
Silicon photonics and integrated optical circuits are pivotal technologies in the post-Moore era, enabling high-speed and low-latency applications. However, the efficient coupling of optical signals from photonic chips to optical fibers remains a significant challenge due to the mismatch in fiber and waveguide dimensioSilicon photonics and integrated optical circuits are pivotal technologies in the post-Moore era, enabling high-speed and low-latency applications. However, the efficient coupling of optical signals from photonic chips to optical fibers remains a significant challenge due to the mismatch in fiber and waveguide dimensions. Grating Couplers (GCs) present a promising solution due to their flexible coupling position selection, compact structure, and ease of wafer-level testing. This paper aims to design a high-efficiency, compact, and cost-effective GC using shape optimization theories and methods to provide an excellent interconnect solution for silicon photonic chips. This paper designs a double-layer grating coupler with different grating teeth and slots formed through two etching processes, enhancing the coupler's directionality. To address the challenge of local optima during the optimization process and enhance the probability of finding the global optimal solution, a random perturbation method is employed to generate a series of initial devices that are uniformly distributed in the solution space. Each element of the parameter vector undergoes random addition or subtraction of a uniformly distributed random number, ensuring that each element is randomized in the same manner, independent of its position in the vector. This approach promotes a more comprehensive exploration of the solution space and facilitates the identification of the global optimal solution. To overcome the inefficiency of optimizing multiple devices simultaneously due to noise addition, a parameter update rate regulation strategy is introduced to accelerate the optimization process. During the initial optimization stage, a larger update rate α is employed to achieve rapid convergence. However, as the iteration progresses, α is gradually decreased to prevent missing the global optimal solution. This cosine decay strategy allows for swift exploration of the solution space during the early stages of optimization while enabling refined parameter adjustment in the later stages, thereby increasing the likelihood of converging to the global optimal solution. Compared to other decay methods, this strategy demonstrates superior performance in parallel optimization of multiple devices. The adjustment strategy for the update rate is crucial for the design of the optimization algorithm, as it requires balancing convergence speed and stability to ensure efficient and accurate identification of the global optimal solution. The cosine decay strategy not only ensures the stability of model training but also enhances the search capability for the global optimal solution. The decay path of the update rate follows the characteristics of the cosine function, experiencing a rapid decline in the initial stage to capture the preliminary optimization direction and a gradual decline in the later stages for fine-tuning parameters, thereby improving the precision of locating the global optimal solution. The introduction of a dynamic parameter update rate in the shape optimization algorithm primarily involves larger update steps in the early stage and smaller steps in the later stage. This dynamic adjustment offers two main improvements over static updates: rational control of iteration count, reducing the time cost of optimization, and rapid convergence and elimination of instability, facilitating swift convergence in the early stage and preventing instability in later stages. By employing this optimized parameter update rate regulation strategy, the shape optimization algorithm for grating couplers effectively explores the solution space and converges to the global optimal solution with high efficiency and accuracy. The specific steps are as follows: establishing a parametric model of the double-layer grating coupler and forming a parameter vector by recording vertex coordinates; generating a series of initial devices uniformly distributed in the solution space through random perturbation; introducing a parameter update rate regulation strategy, adopting a larger update rate initially for rapid convergence and gradually decreasing it later to avoid missing the global optimal solution; obtaining a grating coupler structure that meets the design conditions through iterative optimization. The designed double-etched grating coupler achieves a high coupling efficiency of 0.887 in the C band with a 3 dB bandwidth of 28 nm. Compared to recently reported silicon-based grating couplers, this design achieves the highest known coupling efficiency on a 220 nm thick SOI platform without the need for additional material layers. In the case of a 10 nm etching depth error, experimental testing results show that the peak single-port coupling efficiency of the double-etched grating coupler is -3.62 dB (43.5%) at 1 548 nm. The proposed design method can achieve a high-efficiency, compact, and cost-effective grating coupler, providing an excellent interconnect solution for silicon photonic chips and offering new insights into the design and application of grating couplers..
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0113001 (2025)
Ultrafast Optics
Streaking Time Delay and the Oscillation Amplitude of the Momentum Shift
Mengfei XIE, and Weichao JIANG
In usual attosecond streaking schemes, an Extreme Ultraviolet (XUV) pulse of a few hundred attoseconds serving as a pump and a phase-controlled few-cycle Infrared (IR) pulse as a probe. XUV photon can excite the bound electron in atoms to continuum states, resulting in ionization. The ionized electrons can then be acceIn usual attosecond streaking schemes, an Extreme Ultraviolet (XUV) pulse of a few hundred attoseconds serving as a pump and a phase-controlled few-cycle Infrared (IR) pulse as a probe. XUV photon can excite the bound electron in atoms to continuum states, resulting in ionization. The ionized electrons can then be accelerated, decelerated, or deflected by the external IR laser field, generating attosecond-level temporal resolution. In the presence of a polarized IR laser pulse, ionization of a bound electron is achieved by absorbing an XUV photon, the energy of the photoelectron ejected forward along the laser polarization is affected by the attosecond streaking of the external IR pulse and depends on the phase of the IR pulse at the moment of ionization. By analyzing attosecond streaking spectra, the photoemission delay can be determined from the final kinetic energy oscillation associated with the relative delay between XUV and IR fields. Both the streaking time delay and amplitude of the energy oscillation depend on the coupling of the potential and the detected IR field. The determination of streaking time delay is a complex task that involves taking into account a variety of effects, such as the Eisenbud-Wigner-Smith (EWS) time delay,Coulomb-Laser Coupling (CLC), electron correlation and dipole-laser coupling. In the context of streaking time delay for ground-state hydrogenic atoms, it is commonly acknowledged that two factors account for this delay. The first is the EWS delay, resulting from the potential's short-range behavior. The second is the CLC delay, due to the joint influence of the IR pulse and the long-range Coulomb potential. Attosecond streaking provides unprecedented insights into the dynamics of time-resolved photoelectron emission in atoms. Moreover, in addition to conventional linearly polarized fields, researchers have recently combined bicircular fields with streaking. This approach enables ionization-time retrieval with remarkable few-attosecond precision. While exploring the streaking dynamics, most of the aforementioned reports have focused mainly on the streaking time delay and have not discussed the oscillation amplitude of the momentum shift of the electron, which defines the strength of the IR field if measured from the spectra, explicitly. However, to truly understand the dynamics, we need to consider the oscillation amplitude
Acta Photonica Sinica
- Publication Date: Jan. 25, 2025
- Vol. 54, Issue 1, 0132001 (2025)
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