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2025
Volume: 54 Issue 4
22 Article(s)
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Interferogram Filtering Method for Interferometric Spectral Imager Based on VMD-DWT
Xiangyu GAO, Juan LI, Can YU, Runjia LIU, Xin GENG, and Shuang WANG
Interferometric spectral imagers enable simultaneous acquisition of two-dimensional spatial information and one-dimensional spectral data of targets. It offers significant advantages, including high throughput, multi-channel capability and superior resolution, with broad applications in agricultural vegetation analysisInterferometric spectral imagers enable simultaneous acquisition of two-dimensional spatial information and one-dimensional spectral data of targets. It offers significant advantages, including high throughput, multi-channel capability and superior resolution, with broad applications in agricultural vegetation analysis, soil composition assessment, marine biology studies, atmospheric monitoring and mineralogical investigations. The workflow involves dispersion, interference and imaging to capture target's images and interference information, from which spectral information is reconstructed.However, during the acquisition of interferograms, deviations arise due to factors such as the non-uniform response of the detector itself, variations in signal processing circuits across different sub-regions, environmental fluctuations affecting the detector's operation, and non-uniformity in optical energy transmission. These deviations prevent the interferograms from accurately representing the target's spatial and spectral characteristics, resulting in spectral distortion during reconstruction. Consequently, prior to spectral reconstruction, interferogram correction must be performed through steps including dark current correction, response non-uniformity compensation, and bad pixel correction to mitigate the impact of various errors on the reconstructed spectrum.The corrected interferogram represents a superposition of high-frequency information and low-frequency background noise that varies with optical path difference. The low-frequency component adversely affects the accuracy of spectral reconstruction and should be eliminated through filtering. Current interferogram filtering methodologies primarily encompass differential method, fitting method, and Empirical Mode Decomposition (EMD). While differential method inherently compromises interference curve symmetry, fitting-based method demands prior knowledge of signal characteristics and empirical mode decomposition has drawbacks such as endpoint effects and mode mixing.Variational Mode Decomposition (VMD) proposed by DRAGOMIRETSKIY K et al in 2014, is an adaptive, fully non-recursive modal variational method based on empirical mode decomposition, which has the advantages of determining the number of modes and suppressing mode mixing. Discrete Wavelet Transform (DWT) proposed by MALLAT S G in 1989, is a method that can extract local features of signals and achieve multi-resolution analysis, particularly suitable for capturing the non-stationary characteristics of signals, with advantages in multi-scale and localized analysis. To address the constraints of existing filtering methods, this paper proposes a combined VMD-DWT filtering method. Through the synergistic application of VMD and DWT, low-frequency component is systematically separated while high-frequency components are effectively extracted, achieving enhanced spectral reconstruction accuracy. First, VMD is performed on the interferogram to determine whether the correlation coefficient of each mode component exceeds the threshold, thus obtaining the optimal number of modes. Second, the interferogram undergoes VMD under the optimal number of modes, filtering out mode component with relatively high value and low center frequency to represent the low-frequency component of the interferogram. Then, DWT is used to separate the residual signals in the mode component to obtain low-frequency component. Finally, the low-frequency component is directly subtracted from the interferogram to obtain an interferogram containing only high-frequency information, completing the filtering process.Experimental validation is conducted using the HJ-2A hyperspectral imager, with the Signal-to-Noise Ratio (SNR) and the Relative Quadratic Spectral Error (RQE) of the reconstructed spectrum in the spatial dimension serving as evaluation metrics. The results demonstrate that the advantages of fitting filtering are intuitive results and fast computational speed, while the disadvantages include the need for prior information. Specifically, polynomial fitting cannot fully represent the low-frequency component, leading to significant fluctuations in the fitting curve and larger errors at both ends, which affect the filtering effect. The SNR of the restored spectrum in the spatial dimension after fitting filtering is 28.119 3, and the RQE is 0.005 268. The advantages of EMD filtering include adaptability, while the disadvantages include difficulties in determining the values of the upper and lower envelope lines at endpoints, leading to endpoint effects. The decomposed mode components cannot represent an independent oscillation mode, resulting in mode mixing. Both endpoint effects and mode mixing affect the filtering effect. The SNR of the restored spectrum in the spatial dimension after EMD filtering is 28.779 6, and the RQE is 0.004 887. The advantages of VMD filtering include the ability to determine the number of modes and suppress mode mixing, while the disadvantages include the influence of the penalty factor and the number of modes on the filtering effect. The SNR of the restored spectrum in the spatial dimension after VMD filtering is 29.054 2, and the RQE is 0.004 698. The advantages of the VMD-DWT combined filtering method include the ability to determine the number of modes, suppress mode mixing, and provide multi-scale and localized analysis. The SNR of the restored spectrum in the spatial dimension after VMD-DWT combined filtering method is 29.075 2, and the RQE is 0.004 683.The proposed VMD-DWT combined filtering method demonstrates superior performance compared to conventional approaches, showing SNR improvements of 3.40%, 1.03%, and 0.07% over fitting filtering, EMD filtering, and standalone VMD filtering, respectively. Concurrently, it achieves RQE reductions of 11.10%, 4.17%, and 0.32% for these respective methods. This innovative approach effectively eliminates low-frequency component while preserving critical spectral features, thereby improving spectral restoration accuracy and enhancing the quantitative measurement capabilities of interferometric imaging spectrometers. The combined advantages of noise suppression and signal fidelity maintenance make this methodology particularly valuable for high-precision spectral analysis applications..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0410001 (2025)
Hyperspectral Image Classification Method Based on Dynamic Graph-spectral Feature Extraction
Chenjie XU, Dan LI, and Fanqiang KONG
Hyperspectral remote sensing integrates spatial and spectral imaging technologies to capture continuous spectral image data containing spatial and spectral information. Hyperspectral image (HSI) classification is one of the key studies in hyperspectral interpretation. HSI classification has been applied in military tarHyperspectral remote sensing integrates spatial and spectral imaging technologies to capture continuous spectral image data containing spatial and spectral information. Hyperspectral image (HSI) classification is one of the key studies in hyperspectral interpretation. HSI classification has been applied in military target recognition, land resources and other fields. Despite significant advancements in machine learning and deep learning methods, challenges persist due to the high dimensionality of HSI data, limited training samples, and the inherent complexity of spatial and spectral features.Aiming at the problem of low classification accuracy caused by limited samples in HSI classification, this study proposes a novel HSI classification framework based on Dynamic Graph-Spectral Feature Extraction (DGSFEC). The proposed method enhances the classification accuracy by integrating dynamic graph construction, dynamic graph convolution spatial feature extraction, and hierarchical spectral feature modeling within a unified architecture. The contributions of this work are threefold: (1) the introduction of a novel dynamic graph construction method that adaptively captures spatial relationships, (2) the development of a multi-resolution dynamic graph feature extraction network that enhances the perception of cross-domain spatial features, and (3) the implementation of multi-level spectral feature extraction network that combines shallow local and deep global features to achieve refined spectral information processing. The Dynamic Graph Construction (DGC) addresses the limitations of static graph-based methods that fail to capture the spatial variability within HSI. By utilizing a sliding window mechanism, the dynamic graph adjusts adaptively to the spatial relationships of pixels, incorporating spatial similarity and distance information. This method effectively captures both local and global spatial dependencies, resulting in a more accurate and representative graph structure for HSI. Unlike static graphs, the dynamic approach offers flexibility and computational efficiency, ensuring the graph better reflects real spatial relationships. The DGSFEC model is structured into two synergistic branches. The Dynamic Graph Feature Extraction Network (DGCFN) is designed to effectively extract spatial information at different resolutions through dynamic graph convolutions, and combine with dynamic spatial convolutions and conditional position coding to enhance the perception of cross-domain spatial features. This module integrates fine-grained local features and global spatial information to provide richer spatial feature descriptions for HSI classification. Simultaneously, the Region-Global Spectral Feature Network (RGSFN) focuses on both local and global spectral feature modeling. RGSFN employs a multi-level convolutional structure, which is then combined with 3D Transformer encoders cross-layer feature fusion mechanism to integrate shallow local features with deep global features. This branch enhances the model's capacity to identify subtle spectral differences between classes. To further improve classification accuracy, the Cross Attention Feature Fusion (CAFF) network is proposed, which combines spatial and spectral features extracted from the two branches. The CAFF module employs a cross-attention mechanism to dynamically align spatial and spectral information, capturing the complementary relationships between the two feature types. This fusion enhances the model’s robustness in distinguishing complex patterns and improves generalizability across datasets with varying spatial and spectral characteristics.Extensive experiments were conducted on three benchmark hyperspectral datasets Indian Pines, University of Pavia, and Salinas. Results demonstrate that the DGSFEC framework outperforms state-of-the-art methods, achieving higher Overall Accuracy (OA), Average Accuracy (AA), and Kappa coefficients across all datasets. Notably, the proposed method achieves smoother and accurate classification maps in complex scenarios, highlighting its robustness and generalizability. This is attributed to integrating cross-domain spatial features and global spectral similarity features from DGCFN and RGSFN, ensuring classification performance on complex scenes. In addition, the DGFCN decreases computational complexity, attributed to its dynamic graph construction and feature optimization strategies. Comparative analyses reveal that the DGSFEC balances computational demands and accuracy more effectively than existing graph-based and transformer-based approaches.In conclusion, the DGSFEC method offers a classification performance advancement in HSI classification by effectively utilizing limited training samples and extracting both spatial and spectral features. The future work will focus on optimizing the training cost and exploring limited sample, low-parameter classification models to further enhance the comprehensive performance of HSI classification..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0410002 (2025)
MSP-YOLACT：Instance Segmentation Model for Multimodal PET/CT Medical Images of Lung Tumors
Tao ZHOU, Wenwen CHAI, Yaxing WANG, Kaixiong CHEN, Huiling LU, and Daozong SHI
With the development of medical image technology, multimodal medical image instance segmentation is a research hotspot. Existing instances segmentation model of multimodal medical image does not fully consider the complementary information of multimodal images lesions. To address the issues of low contrast and blurred With the development of medical image technology, multimodal medical image instance segmentation is a research hotspot. Existing instances segmentation model of multimodal medical image does not fully consider the complementary information of multimodal images lesions. To address the issues of low contrast and blurred boundary of lesion information in lung tumor medical images, the instance segmentation model is proposed for PET/CT lung tumor medical images in this paper. The main contributions of the model include the following 3 parts. Firstly, in order to adequately fully utilize the common features of lesions in different modal images for lesion morphological enhancement, a multimodal feature mixer is designed. The module adaptively learns the common features related to lesion area through the PET and CT 2 branches. Specifically, it first normalizes the input PET and CT feature maps to make the data distribution more stable. Then, it adopts the self-attention mechanism to extract the PET/CT branche features. This mechanism enables the model to focus on different parts of the features and capture more discriminative information. After that, it fuses the features of lesions areas learned from PET and CT branches into PET/CT branches pixel by pixel. By using a weighted fusion method, the important features are emphasized, thereby highlighting the features of lesions areas and making the lesion regions stand out more clearly in the images. Secondly, in order to increase the lesion area attention, the enhanced feature pyramid is designed, which includes an enhanced feature fusion module and a multi-scale feature fusion device. For the enhanced feature fusion module, in the top-down fusion process, the module focuses on the semantic information of the high-level feature map while suppressing the noise factor. It does this by leveraging self-attention mechanisms to selectively emphasize relevant features. For the multi-scale feature fusion device, which receives the coarse and fine information of PET and CT branch features, the module effectively fuses the fore-back ground prominent features, fills the lowest pyramid feature map information, and enhances the learning ability of image morphological information by using dedicated convolutional operations for better feature extraction. Finally, in order to enhance the localization and boundary characterization ability of the model, a parallel feature enhancement prediction head module is designed. This structure reconstructs the anchor frame and mask coefficient branches. Specifically, the anchor frame branch generates a different proportion of anchor frame for each pixel based on the learned feature distribution, and the mask coefficient branch realizes a one-to-one corresponding to the mask by accurately predicting the coefficient of each mask, so as to precisely locate the lesion area. Additionally, global and local feature enhancement modules are introduced to further enhance the lesion areas in the feature map, and thus significantly improve the ability to identify the lesion regions and lesion boundaries. The clinical multimodal lung tumor medical image dataset is used to verify the validity of the model. PET/CT single mode is used to detect and segment lung tumor lesion area, the mAPdet ,mAPseg are 58.25 and 59.45, respectively. PET/CT and CT modes are used to detect and segment lung tumor lesion area, the mAPdet, mAPseg are 57.59 and 59.18, respectively. PET/CT and PET modes are used to detect and segment lung tumor lesion area, the mAPdet, mAPseg are 58.31 and 59.32, respectively. The experimental results showed that the APdet, APseg, ARdet, ARseg, mAPdet, and mAPseg of the proposed model are 64.55%, 65.53%, 51.47%, 52.28%, 64.37% and 65.41%, respectively, for the lung tumor lesions area detection and segmentation. This model can achieve accurate detection and segmentation of lung tumor lesion area, which is of positive significance for automated auxiliary diagnosis of lung tumors..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0410003 (2025)
Application of Frequency Stabilization and Frequency Shifting Technology Based on Electro-optic Modulation in Differential Absorption Lidar
Ao HE, Linmei LIU, Guangbao YU, Yong YANG, Xin LIN, Zhaoxiang LIN, and Faquan LI
As the greenhouse effect becomes more severe, high-precision detection of atmospheric carbon dioxide column concentration has become increasingly critical. Differential Absorption Lidar (DIAL) has been widely used in atmospheric detection due to its high sensitivity and high spatiotemporal resolution. However, traditioAs the greenhouse effect becomes more severe, high-precision detection of atmospheric carbon dioxide column concentration has become increasingly critical. Differential Absorption Lidar (DIAL) has been widely used in atmospheric detection due to its high sensitivity and high spatiotemporal resolution. However, traditional DIAL systems still face challenges, including the complexity of seed laser frequency stabilization and the instability of the
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0414001 (2025)
Multifunctional Devices Based on Vanadium Dioxide-assisted Metamaterials
Sha LI, Yingjue CAO, Xiangjun LI, Le ZHANG, Jining LI, and Dexian YAN
Terahertz technology has garnered significant attention from researchers due to its outstanding properties and potential. Currently, terahertz waves are extensively used in various fields, including communications, medical imaging, radar, and more. As a result, terahertz technology plays a crucial role in the advancemeTerahertz technology has garnered significant attention from researchers due to its outstanding properties and potential. Currently, terahertz waves are extensively used in various fields, including communications, medical imaging, radar, and more. As a result, terahertz technology plays a crucial role in the advancement of modern society. Alongside terahertz technology, metamaterials have also emerged as a key area of research. Traditional natural materials possess certain inherent limitations, such as difficulty in modifying their structures, limited operational ranges, and fixed properties. In contrast, metamaterials offer superior characteristics and advantages. These artificial composite materials, designed with tailored structures, can be easily fabricated and modified to meet specific operational requirements. Metamaterials exhibit extraordinary physical properties that natural materials do not, overcoming the limitations of conventional materials and providing new possibilities for technological innovation. Currently, metamaterials are widely applied in areas such as lenses, energy absorption, and antennas. When integrated with terahertz technology, metamaterials enable the design of advanced devices, including absorbers, filters, and sensors, further expanding the potential of terahertz applications. Vanadium dioxide (VO?) exhibits remarkable phase transition characteristics. When external factors like temperature, electric field, or light field are altered, the conductivity of vanadium dioxide can change by 4 to 5 orders of magnitude, triggering a phase transition. Building on these phase transition properties, this paper proposes a multifunctional device based on vanadium dioxide metamaterials. This device is capable of achieving three key functions: broadband absorption, linear-to-circular polarization conversion, and linear-to-linear polarization conversion. Additionally, the device allows for switching between these functions by altering the external environment. Simulations and data analysis are carried out using the commercial software CST Studio Suite in the frequency range of 0.1 to 6 THz. The results show that when the conductivity of vanadium dioxide is 2×10? S/m, it is in its metallic state, and the designed structure achieves broadband absorption in the frequency range of 2.11~4.89 THz, with a relative bandwidth of 79.43%. When the conductivity of vanadium dioxide is reduced to 20 S/m, it transitions to an insulating state. The structure designed in this paper achieves Linear-To-Circular (LTC) polarization conversion across multiple frequency bands, including 1.02~1.21 THz, 1.39~2.34 THz, 2.81~3.44 THz, and 3.60 THz. It also enables Linear-To-Linear (LTL) polarization conversion in the frequency ranges of 1.21~1.39 THz, 2.46~2.81 THz, and 3.44~3.57 THz. Additionally, the paper investigates the effects of terahertz wave incidence angle and polarization angle on the structural absorption, as well as how structural parameters influence both absorption and polarization conversion characteristics. The results show that, when the incidence angle is within the range of 0° to 40°, the designed structure maintains high absorption performance. Furthermore, the absorption performance remains unaffected by changes in the polarization angle, demonstrating that the device exhibits polarization insensitivity. In summary, the device proposed in this paper not only realizes the three functions of broadband absorption, linear-to-circular polarization conversion, and linear-to-linear polarization conversion, but it also features a simple design and is easy to manufacture. These advantages position the device as a promising candidate for potential applications in terahertz communication, imaging, and other intelligent fields..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0416001 (2025)
Large-scale Fabrication of SiO2 APC and Patterning Application by Spray Coating
Congcong CHI, Xinggen XU, Xin XU, Danjie ZHANG, Jiahao LI, and Jiangxue REN
Photonic crystals have attracted great attention from researchers and scientists all over the world due to their distinct optical characteristics. There are two different kinds of photonic crystals, crystalline Photonic Crystals (PC) and Amorphous Photonic Crystals (APC). In comparison to the former, the amorphous photPhotonic crystals have attracted great attention from researchers and scientists all over the world due to their distinct optical characteristics. There are two different kinds of photonic crystals, crystalline Photonic Crystals (PC) and Amorphous Photonic Crystals (APC). In comparison to the former, the amorphous photonic crystal can present a soft and angle-independent structural color. Among the different nano particles, nano-silica is widely used in the assembly of amorphous photonic crystals due to its simple preparation process. Spray coating is a common method for fabricating amorphous photonic crystals, which benefits for large-scale construction of APCs. However, during spraying, the rapid evaporation of the solution leads to the formation of a long-range disordered structure, which leads to incoherent scattering and finally reduces color saturation. In order to resolve the above problem, large-scale construction of SiO2 APCs on Polyvinyl Chloride (PVC) substrate was made by spray coating process herein. Monodisperse silica nanoparticles were first prepared by the modified stober method and dispersed in ethanol solution containing Acetylene Carbon Black (ACET). A certain amount of Polyvinylpyrrolidone (PVP) was added to improve the dispersion of the system to avoid strong agglomeration between the nanoparticles, which was fully sonicated and spray-coated. To protect the amorphous photonic crystal structure, aqueous acrylic resin was sprayed on the surface of the assembled APCs. The resin was able to encapsulate the microspheres well without destroying the internal structure of the APCs. To realize the patterned application of the APCs, a mask was added to block part of the spray liquid, and a simple spraying process was used for rapid construction of large-area SiO2 APCs and patterning application. It is found that APCs thickness has a significant effect on color saturation. The color saturation decreases with increased number of coating times (from 1 to 3 layers), which is attributed to more incoherent scattering for more layers. The addition of ACET can reduce incoherent scattering and results in enhanced color saturation. PVP is helpful for increasing the lattice spacing of photonic crystals, which finally improves the orderliness of APC structure. At the PVP dosage of 1.5 wt%, best color rendering effect with good color saturation is shown. Furthermore, the aqueous acrylic resin can penetrate into the gaps among the SiO2 microspheres to accomplish effective encapsulation. This not only prevents the silica microspheres from erosion by external environment, but also significantly improves the overall structural stability. Thus, a new method of hollow template technique for fast and large-scale fabrication of different kinds of patterns is provided. This study has guiding significance for the preparation of stable and highly saturated structural color films, which shows great potential in the fields of color display, functional coating and textile materials, et al..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0416002 (2025)
Characteristics of Terahertz Wave Frequency Upconversion Detection Based on RbTiOPO4 Crystal
Na MING, Shuzhen FAN, Xingyu ZHANG, Xiaohan CHEN, Zhenhua CONG, Zhaojun LIU, Dechun LI, Liyuan GUO, Binzhe JIAO, Jiasheng YUAN, Shiwu WANG, Kaiyu WANG, Naichang LIU, and Xutao DAI
Terahertz (THz) frequency upconversion detection technology is an important subject in the field of THz measurement. It offers several advantages, including the capability for room-temperature operation, fast response, high sensitivity, and high dynamic range.The nonlinear inorganic crystals suitable for THz frequency Terahertz (THz) frequency upconversion detection technology is an important subject in the field of THz measurement. It offers several advantages, including the capability for room-temperature operation, fast response, high sensitivity, and high dynamic range.The nonlinear inorganic crystals suitable for THz frequency upconversion detection include MgO∶LiNbO3 (LN), KTiOPO4 (KTP), KTiOAsO4 (KTA), and RbTiOPO4 (RTP). The spectral range of the THz parametric source based on LN crystal can be 0.7~2.9 THz, while in the frequency range from 3.0 to 6.5 THz, KTP, KTA, and RTP crystals must be used in combination to achieve essential coverage. Currently, the characteristics of the THz frequency upconversion detection based on LN and KTP crystals have been reported. In this paper, the THz wave frequency upconversion detection characteristics based on RTP crystal are studied.The experimental setup of the THz parametric upconversion detection based on RTP crystal was mainly composed of three parts the pump source, the THz parametric generator, and the THz frequency upconversion detection part. The pumping source was a 1 064.1 nm Q-switched laser with a pulse width of 420 ps and a pulse repetition rate of 1 Hz. Two identical RTP crystals were used in the experiment. One crystal was used as the THz generator to generate THz waves, the other was used to realize frequency upconversion detection.The physical base of THz wave generation and frequency upconversion detection based on RTP crystal is Stimulated Polariton Scattering (SPS). The necessary condition for SPS is the existence of transverse A1 modes which are both infrared and Raman active in the crystal. The THz wave generation in RTP crystal was caused by the stimulated scattering of the polaritons associated with the most intensive transverse A1 mode of 271 cm-1. But the existing of many much weaker transverse A1 modes of 211 cm-1, 170 cm-1, 161 cm-1, 145 cm-1, 107 cm-1, 83 cm-1, 51 cm-1 made the Stokes and terahertz waves intermittently tunable. So the generated THz waves were mainly in four discrete wavelength regions, which were from 3.01 to 3.09 THz, from 3.53 to 4.17 THz, from 4.54 to 4.66 THz, and from 5.35 to 5.89 THz. The THz waves with center frequencies of 3.84 THz and 5.58 THz were selected for measurement in this study.In the SPS process, the pumping, Stokes, and terahertz waves must obey the energy conservation law and momentum conservation law. Because the refractive indexes of RTP crystal in the terahertz range are very large, only noncollinear phase matching can be realized. The angle between the pumping and Stokes beams (expressed as θ) and the angle between the pumping and THz beams (expressed as β) are dependent on the THz frequency. θ is very small, usually between a few tenths of a degree and a few degrees. However, β is rather large, distributed around 58°. The shape of the RTP crystal in the X-Y plane was designed as isosceles trapezoid with a base angle of 58°, a baseline length of 18.0 mm, and a waist length of 11.8 mm. The thickness in the Z direction was 5.0 mm. Two waist surfaces were both AR coated in the range of 1 060~1 100 nm. When the crystal was used as the THz generator, the pump light was vertically incident from one waist of the isosceles trapezoid, reflected by the bottom side, and left the crystal from the other waist. It was because of the design of the isosceles trapezoid with a base angle of 58° that the generated terahertz wave could be output almost vertically from the bottom side.The THz wave was focused and injected into the second RTP crystal through two off-axis parabolic mirrors (OAP1 and OAP2). The THz energy was attenuated by adding black polyethylene terephthalate (PET) plates between OAP1 and OAP2. A vertical slit was set in the THz optical path between the two parabolic mirrors, and the specific center frequency and bandwidth of the injected THz wave were selected by adjusting the transverse position and width of the slit, respectively. The upconverted signal spectral composition and intensity were detected by an optical spectrum analyzer and a highly sensitive photodiode, respectively.The upconverted signal pulse energy (expressed as the equivalent voltage displayed on the oscilloscope) showed a good linear relationship with the input THz pulse energy. The minimum detectable energy achieved at 3.84 THz was 2.29 fJ and the dynamic range was 52.7 dB. The minimum detectable energy obtained at 5.58 THz was 0.311 fJ and the dynamic range was 56.3 dB. The interaction length within the RTP crystal for the pump light, THz wave, and upconverted Stokes signal did not exceed 15 mm. The minimum detectable pulse peak power obtained from the RTP-based THz frequency upconversion detection system was comparable to that of the KTP-based system, while the interaction length for the pump light, THz wave, and upconverted Stokes signal in KTP crystal was larger than 90 mm. Regarding the frequency coverage of inorganic crystal frequency upconversion detection systems, the tuning range of a single crystal is discontinuous, the combination uses of RTP, KTP and KTA crystals can cover the spectral range from 3.0 to 6.5 THz..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0419001 (2025)
Characteristics of the Light Field Near Focal Point of a Nonparaxial Convergent Beam Limited by a Circular Aperture
Xuan GUO, and Fuyuan GUO
In the classical diffraction theory, the Debye integral is an important formula for analyzing the characteristics of the light field near the focal point of a converging light wave. However, since the effect of the inclination angle of the wave vector of the convergent light wave on the diffraction integral formula is In the classical diffraction theory, the Debye integral is an important formula for analyzing the characteristics of the light field near the focal point of a converging light wave. However, since the effect of the inclination angle of the wave vector of the convergent light wave on the diffraction integral formula is ignored, the Debye integral is an approximation formula. In order to further clarify the necessity of the inclination factor on observation side of the diffraction integral formula for calculating the characteristics of light field near the focal point of nonparaxial convergent light wave, a nonparaxial convergent light wave whose amplitude varies with the square root of the cosine of the inclination angle of wave vector limited by a circular aperture is used as the diffraction source, the diffraction integral formula with normalized inclination factor expressed by the square root of the cosine of inclination angle on observation side and the Debye integral are engaged to analyze the characteristics of light field on the focal plane of this diffraction source respectively.Based on the diffraction integral formula with the normalized inclination factor on observation side expressed by the square root of the cosine of the inclination angle of wave vector, the functional expression of light field distribution on focal plane of this diffraction source is presented. Such light field distribution on the focal plane of this special diffracted source is in the form of the Airy spot. Moreover, the total power of the focal plane light field is equal to the total power of the light field of this special diffracted source. It satisfies the law of conservation of radiation energy of traveling wave field, and verifies the rationality of the normalized inclination factor expressed by the cosine square root of the inclination angle on observation side. It shows that the diffraction integral formula with the normalized inclination factor expressed by the square root of the cosine of inclination angle on observation side is suitable for analyzing the characteristics of optical field on focal plane of paraxial and nonparaxial converging light wave.When the Debye integral is used to analyze the characteristics of light field on the focal plane of the same diffraction source, the light field distribution on the focal plane of the same diffraction source deviates from the Airy spot form, the amplitude of focal point deduced by the Debye integral is greater than the actual value, and the total power of the focal plane light field is greater than the total power of the light field of this special diffraction source. It does not satisfy the law of conservation of energy, and thus identifies the limitation of the Debye integral. It shows that the Debye integral is not suitable for analyzing the characteristics of light field on focal plane of the nonparaxial convergent light wave.When the diffraction integral formula with normalized inclination factor expressed by the square root of the cosine of inclination angle on observation side is used to analyze the characteristics of light field on the vertical facet which very close to the focal plane and the characteristics of light field at the point on axis which very close to the focal point of this diffraction source, the amplitudes of light field on several vertical facet which very close to the focal plane of the nonparaxial convergent light wave with a maximum inclination angle of (1/3)π rad are computed, the amplitudes and phases of light field at point on axis which very close to the focal point of this nonparaxial convergent light wave with a maximum inclination angle of (1/3)π rad are also computed. If the maximum inclination angle of this nonparaxial convergent light wave is equal to theoretical value 0.5π rad, the expression of complex amplitude of light field at point on axis which very close to the focal point is derived, the characteristics of amplitude and phase of light field at point on axis which very close to the focal point of this nonparaxial convergent light wave are presented. Thereinto, when the absolute value of axial defocusing is far greater than the wavelength of light wave, the characteristic of phase of light field at point on axis of this nonparaxial convergent light wave with a maximum inclination of 0.5π rad is similar to the Gouy phase shift of the fundamental mode Gaussian beam, this characteristic has its physical meaning and indicates that this formula is worthy of further study.Based on the condition of paraxial approximation, the expression of complex amplitude of light field at point on axis which very close to the focal point of a paraxial convergent light wave whose amplitude varies with the square root of the cosine of the inclination angle of wave vector limited by a circular aperture is derived. The characteristics of amplitude and phase of light field at the point on axis which very close to the focal point of this paraxial convergent light wave are also presented. It shows that the diffraction integral formula with the normalized inclination factor expressed by the square root of the cosine of inclination angle on observation side is suitable for analyzing the characteristics of light field near the focal point which diffracted from a paraxial convergent light wave and a nonparaxial convergent light wave whose amplitude varies with the square root of the cosine of the inclination angle of wave vector within a certain inclination angle..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0426002 (2025)
Fiber Optics and Optical Communications
High-gain L-band Extended Fiber Amplifier Using Bismuth-erbium Co-doped Fiber
Zhikai WU, Xinyong DONG, Yongfang ZOU, Jianxiang WEN, Tingyun WANG, Song WANG, Yuncai WANG, and Yuwen QIN
The rapid development of emerging technologies such as 5G, the Internet of Things, and artificial intelligence has created an ever-growing demand for high-capacity data transmission, increasing the pressure on current optical fiber systems. However, the bandwidth limitations of optical fiber amplifiers constrain the nuThe rapid development of emerging technologies such as 5G, the Internet of Things, and artificial intelligence has created an ever-growing demand for high-capacity data transmission, increasing the pressure on current optical fiber systems. However, the bandwidth limitations of optical fiber amplifiers constrain the number of usable channels, creating a bottleneck in expanding system capacity. Enhancing the gain bandwidth of optical fiber amplifiers is therefore an effective approach to increase the transmission capacity. In long-distance fiber transmission systems, the C-band/L-band Erbium-Doped Fiber Amplifier (EDFA) configuration is often employed to amplify optical signals across parallel structure. However, traditional L-band EDFAs generally cover a gain bandwidth of only 40 nm (1 565~1 605 nm), while the International Telecommunication Union defines the L-band as 60 nm (1 565~1 625 nm). This has driven significant interest in extending the L-band gain bandwidth toward longer wavelengths. As a result, recent years have seen growing research efforts toward developing L-band extended EDFAs. However, the development of L-band extended EDFAs still faces challenges such as limited gain bandwidth, insufficient gain levels, and poor gain flatness.In this study, we explore the gain extension properties of bismuth-erbium co-doped fiber (BEDF), which offers promising potential for extending the gain bandwidth of L-band EDFAs. We designed and developed a two-stage L-band extended BEDF amplifier, which incorporates a double-pass amplification structure in the main amplifier stage. This configuration not only aims to broaden the gain bandwidth but also enhances the gain performance and suppresses noise figure effectively. Our research confirmed that BEDF can effectively extend the L-band gain bandwidth, providing new insights for further advancements in L-band extended EDFA technology. To achieve the L-band gain extension, co-doping erbium-doped fibers with elements such as phosphorus (P), ytterbium (Yb), and aluminum (Al) can shift the Excited State Absorption (ESA) spectrum of erbium ions, effectively broadening the gain spectrum into the longer L-band wavelengths. The addition of bismuth (Bi) ions into the fiber composition further enhances the emission cross-section of erbium ions, contributing significantly to the extended gain bandwidth. By incorporating Bi ions, the BEDF amplifies more effectively, as these ions both broaden the gain bandwidth and increase emission intensity.To enhance gain and suppress noise figure, we implemented a two-stage L-band extended bismuth-erbium co-doped fiber amplifier, where the main amplifier and pre-amplifier stages are connected via a circulator. The pre-amplifier uses a 2 m-long Er-doped fiber pumped by a 980 nm semiconductor laser at 100 mW. The main amplifier consists of a segment of BEDF that is bidirectionally pumped by two 1 480 nm semiconductor lasers through a Wavelength Division Multiplexer (WDM), with forward and backward pump powers of 400 mW and 500 mW, respectively. A Faraday Rotator Mirror (FRM) is positioned at the end of the amplifier, which reflects the amplified signal back through the BEDF, effectively enhancing the gain performance.The study tested BEDF lengths of 3.6 m, 5.8 m, and 6.8 m, analyzing gain and noise figure characteristics for each configuration. Results indicated that the gain spectra for all three fiber lengths appeared in the range of 1 560~1 620 nm, with notable shifts as fiber length increased. Specifically, longer BEDF lengths resulted in a gain peak shift towards longer wavelengths, while gain at shorter wavelengths diminished, leading to a narrowing of the gain bandwidth. At a BEDF length of 6.8 m, the gain level was significantly lower than that at 5.8 m, suggesting an optimal BEDF length for effective amplification. Across all BEDF lengths, the amplifier achieved a gain level above 11.36 dB at 1 620 nm, demonstrating effective L-band gain bandwidth extension, which is attributed to two intrinsic mechanisms: the broadening of the emission cross-section of erbium ions by Bi ions, and the energy transfer from Bismuth Active Centers (BACs) to erbium ions. This transfer enhances emission intensity and efficiency, especially in the presence of BAC-Ge, which exhibits an emission peak at 1 610 nm, directly supporting L-band gain bandwidth extension. The double-pass amplification structure further enhances gain and suppress noise figure. The signal reflected by the FRM undergoes a second amplification in the BEDF, effectively increasing the overall length of the gain fiber. Testing with double-pass BEDF lengths of 3.6 m and 4.8 m showed that the 3.6 m BEDF achieved a wider 20 dB gain bandwidth, higher maximum gain, and a lower minimum noise figure, although with slightly lower gain flatness compared to the 4.8 m configuration. The 4.8 m BEDF showed a slight reduction in 20 dB gain bandwidth and maximum gain, but with improved average gain and gain flatness.Comparison of the single-stage main amplifier and the two-stage amplifier within the 1 560~1 615 nm range revealed that the two-stage amplifier delivered a higher gain than the main amplifier, with comparable gains in the 1 615~1 620 nm range. This is because the pre-amplifier increases the overall pump power, resulting in a shift of the central wavelength towards shorter wavelengths and thus enhancing gain at shorter wavelengths while slightly reducing gain at longer wavelengths. In terms of noise performance, the two-stage amplifier exhibited a lower noise figure within the 1 560~1 610 nm range compared to the main amplifier, with similar noise levels in the 1 610~1 620 nm range. The noise figure improvement is attributed to the primary role of the first stage in determining noise figure, where the pre-amplifier contributes to increased gain and noise suppression.This study achieved a 20 dB gain bandwidth of 63 nm (1 555~1 618 nm), a maximum gain of 52.84 dB, a minimum noise figure of 4.23 dB, and a 3 dB flatness gain bandwidth of 35 nm (1 565~1 600 nm). Compared with recently reported L-band extended EDFAs, our configuration achieved a broader 3-dB flatness gain bandwidth, while maintaining high gain and wide high-gain bandwidth. These results underscore the potential of BEDF for advancing optical amplifier technology and supporting high-capacity, long-distance optical transmission systems. Further optimization in the co-doping composition and ratio of elements in BEDF is expected to extend gain bandwidth even further, paving the way for innovations in optical amplifier design..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0406001 (2025)
Highly Sensitive Refractive Index Sensor Based on Hybrid Optical Waveguide Field Enhancement
Zhengda LI, Hui LI, Chengzhao DENG, Xueying LIN, Yao LU, Ying XIAO, and Hongdan WAN
Optical fiber sensors are capable of converting the external environmental information they perceive into optical signal output in a specific pattern, which can provide the measured information while detecting and feeding back real-time changes in the external environment. When the environment changes, such as alteratiOptical fiber sensors are capable of converting the external environmental information they perceive into optical signal output in a specific pattern, which can provide the measured information while detecting and feeding back real-time changes in the external environment. When the environment changes, such as alterations in Refractive Index (RI), pH, temperature, humidity, biomolecules, etc, some characteristics of the optical wave signal inside the optical fiber sensing unit will also change. The fiber RI sensor based on the hybrid optical waveguide utilizes the cascading of different fiber waveguides to excite the intermodal interference effect, and has the advantages of small size and high sensitivity. However, there is still room for improvement in RI sensitivity, and one effective approach is to employ fiber micro-nano processing techniques to boost the evanescent field intensity, thus enhancing the RI sensitivity. Due to the merits of a simple manufacturing process, low cost, and high stability, this kind of sensor can be applied in a variety of complex industrial environments. This paper proposes and investigates a high-sensitivity RI sensor based on a Single Mode Fiber-Photonic Crystal Fiber-Single Mode Fiber (SMF-PCF-SMF) hybrid optical waveguide. This structure is axially fusion-spliced in sequence by SMF, PCF, and SMF, the length of the collapse area can be kept within the range of 150~160 μm through controlling the welding parameters, and enables the mutual excitation and coupling of the core mode and the cladding mode in the two-side collapse area. Theoretically analyze the sensing principle, and utilize the beam propagation algorithm to simulate the light-field energy distribution and exchange process, thereby obtaining its output spectrum. The simulation results indicate that when the light field enters the PCF from SMF, the high-order mode in the PCF is excited as it passes through the first collapsed area, with the energy being reduced and energy exchange taking place. After traveling a certain distance within the PCF, it reaches the second collapsed area. At the junction with the other SMF, the light converges and enters SMF for transmission, and the light energy is enhanced. Furthermore, when the PCF section is tapered, compressing the cladding thickness of the PCF section can increase the sensitivity of the structure to the change in the external RI, enhance the spectral redshift of the SMF-PCF-SMF hybrid optical waveguide structure, and raise its RI sensing sensitivity. The methods for achieving the enhancement of the evanescent field are investigated. Based on three micro-nano processing techniques, namely the fiber cladding polishing method, the hydrofluoric (HF) acid fiber cladding etching method and the fused biconical taper method, the device preparation is accomplished. The interference spectral characteristics of the SMF-PCF-SMF hybrid optical waveguide structure obtained through three processing techniques are experimentally tested and compared. Moreover, a microfluidic fixture (manufactured by using polytetrafluoroethylene material in combination with precision micromachining methods) is utilized to package the sensor. The RI sensing characteristics of liquid samples in SMF-PCF-SMF structures with three polishing depths, in SMF-PCF-SMF structures with HF acid etching, and in SMF-PCF-SMF structures where the cladding thickness of the PCF in the middle section is compressed by the fused biconical taper method are respectively tested and compared. The experimental results demonstrate that all three micro-nano processing methods have enhanced the linearity of RI sensing. Furthermore, by compressing the cladding thickness of the PCF in the middle section of the SMF-PCF-SMF structure through the fused biconical taper method, the obtained RI sensitivity of the liquid sample is the highest, reaching 1 405.36 nm/RIU, which is approximately 24 times higher. This RI sensor is characterized by its small size, easy integration and high sensitivity, and it has significant application value in the fields of RI sensing, industrial production, etc..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0406002 (2025)
Analysis of Full-mode Acousto-optic Coupling in Stimulated Brillouin Scattering of Few-mode Fibers
Ziming HUA, Lijuan ZHAO, Zhiyuan XIE, and Zhiniu XU
In Brillouin sensing systems, single-mode fibers are commonly used. However, the input optical power of single-mode fibers is limited by the stimulated Brillouin scattering effect, which constrains their sensing performance. Compared with single-mode fibers, few-mode fibers have larger core radius, allowing higher inpuIn Brillouin sensing systems, single-mode fibers are commonly used. However, the input optical power of single-mode fibers is limited by the stimulated Brillouin scattering effect, which constrains their sensing performance. Compared with single-mode fibers, few-mode fibers have larger core radius, allowing higher input optical power and enabling the transmission of multiple relatively independent modes simultaneously, which holds the potential for improved sensing performance. Brillouin scattering in few-mode fibers can be characterized by parameters such as Brillouin gain peak, Brillouin frequency shift, and linewidth. These parameters vary with modes and exhibit different response characteristics to various physical quantities. Brillouin scattering in few-mode fibers offers the potential for simultaneous measurement of multiple physical quantities. Therefore, studying the Brillouin scattering characteristics of different modes in few-mode fibers is crucial, as it helps to better understand how each mode contributes to the overall scattering response and enables the optimization of the fiber's performance for multi-parameter sensing applications. However, existing research still has certain limitations. Reports on the full-mode acousto-optic mode coupling in few-mode fibers, as well as the effects of core radius and doping concentration on their stimulated Brillouin scattering, are scarce. To investigate the impact of core radius and doping concentration on the full-mode acousto-optic mode coupling of stimulated Brillouin scattering in few-mode fibers, this paper introduces the optical and acoustic waveguide characteristic equations of few-mode fibers under satisfying the core and cladding boundary conditions. Different optical and acoustic mode distributions and normalized field distributions are analyzed using characteristic functions and finite element methods. The relationship between the number of acoustic modes and the normalized acoustic frequency values in few-mode fibers is explored. The full-mode Brillouin gain spectrum of four-mode fibers is analyzed, and the effects of core radius and germanium doping concentration on the Brillouin frequency shift, linewidth, and gain spectrum in few-mode fibers are studied. The variation patterns of Brillouin frequency shift and linewidth in full-mode acousto-optic coupling of ten-mode fibers are summarized. In this paper, the simulations were implemented using MATLAB and COMSOL Multiphysics software. The research results indicate that there is a quadratic relationship between the number of acoustic modes in few-mode fibers and the acoustic normalized frequency value. The full-mode acousto-optic coupling in few-mode fibers can be divided into effective and ineffective acousto-optic mode coupling. Effective acousto-optic coupling can be divided into two cases: one is the coupling between the Lln mode family (l=0) and any LPmn mode, and the other is the coupling between the Lln mode family (l=2m) and the LPmn mode (m>0). When the germanium doping concentration is between 1.5% and 4.0%, the linewidth of the full-mode acousto-optic mode coupling increases with the increase of germanium doping concentration, and when the concentration exceeds 2.0%, the linewidth shows a linear increasing trend. Brillouin frequency shift variation: when the LPmn mode families are coupled with the L0n mode family, the Brillouin frequency shift decreases with increasing optical mode order; when the LPmn mode families (m>0) are coupled with the Lln mode families (l=2m), the Brillouin frequency shift approaches the total Brillouin gain spectrum frequency shift. Line width variation: when the LPmn mode families (m=0) are coupled with the Lln mode families (l=0), the line widths of the LP02 and LP03 modes are much larger than those of the LP01 mode; when the LPmn mode families (m>0) are coupled with the Lln mode families (l=0) and the Lln mode families (l=2m) respectively, and the linewidth of the same optical mode family LPmn increases as the optical mode order increases when it is coupled with the acoustic mode. The research results of this paper are expected to provide certain reference value for stimulated Brillouin scattering in few-mode fibers..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0406003 (2025)
Optical Fiber Polymer Microcavity Humidity Sensor Based on Laser-induced Waveguide Self-growth Technology
Huaijin ZHENG, Shengjie LI, Yixin ZHU, Qianhao TANG, Yongqin YU, Chunbo LI, Chenlin DU, and Shuangchen RUAN
Humidity sensors are crucial in a multitude of sectors, including environmental monitoring, industrial processes, and life sciences. As technology advances, the demand for enhanced performance in humidity sensors has been on the rise. Compared to conventional capacitive and resistive humidity sensors, optical fiber humHumidity sensors are crucial in a multitude of sectors, including environmental monitoring, industrial processes, and life sciences. As technology advances, the demand for enhanced performance in humidity sensors has been on the rise. Compared to conventional capacitive and resistive humidity sensors, optical fiber humidity sensors have recently gained significant attention due to their compact size, swift response, robust anti-interference capabilities, extended transmission range, heightened sensitivity, and remarkable stability. Among them, Fabry-Perot microcavity sensors crafted on optical fiber facets have become a focus of research, particularly due to their straightforward fabrication and integration with humidity-responsive materials. Laser-induced waveguide self-growth technology, a method capable of fabricating beam-profile waveguides within photosensitive medium materials according to the intensity distribution of the incident beam, enables rapid preparation of sensors with polymer microcavity structures on optical fiber facets. The photosensitive polymers on the fiber facets undergo chain polymerization reactions induced by laser light, forming a highly cross-linked three-dimensional network structure. The humidity-sensing mechanism of polymer microcavity humidity sensors stems from the swelling or dehydration of the internal three-dimensional network structure upon environmental humidity changes, which leads to variations in the cavity length and refractive index of the polymer, thereby inducing spectral wavelength shifts in the interference spectra. Monitoring these wavelength shifts enables humidity sensing. This paper, embarking from the sensor's structure and principle, delves into the detailed investigation of the sensor's manufacturing process, humidity sensing performance, and respiratory monitoring. It proposes a cost-effective and rapidly fabricable humidity sensor with a polymer microcavity structure on the optical fiber facet. Leveraging laser-induced waveguide self-growth technology, the paper introduces a secondary self-growth method to achieve a higher surface-to-volume ratio and interference spectral contrast. Through precise control of laser power, exposure time, and utilizing the integrated stepper motor of a fusion splicer, a polymer microcavity with a cavity length of 47.65 μm was successfully created on the fiber facet. Changes in the cavity length and refractive index of the polymer microcavity in response to ambient humidity variations result in interference spectral shifts, enabling the accurate detection of relative humidity through spectral shift demodulation. The experimental results indicate that the sensor exhibits an interference spectral contrast of 21.6 dB and a free spectral range of 16.6 nm. It possesses an average sensitivity of 150 pm/%RH within a relative humidity range of 40%RH to 90%RH. Linear fitting of the mean wavelength shifts corresponding to different humidity points in multiple humidity rise-and-fall experiments yields a linearity of 0.985 83. During a 5-hour stability test, the maximum fluctuation in wavelength and reflected optical intensity was 0.208 nm and 0.082 dB, respectively. To further explore the sensor's practical application potential, a real-time breathing monitoring experiment lasting 3 minutes was conducted, revealing a response time of merely 0.8 s and a recovery time of 5.7 s. In summary, the proposed optical fiber humidity sensor, with its simple fabrication process, high sensitivity, rapid response, and excellent stability, shows great potential for applications in meteorological environmental monitoring and wearable healthcare fields. In the future, it is anticipated that the integration of polymers with micro- and nano-structured materials, such as graphene and MoS2, will be employed to fabricate end-face microstructures, thereby achieving an enhancement in humidity sensitivity..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0406004 (2025)
Vehicle-induced Modal Monitoring of Bridges Using Partially Distributed FBG Strain Sensor Arrays
Zhenhui DUAN, Faxiang ZHANG, Shaodong JIANG, Zhihui SUN, Zhaoying LIU, and Yongxu SHEN
Bridges are a crucial component of urban transportation systems, and their safety is directly linked to the life safety of road users. Real-time vibration and frequency monitoring of bridge structures can help identify potential issues at an early stage. By analyzing natural frequencies and vibration modes of bridges, Bridges are a crucial component of urban transportation systems, and their safety is directly linked to the life safety of road users. Real-time vibration and frequency monitoring of bridge structures can help identify potential issues at an early stage. By analyzing natural frequencies and vibration modes of bridges, early warnings for damage detection can be provided. While electronic accelerometers are commonly used, they are point-based and only offer localized measurements. Fiber optic strain sensors have recently attracted attention for vibration mode monitoring of bridges. To address challenges associated with high monitoring costs, complex sensor deployment, and intricate construction when using fiber optic sensors for bridge vibration mode monitoring, a monitoring system is proposed based on a localized array of Fiber Bragg Grating (FBG) strain sensors for the online vibration mode monitoring of medium- and small-span bridges. A high-resolution, long-range FBG strain gauge array is designed, consisting of a single optical fiber embedded with FBG sensors. The fiber is coated with a Glass Fiber Reinforced Polymer (GFRP) layer for protection and structural reinforcement. The sensor array contains seven monitoring points, each equipped with an FBG strain sensor. The sensors are mounted using a Fiber Bragg Grating (FBG) -based platform, which facilitates both sensor fixation and prestress adjustment. A Gray Wolf Optimization (GWO) -based Variational Mode Decomposition (VMD) method is introduced for the extraction of bridge modal parameters, including frequency, damping ratio, and mode shape. Through bridge model simulation experiments, the FBG strain gauge array is installed using a bonding technique. After extracting the bridge's resonance signals via the GWO-VMD method, a secondary interpolation process is employed to fit and extract strain values corresponding to the peak and trough points for the first-order modal analysis. The traditional Stochastic Subspace Identification (SSI) method is then utilized for first-order modal extraction, and the results from both methods are compared. The Pearson correlation coefficient between the two methods is 0.943 75, highlighting the effectiveness of the globally installed FBG strain gauge array in combination with the GWO-VMD method for modal parameter extraction. For practical validation, the system was tested on the Nansongshuigang Bridge along the national key highway from Rizhao to Nanyang. The FBG strain gauge array was positioned at the midspan of the bridge, with four sensor arrays evenly distributed along the bridge's underside. The results demonstrate that the designed FBG strain gauge array effectively captures strain signals induced by passing vehicles, achieving a dynamic strain resolution of 0.1 με. The GWO-VMD method successfully isolates the non-stationary vehicle-induced signals from the bridge's resonance signals. The second Intrinsic Mode Function (IMF2), which encapsulates the bridge's modal behavior, is used for modal analysis. Peak-trough analysis of IMF2 reveals a curve characterized by an initial increase followed by a decrease, with the peak corresponding to the midspan of the bridge and the trough exhibiting the opposite trend. Normalization of the results indicates that the first mode shape derived from the proposed method has a Pearson correlation coefficient of 0.95248 with the finite element simulation results. When compared to traditional electronic sensor monitoring methods, the proposed FBG strain gauge array demonstrates a first-order modal frequency error of less than 1% and a damping ratio error of less than 5% under vehicle excitation. The bridge's natural frequency exhibits a pattern of initial decrease followed by an increase, while temperature variations show an opposite trend. Additionally, the FBG strain gauge array is sensitive to small variations in the bridge's peak frequency due to temperature changes. The online monitoring system and methodology presented in this study enable the extraction of key bridge modal information-such as natural frequency, mode shape, and damping ratio-using a minimal number of strategically placed sensors. This approach facilitates real-time monitoring of the bridge's dynamic characteristics with low-cost implementation, providing a novel approach for assessing the structural health of bridges..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0406005 (2025)
Pressure Type Ocean Wave Height Sensor Based on Fiber Bragg Grating
Zhaokun LIU, Dawei DU, Qiang ZHAO, Dongying CHEN, Li HUI, Wen ZHANG, Peng SUN, and Fengxiang GUO
Ocean waves are one of the most important and complex elements in ocean hydrology. Effective monitoring of ocean waves is crucial for various applications, including nearshore production, marine scientific research, and the prediction of undersea earthquakes. In this paper, a pressure-type ocean wave height sensor baseOcean waves are one of the most important and complex elements in ocean hydrology. Effective monitoring of ocean waves is crucial for various applications, including nearshore production, marine scientific research, and the prediction of undersea earthquakes. In this paper, a pressure-type ocean wave height sensor based on Fiber Bragg Grating (FBG) is designed for nearshore wave height measurement. The sensor's pressure measurement principle is based on the elastic diaphragm made of beryllium bronze. The external seawater pressure acts directly on the elastic diaphragm, which makes the center of the diaphragm change in deflection. Then it changes the effective length of the Pressure Fiber Bragg Grating (P-FBG), leading to the blue shift of its center wavelength, and detects the seawater pressure through the measurement of the wavelength change amount. For temperature compensation, a Temperature Fiber Bragg Grating (T-FBG) is positioned within the same cavity as the Pressure-Measuring Grating (P-FBG). The T-FBG is solely sensitive to temperature. By correlating the center wavelength drifts of the P-FBG and T-FBG through a dedicated formula, the influence of temperature variations on pressure measurements can be dynamically compensated, thereby enhancing measurement accuracy. Utilize the relationship between the underwater pressure wave and the surface wave height to achieve compensation of the wave pressure value, and then calculate the wave height through the upper spanning zero method. The optimal design parameters of the sensor are determined by combining previous laboratory research and simulation analysis. A finite element simulation analysis is conducted after constructing the sensor model to verify its feasibility. Three sinusoidal pressure signals with different amplitudes and periods are used to simulate small, medium, and large waves in the real ocean environment. The simulation results demonstrated that the elastic diaphragm exhibited good stability and responsiveness under the three types of positive pressure signals, with a response time of only 3.8 ms, significantly lower than the actual wave collection frequency, indicating that the response time had a negligible impact on measurement results. When the external ambient temperature changed abruptly from 15 ℃ to 20 ℃, the simulation results indicated that the average absolute temperature difference between the P-FBG and the T-FBG is only 0.03 ℃, suggesting that both experienced the same temperature change. Performance calibration experiments for pressure and temperature are conducted on the FBG ocean wave height sensor, revealing that the P-FBG wave sensor had a sensitivity of -9.486 nm/MPa, a linear correlation coefficient of 0.999 97, and a pressure resolution of 0.000 1 MPa (water depth resolution is 1 cm) within the pressure measurement range of 0 to 0.3 MPa. The temperature sensitivities of the P-FBG and the T-FBG were 24.9 pm/℃ and 30.6 pm/℃, respectively, with resolutions better than 0.04 ℃ and a linear correlation coefficient of 0.999 87 within the temperature measurement range of 0 to 35 ℃. In the hydrostatic test, the measured water level fitted the actual value with a degree of fit of 99.919%. The calibration tests confirmed that the FBG ocean wave height sensor can achieve high-precision wave measurement. This paper proposes a zero-line selection method based on the Smoothed Prior Approach (SPA), specifically designed to address the characteristics of measured ocean wave data. The proposed method effectively resolves the zero omission issue and exhibits superior applicability compared to conventional approaches. A five-day field test of the FBG ocean wave height sensor and the SBF3-2 wave buoy was conducted at the Luhaifeng Ocean Ranch. The results showed that the sensor and the SBF3-2 wave buoy obtained the same trend in wave height values, with a correlation coefficient of more than 0.85. The proposed pressure-type fiber grating ocean wave height sensor features underwater passivity, anti-electromagnetic interference, and fast signal transmission, offering a novel optical measurement method for nearshore wave height measurement and demonstrating promising application prospects..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0406006 (2025)
Instrumentation, Measurement and Metrology
Research on Absolute Measurement Technology of Infrared Radiance Based on Frequency Transfer
Yuhao LIU, Maopeng XIA, Xiaobing ZHENG, and Wenchao ZHAI
The prerequisite for achieving precise mid-infrared (3 000~5 000 nm) radiation measurement is calibration. In recent years, the technology using correlated photons generated by Spontaneous Parametric Down-Conversion (SPDC) for calibration has attracted widespread attention as an absolute calibration method requiring noThe prerequisite for achieving precise mid-infrared (3 000~5 000 nm) radiation measurement is calibration. In recent years, the technology using correlated photons generated by Spontaneous Parametric Down-Conversion (SPDC) for calibration has attracted widespread attention as an absolute calibration method requiring no external standard transfer. This approach, based on an objective physical effect, enables self-calibration. However, limited by the absence of infrared single-photon detectors, research surveys indicate its primary applications have been confined to calibration within the visible-near-infrared spectral range. Up-conversion technology, as a novel infrared single-photon detection method, enables the detection of correlated photon signals in the mid-infrared band.This paper first introduces the principle and mathematical expression for determining channel detection efficiency through correlated photon calibration. Subsequently, an experimental optical path combining parametric down-conversion and up-conversion technologies was constructed, with detailed descriptions of the setup procedures, particularly emphasizing the principles of correlated photon focusing and beam combining. The experiment initially utilized a 775 nm laser to pump a Periodically Poled Lithium Niobate (PPLN) crystal, generating 1 005 nm/3 390 nm correlated photon pairs via SPDC. The 1 005 nm signal photons were routed through the trigger path, while the 3 390 nm idler photons were directed to the measurement path. To address the challenge of lacking mid-infrared single-photon detectors, the 3 390 nm idler photons in the measurement path were up-converted to 810 nm through sum-frequency generation with 1 064 nm pump light in another PPLN crystal. Silicon-based near-infrared single-photon detectors of the same model were used to detect the 1 005 nm and 810 nm photons separately. The outputs from both near-infrared single-photon detectors were connected to a Time-to-Digital Converter (TDC) for time-correlated coincidence measurements. Practical experimental issues including dead time, afterpulses, accidental coincidences, and differences in non-common optical paths were analyzed. A 3 390 nm laser was employed to measure the transmittance of non-common optical paths at 3 390 nm. Based on the modified expression for calculating channel efficiency in correlated photon calibration, the detection efficiency of the measurement path for 3 390 nm was determined by removing the influence of non-common optical path efficiency using the measured coincidence counts and trigger photon counts.The study further investigated the limitations on the area and solid angle of mid-infrared radiation in the measurement path. By comparing the acceptance areas and solid angles for mid-infrared radiation between the up-conversion section and the fiber-coupled collection section, it was concluded that the fiber-coupled collection component imposed the primary constraints on the solid angle and area of mid-infrared radiation. Combining the calibrated detection efficiency of the measurement path, an expression for calculating mid-infrared radiance from up-converted photon counts was derived.The calibrated system was used to measure the radiance of a cavity-type blackbody at 3 390 nm under temperatures of 125 ℃, 150 ℃, 175 ℃, and 200 ℃. Experimental results were compared with theoretical radiance values calculated using Planck's formula, revealing a maximum discrepancy of 4.3%. This demonstrates the potential application prospects of combining up-conversion and down-conversion technologies for absolute radiance measurement beyond 3 000 nm. Finally, possible causes for the observed discrepancies were analyzed, including potential uncalibrated blackbody emissivity and slight laser power fluctuations during the experiment. Future optimizations for the system were proposed, such as incorporating laser power stabilization modules, to further enhance measurement accuracy..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0412001 (2025)
Spectral Confocal Displacement Measurement Method Based on SNV Correction of Surface Roughness
Chunyan LI, Wenwen FU, Jihong LIU, Danlin LI, Shaojie WU, Ninglin WANG, and Kaili REN
As a high-precision non-contact measurement method, spectral confocal displacement measurement technology is widely applied in high-precision fields such as semiconductor manufacturing, precision machining, and material science due to its excellent performance in micro-nano displacement and morphology measurements. HowAs a high-precision non-contact measurement method, spectral confocal displacement measurement technology is widely applied in high-precision fields such as semiconductor manufacturing, precision machining, and material science due to its excellent performance in micro-nano displacement and morphology measurements. However, in actual processing, the sample surface is often not an ideally smooth plane. Instead, it is characterized by varying degrees of micro-fluctuations and surface roughness. A complex scattering phenomenon is caused by surface roughness when incident light interacts with the sample surface. The characteristics of the reflected light beam are altered by this scattering, which in turn leads to the peak wavelength positioning of the spectral signals being affected. As a result, measurement errors are introduced, which lead to a reduction in the accuracy of the displacement measurements. Especially in the high-precision measurement scene, this situation has been recognized as an important bottleneck restricting the improvement of system performance. To enhance the applicability and measurement accuracy of spectral confocal displacement measurement systems, the influence of surface roughness on displacement measurements is systematically analyzed, and an innovative error correction algorithm model is proposed in this paper to address this challenge. Firstly, the working principle of spectral confocal displacement measurement system is introduced. Then, based on the light scattering theory, the small slope approximation (SSA) method is used to analyze the theory. The SSA method can be utilized to analyze the variations in reflectivity characteristics resulting from the interaction between a light beam and a rough surface. Additionally, it can be employed to establish a mathematical relationship model between surface roughness and displacement measurement error. The theoretical foundation is provided by this model for understanding the influence of surface roughness on spectral confocal measurement. Subsequently, the displacement measurement error caused by the drift of the peak wavelength curve due to surface roughness is comprehensively analyzed through theoretical investigation and simulation. The simulation results indicate that the measurement accuracy of the spectral confocal system is reduced by surface roughness. Specifically, with the increase of surface roughness, the deviation of axial displacement becomes more significant. To address this issue, an error correction method combining Standard Normal Variate (SNV) transformation and machine learning techniques is proposed in this paper. Firstly, the SNV transformation is applied to preprocess the spectral data, and a roughness error correction model is established to improve the accuracy of spectral peak identification. To further refine the correction process, the Support Vector Machine (SVM) algorithm is employed to enhance accuracy. The SVM model is trained on spectral data corresponding to different roughness levels to correct the displacement measurement error caused by surface roughness. Carbon steel samples with controlled surface roughness values of 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, and 35 nm are used as test objects for experimental validation. The experimental results validate the effectiveness of the modified model: the relative standard deviation of displacement measurement values is reduced from 1.59% to 0.12% following the application of the SNV-SVM correction method. The significant reduction in measurement deviation demonstrates that the correction model proposed in this study effectively mitigates the influence of surface roughness on spectral confocal displacement measurement. The research results presented in this paper provide a theoretical and practical basis for enhancing the measurement accuracy and applicability of the spectral confocal system. By addressing the challenge of surface roughness, high accuracy is maintained by the system, even when samples with varying roughness levels are measured, through the proposed calibration algorithm. In most cases, the research results presented in this paper hold significant guiding value for improving the measurement accuracy of the system and its applicability to a range of different samples..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0412002 (2025)
Optical Design and Fabrication
Design of a Wide-field Multispectral Imaging Optical System for Martian Dust Storm Detection
Hao GUO, Jianfeng YANG, Xiaolong MA, and Juan LÜ
To achieve comprehensive real-time monitoring and forecasting of Martian dust storms, this study addresses the technical challenges posed by the Mars Color Imager (MARCI) aboard NASA's Mars Reconnaissance Orbiter (MRO), particularly its intricate spectral separation architecture and stringent manufacturing/alignmenTo achieve comprehensive real-time monitoring and forecasting of Martian dust storms, this study addresses the technical challenges posed by the Mars Color Imager (MARCI) aboard NASA's Mars Reconnaissance Orbiter (MRO), particularly its intricate spectral separation architecture and stringent manufacturing/alignment requirements, along with the limitations of China's Tianwen-1 Moderate Resolution Imaging Camera (MoRIC) regarding constrained field-of-view and narrow spectral coverage. We present a novel wide-angle multispectral imaging system designed for subsequent Mars exploration missions. The optical configuration comprises nine spherical lenses, achieving 130° cross-track and 30° along-track fields of view. It operates across a 260~750 nm spectral range with F/11 aperture, 13 mm focal length, 68 mm maximum clear aperture, and 185 mm total length-all meeting design specifications. This design employs a typical reverse magnification structure, utilizing the principal ray tracing formula and the reverse magnification front and rear group chromatic aberration balance relationship. Common ultraviolet optical crystals are incorporated to achieve wide-angle achromatic imaging. After optimizing the optical system, the maximum RMS radius of the spot diagram across the entire field of view does not exceed 10.3 micrometers. The Modulation Transfer Function (MTF) value at the cutoff frequency of 46 line pairs per millimeter across the full field of view exceeds 0.46, indicating excellent imaging quality. Additionally, the relative illuminance on the image plane remains above 74% across the entire field of view, fulfilling the design requirements. Given that the system incorporates three sapphire uniaxial crystals, which may lead to birefringence, CODE V was utilized to simulate two groups of o-light and e-light propagating through the optical system. Light tracing was conducted and analyzed for both groups, revealing that birefringence significantly impacts the optical system's performance. Based on the above, a composite filter integrating both polarization and spectral filtering has been designed. This enables the system to perform multispectral imaging across six spectral bands within the wavelength range of 260 nm to 750 nm, while eliminating the influence of birefringence on the optical system. Moreover, the tolerance analysis reveals that over 90% of the products achieve an MTF value of 0.3 or above at the cutoff frequency of 46 lp/mm across the full field of view, indicating excellent engineering feasibility. Lastly, thermal analysis of the system confirms that within the operational temperature range of -40 °C to 50 °C, the image plane displacement within the depth of field and MTF degradation remain insignificant, showcasing good thermal stability. In summary, the system possesses considerable practical worth and offers an invaluable reference point for the design of subsequent Mars sandstorm detection optical systems..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0422001 (2025)
High-power 2π -LED Standard Lamp Calibrated Based on Total Luminous Flux
Wenrui DUAN, Yi LI, Yang BAI, Limin WEI, Xiaoying ZHANG, Peng ZANG, Heng ZHANG, Yingxin ZHANG, and Ken CHENG
Since the beginning of the 21st century, LED lamps have rapidly replaced traditional incandescent lamps and fluorescent lamps due to the advantages of energy efficiency, long lifespan and environmental benefits, and have become the new standard in the global lighting industry.As LED technology matures and costs decreasSince the beginning of the 21st century, LED lamps have rapidly replaced traditional incandescent lamps and fluorescent lamps due to the advantages of energy efficiency, long lifespan and environmental benefits, and have become the new standard in the global lighting industry.As LED technology matures and costs decrease, the LED lighting industry has experienced significant growth, leading to an increasing demand for standardization. In particular, accurate assessment of the total luminous flux has become a critical task to promote standardization.At present, the usual practice of evaluating the total luminous flux of LED lamps is to compare the total luminous flux of LED lamps with incandescent lamps as the standard. The results show that with the current standards, the measurement error of LED lamps is more than 5%, while the error of incandescent lamp is only 1%. This discrepancy indicates that there is significant uncertainty in using incandescent lamps as a standard for LED measurement, and the present metrological standards cannot meet the standardization needs for LED lamps. Therefore, the development of LED standard lamp is of great significance to simplify the measurement process, reduce the error of total luminous flux measurement, and shorten the transfer chain of the measured value.Many national metrology institutes have actively participated in the research and development of LED standard lights and have made remarkable progress. In present, 2π-LED standard lamps with excellent luminosity stability have been developed in Germany, Russia, the United States, and Japan. Those results provide important support for the establishment of a photometric value transfer system based on LED standard lamps, but the total luminous flux values of the successfully developed 2π-LED standard lamps are generally small, with a maximum of only 2 000 lm. When these low-flux standard lamps are employed to calibrate high-flux LED lamps with luminous flux more than 2 000 lm (such as street lamps and square lighting lamps), it is easy to introduce nonlinear errors in the photometric measurement, resulting in higher uncertainty, which affects the reliability of the photometric value transfer system based on LED standard lamps. Thus, it is particularly important to develop high-flux 2π-LED standard lamps to enhance the measurement accuracy and standardization level of high-flux LED lamps.This study presents a novel high-flux 2π-LED standard lamp designed with 80 LED chips arranged in a concentric circular structure and encapsulated in a circular aluminum lamp panel with the diameter of 162.9 mm and the thickness of 3.0 mm, together with a Thermoelectric Cooler (TEC) for precise temperature control. The lamp has a same color temperature of 4 100 K as the L41 standard lighting and achieves a total luminous flux of 3 000 lm. The temperature field simulations with finite element analysis method were conducted to investigate the temperature distribution and evolution of the 2π-LED standard lamp, which provides an important basis for assessing its reliability with long-term operation. The changes in surface temperatures of the LED chips and the aluminum lamp panel over time are studied experimentally, as well as variations in the total luminous flux, the instability, and the non-repeatability with respect to operating time. The adaptability of 2π-LED standard lamp to TEC cooling temperature and ambient temperature, and the uniformity of spatial luminous intensity are also analyzed. The experimental results show that the technical requirements of the 2π-LED standard lamp are superior to the incandescent standard lamp specified in the ″Standard Method for Total Luminous Flux Measurement of Incandescent Lamps″ (JJG 247-2008) in terms of the ambient temperature adaptability, the luminosity stability, the repeatability, and the luminous uniformity. Within the temperature range of 20 ℃ to 30 ℃, the 2π-LED standard lamp exhibited excellent adaptability to TEC cooling and ambient temperatures, with the temperature coefficients are about 0.10%/°C and 0.01%/℃, respectively. Under conditions of the TEC cooling temperature is 27 ℃ and the ambient temperature is 25 ℃, the preheating time is 150 seconds, and the total luminous flux reaches 3 008 lm. Within 8 h, the photometric instability is only 0.046%.The repeatable and the maximum deviation of the luminous intensity are only 0.048% and 1.9%, respectively, with the color space uniformity is 0.001 2. The research results have significant practical implications for enhancing the photometric parameters of high-power 2π-LED lights, and can provide new insights and foundations for the development of LED lighting technology and the establishment of relevant standards..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0422002 (2025)
Design of Long Working Distance and Wide Wavelength Range Microscope Objective for Industrial Detection
Yangyu SHEN, Teng QIN, Jiyan ZHANG, Muwang HUANG, Liting SUN, Zhengyu LIN, and Tianhao CAO
In the field of industrial detection, the performance of long working distance microscope objectives is of crucial importance, especially in high precision and high resolution application scenarios. Traditional long working distance objectives are often limited by a small numerical aperture and a narrow spectral range,In the field of industrial detection, the performance of long working distance microscope objectives is of crucial importance, especially in high precision and high resolution application scenarios. Traditional long working distance objectives are often limited by a small numerical aperture and a narrow spectral range, which restricts their application in some complex industrial inspection tasks.To overcome these limitations, this article proposes an innovative design scheme that is based on the typical structure of long working distance microscope objectives to optimize the initial structure and performance of the objective. Starting from the requirements of design parameters are based on the theories of third-order aberrations and first-order chromatic aberrations, combines the typical structure of existing long working distance microscope objectives and uses the analytical method to solve the initial structure of the long working distance and large numerical aperture microscope objective, and then optimizes the obtained initial structure. Through this design method, this article successfully designs and optimizes a 20×long working distance and wide wavelength range microscope objective. The design of this objective fully considers key parameters, including working distance, numerical aperture, distortion, and working wavelength range, to ensure its performance in practical applications. The working distance of the designed objective reaches 22.2 mm, the numerical aperture is 0.42, the distortion is controlled within 0.6%, and the working wavelength range is from 430 nm to 900 nm, covering a wide wavelength range from ultraviolet to near-infrared, making it adaptable to various inspection requirements. The lens design must achieve high imaging quality in simulation while complying with manufacturing constraints. The processing and assembly errors of various optical components during the manufacturing process are important factors affecting the overall precision. To ensure the compatibility of the objective with the tube lens system, this article also designs a matching tube lens. Through testing, the results show that this combination can provide high-quality images and meet the high standards of industrial inspection. In addition, to solve the possible deviations in the manufacturing and assembly process, this article also conducts core adjustment and mechanical structure design to ensure the stability and reliability of the objective's performance. The objective after centering can clearly observe the diffraction rings of the star point and they are standard circles, indicating that the objective has good imaging quality and no obvious spherical aberration or coma. The final performance test results show that the resolution of this microscope objective reaches 1 400 lp/mm under both green and white detection light sources, and the infrared defocus at 900 nm wavelength is only 2 μm. Based on the actual processing conditions, a tolerance analysis is conducted, mainly selecting the thickness deviation of the lens, the deviation of the curvature radius, and the eccentricity and tilt deviation between the lens groups as the research objects, and using the root mean square radius of the diffraction spot as the evaluation index, 500 times of Monte Carlo simulation analysis is carried out. The tolerance sensitive items mainly exist in the eccentricity and tilt of the three-lens cemented lens, and their change amounts are all below 5%, which can meet the actual manufacturing requirements.In addition, the designed objective not only has the advantage of long working distance but also has the characteristics of high resolution and wide spectral range, which has certain reference value for the design and manufacture of long working distance and large numerical aperture microscope objectives. With the increasing demand for industrial automation and precision manufacturing, the development of such high-performance objectives will help improve production efficiency and product quality, and further promote the development of industrial inspection technology to a higher level..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0422003 (2025)
Optical Device
High Flatness Microwave Frequency Comb Based on Stimulated Brillouin Scattering Effect
Weichun QU, Ruijia CHEN, Yang LI, Zhiyuan XU, and Zuxing ZHANG
In the fusion field of optoelectronics and microwave technology, Microwave Frequency Comb (MFC) is a kind of crucial high-precision measurement and signal processing tool, and its continuous optimization has been the core issue of scientific research and technical development. With its precise frequency interval and wiIn the fusion field of optoelectronics and microwave technology, Microwave Frequency Comb (MFC) is a kind of crucial high-precision measurement and signal processing tool, and its continuous optimization has been the core issue of scientific research and technical development. With its precise frequency interval and wide coverage, MFC has shown great application potential in many fields such as wireless communication. However, most of the traditional MFC techniques rely on electric filters for frequency selection, leading to unstable frequency comb interval, frequency drift, and vulnerability to electromagnetic interference. These problems are especially prominent in precision measurement and high-speed signal processing scenarios, which limits the further improvement of MFC performance and the expansion of application range. In this paper, an Active Mode-Locked Optoelectronic Oscillator (AML-OEO) based on stimulated Brillouin scattering is proposed. The Stimulated Brillouin Scattering (SBS) and the optoelectronic oscillator are used in the experimental device, and the active mode-locking technology is innovated used to generate the high flatness microwave frequency comb.The main frequency oscillation of the microwave frequency comb is realized by the selective sideband amplification characteristic of stimulated Brillouin scattering in highly nonlinear optical fibers. The active mode-locking of the microwave frequency comb is generated by injecting an external modulated signal to the Mach-Zehnder modulator. Compared with the traditional method, the OEO uses SBS instead of the electric filter, which not only avoids the frequency drift problem, but also improves the stability and flatness of the MFC. The center frequency of this optoelectronic oscillator is determined by the Brillouin frequency shift, therefore, it is only necessary to simply adjust the output frequency of the semiconductor laser to achieve the tuning of the center frequency. By injecting the same external microwave signal as the Free Spectrum Range (FSR), the mode locking is realized, the MFC with adjustable center frequency and high flatness is generated. By varying the output wavelength of the tunable laser and the length of the delayed fiber, the generated microwave frequency comb can be tuned within a range of 307.1 MHz, which is determined by the gain bandwidth of the EDFA. But outside this range, as it is not within the gain bandwidth of the EDFA, the pump power is reduced, and a stable frequency comb cannot be formed, therefore, EDFA is the key device in the experimental device. In addition, through changing the length of the OEO cavity, adding a delay optical fiber with a length of 30 m, 300 m and 650 m in the loop, the MFCs with different frequency intervals of 202.92 kHz, 190.42 kHz and 123.19 kHz have been obtained, the relationship between cavity length and microwave frequency comb interval is inversely proportional. The stability of the frequency comb with center frequencies of 9.207 4 GHz and 9.514 5 GHz is also measured in the experiment. By measuring the frequency variation within 30 minutes at 5 minute intervals, it is proved that the proposed microwave frequency comb based on stimulated Brillouin scattering photoelectric oscillator has good stability and the superiority of stimulated Brillouin scattering based AML-OEO.The AML-OEO technique based on SBS effect proposed in this paper provides a novel and effective method for the generation of MFC. The proposed method reveals a method for generating microwave frequency combs with different intervals, which has potential applications in the field of high-precision measurement. The design of AML-OEO will be further optimized and improved to further broaden the tuning range of MFC, improve its flatness and stability, and better meet the needs of more diverse application scenarios. The proposed AML-OEO technology is expected to promote the development of the intersection of optoelectronics and microwave technology..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0423001 (2025)
Physical Optics
Fast On-line Optimization for Multifocal Modulation with Controllable Position and Intensity Ratio
Xiaonan WANG, and Jian LIN
Precise control of multi-focal intensity is crucial for enhancing the performance of optical systems in various optical applications. However, existing multifocal modulation techniques often exhibit discrepancies between theoretical calculations and experimental outcomes, making it challenging to achieve the desired unPrecise control of multi-focal intensity is crucial for enhancing the performance of optical systems in various optical applications. However, existing multifocal modulation techniques often exhibit discrepancies between theoretical calculations and experimental outcomes, making it challenging to achieve the desired uniformity of focal intensity and relative diffraction efficiency. This discrepancy may stem from system errors, non-ideal factors in the optical path, and algorithmic limitations. Therefore, it is essential to account for the actual parameters of the experimental system in calculations to ensure consistency between theoretical and experimental results, thereby enhancing the accuracy and reliability of the system.To address this issue, this paper proposes a fast online optimization method that enables effective control over the number, position, and intensity ratio of multiple focal points through iterative phase modulation optimization. The optimization process of this method is as follows. First, the modulation phase map corresponding to the target focal distribution is obtained through theoretical calculations and loaded onto a Spatial Light Modulator (SLM). Then, focal plane imaging is performed using a camera to analyze the intensity distribution of the focal points. Based on the analysis results, the phase modulation pattern is optimized and reloaded onto the SLM. By iteratively refining the modulation pattern multiple times, precise control of multi-focal intensity is achieved. The optimization algorithm adopts an intensity coefficient-based update strategy, gradually adjusting the intensity weighting of each focal point to ensure a more uniform intensity distribution among them. The primary advantage of the online optimization method is its ability to dynamically adjust and optimize phase patterns in real time, ensuring that the expected target is achieved promptly during the optimization process. However, when the same optimized phase pattern is reloaded for testing, slight degradation in uniformity may occur.To verify the effectiveness of this method, experiments were conducted using a collimated Gaussian beam with a wavelength of 638 nm, modulated by an SLM (Thorlabs EXULUS-HD2, 1 200×1 920 pixels) and focused through a high numerical aperture (NA=0.70) objective lens. The intensity distribution at the focal plane was recorded using a CMOS camera. The method's effectiveness was validated by generating focal spot arrays in 3×3, 4×4, and 5×5 matrix configurations, as well as in irregular distributions. Experimental results demonstrated that after optimization, the uniformity of the 3×3, 4×4, and 5×5 focal spot arrays increased to 97.12%, 94.51%, and 88.47%, respectively, compared to pre-optimization values of only 35.65%, 17.15%, and 14.10%. Simultaneously, the relative diffraction efficiency improved to 83.86%, 64.17%, and 57.95%, respectively, confirming the method's effectiveness in energy utilization. Compared with the traditional Gerchberg-Saxton (GS) iterative algorithm, the proposed method exhibits a significantly faster optimization convergence rate. For instance, when constructing a 3×3 focal spot array, the GS algorithm typically requires over 100 iterations to achieve uniformity between 20% and 60%, whereas the proposed method achieves 97.12% uniformity with only 31 iterations. Furthermore, this method allows flexible control over the intensity ratio between focal points. In the experiments, a "W"-shaped multi-focal pattern was generated, successfully achieving an intensity ratio of 3∶1∶2, demonstrating the feasibility of the proposed method in controlled multi-focal intensity modulation.The proposed online optimization method enables real-time control over the number, position, and intensity ratio of multiple focal points while significantly improving focal array uniformity and diffraction efficiency, thereby reducing the computational burden associated with traditional iterative algorithms. This method provides a highly efficient and reliable solution for multi-focal modulation and has broad application potential in optical manipulation, parallel direct laser writing, optogenetics, and super-resolution imaging. By achieving precise control over the number, position, and intensity ratio of multiple focal points, the proposed method significantly enhances the performance of optical systems..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0426001 (2025)
Ultrafast Optics
Research on Ultrafast Diagnosis Technology for CUP-VISAR Velocity Field Based on Dynamic Fusion TV-TNN Model
Xi WANG, Xueyin ZHAO, Miao LI, Feng WANG, Yulong LI, Zanyang GUAN, Zhaohui GUO, Xinru ZHANG, and Wenliang LEI
Ultrafast diagnostics for shockwave velocity field distributions in Inertial Confinement Fusion (ICF) experiments provide critical insights into the target pellet's compression state, offering reliable reference data for optimizing the implosion process. However, the accurate reconstruction of dynamic fringe distriUltrafast diagnostics for shockwave velocity field distributions in Inertial Confinement Fusion (ICF) experiments provide critical insights into the target pellet's compression state, offering reliable reference data for optimizing the implosion process. However, the accurate reconstruction of dynamic fringe distributions under complex noise conditions remains a significant challenge for current Compressed Ultrafast Photography-Velocity Interferometer System for Any Reflector (CUP-VISAR) systems. To tackle this issue, this paper proposes a dynamic fusion model that integrates Total Variation (TV) and Tensor Nuclear Norm (TNN) as prior constraints. This approach leverages TV's capability for edge preservation and noise suppression with TNN's effectiveness in enhancing low-rank structures in multidimensional data. This approach yields a robust reconstruction framework capable of accurately resolving dynamic velocity fields while effectively suppressing complex and time-correlated noise frequently encountered in ultrafast imaging systems such as CUP-VISAR.Methodologically, the TV-TNN model is framed within the Alternating Direction Method of Multipliers (ADMM), which decomposes the optimization problem into more tractable sub-problems. The ADMM framework allows for efficient decomposition of the overall problem, with each sub-problem focusing on a specific aspect of the data. For example, one sub-problem focuses on denoising the data using TV, while another handles low-rank reconstruction using TNN. By solving these sub-problems iteratively and adjusting the weights dynamically, the TV-TNN model achieves a high degree of accuracy and robustness, even in low Signal-to-noise Ratio (SNR) environments.Experimental validation was conducted using data from the Shenguang-III prototype, a high-power laser system designed for ICF experiments. The results demonstrate that the TV-TNN model significantly outperforms mainstream models such as ADMM-TV and E-3DTV in terms of both velocity field reconstruction accuracy and robustness to noise. In one experiment, the TV-TNN model reduced the maximum relative velocity error to 4.46%, a substantial improvement compared to the 21.25% error observed with E-3DTV and the 72.51% error with ADMM-TV. These findings highlight the model's ability to accurately capture the dynamic behavior of shockwaves, even under challenging experimental conditions with significant noise interference.Furthermore, the model was applied to two different experimental scenarios: one with a relatively stable fringe pattern and another with significant fringe contraction and motion. In both cases, the TV-TNN model demonstrated superior performance in reconstructing the dynamic evolution of the fringe patterns. In the more complex scenario, where the fringe patterns exhibited pronounced contraction and temporal changes, the TV-TNN model was able to successfully capture the intricate details of the fringe dynamics, providing a clear and accurate reconstruction of the velocity field. This indicates that the model is not only robust in handling noise but also highly adaptable to different types of dynamic changes in the data.The TV-TNN model's ability to handle multidimensional data in a highly dynamic environment makes it particularly ideally suited for applications beyond just CUP-VISAR. For example, it could be applied to other ultrafast imaging systems that require the reconstruction of complex, temporally evolving structures, such as in material science studies or high-speed fluid dynamics experiments. Moreover, the model's ability to capture and reconstruct fine details in the velocity field makes it an excellent tool for diagnosing instabilities in ICF experiments, which are critical for optimizing target design and improving overall fusion efficiency.In conclusion, this paper presents a novel dynamic fusion model combining Total Variation and Tensor Nuclear Norm to address the challenges of reconstructing shockwave velocity fields under complex noise conditions. The TV-TNN model's two key innovations—dual-layer processing of spatial and temporal information and dynamic weight adjustment—enable it to outperform existing models in terms of both accuracy and robustness. Experimental results from the Shenguang-III prototype confirm the model's effectiveness, reducing velocity error to as low as 4.46%. The TV-TNN model not only enhances the diagnostic capabilities of CUP-VISAR systems but also holds potential for broader applications in ultrafast imaging and dynamic diagnostics under extreme transient conditions..
Acta Photonica Sinica
- Publication Date: Apr. 25, 2025
- Vol. 54, Issue 4, 0432001 (2025)
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