As a weak ultraviolet (UV) detector with unique advantages, SiC avalanche photodiodes (APDs) are imperative in many key fields, such as environmental monitoring, corona detection, missile plume detection, deep space detection, and ultraviolet communication. A SiC APD is highly susceptible to irreversible thermal breakdown as its current is extremely sensitive to the bias voltage when it works under the condition of a critical electric field. Therefore, the overbias voltage withstanding capability of a SiC APD is a key issue affecting the working stability of the APD. In addition, the dark count rate is an important parameter that determines the detection sensitivity of the APD in weak UV detection. However, the reported SiC APDs exhibit low overbias voltage withstanding capabilities and high dark count rates. SiC APDs with high overbias voltage withstanding capabilities and low dark count rate have been designed and fabricated in this study.

In this study, SiC separated-absorption-charge-multiplication (SACM) APDs have been designed and fabricated. The SiC APDs are fabricated on n⁺ type 4H-SiC substrates (Fig. 1). The epitaxial structure of the SiC APDs consists of a 10-μm p type contact layer, a 0.65-μm n^- type multiplication layer, a 0.15-μm n type charge control layer, a 0.6-μm n^- type absorption layer, and a 0.2-μm n type contact layer from bottom to top. The fabrication process starts with mesa etching down to the multiplication layer (to an etching depth of 1.05 μm) by inductively coupled plasma etching. The photoresist reflow technique is employed to obtain a positive beveled mesa (with a small slope angle of about 5°) and thereby prevent mesa edge breakdown. Then, the epitaxial wafer is etched to the bottom contact layer. Subsequently, the APD surface is passivated by a thermal oxidation layer and then by a SiO₂ layer deposited by plasma-enhanced chemical vapor deposition. Both the n-type and p-type Ohmic contact electrodes adopt Ni/Ti/Al/Au (35 nm/50 nm/100 nm/100 nm) layers deposited by e-beam evaporation. Finally, the epitaxial structure is annealed by rapid thermal annealing at 850 ℃ for 3 min in N₂ atmosphere.

In this study, a reach-through SiC SACM APD is designed and fabricated. When the device undergoes avalanche breakdown, the electric field extends from the multiplication layer to the absorption layer and the charge control layer. The change rate of the electric field at the multiplication layer decreases, and the avalanche current exhibits a smaller slope accordingly, which is conducive to improving the over-bias voltage withstanding capability of APDs. Moreover, APDs with a small-slope avalanche current can alleviate the breakdown voltage fluctuation among the pixels in the UV imaging array, which is of great significance for high-quality weak UV imaging. In addition, partial mesa etching adopted for the SiC SACM APD designed in this study not only ensures the reliable operation of the device but also increases the fill factor of the device to about 60%, which is beneficial for improving the integration level of imaging array chips.

Solar blind ultraviolet (UV) detectors based on AlGaN ternary compound semiconductors have attracted much attention due to their great application potential in fields such as precision guidance, missile warning, spacecraft tracking, open flame monitoring, bioimaging, and UV secure communication. In increasingly complex target environments and short-range non-line-of-sight optical communication systems, UV detectors with high sensitivity and wide working bandwidth are required. At the same time, new material structure designs and device structure research make the UV detectors have higher performance and wider application. In this work, metal Pt with a work function of 5.36 eV is deposited on the surface of a p-GaN layer with a work function of 7.5 eV on the upper surface of GaN/AlGaN material without annealing. The Schottky contact is formed to replace the Ohmic contact formed by the traditional deposition of Ni/Au, Ti/Pt/Au, and other multilayer metals in an AlGaN-based PIN device and annealed at a high temperature. The p-GaN material forms a Schottky barrier with an energy band bending downward on the side contacting with Pt and combined with the PIN structure of the AlGaN material itself. An SB-PIN heterojunction structure is formed in the device, which changes the energy band, the built-in electric field, and the carrier transport mechanism of the device compared with PIN and SBD devices and results in a new operating mechanism and photoelectric characteristics of the device. The device has a high responsivity under a positive bias voltage and realizes dual-band detection (275 nm and 365 nm).

After the wafer cleaning, a device mesa with a diameter of 700 μm is defined by reactive ion etching (RIE). Ti/Al/Ni/Au metal layers are deposited on the n+-AlGaN layer by an e-beam evaporator, and the sample is then annealed at 550 ℃ to form an ohmic contact. Then, SiO2/SiNx composite dielectric film is grown to passivate the side wall of the device and the n+-AlGaN surface of the lower mesa surface to reduce surface leakage. After the window in the upper mesa surface is opened by lithography and etching process, Pt is deposited on the surface of the p-GaN layer to form a device with an SB-PIN structure. In order to compare the differences between the device prepared in this work and the traditional PIN device, a traditional PIN device is simultaneously fabricated with the same AlGaN material. The PIN device is prepared by depositing multiple layers of Ni/Au/Ni/Au metal on the p-GaN surface of the upper mesa surface after a lower electrode is prepared, and then an Ohmic contact upper electrode is developed after rapid annealing at 850 ℃ in O2 atmosphere. Finally, the device is prepared after passivation film growth and electrode opening.

The UV photodetector based on Pt/p-GAN/AlGaN heterojunction proposed in this paper can realize dual-band (solar blind UV and visible blind UV) detection, and the device can be switched between photovoltaic and photoconductive modes by adjusting the bias voltage. In negative bias voltage, the PIN barrier becomes stronger, and the external voltage drop mainly acts on the PIN depletion region. The surface Schottky junction is smaller. As the direction of the external electric field and the Schottky junction electric field is opposite, the Schottky junction which reduces the resistance of photon-generated carriers under a light of 275 nm is weakened. The device has a responsivity and detectivity that are slightly smaller than those of the PIN structure detector, which can be used as a high-speed solar blind UV photovoltaic detector. Under a high positive bias voltage, the direction of the Schottky junction built-in electric field and the external electric field is the same, and the band bending of p-GaN contacting with Pt is stronger.At the same time, the PIN depletion region is narrowed, which makes the overall built-in electric field of the device smaller andlets transmission and collection of photon-generated carriers controlled by the external electric field. As a result, the device operating mechanism is changed to the photoconductive mode, and the detector operates as a high-sensitivity, high-gain, solar-blind, and vision-blind UV photoconductive detector, which makes the proposed UV photodetector more promising for dual-band, high-speed, and high-gain applications.

Amid the rapid development of modern communication networks, high-order modulation formats, such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM), have been used widely for large capacity and high-speed data transmission. However, compared with binary signals, high-order modulated signals are easily degraded by channel crosstalk noise and amplified spontaneous emission (ASE) noise. In this case, all-optical regeneration technology can help improve the optical signal-to-noise ratio (OSNR) directly in the optical domain. All-optical amplitude or phase regeneration can usually be achieved by some optical structures with nonlinear effects, such as the nonlinear optical loop mirror (NOLM), the Mach-Zehnder interferometer (MZI), the phase-sensitive amplifier (PSA), and the semiconductor optical amplifier (SOA). In the process of all-optical amplitude regeneration, the conversion of amplitude noise to phase perturbation is always adopted to a certain extent. Therefore, phase-preserving amplitude regeneration (PPAR) schemes have been put forward for QPSK and QAM signals. Nevertheless, phase perturbation (larger than 3.8°) remains. The objective of the paper is to present an intact PPAR scheme without phase perturbation.

This paper proposes an optical phase conjugator (OPC)-assisted NOLM (OPC-NOLM) PPAR scheme, in which the reflected signal from the NOLM unit is used to achieve a stepwise power transfer function (PTF) and the OPC is utilized to compensate for the phase perturbation. The optical field output from the OPC-NOLM regenerator is derived and used to analyze the phase-preserving mechanism of the regenerator from the two aspects of amplitude and phase. The structural parameters of the OPC-NOLM regenerator are optimized by the PTF and phase perturbation curves. Then, an OPC-NOLM regenerator simulation platform for optical 16QAM signals is built to verify the intact PPAR performance of the proposed scheme by comparison with the NOLM scheme.

To further eliminate the residual phase perturbation of the currently available PPAR schemes, this paper proposes a novel OPC-NOLM scheme capable of intact phase preservation for input signals. The optical field output from the OPC-NOLM regenerator is derived and then used to explain the phase-preserving mechanism of the regenerator from the two aspects of amplitude and phase. According to the PTF and phase transfer curves of the OPC-NOLM regenerator, this paper optimizes the structural parameters of the regenerator and calculates its phase perturbation (0.002°). With 16QAM signals as an example, the NRR performance of the OPC-NOLM regeneration scheme is simulated. According to the simulation results, the proposed scheme achieves an NRR 3.8 dB higher than that of the NOLM structure without the OPC under an input SNR of 15 dB.

Clinical data have demonstrated that respiratory rate (RR) is an important predictor of serious diseases including heart defects, heart failure, metabolic acidosis, and sleep apnea syndrome. Much important information related to physical conditions can be obtained by analyzing respiratory data. Flexible wearable devices can meet the needs of clinical medicine and health monitoring, which have attracted extensive attention. The most popular respiratory monitoring devices are based on electronic sensors, and cannot be employed in electromagnetic interference environments such as magnetic resonance imaging and computed tomography. In this regard, fiber optic sensors featuring high sensitivity, electromagnetic interference resistance, and corrosion resistance can overcome these challenges. Wearable respiratory sensing devices based on fiber sensors are mainly divided into curvature sensing and humidity sensing according to the principles. For the respiratory monitoring devices based on the humidity sensing principle, the optical fiber sensors have to be coated with moisture-sensitive materials, which have disadvantages such as time-consuming functionalization processes, uneven coating, and poor long-term stability in different degrees. In contrast, the respiratory monitoring system based on the curvature sensing principle is simpler and more stable. However, the compactness and sensitivity of the sensor still have great room for improvement. An optical fiber curvature sensor with ultra-high sensitivity and more compact size using two types of multimode fibers with mismatched core diameters is designed in this paper. Then, the proposed sandwich multimode fiber interferometer is integrated into an elastic waistband for respiratory sensing. The respiratory monitoring device is expected to be widely applied, with great potential in strong electromagnetic fields, radioactive examination environments (such as magnetic resonance imaging system and computed tomography), and sleep quality monitoring.

First, the proposed sandwich multimode fiber Mach-Zehnder interferometer is made by sandwiching the graded-index multimode fiber (GIMMF) between two pieces of very short stepped-index multimode fibers (SIMMFs) spliced with input-single mode fiber (SMF) and output-SMF, thus forming a SIMMF-GIMMF-SIMMF sensor structure. The core diameters of the SIMMFs and GIMMF are 105 μm and 50 μm respectively, and their cladding diameters are both 125 μm. Then, the effect of interference lengths on the curvature response of the SIMMF-GIMMF-SIMMF sensor is studied, and the optimal sensor parameters are selected according to the experimental results. After that, the designed interferometer is integrated into an elastic waistband with ultraviolet (UV) glue and fixed on the human abdomen. The respiratory signals of the volunteers are acquired in real time by monitoring the intensity changes of characteristic peaks in the transmission spectra of the sensor. The signals are denoised by low-pass filter, and the respiratory frequency is obtained by short-time Fourier transform (STFT). Finally, a series of respiratory sensing experiments (such as fast breathing, slow breathing, shallow breathing, and respiratory arrest) are conducted on multiple volunteers to verify the feasibility of the wearable respiratory sensor.

In this paper, a wearable respiratory sensor based on sandwich multimode fiber interferometer is proposed. The sensor unit is made by splicing a GIMMF with length of 1-3 mm between two SIMMFs with lengths of 1 mm. Due to the mismatching core diameters of GIMMF and SIMMF, the fiber Mach-Zehnder interference optical path is achieved. The interference peak intensity of the sensor is very sensitive to micro-bending, with a maximum sensitivity of -74.03 dB/m^-1 at the curvature range of 0-2.36 m^-1. Then, the sandwich multimode fiber optic interferometer is integrated into the elastic waistband and fixed on the human abdomen, and the respiratory signals can be obtained in real time accurately by monitoring the intensity change of the characteristic peaks in the transmission spectrum of the sensor. Experimental results show that the sensor can distinguish different respiratory conditions with universal applicability. The respiratory sensor is characterized by extremely compact structure, baseline drift without signals, high sensitivity, simple fabrication, low cost, easy integration, and electromagnetic interference resistance. It can be employed in strong electromagnetic fields or radioactive examination environments, such as magnetic resonance imaging systems and computed tomography.

The large-scale commercialization of 5G networks promotes the development of fiber optical communication, cloud computing, and Internet of Things technologies. To realize the optical communication systems with high capacity and high spectrum efficiency, higher-order modulation formats are required. However, the higher-order modulation formats require high signal-to-noise ratios (SNR) to ensure communication quality, which will limit the transmission distance. Within the same amount of time, the faster-than-Nyquist (FTN) technology can transmit more signals than the Nyquist system with the same modulation format. Thus, this technology becomes a key technology for the next-generation optical communication networks with the advantages of high spectral efficiency and large capacity. The inter-symbol interference is artificially introduced into the FTN system to make the symbol interval between two adjacent pulses much smaller than the corresponding Nyquist symbol period. Thus, high-speed digital signal processing (DSP) unit, which can equalize and compensate the signal impairments efficiently in electrical domain, is a key module in FTN systems. It can improve the signal quality and support the optical fiber communication systems with ultra-high capacity. Pilot-aided carrier phase recovery (PA-CPR) is an important DSP algorithm for optical receiver, which is used to compensate the frequency deviation between the lasers on both sides of the transceiver and the phase noise generated by the laser linewidth. In present study, we report a two-stage electric-domain pilot-aided carrier phase estimation algorithm, named PA-Viterbi-ML, in which the PA-CPR algorithm is combined with the Viterbi-based maximum likelihood (ML) estimation algorithm. The simulation results verify that the proposed two-stage PA-Viterbi-ML algorithm can effectively track the phase noise when the Mazo limit is not exceeded in the FTN-16QAM system.

The proposed PA-Viterbi-ML, which combines the PA-CPR algorithm with the Viterbi-based maximum likelihood estimation algorithm, can effectively overcome the intersymbol interference (ISI) introduced by FTN technology. The first stage of the algorithm can estimate and compensate most of the phase noise in FTN-16QAM system. In order to compensate the residual phase deviation, the ML phase estimation is used as the second stage of the phase recovery algorithm to obtain a more refined phase estimate value. However, the ML phase estimation will also fail under the influence of the ISI introduced by FTN system, so the Viterbi algorithm is cascaded with it to remove the influence of ISI, and the estimated value of the phase noise close to the real value is obtained. During the simulation, the pilot-signal-ratio (PSR) and the bandwidth of the low-pass filter (B_LPF), two important parameters of the PA-Viterbi-ML algorithm, are optimized first to achieve the optimal system performance. Then, the performance of the proposed algorithm for tracking system phase noise within the Mazo limit is shown. Finally, the maximum linewidth tolerance of the algorithm is determined at the threshold of the bit error rate.

In the present study, a two-stage electric-domain pilot-aided carrier phase estimation algorithm, named PA-Viterbi-ML, is proposed, in which the PA-CPR algorithm is combined with the Viterbi-based maximum likelihood estimation algorithm. The PA-Viterbi-ML algorithm will occupy about 1.7% of the bandwidth, which can effectively overcome the ISI introduced by the FTN technology. The simulation is taken to verify that the proposed two-stage PA-Viterbi-ML algorithm can effectively track the phase noise when the Mazo limit is not exceeded in the FTN-16QAM system. The simulation results show that the OSNR penalty requirements of the proposed algorithm are smaller than those of the traditional PA-CPR when the linewidth tolerance values are the same, which means the performance of the linewidth tolerance of the proposed algorithm is better. The maximum linewidth tolerance value is defined when the BER and the OSNR penalty are equal to 2×10^-2 and 1 dB, respectively. When the accelerating factor α is as low as 0.833, the maximum linewidth tolerance value is about 5×10^-4 for the PA-Viterbi-ML algorithm, and the corresponding value for the traditional PA-CPR algorithm is about 1×10^-4.

The random fluctuation of the fiber Fabry-Perot tunable filter (FFP-TF) is easily intensified by the variation of ambient temperature, ultimately reducing the accuracy of the fiber Bragg grating (FBG) demodulation system. At present, the common solutions are the demodulation method combining the Fabry-Perot (F-P) etalon with reference grating, the demodulation method based on composite wavelength reference with acetylene gas cell, and so on. Although these methods can improve the demodulation accuracy of the system to a certain extent, the added hardware greatly increases the cost of the demodulation system. In addition, these methods are susceptible to ambient temperature. This study proposes a novel software-supported FBG demodulation method based on an improved AdaBoost algorithm. Specifically, the AdaBoost ensemble learning framework is used to construct a temperature drift model of the tunable filter. In the iteration process of the traditional AdaBoost, the weight of the generated weak learner is directly determined by its error rate, with no direct correlation between each two adjacent weak learners. In other words, the performance of the current generated weak learner is not directly affected by the weak learner generated by the previous round of iteration, and it cannot directly affect the results of the next round of iteration either. Consequently, the performance of the generated weak learners is likely to be random, which is unfavorable for the performance of the ensemble model. To solve this problem, this study proposes a dynamic weight update strategy for weak learners based on their error rate differences to accurately compensate the F-P tunable filter.

In this study, the AdaBoost ensemble learning framework is utilized to compensate the demodulation system. Specifically, data on the temperature drift characteristics of the tunable filter in a variable temperature environment are obtained, and the characteristics and labels of the data are determined. Subsequently, the AdaBoost algorithm is used to model the data. The AdaBoost algorithm framework is improved, and weight update steps are added to the AdaBoost iteration process. After the weight update coefficient is calculated with the difference between the error rates of two adjacent weak learners, it is utilized to update the weight coefficient of the current weak learner and ultimately to obtain a close correlation between each two adjacent weak learners. Then, the temperature drift data are modeled in the improved AdaBoost algorithm framework, and the accuracy and stability of the improved model are verified in different variable temperature environments. Finally, the proposed improved algorithm is compared with the common machine learning-based algorithms in the same environment to verify the effectiveness of the proposed algorithm.

By modeling the temperature drift characteristics of the tunable F-P filter and improving the traditional AdaBoost ensemble learning framework, this study proposes a new dynamic weight update strategy based on the error rate differences among weak learners. Furthermore, experiments of temperature drift compensation are carried out in two environments: cooling-heating and cooling. The wavelength shift of the tunable F-P filter is accurately compensated in variable temperature environments. Experimental verification reveals that the improved ensemble model offers the advantages of high accuracy and favorable stability, and it significantly outperforms the traditional AdaBoost algorithm and other traditional machine learning-based algorithms in variable temperature environments. In addition, compared with the traditional temperature drift compensation method for tunable filters based on the etalon and gas cell, the proposed temperature drift compensation method, with no need to add additional hardware to the existing demodulation system, is readily portable and boasts high economic practicability.

Optical tomography aims to reconstruct the cross-sectional distribution from numerous projections along various orientations. Due to its 'hard-field', high spatial and temporal resolution, this technique has been widely used in multi-phase flow monitoring, temperature and species concentration measurement and functional tissue imaging. Optical tomography adopts light emitters to emit laser beams, which are attenuated by the medium. The outgoing light is then detected by photosensitive receivers. Reconstruction algorithms are used to reconstruct the absorption distribution of the medium. Intrusively, increasing light beams and receivers will improve the reconstruction performance. However, this approach is not appropriate when the light access or installation space is limited. Meanwhile, the reported tomography sensors usually have regular arrangement, which forms a non-uniform sensitivity matrix and the region of interest (ROI) is detected unevenly. In this work, we propose an optimization method based on uniformity coefficient and genetic algorithm (GA). We hope our method can provide an optimized sensor configuration that has a uniform sensitivity matrix and improved reconstruction performance.

Sensitivity matrix relates the practical distribution to the numerous projections, which is important for image reconstruction. It is well recognized that uniform sensitivity matrix promises improved reconstruction performance. While the reconstructed images have large error when the matrix has low uniformity. In this work, uniformity coefficient is introduced to represent the uniformity of the matrix. Meanwhile, we assume that the uniformity coefficient is directly related to the quality of image reconstruction, namely, lower uniformity coefficient leads to improved reconstruction performance, and larger value leads to deteriorated performance. The optimization procedure mainly includes the following steps. Firstly, reconstruction with 60 configurations and 10 distributions are implemented to verify the effectiveness of the uniformity coefficient as a predictor. The number of the light emitters and receivers are both 25. Secondly, we adopt GA to optimize the arrangement of the emitters and receivers. The fitness function is set as the uniformity coefficient. Finally, we analyze the optimized configuration and compare its reconstruction performance with the random and regular configurations.

This paper presents an optimization method for optical tomography sensor configuration based on GA. The following conclusions can be concluded. Firstly, simulation experiments of randomly generated configurations and distributions verify that the uniformity coefficient is an effective predictor for reconstruction performance. Configuration with low uniformity coefficient has uniform sensitivity matrix and beam arrangement, and improved reconstruction performance. On the contrary, configuration with large uniformity coefficient has uneven beam arrangement, and its reconstruction performance is deteriorated. Secondly, GA is used to implement the optimization, and we take the uniformity coefficient as the fitness function. The optimized configuration provided by GA has a uniformity coefficient of 0.288. Different distributions have been considered and the reconstruction results indicate the superiority of the optimized configuration over the random and regular configurations. The optimization method has been proven to be effective. Thirdly, reconstruction results display that the practical distribution has significant influence on the performance of the configurations. Since the uniformity coefficient is only related to the configuration, the optimization results are independent to the practical distribution and this optimization method can be used in the applications where the priori information of the distributions is difficult to obtain.

Stereo images are usually acquired by changing the position of a single camera in the scene or by using two or even more cameras fixed at the same platform. Multi-camera systems are large and costly, and due to slight differences between each camera in terms of focal length, zoom level, camera gain, and so on, there are inevitable intensity differences between the matching points of the stereo images. Researchers have adopted methods combining a single camera and several optics devices to achieve stereo vision, so as to avoid this problem. The core function of the optics devices is to develop a single camera system with different imaging views. Various optical components have been reported in studies, for example, by rotating a flat glass plate or some plane mirrors placed in front of a single camera, or using a single camera pointing at a biprism or some multiple parabolic mirrors. However, optics rotation systems need to address the accuracy of mechanical movement, and biprism systems need to solve the problem of how to get the same image size. Parabolic mirror systems using multiple curvatures involve complex mirroring mechanisms, and the mirrors with multiple curvatures make the system difficult to be compact. Therefore, it is of research value and significance to seek more direct methods to realize a single-camera stereo vision system with a simpler and more compact structure. In this study, a single-camera stereo image acquisition system using an optical axis movable liquid crystal lens is presented. The optical axis position of the liquid crystal lens can be controlled by adjusting the voltage, and thus the stereo vision in a single camera can be achieved. Although it is consistent with the purpose of multi-view imaging through rotation, mirror reflection, biprism, etc., the mechanism of the designed system is simple. It allows the system to be used without mechanical movement during image acquisition, so as to reduce the complexity of the system. The liquid crystal lens is thin and light, and fits closely to the camera lens, which makes the system compact and enables a low cost.

The system consists of a fixed camera module and a liquid crystal lens with a polarizer attached. First, the structure of the liquid crystal lens is described, and a polarized interference optical path is built to analyze optical axis movement properties, including the magnitude of the motion at the corresponding drive voltage as well as the aberration and optical power. Then, the effect of the optical axis position change of the liquid crystal lens on the overall system is analyzed, and the relationship between disparity and depth is derived through the pinhole camera model. Finally, the system is used to acquire stereo images, and the disparity is calculated by the optical flow algorithm. The depth information of the scene is inferred from the disparity information in the close-range experiment, and error analysis is performed. In addition, the problems of other disparity acquisition algorithms in the designed system are illustrated.

In this study, a stereo image acquisition system is proposed. The system consists of a fixed camera module and a liquid crystal lens with a polarizer attached. We adjust the voltage to move the optical axis of the liquid crystal lens, capture images, and use optical flow algorithm to obtain disparity information. In addition, we analyze the effect of the optical axis movement of the liquid crystal lens on the overall optical axis of the system, and derive the relationship between the disparity and the depth. In the experiment, the feasibility of the system is demonstrated by verifying the existence of the disparity in the acquired images, and depth acquisition of close-range objects is performed. Experimental results show that the optical axis movement function of the liquid crystal lens can be used to move the overall optical axis of the system to achieve stereo vision. The designed system does not require any mechanical movement, and features a simple and compact structure and a low cost. Therefore, it provides a new method for the acquisition of stereo images.

The complex environment and poor image acquisition quality of aerospace assembly sites pose great difficulty to the detection and identification of assembly reference holes due to reflection, cutting tool traces, uneven light, threaded holes with chamfers, embedded contours, occlusion, and other conditions. To address the above problems, this paper proposes a technique of reference hole detection based on saliency detection. In the process of aerospace assembly, the traditional assembly methods relying on manual operations or special fixtures are transformed into digital and flexible assembly to improve assembly efficiency and ensure assembly accuracy. The references on components need to be measured before assembly. Although contact measurement offers high accuracy, it is slow and inefficient. Therefore, non-contact measurement is often chosen, and assembly components are measured by machine vision systems, namely that the positioning references on the components are measured by vision. The above methods are based on the acquisition of the edge points of the reference holes. Nevertheless, Canny edge detection, threshold segmentation, and other image preprocessing methods can hardly accurately extract the actual edge points of reference holes in actual working conditions due to reflection, cutting tool traces, uneven light, threaded holes with chamfers, embedded contours, occlusion, and other conditions. For the above reason, this paper assumes that the image can be transformed into a saliency map before detection and accurate contour positioning can be achieved by principal component analysis and sub-pixel extraction.

The traditional process of reference hole detection mainly consists of image preprocessing, contour extraction, and contour recognition. Among them, image preprocessing is to reduce noise and highlight the reference hole in the image. Contour extraction is to obtain all the contours in the preprocessed image, although a lot of them are not the contour of the reference hole. Contour recognition is to eliminate all such contours from all the contours to be detected to retain the contour of the reference hole and ultimately achieve accurate detection of the reference hole. Images taken at the actual assembly site indicate that the traditional detection methods can hardly accurately extract the contour of the reference hole, resulting in the false-positive and false-negative detection of the reference hole. To overcome the above problem, this paper preprocesses the image by saliency detection to transform the image into a saliency map, highlights the saliency of the reference hole in the image, and extracts the region of interest regarding the reference hole from the saliency map with the Hough circle detection algorithm. Subsequently, double-threshold contour segmentation is performed, and principal component analysis is conducted to extract the pixel-level contour of the reference hole. Then, the Bazen method is employed to extract the sub-pixel contour of the reference hole. Finally, the reference hole is positioned with high precision according to the principle of random sample consensus (RANSAC).

The proposed method of reference hole detection based on saliency detection can be applied to detect reference holes under reflection, cutting tool traces, uneven light, threaded holes with chamfers, embedded contours, occlusion, and other conditions, in which it still ensures the robustness of detection. Experimental verification shows that the positioning error of the noisy image is 0.202 pixel, and that of the real reference hole is 0.027 mm. The method has a bright application prospect in industrial assembly reference, processing, and the positioning hole of a task. Compared with existing methods, this method can detect reference holes that are difficult to detect for most of the traditional methods. On the basis of a known size of a reference hole to be detected, the minimum and maximum radius thresholds can be set for the Hough circle detection algorithm to guarantee the robustness of the proposed detection method.

Time-of-flight distance measurement based on a dual-comb approach is widely applied in the fields of laser radar, topography scanning, and vibration measurement by using two femtosecond lasers with a small repetition frequency difference for asynchronous optical sampling (ASOPS). In this manner, the high temporal resolution, comb-shaped spectrum, and ultra-low noise performance of femtosecond lasers can be fully utilized. However, the update rate of a dual-comb ranging system based on ASOPS is limited to a few kilohertz (determined by the repetition frequency difference) so as to avoid insufficient optical sampling. Given the extremely small duty cycle determined by the ratio of the femtosecond pulse width to the millisecond sampling period, most of the sampling time during a full sampling cycle is wasted in the process of pulse walk-off. To solve this problem, an electro-optical modulator (EOM) is added to the ASOPS system to modulate the repetition frequency periodically in this work. The so-called electronically controlled optical sampling (ECOPS) approach breaks the update rate limitation in the ASOPS system and can increase the update rate to hundreds of kilohertz, further enriching the application fields of dual-comb distance measurement technology.

ECOPS uses two lasers with tightly phase-locked repetition frequency (f_r) as the light source. One is called a local laser with an EOM inserted in the cavity, and the other is called a signal laser. The EOM in the cavity is used to modulate the repetition frequency of the local laser. As a square wave is imposed on the EOM, the repetition frequency is switched between f_r-Δf_r and f_r+Δf_r, and the modulation period is determined by the square wave modulation frequency f_m. Therefore, the repetition frequency difference between the two lasers switches between -Δf_r and Δf_r at the modulation frequency f_m. Different from ASOPS with only a fixed Δf_r, the rapid switching of ±Δf_r effectively drives the output pulse of the local laser to scan back and forth on both sides of the output pulse of the signal laser, resulting in a controlled, bounded optical sampling and avoiding the unwanted pulse walk-off. The update rate is determined by the modulation frequency f_m, which breaks the limitation of the ASOPS-based measurement system where the update rate is determined by the repetition frequency difference Δf_r. In the experiment, a pair of nonlinear polarization rotation (NPR) mode-locked fiber lasers with a repetition frequency of ~158 MHz are selected as the signal laser and the local laser. In the local laser, the pulse duration is 140 fs, the spectral width is 21 nm, the central wavelength is 1569 nm and the average power is 20 mW. As for the signal laser, the pulse duration is 92 fs, the spectral width is 51 nm, the central wavelength is 1557 nm and the average power is 50 mW. Part of the output of the two lasers is combined and directed to a balanced optical cross-correlator (BOC), which detects the relative timing error between the two lasers with sub-femtosecond resolution. The error signal is fed back to the end mirror mounted on a fast piezo-actuator such that the repetition frequencies of the two fiber lasers are tightly phase-locked, and the residual timing jitter is lower than a few femtoseconds. After the phase locking of repetition frequencies is established, the main parts of the laser output are directed to the distance measurement module. A mechanical delay line is used to adjust the optical path from the output of one laser so as to make sure that the pulses from the two lasers overlap in time. As square wave modulation is applied to the EOM, the signal pulses naturally scan back and forth on both sides of the local pulses, which enables ECOPS-based distance measurement.

This paper proposes a dual-comb absolute distance measurement system based on ECOPS and selects two NPR mode-locked lasers with a repetition frequency of ~158 MHz. In the experiment, their repetition frequencies are locked by the synchronization module. After the two femtosecond pulse trains are aligned through the spatial delay line, the sampling pulse is moved back and forth on both sides of the signal pulse by adding a square wave to the EOM in the local laser, which overcomes the problem in the ASOPS approach where the update rate is limited by the repetition frequency difference. The update rate of the experimental setup can be up to 200 kHz, and the measurement accuracy can reach 16.7 nm with an average time of 20.5 ms in the measurement of an absolute distance of 41 μm. In theory, when the update rate is reduced to 40 kHz, the detectable range can reach 1.75 mm, which meets the detection requirements of trenches in most micromechanical structures, semiconductor devices, and other micro-nano devices. The experiments show that the system has good repeatability, reproducibility, stability, and accuracy. The trench depth in a micromechanical structure is measured with the designed system. The dual-comb system based on ECOPS can be widely used in micro-distance measurement, and also has potential application prospects in the fields of 3D topography scanning, such as surface profilometry and flatness analysis.

Due to the advantages of high sensitivity and electromagnetic interference resistance, the fiber-optic current sensor based on the Faraday effect has received extensive attention and is one of the research focuses in current measurement. However, there is high random linear birefringence inside the sensing coil, which seriously affects the sensitivity of the sensor. The spun fiber is widely used in the field of current sensing to overcome the influence of random linear birefringence on sensitivity. Moreover, the method of adding a Faraday rotator mirror (FRM) to the end of the sensor's induction coil can also be used to analyze the effect of random linear birefringence. An FRM induces light reflection and polarization rotation of 90° so that the outgoing light is orthogonally polarized to the incoming light. Polarization modulation can be eliminated when orthogonally polarized light undergoes reciprocal birefringence. To remove the influence of random linear birefringence on the measurement of fibers' magneto-optical properties, we prepare spun fibers with four helical pitches and unspun fibers and build an FRM-based testing system for magneto-optical properties of fibers.

The optical fiber samples used are homemade low-birefringence spun fibers. During the fiber drawing process, the fiber preforms are rotated at 1000 r·min^-1, 667 r·min^-1, 500 r·min^-1, and 333 r·min^-1, and spun fibers with a pitch of 1.0 mm, 1.5 mm, 2.0 mm, and 3.0 mm are obtained, respectively. The wavelength of the light source used in the experiment is 1310 nm. A magneto-optical property testing system based on FRM is built to accurately measure the extinction ratio and Verdet constant of fiber samples. Specifically, the extinction ratio characteristics of spun fibers with different helical pitches and unspun fibers without and with the FRM are measured. Afterward, the Faraday rotation angles and Verdet constants of spun fibers with different helical pitches and unspun fibers without and with the FRM are tested and compared with the theoretical values.

In the case of a light source of 1310 nm and no FRM, the Verdet constant of the spun fiber is larger as its helical pitch decreases. In particular, when the pitch is 1.0 mm, its Verdet constant [0.8304 rad·(T?m)^-1] is about 3.43% higher than that of the unspun fiber [0.8029 rad·(T?m)^-1]. When an FRM is applied, the measured Verdet constants of different fiber samples are improved to a certain extent, especially for the spun fiber with a pitch of 1.0 mm, whose Verdet constant is improved by 7.50%. In addition, the mean square deviation of the measured Verdet constants of different fibers with FRM is 0.61%, which is less than 0.99% in the case of no FRM. It is indicated that the introduction of FRM can cancel the random linear birefringence in the fiber and improve the measurement stability of the Verdet constant of fibers with different helical pitches. The FRM measurements of the Verdet constant for spun fibers with pitches of 1.0 mm, 1.5 mm, 2.0 mm, and 3.0 mm are improved by 3.94%, 4.72%, 4.24%, and 5.63%, respectively. The addition of FRM can reduce the influence of random linear birefringence on the measurement of the magneto-optical properties of doped fibers and further increase the measurement accuracy of the Verdet constant of doped fibers.

As a new type of phase modulator, the liquid-crystal spatial light modulator (LC-SLM) has been widely used in adaptive optics, optical communication, optical tweezers, and digital holography. However, the phase modulation characteristics of LC-SLMs are different generally, and the accuracy of phase modulation will be affected by their transportation processes and application environments. Therefore, it is essential to measure and calibrate the phase modulation characteristics of an LC-SLM before using the device for phase modulation and compensation. Nevertheless, the currently used measurement methods still suffer from limitations. Traditional radial shearing interferometry and Twyman-Green interferometry are usually inefficient and difficult to meet the requirements for rapid detection. Commercial Fizeau interferometers can only measure the phase modulation characteristics of the LC-SLM at a fixed light wavelength. In addition, these methods ignore the influence of light wavelength on phase modulation characteristics. In this study, a fast measurement and calibration method based on digital holography is proposed, and the phase modulation characteristics of the LC-SLM at different light wavelengths are systematically evaluated. Digital holography exhibits excellent measurement accuracy and efficiency. We expect that our method can be helpful in improving the accuracy of LC-SLMs in phase modulation and compensation.

Digital holography is used to measure and calibrate an LC-SLM in this study. First, an experiment setup of a digital holography system is developed, in which the LC-SLM is used as an object. Then, a driving image with gray levels of 0-255 is loaded on the LC-SLM, and a digital hologram is recorded on the image plane. The phase distribution of the object wave can be obtained by using the reconstruction algorithm, and the relationship between the phase and the gray level of the LC-SLM at the specific wavelength can be determined. Next, without changing the structure of the setup, a comparative experiment is carried out using Twyman-Green interferometry, which requires 52 interference images to obtain the phase modulation characteristics. Afterwards, the inverse interpolation method is used to linearly correct the phase modulation curve and improve the driving accuracy of the LC-SLM. Finally, the formula of the phase correction coefficient at the specific wavelength is theoretically derived and experimentally verified by recalibrating the phase modulation characteristics at a non-standard light wavelength.

In this study, a fast measurement method based on digital holography is proposed to calibrate LC-SLMs. With this method, the phase modulation characteristics at a specific wavelength can be measured in real time by using only one digital image-plane hologram. This method improves the measurement efficiency thanks to the simple system structure and no need for diffraction propagation calculation. Without changing the structure of the setup, a comparative experiment which uses Twyman-Green interferometry is carried out to verify that digital holography has higher measurement accuracy. The experimental results show that the phase modulation range of the LC-SLM is 0-6.185 rad at the standard light wavelength of 633 nm, and the nonlinear error of the phase modulation characteristics is reduced to 2.45% by the inverse interpolation method, which effectively improves the linear driving accuracy of the device. Depending on the wavelength response characteristics of the LC-SLM, a phase correction coefficient model at the specific wavelength is built, and the actual phase modulation range of the LC-SLM at a non-standard light wavelength of 670 nm is corrected. This study verifies the feasibility of using an LC-SLM for phase correction in dual-wavelength interference measurement systems.

The Shack-Hartmann wavefront sensor is widely used due to its advantages of simple structure, high utilization rate of light energy, and fast detection speed. In practical application scenarios, affected by factors such as skylight background, atmospheric turbulence intensity, return characteristics of beacon light, detector noise, etc., the spot array images collected by the Shack-Hartmann wavefront sensor often have uneven sub-aperture spot intensity distributions, with low signal-to-noise ratios (SNR). In this case, it is difficult to accurately extract the centroid of a light spot, and the detection accuracy of the wavefront decreases. To solve the localization problem of the sub-spots of spot array images with a low SNR, researchers have proposed several improved methods, such as the thresholding centroid method, weighted centroid method, intensity weighted centroid method, cross-correlation algorithm, frequency domain method, local adaptive threshold method, windowed thresholding centroid method, and windowed thresholding weighted centroid method. However, when the near-field light intensity of the beam to be measured fluctuates dynamically, and the detector noise, image background noise, and other interfering noise signals change dynamically, the effective optical signal and noise signal of a spot array image fluctuate in time and space. When the spot centroid is selected, the algorithm parameters need to be dynamically adjusted to ensure the centroid extraction accuracy of the sub-spot. This algorithm mechanism significantly increases the complexity of the centroid extraction algorithm, and there are also problems with optimal parameter selection and dynamic setting, which will eventually lead to a decrease in the wavefront restoration accuracy of the sensor.The efficient centroid extraction of the sub-spot when the near-field intensity of the incident beam dynamically fluctuates requires a centroid localization method for the low-SNR sub-spot image collected by the Shack-Hartmann wavefront sensor with high adaptability.

When the wavefront sensor collects the sub-spot image, the detector introduces signal photon noise, background photon noise and readout noise, etc., due to factors such as the environment and the quantum characteristics of the photodetector. According to the characteristics of detection noise, photodetector noise is generally represented by a Poisson-Gaussian model. In this model, the signal-related noise introduced by the quantum characteristics of the sensor is modeled by Poisson distribution, and the signal-independent noise is modeled by Gaussian distribution. According to the signal characteristics and noise characteristics of the photodetector, this paper proposes a method to extract the centroid of a sub-spot based on variance-stabilizing transformation (VST). It converts the Poisson-Gaussian noise that varies with the signal into Gaussian noise with a fixed variance. An improved block-matching and 3D filtering (BM3D) method, i.e., noise feedback block-matching and 3D collaborative filtering (NFBM3D), is used to remove the noise of the spot array image, and then sub-spot centroid extraction and wavefront restoration are performed.

Through simulation and experiments, it is confirmed that the method proposed in this paper can effectively extract the light spot signal data in the low-SNR spot array image collected by the Shack-Hartmann wavefront sensor. It can avoid noise interference in the image and fully improve the centroid localization accuracy and stability of the sub-aperture spot. Compared with the traditional adaptive threshold and other methods, this method can improve the centroid extraction and wavefront restoration accuracy by more than 2 times when the peak SNR of the sub-spot image is lower than 6. This algorithm is expected to meet the real-time requirements of the centroid extraction of the adaptive optics system after the accelerated processing of similar and fast search and matching.

Holes are the most common form of parts in machining and manufacturing. Enterprises have been long suffered from the inner surface defect detection of holes, especially the measurement of inner surface size. With the development of image processing technology and miniaturization of imaging elements, inner surface defect detection based on endoscopic images has been widely used in mechanical manufacturing, aerospace, and other fields. When endoscopes are applied to detect the inner surface defects, the real inner surface is parallel to the main optical axis of the lens and exhibits a circular scattering distribution on the imaging plane, which is not conducive to identifying the contour and size of the defects. Aiming at the difficulty in measuring the inner surface defect size of the hole, this paper proposes an endoscope image correction method to realize the inner surface size measurement.

According to the mapping relationship between spatial coordinates and phase plane coordinates in the endoscopic imaging system, the endoscopic image distortion is divided into circumferential distortion and axial distortion. In the case of adding geometric constraints in the application scene, the parameters in the correction model are simplified to a central coordinate and a nonlinear parameter, which improves the correction accuracy of the edge region of the endoscope image. The Hough transform and difference expansion are adopted to process the endoscopic image to realize the circumferential correction, and the axial correction is performed by pixel calibration and nonlinear stretching of the circumferential corrected image. The paper utilizes the neural network algorithm to fit the axial correction function including the relative positions of pixels and apertures based on the calibration experiments of different apertures, thus overcoming the dependence of the axial correction on the calibration results. Additionally, the measurement of inner surface defects of holes without calibration conditions is obtained.

In this paper, a method for distortion correction and measurement of in-hole images based on the endoscopic imaging principle is proposed. The method of circumferential correction and axial correction is adopted to restore the endoscopic image to an orthographic image that conforms to the visual habit and measure the size. The feasibility of the method is verified by experiments and the factors affecting the detection accuracy are discussed. The specific conclusions are as follows. 1) The method based on image processing can effectively realize the endoscopic image correction and the defect measurement, and the accuracy is high. 2) The measurement method is not sensitive to the aperture, and the measurement accuracy is not affected by the test object, so the method has good applicability. 3) The coordinates of the image center and the axial distance growth function are the key parameters that affect the accuracy of image correction and defect measurement. For further improvement of detection accuracy, a textured calibration paper can be placed on the inner surface of the hole during the detection to improve the accuracy of circle center detection and axial distance growth function fitting. As the proposed correction method has good versatility and high accuracy, it can be used for automatic detection and intelligent detection of holes and pipelines.

Since its development in the last century, the performance of synchrotron light sources has been-increasingly improved, providing a new and efficient platform for research in many fundamental disciplines such as physics, chemistry, materials science, and life sciences, and helping to achieve many cutting-edge results. In synchrotron light sources and X-ray free electron laser devices, grating monochromators and spectrometers are crucial for both beamlines and experimental stations. Monochromators variable-line-spacing (VLS) grating is simple and easily achieve high spectral resolution and transmission efficiency. Thus, VLS gratings have become the dominant

The main methods for measuring grating line densities include interferometry, diffraction, and long trace profiler (LTP) methods. These methods have their advantages and disadvantages. To meet the need of measuring line density of VLS grating, LTP with stitched data is used. In order to complete the Hefei Light Source photoelectron spectral beam line maintenance project, the Hefei Light Source independently developed a VLS grating. In order to characterize the line density more precisely, this paper proposes an improved stitching measurement method using LTP. In particular, the incident angle of each segment is inverted to improve based on the proposed stitching measurement method of LTP. In fact, for previous stitching methods, the angle of incidence at the central of each segment was determined from the angle of the deflector and the relative diffraction angle within each segment, which was based on the angle value given by the LTP detector. However, the angular error of the deflector is not negligible. In this method, first, taking the midpoint of the VLS grating as a reference point, the line density and incidence angle of the reference point is determined. Moreover, with the data of the reference point, the line densities of other positions of this segment are measured. Second, it is to measure the line density distribution of the next segment.

The LTP stitching measurement method is used to test VLS grating parameters using a segmented overlapping data processing method, which avoids angular errors in the deflector and provides a significant improvement in repeatability. The consistency of the test results is better than 1.13×10^-6 (RMS) for the same VLS grating using different overlapping rates, and the PV value of the repeatability deviation of the test data decreases significantly with increasing overlapping rate. Therefore, a reasonable selection of step length and the overlapping ratio of two adjacent segments can improve measurement accuracy while suppressing splicing errors and ensuring a certain level of measurement efficiency. However, due to the relative accuracy of the turntable, the deviation of the absolute value of the central density is about 0.1 lp/mm, which needs to be improved by using the relative calibration method. This will be followed by an experimental approach to investigate the effect of central line density error measurements on variable-line-spacing grating parameters and experimental verification.

Lithographic tool is an important device for large-scale IC manufacturing. Its function is to transfer mask patterns into photoresists on wafers. Nowadays, the designed feature size of IC is below 10 nm, and the number of transistors of an IC is as high as tens of billions. With the demand for high integration and good performance, the physical design of IC continues to shrink, and lithographic printability has become one of the critical issues in IC design and manufacturing. Affected by the layout design and lithography process, the lithography results of some patterns in the layout are quite different from that of the target patterns, which results in short-circuit or open-circuit problems. These problems will cause lithography hotspots. In order to reduce lithography hotspots, hotspot detection and layout correction are carried out in turn in the layout design phase. The performance of the hotspot detection affects the period and yield of IC manufacturing. Hotspot detection is one of the important techniques for IC design and manufacturing. For available hotspot detection methods, the hotspot detection method based on lithography simulation is time-consuming, and the hotspot detection method based on pattern matching is invalid for unknown hotspot patterns. The hotspot detection method based on machine learning has good performance in speed and accuracy and has been widely studied. Transfer learning has been applied in the hotspot detection method based on machine learning and achieved positive model performance. Model performance and model training time affect the application of the hotspot detection method based on transfer learning. In this study, a lithography hotspot detection method based on a pre-trained VGG11 model is proposed. The proposed method helps to improve the model performance and model training time.

In this study, we adopt a transfer learning strategy for model training of hotspot detection. First, the ImageNet dataset is used to pre-train the VGG11 model, and the pre-trained VGG11 model is used as the model to be trained for hotspot detection. Then, the network architecture of the pre-trained VGG11 model is fine-tuned to make it suitable for hotspot detection. In the data preparation phase, pattern down-sampling and data balance are employed to prepare data for model training. In the model training phase, the strategy of preserving pre-trained model weights and freezing convolutional layers is adopted for model training. The trained model is suitable for hotspot detection.

A lithography hotspot detection method based on a pre-trained VGG11 model is proposed in this study. A transfer learning strategy is adopted for model training. The proposed method uses a VGG11 network trained by the ImageNet dataset as the pre-trained model, and the network architecture of the pre-trained VGG11 model is fine-tuned to make it suitable for hotspot detection. Model training is performed by using a strategy of preserving pre-trained model weights and freezing convolutional layers. ICCAD 2012 benchmark suite is used for model training and model tests. Compared with that of available methods, the results show that the model of the proposed method has better comprehensive performance and requires less model training time. The average accuracy, recall, precision, and F₁ score of the model reach 98.9%, 98.2%, 89.5%, and 93.3%. In addition, the model training time of the proposed method is only 279 s. The proposed method helps to improve the efficiency of hotspot detection and shortens the period of integrated circuit manufacturing.

The 8-12 μm long-wave infrared (LWIR) laser, which is within the atmospheric transmission window and the eye-safe range and demonstrates a higher transmittance in atmospheric media (Fig. 1), has critical applications in various fields, such as directed infrared countermeasures, environmental monitoring, lidar, and surgery. For example, the laser in this LWIR band plays an important role in environmental monitoring and differential absorption lidar because this band covers the fundamental absorption bands of many gas molecules, such as H₂O, CO₂, NH3, and O₃. In terms of medical treatment, the 8-12 μm LWIR laser, with a large absorption coefficient and a shallow penetration depth in water and other components of biological tissues, serves as a unique and effective tool in biological tissue treatment. In addition, high-energy 8-12 μm LWIR lasers are in high demand in the field of defense.

Specifically, the working principles and characteristics of the second-order nonlinear frequency conversion techniques, including optical parametric generation (OPG), optical parametric oscillation (OPO), difference frequency generation (DFG), and optical parametric amplification (OPA), are described (Fig. 3). Subsequently, the physical and nonlinear optical properties, including nonlinear coefficient, transparency range, thermal conductivity, and damage threshold, of commonly used nonlinear crystals, such as ZnGeP2, BaGa4Se7, CdSe, GaSe, LiGaS2, orientation-patterned GaAs, and orientation-patterned GaP, are summarized (Table 1). Then, the detailed properties of different crystals and the output characteristics of the corresponding LWIR laser based on the crystals are analyzed. The research progress analysis shows that LWIR lasers based on second-order nonlinear frequency conversion have achieved femtosecond, picosecond, and nanosecond output in pulse width and repetition rates ranging from several hertz to megahertz. However, due to the low inherent quantum conversion efficiency of nonlinear frequency conversion towards the LWIR band (pumped by 1-3 μm near- and mid-infrared lasers), the output energy of the LWIR lasers is mainly at the microjoule and millijoule levels at present (Fig. 10). Finally, the opportunities and challenges for LWIR lasers based on second-order frequency conversion techniques are discussed, and the potential method of LWIR lasing via Raman conversion based on the third-order nonlinear effect and its prospect are presented.

Crystalline LWIR lasers based on second-order nonlinear frequency conversion techniques have made outstanding achievements in ultrashort pulse width, high repetition rate, wide wavelength tuning range, and high peak power. The improvement of crystal growth technique, the emergence of new types of nonlinear optical crystals, and the development of currently available crystals with higher optical quality and larger volume crystals pave the way for the further improvement of the power and conversion efficiencies of LWIR lasers. In addition to the above reviewed second-order nonlinear frequency conversion techniques, diamond Raman lasers (based on the third-order nonlinear optical effect) with an extremely wide spectral transmission range and an extremely high thermal conductivity are considered a promising way of wavelength conversion from short-wave to long-wave.

As class-B lasers, distributed feedback semiconductor lasers (DFB-SLs) can output chaotic laser under external disturbances, such as external optical injection and optoelectronic feedback, and the bandwidth is up to GHz. Therefore, DFB-SLs are widely applied in many fields, such as secure communication and physical entropy sources for generating random physical numbers. However, the chaotic laser output from DFB-SLs has weak periodicity and time-delay signature (TDS) due to optical feedback and optical injection. This would reduce the quality of the random numbers generated with chaotic laser sources and restrict the applications of chaotic laser. In addition, the bandwidth (BW) of the chaotic laser determines the transmission rate of secure communication. For the above reasons, the TDS and BW are two important parameters that affect chaotic laser's applications and are often used to characterize the chaotic characteristics of chaotic laser. This paper presents a semiconductor laser system with external unidirectional dual-path optical injection and phase-modulated optical feedback (SL-EUDOI-PMOF) and investigates its effectiveness in suppressing the TDS and broadening the BW of chaotic laser. The results of this paper are significant for achieving information confidentiality and high-speed transmission in chaotic laser-based secure communication.

This paper presents a scheme of semiconductor lasers. Specifically, a DFB-SL with an external-cavity optical feedback is used as the master laser, while a DFB-SL with the PMOF is taken as the slave laser. Subsequently, the chaotic laser output from the master laser is injected into the slave laser through two paths. The SL-EUDOI-PMOF is thereby obtained. Then, the influences of parameters, including the external optical injection coefficients and the feedback coefficients, on the TDS of the chaotic laser output from the SL-EUDOI-PMOF are numerically investigated. The time-delay eigenvalue β is defined as the maximum value of the time-delay eigenpeaks in the autocorrelation function curve of the chaotic laser. When β<0.2, the TDS is suppressed. Furthermore, the BW of the chaotic laser is examined under the parameters enabling effective suppression of the TDS.

This paper proposes the SL-EUDOI-PMOF system for suppressing the TDS and broadening the BW of chaotic laser. For this purpose, the influences of the system's parameters on the TDS are numerically investigated, and the results are physically analyzed. The results show that in the selected parameter value ranges, the time-delay eigenvalue β decreases first and then increases as the feedback coefficient Km or the pumping factor of the master laser increases. The value of β decreases as the feedback coefficient Ks or the two injection coefficients increase. Moreover, the value of β increases first and then decreases with an increasing frequency detuning, and it decreases first, then varies in a gentle manner, and rises slightly higher afterwards with the increase in the pumping factor of the slave laser. The optimal parameter value ranges for suppressing the TDS effectively are obtained accordingly. Then, the BW is investigated under the parameters enabling effective suppression of the TDS, and the result is physically analyzed. The analysis results show that the value of the BW increases rapidly first and then decreases slowly as the feedback coefficient Km or the pumping factor of the slave laser increases. It increases with an increasing injection coefficient K2 or an increasing feedback coefficient Ks. The value of the BW increases first and then varies gently as the pumping factor of the master laser rises, and it increases gradually first and then decreases rapidly with an increasing frequency detuning. The maximum value of the 3-dB BW of the chaotic laser obtained is about 20 GHz.

Laser speckle projection systems have been widely used in various fields, including but not limited to three-dimensional (3D) reconstruction, industrial detection, and gesture recognition. According to the number of infrared cameras, laser speckle projection systems are generally divided into two categories: the binocular mode and the monocular mode. A binocular laser speckle projection system consists of a laser speckle projector and two infrared cameras. The feature information provided by random speckle patterns is sufficient to match images in textureless areas, which significantly improves the accuracy and stability of binocular stereo vision systems. Moreover, speckle patterns in the infrared spectrum minimize the impact of the ambient light. However, the cost of binocular laser speckle projection systems is typically high, and the calibration process is complex. Compared with their binocular counterparts, monocular laser speckle projection systems are more compact and cost-effective. Due to the lack of reference speckle patterns, monocular laser speckle projection systems generally use a precise range finder to capture speckle images at different standard distances in advance. The measurement process is complex, and the deviation of the optical axis cannot be corrected online. To solve the aforementioned problems, this paper proposes a calibration method for the extrinsic parameters of monocular laser speckle projection systems. The virtual speckle image of the projector is generated by calculating the pose relationship between the infrared camera and the laser speckle projector. Only a calibration board with corner features is required in the proposed calibration process, rather than the precise range finder. With this method, a monocular laser speckle projection system becomes equivalent to a binocular stereo vision system with speckle images.

First, a simple calibration board with corner features is designed. These features only occupy a small part of the calibration board, which leaves sufficient area for the speckle pattern. The plane equation of the calibration board in the camera coordinate system is calculated by extracting the coordinates of corner features in the image. Then, the laser speckle projector projects a random speckle pattern to the calibration board in different poses, and the infrared camera captures speckle images. Next, the digital image correlation (DIC) method is utilized to determine the corresponding speckle points in different speckle images. According to the plane equations of the calibration board, those speckle points are projected to corresponding planes, whose 3D coordinates can be obtained in the camera coordinate system. The straight lines fitted by corresponding speckle points pass through the center of the laser transmitter in the projector, which is regarded as the optical center of the projector. Therefore, the optical center and axis of the projector in the camera coordinate system are estimated by fitting corresponding lines. Finally, the pose relationship between the camera and the projector is solved and optimized. The virtual speckle image of the projector is generated by constructing the equation of planar homography. Through the aforementioned process, a monocular laser speckle projection system can be equivalent to a binocular stereo vision system with speckle images.

In this paper, a simple and efficient calibration method for the extrinsic parameters of monocular laser speckle projection systems is proposed. The 3D coordinates of the corresponding speckle points are calculated by adjusting the pose of the calibration board. Then, the relationship between the infrared camera and the laser speckle projector is solved and optimized to generate the virtual speckle image of the projector. The pose relationship of the monocular laser speckle projection system can be easily calibrated with the help of a calibration board with corner features, which improves calibration efficiency and reduces calibration costs. Generating the virtual speckle images of the projector enables the monocular laser speckle projection system to be equivalent to a binocular stereo vision system with speckle images, which significantly improves depth measurement accuracy. Simultaneously, the deviation of the optical axis can be corrected online. The experimental results show that the measurement errors of displacement and sphere radii are less than 0.16 mm and 0.13 mm, respectively. Within a certain depth range, the reconstruction results of the proposed method are significantly better than those of Astra-Pro. The proposed method can well improve the calibration efficiency and depth measurement accuracy of monocular laser speckle projection systems.

Fiber optical parametric chirped pulse amplification (FOPCPA) is a widely studied ultrashort pulse amplification technique. The FOPCPA can provide excellent gain bandwidth and achieve ultrashort pulse amplification with a more compact and stable system design. The basic principle of the operation relies on a degenerate phase-matched four-wave mixing process involving one strong narrow-bandwidth pump wave, a weak stretched signal, and a generated idler wave. The FOPCPA process can be described by the nonlinear Schrodinger equation. However, the FOPCPA system is highly sensitive to the initial parameters and fiber parameters. Consequently, the traditional numerical methods (i.e., split-step Fourier method and finite-difference method) of analyzing the ultrashort CPA in an FOPCPA system require a huge amount of computation and become less efficient. Nowadays, deep learning (DL) methods have been developed to model and predict nonlinear pulse dynamics and thereby reap the benefits of purely data-driven methods without any underlying governing equations. This study focuses on modeling the ultrashort CPA in fiber by a DL method. The proposed method is expected to broaden the application of DL methods in the prediction of laser behavior and provide an alternative for studying the characteristics of ultrashort pulses in fiber.

A deep convolutional neural network is constructed in the present study. This network contains three parts: five convolutional blocks, a reshaping layer, and three fully connected layers (Fig. 3). Each convolutional block contains a one-dimensional (1d) convolutional layer, a batch normalization layer, a rectified linear unit activation function, and a 1d max pooling layer. The intensity distribution of the initial chirped pulse is used as the input of the neural network. After five convolutional blocks and three fully connected layers, the predicted ultrashort pulse propagation is obtained. For better feature extraction, the real and imaginary parts of the initial pulse are simultaneously used as the input of the deep convolutional neural network. The weights and biases of the proposed network are updated by the back-propagation of the root-mean-square error between the predicted pulse propagation intensity and the ground truth. In the training phase, this study uses the Adam optimizer and sets the learning rate of the network to 0.0001. The whole program is implemented in the Pytorch framework with a 2080Ti GPU. Four cases are considered to test the performance of the proposed network (Table 2). In all these cases, the training sets and testing sets are independent of each other, namely that no duplicate samples are used.

In this study, a DL method is employed to model ultrashort CPA in fiber. A deep convolutional neural network that consists of convolutional blocks and fully connected layers is designed to predict ultrashort pulse propagation under different initial parameters with high precision. Specifically, the paper analyzes the propagation characteristics of the chirped ultrashort pulse and the influence of initial chirp on pulse evolution. The prediction precision and computation efficiency of the proposed method are further studied under different initial pulse parameters. Without compromising generality, the study selects the case of different initial pulse power, width, and chirp to present the testing results. The results show that the neural network constructed performs well in both prediction precision and computation efficiency. On 500 independent testing samples, the proposed deep convolutional neural network achieves normalized root-mean-square errors smaller than 0.0584 and takes less than 1/10 the computation time of the traditional split-step Fourier method. The proposed method extends the application of DL methods in laser technologies and ultrafast optics and provides an alternative for modeling ultrashort pulse propagation in fiber.

With the development of information technology, electronic devices are widely used while leading to many electromagnetic interference problems. In addition, useless electromagnetic waves may pose a potential threat to human health. Therefore, electromagnetic absorbing materials have been developed to eliminate electromagnetic interference and provide information security. Conventional coated absorbing materials usually have a narrow and fixed absorbing band and are susceptible to external environmental influences. Compared with traditional absorbers, metamaterial absorbers have a larger absorbing bandwidth, a stronger absorbing capacity, and a lower thickness. To solve the problem that common absorbing materials have a high thickness and a narrow absorption band, a new dipole square ring crossed element structure with a large bandwidth and a low thickness is designed in this paper. This new structure shows good stability and high-frequency characteristics.

In this paper, the relationship between the electromagnetic parameters and the reflectivity of the absorber is calculated by the finite-difference time-domain method through simulation with CST software. The equivalent electromagnetic parameters of the absorber are obtained by inversion according to the equivalent medium theory. The cell size and circuit parameters of the dipole square ring crossed element structure are optimized by the equivalent circuit model of the absorber. The influence of two main parameters on reflectivity is studied. The surface current, electric energy density, and magnetic energy density of a unit cell at the operating frequency are simulated to analyze the working mechanism. A high-impedance surface comprising a lossy frequency selective surface (FSS) is employed to design a broadband microwave metamaterial absorber. The dipole square ring cross element structure is designed. Conductive paste and alumina ceramic are selected as the FSS raw material and the dielectric layer, respectively. Firstly, the alumina ceramic is used to make the dielectric substrate so that the limit thickness can be reduced. Secondly, the conductive paste is applied to the dielectric layer by the screen printing method, and the surface square resistance of the conductive paste is 60 Ω/sp. Finally, the reflection coefficient of the sample is measured by the free-space method in a microwave darkroom with a double-ridged horn antenna and a network analyzer.

In this paper, a thin wideband metamaterial absorber is designed and fabricated with a dipole square ring cross element structure. The reflectivities of three metamaterial absorbers based on cross element structures are solved using CST software through time-domain finite integration, and the effect of the metasurface structure on the reflectivity is investigated. According to the analysis of the surface current distribution and electromagnetic loss density at the resonance frequency, the absorbing loss mechanism is made clear, namely that the existence of the gaps makes the current fail to flow, and positive and negative electrons gather on the two sides of the gaps respectively to form dipoles able to generate a strong electric field. The energy of the incident electromagnetic wave is rapidly lost in the electric field enhancement region. The equivalent dielectric constant, equivalent permeability, and equivalent impedance of the absorber are obtained by inversion in light of the equivalent medium theory, and it is found that the loss mechanism of the metamaterial absorber is the excitation of magnetic resonance. The simulation and experimental results are in good agreement. The experimental results have a small deviation from the simulation results because the simulation model is an infinite one and has ideal boundary conditions. The metamaterial absorber has a reflectivity less than -10 dB in the 11.0-18.0 GHz band with a thickness of 1.6 mm, and the microwave absorption peaks at 12.7 GHz. The simplicity of the raw materials produced in this study and the feasibility of the metamaterial absorber fabrication make the large-scale application of the designed absorber possible.

Diffractive optical elements (DOEs) are widely applied in light distribution control such as laser beam shaping, structured light illumination, and beam splitter. Various methods can be utilized to design DOEs, such as Gerchberg-Saxton (GS) algorithm, simulated annealing algorithm (SAA), and Genetic algorithm (GA). These traditional methods can design DOE effectively for a group of initial parameters, such as beam waist radius, wavelength, size of target plane, and distance between DOE and target plane. However, when any parameter is changed, the new phase profile needs to be recalculated, which is time-consuming, especially by global optimization algorithms such as SAA and GA. To overcome the disadvantages, this paper employs a machine learning algorithm to design the DOEs with continuous phase distribution. The mapping relationship of system parameters such as waist radius, size of target plane, and distance between DOE and target plane with DOE phase coefficients is constructed by the neural network. With this relationship, the DOE phase coefficients can be predicted automatically when a set of system parameters are given. It overcomes the limitation of the traditional design methods which need to recalculate the phase distribution when the parameters are changed.

Machine learning algorithm is employed to design the DOE with continuous phase distribution, which can be used for laser shaping. Firstly, the gird energy mapping method is applied to calculate the phase distribution data of the DOEs with a set of initial parameters including waist radius, size of target plane, and distance between DOE and target plane. The DOE enables the laser to generate uniform laser irradiance distribution. Secondly, the phase distribution data of DOE elements are fitted into a polynomial. Then 10000 sets of initial parameters are generated. With the 10000 sets of initial parameters, the phase distribution data of 10000 sets of DOEs are calculated by grid energy mapping. The initial parameters of the DOE and DOE phase coefficients are taken as input and output data to train the neural network, respectively. The trained network constructs a mapping relationship between system parameters and phase coefficients. With this relationship, the DOE phase coefficients can be predicted automatically when a set of system parameters are given.

The machine learning algorithm is employed to design DOEs with continuous phase distribution. The parameters of the DOE and DOE phase coefficients are acquired automatically as input and output data respectively to train the neural network. The trained network constructs a mapping relationship between system parameters and phase coefficients. With the relationship, the DOE phase coefficients can be predicted automatically when a set of system parameters are given. The results show that the prediction accuracy of the phase coefficient is above 99.9% within the trained range of the system parameters. When all parameters are expanded by 80% and 55% in both forward and reverse directions based on the pre-trained range, the prediction accuracy remains above 99.5% and 97.5%, respectively. It is also shown that the size of the target plane has the most obvious influence on the prediction accuracy when the size of the target plane is smaller than the predetermined size. In future work, the method may be extended to design the DOE with discontinuous phase distribution.

In bionics research, micron-level double-layer composite structures can usually show better mechanical, optical, and chemical properties than single-layer structures. Designing and constructing these unique biomimetic microstructure surfaces for human use is a hot research topic in recent years. The traditional photolithography technology is very convenient and has the advantage of simple process when it is applied to prepare the micron-level single-layer structures. However, when the traditional photolithography technology is adopted to prepare the micron-level double-layer composite structures, it needs to use the overlay lithography process for many times, which will greatly increase the manufacturing difficulty and processing cost of the microstructure. To overcome the above difficulties, researchers have developed a variety of microstructure processing methods, such as dry/wet etching, nanoimprinting, 3D printing, self-assembly, laser processing, photolithography, replication molding, and electrospinning. A variety of single-layer microstructure surfaces can be prepared by using these technology combinations, and even multi-scale micron-level composite structure surfaces can be prepared. However, the combinations often lead to more cumbersome processing procedures and higher costs of micron-level composite structures. To solve these problems, a method to adjust the exposure efficiency of photoresist by changing the width of the light transmitting part on the mask is proposed. By this method, the micron-level double-layer composite structures can be obtained on the positive photoresist with only one exposure and one development, which greatly reduces the processing difficulty and manufacturing costs of such structures and provides a new strategy for fabricating multi-scale micron-level composite structures.

Two lithographic masks with different parameters are designed and purchased from the 55th Research Institute of China Electronics Technology Group Corporation. The pattern of the mask and the schematic diagram of the photolithography process are shown in Fig. 1 and Fig. 2, respectively. The specific experimental process is detailed as follows. The K₉ glass substrate is cleaned by an ultrasonic cleaner in acetone, ethanol, and deionized water for 5 min each and then dried with nitrogen flow. The AZ9260 photoresist film with a thickness of 8 μm is spin-coated on a 5.08-cm K₉ substrate at 2500 r/min for 40 s using Laurell WS-650Mz spin coater. After standing at room temperature for 10 min, the substrate is placed on a 65 ℃ hot plate for 5 min, a 95 °C hot plate for 10 min and a 110 °C hot plate for 5 min, and finally cooled to room temperature. The ultraviolet (UV) lithographic exposure process is performed on Midas MDA-400M. The exposure energy of different samples is set between 125 mJ/cm² and 240 mJ/cm². The photoresist development is carried out with AZ400K developer (the volume ratio of AZ400K developer to deionised water is 1∶3) after UV exposure. Leica Microsystems DM8000M optical microscope is used to characterize the obtained samples. In addition, the light field distribution during mask exposure is analyzed by the finite-difference time-domain method to find out the formation mechanism of the micron-level double-layer composite structures.

The application of the one-step photolithographic preparation technique proposed in this paper can effectively reduce the difficulty in fabricating micron-level double-layer composite structures. The experimental results show that the fabrication process of the micron-level double-layer composite structures using the proposed method is very simple compared with that using overlay lithography technology, and only one exposure and one development process are needed. The maximum exposure energy of 8 μm thick AZ9260 photoresist should not be higher than 160 mJ/cm² to obtain micro-level double-layer composite structures. The light field distribution behind the mask during lithography exposure is analyzed by the finite-difference time-domain method. The simulation results show that the exposure efficiency of the photoresist under 4 μm narrow slit is lower than that under 40 μm wide slit, which is also the fundamental reason why the micron-level double-layer composite structures can be prepared by one-step lithography technology. The developer renewal speed in the 4 μm narrow slit is less than that in the 40 μm wide slit, further promoting the formation of the micron-level double-layer composite structures. During the mask lithography, the photoresist under the wide light transmitting area on the mask will be developed to the substrate faster than that under the narrow light transmitting area. According to this rule, various masks can be designed, and it is expected to prepare a variety of micron-level double-layer composite structures or even micron-level multi-layer composite structures by this method. The masks shown in this paper have reference significance for preparing various micron-level multi-layer composite structures.

In recent years, the terahertz (THz) band has attracted extensive attention from researchers due to its potential of realizing high-speed and high-capacity wireless communication systems. The multiplexing technology has great research prospects in improving the communication rate and system capacity. The electromagnetic wave (EMW) carrying the orbital angular momentum (OAM) is called the OAM wave. OAM can be used as a new information carrier to provide an additional dimension for spatial multiplexing. The metasurface can effectively control the amplitude, phase, and polarization of EMW, and according to the main types of materials used, it can be divided into the metal and dielectric metasurfaces. Compared with the metal metasurface, the dielectric metasurface has the advantages of smaller ohmic loss, lower costs, easier processing and manufacturing, and higher transmission efficiency. Most previous research focused on generating an OAM beam or realizing OAM beam multiplexing by the metal metasurface, and hence, generating OAM beams and further realizing OAM beam multiplexing based on the dielectric metasurface have become the research hotspots. The methods of OAM beam multiplexing based on the dielectric metasurface have the disadvantages of OAM waves carrying the same information, the limited number of multiplexing channels, and the complexity and high cost of the communication system. Although the above problems can be effectively solved on the basis of the angle-multiplexed dielectric metasurface by converting multiple incident waves with different angles into orthogonal OAM coaxial beams, the current angle-multiplexed dielectric metasurface only works at a single frequency. Once the incident wave frequency changes, the generated OAM waves will deviate from the axis. In addition, the existing research focuses on the optical frequency band, and few studies combine two or more physical dimensions to achieve multiplexing. We need to study the realization of dual-dimensional or multi-dimensional multi-channel multiplexing in the terahertz band based on the dielectric metasurface and the expansion of the working bandwidth of the dielectric metasurface. Therefore, this paper proposes a dielectric metasurface, on the basis of which the dual-dimensional multi-channel multiplexing can be realized by the combination of OAM and frequency dimensions. Theoretically, the simultaneous transmission of 4×N-channel (N is any positive integer) orthogonal coaxial beams can be realized. The proposed dielectric metasurface has potential application value in the field of high-speed and high-capacity terahertz communication.

First, the designed dielectric unit cells of the metasurface are composed of silicon pillars and substrates, and unit cells with different rotation angles are simulated on CST Microwave Studio. Periodic boundaries are set in the x-axis and y-axis directions; two Floquet ports are set in the z-axis direction, and the excitation is set as the left circularly polarized (LCP) wave in the negative direction of the z-axis. Then, for topological charges l₁=0, l₂=-2, l₃=+2, and l₄=-4, according to the theoretical formula, the required phase distribution of the proposed metasurface is calculated. After that, the metasurface is designed on the basis of the Pancharatnam-Berry (PB) phase principle and the dielectric unit cell. Then, to verify the designed metasurface, we take frequencies f₁=0.35 THz, f₂=0.3 THz, and f₃=0.25 THz to calculate the corresponding incident angle simultaneously. Finally, far-field amplitude and phase patterns of right circularly polarized (RCP) transmission waves are simulated on CST Microwave Studio when three-channel circularly polarized (CP) plane waves with different frequencies and incident angles are obliquely incident on the metasurface in four directions.

In this paper, a dielectric metasurface working at 0.25-0.35 THz is proposed. When N-channel CP plane waves with different frequencies and incident angles are obliquely incident on the metasurface in four directions, in the direction perpendicular to the metasurface, 4×N-channel cross-polarized transmission waves are converted into coaxial beams that are orthogonal to each other in topology or frequency, namely that the 4×N channel multiplexing is realized. For simulation verification, we assume f₁=0.35 THz, f₂=0.3 THz, and f₃=0.25 THz. The simulations show that when four-channel LCP plane waves with frequency f₁, f₂, or f₃ are obliquely incident on the metasurface along the ±x and ±y axes, four topologically orthogonal coaxial beams with frequency f₁, f₂, or f₃ in the direction perpendicular to the metasurface are generated. At the same time, the generated three groups of beams are orthogonal to each other in frequency. It can be seen that on the basis of the designed dielectric metasurface, 12-channel incident waves are successfully converted into coaxial beams with topology or frequency orthogonality. In other words, 12-channel multiplexing is realized by the combination of OAM and frequency dimensions. The designed dielectric metasurface has potential application value in the field of high-speed high-capacity terahertz communication.

As people tend to pay much attention to the ecological environment, the detection of liquid salinity in hydrological environments has gradually become an indispensable part of research fields such as agricultural planting, aquaculture, and monitoring of the marine environment. Surface plasmon resonance (SPR) technology has unique advantages in liquid salinity detection due to its fast response speed and high precision. The optical fiber SPR sensor combines optical fiber and plasma technology to overcome the limitations of prism-type SPR sensors. It not only has the advantages of simple structure and water resistance of optical fiber sensors but also has a wide detection range and high detection sensitivity of surface plasmon technology and convenient operation. However, traditional optical fiber SPR sensors based on multi-mode or single-mode single metal film structures generally face many problems. Specifically, the metal film is easy to fall off, and the detection sensitivity is low. In addition, the stability is poor. Indium has excellent electrical properties and positive flexibility. Therefore, it is an important optoelectronic material and is easy to form a firm coated on a metal surface. Furthermore, it is not easy to react with alkaline solutions and has good corrosion resistance. Therefore, this paper uses the strong coupling characteristics of a three-core fiber and the principle of SPR to design a tapered fiber SPR sensor based on a silver film and a high-purity indium film, which can improve the sensitivity and corrosion resistance of the sensor.

This thesis is based on the theory of an optical fiber SPR sensor, and the propagation law of light in a tapered fiber is studied. The total reflection coefficient of the sensor is obtained by using the Fresnel formula, and the influence of the diameter of the cone waist region, the length of the sensing region, and the thickness of the metal film on the sensor is analyzed by numerical calculation, and the optimal parameters are determined. The sensor is fabricated according to the numerical values determined by the simulation, and the optical fiber is tapered by a hydrogen flame fusion taper. The silver film and the high-purity indium film are prepared by magnetron sputtering and molecular vapor deposition, respectively. First, the optical fiber SPR sensor with a single silver film structure is studied, and the change in its resonance peak is observed by dropping salt solutions of different mass fractions. Then, under the same experimental conditions, the sensing performance of the optical fiber SPR sensor with silver film and high-purity indium film structures is studied. After comparing the two sets of experimental data, a conclusion is drawn.

In this paper, an Ag/In structure SPR sensor based on a three-core fiber is designed. The multi-core fiber SPR sensing mechanism is theoretically analyzed, and the system structure parameters are determined. In addition, two fiber SPR sensors with only a single silver film structure and a silver film plus high-purity indium structure are experimentally studied. By comparing the experimental results, the resonance peak of the Ag/In structure SPR sensor shows a larger wavelength shift, and its refractive index sensitivity is about 44% higher than that of the Ag structure fiber SPR sensor in a mass fraction measurement range of 1.4%-3.6%, which can realize salinity measurement in a full depth range of 0--5000 m. Therefore, the introduction of high-purity indium outside the silver film can improve the sensitivity and stability of the sensor, and the sensor can be applied in fields such as environmental monitoring, biomolecular measurement, and climate prediction.

In the past 40 years, tunable diode laser absorption spectroscopy has developed into a remarkable optical diagnostic technique because it can provide a reliable, fast, time-resolved, sensitive, non-intrusive in situ measurement for various gas parameters, such as concentration, temperature, pressure, velocity, mass flux, and density. Hence, it has been widely used in many fields including environmental monitoring, combustion diagnosis, industrial process control, and biomedical sensing. In some special applications, there is not only gas but also some particles and liquid droplets in the measurement area, such as the soot in the flow field of combustion and fine particles (PM_2.5 and PM₁₀) in the atmospheric air. Then the variation in the intensity of the tunable diode laser is induced by both the absorption of the target gas molecule and the extinction/scattering caused by particles and liquid droplets. Thus, how to extract the absorption spectrum of the target absorption line of the measured gas molecule from the mixed signal is significant for the development of gas sensors based on tunable diode laser absorption spectroscopy. Here we design a gas-solid two-phase optical sample cell and apply it to simulate a gas detection environment in the presence of both particulate matter and gas. Temperature and acetylene content are measured in the gas-solid two-phase optical sample cell, which can demonstrate how to detect gas in a particulate environment.

The gas-solid two-phase optical sample cell used in the experiment is made of stainless steel and located in a ceramic fiber furnace whose temperature is controlled by a custom-made temperature control system. The heated static cell has a height of 290 mm and a diameter of 120 mm. The JGS3 quartz rods with a length of 90 mm and a diameter of 52 mm are inserted into the cell, and each quartz rod has an inclination of 1.5^o against the vertical direction to further reduce interference fringe noise. The temperature in the sample cell is measured by three K-type thermocouples with accuracies of ±1% and precision of 0.1 K. In the experiment, the C₂H₂-N₂ mixtures which are controlled by a mass flowmeter carry the quartz sand particles with a diameter of 125 μm into the gas-solid two-phase optical sample cell. A distributed feedback (DFB) laser with a central wavelength of 1540 nm is used to detect the absorption spectra of the target acetylene lines at 6489.07 cm^-1 and 6490.02 cm^-1. A multi-pass absorption technique is combined in the system to enhance performance. The temperature is inferred from the integrated absorption ratio of the two lines, and the acetylene content is determined from the integrated absorption of the line at 6490.02 cm^-1 due to its high absorption intensity.

In this work, temperature and acetylene content are simultaneously measured by means of a DFB laser with a central wavelength of 1540 nm in a particulate environment which is simulated by a gas-solid two-phase optical sample cell. Two target acetylene lines at 6489.07 cm^-1 and 6490.02 cm^-1 are chosen as the optimum line pair for the tunable diode laser absorption spectroscopy sensor to detect temperatures in the target range of 500-1000 K at atmospheric pressure. The comparison of the values measured by the sensor with the well-controlled gas temperature and acetylene content confirm the accuracy and reliability of the sensor. At present, the optical path length is only 0.36 m. The short path length limits the detection sensitivity of the sensor. In the future, the detection sensitivity will be decreased due to the increase in the reflection time of multi-pass absorption. Then the system can be used to detect the gas volume fraction and temperature in the flow fields of real flames or internal combustion engines, where both gas and particles coexist.

As a typical numerical representation of geometric models, the triangular mesh is widely used in additive manufacturing, inverse design, and finite element analysis. The triangular mesh model is directly reconstructed based on industrial CT images, which allows for the reconstruction of 3D representations of parts with complicated internal cavity structures. However, current algorithms for reconstructing triangular mesh models based on industrial CT images, for example, marching cube (MC) algorithm, have problems such as loss of sharp features, many long-narrow triangles, and a large number of triangular surfaces. In this paper, we propose an adaptive 3D mesh model reconstruction method to simultaneously address these issues while improving the quality of the reconstructed triangular mesh model from industrial CT images.

First, a bilateral filter and an OTSU algorithm are utilized to preprocess industrial CT images, so as to denoise and determine the value of the isosurface. Second, an octree structure is used to confirm the voxels; the octree is created top-down recursively, while non-boundary voxels are deleted to save storage space. The quadratic error function (QEF) is then applied to each boundary voxel of the octree to produce a feature point, and the octree is simplified by merging the feature points from the bottom up. Third, a quadrilateral formed by four adjacent feature points is divided into two triangular meshes. In order to validate the performance of the proposed algorithm, experiments are performed using a cubic dataset and two groups of real industrial CT images.

This paper proposes an adaptive 3D mesh model reconstruction algorithm to deal with the problems of sharp feature loss, many long-narrow triangles, and a large number of triangular surfaces in the reconstructed triangular mesh model of industrial CT images obtained from X-rays. Firstly, the image is denoised by the bilateral filter, and the value of the isosurface is determined using the OTSU algorithm. Then, voxels are organized by an octree structure, and the octree is generated top-down; feature points are generated by minimizing a quadratic error function (QEF), and an adaptive octree is constructed by merging feature points bottom-up. Finally, triangular meshes are generated by dividing the quadrilateral formed by four adjacent feature points. The algorithm in this paper checks its ability to keep sharp features compared with the MC algorithm. Compared with vertex clustering and edge-shrinking mesh simplification algorithms, the algorithm in the present paper can keep features and guarantee the quality of simplified meshes. Under the given simplified parameters, the method used in this research can adaptively extract the isosurface in voxels of different sizes based on the local characteristics of the object and achieve the reconstruction of an adaptive 3D mesh model. From the experimental results, it is found that the simplification rate of the triangular mesh model generated by the algorithm in this paper can be as high as 90%, and the average proportion of mesh quality higher than 0.3 after simplification reaches 99%. Compared with the conventional mesh method, the proposed method can maintain the sharp features of the model while simplifying the mesh, reducing the number of long-narrow triangles, and improving the quality of the reconstructed triangular mesh model from industrial CT images.

In laser inertial confinement fusion, high-precision X-ray imaging diagnostic instrument has become the key to observing the implosion process and quantitatively inverting the implosion parameters. It plays an important role in the research on irradiation uniformity, implosion compression symmetry, hydrodynamic instability, and fuel mixing. Rayleigh-Taylor (RT) instability during implosion is a non-linear high-gain transient process, which requires high spatial resolution, large effective field of view, and high temporal resolution of the diagnostic system. RT instability experiments are typically performed using plan-modulated samples with low amplitude and high spatial frequency sine periods. Higher spatial resolution helps reveal early phenomena of hydrodynamic instability. Currently, diagnostic X-ray imaging equipment widely used in diagnostic science mainly includes pinhole camera, Kirkpatrick-Baez (KB) microscope, Wolter microscope, and spherically bent crystal. Affected by initial configuration and optical processing capabilities, the optimum spatial resolution is 3-5 μm, and the effective field of view is limited to the order of hundreds of microns to millimeters. Improving the spatial resolution of diagnostic equipment at the submicron level is favorable for revealing the phenomena and detailed features that are difficult to observe in implosion diagnostics. In particular, it may enhance the ability to observe low amplitude and high spatial frequency sine samples in the study of RT instability. Wolter microscope is an ideal optical configuration for high-precision X-ray imaging diagnostics due to its high spatial resolution and high optical collection efficiency. However, it is difficult to directly apply the Wolter configuration to laser fusion research. Most of the previous development experience focused on the development of full-aperture Wolter mirrors and imaging systems. It is difficult to obtain the theoretically designed ultra-high spatial resolution since small aperture and closed quadric mirror are hard to be processed. Errors in the form and roughness of the mirror surface directly influence the performance of the Wolter configuration.

A submicron resolution X-ray microscope is designed for high-precision RT instability diagnostics. By improving the Wolter configuration, this paper transforms the closed inner surface that could not be directly processed and tested into an open outer surface that could be directly processed and tested by using part of the sector. The improved Wolter configuration is a double mirror structure based on a rotating hyperboloid mirror and a rotating ellipsoid mirror. It still has the technical features of the original Wolter configuration and can meet the technological requirements of high-precision optical treatment, inspection, and coating. A Wolter microscope system with large grazing angle and high magnification is designed. The main structural parameters of the system, such as object distance, grazing angle, magnification, and mirror size, are optimized by theoretical derivation and ray-tracing simulation. A large grazing angle and high reflectivity at the specific energy point can be achieved by coating periodic Cr/C multilayer films on the mirror surface. The ray-tracing simulation verifies the optical structural parameters and evaluates the imaging performance of the system.

The design and verification of a 2.5 keV submicron resolution modified Wolter microscope has been completed. The system working energy point is designed as 2.5 keV with a grazing angle of 2.0°, and the system magnification factor is 35×. Limited by the angular bandwidth of the multilayer films, the effective field of view is about ±0.35 mm. At the current technical levels, the mirror slope error is 1 μrad, surface shape accuracy is λ/43,and the roughness is 0.3 nm. In this condition, the resolution of the central field of view is about 0.63 μm, and the spatial resolution over the full field of view is better than 1 μm, which satisfies the designed submicron resolution. At the same time, if the accuracy of the surface shape increases to λ/85, the system can achieve imaging ability near the diffraction limit. The system is characterized by high collection efficiency and the geometric solid angle is 3.73×10-5 sr, without considering the reflectivity of multilayer films. While considering it, the response efficiency of the system reaches a peak of 1.52×10-5 sr and is greater than 7.55×10-6 sr in the field of ±0.28 mm.

The design of a submicron resolution X-ray microscope based on an open Wolter configuration is systematically described. The optical structure, design methodology, and performance characteristics of the microscope are presented in detail. A set of 2.5 keV submicron X-ray microscope parameters for RT instability diagnostics is provided. At the same time, it is pointed out that since the open configuration uses a portion of the mirror for imaging, the solid angle is smaller than that of the original configuration, but it is still larger than that of the pinhole camera and KB microscope commonly used in diagnostics. With the improvement in the super smooth rotary quadric mirror processing technology and a further increase in the effective mirror width, the geometric solid angle of the microscope can be greatly raised. This study extends the application of the Wolter configuration to high-precision radiographic imaging diagnostics. An X-ray optical configuration with a large field of view, high spatial resolution, and high collection efficiency is provided, which can effectively compensate for the shortcomings of existing diagnostic equipment. In the future, it is expected to play an important role in studying the growth of disturbance in low amplitude and high spatial frequency planetary modulated targets driven by long laser pulses.