Based on the measured remote sensing reflectance (Rrs) and phytoplankton pigment concentrations collected during seven cruises in the Bohai Sea, Yellow Sea and East China Sea from 2016 to 2018, we established the inversion models of total chlorophyll-a, chlorophyll-b, total chlorophyll-c, photoprotective carotenoid, and photosynthetic carotenoid concentrations from the geostationary ocean color imager (GOCI) Rrs products in the coastal waters of China. Furthermore, we obtained the concentration distribution of each pigment from 2014 to 2018 in the Bohai Sea, Yellow Sea and East China Sea. The research results showed that the inversion model established by the Rrs band combinations could achieve the quantitative inversion of pigment concentration, and the inversion accuracy of the established model was relatively high (R2>0.72). As can be seen from the satellite inversion results, the phytoplankton pigment concentration presented a trend of decreasing from nearshore to offshore waters, displaying a significant seasonal variation. In conclusion, the inversion model of phytoplankton pigment concentration established in this paper can provide method support for the in-depth understanding of phytoplankton population structure and spatial-temporal variation rules in the coastal waters of China.

This paper proposed a deep learning model based on a semantic segmentation neural network (UNet) for extracting floating macroalgae blooms effectively from the data of Geostationary Ocean Color Imager (GOCI) satellite sensors, achieving the end-to-end and pixel-to-pixel segmentation and recognition of the information of floating macroalgae blooms. The validation results show that the average recognition accuracy of the deep learning model for floating macroalgae blooms in the validation set can reach 88.54%. Compared with existing methods for detecting floating macroalgae blooms, including normalized difference vegetation index (NDVI) and alternative floating algae index (AFAI), the constructed model based on the UNet for monitoring floating macroalgae blooms has high accuracy and is less affected by clouds. Consequently, the recognition results of the UNet based model for floating macroalgae blooms are successfully applied to analyzing the outbreak process of floating macroalgae blooms in the East China Sea in 2017. The proposed model indicates a good applicability.

High-performance volume holographic gratings are important coupling elements for holographic waveguides, and small angular bandwidth and low average diffraction efficiency are major factors restricting the performance of volume holographic gratings. In this paper, a volume holographic grating with large angular bandwidth and high diffraction efficiency is designed and prepared based on asymmetrical tilt recording. First, we discuss the relationship between the recording parameters and the diffraction efficiency of the volume holographic grating under transverse electric mode light and transverse magnetic mode light to find the range of recording parameters at high average diffraction efficiency. Then, the relationship between the recording parameters in this range and the angular bandwidth of the volume holographic grating is further analyzed, thereby determining the best recording parameters to obtain a volume holographic grating with large angular bandwidth and high diffraction efficiency. The experimental results show that when the incident angle of the reference light is 25° and the incident angle of the signal light is 30°, the angular bandwidth of the prepared volume holographic grating reaches ±14° and the diffraction efficiency is 82%.

Acoustic sensor and photoacoustic cell are key components of laser-based photoacoustic spectroscopy. In this paper, we proposed an air-backed mandrel fiber optic microphone with a resonant photoacoustic tube, combining the technologies of fiber Michelson interferometer, phase generated carrier demodulation, and longitudinal resonant photoacoustic cell. In the microphone, we fabricated a sensing arm by wrapping a 10-meter ultra-thin single-mode fiber on a copper capillary tube that was also utilized as the resonant tube of the photoacoustic cell. Besides, a 5-cm reference arm has been isolated from sound and vibration. Furthermore, the resonant frequency of the microphone was designed to be slightly lower than the first-order longitudinal resonant frequency of the photoacoustic cell to achieve quasi-double resonance as the resonant frequency of the microphone was stable. The experimental results show that the minimum resolution is 0.69 μPa/Hz 1/2 at the resonant frequency of 1443 Hz, and the linearity of acoustic pressure and voltage response is 99.98% (5 mPa-3 Pa) with a dynamic range of 112.52 dB at 1 kHz. In conclusion, the microphone can be applied for trace gas detection in high-temperature, explosive, or high-electromagnetic-interference environments.

Aiming at the deficiency of the design of irradiance attenuators for solar simulators, we proposed a new design method of attenuators to improve their irradiance uniformity. According to the theory of Etendue, the mesh area of an attenuator was calculated and the radiation flux in the convergent optical path was modulated. Furthermore, the radiation distribution on the attenuator was theoretically analyzed by the zoned method of condenser. Then, the structure of non-uniform mesh distribution was designed to modulate the radiation distribution in the convergent optical path and the specific design parameters were provided. In addition, the effects of attenuators with uniform and non-uniform mesh distributions on radiation modulation and irradiance uniformity were compared and analyzed. The results suggest that the irradiance of 0.1--1.0 solar constant can be output continuously after an attenuator with non-uniform mesh distribution was installed and the power of xenon lamp was adjusted. Besides, after the modulation of radiation distribution, the maximum reduction rate of irradiance non-uniformity in the spots is 48.7%, and the wide adjustment of irradiance at low irradiance non-uniformity can be realized, which has certain reference significance for improving the performance of solar simulators.

Aiming at the wide-field-of-view and high-resolution imaging requirements of the airborne photoelectric imaging systems, we design a multi-scale wide-field-of-view and high-resolution optical imaging system based on a concentric spherical lens. The optical system includes a large-scale concentric spherical lens and a small-scale secondary camera array, and it has the advantage of compact structure. According to the spherical aberration and chromatic aberration of the concentric spherical lens, in combination with the small-scale cameras, the aberration is further corrected for the segmentation of the field of view, achieving wide-field-of-view and high-resolution imaging. Furthermore, the whole system is subjected to experiments under stress and at high and low temperatures. The experimental results show that the imaging system has good stability. Besides, the value of modulation transfer function in the full field of view is always close to the diffraction limit of the system, and the square root of the speckle radius is smaller than the pixel size of the detector, indicating good imaging effect of the system. In conclusion, the traditional airborne imaging system cannot satisfy both wide-field-of-view and high resolution at the same time, and provides a new idea for optical imaging system design.

In different unwrapping algorithms, the minimum cost flow (MCF) unwrapping method can limit the long-range diffusion of the residue error. Further, it can first limit the error to the low coherent region, which ensures that the unwrapping results in the high coherence region are not disturbed, and its accuracy is high. However, when the number of residues is large, the calculation efficiency is very low. To decrease the unwrapping time, a residue preprocessing method is proposed. In this method, the residues are regarded as positive and negative charges. Moreover, the electric field force is used to guide the residues with different signs close to each other to mutually offset, thus significantly reducing the number of residues and improving the efficiency of unwrapping calculations. Simulated and experimental data show that the residue preprocessing has little influence on the phase unwrapping accuracy. When the number of residues exceeds 3000, the residue preprocessing significantly improves the phase unwrapping efficiency.

In the 3D reconstruction of color structured light, the non-linear coupling of systems, the topological structure of the test surface, and other factors will affect the decoding of structured light, which can lead to the missed detection of modulation fringes and the false recognition of color codes. To solve the problem, we proposed the 3D reconstruction method of color-coded structured light. First, filtering differential projection was performed on the modulation fringes through the YUV color channel, and then the center feature lines of the fringes were extracted after an analysis of the waveform distribution of the fringes. Furthermore, the information values of color codes of the fringes were accurately acquired by virtue of the color clustering method. Finally, in order to establish the corresponding relationship between the codewords of coded fringes and those of modulation fringes, we put forward an optimal matching method based on the combination of sequence features, and combined with the mathematical model of binocular depth perception, we solved the three-dimensional information value of the codewords of coded fringes. The experimental results show that this method has a low rate of missed detection, a high recognition rate of color codes, and strong anti-interference and robustness.

In this study, the damage mechanism of thin film components is revealed by reconstructing the three-dimensional (3D) micro-morphology of a damaged optical film surface and by analyzing and studying the damage process and image damage characteristics of thin film components after laser irradiation. The interference microscopic 3D cloud data of the damaged thin film surface is collected based on the principle of white light interference microscopy. The triangular mesh model of the damaged surface is constructed using the Delaunay triangulation method. Moreover, the 3D micro-topography of the damaged surface is reproduced through a visual simulation. The results show that the surface damage areas of HfO2 films tested in the experiments are pit-shaped. The damage morphology is irregular (i.e., many bulges and cracks exist inside). In addition, the edge steepness greatly changes, and more burrs are observed. Compared with the reconstructed image, processing results of the VEECO Vision software, and test results of the Taylor-Hobson non-contact profiler, the reconstructed image can more intuitively and comprehensively reproduce the micro-morphology of a damaged surface. The results provide technical support for analyzing the morphology of the damaged surface and controlling the preparation process of films with high-damage threshold.

Infrared thermal radiation light source has important applications value in such fields as photoacoustic spectroscopy detection, and its light distribution characteristic is the basis of subsequent optical system design. In this paper, according to the variation law of radiant flux of the infrared thermal radiation light source under water cooling, a method for quickly obtaining the light distribution characteristic of a high-power infrared thermal radiation light source is proposed and verified. The experimental results show that the radiant flux of the light source is at an angle of 80°, and the radiant flux on both sides decreases slowly, and the energy is dispersed in the range of 0°--130°, 54.5% of which is concentrated in the range of 50°--110°. Furthermore, the accuracy and effectiveness of the rapid measurement method are verified through a comparative experiment of the light distribution characteristics under stable radiation for a long time. Beyond that, breaking the limit of light source angle in the ZEMAX software, the measured data is used to build a model and simulate the experimental process for verification. In conclusion, in the absence of the light distribution curve of the high-power infrared thermal radiation source, the proposed method can obtain the light distribution characteristics simply and quickly, providing initial conditions for optical design.

Breast cancer is one of the malignant tumors with the highest mortality among women globally and its early detection helps to increase the survival rate of patients. In this paper, we mainly used the target detection network in deep learning to locate and classify tumor lesion areas in the X-ray mammography images. Then, the Mask R-CNN network was taken as the target detection model for the improvement of its benchmark network D-ShuffleNet. Furthermore, a new network Mask R-CNN-II was proposed, to which the transfer learning algorithm was applied. Finally, it was experimentally demonstrated that the Mask R-CNN-II network had higher detection accuracy than the Mask R-CNN network. Besides, we also found that the proposed benchmark network, the idea of image fusion applied, and the transfer learning algorithm were effective. In conclusion, the network proposed in this paper is beneficial to improve the localization and classification of breast tumors and can provide auxiliary diagnostic advice for radiologists, which has certain clinical application value.

Environmental vibrations introduce random tilt and phase-shifting errors in interferometric measurement, which reduce the accuracy of the final measurements. To reduce the influence of these vibrations, this article proposes anti-vibration interferometric shape measurement based on tilt phase. First, the interferograms are Fourier-transformed into the frequency domain. Next, the peak coordinates are located at subpixel precision by a frequency-domain subdivision operation and the vibration tilt plane is extracted. Finally, the phase distribution is extracted by a least-squares algorithm. Experimental results show that the phase distributions retrieved by the method are highly consistent with those obtained by the simultaneous phase-shifting interferometry, with very small deviations of the peak-to-valley and root mean square values. Moreover, the method does not need to modify the hardware, and can provide a low-cost, high-precision solution for phase-shifting interferometry under vibration environment.

We experimentally and theoretically investigated the mode competition and output power characteristics in the terahertz quantum cascade lasers of first-order distributed feedback based on a buried grating. The simulations obtained the relationships of the etching depth of a buried grating with the waveguide loss, optical confinement factor, radiation loss, and radiation efficiency of two band-edge modes. Theoretical calculations have obtained the relationship between buried grating corrosion depth and waveguide loss, optical confinement factor, radiation loss, and radiation efficiency of the two band-edge modes. Theoretical calculations show that the distributed feedback structure of the buried grating can adjust the threshold gain and radiation efficiency of the laser while ensuring the stable single-mode operation of the laser in the high-band side mode by changing the corrosion depth. Experimental and test results show that the laser radiation wavelength is proportional to the period of the buried grating and the laser could operate stably in a single mode in the whole dynamic range. The single-mode laser can cover the range from 86.2 μm to 91.7 μm, the side mode rejection ratio can reach 25 dB, and the maximum output power is 9.1 mW. This work is helpful for the development of high-performance single-mode terahertz lasers and phase-locked laser arrays.

In this paper, we propose a light-excited, dynamically switchable, dual-band terahertz metamaterial absorber based on a square ring-wire structure. By adjusting the insulation/conduction state of photosensitive silicon and germanium chips embedded in the gaps of the structure, the absorber can be switched freely between three dual-band perfect-absorption states without changing its structure. The results show that when there is no laser pumping, the absorber works in a dual-band absorption state at 0.987 THz and 1.767 THz; when using 800 nm laser pumping, it switches to dual-band absorption at 0.717 THz and 1.444 THz; and when using 1550 nm laser pumping, it switches to dual-band absorption at 0.716 THz and 1.767 THz. In addition, the three switchable dual-band absorption mechanism is explained from equivalent circuit, impedance matching and current distributions, and the polarization-insensitive absorption characteristics are discussed.

In this study, a wide-coverage and high-resolution airborne camera optical system is designed for wide-area and efficient aerial work demand of airborne camera. The optical system adopts a novel cascade structure of optical imaging consisting of a fore monocentric symmetric objective and a relay lens array. The fore monocentric symmetric objective obtains a wide field-of-view curved surface image with uniform residual aberrations. The relay lens array simultaneously performs the field-of-view subdivision, residual aberration correction, and relay imaging for the curved surface image. The optical system in the designed airborne camera has 60 mm focal length, 3.4 F number, and 132° field-of-view angle. The relationship between the imaging performance of the airborne camera optical system and the wide-field curved image at different flight altitudes is studied herein based on the first-order theory and aberration characteristics. A method for achieving clear imaging at different flight altitudes is also obtained. The image performance evaluation results show a good image quality when the optimized design system performed ground observation at low, hollow, and high altitudes. The modulation transfer function reaches 0.4 at the Nyquist frequency of 230 lp/mm, and the root mean square radius of the system's ray tracing spot diagram is less than 1.6 μm, verifying that the designed cascade optical imaging system is suitable for airborne cameras with different flight altitudes.

In this paper, a static correction method based on a fixed corrector and a lens array is proposed to correct the aberrations introduced by an onboard conformal window in a large scanning field of view. To be specific, first, a fixed corrector is used to correct the static aberrations. Then, a fixed lens array is placed in front of the image plane and the lens units in the array are respectively employed to correct the dynamic aberrations at different scanning angles. Finally, an aberration corrector in an onboard conformal optical system is designed based on the proposed method. The design results show that the proposed method can well correct the aberrations introduced by the conformal window in the scanning field of view of ±42°. Compared with other dynamic or static correction methods, the proposed method can realize the aberration correction of an onboard conformal optical system in a large scanning field of view, reduce the mass of the system, and improve the stability of the system.

Aiming at the problem of difficult adjustment of prism-grating-prism (PGP) imaging spectrometer, the method of correcting the chromatic aberration of PGP imaging spectrometer is used to ensure that the detector image plane is perpendicular to the optical axis, and a wide-band achromatic PGP system is designed in this paper. Starting from the wide-band apochromatic theory, the minimum theoretical chromatic aberration of the three glass material combinations is calculated, which provides theoretical support for the apochromatic optical design. Using the optical design software to optimize the initial structure, the results show that the secondary spectrum of the PGP system has been well corrected, and the CCD of the detector does not need to be tilted, which is more convenient for later installation and adjustment. The coverage spectrum width is 400--1000 nm, the field of view is 9.2 mm, the spatial resolution is better than 10 μm, the spectral resolution is better than 2.8 nm, and the optical transfer function is greater than 0.7, which is close to the diffraction limit, meeting imaging requirements.

A scheme for phase-matched passive-decoy-state quantum key distribution is proposed based on the heralded single photon source. In the scheme, both communication parties only need to generate one-intensity signals. The detection results of the third party are divided into four sets according to the response of the local detectors of both communication parties. These four sets not only serve as the signal state and the decoy state, but also jointly participate in parameter estimation and key generation. Thus, the difficulty of system implementation is reduced and the performance of the scheme is improved. The simulation results show that the maximum safe transmission distance of the phase-matched passive-decoy-state scheme can reach 552 km, and the performance is close to those of the existing phase-matched active-decoy-state schemes. Moreover, there is no need to generate decoy states actively. To some extent, this scheme overcomes the drawback that the phase-matched active-decoy-state scheme relies on the detection efficiency heavily and makes the performance more stable. As the data length decreases, the transmission performance of the scheme is declined. Even if the data length drops to 10 7, the maximum safe transmission distance of the scheme can still reach 507 km.

In this paper, a set of inversion algorithms for the micro-motion parameters in a retroreflector array was established based on the time-frequency signals of laser dynamic speckles. Firstly, the real-time intensitys formula of laser speckle from a retroreflector and a retroreflector array were derived by a physical optics method. Then, based on the short-time Fourier transform, the formation mechanism and digital characteristics of the speckle power spectrum were studied. Finally, the spectral correlation method and time-frequency to phase-amplitude transform algorithm were proposed, and the algorithm was used to extract the period and amplitude distribution of the time-frequency spectral lines of dynamic speckles. Besides, we made an inversion on the spin period and the direction of the rotation axis of a target in three typical motion states. The results show that the proposed inversion algorithm can obtain the rotation period and line-of-sight angle with high accuracy through speckle intensity signals in a few periods, but the inversion accuracy for the azimuth angle of the rotation axis is relatively poor. It follows that more observation data are required for satisfactory results.

Information in visualizing flow field is needed for optimization in the aerodynamic opening process of parachute and in the process of optimizing aerodynamic shape of commercial aircraft. In this paper, based on the principle that atmospheric disturbance can lead to deflection of the background light, high-precision atmospheric disturbance detection method for moving objects is established. The method is composed of integral pixel crossing search, Newton-Raphson sub-pixel location, and solution of disturbing equation. The theoretical disturbance detection accuracy in ideal conditions is verified using numerical speckle image. Using the speckle image as the background in the laboratory, the method and the traditional schlieren monitoring method are used to detect the disturbance of the high-pressure tracheal outlet airflow. The results show that the accuracy of the method is high, and it can provide a more applicable way for visual monitoring outside the laboratory.

In order to meet the high-frequency on-orbit calibration requirements of the on-board load, the site automatic calibration technology is often used to invert the multi-spectral reflectance into the hyper spectral reflectance, so it is particularly important to improve the precision of automatic calibration to improve the inversion accuracy of spectrum. In this paper, we calculate the reflectance from September 2018 to September 2019 using a channel-type automated observation instrument deployed in the Dunhuang radiometric calibration site. Furthermore, we divide the data into 6 groups according to the difference in the solar zenith angles during the measurement process and employ the model of bidirectional reflectance distribution function (BRDF) for the consistency analysis of spectral shapes at different solar zenith angles. The experimental results show that the BRDF model is effective for correcting the data at different incident angles.

Molecular spectral parameters are fundamental data for IR radiation calculation. Currently, there is no such gas spectral database as the high resolution transmission molecular absorption database (HITRAN database) in China. In this paper, supported by the National Numerical Windtunnel (NNW) project, we developed the calculation codes of high-temperature air spectral parameters to support the calculation of target IR radiation. To be specific, NO is one of the products of chemical reactions in the high-temperature air flow field of hypersonic vehicles, and the radiation generated by its vibration-rotation levels is in the typical band of infrared detectors. Based on the molecular potential and permanent dipole moment of split NO calculated by the ab initio method, we calculated the line intensity of NO at 8000 K, and the theoretical calculation results were in a good agreement with the values from the HITRAN database at 300 K and 3000 K. Moreover, we calculated the absorption coefficient of NO at the X 2Π1/2 state at 296 K and 2000 K by employing the narrow-band model. It turns out that without the help of experimental spectral constants, the proposed method can obtain more line positions than the HITRAN database in both low and high vibration levels, which can provide the high-temperature spectral data of NO for the calculation of target radiation characteristics in the NNW project.

Ultrafast imaging is an important method for studying ultrafast phenomena such as explosions and high-voltage discharges. All-optical framing imaging has great development prospects because it can overcome the time limitation of optical-to-electric signal conversion. In this study, an all-optical spatial framing imaging system is constructed using a diffractive optical element and a band pass filter, the array framing imaging is successfully realized, and the results of different bands are analyzed. The experimental results show that the designed all-optical spatial framing imaging system can realize 16-framing imaging in a 4×4 array in different wavelength bands. The relative standard deviation of the non-uniformity between the image frames is 7.4%, and the average deviation of the non-uniformity within a frame is 2.83%. Further, the modulation transfer function of the all-optical framing imaging system is 0.991 at resolution of 35 lp/mm.

Under the 45°/0° illumination observation conditions of various light sources, an evaluation method and measuring device are developed to measure the flash effect of metallic coatings. Based on different color distributions and sparkle grades, 39 metallic paints are selected to create the sample database. Multispectral images of the collected samples are corrected to meet the visual response of the observer under different light sources. The sparkle points and background are separated by setting the threshold related to the images. An algorithm for the sparkle area, intensity, and grade is developed by calibrating the test data of BYKmac. The correlation between the sparkle grade measured by experimental setup and visual evaluation data is further evaluated under light sources D65 and A. Experimental results show that the correlation coefficient between the experimental device and visual data are 0.848 and 0.851 under light sources D65 and A, respectively. Moreover, the measurement effect of the experimental device is better than that of the existing measurement equipment.