Laser induced fluorescence (LIF) is an effective remote sensing technology for detecting oil spills on the sea surface. Bidirectional reflectance and reradiation distribution function (BRRDF) can describe the quantitative relationship between the incident laser and the emission fluorescence on the surface of mediums, which provides theoretical guidance for LIF technology to detect oil spill pollution on the sea surface. Based on Monte Carlo method, the photon transmission model of oil-in-water emulsion is established. The intrinsic optical parameters of oil-in-water emulsion are calculated by using the Mie scattering theory. The product of BRRDF and the cosine of photon emission and incident zenith angles XBRRDFcos θrcos θi is analyzed under different emulsifying time, fluorescence wavelength, and detection receiving angle. Simulation results show that with the increase of emulsifying time of oil-in-water emulsion, XBRRDFcos θr cos θi is an overall upward trend, and it is larger at the fluorescence wavelength where the fluorescence quantum yield is larger. The receiving optical power received by the transceiver coaxial LIF system is affected by its detection-receiving angles, thickness and concentration of oil-in-water emulsion, When the detection receiving angle is less than 50°, the received optical power of the system is larger.

Regarding the problem that the distribution pattern of polarized light is complex in test environments and the divergence of feature points reduces the accuracy of fitting solar meridian using the least square method, we propose a solar meridian extraction method based on Hough transformation. First, we build a polarized light distribution pattern test system and obtain the polarization azimuth distribution model based on the Stokes vector principle. By setting the feature threshold, we obtain the feature area of the solar meridian. Then, we detect the feature area edge and direction of the edge lines using the Canny operator and Hough transformation, respectively. Next, we calculate the solar meridian azimuth by combining the symmetry distribution relationship. Through the test experiment, we compare the measurement accuracy of the proposed method with that of the least square method. The experimental results show that the proposed method has higher measurement accuracy than the least square method. When the feature threshold is 3N≤6, the measurement correctness and measurement precision of the proposed method improved by 5.53%-87.84% and 1.81%-92.68%, respectively, compared with the least square method.

In this work, a high-precision, high-stability refrigeration system for a scientific-grade bare CCD detector was developed to reduce the thermal noise and dark current of the detectors. A temperature control method for the detector comprising a low-temperature circulator and a thin-film electric heater was proposed, which could reduce the influence of the test light source and environmental temperature change on device performance. Experiment results reveal that the cooling rate of the refrigeration system is less than 0.6 ℃/min and the temperature control accuracy is better than ±0.08 ℃. The refrigeration system was applied to the electro-optical performance test of an array detector for a directional polarimetric camera (DPC). The results revealed that the working temperature increased by 6.5 ℃, the dark current increased approximately 1-fold, and the dark current at 20 ℃ was approximately 6.93 times that at 0 ℃. The quantum efficiency in the near-infrared band was considerably affected by temperature. The change rates of temperature-induced quantum efficiency in the working range of 0--15 ℃ at the 810- and 900-nm bands are 0.2993% ℃ -1 and 0.4575% ℃ -1, respectively. The study of the temperature characteristics of the detector dark current and spectral responsivity provides a basis for the temperature correction of the in-orbit radiation calibration of the DPC.

This study design a novel power detector based on three-dimensional porous graphene (3DPG) and cholesteric liquid crystal microcapsule (CLCM) to measure high-intensity THz waves. The 3DPG is characterized by its high absorptivity, which is more than 97% at the frequency of 0.5-1.5 THz. The thermochromic properties of temperature-supersensitive CLCM are used for visual quantitative study of the THz power under steady state condition. The THz power intensity is as high as 2.77×10 2 mW/cm 2, where the minimal detectable THz power is only 0.009 mW. Further analysis reveals that there is a linear relationship between the THz power and the Hue value of the CLCM after the 3DPG is sputtered with a small amount of gold nanoparticles. The visual detector is simple, portable, inexpensive, efficient, and practically applicable. Further, it can be used in THz system alignment and beam analysis as well as THz imaging and sensing.

A high-precision reconstruction algorithm is proposed for ptychographic iterative engine (PIE) with highly tilted illumination. By replacing the highly tilted illumination with exiting light field of vertical illumination and modifying the optical transfer function in free space, a computing formula suitable for calculating the large-angle light propagation is obtained together with the corresponding iterative reconstruction algorithm. This proposed method successfully avoids the under-sampling of steep phase ramp of light beam on a sample plane while ensuring the computing accuracy for the diffraction intensity of the non-paraxial light beam on recording plane, overcoming the most significant technique problem that hinders the further improvement in reconstruction accuracy and the applications of single-shot PIE in various research fields.

In this paper, aiming at the feasibility of the visible and infrared large-aperture membrane diffractive optical systems in space application, we present a simulation model and fast analysis and calculation method of large-aperture diffraction optical imaging characteristics based on the two-dimensional finite difference time domain. First, we fully consider the electromagnetic field modulation characteristics of the subwavelength microstructure size of the large-aperture diffractive optical element, and establish the imaging characteristics analysis model of the sub-wavelength structure size diffractive optical system. Then, through vectorization programming and GPU acceleration calculation, we propose a fast simulation calculation method to solve the problem caused by vector calculation. Finally, we take a large-aperture diffractive optical system as an example and verified the imaging characteristics using the established imaging characteristics analysis method. Experimental results reveal that the proposed method can effectively characterize the imaging characteristics of the subwavelength microstructure diffractive optical system, and realize a rapid analysis of the imaging performance of diffractive optical elements.

Currently, narrow linewidth fiber lasers with output powers of several kilowatts have been developed; however, their power improvement and system stability are still majorly restricted by stimulated Brillouin scattering (SBS). Based on our previous verification, the tilted fiber Bragg grating (TFBG) can effectively filter the backward-propagating Stokes wave owing to SBS in fibers. This study further verified its suppression effect on SBS through experiments. With an optimized and fabricated TFBG being inserted between a single-mode fiber (SMF) amplifier and a 150-m-length single-mode energy-transmitting fiber, the backward-propagating Stokes wave is strongly rejected with a filtering ratio of approximately 10 dB. Meanwhile, an obvious increasing of the SBS threshold is observed with a value 1.25 times than that in the case without the TFBG, which increases the effective laser output power by approximately 20%. This study provides new insights into the SBS suppression in fibers. Furthermore, it is extremely useful in the further power scaling of high-power narrow bandwidth all-fiber lasers. It also has potential applications in fiber-optic communication and fiber-optic sensing.

The Doppler lidar has been widely used in the detection of atmospheric wind field. An optical fiber Mach-Zehnder interferometer (MZI) used as a new frequency discrimination system for the Doppler lidar, has the advantages of small size, light weight, and high stability. To address the problems of serious energy coupling loss and low coupling efficiency when the commonly used single-mode fiber MZI is directly coupled with the multimode fiber of the telescope receiving system, the multimode fiber MZI is proposed as a frequency discrimination system to improve the coupling efficiency and increase the energy and detection height. In order to solve the problem of interference quality degradation caused by the multimode fiber MZI, the quasi single mode output of the multimode fiber is realized by adjusting the incident angle and disturbing mode. Theoretical calculation results show that the incident angle of the quasi single mode output of the multimode fiber used in the experiment is 0.8° and the diameter of the disturbed mode is 2.2 cm, which are experimentally verified. Through the simulation and comparative analysis of the signal-to-noise ratio of Doppler lidar wind measurement, we find that the multimode fiber MZI used as a frequency discriminator will effectively improve the system detection performance.

To improve the measurement accuracy of objects moving at ultrahigh speeds, a correction amount δ is added to optimize the traditional calculation model based on analyzing the test process of a high speed photography system. Combining the two models before and after optimization, the theoretical and actual trajectory velocities of moving objects are simulated under different speed intervals, correction amount δ, distance between camera reflection screen hz, and other important parameters. The analysis shows that the proposed model exhibits accurately corrected movements of high speed and ultrahigh speed objects, and the correction effect improved with the increasing model speed. Analyzing the relationship between the distance and the measurement error reveals that when the hz value is greater than 30 m, the relative deviation is less than 1%. The field test results are consistent with the proposed calculation model and simulation model.

Herein, a cell segmentation model is proposed based on the U-Net and combination of residual block and attention mechanism. This model aims at addressing the problems of uneven brightness and low contrast between a cell and background of cell images collected using a phase contrast microscope. First, the U-Net with encoder-decoder structure is used to conduct the initial segmentation on cell images. Hereafter, the residual block is introduced into the U-Net to strengthen the propagation ability of the features and extract more cell-detail information. Finally, the attention mechanism is used to increase the weight of the cell area and reduce the interference of uneven brightness and low contrast on the model. The experimental results show that compared with other models, the proposed model exhibit better segmentation results in visual effects and objective evaluation indicators.

Traditional terahertz time-domain spectral imaging is performed by finding a single characteristic parameter of the imaging target. This method has great limitations,it cannot flexibly compare the advantages and disadvantages of imaging with various characteristic parameters, and it is difficult to form a clear contrast to imaging target. This paper proposes a new imaging method, multi-feature parameter imaging, for terahertz non-destructive detection of high temperature composite material bonding defects. This method can improve the overall imaging effect by changing the weight of a single characteristic parameter in the imaging channel. Based on the multi-feature parameter imaging method, the thickness of the upper adhesive layer of 0.05 mm and the lower adhesive layer of 0.15 mm are preset for debonding defect recognition. Compared with the single-feature parameter imaging method, the image contrast is increased by 2 times.

Digital mean filter can effectively reduce noise and improve signal-to-noise ratio in high-precision temperature measurement systems, but it will also cause signal distortion and become a source of uncertainty. The collected temperature signal is generally discrete-time series signal, so the existing filter evaluation method is difficult to quantify the distortion caused by this kind of signal. In order to solve this problem, the temperature variation characteristics of the object with slowly varying temperature are analyzed in this work. The typical temperature signal sequence of the measured object is collected by low-noise and high-precision measuring instrument, and then the input sequence of digital mean filter is constructed. A method to evaluate the uncertainty introduced by digital mean filter in high precision temperature measurement system is established. Temperature measurement experiments on a black-body under stable temperature conditions are performed. The temperature range of this black-body is (30.874±0.002)℃, and its rate of the temperature change obeys normal distribution function, which verifies the correctness of the proposed temperature characteristics model. Fluke 1595A thermometer is used to collect typical temperature sequences, and the uncertainty of digital mean filter is analyzed. Under the condition of different sampling intervals and filter lengths, the uncertainty introduced by digital mean filter in high-precision temperature measurement system is difficult to evaluate is basically solved.

The ground validation of radiometric calibration results of optical sensors is comprehensive affected by many uncertainty factors in earth-atmosphere system, which leads to the difference of single verification results in single site, and unable to effectively compare and comprehensively analyze the results. In this study, in consideration of the influence of target characteristics, atmospheric environment, and other factors on the single verification results of single site, a comprehensive weighting method based on weighted average value with the uncertainty of verification results as the weight is proposed to obtain the key comparative reference value (KCRV) closer to the real value, so as to realize the comparison and comprehensive analysis of single site verification results. Meanwhile, the radiometric calibration results of ZY-3/MUX made in China are comprehensively verified based on the ground targets with different reflection characteristics in the national high resolution remote sensing integrated calibration field. The results show that the KCRVs of 12 validation samples in blue, green, red and near-infrared bands are 3.88%、5.42%、6.14%、9.81% respectively, the corresponding uncertainty is 1.79%, 1.87%, 1.96%, and 2.02%, respectively. Among them, the equivalence degrees of 9 validation samples are less than 0.1, prove that the validation results of these samples are closer to KCRV.

Herein, a measurement method is proposed to obtain the binocular visual position and attitude of a target during the removal of a spatial non-cooperative target. First, an algorithm measuring the position and attitude based on binocular vision is designed. The algorithm detects the docking ring on the target surface based on arc-support line segments. Moreover, it establishes the detection and sifting mechanism of the docking ring in complex environments using polar line constraints and optical flow method. Further, regular marking points with prominent features on the target surface are extracted based on a two-pass algorithm coupled with area and curvature constraints. The measurement algorithm also builds the target coordinate system using the docking ring plane after the execution of a 3-D reconstruction method and regular marking points. Then, the pose relation under the world coordinate system is calculated. Hereafter, the algorithm is transplanted into the DSP6678 information processing platform, and the real-time continuous measurement of the positions and attitudes are realized for a spatial non-cooperative target. The effectiveness of the proposed method is verified using three small satellite model experiments under different working conditions.

A new technique for surface shape measurement is proposed, which is based on the Brox optical flow algorithm. The measurement system consists of a projector, a CCD camera and the measured object. First, one fringe pattern is projected onto a reference plane at a small angle and then two fringe images are recorded using the CCD camera before and after the measured object is added, respectively. Based on the analysis of the spatial geometry of the measurement system, the spatial relationship between the height of the object plane and the displacements of the observing point, the projector and the camera is established, in which the displacement of the observing point is obtained by estimating the optical flow distribution between two images based on the Brox optical flow algorithm. Aiming at the problem caused by the measurement system that the measurement result is not consistent with the actual shape, we analyze the cause of the error through the numerical simulation with the spherical crown geometrical model and propose a modified method. The theoretical simulation and the actual measurement verify the feasibility of the proposed method. By comparing with the Fourier transform method, it is proved that the proposed method can accurately restore the height information of the object with only two frames of images. The experimental process is simple, the measurement time is effectively shortened, and it is suitable for a dynamic measurement.

The calibration of optical axis parallelism plays an important role in multi-sensor photoelectric systems. Herein, a method for calibrating the optical axis parallelism based on interference fringes is proposed for calibrating the optical axes of passive multi-sensor photoelectric devices to meet the general application requirements of optical systems. The reference optical axis is established based on a collimation method. Newton ring interference fringes form by the reflection of the receiving optical system are fitted. Thus, the angle between the optical axis is calculated. The calibration axis of a laser distance-measuring machine is used to calibrate the optical axis of a transmitter and receiver. Experiment results show that the measurement accuracy for the receiving optical axis parallelism and the reference optical axis is better than 5″, satisfying the application requirements for calibrating multi-sensor photoelectric systems. The results also verify the effectiveness of the proposed method, whose visual calibration process can improve the calibration efficiency of passive optical devices and has the advantages of simple operation, practicability, and high measurement accuracy.

In this study, self-organized nanoripples on photoresist surfaces are produced through low-energy ion bombardment (IB). Subsequently, the IB-induced photoresist nanoripples are considered as masks for fabricating subwavelength nanostructures on fused silica surfaces by reactive ion beam etching technique. Compared with pure-IB-induced fused silica nanoripples, the amplitude and aspect ratio of the proposed nanostructures increases significantly. The transmittance of the subwavelength nanostructured fused silica surfaces increases to approximately 94% for wavelengths from 600 nm to 1300 nm. Preliminary results reveal the potential of ion bombardment in the preparation of functional surface nanostructures.

Aiming at that the requirements advanced driving assistance system for vehicle forward looking depth of field information, this paper proposes a scene depth estimation method based on monocular vision under the framework of unsupervised learning. In order to reduce the influence of forward looking targets with diverse sizes on the depth estimation results, the proposed method uses a pyramid structure to preprocess the input image. In the training process, the depth estimation problem is transformed into an image reconstruction problem, and a new loss function is designed using binocular images instead of the true depth label, which solves the problem that the depth data of the real scene is difficult to obtain. The size of disparity map and original input image is unified, which improves the hole phenomenon in depth map and improves the accuracy of scene depth estimation. The quantitative and qualitative comparison results on the KITTI and Make3D datasets show that the proposed method can obtain high accuracy absolute depth of field data and has good generalization ability, Experimental results in real road scenes show that the proposed method can obtain pixel level depth of field information from a single vehicle forward looking image.

The target rotation and scale change lead to the target losing and precision decrease greatly. Therefore, it has become a hot topic to solve the rotation and scale change of the target in the processing of moving. This paper presents a target tracking algorithm with rotation characteristics. Based on the BACF (background-aware correlation filter) proposed by Hamed et al., the proposed algorithm retains the localization in BACF algorithm, and converts the target features under Cartesian coordinates into that under polar coordinates. The Fourier-merlin formula is used to calculate the changes in rotation angle and target scale, and the dataset POT is used to verification and comparison. It is found that, in the improved algorithm, the rectangle box can rotate with the target when the target is rotating. The accuracy and success rate of the algorithm in this paper are greatly improved on the POT dataset, which are 0.6561 and 0.5930, respectively, and the rotation characteristic accuracy and success rate are 0.9619 and 0.8527, respectively.

The changes in ambient temperature will cause thermal focus shift in the optical system, which leads to the instability of the image quality. Due to the limitation of materials, the athermal design process and results of optical system in deep ultraviolet band are very complex. Therefore, we propose a method combining split design and single-layer diffractive optical element to realize the athermal design of deep ultraviolet optical system. In this method, the solution of the athermalized and achromatic equations are first presented, and the results are used to split and recombine the deep ultraviolet optical system, which simplifies the athermal design process. Then a single layer diffractive optical element is added to the combined system to simplify the design results. With this method, a deep ultraviolet reconnaissance camera lens with a focal length of 110 mm, an F-number of 3.5, and a temperature of -60--100 ℃ is designed athermally. When the Nyquist frequency of the obtained system is 18.5 lp/mm, the modulation transfer functions are all greater than 0.65. The results show that this method can solve the thermal defocusing of the deep ultraviolet transmission optical system in a wide temperature range, and at the same time can improve the optical performance of the system.

In order to realize the requirements of the sensitivity, detection timeliness, aperture and total length of an optical detection system in the space debris detection scenario, we design a space optical detection system with a large relative aperture and large field of view. Moreover, based on the detector index and target characteristics, we determine the design parameters and realize the detection of 12.5 magnitude stars. The system uses an asymmetrical double Gaussian lens group optical structure and has the working band of 450-850 nm, field of view of 20°, F number of 1.05, and entrance pupil diameter of 150 mm. We design the front surface of a lens in the system based on an XY polynomial freeform surface. The design and analysis results show that the diffuse spot of the system is within the 2×2 detection pixels, the surrounding full field-of-view energy ratio within the 2×2 detection pixels is larger than 86%, and the maximum distortion is less than 1.4% . The optical detection system possesses a compact structure, a reasonable aperture, good detection effect, high detection sensitivity, and strong timeliness. The performance of the adopted materials meets the conditions of space use and matches up with the optical processing capability. The designed detection system can be used for the accurate detection of space debris.

The aspheric test via finite distance null compensation is limited by too many adjustment factors and low test precision. Here we establish an aspheric test system by integrating front null compensation with back null compensation, in which a compensation lens is used before and after the spherical center of the mirror to be tested and thus the front and back areas show aberration correlation. First, based on the aberration theory, the optical parameters of two compensation lenses are theoretically derived and solved, and the relationship between the initial parameters and the normalized data is analyzed. Then the normalized results are scaled and optimized by the optical design software. Four concave parabolic mirrors with different curvature radii are designed under different magnifications of front null compensation lenses. The maximum aperture and the maximum relative aperture of the aspheric mirror are obtained. The compensation lens with a 1/10 aperture ratio is used to realize the aspheric test of an aspheric mirror with a large aperture of 3.7 m and a large relative aperture of 1/1.2, and the peak-valley (PV) value of the residual wavefront error is superior to 0.1λ (λ=633 nm). The tolerance analysis confirms that the test system is feasible. The principle experiment is conducted for the parabolic mirror with an aperture of 500 mm and a relative aperture of 1/1. The maximum diameter of the aspheric mirror and the difficulty of implementation are compared between the proposed method and the classical configuration. The PV value and the RMS value of the designed wavefront error of the test system combining front and back null compensation is 0.061λ and 0.009λ, respectively. The RMS value is superior to λ/40. The test system combining front and back null compensation is suitable for the high-precision test of aspheric surfaces with 4 m large diameter and large relative aperture.

Freeform lenses are used as freeform surface beam splitters to generate discrete spot arrays with arbitrary energy proportions. According to the law of conservation of energy, the incident beam is divided into a series of subregions corresponding to the spot array. For each subarea, variable separation is used to acquire the ray map between the input beam within the subarea and the corresponding spot. Further, a least squares method is used to construct the freeform surface following the ray map. Two design examples are provided to demonstrate the capabilities of freeform beam splitters: one could generate a Gaussian spot array with an equal energy proportion and the other could generate a square top-hat spot array with a predefined non-uniform energy proportion. Both freeform beam splitters exhibit light output ratios of over 89% considering Fresnel losses.

To solve the problems of low ship-target detection rate and low multi-spectral near-infrared (NIR) band utilization rate of optical remote sensors in complex background, a novel algorithm for saliency detection of ship targets based on four-band multi-spectral remote sensing images is proposed. The proposed algorithm employs the features of visual images in four-band remote sensing data that have rich color information as well as NIR images that have good ability to describe details. First, the three bands of blue, green, and red images are transformed into the CIE-Lab color space. Then, the NIR image is decomposed via the non-subsampled contourlet transform. The obtained high-frequency components are nonlinearly enhanced to suppress noise and enhance details, and the low-frequency components are enhanced via unsharp masking to improve the uniformity of image brightness. The high-frequency components and low-frequency components are combined with the brightness images in Lab space to obtain a new Lab image. Finally, the maximum symmetric surround model is applied to the new Lab image to obtain a saliency image of the ship target. The experimental results show that the proposed algorithm can fully suppress the complex background information of clutter interferences, such as cloud waves and sea wakes, and it also can highlight ship targets in low contrast backgrounds. The proposed algorithm has good precision and recall.

Glint from the assembled units of a satellite body is an embodiment of the individual characteristics of the satellite. By combining the characteristics of satellite motion with the mirror-reflection characteristics of materials, we have developed a method for determining in principle the direction of the mirror reflection and the reflectance of the satellite body, as well as for giving the area of the mirror-reflection component. Based on this, we propose a quantitative analysis of the glint from satellite-body components, using a multivariate model to describe the optical-scattering characteristics of the satellite, which we have verified with actual data. Experimental results show that the multivariate method is more comprehensive than the binary method for describing satellite optical-scattering characteristics, and is more applicable in principle. Specifically, it can reflect the unique optical-scattering characteristics of an individual satellite, and the accuracy obtained after adding the mirror-reflection component can be increased by 0.111 magnitude. In engineering applications, depending upon the actual observational situation, the model elements that describe the light-scattering characteristics of the satellite can be adjusted to simulate the optical-scattering characteristics of any target. The multivariate model not only provides a reference for explaining the glint phenomenon of satellites but also it can serve as a basis for target identification and working-state analysis.

Herein, the sea surface environment was simulated and a snapshot hyperspectral camera was used to detect the reflection spectra of spilled oil films with different thicknesses at the sea surface. First, based on hyperspectral images obtained using the snapshot hyperspectral camera, the reflection spectra of oil films were obtained. Hereafter, the wavelet transform was used for analysis, and the Coif5 wavelet base was used as the discrete wavelet transform to obtain the detail coefficient of the 9th layer reconstructed signal to determine the difference in oil film thickness. The oil film thickness and detail coefficient exhibited a good linear relationship, thus enabling accurate identification of the oil film thickness at different locations in a large-scale spilled oil accident. The snapshot hyperspectral detection method used herein significantly improves the spilled oil analysis efficiency and provides references for the rapid detection and real-time monitoring of spilled oil disasters.