Spaceborne two-dimensional turntable is an important part of the laser communication pointing structure and one of the key components affecting the performance of the satellite-based equipment, which requires reasonable structural design and correct stiffness analysis, and the correctness of the design analysis is verified by comparing the test results with the simulation results. Firstly, the structural form of spaceborne two-dimensional turntable was introduced, two structural form of pendulum mirror assembly was compared and analyzed based on Isight integration and optimization method, the stiffness design of the U-shaped frame and shaft system was completed. Then, finite element modeling of the two-dimensional turntable was conducted and the spring unit equivalent modeling of the bearing was focused on. Finally, the stiffness analysis and test verification of the spaceborne two-dimensional turntable were carried out. The error between the natural frequency of the two-dimensional turntable in the X, Y, and Z directions of the simulation calculation and the test result was within 6%, indicating that the equivalent spring unit can accurately simulate the mechanical characteristics of the bearing joint. The dynamics test of the two-dimensional turntable shows that the mass constraint and stiffness requirements of the two-dimensional turntable are ensured, the structural stiffness of each component part is reasonably designed, and the mechanical resistance of the two-dimensional turntable meets the mission requirements of laser communication.
An e-nose system based on near infrared spectral absorption technology which can be used to detect target gases accurately and efficiently was designed and implemented in this paper. The e-nose was composed of near infrared laser emission unit, gas cell unit, system controller unit, and human-machine interface. Principal Component Analysis (PCA) algorithm and Back Propagation (BP) neural network for analyzing the collected date were integrated into the upper computer software by the MATLAB Script node supplied by LabVIEW. The results indicate that the e-nose is stable, and its accuracy is 0.0001, when the time of the networks training reach more than 1 000. The recognition accuracy rate in recognizing the white vinegar, rice vinegar and apple vinegar is 100%, which can achieves the design goal of high-precision, high-stability, high resolution, and behaves good application prospects.
Computer aided alignment (CAA) plays an important role in the assembly of optical systems. However, most proposed CAA methods are based on wavefront aberrations, which need the aid of an additional high precision wavefront sensor (WFS). Based on the ellipticity distribution of stellar image in different fields of view (FOVs), a misalignments calculation method for aligning optical systems efficiently without WFSs was proposed, which only needed an image sensor such as CCD/CMOS. Based on the nodal aberration theory, the theoretical basis of the proposed method was introduced and the ellipticity distribution of stellar image under the effect of misalignments and FOVs was analyzed, which revealed the essence of the proposed method. The ellipticity distribution could be expressed as a function of misalignments, just like the wavefront aberrations. Based on those, the process of misalignments calculation for optical systems was established and it turned to be a multi-objective optimization problem which could be solved by optimization algorithms. The method was tested in Hilbert telescope, which was a typical two-mirror optical system, and four lateral misalignments were used to be calculated according to the ellipticity distribution in three FOVs. The simulation results show that the precision of misalignments calculation can reach up to micron level and verify the correctness of the proposed method, which can promote the application of CAA and provide new insights for non-WFS optical alignment.
The accuracy of absolute distance measurement is of great significance to the fields of aerospace technology, precision equipment processing, satellite formation, and planetary space positioning. The frequency sweeping interferometry (FSI) ranging technology based on tunable lasers has become an international research hotspot in recent year. It has the advantages of breaking 2π ambiguity, no dead zone of measurement, non-touch and independent of guide rail. The principle of FSI ranging, types and performance of some devices in the ranging system were briefly introduced, such as tunable lasers, detectors, etc. The factors that affect the uncertainty of the ranging system, including non-linear frequency sweep, Doppler frequency shift, dispersion mismatch, etc. were analyzed. Corresponding compensation methods for influencing uncertainty factors were discussed, and measurement results after compensation were compared and summarized.
The large aperture fast steering mirror(FSM) are often used in the fields of space optical communications and laser weapons. The large aperture fast steering mirror surface figure test system with an effective aperture of 400 mm has a working wavelength of 633 nm. The system is mainly composed of an off-axis beam expanding system and a focal plane system to achieve real-time detection of the mirror surface figure error when the large aperture FSM is working. In this paper, the design parameters of the optical system and the selection of optical element parameters were described. The optical system was designed and simulated, and the effect of temperature variation on the optical system was obtained based on the integrated optical-mechanical-thermal analysis. The test results show that the large aperture fast steering mirror surface figure test system can achieve real-time recording and high precision measurement, meanwhile, the system has stability in a working environment with temperature changes. The stability of the surface figure test system is 0.048λ (RMS, λ=633 nm).
In order to improve the image stability accuracy of the photoelectric servo stabilization platform, a compensation method of friction observation based on the running angle position information of the mechanism was proposed according to the principle of friction disturbance on the platform. Firstly, the precise control model of the photoelectric servo platform was established, and the friction torque of the platform was analyzed by using LuGre friction model, and then, the acceleration was estimated by the least square method based on the angular position information of the mechanism, and a disturbance observer based on the angular position information of the mechanism was designed to compensate the friction observation. Finally, the system simulation platform was established to perform simulation and experimental verification in Matlab/simulink. During the experiment, the mirror mechanism was installed on the simulation turntable, and a 10 Hz sinusoidal signal was applied to the turntable to compare and analyze the before/after experimental data of friction observation compensation. The experimental results show that when the simulation flight turntable moves sinusoidal at 10 Hz frequency, at friction observation of the before/after compensation, the image stabilization accuracy of the platform is reduced form 26 μrad to 17 μrad, and the performance is improved by 34.6%. This method can effectively enhance the anti-interference ability of the system, and improve the stability of the system.
In view of the large difference in the intensity of single channel multi-grating reflection spectrum of demodulator, which leads to the failure of peak-seeking or the increase of peak-seeking error, a method of multiple exposure demodulation in different exposure cycles was proposed to adjust the peak value, and the peak-seeking threshold was determined according to the histogram of the spectral data. The influence of peak value of spectrum on the stability of peak-seeking was analyzed, the adaptive adjustment rules of exposure cycle and peak-seeking threshold were established, and the adaptive peak-seeking demodulation algorithm was realized by LabVIEW software. Through the actual fiber grating sensor test, the automatic exposure and peak-seeking demodulation of the reflection spectrum with large difference could be completed. On the premise of ensuring the peak-seeking stability, the number of spectral peak identification, the adaptive ability of the demodulation system and the working reliability were effectively improved. The experimental results show that the peak stability is the highest when the peak value is in the range of 70%-90% of the saturation intensity. The standard deviation of the center wavelength obtained by peak-seeking is within 0.5 pm, the stability is increased by 50% compared with single exposure demodulation, and the program running time is within 100 ms, which can realize fast demodulation.
According to the design requirements of the birefringence of polarization-maintaining fiber in the non-line-of-sight azimuth transmission system, the influence of different types of polarization-maintaining fiber parameters on the birefringence is emphatically analyzed. Firstly, the stress-optical coupling relationship of polarization-maintaining fiber was deduced based on the stress-strain, variational principle and the stress-photoelastic effect. Then, the influence of different factors on the birefringence of polarization-maintaining fiber was investigated by means of finite element analysis software, and two kinds of polarization-maintaining fiber (Panda polarization-maintaining fiber and Bow-tie polarization-maintaining fiber) were compared and analyzed. The results show that the higher birefringence value near the center of the core can be obtained by a variety of methods, such as reducing the distance between the core and the stress zone, increasing cladding radius when fixing core size, or increasing the reference temperature of the polarization-maintaining fiber. Simultaneously, the birefringence of the bow-tie polarization-maintaining fiber is larger in the same conditions. The research results can provide some reference for the design and selection of polarization-maintaining fiber in the non-line-of-sight azimuth transmission system.
An FBG spectral demodulation method based on deep learning was studied. The Convolutional Neural Networks(CNN) model was used to deal with the nonlinear sequence model of the overlapping spectrum, and the central wavelength of the overlapping spectrum was predicted and identified through a one-dimensional convolutional neural network. And a parallel structure of the overlapping spectrum data automatic acquisition experimental system was built to verify the high-precision demodulation of the center wavelength of the overlapping spectrum. The experiment analyzes the effects of training samples and epoch times on training time, testing time, and demodulation accuracy, and tests the computational demodulation time of the model after training. The demodulation accuracy and test time were compared with other demodulation algorithms. At the same time, the demodulation model algorithm and the peak finding algorithm at the highest point were used to compare the center wavelength value and analyze the error for the same set of spectral data. The experimental results show that the root means square error of the demodulation model is 0.082 58 pm, and the demodulation calculation time is 30.886 ms, which is used Intel(R) Core (TM) i7-8550U CPU. The research results show that the convolutional neural network model is reasonable for the accuracy of the central wavelength demodulation results of the overlapping spectrum. Compared with other algorithms, the demodulation algorithm in this article has advantages in demodulation accuracy and time. The model size is less than 400 kB, and the required computing power is small. It can be deployed in small embedded devices. It has good application prospects in large-scale airborne sensor networks and structural health monitoring.
Aiming at the problem of poor confidence propagation decoding performance of polarization codes caused by atmospheric turbulence in wireless optical communication, a Deep Neural Networks-Normalized and Offset Min-Sum (DNN-NOMS) decoding method under wireless optical communication was proposed. First, the factor graph of the traditional belief propagation decoding algorithm for polarized codes had been transformed into Tanner graphs which similar to Low-density Parity Check (LDPC) codes. The Tanner graphs were expanded and transformed into Deep Neural Network (DNN) graphical representations. The Min-Sum (MS) decoding method added the normalization factor and the offset factor, at the same time to the edge weights of the Tanner graph were given, which simplified the calculation method of the log likelihood ratio of the polarization code. By limiting the number of training parameters, the factor parameters were selected under the condition of the minimum loss function, and trained to obtain the optimal normalization factor and offset factor of the decoding model . The simulation results show that under different atmospheric turbulence intensities, the decoding method can select better normalization factor and offset factor parameters under the premise of sacrificing smaller storage space, so as to obtain better error codes. The DNN-NOMS decoding method can produce a performance gain of 0.21-3.56 dB and reduce the number of iterations by 87.5% when the error rate is 10-4.
Phase sensitive optical time-domain reflectometry system due to its advantages of distributed optical fiber sensing technology has a high application prospect in low frequency monitoring fields such as distributed hydrophone, fracture micro-seismic detection and natural disaster warning. The problem of different clock source of pulse chopper signal and frequency modulated signal in the system was verified and the influence was analyzed theoretically in this paper. A dual-channel synchronous clock source was designed to generate pulse chopper signal and frequency modulation signal to reduce the random low-frequency phase noise of frequency modulation signal in each pulse repetition period and improve the phase stability of the detection pulse light. The acousti-optic modulator of typical phase-sensitive optical time-domain reflectometry system based on heterodyne coherent detection was driven by clock homology and clock non-homology, a signal generator drives a piezoelectric ceramic wrapped in optical fibers to generate disturbance signals in different frequency bands. The experimental results show that under the same test conditions, the former is superior to the latter in the aspects of SNR, phase demodulation quality and frequency response in low frequency band. The minimum response frequency is 0.1 Hz, which is 2 orders of magnitude higher than the latter, and reduces the interference of low frequency noise in the system. The method was easy to implement and compatible with the existing low frequency performance optimization methods or structures to further improve the low frequency response performance of the system.
The subsurface damage of workpiece formed in lapping and polishing process is the main reference to evaluate the quality of processing technology and decide the machining allowance. Therefore, the accurate prediction of subsurface damage is helpful to improve the machining efficiency. Discrete element method (DEM) was used to simulate the subsurface damage of the fixed abrasive lapping process of typical soft and brittle material ZnS, and the depth of subsurface microcrack layer after diamond machining with different particle sizes was predicted. The angle polishing method was used to polish the workpiece to an inclined plane as the sub surface damage observation plane. The corrosion of hydrochloric acid makes the subsurface microcracks appear. Under the metallographic microscope, the end point of microcracks disappearance was found and converted into the depth of subsurface microcrack layer, and the simulation results were verified by experiments. The results show that the predicted values of the depth of subsurface microcrack layer caused by grain size of 5 μm, 15 μm, 25 μm and 30 μm are 2.28 μm, 3.62 μm, 5.93 μm and 7.82 μm respectively, and the measured values of angle polishing method are 2.02 μm, 3.98 μm, 6.27 μm and 8.27 μm respectively. The results show that the wear particle size has a great influence on the subsurface damage of ZnS. With the increase of wear particle size, the depth and number of micro cracks increase. The deviation between predicted value of discrete element method and measured value is 5% - 15%. The subsurface damage of soft and brittle material ZnS after processing can be accurately predicted by using the discrete element method, which provides a reference for the formulation of polishing process.
A refractive index sensor based on single-mode fiber (SMF) four-core fiber (FCF) and thin-core fiber (TCF) is proposed, forming a SMF-FCF-TCF Michelson interference structure. The optical fibers are spliced by direct splicing. Because of the mismatch of the diameter of the optical fibers’ cores, light excitation and coupling will be induced at the splicing part. The end face of the TCF is coated with a layer of silver film and protected with ultraviolet curing glue to enhance the reflectivity of the light at the end face. The four-core fiber is used as a coupler in the sensing structure, which excites more light into the cladding of the TCF, improving the sensitivity of the sensor. The refractive index and temperature sensing characteristics of the sensor were investigated experimentally. The experimental results show that the sensitivity in the refractive index range of 1.3333 to 1.3794 is 137.317 nm/RIU, the linearity is 0.999, and the temperature has little effect on the sensor. The sensing structure has a simple welding method and has certain application prospects in the field of refractive index measurement.
Polarization and phase controlled beam splitter is an indispensable optical element in free space quantum communication system. Its performance directly affects the communication quality and determines the communication error rate. Based on the theory of equivalent layer design, the special film structure of ''dielectric+metal+dielectric'' is adopted, and Ag metal material and SiO2, Al2O3, Ta2O5 dielectric material are selected to realize 45° incident angle on quartz substrate, and the average transmittance/reflectance ratio is 8.5: 91.5 in the wavelength range from 1500 nm to 1600 nm. Phase is controlled at 1530, 1540, 1550, 1560 nm. By optimizing the deposition process, the splitter film samples are prepared by electron beam evaporation with ion assisted technology. The test results show that the average transmittance/reflectance ratio is 8.53: 91.47 in the wavelength range of 1500-1600 nm under the condition of 45° incidence. The transmitted phase difference controlled within 5.02° and the reflected phase controlled with 8.05° in the range of 1530, 1540, 1550, 1560 nm, which meets the requirements of spectral energy splitting ratio and phase control of communication system. In addition, the film passed the corresponding environmental test, which meets the reliability requirements.
Local cleavage of indium antimonide (InSb) chips restricts the improvement of the yield of InSb infrared focal plane detectors (IRFPAs) under cyclic liquid nitrogen shocking tests. Stress concentration effect may appear in isolation troughs surrounding mesa-junction photosensitive units, drives the dislocation line to nucleate and to propagate, ultimately to punch through InSb chips. In order to analyze quantitatively the influence of isolation troughs on the cleavage of the InSb chip, a structural model of InSb IRFPAs was established, and the in-plane normal stress distribution on the InSb front surface was obtained. Stress concentration phenomena appear on the bottom of V-shaped isolation trough added. Then, the assumed initial cracks with different lengths at the bottom of V-shaped isolation trough were put, here the preset initial cracks were employed to describe dislocation lines in InSb wafers, and were perpendicular to the InSb chips, and obtained the relationship between the energy release rates and the preset crack length. After analyzing these results, the in-plane stress concentration phenomena appears exactly at the bottom of V-shaped isolation trough, and originates from the added V-shaped isolation trough; the enlarged stress at the bottom of V-shaped isolation trough could drive the dislocation lines in the InSb chip to grow and to punch through the InSb chip, thus, the macro cleavage of InSb chip is created; once the preset cracks connect directly with the bottom of V-shaped isolation trough, cleavage of InSb chips is more likely to appear. All these conclusions provide a new perspective to understand the cleavage of InSb chips.
Quantum dots have attracted much attention in recent years because of their excellent photoelectric properties. However, the large-scale application of quantum dots has yet to be developed due to its processing technology and stability. The emergence of quantum dot-polymer nanocomposites effectively makes up for this problem. It is an effective method to solve the current application problems of quantum dots by disperses quantum dots into organic polymers to form nanocomposites and integrates the respective advantages of quantum dots and polymers. It has significant development potential. The main preparation technology of quantum dots was introduced, on this basis, the preparation methods of QD-polymer composites and their applications in lasers, light emitting diodes, photodetectors, QD-TVs and other optoelectronic devices were summarized, and finally its application in the field of optoelectronic device was prospected.
A multimode, small, Wide-Field-of-View, lightweight, low-cost photodetector is the key to develop the next generation precision-guided munitions. As one of the important representatives, the Wide-Field-of-View photoelectric detection system based on artificial compound eye has been highly valued by the U.S. Air Force. Through a number of innovative research projects, the U.S. military expects to develop a multi-aperture Wide-Field-of-View artificial compound eye photodetector, which can be used for autonomous guided munitions and strengthen the environmental perception, penetration and confrontation ability, and combat effectiveness of the next generation of precision guided munitions. The development requirements of precision guided munitions, as well as the structure and characteristics of compound eye, were briefly described. The application expectations and assumptions of artificial compound eye in precision-guided weapons in the U.S. military were introduced. The advantages of artificial compound eye in urban combat weapons and equipment were briefly discussed. The progress of foreign artificial compound eye in infrared precision-guided munitions was discussed, as well as the research and development process. The application progress of foreign artificial compound eye in semi-active laser-guided munitions was described. Finally, the research suggestions on the application of artificial compound eye in precision-guided weapons and equipment were given.
The metalenses with focusing and imaging based on metasurfaces, which can manipulate the amplitude, phase, and polarization of light waves, have attracted enormous attentions. A high numerical aperture bifocal metalens with regulatory focusing intensity was designed, and both theoretical analysis and simulation verification were demonstrated. The simulation results reveal that the designed metalens can focus circularly polarized incident light efficiently to a spot of full width at half-maximum as small as ~0.44λ, and the corresponding numerical aperture reaches up to 0.95. Besides, the relative intensity of two focal points can be adjusted flexibly through changing the polarization states of the incident light, which is unlike previous bifocal metalenses with repatterned intensity. More importantly, when the circularly polarized light is incident, the focusing efficiency is both up to 65%, and it is available for a relatively broad frequency range from 0.8 to 1.2 THz and a wide incident angles of 0°-20°. This work provides an important idea for designing the multi-focal metalenses, and it will also have high application value in many fields such as multi-imaging system, optical tomography.
Bipod structure has the characteristics of static support and can isolate the additional mechanical load. Therefore, it has become one of the common support forms of large aperture space camera mirror assembly. When installing and adjusting on the ground, the surface figure of the mirror supported by the Bipod decreases due to the action of gravity. After the space camera enters the orbit, the surface figure of the mirror will change again with the release of gravity deformation. The gravity error of the mirror assembly is evaluated by finite element analysis method, and its accuracy is difficult to meet the requirements of high-quality and high-resolution imaging. At the same time, the gravity unloading scheme used in the mirror processing process is also difficult to be used to the component stage. In order to solve the problems of aliasing of assembly error and trefoil aberration and insufficient accuracy of spherical aberration test by detection light path in the process of gravity error test, a test scheme combining turnover and unloading was proposed. Based on the orthogonality of different aberrations, individual tests could be carried out to obtain each aberration item by item. Through the gravity turnover test of the mirror, the trefoil aberration in the assembly error and gravity error was separated. The unloading device with certain accuracy was designed. Through the comparative test before and after unloading, the spherical aberration caused by gravity, was obtained. By adopting the above scheme, the measurement of all gravity errors could be realized. The 1.3 m high-lightweight mirror assembly was tested. The gravity error surface figure (rms) and on-orbit surface figure (rms) are 0.192λ(λ=0.6328 μm) and 0.023λ, respectively.
MTF test is an important part of the camera manufacture process. General practice of MTF test needs to use the parallel light tube. Self-collimating MTF test method does not rely on the parallel light tube. Firstly, the principle and construction plan of the self-collimating MTF test system were introduced. Secondly, the curve measured by parallel light tube and self-collimating MTF test were compared. It is obtained that the self-collimating MTF test system has high sensitivity to the off-focus, this method can be used to determine the focal plane position. Thirdly, the phenomenon of the deflection of the self-collimating target fringe were analyzed, which will affect the MTF test result. The formula of the deflection angle of target fringe and the formula of the influence of fringe deflection on MTF test result were given. Design method of the self-collimating target of off-axis system were given. With this method, the deviation of MTF caused by the deflection of the target fringe can be reduced to a negligible level. Splicing relationship between the detector and the test target of self-collimating MTF test system were discussed, influence of detector's filter on focal plane position was analyzed, and design and splicing program of the detector and the target were proposed. Finally, the application of self-collimating MTF test on a camera were summarized. The result obtained by this study can be used to other camera in the future.
C/SiC composite material was applied to design of secondary mirror bearing cylinder in large aperture space telescope. Firstly, the material characteristics of C/SiC composite material and the applications in space optical remote sensor were introduced. Secondly, taking secondary mirror bearing cylinder of a large aperture space telescope as an example, the weight and structural-thermal performance based on different materials were compared. The analysis results indicate that C/SiC reduces the weight of secondary mirror bearing cylinder to 32 kg by 45.5% compared with titanium alloy cylinder. With natural frequency of 204 Hz, the structure meets the design requirement and can control the influence of thermal deformation on the shape of mirror. Finally, the C/SiC secondary mirror bearing cylinder was developed and the main physical properties were tested, and the mechanical vibration test was carried out, and the three-coordinate measurement data of the structure before and after vibration were compared. The results show that the fundamental frequency is excellent. Moreover, the frequency drift is less than 1% before and after the vibration test, and the micro displacement of the structure is in micron level. This paper provides several reference value for the design of large size integral bearing structure of space remote sensor by using C/SiC.
In order to solve the situation that the 2.4-meter telescope cannot switch Nasmyth focus and Cassgrain focus intelligently, a switching system was designed to switch focuses rapidly to improve the observation ability. Firstly, a M3 switching system model was built, the composition and implementation of the system were discussed in detail. Then precision analysis and finite element analysis were performed to optimize design. Finally, the calibration and test were carried out through experiment. Finite element analysis shows that under the action of gravity, the mirror surface shape RMSλ/60 (λ=632.8 nm). The first-order modal of system reaches 34.71 Hz, the system has good stiffness. Experimental measurement shows that azimuth structure and altitude structure repetition precision are superior to 0.5″, translation stage repetition precision is 1 μm. The switching system meets the performance requirements and has been installed in the telescope, which has been used for observation. It also provides a reference for the design of other telescopes.
Infrared imaging is an important means of modern battlefield reconnaissance, and target recognition technology based on infrared images can provide important support for intelligence interpretation. Aiming at target recognition in infrared images, a method based on selected deep features was proposed. A ResNet with a proper structure was designed to perform feature learning on infrared images, and the output feature maps from each convolutional layer was vectorized to obtain a corresponding feature vector. For the deep feature vectors of different feature maps, their correlations with the original image were evaluated based on the Spearman rank correlation coefficient. Afterwards, several deep features with high correlations were selected through the threshold decision algorithm. The deep features obtained after selection can eliminate unnecessary redundant components, thereby improving the accuracy and robustness of subsequent classification. The joint sparse representation model was used to characterize and classify the selected deep features, and finally the category of the sample can be identified. Therefore, the proposed method can effectively combine the discrimination of the multi-level deep features learned from ResNet, thereby improving the final recognition performance. The experiments were carried out in the public mid-wave infrared target image dataset (MWIR), using the original test samples, simulated noisy samples and simulated occluded samples to test and analyze the performance of the method. The experimental results show that the proposed method can achieve stronger effectiveness and robustness compared with some existing infrared target recognition ones.
Fire detecting system is a important part of the fire extinguishing and the explosion suppression equipments which can be used to protect the safety of people when the fire broke out in special vehicles. The systems based on temperature sensor and non-imaging photo-detector which are usually used now have some problems because the space structure of engine bay is complex. For example, the system based on temperature sensor often fails to find the fire because the distance between senor and fire is too large, and it is impossible for full coverage of the Engine Bay space. Also, the fire detecting system based on linear or spot heat detector often fail to detect the fire in time used for engine bay overheat protection. And the system based on non-imaging photo-detector often fails to alarm the fire because the sensitivity is too high. So the fire and explosion seriously threaten the safety of armored vehicles and the people in them. To solve the problem, a detecting system based on infrared imaging technology is designed. A infrared-imaging detector is employed. A display and control unit is used to monitor the engine bay temperature. The temperature threshold can be preset for early warning of high temperature and fire alarm. Engine bay overheat protection system based on infrared imaging technology is tested by normal blaze experimental platform which is made up of a normal brazier and a exhaust equipment. Experiment results show that the system can generate alarm signals when the temperature in engine bay space is higher than the temperature threshold. And fire response time based on this infrared imaging system is less than 5 s, which can meet the requirement of relevant National Military Standards of China. As a result, the system can be used for fire early warning, and can meet the requirements for engine bay unreported fire. This system can be used to protect armored vehicles and safety of people in them.
An novel infrared dim and small target detection algorithm, called J-MSF, based on multi-channel and multi-scale feature fusion was proposed, which solved the problem that the classical infrared dim and small target detection algorithm based on deep learning cannot detect because the target information disappeared in the upper receptive field. Firstly, a new multi-channel Janet structure was proposed to design the J-MSF backbone extraction framework. Secondly, a descending threshold feature pyramid pooling structure (DSPP) was exploited, and a multi-scale fusion detection strategy was conducted. Finally, the Gauss loss optimization function was designed. The experimental results show that the recall rate and the AP value of the proposed algorithm are improved by 9.07%, 9.89% and 1.67%, 3.16%, respectively, compared with those of YOLOv3 and YOLOv4 algorithms in "a dataset for infrared detection and tracking of dim and small aircraft targets underground/air background". The proposed algorithm can be effectively applied to infrared dim and small target detection, shows good robustness and adaptability, and is better than the state of the art algorithms.
With the progress of long-wave infrared detector technology, space-based infrared remote sensing satellite technology has developed rapidly by selecting Aerospace large array long-wave infrared detector. The response of long-wave infrared detectors is strongly nonlinear, which leads to large correction errors in traditional non-uniformity correction algorithms based on linear models, thereby affecting the effectiveness of the satellite in orbit. Aiming at this problem, combing the statistical analysis results of a large number of laboratory data and satellites in orbit data, a new long-wave infrared detector nonlinear response model was established, and an new non-uniformity correction algorithm based on improved Gamma curve was proposed accordingly, which was in order to effectively overcome the influence of the strong nonlinear response of infrared detector on the correction accuracy. Firstly, the nonlinear response model based on improved Gamma curve for infrared detector was established, and the nonlinear compression mapping operation on the original image data was completed to achieve linearization of the infrared detector response. Secondly, the linearization algorithm was used to implement non-uniformity correction based on space-borne calibration technology. Meanwhile, the calibration temperature point was dynamically updated according to the established model parameters. Finally, the actual image after non-uniformity correction was restored by inverse nonlinear compression mapping. The experimental result based on artificial blackbody and on orbit real infrared images show that the proposed algorithm can effectively solve the influence of strong nonlinearity of infrared detector response on the correction accuracy, and the visual effect and quantitative index of the corrected image are better than the traditional space-borne non-uniformity correction algorithm.
The conventional diffraction spectrum imaging system adopts the single channel scheme, which mainly carries out simulation and spectral imaging experiments for simple graphic targets or gas targets with known spectral characteristics. When the target is a complex scene such as natural scene, the spectral solution effect and accuracy of the imaging system are difficult to ensure. For the imaging of complex scenery, a dual channel visible and near-infrared diffraction computational imaging spectrometer system was designed. Based on the conventional single channel diffraction imaging spectrometer system, adding a panchromatic camera imaging coluld provide panchromatic information and a priori knowledge of complex scenes for diffraction imaging channels. The data of the two channels were jointly processed to improve the final spectral data inversion effect and inversion accuracy. The system composition and basic principle were introduced, the system performance was analyzed, and the imaging process of the system was simulated by using the simulation program. A verification device for the principle of visible and near-infrared diffractive computational imaging spectrometer system was built in the laboratory. Spectral restoration was carried out on the visible and near-infrared aliasing spectral data of 450-800 nm. Using the spectral curve of the color plate tested by ocean optics spectrometer as the standard spectral line, compared with the restored spectral data, the average accuracy of the retrieved spectral data was better than 90%. Through theoretical analysis, system simulation and imaging experiment, the correctness and feasibility of the system principle were verified. It can obtain better spectral solution effect and accuracy of complex scenery, and improve the application potential and application value of diffraction imaging spectral system.
Aiming at the requirements of the mode and operating temperature of vertical-cavity surface-emitting laser (VCSEL) used as the laser source system of the atomic clock (Cesium) chip, the 894.6 nm oxide-confined fundamental transverse mode VCSEL that could operate at high temperature was reported. By reducing the diameter of the oxide aperture of the VCSEL to 3 μm, the higher order transverse modes could be suppressed, which guaranteed the fundamental transverse mode and low threshold current of the VCSEL. Through the structural design that the cavity mode and the material gain was detuned by 12 nm at room temperature, the emission wavelength of the device could match with the atomic energy level and the operating mode was stable at a high temperature of 50-65 ℃. The obtained VCSEL shows a center wavelength of 894.6 nm, a side mode suppression ratio (SMSR) larger than 35 dB, a fundamental transverse mode power of 0.75 mW and a far-field divergence angle of 11.4° when the operating temperature is 55 ℃ and the injection current is 1.8 mA. At the temperature of 65 ℃, the SMSR is larger than 25 dB and transverse mode power is larger than 0.1 mW. The development of the high temperature fundamental transverse mode VCSEL has great potential in chip atomic clocks.
A widely tunable long-infrared optical parametric oscillator (OPO) based on ZnGeP2 (ZGP) with nanosecond pulses output in dual bands was reported in this paper. As a fundamental laser of ZGP-OPO, KTP- OPO of 2.1 μm based on type Ⅱ phase match was pumped by 1064 nm fundamental delivering <10 ns pulses at 50 Hz. Furthermore, the ZGP-OPO of 7-11 μm based on type Ⅰ phase match was designed. Continuous-tunable signal wavelengths of 2.815-2.963 μm corresponding idle wavelengths of 7.82-9.08 μm was obtained using angle tuning of ZGP. Continuous-tunable signal wavelengths of 2.624-2.662 μm and 2.745-2.956 μm, which correspond the idle wavelengths of 7.94-9.07 μm and 10.20-10.82 μm respectively , were obtained using pump wavelengths tuning. The single-pulse energy was 0.8 mJ at 8.03 μm, pump to idler conversion efficiency was 9.4%.
To explore the degradation and mechanism of 850 nm high-speed vertical-cavity surface-emitting laser in space radiation environment, the degradation of light output power and threshold current were obtained by Gamma ray and 10 MeV proton irradiation. The physical mechanism of VCSEL parameter degradation caused by radiation was analyzed. In addition, 236 h forward-bias annealing research was also carried out. The results show that VCSEL is not sensitive to the total dose effect caused by gamma rays, the photoelectric properties have a certain degree of recovery due to the deposition energy promoting the order of the crystals near the quantum well interface within a certain dose range. But threshold current and external quantum efficiency of VCSEL are degraded in varying degrees under proton irradiation, the threshold current damage factor is calculated to be 1.468 × 10-15 cm2/p. After 20 mA forward-bias annealing, the threshold current is restored by 20%, and the optical output power is restored by 10% at 25 mA injection current. The degradation of threshold current and external quantum efficiency is attributed to the non-radiative recombination center introduced by proton irradiation. These results provide support for the application of VCSEL and the data communication and instrument system containing VCSEL in harsh space radiation environment.
The polarization control of vertical cavity surface emitting laser (VCSEL) with surface grating structure was studied. After introducing the surface grating, the polarization-dependent mirror loss under different etching depths was simulated. The results show that the etching depth of the surface grating can achieve stable polarization in the range of 44 nm to 130 nm, which has a large fabrication tolerance. For the fundamental transverse mode, the orthogonal polarization suppression ratio (OPSR) of the surface grating VCSEL is more than 20 dB, and the peak-to-peak OPSR of the polarization-resolved spectrum reaches 40 dB. Effective polarization control can also be achieved even for multimode VCSEL. In order to further verify the effect of the grating on polarization control, two surface gratings with mutually perpendicular directions were fabricated. The OPSRs of the VCSEL with the two directional gratings were more than 20 dB. The test results show that surface grating is an effective means for VCSEL to achieve stable polarization.
In the high power synthesis schemes based on fiber signal combiner, it is one of the urgent problems to be solved in the current laser field to maintain good beam quality after beam combining. A kind of high beam quality fiber signal combiner was developed. Firstly, the model of a 3×1 fiber signal combiner was established by using simulation software, and the factors affecting the beam quality and transmission efficiency of combiner were simulated, and the theoretical values of optimum parameters of combiner were obtained. Secondly, based on the etched fiber cladding technology, a high beam quality 3×1 fiber signal combiner with an output fiber of 50/400 μm (NA=0.12) was fabricated by using the taper-fused fiber bundles technology according to simulation results. Finally, three 3 kW fiber lasers were used to test the combiner. Under the condition that the total input power is 8.95 kW, the output power after beam combining is 8.62 kW, the overall transmission efficiency is more than 96%, and the beam quality is M2=4.035.
The interaction between the femtosecond laser pulses and the gas plasma has been widely used to generate strong and broadband THz pulse radiation. A femtosecond laser pulse with slow turn-on, rapid turn-off shape was used to generate THz radiation by interacting with gas plasma. Based on the plasma current model, the properties of the THz generation from such scheme were investigated in detail. Because the electrons were accelerated to a fast velocity by such specially shaped laser pulses, their motions form a fast oscillation current, which emits electromagnetic wave with frequency in the THz region. The results show that such scheme can generate stronger and broader THz radiation than the normal two-color femtosecond laser scheme, although it loses some energy of the laser pulse. This proposal might offer a new way to develop plasma-based broadband THz radiation source.
Due to low laser absorption rate and high thermal conductivity, the properties of micro-structure of aluminum alloy formed by laser melting deposition (LMD) are greatly affected by temperature field. In order to analyze the temperature field of annular beam LMD aluminum alloy molten pool and its’ influence, optimize the forming quality and the performance of forming parts, the Ar-supplied protective LMD technology was used, the AlSi10Mg aluminum alloy forming experiment was carried out. The shape and change of the temperature field of molten pool were systematically analyzed, as well as the influence mechanism of the temperature field on the forming quality, porosity, the properties of micro-structure. The results show that the overall shape of the temperature field of the ring beam LMD aluminum alloy molten pool is "half crescent" with the opening in the scanning direction. With the increase of laser power, the temperature field shape becomes more and more sharp, and its high temperature rate, temperature gradient and average temperature also increase accordingly. The increase of the average temperature of the temperature field can increase the laser absorption rate, coarsen the micro-structure and reduce the micro-hardness. The temperature field affects the porosity rate of the formed part significantly and thus changing the tensile properties. Finally, when the average temperature is 857.7 ℃, the porosity rate is reduced to 2.1%, and the tensile strength is 305.6 MPa, and the elongation rate is 5.7%, which is 52.5% higher than the casting. It provides theoretical guidance for LMD aluminum alloy temperature field and properties of micro-structure control.
Laser cleaning of rust layer on the surface of carbon steel was studied by fiber laser, the effect of laser scanning speed on the removal quality of rust layer was studied by white light interferometer, optical microscope and Raman spectrometer. The results show that, when the laser scanning speed is less than 2 000 mm/s, the high spot overlap rate and strong heat accumulation effect, lead to the melting and recondensation of the substrate surface, and a secondary oxidation occurs on the sample surface, which result the formation of a complex iron oxide film, at the same time, the surface roughness of the sample is the smallest. When the laser scanning speed is increased to 3000 mm/s, the rust layer on the surface of the sample is completely removed, the color of the metal substrate is exposed, and the secondary oxidation on the surface of the substrate is weakened. When the scanning speed continues to increase, due to the low spot overlap rate, the laser energy absorbed by the rust layer is less, only part of the rust layer is removed, the residual rust layer begins to appear on the surface of the sample, and with the increase of scanning speed, the residual rust layer and surface roughness increase. Better rust removal effect can be obtained by adjusting the scanning speed, after optimization the process, when the laser power is 120 W, the rust removal efficiency reaches 1.5 m2/h.
In the package of infrared device some components made of transparent materials are often needed, dicing and marking such components by using pulse laser are favorite, whereas the processing parameters should be precisely optimized. Targeting on the laser marking of IR grade Conning glass for packaging of infrared focal plane arrays, the essential parameters including ablation threshold, beam features of laser processing head and geometrical error caused by scanning angle, were measured and analyzed. Suitable strategies and parameters for the laser marking process were gained based on the results, and applied to practical operation successfully. Those strategies and parameter setting methods could be extended to the laser marking of other transparent materials.
In the laser wireless power transmission(LWPT), the wavelength, laser power and temperature of the power transfer laser have a significant influence on the output characteristics of the photocell. The maximum power point tracking(MPPT) can solve the power mismatch problem under the influence of the above factors and improve the DC-DC efficiency of the system. In this paper, an integrated simulation system of MPPT was built for LWPT. The comprehensive influence of wavelength, laser power and temperature on the output characteristics of GaAs photocell was coupled. The output characteristics of photocell under various conditions such as power matching, power mismatch and MPPT modulation could be analyzed at the same time. Firstly, the physical laws of photocell under different wavelength, laser power and temperature were studied. When the wavelength increased, the conversion efficiency ηmax reached the maximum value of 50% at about 850 nm, and then ηmax decreased rapidly because the photon energy was less than the GaAs band gap. Power increased, ηmax was basically unchanged, and the maximum power matching resistance RLmax decreased. The temperature rised, ηmax and RLmax decreased continuously. In addition, the output characteristics of the photocell in the case of power mismatch were studied. The conversion efficiency of the photocell decreased to different degrees compared with the power matching. According to the output characteristics of the photocell, the MPPT circuit was designed in the simulation system, and the maximum power tracking was carried out by using the perturb and observation algorithm. After modulation in MPPT system, the photocell can work at the maximum power point of power matching, and the energy utilization rate of photocell can reach 99.93%. The research results have important guiding significance for the practical application of laser energy transmission.
In recent years, lidar applications had put forward higher requirements for detection distance and sensitivity. As an ideal light source, 905 nm semiconductor lasers also urgently needed to improve the peak power and beam quality. In this context, the effects of different gain region types and waveguide structures on beam quality and power efficiency of 905 nm tunnel-junction pulsed semiconductor lasers were investigated based on asymmetric large optical cavity structures. By optimizing the gain region type and waveguide structure, the bulk resistance and internal loss were reduced. The ability to limit carrier leakage was enhanced, and the peak power and electro-optical efficiency of the device working at high currents were improved. By increasing the threshold gain ratio of the multimode to the fundamental mode, the high-order mode lasing was suppressed, and the far-field divergence angle was reduced. On this basis, the developed quadruple-active regions semiconductor laser with 800 μm cavity length and 200 μm electrode achieved a peak power output of 177 W at a pulse current intensity of 41.6 A in pulse power test with a pulse width of 100 ns and a repetition rate of 1 kHz; fundamental mode emitting in the vertical direction, the full width at half maximμm far-field divergence angle was 24.3°.
Microcavity optical frequency comb (also called the microcavity comb), a subversive technology, is an integrated light source produced from a four-wave mixing process in a nonlinear optical microcavity. As a precision device with excellent properties of optical frequency, microcavity combs can be extensively applied in many fields such as molecular spectroscopy, coherent communication, LiDAR, metrology, and lightweight equipment for airborne system. Here, the fabrication of integrated silicon nitride (Si3N4) microcavity optical frequency comb devices was reported. The balance between the stress, thickness and stoichiometry of Si3N4 was well controlled. A reliable method was proposed to fabricate Si3N4 optical film with enough thickness and stoichiometry to meet the requirements of anomalous dispersion and reducing light absorption. The modified technology of Damascene process with microstructures to decline the stress of thick Si3N4 film was developed to reduce defects. Furthermore, the mask via with a 30 nm thick alumina compensation layer was optimized and a practicable etching process was used for fabricating Si3N4 microresonators with sub-15 nm roughness of lateral walls of microring and waveguide. The experimental results show a high quality of Si3N4 microcavity. Additionally, a coherent Kerr optical frequency comb spectrum can be produced with a wide spectral range from 1480 nm to 1640 nm via dual light pumping.
Microwave signals with low phase noise are indispensable in wireless communication, radar, time and frequency metrology, and deep astronomy. Photonic microwave signal generation approaches are promising. They can break through the bottleneck of classical microwave signal generation methods in many aspects, such as carrier frequency, tunability, phase noise, integrability, and power consumption. The early systems of photonic microwave signal generation were complex, the lasers and related equipment were bulky, which restricted their application outside of the laboratories. In recent years, optical frequency combs based on microcavities develop rapidly. With the advantages of microcombs such as low loss, small size, and ultrahigh stability, the application of microwave photonics based on microcombs has attracted extensive attention of researchers and the systems of photonic microwave signal generation have been intensively studied. In this paper, we reviewed the topic of low phase noise photonic microwave generation with microcombs and prospected the challenges and development trend in the future application of the systems of photonic microwave signal based on microcombs at the end.
Chip-scale optical frequency combs based on microresonators have great potentials in spectroscopy, microwave photonics, optical atomic clocks and coherent optical communications. The non-centrosymmetric wurtzite crystal structure of aluminum nitride (AlN) and gallium nitride (GaN) allows them to exhibit both second- and third-order nonlinear optical coefficients, together with wide transparency window and large refractive index contrast against sapphire substrate, making III-nitrides an attractive platform for nonlinear photonics. The basic properties of AlN and GaN microresonators as well as recent advances in III-nitride-based microresonator frequency combs are presented, including broadband frequency comb generation and optical parametric oscillation in AlN microresonators, and soliton microcomb generation in GaN microresonators.
Optical solitons are wavepackets that can sustain the shape via a nonlinear refractive index potential well. They exist in a wide range of optical systems spanning optical fibers, fiber lasers and parametric oscillators. Recently, a new type optical solitons have been observed in coherently pumped high-Q microcavities. The observation of microcavity optical solitons provides a well-controlled experimental platform to study soliton physics. Microcavity optical solitons also endow an array of highly stable spectral lines in the frequency domain, which advance the miniaturization of frequency comb systems. These soliton microcombs have been self-reference stabilized and could enable many chip-based applications including optical frequency synthesizers, optical atomic clocks, data transmission, spectrometer and LiDAR in the near future. Here, the fundamental of microcavity optical solitons was introduced, with a special focus on soliton interaction dynamics. The microcavity dual-comb measurement based applications in fast imaging and mid-infrared gas spectroscopy were also discussed.
To cope with the ever-increasing requirements on transmission capacity, spectral utilization, energy efficiency, small volume, and system simplicity, wavelength-division multiplexing (WDM) optical fiber communication systems need more advanced laser sources than those conventional laser modules used today. Kerr optical frequency comb generated in integrated on-chip micro-cavity provides a promising candidate as the next generation WDM laser source, thanks to its advantages including broadband spectrum, large number of comb lines, matched frequency interval with WDM channels, highly stable frequency, low phase noise, compatibility for chip integration, and low-cost volume production. The fundamental physics of Kerr optical frequency comb was reviewed and the fabrication methods of various Kerr optical frequency comb devices were introduced. Moreover, the unique merits of Kerr optical frequency comb were discussed, such as high spectral purity and compatibility for chip integration, which could facilitate WDM optical communication in the scenarios of long-haul coherent transmissions and data center interconnects.
Based on ultra-high quality factor(Q) and nonlinear optical microcavities, optical microcombs(microcavity optical frequency comb) have enabled a variety of important applications including high volume optical communications, optical data center, photonic neuromorphic computation and massive parallel LIDAR. Whispering gallery mode (WGM) microcavities stand for an important platform for studying the microcavity optical frequency comb technology, particularly having record ultra-high Q factors as well as the ultra-high finesse. It can realize ultra-narrow linewidth lasers and optical frequency combs, and photonic microwaves for synthesizing ultra-low noise. Here we developed highQ WGM microcavities from a silica (SiO2) rod fused and shaped with the CO2 laser. The quality factor is above 108 with a free spectrum range at the level of 10 GHz. The cavity resonances as well as the coupling ideality have been characterized, where a degradation of Q factors in a humid environment was observed and recovered with a second annealing process. Moreover, Kerr comb generation was demonstrated in such SiO2 microcavities, which at the moment is mostly in a noisy state governed by the modulation instability regime. Yet the footprint of the cavity soliton state was experimentally observed as a “soliton step” signal. The results indicate that a low-noise and fully coherent soliton microcomb is potentially accessible in home developed SiO2 microcavities, and is readily for comb-related applications.
Optical resonators with high quality (Q) factor can restrict light in a small mode volume for a long time, greatly enhancing the interaction between light and matter, and becoming an important component with great potential in integrated optical devices. Focusing on the silicon nitride material platform, which is currently widely used in the field of integrated nonlinear optics, in order to solve the problem of large scattering loss in the large size on-chip silicon nitride microring resonator caused by the stitching error, the surface roughness and other factors, a series of fabrication process improvements were made to improve the quality factor of the large size silicon nitride microring resonator. The results show that the scattering loss of the silicon nitride waveguide can be effectively reduced by thin film redeposition process, and the intrinsic Q of the large size silicon nitride microring resonator with a radius of 560 μm is increased by 26% on average. Thanks to the improved Q of the large size microring resonator, the frequency comb with the repetition rate of 40 GHz is realized in the on-chip silicon nitride microring resonator.
Self-reference Dissipative Kerr Solitons (DKSs) based on optical microring resonators have a wide range of applications, such as frequency synthesizers, coherent communication, astronomical spectrometer calibration, precision measurements, optical clocks, dual-comb spectroscopy, etc. The directly accessing octave-spanning DKS has been obtained in silicon nitride and lithium niobate microresonators. Here, a simple method that can directly access the octave-spanning DKS in an aluminum nitride (AlN) microring resonator via a single pump was proposed. The TE00 and TE10 modes act as the pump resonance and auxiliary resonance modes, respectively, which had the resonant frequencies close to each other, and the auxiliary mode on red detuning side could effectively balance the thermal drag effect during the formation of soliton. The pump wavelength was tuned slowly to access a stable soliton comb with a bandwidth of 1100-2300 nm and the maximum soliton existence range of 10.4 GHz (83 pm), which was the first time an octave-spanning Kerr soliton had been obtained on the AlN platform. The stable octave-spanning DKS with large soliton accessing window could be obtained in this scheme using a single pump, which was different from other schemes with additional complex controls means and equipments.
Chalcogenide glass integrated microresonators (chalcogenide microresonators) have attracted great attention in nonlinear integrated photonics in recent years because of their high linear refraction index, high nonlinearity coefficient, ultra-wide transmittance window, low thermo-optic coefficient, and precisely regulated dispersion with conventional semiconductor micro-nanofabrication technology. Recently, the researchers from the Sun Yat-sun University developed a novel chalcogenide glass (Ge25Sb10S65) material platform and realized a series of high-quality chalcogenide integrated photonic devices. The progress of integrated soliton microcombs generation and regulation based on chalcogenide microresonators was reviewed. The integrated chalcogenide microresonators with high-quality factors(Q>106) were achieved by a modified nanofabrication process. Furthermore, mode-locked soliton microcombs with a low pump power and a widely tunable Kerr-Raman comb were achieved by precisely controlling the dispersion , respectively.
Optical frequency combs (OFCs) based on optical microcavities have the characteristics of low threshold, wide spectrum, and compact structure, and have important application prospect in the fields of precision measurement, sensing and et al. Therefore, microcavity based OFC has become an international research hotspot in recent years. At present, relevant researches focus on the generation principle and application of mode-locked OFCs in the infrared band. Although the OFCs in the visible light band have special applications in the fields of precision spectroscopy, atomic clocks and biomedicine, the realization of visible light OFCs is extremely challenging. Based on a brief description of the generation principle of OFCs, this paper introduces the main challenges of realizing OFCs in the visible light band, and the current research progress of three implementation schemes, including the use of the second-order and third-order nonlinear effects of materials, the regulation of the geometric dispersion of the microcavity, and the regulation of the dispersion via modal strong coupling effect to generate visible light frequency combs.
Optical frequency comb (OFC) is the spectrum structure composed of a set of discrete and equally spaced frequency components, which has been widely used in many areas such as spectroscopy, precision measurement, optical communication and sensing as the natural scale for spectral analysis. According to its generation methods, OFC can be generated in three ways, including mode-locked laser based OFC, Kerr microresonator OFC and electro-optic frequency comb (EOFC). EOFC has been greatly developed because of its advantages including remarkable tunability of frequency spacing, high comb line power, as well as the accessible conversion from microwave to optical wave. However, there are some drawbacks in conventional EOFC generator, for instance, the bulk size and required high power, which limit its further development. As the micro/nanofabrication technology gradually grows, more and more materials are applied into integrated chip-scale optical devices, including Si, Silicon Nitride, Aluminum Nitride, Indium Phosphide, Lithium Niobate and Aluminium Gallium Arsenide. Integrated EOFC possesses the excellent characteristics, such as small volume and low power consumption, which is an important device for optoelectronic integrated chip. The research status of the integrated EOFC is reviewed in this paper. First, the classification of optical frequency comb, as well as detailed content about generation mechanism of EOFC are introduced. Next, the information comprising various material platforms, corresponding devices performance metrics and applications about EOFC is presented. Finally, the future research directions are prospected in view of the existing problems of integrated EOFC.
Point cloud registration is one of the key technologies for 3D reconstruction. To address the problems of the iterative closest point algorithm (ICP) in point cloud matching, which requires high initial position and low speed, a point cloud registration method based on adaptive local neighborhood feature point extraction and matching was proposed. Firstly, according to the relationship between the local surface change factor and the average change factor, feature points were adaptively extracted. Then, the fast point feature histogram (FPFH) was used to comprehensively describe the local information of each feature point, the coarse alignment was achieved combining with the random sampling consistency (RANSAC) algorithm. Finally, according to the obtained initial transformation and feature point based ICP algorithm, the fine alignment was achieved. The alignment experiments were conducted on the Stanford dataset, noisy point cloud and scene point cloud. The experimental results demonstrate that the proposed feature point extraction algorithm can effectively extract the features of the point cloud, and by comparing with other feature point detection methods, the proposed method has higher alignment accuracy and alignment speed in coarse alignment with better noise immunity; compared with the ICP algorithm, the registration speed of the feature point based-ICP algorithm in the Stanford data set and scene point cloud is increased by about 10 times. In the noisy point cloud, the registration can be performed efficiently according to the extracted feature points. This research has certain guiding significance for improving the efficiency of target matching in 3D reconstruction and target recognition.
ZY3-03 is a land remote sensing satellite for 1: 50 000 stereo mapping built by the Ministry of Natural Resources. It is equipped with operational laser altimeter, which is mainly used to obtain high-precision elevation control points. In this paper, aiming at the laser altimetry data of ZY3-03 satellite, the standardized surveying processing flow and the method of extracting elevation control points is studied. Moreover, the accuracy verification in Sunid Right Banner of Inner Mongolia and Suzhou of Jiangsu Province is implemented, and the combined surveying and mapping application in Heilongjiang and Hebei areas is experimented and validated. The accuracy verification results show that the elevation accuracy of ZY3-03 laser points in the flat area of Sunid Right Banner in Inner Mongolia is (0.051±0.232) m, and the overall accuracy of the laser points in the urban area of Suzhou, Jiangsu is (0.414±6.213) m, and the elevation accuracy after elevation control points extraction is (-0.526±0.624) m, which can meet the elevation control requirement of 1:50 000 mapping; The application of combined surveying and mapping shows that the elevation accuracy of stereo images can be improved from 5.27 m to 2.58 m in flat area of Heilongjiang and from 11.25 m to 4.45 m in Taihang mountain area of Hebei by using laser elevation control points derived from the ZY3-03 satellite. It is concluded that the elevation accuracy of stereo images can be effectively improved by using laser elevation control points of ZY3-03 satellite in both flat and mountainous areas, and the requirement of 1:50 000 mapping can be met.
In order to improve the practicability of JTC optical image encryption system, solve its noise problem, improve its encryption efficiency and security, a multi-image optical encryption method based on computer generated hologram (CGH) and the frequency shift of Fourier transform was proposed. Firstly, each image with different size and type was modulated by random phase and Fourier transform, then the frequency spectrum of multiple images was modulated by frequency-shift phase and superimposed to be encoded into binary real-value CGH, finally the CGH was encrypted into joint power spectrum by JTC optical image encryption system. In the decryption process, the encrypted image was decrypted by 4F system to obtain the CGH. The binary real-value CGH has strong noise resistance to eliminate the effects of noise. Then, multiple decrypted images can be obtained after Fourier transform. Simulation results show that the proposed method can encrypt and decrypt multiple images of different sizes and types in parallel, and has high encryption efficiency. Meanwhile, multiple images with mutual keys and double optical keys ensure the security of the encryption system.
A Combined Atmospheric Radiation Transfer (CART) software was developed, which could be used to calculate the spectral transmittance and background radiation (including ambient scattered solar radiation and thermal radiation) of the atmosphere from visible to far infrared wavelength bands based on atmospheric parameters. The multi-dimensional variations of atmospheric transmittance and background radiation in a certain wavelength-band with zenith angle and distance were paid special attention by researches in optical engineering area. Thus, it was necessary to quickly calculate the scene atmospheric optical characteristics. The newly two-dimensional scene fast computing function of atmospheric radiative transfer by using CART was mainly introduced. According to the characteristics of discrete coordinate method (DISORT) which could simutaniously output the radiance at various zenith angles and azimuth angles, the program to calculate the multiple-scattered atmospheric radiance was designed at each direction (azimuth and zenith angles) simultaneously, which greatly improved the calculation efficiency. As the atmospheric transmittance and heat radiation change slowly with space position, sampling calculation and spline interpolation were adopted to save the calculation time greatly while keeping the calculation accuracy. For a scene with more than 10000 times calculations, the speed was 2-3 orders of magnitude faster than before. It will be usefull in the calculation of atmospheric radiative transfer scenarios in practical engineering applications.