Abstract:Compared with the infrared photon detectors fabricated from other materials, the infrared detectors fabricated from mercury cadmium telluride (MCT) have several advantages of highly tunable bandgap, high quantum efficiency and R0A approaching the theoretical limit. The main drawback of the MCT detectors is that they have need to use cryogenic cooling to suppress the thermal-induced free carriers resulting in noise. It is desirable that the MCT detectors can operate at high operating temperatures (HOT) without sacrificing their performance. The HOT/MCT detectors are designed mainly to suppress Auger processes so as to reduce noise and degrade cryogenic cooling requirement. Starting from related basic concepts, the understanding of the physical mechanism of HOT/MCT is discussed and the development of the HOT/MCT detection technology in recent years is presented.

Abstract:The precision of infrared temperature measurement instruments and the surface emissivity of the tested object have a great influence on the accuracy of infrared radiation measurement. In order to improve the calculation accuracy of surface emissivity, an infrared thermal imager is calibrated through a standard blackbody. Then, the temperature is measured with the calibrated infrared thermal imager, and the surface emissivity of the object to be tested is calculated. By using the calibration method of the infrared thermal imager based on neural network to calculate the target emissivity, the system error of the infrared thermal imager is effectively eliminated. The measuring equipment is simple and its measurement result is accurate. At the same time, the precise measurement of the temperature and emissivity has laid the basis for the development of infrared stealth materials.

Abstract:When a spaceborne visible camera is imaging the objects on the ground, the radiation both inside and outside its field of view may bring stray light to the image plane of it. The stray light may degrade the imaging quality of the camera and even affect the normal operation of the camera. To improve the imaging performance of the camera, the generation mechanism and suppression measures of the stray light in a visible light system are presented. An optical and mechanical model of the camera is established. The stray light in the system is analyzed. The design of the baffle system is improved. Finally, a stray light measurement setup is used to measure the veiling glare index of the camera lens. Thus, the modeling analysis result is verified.

Abstract:A primary mirror is a main imaging part of a space camera. Both its surface figure error and position error will decide the quality of imaging. A circular mirror assembly with an aperture of Φ660 mm is designed. It is restricted completely according to the six-point orientation principle. Its backside is supported by three fixers which are flexible multilayer frameworks. By reasonably distributing the freedom and rigidity of each supporting fixer and unloading the stress due to different expansion coefficients of different materials when temperature is changing, the distortion of the primary mirror assembly is symmetrical. The Finite Element Method (FEM) analysis result shows that the primary mirror assembly has a surface figure error (root-mean-square (RMS) value) up to 1/50λ (λ=632.8 nm), a first mode of 249 Hz and good dynamic rigidity.

Abstract:Optical mirrors are the important components of a space remote sensor. Their rigid body displacement and surface figure errors are the key indexes for evaluating their environment adaptability. A method for extracting the rigid body displacement and surface figure errors of mirrors from the Finite Element Analysis (FEA) result by the Pantran/Nastran software is presented. First, the raw data output from Patran/Nastran are preprocessed to eliminate their error. Then, a coordinate transformation method and a method of normal equations are used to calculate the rigid body displacement. No ill conditioned matrix problems are found in calculation. Finally, a spherical fitting method is used to calculate the surface figure error. The spherical fitting problem is converted to the optimization problem with three variables and the Gauss-Newton method is used to carry out numerical calculation. The experimental result shows that this method is concise, accurate and fast.

Abstract:In practical applications, the accuracy of optical remote sensing data is often reduced because of the influence of atmospheric effect. The sensitivity of Shortwave Infrared Perpendicular Water Stress Index (SPSI) to atmospheric effect is evaluated by using the data from the Moderate Resolution Imaging Spectroradiometer (MODIS), the corresponding ground reflectivity product and the atmospheric transfer model simulation. The result shows that the atmospheric effect can make the reflectivity in near infrared and shortwave infrared bands increase linearly. However, since it has less influence on the typical triangular distribution and the geometric relationship among the pixels in near infrared and shortwave infrared spectral space, the influence of atmospheric effect on soil moisture inversion can be compensated to a certain extent. Therefore, the uncertainty brought by atmospheric effect to SPSI and relative soil moisture can be ignored in practical applications and the operational process can be simplified.

Abstract:Because of high communication speed and weak echo signals, cat's eye modulating retro-reflecting laser communication has strict requirements of low noise, high gain and wide bandwidth for receiving amplifier circuits. Firstly, the influence of link models for cat's eye modulating retro-reflecting laser communication and Signal-to-Noise Ratio (SNR) on Bit Error Rate (BER) is analyzed. Then, the parameters such as circuit bandwidth, gain and noise limit are analyzed according to the performance specifications of the system. On this basis, a receiving amplifier circuit consisting of a transimpedance preamplifier module and a main amplifier module with adjustable gain is designed. Finally, its total noise is calculated and its response characteristics are simulated. It has the bandwidth of 140 MHz, the signal gain greater than 70 dB and the total noise current about 1.34×10-7A. The result shows that the design of the circuit can meet the requirement of following data processing of the system.

Abstract:In atmospheric wireless optical communication, the droplets in atmosphere may cause laser signals to attenuate severely, thus resulting in the attenuation of the temporal spread and energy of pulse signals. A Monte Carlo method is used to simulate the transmission of 1.064 μm laser pulses in fog. In the simulation, the scattering phase function values of light in the fog with different visibility are calculated according to the Mie scattering theory. The corresponding relation between the random number and the scattering angle is obtained through random sampling. Compared with the method in which the H-G phase function is used to represent the Mie scattering characteristics of particles to light approximately, this method is more accurate. On this basis, the impulse response characteristics of laser signals at different receiving view angles under different fog environment are simulated.

Abstract:Aiming at the features of concentrated gray-level distribution and low contrast of infrared images, a contrast enhancement algorithm based on an improved histogram equalization method is proposed. Firstly, a raw 16 bit image is mapped to 8 bit image A through linear contrast enhancement. Secondly, the raw 16 bit image is mapped to 8 bit image B by using the improved platform histogram equalization method. Thirdly, the weights of image A and image B are determined according to the gray-level range of the input images. Finally, the determined weights are used to combine image A and image B into the final contrast-enhanced image. This method has overcome the drawbacks of great noise and light mutation in traditional platform histogram equalization algorithms. Because the traditional gray-level transform enhancement algorithm is incorporated, it can adjust the contrast of the image adaptively according to the gray-level distribution of the object and background in the whole image. The experimental result shows that the proposed algorithm can keep the entire information in the image effectively while enhancing the contrast of the object.