Compared to current radio-frequency technology, Free-Space Optical (FSO) communication holds tremendous advantages, such as higher data rates, high security, light weight, smaller size, and less power. FSO communication has attracted considerable attention for space activities. Optical antenna and Acquisition, Pointing and Tracking (APT) system are crucial technologies of FSO communication. The tasks of optical antenna include narrowing the diverging angle of transmitted beam, assure the wavefront quality of beam transmitted and received, and improve angular resolution for fine pointing. For a Chinese space mission, a 4-mirror off-axis optical system coupled with Coude path is designed. The telescope is mounted on a 2-axis gimbal mechanism to implement coarse pointing. The optical terminals were launched in 2018. The ground-to-satellite and inter-satellite bi-directional communication demonstrations are carried out successfully. The performance of the optical terminal satisfies all requirements of the mission.
This dissertation discusses the simulation and experimental research concerning an integrated infrared detector supporting structure. We verified this structure's heat-transfer characteristics and thermal stress by setting test benches and a vacuum-water cooling system. We tested the temperature on both ends of the supporting structure. The conclusion drawn from comparing the calculated value with the test result is, this integrated infrared detector supporting structure can transmit the cooling capacity from the cold head to the infrared detector with coupling temperature difference in 2K; x-y-z slot for releasing stress can effectively reduce the thermal deformation caused by thermal stress, meeting the demand of infrared detector imaging.
By reducing the working temperature of opto-mechanical structures, the infrared system signal to noise ration can be effectively improved. Normally, structures close to the focal plane radiate more noise. Signal to noise ration can be improved by cooling the opto-mechanical in front of the focal plane. In this paper, an infrared lens working at both room and low temperature is designed. The first four lenses work at room temperature and the other three lenses close to the focal plane work at 200K. The cryogenic lens is cooled by space pulse tube cryocooler. UG TMG thermal analysis results show that the working temperature of each lens meets the requirements, and the temperature gradient of each lens is less than 0.2K. Modal analysis results show that the first modal frequency is 97.2Hz. The finite element analysis results show that the maximum stress of the structure is 227.8MPa, which is far less than the allowable stress of TC4. Thermal-structure-optical analysis show that the system MTF has a small decrease from root temperature to working temperature. By adjusting the position of the focal plane, the system MTF can recover to the design results. The results of the vacuum experiment show that the working temperature of the lens is close to the simulation results. The wavefront error is measured both at room temperature and working temperature. The average wavefront error is 0.060λ (λ=632.8nm) at working temperature and 0.070λ at room temperature. Experiment results verify the correctness of the thermal design and opto-mechanical design.
The infrared focal plane assemblies (FPA) are usually integrated in ambient temperature, and operate in 80K~40K. Mechanical cryocoolers are typically used to provide the cryogenic temperature. The mechanical designs of FPA have to take a variety of constraints into consideration, such as thermal stresses, temperature uniformity of detectors, vibration from cold fingers, stray light of dewar (optical stops), flatness of the detector arrays, thermal insulation, severe vibrations during launch and optical alignment stability. For large format focal plane assemblies, the stress and deformation caused by thermal mismatch of HgCdTe detector chip, silicon readout integrated circuit (ROIC) and mounting structures become worse. This paper presents an innovative thermo-mechanical designed for Visual and Infrared Multispectral Imager (VIMI) onboard GaoFen-5 satellite. It proposed a symmetrical sandwich configuration to ensure the flatness of detector arrays, and a flexure adaptor plate module made of molybdenum-copper to mitigate thermal stress of detectors and Low Temperature Co-fired Ceramic (LTCC). Finally, a series of mechanical and thermal tests were conducted to verify the reliability of the thermo-mechanical design of FPA.
In order to obtain reflector supporting structure with high stiffness and thermal stability, a “2-2-1-1” kinematic supporting structure is designed for a 350mm mirror of space remote sensor. Firstly, the kinematic principle of the “2-2-1-1” supporting structure is studied, and the dynamic model of the “2-2-1-1” supporting structure is deduced, the relationship between the supporting stiffness of the mirror assembly and the parameters of the length, diameter and span of the ball-end supporting rod is given. According to the relationship between the supporting stiffness of the mirror assembly and the length and diameter of the supporting rod, a “2-2-1-1” supporting structure of the mirror assembly is designed. The mechanical simulation of the design results is carried out by means of finite element analysis, and then the test of the actual supporting structure is carried out. The experimental results show that the RMS value of the reflector supported by “2-2-1-1” supporting structure is better than λ/40(λ=632.8nm), The fundamental frequencies of the components in three directions are respectively 231Hz, 319Hz and 365Hz, it is close to the results of finite element analysis and theoretical calculation. The rationality of the designing of the “2-2-1-1” supporting structure of the reflector is proved, which meets the imaging requirements of the space remote sensor.
The optical surface error is one of the crucial factors to guarantee the image quality of space telescopes. The growing of aperture and pursuing of lightweight structure result in a relatively lightweight and flexible telescope mirror, whose surface figure is vulnerable to micro-vibration disturbances. Unfortunately, there are a variety of disturbances in space, such as control moment gyroscopes, momentum wheels, and cryocoolers. In order to investigate the dynamic optical surface error result from disturbances, a method based on mode superposition theory and Zernike polynomials (ZPs) fitting is proposed. On the basis of normal mode theory, the responses of an opto-mechanical structure under forced excitations can be approximated by the sum of a series of single normal modes of the system. Meanwhile, for each mode shape of an optical surface, it can be fitted as a linear combination of ZPs, which are typically used to describe optical surface errors and represent the aberrations of optical systems. Therefore, the dynamic Zernike coefficients can be computed by the superposition of the Zernike coefficients of each single mode shape. From the combination of normal mode theory and ZPs fitting technique, the relationship between the dynamic responses of optical surface error and micro-vibration disturbances is established. So, the influence of micro-vibration on the aberrations of optical system can be predicted straightforwardly. Finally, an example is given to show the effectiveness of the proposed method.
The space-borne Fourier Transform Spectrometers (FTS) are widely used for atmospheric studies and planetary
explorations. An adapted version of the classical Michelson interferometer have succeeded in several space missions,
which utilized a rotating arm carrying a pair of cube corner retro-reflectors to produce a variable optical path difference
(OPD), and a metrology laser source to generate the trigger signals. One characteristic of this kind of FTS is that it is
highly sensitive to micro-vibration disturbances. However, a variety of mechanical disturbances are present as the
satellite is in orbit, such as flying wheels, pointing mechanisms and cryocoolers. Therefore, this paper investigates the
influence of micro-vibration on the space-borne FTS. Firstly, the interferogram of metrology laser under harmonic
disturbances is analyzed. The results show that the zero crossings of interferogram shift periodically, and it gives rise to
ghost lines in the retrieved spectra. The amplitudes of ghost lines increase rapidly with the increasing of micro-vibration
levels. As to the system that employs the constant OPD sampling strategy, the effect of zero-crossing shifting is reduced
significantly. Nevertheless, the time delays between the reference signal and the main signal acquisition are inevitable
because of the electronic circuit. Thus, the effect of time delays on the interferogram and eventually on the spectra is
simulated. The analysis suggests that the amplitudes of ghost line in spectra increase with the increasing of time delay
intervals.
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