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According to the feature of receptor system of coherent field imaging technique, also known as Fourier telescope, the influence of the deviation of receptors on optical transfer function (OTF) of coherent field imaging technique is investigated, and the relation between OTF and optical distance mean square deviation of receptor is also derived, which indicates that the OTF of coherent field imaging technique is a negative index function of square of product of optical distance mean square deviation and frequency difference. It can provide theoretical basis of determining the accuracy of receptor system.
According to the feature of receptor system of coherent field imaging technique, also known as Fourier telescope, the influence of the deviation of receptors on optical transfer function (OTF) of coherent field imaging technique is investigated, and the relation between OTF and optical distance mean square deviation of receptor is also derived, which indicates that the OTF of coherent field imaging technique is a negative index function of square of product of optical distance mean square deviation and frequency difference. It can provide theoretical basis of determining the accuracy of receptor system.
Coherent field imaging is based on the assumption of equal transmitting apertures spacing and equal spectrum of laser, and high-resolution image is reconstructed by iteratively computing the frequency spectrum. However, the inevitable transmitting aperture spacing error of laser is a key factor to affect the coherent field imaging quality in the application. Aiming at the problem of degrading imaging quality caused by the transmitting aperture spacing error, we discuss the mechanism of influence of aperture spacing error on imaging quality and propose a novel method of improving imaging quality from the perspective of suppressing the influence of transmitting aperture spacing error. Firstly, the mechanism of the influence of aperture spacing error on imaging quality and laser echo spectrum is analyzed in detail. Then we derive a frequency spectrum error propagation model. Based on the model, the iterative effect of frequency spectrum error is investigated and the trend of variation in imaging quality with frequency spectrum error is given. We propose a theoretical equation, in which the transmitting aperture spacing error has no influence on frequency spectrum nor imaging quality. To solve the above equation, an optimized method of linear programming is proposed and the optimized matrix of aperture spacing error is obtained. In practice, the influence of aperture spacing error on imaging quality can be largely counteracted by reasonably allocating aperture spacing error according to the optimized spacing error matrix. The correctness and validity of the theoretical model are verified by a simulation experiment. The results show that the Strehl ratio of imaging quality index can be improved by about 100% through using the proposed method, the greater the aperture spacing error, the higher the Strehl ratio of imaging quality index obtained by the method will be. In addition, the method is easy to use practically and less costly as well. Finally, it is concluded that the proposed method can easily and effectively counteract the degrading effect of aperture spacing error on imaging quality. The research can provide a theoretical basis for improving imaging quality and reasonably assigning transmitter aperture spacing accuracy of coherent field imaging telescope.
Coherent imaging with a multi-beam laser is considered as a key technique in ground based imaging. The image quality is directly determined by stability and consistency of each beam in transmitter. Although the stabilities of laser frequency and the drifting compensation methods have been studied previously, they mostly focused on the laser source. In most cases, especially in large transmitter array, however, transmitted beams are always disturbed by different influential factors, such as frequency drift induced by acoustic-optical modulation (AOM) and high power driven amplification. Therefore this kind of frequency drifting needs further rectification. Aiming at this problem, in this paper we propose two new methods called dynamic demodulation and dependence range demodulation. Firstly, the dynamic demodulation takes the whole drifting frequency drift as a changing procedure. It is believed that the beat frequency drifted at any position still carries the target information, so the system demodulates the signal at that drifted position. According to this method, the response speed of the demodulation system should be very high. But in a real system this acquisition is too high to be satisfied. It cannot work as quickly as expected. In computer simulation some slow varying drifts are induced at the beat frequency and the variation is distributed only in three parts of spatial frequency of transmitter interfering array. Simulation results show that this method may well compensate for slow drifting beat frequency. While its response speed is often limited by hardware system. On the other hand, for the dependence range demodulation, the beat drifting range is considered as a useful district, in which all the beat energy is added and demodulated at a preset position. An experiment is carried out to verify this method. The result demonstrates that it can well restrict the beat frequency drift within 100 Hz, which often happens in the procedure of AOM and driving amplification. Besides the laboratory setup research, the field experiments in 200 m and 1.5 km range are also carried out. The dependence range demodulation is proved to be well performed as well. The resolution of the 25 cm simulated target in 1.5 km reaches 0.008 rad. In the consideration of real system, the imaging range is further expanded and the amplifier power is stronger. The field experiments reveal that this demodulation method is applicable in such a condition. Therefore the research in this article provides some new techniques for the remote high resolution imaging in multi-beam laser interfering imaging.
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