Mr. Qi Zhao Profile

Qi Zhao

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Proceedings Article | 24 October 2017 Paper

Research on laser spot location algorithm in weak turbulence

Qi Zhao, Dai Zhang, Qingsong Zhao

Proceedings Volume 10462, 104624I (2017) https://doi.org/10.1117/12.2285556

KEYWORDS: Atmospheric turbulence

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Laser communication has become the main driving force for the development of modern wireless optical communication technology with the characteristics of large communication capacity, good concealment and good directivity. Acquisition, pointing and tracking system which is refer to as the APT system is the key technology to the laser communication, and the detection and processing technology is one of the key technologies of APT system. The effect of scintillation and laser spot drift caused by atmospheric turbulence seriously affects the laser spot positioning accuracy of laser communication APT system, which will affect the performance of laser communication system. It is very important to choose the appropriate spot pre-processing method and the best method to improve the positioning accuracy of laser communication system. An ideal spot image with known center coordinates was generated artificially and the MATLAB was used to simulate the atmospheric turbulence to make the laser spot close to the real atmosphere. Chosing median filtering and mean filtering method to the denoising pretreatment to get filtered image. Using iterative threshold method to obtain the binary image. Through 3 common spot positioning method like the gray centroid method, circle fitting method and the Gaussian fitting method to calculate the centroid of the binary image. After getting the central coordinates, the results were compared and analyzed. Experimental results showed that mean filtering is better than median filter to filter noise of the laser spot. Compared with other methods, the centroid accuracy obtained by the gray centroid method had larger deviation due to the process of filtering the noise was not completely suppressed. The laser spot center calculating by the Gaussian fitting method were with higher positioning accuracy. According to the calculation results, the applicable conditions of different spot location algorithms were given.

Proceedings Article | 31 October 2016 Paper

Depolarization of laser beam propagating through atmosphere based on multiple Rayleigh scattering model

Dai Zhang, Shiqi Hao, Lei Wang, Qi Zhao

Proceedings Volume 10021, 100211Y (2016) https://doi.org/10.1117/12.2247673

KEYWORDS: Laser scattering, Atmospheric propagation, Polarization, Scattering, Atmospheric modeling, Light scattering, Rayleigh scattering, Signal attenuation, Photons, Clouds

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An analytical formula model which is used to describe laser beam’s depolarization characteristics is solved based on multiple Rayleigh scattering model. Firstly, by using Stokes vector to characterize intensity and polarization, while at the same time using Mueller matrix and rotation matrix to characterize polarization changing in scattering procedure, a single scattering model is built. Then, a multiple scattering model is built considering the effects of atmospheric absorbing and scattering attenuation. The received light’s Stokes vectors through multiple scattering procedure are separately solved. At last, on the basis of multiple scattering vectors, the depolarization characteristics of laser beam propagation through atmosphere are estimated though calculating ratio of depolarization and polarized angle shifting. The numerical analysis based on analytical conclusion of this paper shows that for a horizontal polarized laser beam, its ratio of depolarization is about 1% and polarized angle shifting is about 0.3。 when propagates through atmosphere and arrives into the receiver on the ground, and both the above characteristics have only a small change compared with the change of cloud’s depth. The findings of these research show that Rayleigh scattering from atmosphere has a weak effect on the laser beam’s polarization status. The multiple scattering model and Stokes vector analytical formulas raised in the paper could also be used to study the depolarization characteristics of ellipse polarized laser beam and partially polarized laser beam propagating through atmosphere. The research findings of this paper will have theoretical guiding significances in the domain of laser communication, laser detection and laser imaging.

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