Mechanism And Experimental Study Of Wavefront Distortion On Mixing Efficiency

In an optical wireless coherent communication system,after the signal beam is transmitted through a turbulent channel,the wavefront distortion caused by environmental factors such as atmospheric turbulence becomes the main factor affecting the performance of the coherent detection system at the receiving end.To explore the influence mechanism of wavefront distortion and its correction on the performance of the optical wireless coherent detection system,this dissertation takes the optical wireless coherent detection system as the research background,and studies the wavefront distortion and its correction caused by environmental factors such as atmospheric turbulence.The main work of this dissertation is as follows:1、According to the basic theory of optical wireless coherent detection and atmospheric turbulence,the principle mechanism of wavefront distortion in Kolmogorov and non-Kolmogorov atmospheric turbulence is expounded,and the mathematical models of the wavefront distortion of multi-beam transmission and the mixing efficiency,signal-to-noise ratio(SNR)and bit error rate(BER)of the optical wireless coherent detection system in different atmospheric turbulence models are established.The effects of spectral power law exponent,zenith angle,transmission distance,and Fresnel zone on optical wireless coherent detection system performance are analyzed by simulation.2、Using the model method of C-Zernike,Chebyshev,Legendre,and S-Zernike polynomials to describe and evaluate the wavefront distortion,establish the transmission model of the signal beam in the atmospheric turbulence according to the principle of Fresnel diffraction,and the light field distribution reaching the receiving end system calculated;In addition,the impact of wavefront distortion on alignment deviation at the receiving end is analyzed.Then,the impact of wavefront distortion and its correction on the performance of the optical wireless coherent detection system was simulated and analyzed,including the impact of high-order wavefront distortion and its correction compensation amount on the performance of the optical wireless coherent detection system at the receiving end,and the demand limit of the performance index of the optical wireless coherent detection system on the wavefront distortion and its correction order.3、The wavefront distortion correction experiments of indoor and outdoor(600 m,1.2 km,and 1.3 km)optical wireless coherent detection systems were built.Taking the wavefront peak-to-valley(PV)value,root mean square(RMS)value,mixing efficiency,BER,and IF signal as evaluation indicators,the wavefront distortion correction experiment in the optical wireless coherent detection system is carried out,and the influence of multi-beam transmission on the performance of optical wireless coherent detection system at the receiving end is verified.Wavefront distortion and wavefront jitter can be suppressed by using multi-beam transmission wavefront superposition,which effectively improves the performance of an optical wireless coherent detection system.The research results show that the correction and compensation of wavefront distortion can effectively improve the performance of the optical wireless coherent detection system at the receiving end,the mixing gain obtained by homodyne detection is about 3 dB higher than that obtained by heterodyne detection.However,to achieve optimal mixing efficiency,the wavefront correction order required for homodyne detection is larger than that for heterodyne detection.Correction experiments were carried out on the wavefront distortion of the optical wireless coherent detection system with indoor,600 m,1.2 km,and multi-beam transmission of 1.3 km respectively.Among them,the mixing efficiency of the optical wireless coherent detection system is significantly improved after the wavefront is corrected to the tilt,defocus and astigmatism items,and compared with the single-beam transmission,the wavefront PV value and RMS value variance of the two-beam transmission are reduced by 1.45 μm and 0.04μm2.The research work in this dissertation lays a theoretical foundation for analyzing the influence of the performance of the optical wireless coherent detection system,and provides a quantitative reference for the optical wireless coherent detection system to achieve optimal wavefront correction.