Research On Coupling Efficiency Of Partially Coherent Light-single Mode Fiber In Atmospheric Turbulence Channel

The coupling technology of space laser to single mode fiber is the key technologies of free space optical communication,but high-efficiency coupling is always very difficult.The main reason is that the core size of the single mode fiber is small,and the focused spot is difficult to accurately align with the single mode fiber.At the same time,the transmission quality of the space laser is easily affected by atmospheric turbulence,and the beam wavefront phase distortion is serious,how to improve coupling efficiency is an urgent problem to be solved.Existing research results mostly use fully coherent light as the light source model to research the coupling efficiency.However,in practical applications,the laser is affected by the thermal effect,the size of the resonator,and its own line width,and the light emitted by the laser has time and space expansion.Therefore,the light emitted by the laser is a partially coherent light source.In this paper,the partially coherent light is used as the light source model.In the case of atmospheric turbulence channels,the coupling efficiency of partially coherent light and single mode fiber is researched.The specific work includes three aspects:(1)Under the Kolmogorov turbulence spectrum model,using the cross spectral density function of partially coherent light,the mathematical model of the coupling efficiency of partially coherent light-single lens-single mode fiber under the horizontal link is derived,and the coupling efficiency of partially coherent light-lens array-single mode fiber array is researched.The influence of light source coherence,wavelength,turbulence intensity,transmission distance,number of sub-apertures of the lens array and Dlp H·on the coupling efficiency is analyzed.On this basis,the coupling power and the relative fluctuation variance of the coupling power are researched.(2)Based on the ITU-R atmospheric structure constant model,based on the theoretical basis of horizontal link coupling,the theoretical model of partially coherent light-single lens-single mode fiber coupling under the slant link is established.Due to the asymmetry of the slant link,the coupling efficiency of the partially coherent light-lens array-single mode fiber array of the slant link will be researched from both the uplink and the downlink.The influence of light source coherence,wind speed,transmission distance,zenith angle,number of sub-apertures of the lens array and D/?_F coupling efficiency are analyzed,and the coupling power and the relative fluctuation variance of the coupling power are further researched.(3)Based on the coupling of horizontal and slant links,according to the structure function of different aberrations,the effect of aberration correction in atmospheric turbulence on the coupling efficiency of partially coherent light-single mode fiber is researched.The effects of correcting tilt,defocus,astigmatism,and coma aberration on the coupling efficiency of partial coherent light-single lens-single mode fiber and partial coherent light-lens array-single mode fiber array of horizontal link and slant link are analyzed.The research results show that,lens array reception and aberration correction can significantly improve the coupling efficiency of partially coherent light.Take the lens array with a sub-aperture of 7 and the 6th-order aberration before correction as an example,when the two work together,in the case of a light source with a coherence of 0.04 m,for a horizontal link,when the transmission distance is 5 km,the coupling efficiency is increased from 27.7%to58.9%;For the slant link,when the transmission distance is 10 km,the downlink coupling efficiency is increased from 10.9%to 50.7%,and the uplink coupling efficiency is increased from 19.4%to 56.4%.Compared with fully coherent light,partially coherent light has a smaller relative variance of coupling power,and the power of partially coherent light coupled into the single mode fiber is more stable.The research work in this paper has laid the theoretical foundation for the design and parameter optimization of the partially coherent light-single mode fiber coupling system of the horizontal link and the slant link in the atmospheric turbulence channel.