Research On Mode Conversion To Improve Coupling Efficiency On Single-mode Fiber

In the wireless laser communication system,the spatial light mode is affected by atmospheric turbulence,which causes its degradation,resulting in the incomplete matching of the mode field of the single-mode fiber coupled with the receiving part,thereby reducing the coupling efficiency of the space light-single-mode fiber.Therefore,it is particularly important to match the beam mode with the fiber mode.In this paper,the conversion method of free space light mode is studied with the liquid crystal spatial light modulator as the core structure,the conversion transfer function is optimized,and the mode conversion is used to experimentally improve the coupling efficiency of single mode fiber.The main tasks are as follows:1.The mode theory of optical fiber is analyzed and the expression of the transfer function of the exchange between high and low order modes is deduced,which provides a theoretical basis for the establishment of the conversion model in the later period.2.In the simulation,the binary phase is used to excite between the LP01 mode and the high-order modes,which is used as the high-order mode after turbulence degradation.Then,the binary phase is used to realize the conversion between the high-order modes and the LP01 mode,and the conversion transfer function and corresponding conversion efficiency evaluation indicators are established.3.The simulated annealing algorithm is used to optimize the phase of the transfer function by random disturbance,and the effects of initial temperature,annealing coefficient,disturbance times and specific parameters of the optimized area on the mode conversion efficiency are analyzed.Taking conversion efficiency as the evaluation index,the optimal conversion transfer function and its phase hologram optimized by the simulated annealing algorithm are established,and the conversion efficiency after pure binary phase conversion has been greatly improved.Combined with simulated annealing algorithm optimization,the mode conversion system is improved.4.The coupling experiment platform under mode conversion is built to study the effect of mode conversion on coupling efficiency.The experimental pre-processing obtained the degraded higher-order modes after turbulence,and then the results of mode conversion when loading optimized conversion transfer function hologram and without optimization are compared and analyzed.The results show that the mode conversion scheme can effectively improve the coupling efficiency of space light-single-mode fiber.The conversion efficiency obtained by the mode conversion system optimized by the simulated annealing algorithm is as high as 59.88%.In the mode conversion experiment,the peripheral diffraction of each order mode conversion is reduced compared to the pure binary conversion.When the mode conversion scheme is applied to the coupling experiment system,compared with the coupling efficiency without the optimized mode conversion,the coupling efficiency after using the simulated annealing algorithm to optimize the mode conversion has increased from 49.19%to over 60.29%.Therefore,mode conversion has great value for improvement in coupling efficiency.