AS-SPGD algorithm to improve convergence performance for fiber coupling in free space optical communication

In free space optical (FSO) communication, atmospheric turbulence will introduce aberration to the laser wavefront, which will reduce the coupling efficiency of space light to optical fiber. The system based on blind optimization principle can realize wavefront aberration compensation by using actuators such as fast steering mirror (FSM) and deformable mirror (DM), combined with blind optimization control algorithms. Stochastic parallel gradient descent (SPGD) algorithm is widely used in this kind of system. The disadvantages of SPGD algorithm are possibly local convergence and slow convergence. In order to improve the convergence performance of the SPGD algorithm, we proposed improved SPGD algorithms called Adagrad & Staged SPGD algorithm (AS-SPGD). Specifically, to accelerate SPGD algorithm, the Adagrad SPGD algorithm (A-SPGD) integrated Adagrad's adaptive gain strategy is proposed firstly in wavefront tip-tilt aberration compensation using FSM. Then, we consider compensating the wavefront high-order aberration using DM. By adopting the strategy of using SPGD algorithm with different settings in different stages, the Staged SPGD algorithm (S-SPGD) is proposed to improve the local convergence of the traditional SPGD algorithm. The simulation results show that the Adagrad SPGD algorithm converges faster than the traditional SPGD algorithm in the compensation of tip-tilt aberrations of different amplitude; the Staged SPGD algorithm has better global convergence ability than the traditional SPGD algorithm in high-order aberration compensation.