The adaptive coupling of laser beam from space into single-mode fiber (SMF) plays an important role in the fiber-based free space optical communication technology. This paper investigates a kind of adaptive SMF coupling system based on a novel corrector named adaptive fiber coupler, and an improved control algorithm named precisedelayed stochastic parallel gradient descent (PD-SPGD). Compared to the SPGD algorithm used previously, PD-SPGD can compensate for the inherent response delay of the controlled system by setting a precise time delay between the disturbed voltages and the performance metrics. The experimental results show that this new algorithm increases the iteration rate from 3 to 8 kHz and decreases the convergence time from 6.3 to 2.5 ms when the static angular errors was corrected. As a result, the control bandwidth of the sinusoidal angular jitters is more than doubled. At the end, a preliminary atmospheric compensation experiment over a 520 m near-ground propagation path is conducted to verify the effectiveness of the method mentioned above.

No abstract

Multi-aperture receiver with optical combining architecture is an effective approach to overcome the turbulent atmosphere effect on the performance of the free-space optical (FSO) communications, in which how to combine the multiple laser beams received by the sub-apertures efficiently is one of the key technologies. In this paper, we focus on the combining module based on fiber couplers, and propose the all-fiber coherent beam combining (CBC) with two architectures by using active phase locking. To validate the feasibility of the proposed combining module, corresponding experiments and simulations on the CBC of four laser beams are carried out. The experimental results show that the phase differences among the input beams can be compensated and the combining efficiency can be stably promoted by active phase locking in CBC with both of the two architectures. The simulation results show that the combining efficiency fluctuates when turbulent atmosphere is considered, and the effectiveness of the combining module decreases as the turbulence increases. We believe that the combining module proposed in this paper has great potential, and the results can provide significant advices for researchers when building such a multi-aperture receiver with optical combining architecture for FSO commutation systems.