Phase-locking control in all fiber link based on fiber coupler

Abstract: A new technique of phase-locking control in all fiber link based on fiber coupler has been proposed. Laser beams backscattered by the fiber tips of the different outgoing fiber laser beams interfere with each other in the fiber couplers. Meanwhile, the outgoing laser beams interfere with the partial local laser beams in the fiber couplers. These interference results provide metrics for phase-locking control algorithm named stochastic parallel gradient descent (SPGD). Laser beams are then phase-locked on their outgoing fiber tips under such system. Model of such novel phase-locking system for multi-laser-beams is built and steady-state control conditions are discussed. All fiber phase-locking is achieved for two laser beams in our experiment. The two laser beams are collimated and adjusted to overlap and interfere with each other in far field. Interference patterns in far field are collected by high speed camera to judge the control performance. Experimental results show that such technique promotes the fringe visibility of the long-exposure pattern during 10 s from 0.25 in open loop to 0.82 in closed loop, under phase disturb with an amplitude of 4 wavelengths and a frequency of 2 Hz. Fringe visibility of the short exposure pattern rises from 0.65 to 0.98 correspondingly.