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

Phase locking is one of the key issues for fiber laser array applications, like coherent beams combining andfiber laser phased array. These applications can be applied in fiber laser systems like laser radar, target tracking, activeillumination, free-space laser communication and direct energy. Phase locking is aimed at stabilizing the wave-frontphase at the pupil of each element in the fiber laser array, which is mainly caused by the path length fluctuations between uncommon lengths of fiber. Most existing fiber array systems compensate for the piston difference throughmeasuring the output phase of the array by sampling the outgoing beam using free space optical devices outside thearray. To avoid the complex and dumb spatial optical devices, a new technique of phase-locking control in all fiber linkbased on fiber coupler has been proposed. Laser beams backscattered by the fiber tips of the different outgoing fiberlaser beams interfere with each other in the fiber couplers. Meanwhile, the outgoing laser beams interfere with thepartial local laser beams in the fiber couplers. These interference results provide metrics for phase-locking controlalgorithm named stochastic parallel gradient descent (SPGD). Laser beams are then phase-locked on their outgoingfiber tips under such system. Function of the fiber coupler is forming conventional interferometry scheme. Differentfrom existing phase-locking methods based on active phase difference measurement, phase-locking here is achievedthrough optimization algorithm. The main advantage of such technique is avoidance of high-speed phase modulatorsand complex phase demodulation. This provides a potential way to realize phase locking control with light and fiber-integrated scheme. 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 laserbeams are collimated and adjusted to overlap and interfere with each other in far field. Interference patterns in far fieldare collected by high speed camera to judge the control performance. Experimental results show that such techniquepromotes 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. Experimental results prove that the phase locking method proposed here is effective to stabilize the wave-front phase of the fiber laser array.