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摘要:
提出了一种基于光纤耦合器的全光纤链路锁相方法。通过光纤交叉互联使得出射光束与本振激光之间以及光纤端面后向回光相互间在光纤耦合器内进行干涉以获得性能指标，利用随机并行梯度下降算法(SPGD)对性能指标进行盲优化控制，实现多路出射光束在光纤端面处的相位锁定。首先建立了这种锁相方法的模型，讨论其锁相控制的稳态条件。搭建了两路全光纤链路锁相实验，并对两路出射光纤激光进行准直发射，通过相机获得的远场干涉条纹图像来判定锁相精度。实验结果表明，在引入正弦相位扰动幅值4个波长和频率2 Hz的情况下，该锁相方法能够将10 s长曝光图像条纹对比度从开环时的0.25提升至闭环时的0.82，单帧短曝光图像条纹对比度从开环时的0.65提升至闭环时的0.98。
Abstract: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.
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Key words:
- coherent beam combining /
- all fiber link /
- phase locking /
- fiber coupler
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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.
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图 1 光纤链路锁相方案. PM：光纤相位调制器；PM-SMF：保偏单模光纤.
Figure 1. Scheme of all-fiber-link phase-locking for 4 outgoing fiber laser beams. PM: Fiber phase modulator. PM-SMF: polarization-maintaining single-mode fiber.
图 2 双路全光纤链路锁相的实验装置图.
Figure 2. Experimental scheme of all-fiber-link phase-locking for 2 outgoing fiber laser beams.
图 3 综合性能指标v0在开闭环各10 s区间内的变化曲线.
Figure 3. Ensemble metric v0 as a function of time during 10 s under open loop and closed loop.
图 4 开闭环各10 s期间内v1、v2和v3的变化曲线. (a) v1. (b) v2. (c) v3.
Figure 4. v1、v2 and v3 as functions of time during 10 s under open loop and closed loop. (a) v1. (b) v2. (c) v3.
图 5 远场光斑图像分布. (a)开环AFOC1远场短曝光图. (b)开环长曝光图. (c)开环短曝光图. (d)闭环AFOC1短曝光图. (e)闭环长曝光图. (f)闭环短曝光图.
Figure 5. Far-field intensity distributions. (a) Long-exposure pattern under open loop for single aperture of AFOC1. (b) Long-exposure pattern under open loop. (c) Short-exposure pattern under open loop. (d) Short-exposure pattern under closed loop for single aperture of AFOC1. (e) Long-exposure pattern under closed loop. (f) Short-exposure pattern under closed loop.
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