Research On Active Phase Locking And Adaptive Polarization Control Technology Of Fiber Lasers

High power and good beam quality lasers have a wide range of applications in materials processing, industrial machining and medicine. Particularly, fiber lasers have been developed rapidly in recent years due to their essential characters such as excellent beam quality, compact configuration and high conversion efficiency. Because of the restraint for thermal and nonlinear effects, single fiber laser can’t obtain the high energy output and maintain a good beam quality. The successful demonstrations of high power coherent beam combination (CBC) system with phase-locking of master oscillator power amplifier (MOPA) array show a way to achieve high-brightness beyond the limit of single laser source. Phase locking and polarization control play an important role in the coherent beam combination (CBC). Along with increase of the frequency of the phase modulation signal, and the A/D and D/A converter speed, we believe the active phase locking technique has the potential to be scaled to a large-scale array with high output power and good beam quality. So the research of the active phase locking technique based on high speed and parallel processing is convenient for achieving the CBC of a large-scale array with a high efficiency, which is good for the development and application of high energy lasers. The active phase locking and polarization control are studied theoretically and experimentally. The relationship between the active phase locking methods and combining channels and the potential to a large-scale are analyzed. The applications of the active phase locking in fiber laser arrays and slab laser amplifiers and adaptive polarization control technique are discussed, which provides a reference and recommendations for high brightness scaling of the fiber laser and solid state laser.Theoretical model of CBC using SPGD, multi-dithering and single-dithering methods is built based on numerical computing. The control parameter optimization, phase locking time and stability of controlling for phase turbulence are performed by numerical computing. The control bandwidth, convergence time and their dependence on laser numbers are analyzed by dynamical simulation. The feasibility of a large-scale phase array based on these control algorithms are discussed in details. At the same time, the phase error RMS, amplitude of phase disturbance and beam quality of coherent combination in the large-scale array are simulated in theory.Experimental research of active phase locking for CBC using SPGD algorithm has been done. A high speed phase controller of fiber array has been designed and manufactured based on stochastic parallel gradient descent (SPGD) algorithm. The signal processing speed of the FPGA circuit is50MHz and its iteration rate is more than200kHz. The validation experiment shows that the phase controller can perform well in phase locking of five lasers and ten lasers, respectively. An optimization algorithm using a variable gain coefficient is proposed and the phase locking of ten lasers is obtained. The phase locking efficiency of CBC is89.68%and the light intensity in far-field of CBC is stable near the maximum. The feasibility of active phase locking for a large-scale fiber array using SPGD algorithm has been verified experimentally.A novel active phase locking algorithm based on multilevel phase dithering is analized theretically and demonstrated experimentally. Using dual-level rectangular-wave phase modulation, time division multiplexing and parallel processing, the proposed multi-level phase dithering technique(MPD) can achieve the same phase error signal as single/multi-frequency dithering technique, but without coherent demodulation circuit, which has a high convergence speed and is suitable for phase locking of a large-scale array. Experimental investigation on coherent beam combination of ten fiber laser beams is successfully demonstrated, with imposing an additional phase disturbances to mimic the phase noises in the high power fiber amplifiers. The iteration rate of phase detection and contoll is more than200kHz.The phase locking efficiency with large modulated phase noise (phase noise amplitude is±2rad, and its frequency is4kHz) is achieved above97.55%and the accuracy of phase control is improved to λ/42rms. We believe the proposed MPD technique has the potential to be scaled to a large-scale array with high output power and good beam quality. A coherent combining experimental system of thirty fiber lasers has been designed. The phase control module of thirty channels and experimental setting are built.The experimental investigation on the208W coherent beam combining of two slab amplifiers using MPD algorithm shows that the whole system in a closed loop performs well over a long time observation. The visibility of the coherent combined beam profile is about87%and the phase locking efficiency is nearly90.38%and the accuracy of phase control is improved to λ/20rms. Phase locking of high power solid-state amplifiers using MPD algorithm is validated experimentally. CBC technology of slab lasers is studied numerically and theoretically. One of the application fields, i.e., Beam steering with MPD active phase-locking of master oscillator power amplifier (MOPA) array is proposed. In the numerical model, high speed phase locking and two dimensional scanning using fiber MOPA configuration and MPD algorithm can be obtained.Finally, active polarization control of fiber laser is studied numerically and experimentally. The whole polarization control system in the fiber optical path including a seed laser, a polarization control module and a polarization detector has been designed. The polarization detection and control utilizing fast search algorithm and a fiber polarization controller based on piezoelectric ceramics are demonstrated. In addition, the polarization control using SPGD algorithm is validated experimentally. The results have shown that the response time of the polarization control is less than1ms and the fluctuation of light intensity in closed-loop is less than1.5%. A novel fiber phased array based on adaptive polarization and phase locking is proposed. This system using a single detector and MPD algorithm can obtain the polarization and phase locking simultaneously, which is good for high efficiency coherent combination. In the research of polarization control, the CBC of single mode fiber laser in place of PM (polarization maintaining) fiber laser is analyzed, which has the potential to be scaled to a higher brightness.