Coherent Amplified Array Of Narrow-linewidth Nanosecond Fiber Lasers

Fiber lasers have become an important development direction because of their essential characteristics of excellent beam quality, high conversion efficiency, robust performance, convenient heat management, steady operation, and so on. In recent years, narrow-linewidth nanosecond fiber lasers have attracted much attention for a range of applications such as nonlinear frequency generation, LIDAR, and remote sensing. However, the output power/brightness of fiber laser is limited by the thermal effect, nonlinear effect and so on. In order to scale the output power/brightness from a single fiber, the impact of nonlinear effect should be studied in detail and the laser system should be optimized carefully. Coherent amplified array, which can increase the output power of fiber laser while maintaining high beam quality, provides another way of obtaining high bright laser. We theoretically and experimentally study the coherent amplified array of narrow-linewidth nanosecond fiber lasers in this thesis, which contains the following contents:The nonlinear effects for nanosecond laser are investigated firstly, including stimulated Brillouin scattering(SBS), stimulated Raman scattering(SRS) and self-phase modulation(SPM). Impact of pulse width, pulse period, pulse shape and fiber length on SBS threshold for nanosecond laser in optical fiber is numerically studied based on three coupled amplitude equations. Interaction length is proposed, which could be used to model and analyse the SBS threshold of nanosecond laser in optical fiber. The relationship between SBS threshold of nanosecond fiber laser and interaction length is theoretically and experimentally studied. SPM induced spectral broadening is pre-compensated by phase modulation. Impact of SPM pre-compensation on SBS threshold is studied: the linewidth of output laser equals that of the pulsed seed when absolute SPM pre-compensation is implemented, however, SBS threshold decreases at the same time; over SPM pre-compensation can be used to obtain pulsed laser with both narrow linewidth and high SBS threshold.Narrow-linewidth nanosecond fiber amplifiers are studied based on the research of nonlinear effects. High average power of 913 W is obtained, which is the highest average power of narrow linewidth nanosecond laser reported so far, to the best of our knowledge. Single-frequency, single-mode, linearly-polarized, nanosecond fiber laser with peak power of 1.47 k W is generated based on SPM pre-compensation. The single-frequency nanosecond fiber Raman amplifier and the dual-frequency narrow-linewidth nanosecond hybrid fiber amplifier are proposed. A narrow-linewidth nanosecond all fiber Raman amplifier pumped by pulsed laser is built, and a gain of 31 d B is obtained.Then, coherent amplified array of nanosecond fiber lasers is studied in detail. The effects of aberrations on coherent amplified array of nanosecond fiber lasers are analyzed. The key technologies of coherent amplified array are studied. With respect to optical path difference(OPD) controlling, we invent a multi-beam combiner with the function of OPD adjustment. A new OPD controlling method is proposed, where passive fibers are employed to control OPD. For phase locking, impact of the laser pulse on phase controlling is analyzed, and phase controlling methods of pulses with different repetition rate are proposed. We also proposed a phase locking system for coherent amplified arrays with large number of fiber lasers.Experiments of coherent amplified arrays for nanosecond fiber lasers are demonstrated at last. High speed high precision phase controllers with channel of 16 and 32 are designed and manufactured based on Field Programmable Gate Array(FPGA). For pulses with high repetition rate, we obtain high average power of 200 W and 800 W from coherent arrays with two and five amplifiers by using stochastic parallel gradient descent(SPGD) algorithm. Using single frequency dithering technique, seven lasers are actively phase-locked and an average power of 1.2 k W is obtained, which is the highest average power for cohenrent amplified array of pulsed fiber lasers to the best of our knowledge. For pulses with low repetition rate, a novel structure is proposed, and the validity of this method is verified by actively phase locking of two nanosecond all-fiber amplifiers with low repetition rate of 5 k Hz.