Research Of Active Phase-locking Fiber Laser Coherent Combining Technique

Coherent combination of multiple fiber lasers is a main technique to realize high-power and high-brightness fiber laser system. It plays an important role in laser process, laser medical and military application areas. Thus it is significable to investigate the fiber laser coherent combining system with scaling elements and power. In this dissertation, coherent combining technique of master oscillator power amplifier (MOPA) architecture is investigated theoretically and experimentally in-depth.Energy contained in the central lobe is presented, defined as the ratio of energy contained in the central lobe to the whole energy in the far field. The parameter describes the energy distribution of the central lobe and side lobes in the far field. Compared with the conventional parameters of power in the bucket (PIB) and beam propagation factor (BPF), the parameter of energy contained in the central lobe is much more practical in theoretical analysis and experiments. Strehl ratio is defined as the ratio of actual on-axis intensity to that of an ideal beam, which illuminates the precision of optical path alignment and the phase controlling electronics. In this dissertation, we employ energy contained in the central lobe and Stehl ratio as the main evaluation parameters of coherent combining system.A theoretical model of far field intensity distribution is established based on Fraunhofer diffraction theory. The effects of element parameters on far field intensity distribution are analyzed with Matlab software by a hexagonal array as an example, including fill factor, phase, amplitude, polarization, element number and array arrangement. The simulating results indicate that energy contained in the central lobe increases with the enlargement of fill factor. Strehl ratio and energy contained in the central lobe are decreased by the phase error, amplitude and polarization error between elements.A seven-element hexagonal fiber coherent combining system with MOPA architecture is established. The seven signal arms are seven 10 meters polarization maintaining (PM) fibers transmitting master oscillator laser instead of 10W polarization maintaining fiber amplifier. The hexagonal prism is designed as beam combining and splitting component to obtain fill factor of 0.66. The coherent combining system is scalable of element number by increasing the number of prism surfaces or assembling different diameter prisms. The hill climbing method is used as the phase controlling technique. The far field distribution of the seven-element hexagonal array, the four-element rectangular array, the three-element triangle array and the two-element linear array are experimentally researched. Energy contained in the central lobe of the seven- element coherent combining beam is 0.40 (close to the theoretical value of 0.55) and Strehl ratio of the seven-element coherent combining beam is 0.91.The phase controlling techniques of fiber laser coherent combining system are investigated. The four phase controlling techniques are experimentally researched and compared, including hill climbing method, heterodyne method, stochastic parallel gradient descent (SPGD) method and adaptive optics technique. We designed lithium niobate crystal, waveguide lithium niobate, and adaptive deformable mirror as phase modulators. The phase controlling precisions of the four techniques are experimentally researched on the two-element fiber coherent combining system. The phase controlling steady precisions are tested as followings: hill climbing method is 4%, heterodyne method is 10%, SPGD method is tested as 4%, and adaptive optics method is 18%.The thermal effect of a 10-W fiber amplifier is analyzed. The temperature distribution of the moment when the pump is on and the steady-state are deduced and simulated. The temperature of the fiber amplifier in the paper trends to steady-state after the pump is on for 12.3 seconds, and during the proceed the temperature increase by 8.06℃. The phase noises of 10-W polarization maintaining fiber amplifier and 10 meters polarization maintaining fiber are tested. Both majority of the phase noises are at frequency about several kHz. The experimental results illuminate the feasibility and novelty of the 10-meter polarization maintaining fiber simulation the 10-W polarization maintaining fiber amplifier in the research work of fiber laser coherent combination.The applied prospect of fiber laser coherent combining in phased array is introduced. The principle of fiber laser phased array is analyzed. The theoretical model of beam scanning by changing the phase difference is established. The beam scanning of one-dimension and two-dimension square array, and two-dimension hexagonal array are simulated by Matlab.