Research On Laser Beam Combination Based On Non-collinear Brillouin Serial Amplification

Laser beam combination technology, which combines multi-small-energy laser beams together, going through the rapid development in recent years, is an effective method to generate high power, high beam quality and high stability laser radiation. The technology breaks the large volume restriction of single laser system under the condition of high output power, reduces high manufacturing costs, avoid the technical risks of thermal damage and finally, promotes the robustness of the entire laser system. The serial laser beam combination technology based on Stimulated Brillouin scattering, compared to other laser beam combination technology, is close to that one completely coherent laser beam can be achieved physically by the means of incoherent laser beam combination. The advantages of the technical scheme include the independency of each pump light which is free from precise phase control, high system stability and significant scalability. In this dissertation, the main problems including strong signal Brillouin amplification and non-collinear Brillouin amplification are investigated in detail. Based on the theory, the non-collinear Brillouin serial amplification laser combination demo nstration system is designed and built, which makes single high-energy laser pulse generated by the multi-beam power combination come true.Firstly, based on the classic acoustic-optic coupling equation set of Stimulated Brillouin Scattering(SBS), the theoretical model of strong signal Brillouin amplification with the existence of Stimulated Brillouin Scattering Stokes seeds is derived. With this theoretical model, the simulation and analysis on the influence of the pump light on the stimulated scattering from the Stokes seed in the process of strong signal Brillouin amplification is studied explicitly. The participation of the pump light lowers the threshold of Stimulated Brillouin Scattering generated by the Stokes seed. The verification experiment has been designed and conducted. Subsequently, changing the encounter time and the crossing angle between the Stokes seed and the pump light in the SBS media by regular steps, the relation between the occurrence of the Stokes Simulated Brillouin Scattering and the experiment conditions in the strong signal Brillouin amplification process is studies experimentally, which will direct the system structure parameters settings. By changing the Brillouin frequency shift, Brillouin gain linewidth and Brillouin gain coefficient of the SBS media regularly, the influence of the media parameters on the Stokes Stimulated Brillouin Scattering in the process of strong signal Brillouin amplification is theoretically simulated, which provides the reference to the selection of the SBS media.Secondly, the study on the non-collinear Brillouin amplification of strong signal with different crossing angles is conducted on the self-designed laser system. The conclusion that phase match is not necessary to sustain the high energy extraction efficiency with small nonlinear crossing angles(<15?) has been achieved. To address the problem of the rapid decrease of non-collinear Brillouin amplification efficiency resulting from large crossing angle, the theoretical simulation and analysis have been conducted. The solution to non-collinear SBS amplification frequency mismatch by controlling the frequency of the Stokes seed and the acoustic wave to reach the resonance state has been proposed and verified experimentally. Furthermore, a simple but effective and innovative measurement method of Brillouin characteristic parameters of SBS media is proposed. With this method, the Brillouin frequency shift and the Brillouin gain linewidth of the media, which is suitable for the serial laser beam combination, have been measured. Measurement results can provide a reference to the media selection.Finally, on the basis of the research content in the previous chapters, the non-collinear Brillouin serial laser beam combination demonstration system is designed and built modularly. In the laser oscillator-amplification module, composite cavity structure is adopted for the single-longitudinal mode selection; the structure parameters are calculated with the theory of transmission matrix and the structure design is conducted with the assistance o f 3D mapping software Solidworks. In multiple laser pulses parallel amplification module, the double-pass amplification structure is adopted; on the basis of the gain simulation, the specifications of the Nd:YAG rod amplifier is chosen and the power specifications is settled down by the simulation of the laser power discharge waveform. In the non-collinear Brillouin serial amplification power combination module, the encounter manner of the Stokes seed and the pump light in the amplification cell is determined by the structural arrangement of the system. Through the joint debugging after the separate construction of each module, the power combination of 5 pump light and the Stokes seed is realized, with the repetition frequency of 1Hz, the pulse width of 10.6ns and the output energy of 2.96 J. For the entire laser combination system, the energy extraction efficiency of 0.71 and the laser beam combination efficiency of 0.78 are achieved eventually.