Numerical investigation of coherent laser beam combination by stimulated Brillouin scattering

摘要

Serial coherent laser beam combination is studied using one-dimensional transient model of stimulated Brillouin scattering (SBS). Beam combination is achieved by transferring the energy of several pump beams to one Stokes beam. The pulse-width of the Stokes beam, which is approximately equal to that of the pump, can be kept constant by adjusting the meeting position between Stokes beam and pump beams in the cell. Numerical simulation is made for the condition that power density of Stokes beam is larger than that of the pump beam. Simulation results show that the major factors that determine the energy extraction efficiency and the critical value of beam combination are the absorption of the medium and the length of SBS cell. The absorption loss is found to play a leading role, so that the overall energy extraction efficiency and the critical value of beam combination can be increased by using shorter SBS cells and/or medium with lower absorption rate. The simulation chooses CCl_4 as target medium. The SBS cell length is 20cm; 20 beams (wavelength 1064nm, energy 1J, pulse-width 10ns, beam-area 1cm~2) are combined in serial, one of which generates the Stokes seed, others are the pump beams. In order to keep the power density below 100MW/cm~2 to avoid being more than threshold of SBS, the Stokes beams and pump beams must be expanded. The output beam of ～10J energy, ～10ns pulse-width, ～10cm~2 beam area is obtained.