Investigation Of Some Key Problems In Serial Laser Beam Combination Based On Brillouin Amplificatioin

Lasers of high repetition rate, high power and high energy have been found wide applications in defense, scientific research and industry, etc. The traditional way of achieving high energy is such that a laser uses a single active medium to produce a single high energy beam. This approach suffers from several restrictions, which include the volage of single active medium, thermooptic effects, etc., that limit the maximum energy achievable from a single beam laser.In order to walk around these restrictions, the technique of laser beam combination was developed and researched over the past decades. The scheme of serial laser beam combination based on Brillouin amplification was presented in this dissertation, and the main problems including strong signal Brillouin amplification, influence of wave-vector mismatch on the Brillouin amplification and the structural design of beam combination were investigated in detail Firstly, a general mathematical model of collinear Brillouin amplification was constructed from the classic one-dimension transient coupled-wave equation of stimulated Brillouin scattering (SBS). The equation for simulating the influence of wave vector mismatch on the Brillouin amplification was derived. Based on the equations, a general mathematical model of non-collinear Brillouin amplification and the influence of wave vector mismatch on the Brillouin amplification were established.Sceondly, strong signal Brillouin amplification was investigated. Preliminary theoretical analysis of the feasibility was performed, and results show that theoretically it is feasible to achieve collinear strong signal Brillouin amplification. Then the detailed investigation of strong signal Brillouin amplification was carried out both theoretically and experimentally. The results show that the influence of non-focus backward scattering on strong signal Brillouin amplification is significant when the medium absorption loss is not considered. Choosing shorter cell can increase the energy threshold. Also, the study on variation of laser pulse-width shows that choosing the laser beam with larger beam diameter and longer pulse-width will increase the energy threshold. When the non-focus backward scattering is not considered, the influence of medium absorption loss on strong signal Brillouin amplification was significant. It can be reduced by choosing the medium with small absorption coefficient and short medium cell. Therefore, it is important to choose a set of optimized working parameters in the applications of beam combination. Experimental study shows that when the value of total system gain is close to the gain of Brillouin threshold Gth (~25) and the Stokes beam energy is higher than 10 times of pump beam energy, the energy extraction efficiency is higher than 85%. Next, the non-collinear strong signal Brillouin amplification was analyzed in detail both experimentally and theoretically. Using the scheme that there was 50(~87mrad)-cross angle between Stokes beam and pump beam, the energy extraction efficiency and the depletion ratio of pump energy was found both increase with the increasing of the Stokes beam energy. The energy extraction efficiency can reach greater than 80% and the depletion ratio of pump energy greater than 90% when the energy of Stokes beam and pump beam are both larger. Then the influence of cross-angle between Stokes beam and pump beam on Brillouin amplification was studied.The above results show that the constraint condition of strong signal Brillouin amplification is efficiently realized is: the value of total system gain reaches but not exceeds the gain of Billouin threshold.Thirdly, the influence of wave vector mismatch on Brillouin amplification was analyzed theoretically using the mathematical model, and experimental verification was performed. The results show that when the interaction length between two beams is long enough, the tolerancethe of wave vector mismatch angle is smaller than 400 mrad.Fourth, the beam combination scheme consisted of two laser beams was studied. It was found that the energy extraction efficiency increased with the increasing of Stokes beam energy, and the coupling efficiency also increased. In the experiment, up to >80% coupling efficiency was observed. Using 47.1mJ and 41.5mJ pump beams and 88.0mJ Stokes beam, the total energy extraction efficiency of 61.3% was observed and the coupling rates of laser beams was 80.7%.Finally, the constructions of serial laser beam combination were designed, and the influence of system loss on beam combination was analyzed. The scheme of beam combination based on collinear construction and non-collinear construction were designed respectively. The designed schemes of collinear and non-collinear were simulated and the data plots and tables for the scheme design were given. Based on the data plots and tables, the simulation results and layout of corresponding schemes of beam combination were presented. The coupling rates of beam combination reaches to more than 80%. The results of investigation Show that the scheme of beam combination based on collinear construction is suitable to apply when the energy of single laser beam is small, since the interaction length between two beams is longer in it; while the scheme of beam combination based on non-collinear construction is suitable to apply when the energy of single laser beam is larger, since the larger caliber of beam is needed. The system loss due to the optical elements has large influence on the efficiency of construction unit, under the condition of medium with small absorption coefficient and confirmed energy of single laser beam, the number upper limit of laser beams that can be combined is decided by the system loss. In order to meet the need of scale in serial laser beam combination, the combined scheme of beam combination is designed based on the collinear construction module and non-collinear construction module.Through the investigation of this dissertation, some key problems of serial beam combination based on Brillouin amplification are solved, and the feasibility of the scheme is proved. The laser beam obtained from this scheme has such characteristics including single beam in physical domain, smooth space distribution of intensity, needlessness of special optical elements and control equipments, low cost, simple construction, and strong maneuverability. This scheme provides a novel technique route for the development of laser beam combination.