Coherent beam combination of wave-front divided 4 beams by phase controlled stimulated Brillouin scattering phase conjugation mirrors toward the realization of a practical laser fusion driver

Complete text of publication follows. Creating a laser with a high power and high repetition rate is challenging due to thermal problem related to the laser medium. Several studies have sought to overcome these problems but the suggested solutions are inadequate for certain applications, such as a laser fusion driver for practical power generation, because of thermal problems with the laser media. A beam combination method can fundamentally solve the thermal problems because it uses only small-size amplifiers and coherently combines the beams to create a high-power laser. The beam combination laser using stimulated Brillouin scattering phase conjugate mirrors (SBS-PCMs) is a promising technique for this purpose. The SBS-PCM generates a phase conjugate beam which can restore the optical distortions generated by active media in beam path. However, because the SBS process is initiated from thermal noise in the SBS medium, the relative phases between the separate SBS-PCMs are inherently random. With a 'self-phase control' method, any number of beams can be reflected from each SBS-PCM with definite relative phases. The self-phase control method was demonstrated experimentally and expanded to control the relative phases of the beams. We report on the experimental results of combining four beams by using SBS-PCMs with wave-front division. After passing through a four-beam aperture, the beams are combined with the aid of the SBS-PCMs. An interference pattern is generated between a part of the combined output beam and a part of one of the expanded beams, Beam 1. The relative phases are calculated from this interferogram. The output energy was 169 mJ ± 6 mJ when the input energy was 32.2 mJ ± 0.3 mJ. The standard deviations of the phase differences of beams 2, 3, and 4 were measured to be λ/26.5, λ/28.0, and λ/26.1, respectively. Acknowledgements. This work was supported ba a grant from the Nuclear Research and Development Program of the Korea Science and Engineering Foundation, which is funded by the Korean government (MEST) (grant code: M20090078160). It was also supported by the R and D Program through the National Fusion Research Institute of Korea (NFRI) founded by the Government funds, the KAIST High Risk High Return Project, and the International Atomic Energy Agency as a part of the coordinated research project 'Pathway to Energy from Inertial Fusion - An Integrated Approach' (Research Contract NO. 13758/R0).