Stimulated Brillouin Scattering Phase Conjugate Mirror and its Application to Coherent Beam Combined Laser System Producing a High Energy, High Power, High Beam Quality, and High Repetition Rate Output

Abstract

Stimulated Brillouin scattering (SBS) is a nonlinear optical process that generates backward scattered phase conjugate wave (Zel’dovich et al., 1972; Zel’dovich et al., 1985; Damzen et al., 2003; Brignon & Huignard, 2004). A device that generates the phase conjugate wave by the SBS process is called SBS phase conjugate mirror (PCM). An SBS-PCM can compensate wavefront distortion induced by a phase aberrator, such as a laser gain medium; hence, it is widely used in high-energy laser systems to obtain a high-quality beam. Efficient heat dissipation is a major issue in high-energy laser systems, particularly with regard to the high repetition rate. The combination of beams from small laser systems is a constructive approach to this issue. Of the various beam combined systems using SBS-PCMs, the crosstype beam combined system has many outstanding advantages, such as perfect isolation of leak beam, compensation of thermal birefringence, easy alignment and convenient maintenance (Kong et al., 1997, 2005a). Since the SBS wave generates from a thermal noise, it naturally has a random phase with respect to the incident beam. Therefore the phase controlling of the SBS wave is a key technology in the realization of a coherent beam combined system. For this reason, the self-phase control method was proposed and has been developed by Kong et al. (2005a, 2005b, 2005c), which can control the phase of the SBS wave with the simplest composition as well as ease of alignment, no limitations on the number of combined beams, and excellent phase conjugation. Furthermore, the active phase control with a piezoelectric translator (PZT) enables long-term phase stabilization (Kong et al., 2006, 2008). In addition to a random phase characteristic, the distortion that generally occurs in a pulse waveform of an SBS wave is another negative characteristic in terms of the beam combination. Kong et al. (2005d) has overcome this problem with the SBS waveform preservation technique, which is called the prepulse injection method. These works are expected to boost the development of laser systems in term of a high level of energy and