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Energy-limiting processes in powerful Nd:YAG lasers with phase conjugation via stimulated Brillouin scattering

It is shown that the main processes limiting the output energy of powerful YAG:Nd lasers consisted of a master oscillator and multipass amplifier with a phase conjugation mirror via stimulated Brillouin scattering (SBS) are, first, the amplifier self-excitation involving spontaneous Rayleigh scattering in SBS-liquid, and, second, the two-pass amplified spontaneous radiation propagating along active laser rods. Comparison of various optical schemes of the multipass amplifier shows that the maximal output energy of the two-pass amplifier is 3-4 times as high as that of the four-pass one owing to a higher threshold for those phenomena. Under optimal conditions the one-pass low-intensity gain is limited to a level 2 X 104 divided by 3 X 104, and the maximal energy density reaches approximately equals 3.75 J/cm2 and is almost independent of the reflectivity of the SBS-mirror and the input energy density down to approximately equals 0.1 mJ/cm2. It is shown that the self-excitation of the multipass amplifier can result in the spontaneous generation of powerful 15 divided by 30 ns pulses with inhomogeneous intensity cross-section distribution.