Processes limiting the output energy of Nd3+:YAG lasers with a phase-conjugate mirror based on stimulated Brillouin scattering

It was established that the main processes limiting the output energy of powerful Nd³⁺:YAG laser systems, comprising a master oscillator and a multipass amplifier with a phase-conjugate mirror based on stimulated Brillouin scattering (SBS), were: first, self-excitation of the amplifier involving Rayleigh scattering in the SBS medium and, second, two-pass amplified spontaneous emission of radiation propagating along the active elements of the amplifier. A comparison of various multipass amplifier configurations showed that the maximum output energy of a two-pass system was 3 – 4 times higher than the maximum output energy of a four-pass system because of the higher threshold of the appearance of these power-limiting processes. Under optimal conditions the small-signal gain per one pass through the amplifier was ~(2 – 3) × 10⁴. The output energy density then reached ~3.75 J cm^–2 and was practically independent of the reflection coefficient of the phase-conjugate mirror and of the energy density of the input signal (beginning from ~0.1 mJ cm^–2).