We have demonstrated stable operation of a 2-kW Yb:YAG phase-conjugate master oscillator,
power amplifier (PC-MOPA) laser system with a loop phase-conjugate mirror (LPCM). This is
the first demonstration of a CW-input LPCM MOPA operating at a power greater than 1 kW
with a nearly diffraction-limited output beam. The single-pass beam quality incident on the
LPCM varied with the specific operating conditions, but it was typically ~ 20 times diffraction-limited
(XDL). The measured beam quality with a MOPA output power of 1.65 kW was
1.3 XDL.
Spatial-filter pinholes and knife-edge samples were irradiated in vacuum by 1053-nm, 5-20 ns pulses at intensities to 500 GW/cm2. The knife-edge samples were fabricated of plastic, carbon, aluminum, stainless steel, molybdenum, tantalum, gold, and an absorbing glass. Time- resolved two-beam interferometry with a 40-ns probe pulse was used to observe phase shifts in the expanding laser- induced plasma. For al of these materials, at any time during square-pulse irradiation, the phase shift fell exponentially with distance from the edge of the sample.. The expansion was characterized by the propagation velocity V2(pi ) of the contour for a 2(pi) phase shift. To within experimental error, V2(pi ) was constant during irradiation at a particular intensity, and it increased linearly with intensity for intensities < 300 GW/cm2. For metal samples V2(pi ) exhibited an approximate M-0.5 dependence where M is the atomic mass. Plasmas of plastic, carbon, and absorbing glass produced larger phase shifts, and expanded more rapidly, than plasmas of heavy metals. The probe beam and interferometer were also used to observe the closing of pinholes. With planar pinholes, accumulation of on-axis plasma was observed along with the advance of plasma away from the edge of the hole. On-axis closure was not observed in square, 4-leaf pinholes.
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