SPECTRALLY DISPERSED AMPLIFIED SPONTANEOUS EMISSION FOR IMPROVING IRRADIATION UNIFORMITY INTO HIGH-POWER ND-GLASS LASER SYSTEM

An amplified spontaneous emission (ASE) from Nd:glass has been introduced into the high power twelve beam Nd:glass laser system, Gekko XII for obtaining smooth intensity distribution of a focused beam. The angular dispersion of an ASE spectrum with large beam divergence was adopted for efficient beam smoothing without significant reduction of the harmonic conversion efficiency. Temporal evolution of the beam smoothing was evaluated as a function of the beam divergence by using a statistical model of speckle. In Gekko XII, the spectral width and beam divergence of ASE were controlled in a range of 0.4 to 0.6 nm and 6 to 22 times diffraction limited, respectively. Final output energy of 1.3 kJ/beam in a 2.2 ns duration was demonstrated without significant gain reduction and spectral narrowing. The doubling efficiency of 50% was obtained at a low intensity region of around 0.3 GW/cm2 by matching the angular dispersion of spectrum to that of phase matching condition of a frequency conversion crystal. The speckle structures in the focused beam pattern measured at the equivalent target plane were greatly smoothed out due to the introduced angular dispersion of spectrum and the large beam divergence. The standard deviation of the intensity distribution of the focused beam pattern was estimated to be 3.3%.