Performance Of Smoothing By Spectral Dispersion Combined With Distributed Phase Plate On Sg-Ii

Experimental performance of one-dimensional (1D) smoothing by spectral dispersion (SSD) combined with distributed phase plate (DPP) on the ninth beam of SG-II is presented. Without the application of SSD, normalized focal-spot non-uniformity of an 85% energy concentration is about 60%. Then, spectral bandwidth of the 3-ns, 1053-nm laser pulse is broadened to 0.3 nm (as 270GHz in 3 omega) by a 3-GHz modulator and a 10-GHz modulator integrated in the front-end system. Spectral dispersion of 236 mu rad/angstrom is achieved by a Littrow-configuration, 1480-l/mm grating placed between the Phi 40mm faraday isolator and the third Phi 40mm rod-amplifier. By using such SSD, normalized non-uniformity with the same energy concentration is decreased to 16%. A scheme of spatial power spectral density (PSD) in different directions is adopted to analyze the intensity distribution of the far-field irradiation. Based on the spatial PSD analysis, theoretical predictions of spectral peak caused by SSD's color cycles is in excellent agreement with the experimental result. With double-frequency modulation, the amplitude of the spectral peak is reduced by similar to 10dB. The temporal waveform of the 1 omega laser is measured. Waveform distortion criterion defining the frequency modulation to amplitude modulation conversion (FM-to-AM) is about 6% with 1 omega laser energy of similar to 1.8kJ.