Simulations of the propagation of multiple-FM smoothing by spectral dispersion on OMEGA EP

A one-dimensional (1-D) smoothing by spectral dispersion (SSD) system for smoothing focal-spot nonuniformities using multiple modulation frequencies has been commissioned on one long-pulse beamline of OMEGA EP, the first use of such a system in a high-energy laser. Frequency modulation (FM) to amplitude modulation (AM) conversion in the infrared (IR) output, frequency conversion, and final optics affected the accumulation of B -integral in that beamline. Modeling of this FM-to-AM conversion using the code Miro [Morice, O., “Miro: Complete modeling and software for pulse amplification and propagation in high-power laser systems,” Opt. Eng. 42(6), 1530−1541 (2003).] was used as input to set the beamline performance limits for picket (short) pulses with multi-FM SSD applied. This article first describes that modeling. The 1-D SSD analytical model of Chuang [Chuang, Y.-H., “Amplification of broad-bandwidth phase-modulated laser counterpropagating light waves in homogeneous plasma,” Ph.D. thesis, University of Rochester (September 1991).] is first extended to the case of multiple modulators and then used to benchmark Miro simulations. Comparison is also made to an alternative analytic model developed by Hocquet et al. [Hocquet, S., Penninckx, D., Bordenave, E., Gouedard, C. and Jaouen, Y., “FM-to-AM conversion in high-power lasers,” Appl. Opt. 47(18), 3338−3349 (2008).] With the confidence engendered by this benchmarking, Miro results for multi-FM SSD applied on OMEGA EP are then presented. The relevant output section(s) of the OMEGA EP Laser System are described. The additional B -integral in OMEGA EP IR components upstream of the frequency converters due to AM is modeled. The importance of locating the image of the SSD dispersion grating at the frequency converters is demonstrated. Finally, since frequency conversion is not performed in OMEGA EP’s target chamber, the additional AM due to propagation to the target chamber’s vacuum window is modeled.