Spectral broadening of a KrF laser via propagation through Xe in the negative nonlinear index regime

Abstract

In inertial confinement (ICF) experiments at the NIKE [1] laser facility, the high-power krypton fluoride (KrF) laser output beams propagate through long (∼75m) air paths to achieve angular multiplexing, which is required because the KrF medium does not store energy for a sufficiently long time. Recent experiments and simulations have shown that, via stimulated rotational Raman scattering (SRRS), this propagation can spectrally broaden the laser beam well beyond the ∼1 THz laser linewidth normally achieved by the induced spatial incoherence (ISI) technique used in NIKE [2]. These enhanced bandwidths may be enough to suppress the laser-plasma instabilities which limit the maximum intensity that can be incident on the ICF target. In this paper we investigate an alternative technique that achieves spectral broadening by self-phase modulation in Xe gas, which has a large, negative nonlinear refractive index ∼ 248 nm [3], and thus completely avoids transverse filamentation issues. The collective, nonlinear atomic response to the chaotic, non-steady state ISI light is modeled using a two-photon vector model, and the effect of near-resonant behavior on the spectral broadening is studied.