Near and far field intensity behavior of stimulated rotational Raman scattering in hydrogen

Abstract

We report the measurement of unexpected near and far field intensity distributions for stimulated rotational Raman scattering (SRRS) in hydrogen. The Raman process used low pressure hydrogen pumped by the second harmonic of a phase conjugate Nd:YAG laser with a cesium dideuterium arsenate (CD*A) frequency doubling module. The pump beam consists of a uniform, truncated Gaussian beam at 532 nm with 250-mJ energy, 15-ns pulse width, and a beam divergence of 3 mm mrad. The overall Raman energy conversion efficiency was 88% with an approximate conversion of 28% in the first rotational Stokes line, 48% in the second Stokes, and ~12% of the energy in the third Stokes line.1 We observed an increasing near field depth of intensity modulation for successive rotational Stokes lines out to the third order. However, the far field intensity distributions of the unconverted pump and first three Stokes lines remained relatively uniform. The beam divergence was measured using the pinhole method and we observed an overall beam divergence increase of a factor of 2 for the combined output relative to the 532-nm pump input. No degradation in beam quality or divergence was observed in going from the first to the third Stokes line. Preliminary measurements indicate the unexpected result that the beam divergence may actually be decreasing for the higher orders relative to the first Stokes line.