Observation of anomalous low J-value Stokes and anti-Stokes lines in stimulated rotational Raman scattering over long air paths

The authors report their recent observations of low J-value Stokes and anti-Stokes lines from stimulated rotational Raman scattering (SRRS) in long air paths with a large Fresnel number beam at a pump wavelength of 1053 nm. The spectral and temporal dynamics of SRRS is important to our understanding of SRRS as applied to the propagation of large-aperture beams in laser systems for fusion research and in defense applications. An example of this is the proposed implementation of broad bandwidth and spatial incoherence for target irradiation experiments in the field of laser-plasma interactions. Propagation and amplification of laser bandwidths exceeding +-20 cm can seed and possibly amplify the low J-value rotational transition in nitrogen. This can lead to significantly reduced beam quality and divergence comparable to the solid angle of the beam if SRRS growth cannot be controlled. To understand the spectral and temporal dynamics of SRRS, the authors have conducted further experiments using one arm of the Nd:glass Nova laser system at Lawrence Livermore National Laboratory. A spatially uniform linearly polarized section of the 74-cm beam was apodized to 23 cm and down-collimated to 11.5 cm. The beam was propagated over a 75-m air path at atmospheric pressure to a diagnostic station. Spectral measurements were made at the end of the propagation path in both the near field and far field at various intensities. In addition, time-resolved spectra were recorded using a streaked spectrometer to characterize the temporal evolution of the Raman transitions