Abstract

Spectral characteristics of stimulated Raman scattering (SRS) in optical fibers are examined in a regime (nanosecond pulses or longer) for which effects such as self-phase modulation, cross-phase modulation and group velocity dispersion are negligible. A XeF excimer laser was used to pump a multimode fused silica UV optical fiber to observe SRS multiple order Stokes generation. A theoretical model in which SRS is the only nonlinear effect included was developed to simulate the SRS spectra of excimer laser pumped silica fibers. Very good agreement has been achieved on characteristics of SRS spectra between experimental results and computer simulation. The most important parameters to the evolution of SRS spectra in fibers were found to be: the Raman gain curve, and the linewidth and the temporal profile of the input pump. The key concept to understand the formation of SRS spectra is a redshift effect in which energy transfers toward longer wavelength under the Raman gain. Gains from stimulated four-photon mixing (SFPM) were found to be not important in SRS Stokes generation. Only occasionally did we observe new frequency lines rather than the normal Raman lines, which are attributed to phase-matched wavemixing processes.

© 1990 Optical Society of America