Direct measurement of nonlinear frequency chirp of Raman radiation in single-mode optical fibers using a spectral window method

The frequency chirps of the first-Stokes Raman (1.12-μm) and fundamental (1.06-μm) picosecond (~100-psec) pulses propagated through various lengths of single-mode optical fiber have been measured. The fiber lengths were varied from a regime in which walk-off was significant to a regime where it could be neglected. Besides being of positive sign and substantially greater than that of the fundamental, the chirp of the Raman radiation was found to be highly nonlinear, in good qualitative agreement with theoretical predictions.