The Observation Of Chirped Stimulated Raman Scattered Light In Fibers

Abstract

Stimulated Raman scattering (SRS) limits the maximum achievable power in the fiber-grating compression of optical pulses [1]. We have recently experimentally investigated SRS in single-mode fibers in the regime of large Raman Stokes pulse walk off from the pump pulse by the effect of group-velocity dispersion (GVD) [2]. We found that the Raman Stokes pulse was produced within the first 3-4 walkoff lengths, where a walkoff length is defined as the distance in which the Stokes signal passes through one pump pulse width via the action of GVD. For a 21 m length of fiber (3.4 walkoff lengths) we were surprised to find that the Raman pulse was generated with a strong frequency chirp. This is not the first observation of chirped SRS in a fiber [3], but it is the first observation of chirped SRS in an experimental configuration simple enough that there is hope for a relatively straightforward understanding of the physical mechanisms involved. The distinctive features of the present results are that over this short length of fiber, subsequent modifications of the Raman pulse by self­phase modulation (SPM) or GVD are minimal and the pump pulse is only minimally chirped. The wings of a spectrally broadened pump pulse are thus not the source of the chirping of the Raman pulse.