Abstract

According to conventional understanding of stimulated Brillouin scattering (SBS) the scattered (Stokes) emission evolves from spontaneous scattering and is therefore stochastic, [1], gain narrowing being the only expected modification of the seeded spontaneous spectrum. Recently however we noted that above SBS threshold a strong monochromatic component appears in the Stokes spectrum, [2], what means that the spectrum essentially reproduces the coherence of the pump radiation. Evidence of this can in fact also be found in earlier reports on SBS in both bulk and fibre media. It is therefore most surprising that such a singular feature, which is markedly at odds with the classical description of SBS, has for so long not been addressed. Here, we provide physical insight into the nature of this coherent behaviour. We show that it is a manifestation of a phenomenon we term spectral self-phase conjugation (SpSPC), first mentioned in the literature on SBS as long ago as 1983 [3]. It is, as we show, the spectral analogue to well studied spatial self-phase conjugation and like the latter is an inherent property of SBS and of all probability other stimulated scattering phenomena. As such SpSPC provides a unique means by which spectral distortions of an optical beam may be compensated to restore the beam's spectral coherence, in much the same way that spatial self-phase conjugation has been used to restore the spatial coherence of a spatially distorted beam.

© 2007 IEEE