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We study the impact of structural variations (that is slowly varying geometryaberrations and internal strain fields) on the width and shape of thestimulated Brillouin scattering (SBS) resonance in nanoscale waveguides. Wefind that they lead to an homogeneous resonance broadening through two distinctmechanisms: firstly, the acoustic frequency is directly influenced viamechanical nonlinearities; secondly, the optical wave numbers are influencedvia the opto-mechanical nonlinearity leading to an additional acousticfrequency shift via the phase-matching condition. We find that this secondmechanism is proportional to the opto-mechanical coupling and, hence, relatedto the SBS-gain itself. It is absent in intra-mode forward SBS, while it playsa significant role in backward scattering. In backward SBS increasing theopto-acoustic overlap beyond a threshold defined by the fabrication toleranceswill therefore no longer yield the expected quadratic increase in overallStokes amplification. Finally, we illustrate in a numerical example that inbackward SBS and inter-mode forward SBS the existence of two broadeningmechanisms with opposite sign also opens the possibility to compensate theeffect of geometry-induced broadening. Our results can be transferred to othermicro- and nano-structured waveguide geometries such as photonic crystalfibres.

Brillouin resonance broadening due to structural variations in nanoscale waveguides