Managing the resolution bandwidth in Brillouin based spectroscopy

The nonlinear optical effect of stimulated Brillouin scattering (SBS) is due to its rather narrow amplification bandwidth very suitable for the application of optical spectroscopy. Mediated by an acoustic wave in the Brillouin medium an optical pump wave is generating a frequency downshifted gain bandwidth of a few tens of MHz for counter propagating waves. By sweeping this gain through an unknown spectrum one can sample the spectrum with a resolution equal to the SBS gain bandwidth. Compared to grating based technology used in commercially available optical spectrum analyzers which offers a resolution of approximately 1 GHz, this is an increase of the resolution of two orders of magnitude. Since the SBS gain bandwidth is bound to the attenuation of the acoustical phonons in the SBS medium, there is no way to decrease it, unless using a different Brillouin medium. In our contribution we report and discuss a method to increase the resolution of SBS based optical spectroscopy by reducing the SBS gain bandwidth below its natural value regardless of the used fiber material.