Stimulated Brillouin Scattering (SBS) is a well-known phenomenon, where a strong pump wave and a typically weak, counter-propagating signal wave optically interfere to generate, through electrostriction, a traveling longitudinal acoustic wave. The acoustic wave, in turn, couples these optical waves to each other, passing energy from the higher frequency (‘Stokes wave’) to the lower frequency. The SBS interaction is efficient only when the difference between the optical frequencies of the pump and signal waves is very close (within a few tens of MHz) to a fiber-dependent parameter, the Brillouin shift W_B, which is on the order of 2p×11·10⁹ [rad/sec] in silica fibers at room temperature and at telecommunication wavelengths. The shift frequency is determined by the wavelength and fiber characteristics.

Since the SBS process relies on interference of the pump and signal, the relative State of Polarization (SOP) of the electric fields affects the interference efficiency, and hence the amplification process. In previous works, the analytic vector relations between pump and signal were formulated and investigated in the undepleted pump regime, in low birefringence fibers. It has been found both theoretically and experimentally that the output signal’s SOP is “pulled” towards a specific polarization, which is determined by the pump’s signal SOP and power.

In this work a sharp and highly-selective tunable optical band-pass filters, based on SBS amplification in standard fibers, are described and demonstrated. Polarization pulling of the SBS-amplified signal wave is used to increase the selectivity of the filters to 30 dB. Pump broadening via synthesized direct modulation was used to provide a tunable, sharp and uniform amplification window. Pass-band filters of 700 MHz at half maximum of and 1GHz at -20dB, were obtained: Pass-band widths of 700 MHz at half maximum and 1GHz at the -20dB points were obtained. The central frequency, bandwidth and shape of the filter can be arbitrarily set. Compared with scalar SBS-based filters, the polarization-enhanced design provides a higher selectivity and an elevated depletion threshold.