Polarization-Independent Rectangular Microwave Photonic Filter Based on Stimulated Brillouin Scattering

We have demonstrated a rectangular microwave photonic filter (MPF) based on stimulated Brillouin scattering (SBS) effect in optical fiber and offering tunability on bandwidth, central frequency, and selectivity. A sweeping-pump multistage configuration with feedback control is implemented to achieve the rectangular MPF with high selectivity. The obtained 20-dB shape factor is as low as 1.056, which is, to our knowledge, the best reported result for MPF in gigahertz bandwidth. Furthermore, we solve the polarization-dependent SBS gain issue and realize a polarization-independent MPF. The SBS noise is reduced by adopting a multistage configuration to limit the gain at each stage. Finally, the filter selectivity for a four-stage configuration is as high as 57 dB for a 2.1-GHz bandwidth. In this case, the signal-to-noise ratio penalty is only 2.6 dB for a 4-Gbit/s orthogonal frequency division multiplexing signal in the quadrature-phase-shifted keying format.