Tunable Microwave Photonic Filter with Ultra-Narrow Passband and High Out-of-Band Rejection

A novel scheme for a high-performance microwave photonic filter (MPF) based on vector composite stimulated Brillouin scattering (SBS) is proposed and experimentally demonstrated. The proposed MPF achieves an ultra-narrow passband, free from the inherent bandwidth limitations of natural SBS, and exhibits a high out-of-band rejection. By utilizing composite SBS technology (two Brillouin loss spectra are superimposed upon a central Brillouin gain), the MPF's bandwidth is effectively narrowed. However, the superimposed losses lead to a decrease in SBS gain, resulting in reduced out-of-band rejection of the MPF. To achieve narrow bandwidth and high out-of-band rejection simultaneously, vector SBS technology is introduced. In vector SBS, suppression of unamplified spectral components and SBS gain compensation are implemented through polarization state and power adjustments of the pump wave. Benefiting from the suppression of unamplified spectral components, the proposed MPF demonstrates higher out-of-band rejection at the same SBS gain. Consequently, the MPF's response exhibits a remarkably steep slope due to the narrowed bandwidth and increased out-of-band rejection. In the proof-of-concept experiment, the 3-dB and 20-dB bandwidths of the MPF are narrowed to 5.5±0.1 MHz and 14.4±0.2 MHz, respectively, with a high out-of-band rejection of 55 dB. The slope of the MPF response reaches 22.24 dB/oct. Additionally, a tunable central frequency ranging from 11 GHz to 20 GHz is demonstrated. The effectiveness of the MPF is further validated by suppressing two strong interferences, placed 100 MHz away from the signal, by 56 dB while only a minimal 0.3 dB loss is observed in the signal power due to SBS gain.