Flexible tunable microwave photonic filter with a dual ultra-narrow passband based on a dual-wavelength Brillouin laser

This article presents a flexibly tunable microwave photonic filter (MPF) with a dual ultra-narrow passband based on a dual-wavelength and narrow linewidth Brillouin laser. The dual passband of the filter not only exhibits ultra-high frequency selectivity but also allows for flexible and simultaneous tuning of the center frequency and interval of the passbands. In the proposed scheme, the core optical processor of the MPF consists of a dual-ring Brillouin laser resonator, which is composed of a 100-meter main fiber ring cascaded with a 10-meter secondary fiber ring. A dual-tone pump is generated through carrier-suppressed dual sideband modulation and injected into the dual-ring resonator to excite two Brillouin gains, which resonate separately in the dual-ring resonant cavity to generate a dual-wavelength single longitudinal mode Brillouin laser for processing optical RF signals. By selectively amplifying the modulated sideband signal through narrow linewidth Brillouin laser gain, the ultra-narrow dual passband are ultimately demodulated. The resulting dual filtering passband exhibits unparalleled sub-kHz bandwidth and flexible tuning capability. Experimental results demonstrate that the MPF proposed in this paper achieves an ultra-narrow dual passband of 83 Hz with a maximum side-mode suppression ratio of over 28 dB. Within the testing range of vector network analysis, the maximum tuning range of the center frequency is 20 GHz, and the passband interval can be freely tuned from 2-18 GHz. These results highlight the MPF's ability to concurrently filter dual-channel signals with high precision and its significant potential application value in ultrafine microwave frequency spectrum resolution and processing fields, such as instantaneous frequency measurement and microwave photonic sensing.