Microwave photonic filter with a sub-kHz bandwidth based on a double ring Brillouin fiber laser

A scheme to achieve a sub-kHz bandwidth microwave photonic filter (MPF) based on a double-ring Brillouin fiber laser (DR-BFL) is proposed and experimentally verified in this study. The stimulated Brillouin scattering gain spectrum can be efficiently narrowed by the DR-BFL, which comprises a main-ring cavity with a 100-m single-mode fiber (SMF) and a sub-ring cavity with a 10-m SMF. A single passband narrow bandwidth MPF is accomplished by the DR-BFL with a single longitudinal mode laser output using the Vernier effect. The bandwidth of the MPF at the center frequency of 10.735 GHz is 114 Hz, and the corresponding Q-factor is 9.42 × 10 7 , which is the highest Q value to the best of our knowledge. Since the measured linewidth of the DR-BFL is 1.2 Hz at the −20-dB power point, the theoretical bandwidth of the DR-BFL-based MPF is 0.16 Hz. This design provides a new idea for realizing high-precision signal extraction in future communication or sensing systems.