A microwave photonic filter (MPF) is a key processing device used to accurately extract the target signal, eliminate or suppress noise and stray signals, improve the purity of the signal spectrum, and meet the strict transmission performance requirements of modern communication systems. It has advantages such as large bandwidth, low transmission loss, fast processing speed, and strong resistance to electromagnetic interference. With the development of communication systems, various types of dual-band MPF research have emerged. The stimulated Brillouin scattering (SBS) effect is widely used to realize narrow-band MPF due to its ultra-narrow gain spectrum and large suppression ratio. The natural linewidth of the Brillouin scattering gain spectrum is mainly affected by the phonon lifetime in fiber materials, typically between 10?20 MHz. This characteristic limits the application of Brillouin scattering in high-resolution filtering. Therefore, the key to further reducing the filter passband bandwidth lies in achieving a narrower Brillouin gain spectrum. To achieve a passband bandwidth below 1 MHz, other performance aspects, such as the out-of-band rejection ratio, also need to be optimized. We construct a dual-wavelength laser by exciting a second-order Stokes light and combine the excellent narrowing Brillouin gain linewidth effect based on the Brillouin laser resonator with the dual-ring cavity dual-wavelength structure to obtain a dual-channel narrow-bandwidth Brillouin fiber laser microwave photonic filter.

A 10 m unpumped erbium-doped fiber (UP-EDF) is used to form a saturated absorption ring, while a 100 m single-mode fiber (SMF) ring forms a dual-ring cavity structure. The dual-channel narrow-bandwidth microwave photonic filter is created by combining the Vernier effect of the dual-ring cavity with the dual-wavelength Brillouin fiber laser. By using two different cavity lengths to match different resonant modes for mode selection, the filter passband of MPF is narrowed, thus achieving side mode suppression. The radio frequency (RF) signal output by the microwave source remains unchanged at 5 GHz, and the Brillouin gain amplifies the sweep modulation signal, forming two passbands at the center frequencies of 5.735 and 16.475 GHz.

We test the performance of the proposed dual-channel narrow bandwidth Brillouin fiber laser microwave photonic filter. According to the structure shown in Fig. 1, the frequency-shifted modulated signal light and the dual-wavelength single-longitudinal-mode Brillouin laser spectrum are measured, as shown in Fig. 4. The bandwidth and side-mode suppression ratio of the dual-band filter are tested. When the sub-ring cavity is not connected, only the Brillouin laser resonator is used for the filtering test. The passbands of the two comb filters are measured by vector network analyzer (VNA), which are 5.735 and 16.475 GHz, respectively. As shown in Fig. 5(a), the Brillouin gain spectrum is narrowed to a comb, and the comb bandwidth is significantly smaller than the Brillouin gain bandwidth. The out-of-band rejection ratios of the two passbands are 7.8 dB and 9.0 dB, respectively. When the sub-ring cavity is connected, the results are shown in Fig. 5(b). The side mode suppression ratios of 28.1 dB and 27.9 dB are obtained at 5.735 and 16.475 GHz, respectively. Then, the bandwidth of the proposed MPF is tested, and Lorentz fitting is performed on the amplified bandwidth. The 20 dB bandwidth extended at 5.735 GHz is 152 Hz, and the bandwidth extended at 16.475 GHz is 118 Hz, as shown in Fig. 5(b). A long-time filter stability test is conducted. At 5.735 GHz, continuous measurement is performed for 120 min at room temperature of (25±1) ℃, with an interval of 10 min. The experimental results are shown in Fig. 6. The drift of the center frequency is about 150 Hz, and the measured value of the narrow linewidth jitter is about 10 Hz, which may be caused by the sensitivity to ambient temperature and noise. Therefore, it is believed that as the basic performance of the filter, the performance of the filter is relatively stable.

In summary, we propose and experimentally verify a dual-channel narrow-bandwidth Brillouin fiber laser microwave photonic filter based on a second-order Brillouin laser. Two single-mode fibers with different cavity lengths of 100 and 10 m, un-pumped erbium-doped fibers, are used to match different resonant modes for mode selection, forming a Vernier effect that narrows the filtering bandwidth of the MPF and achieves high side mode suppression. This method solves the issue of the comb gain spectrum formed by the long cavity corresponding to the small FSR and ensures that there is only one filter passband in a single gain spectrum. The dual-passband is the signal light generated by the VNA sweep modulation. When passing through the dual-wavelength Brillouin laser, the optical power located in the Brillouin gain spectrum is amplified to achieve frequency selection. By combining this with the dual-ring cavity structure, the passband bandwidth is further narrowed, which results in higher out-of-band suppression, with values of 28.1 dB and 27.9 dB. The final filter bandwidths of the microwave photonic filter are 152 and 118 Hz, with high-frequency precision filtering. Theoretically, as long as the pump power is large enough, third-order or even higher-order Stokes light can be generated, corresponding to a triple-passband or even multi-passband filter. This research is of great significance for the development of microwave photonic filter devices with multi-band collaborative processing and high-frequency resolution. It is expected to break through the physical bottleneck of traditional microwave filters and provide high-performance, intelligent spectrum management solutions for the next generation of communication, sensing, and defense electronic systems.