Research On High-Q Microwave Photonic Filtering Technology Based On Narrow Linewidth Brillouin Stokes Light

Compared to electronic filters,microwave photonic filters(MPFs)can directly process radio frequency signals in the optical domain,with advantages including high operating frequency,strong electromagnetic interference resistance,low loss,large instantaneous bandwidth,tunability and reconfigurability.As an important functional unit in communication systems,MPFs with high Q can provide high-frequency selectivity and can be applied in civilian and military fields such as generating high spectral purity microwave signals,high-sensitivity microwave photon sensors,and high-resolution microwave photon radar.Brillouin Fiber Lasers(BFLs)can achieve narrow linewidth and single-longitudinal-mode(SLM)laser output by narrowing the Brillouin gain spectrum,and have wide-ranging application prospects.BFLs based on a composite cavity can suppress weak excess modes without requiring precise power control or introducing additional losses,thereby achieving SLM narrow linewidth laser output.By combining the narrow linewidth BFL with microwave photonics technology,the high-Q single-passband MPF can be achieved.Therefore,studying MPF based on BFL is of considerable importance.In addition,compared to BFLs based on composite cavity,BFLs based on parity-time(PT)symmetry with long-cavity can further narrow the laser linewidth,and this design have lower costs.This paper mainly analyzes the concepts and principles of high-Q MPFs based on narrow linewidth BFLs.A SLM and narrow linewidth BFL based on a double-ring cavity(DR-BFL)is designed,and a high-Q MPF based on the DR-BFL is proposed.In addition,to further narrow the laser linewidth,a long-cavity BFL based on PT symmetry is proposed.The main work of this paper includes:1.This paper introduces the basic concepts and research progress of MPFs.The theoretical basis of MPF based on BFL is mainly discussed,along with the experimental testing of the Brillouin frequency shift and threshold of the single-mode fiber.2.A SLM and narrow linewidth DR-BFL is designed and experimentally verified.The Stokes light generated by SBS makes numerous round trips.Periodic resonance occurs in the cavity,and the FWHM of each resonance is quite small.The SLM functioning of the DR-BFL is performed via the Vernier effect.The experimental results show that the 3-d B linewidth of the DR-BFL is 13.05Hz,and the optical signal-to-noise ratio is 67d B.3.A high-Q MPF based on a DR-BFL is proposed and experimentally demonstrated.The stimulated Brillouin scattering gain spectrum can be efficiently narrowed by the DR-BFL.A single passband narrow bandwidth MPF is accomplished by the DR-BFL with a single longitudinal mode laser output using the Vernier effect.The test signal can then be filtered out by the BFL.Experimental results show that the 3-d B bandwidth of the MPF based on the DR-BFL is 114Hz,with a maximum Q-factor of 9.42×10~7 and a maximum side-mode suppression ratio of 24.5d B.By further narrowing the linewidth of the optical carrier,the optimal filter bandwidth of this design was measured using a vector network analyzer to be approximately1.2Hz.4.A SLM PT-symmetric BFL based on a lithium niobate phase modulator(LN-PM)Sagnac loop is proposed and experimentally demonstrated.The Sagnac loop,composed of a LN-PM and two polarization controllers,is used to realize PT symmetry and achieve SLM and narrow linewidth BFL output.Compared with the DR-BFL,it has a longer cavity length,which can obtain a narrower laser linewidth.In the experiment,the 3-d B linewidth of the PT-symmetric BFL based on the LN-PM Sagnac loop was measured to be 3.85Hz.The optical signal-to-noise ratio was measured to be 65d B,and the maximum side-mode suppression ratio was 43d B.The wavelength tuning range is 1550-1550.41nm.