Switchable four-wavelength narrow-linewidth fiber laser based on a compound-cavity filtering and long self-injection locking polarization-maintaining delay fiber

The stability, linewidth, and noise of multi-wavelength laser sources are regarded as critical factors determining detection accuracy and sensitivity. A narrow-linewidth and switchable four-wavelength fiber laser is proposed and experimentally demonstrated. The laser is constructed with a triple-coupler dual-ring (TCDR) compound cavity filter and a 30 m self-injection locking polarization-maintaining delay fiber. Two cascaded polarization-maintaining fiber Bragg gratings (PM-FBGs) define four wavelength channels. Flexible switching among four single- and six dual-wavelength lasing states is achieved through polarization hole burning induced by intracavity polarization control. The TCDR filter is constructed from three 2 × 2 optical couplers. It effectively suppresses dense longitudinal modes and ensures stable single-longitudinal-mode operation. Optical signal-to-noise ratios (OSNRs) above 61 dB are obtained for all single-wavelength outputs, and values above 58 dB are achieved for dual-wavelength states. Maximum wavelength drift is restricted to 0.08 nm, and power fluctuation remains within 0.83 dB. Through self-injection locking with a 30 m polarization-maintaining delay fiber, the linewidth is compressed to below 585 Hz, accompanied by a 4.5 kHz reduction in relaxation oscillation frequency. Furthermore, each single-wavelength output exhibits a high degree of polarization above 97%, attributed to the polarization-selective nature of the PM-FBGs. This integrated design of multi-wavelength switching, ultranarrow linewidth, high OSNR, and polarization purity makes the laser highly suitable for advanced applications such as coherent LIDAR, high-resolution sensing, and precision metrology.