Stable four-wavelength switchable ultra-narrow linewidth thulium-holmium co-doped fiber laser by self-injection locking

In response to the demand for multi-wavelength, ultra-narrow linewidth laser sources in fields such as high-speed coherent optical communication and high-precision sensing, this paper proposes and experimentally demonstrates a stabilized four-wavelength switchable ultra-narrow linewidth thulium-holmium co-doped fiber laser based on self-injection locking (SIL). The laser employs superimposed polarization-maintaining fiber Bragg gratings for wavelength selection, incorporates a triple-coupler quadruple-ring compound cavity filter to ensure single-longitudinal-mode operation, and utilizes the nonlinear polarization rotation effect to control intracavity loss for wavelength switching. Experimental results show that the laser can stably switch among four wavelengths, namely 1940.20, 1940.60, 1941.86, and 1942.31 nm. Under single-wavelength operation, the optical signal-to-noise ratio exceeds 72.91 dB for all wavelengths. After applying SIL feedback via a 100 m delay fiber, the linewidth of each wavelength is significantly reduced from approximately 7 kHz to the sub-kHz level, with an optimal value reaching 0.21 kHz. Furthermore, the relative intensity noise is below -135.35 dB/Hz at frequencies above 500 kHz. During a 60 minute stability test, the central wavelength drift was less than 0.007 nm, and the peak power fluctuation was below 0.27 dB. With its excellent comprehensive performance, this laser provides an effective solution for realizing high-performance, multi-wavelength switchable, narrow-linewidth light sources in the 2 μm band.