Stable Four-Wavelength Switchable Narrow-Linewidth Thulium-Holmium Co-Doped Fiber Laser Based on Dual-Cascaded PM-FBGs

This paper proposes a stable four-wavelength switchable narrow-linewidth thulium-holmium co-doped fiber laser based on dual-cascaded polarization-maintaining fiber Bragg gratings (PM-FBGs). Operating in the 1.94 μm band with a linear cavity configuration, wavelength switching is achieved by adjusting two drop-in polarization controllers to introduce polarization-dependent loss. An unpumped section of thulium holmium co-doped fiber serves as the saturable absorber (SA) to ensure single-longitudinal-mode (SLM) operation. Experimental results demonstrate that the laser can be stably switched among four wavelengths: 1940.02 nm, 1940.49 nm, 1941.76 nm, and 1942.15 nm. Each wavelength exhibits an optical signal-to-noise ratio greater than 77.44 dB, wavelength fluctuation below 0.006 nm, and output power fluctuation less than 0.27 dB. Self-heterodyne beating measurements confirm stable SLM operation at all four wavelengths. Linewidth measurements show that the linewidth at each wavelength is below 2 kHz, with a minimum value of 1.25 kHz. Furthermore, the relative intensity noise is below -133.21 dB/Hz for frequencies above 500 kHz, and the slope efficiencies for the wavelengths range from 1.44% to 1.57%. This study indicates that the proposed linear-cavity structure, incorporating dual-cascaded PM-FBGs and an SA, provides an effective solution for realizing highly stable, multi-wavelength switchable, narrow-linewidth fiber lasers.