Self-injection locking assisted four-wavelength switchable sub-kHz ultra-narrow linewidth thulium-doped fiber laser

In this paper, a high-performance, wavelength-switchable thulium-doped fiber laser based on self-injection locking technology is proposed and experimentally demonstrated. A Fabry-Pérot fiber Bragg grating is used as the wavelength selection element, and a double-ring compound cavity is adopted as the mode-selective filter, enabling single longitudinal mode laser output with four wavelength switchable options. The self-injection locking mechanism is introduced and different lengths of delay fibers are used to extend the photon lifetime in the cavity, which results in effectively compressing the laser linewidth. Experiments are then conducted, demonstrating that the output optical signal-to-noise ratio for the four wavelengths is greater than 75.916 dB, with central wavelength and peak power fluctuations, respectively, less than 0.02 nm and 1.217 dB, over a 60 min duration. Under the condition of a 100 m delay fiber, the relaxation oscillation peak exhibits a significant leftward shift, and the intrinsic linewidth of the laser is compressed to less than 0.68 kHz. Moreover, the relative intensity noise remains less than -135.0 dB/Hz at frequencies greater than 200 kHz. This wavelength-switchable, ultra-narrow-linewidth fiber laser demonstrates high potential for many applications in fiber sensing, fiber communications, and spectral analysis.