Stable narrow linewidth single-longitudinal-mode Tm3+-doped fiber laser with compound cavity filter and self-injection feedback

Abstract

A single-longitudinal-mode (SLM) Tm³⁺-doped fiber laser based on self-injection feedback with narrow linewidth and high stability is proposed and demonstrated. The effective free spectral range is effectively expanded using a traditional ring main cavity structure combined with a triple-coupler double-ring compound-cavity (TCDR-CC) filter. The TCDR-CC filter cooperates with a uniform fiber Bragg grating (UFBG) to improve mode-selection and enable SLM operation. Delay fibers with different lengths are introduced outside of the main cavity to form self-injection feedback, and the linewidth compression effect of the self-injection feedback is explored by varying the power of the feedback and the length of the delay fiber. A SLM fiber laser operating at 2048.42 nm is obtained with an optical-signal-to-noise ratio (OSNR) of 77.30 dB. In addition, this laser has high stability because the output power and wavelength fluctuate only slightly — by less than 0.2 mW and 0.02 nm, respectively. It has been confirmed that after the addition of different lengths of delay fiber, the linewidth can be effectively compressed. In particular, the linewidth for a 100-m delay fiber and an integration time of 0.001 s is the least, 1.3 kHz.