We have demonstrated an efficient 1720-nm all-fiber laser with ring-cavity configuration based on commercial Tmdoped silica fiber and 1570-nm in-band pump source. The rate equation model was built up to analyze the laser performance of Tm-doped fiber, which exhibits strong absorption in 1.7-μm region. The results show that efficient laser operation can be achieved through the optimization of output coupling and the length of Tm-doped fiber. By using homemade couplers, we experimentally achieved 2.36-W laser output power under 6-W launched pump power. The slope efficiency with respect to the absorbed pump power and optical efficiency were 50.2% and 39.3%, respectively. Due to the employment of ring resonator, a narrow laser linewidth of ~4 GHz at maximum output power was observed.
We observed a large optical bistability in a single-frequency thulium fiber laser with ring cavity configuration. A piece of unpumped Tm-doped fiber served as nonlinear saturable absorber (SA), which also acted as a narrow-bandpass filter by forming self-induced gratings with counter-propagate lasers and enabled single-frequency laser operation at 1720 nm. Due to the large absorption cross section of thulium ions at 1720 nm, the unpumped Tm-doped fiber has large variable losses, hence resulting in strong optical bistability. With 0.75-m SA fiber, a 4.8-W wide bistable region was achieved. The evolution of bistable region with different lengths of SA fibers was investigated. The bistable region became narrower with decreasing SA fiber length, and totally disappeared at a SA fiber length of 0.15 m. To the best of our knowledge, this is the first observation of optical bistability in thulium fiber lasers.
Stimulated Brillouin scattering (SBS) is a well established method to narrow the laser linewidth to kilohertz level, which however suffers high threshold due to the low SBS gain at the region of 2 μm. The hybrid Brillouin/thulium fiber laser (BTFL) is such an approach which could suppress the laser linewidth with low threshold and high efficiency. In this paper, an ultra-narrow linewidth hybrid Brillouin/thulium fiber laser (BTFL) was demonstrated. Through experimentally optimizing the output coupling, pump scheme, Brillouin pump power and cavity length of the laser, 344-mW laser output with a narrow linewidth of 0.93 kHz was obtained, in which the linewidth of Stokes light was suppressed more than 43 times compared with the 40 kHz linewidth of the Brillouin pump. Besides, the influences of output coupling and pump scheme on the power and linewidth behavior of a single-frequency BTFL were also experimentally investigated, and there exists a performance balance among linewidth narrowing, output power and SBS threshold. The BTFL output power was further boosted to 5.5 W by a one-stage cladding-pumped fiber amplifier, and the corresponding spectral linewidth was broadened to 1.93 kHz. The output coupling exerted a significant influence on the BTFL performance.
A 1720-nm thulium-doped all-fiber laser based on a ring-cavity configuration is demonstrated. The long-wavelength lasing near the 1.9-μm thulium emission peak was suppressed using a wavelength division multiplexer and single-mode– multimode–single-mode (SMS) fiber device, which together served as a short-pass filter instead of the grating devices usually used in 1.7-μm thulium fiber lasers. A stable hundred-milliwatt-level 1720-nm laser output with a narrow spectral linewidth on the order of gigahertz was obtained after optimizing the output coupling, the active fiber length and the SMS device.