Multi-wavelength tunable and stable ring cavity thulium-doped fiber laser based on Sagnac loop filter

Abstract A tunable and stable multi-wavelength ring-cavity thulium-doped fiber laser (TDFL) based on a Sagnac loop using polarization-maintaining thulium-doped fiber (PM-TDF) and 3 × 3 fiber optical coupler (OC) was proposed and successfully realized. For the designed TDFL, the Sagnac loop was fabricated using a 3 × 3 OC, polarization controller (PC) and 2 m PM-TDF; in addition, the PM-TDF was used as a saturable absorber, which has the effect of restraining mode jumping. For the proposed fiber laser, stable and tunable single-, dual-, triple- and quadruple-wavelength lasers could be generated respectively in the experiment. For a single-wavelength laser output, the 1851.2–1866.3 nm tuning range was realized by adjusting the PC. Within this tuning range, the wavelength spacing was more than 1.3 nm, and the maximum power drift was less than 1.4 dB. In the experiment, tunable dual- and triple-wavelength laser emissions could be realized by adjusting polarization conditions. Four different wavelength groups of dual-wavelength laser output could be realized, the minimum and maximum tuning wavelength intervals were 2.6 and 11.5 nm respectively. For the proposed TDFL, three different wavelength groups of triple-wavelength within the 1862.7–1874.7 nm range could be obtained. Finally, quadruple-wavelength laser output was achieved in the experiment with a 30.72 dB signal-to-noise ratio. Over a 10 min observation period at room temperature, the power fluctuations for the single-, dual-, triple- and quadruple-wavelength emissions were less than 0.125, 1.607, 2.397, 2.484 dB, respectively.