Watt-level high-OSNR continuous wave tunable figure-8 holmium-doped fiber laser for OWC systems.

The 2000 nm optical transmission window is gaining significant interest for terrestrial and deep-space optical wireless communication (OWC) due to its exceptionally low atmospheric absorption. Therefore, high-power Holmium-doped fiber lasers (HDFLs) with high optical signal-to-noise ratio (OSNR) are attractive solutions for OWC systems because they can enhance transmission efficiency, improve signal quality over longer transmission distances, and ensure reliable communication in challenging atmospheric conditions by operating in the eye-safe 2000 nm wavelength range. In this work, we propose the design of a watt-level, high-OSNR continuous wave (CW) HDFL tunable in 2022-2140 nm wavelength range based on figure-8 (F8) cavity and a single in-band backward pump source. The performance of F8 HDFL is evaluated considering optimized pumping configuration, length of Holmium-doped fiber (HDF), and Ho3+ ions density. Maximum output powers of 0.842 W, 1.5 W, and 2 W are obtained at lasing wavelength of 2046.7 nm for 20%, 40%, and 60% of coupling ratios, respectively. Highest slope efficiency (SE) of 51.8% is achieved at 2046.7 nm for 60% of coupling ratio considering optimized parameters. OSNR and linewidth (LW) in the range of 30.4-93.3 dB and 18.5-19.9 MHz, respectively are obtained when laser is tuned in 2022-2140 nm wavelength range. A power fluctuation around 0.3 dB at lasing wavelengths of 2022 nm and 2100 nm for fifteen iterations, each taken at five-minute interval is observed for 20% coupling ratio. Finally, the effect of pair induced quenching (PIQ) at laser's output power is analyzed. A negligible penalty of around 7 mW is observed in output power of the laser at 2046.7 nm for 20% coupling ratio.