Tm/Ho co-doped single-frequency fiber laser with ultra-wide tunable range from 1960 nm to 2130 nm

We demonstrate a widely tunable all-fiber Tm/Ho co-doped single-frequency fiber laser (TH-SFFL) operating at the 2-μm band based on an all-fiber spectral filter (AFSF) and a fiber electrically optic-tunable filter (EOTF). The filtering characteristics of the AFSF, formed by a combination of four fiber couplers, are briefly simulated and analyzed. Leveraging the wide-range filtering of the AFSF, the wide emission bandwidth of the Tm/Ho co-doped fiber (THDF), and the broadband tunability of the EOTF, a laser output with a wavelength tuning range from 1960 nm to 2130 nm (170 nm), and a tuning step of less than 0.35 nm is achieved. The laser maintains single-longitudinal-mode (SLM) operation across the entire tuning range with optical signal-to-noise ratio over 60 dB, relative intensity noises below -133 dB/Hz@1∼10 MHz, and linewidths narrower than 11 kHz. To our best knowledge, this is the widest tuning range of 2 μm SFFLs, and is also the first demonstration of extending the tunable SFFLs' wavelength exceeding 2.1 μm. This laser source has effectively covered the atmospheric transmission windows existing from 2030 to 2050 nm and from ∼2080 nm to beyond 2100 nm, which makes it highly valuable for long-distance free-space optical communications.