2-μm-band single-frequency Tm/Ho co-doped fiber laser with several-kHz linewidth in ∼100 nm wavelength-tunable range

A widely wavelength-tunable single-frequency (SF) narrow-linewidth thulium/holmium co-doped fiber laser (THDFL) was developed in this study. A motorized etalon-based fiber optic tunable filter was used to determine the lasing wavelength in a spectral range as large as 99 nm (1950–2049 nm). A 1.08 m long unpumped thulium/holmium co-doped fiber (THDF) and a broadband fiber reflector were combined to form the effective saturable absorber based dynamic narrow-band filter, which achieves single-longitudinal-mode lasing operation of the THDFL over the whole wavelength-tunable range. At any lasing wavelength in the ∼100 nm tunable-range, the THDFL can operate with a highly stable optical spectrum, optical signal to noise ratio of >63 dB, linewidth of <4 kHz, relative intensity noise of ≤−143 dB/[email protected]≥5 MHz, and output power fluctuation of <0.3 dB. The feasibility of Brillouin-laser-based linewidth measurement for broadband wavelength-tuning in the 2-μm-band was validated for the first time. The THDFL exhibits state-of-the-art comprehensive performance compared to the similar 2-μm-band SF fiber lasers reported previously, to the best of our knowledge. The proposed device has significant application potential in the wavelength-division-multiplexing free-space optical communication, high-precision medical surgery, and atmospheric trace-gas detection fields.