In recent years, the development of thulium-doped fiber lasers (TDFL) gradually followed the footsteps of ytterbium-doped and erbium-doped fiber lasers. The tunable range of TDFL is 1400-2200 nm, covering the atmospheric transmission window. In this window, the allowed power transmission of light in free space can be several orders of magnitude higher than that of the other wavelength bands. In particular, optical power transmission exceeds 80% in the 2050 nm wavelength band, making it possible to use TDFL in this band for free-space optical communications and atmospheric Doppler lidar. The TDFL operating wavelength is in the 2 μm band, which is a safe operating band for human eyes, and in which there is a high transmittance atmospheric window and strong absorption peaks of multiple gas molecules and OH^- ions. Therefore, lasers in this band are favored by various application industries, especially in the free-space optical communication field, where human eye safety is a requirement. Single longitudinal mode (SLM) fiber lasers have excellent characteristics, such as high beam quality, good coherence, and narrow linewidth, and are widely used as the preferred light source in multiple important fields. For example, fiber lasers with a narrow linewidth output have been used in ultra-long-range coherent optical communication, fiber optic sensing, optical metrology, high-resolution spectroscopy, and lidar, and have potential applications in optical atomic clocks, fundamental constant measurements, and physics. Therefore, the realization of a stable single longitudinal-mode narrow-linewidth laser source in the 2050 nm band is indispensable.

The proposed structure predominantly consists of a ring main cavity and a compound sub-cavity (Fig.1). The 793 nm pumped source output is input into the ring cavity through the fiber combiner, a 4 m long double-clad thulium-doped fiber is used as the gain medium, and the circulator ensures unidirectional transmission of light inside the ring cavity. A 0.5 m length of unpumped thulium-doped fiber is added to port 2 of the ring as the saturable absorber (SA), making it equivalent to a dynamic self-tracking narrow-band filter, which effectively suppresses the multi-longitudinal mode oscillation, realizing single longitudinal mode operation and compressing the narrow linewidth. A fiber Bragg grating (FBG) is used as a wavelength-selective device. The optical signal reflected by the FBG is injected into a composite double-loop cavity composed of three couplers and a composite double-loop cavity structure (Fig.3), which consists of one 3×3 coupler OC1 and two 2×2 couplers OC2 and OC3 which are connected in sequence. The 3×3 coupler OC1 has a 33∶33∶33 output and divides the input optical signal into three optical signals. The coupling ratio of both 2×2 couplers is 20∶80. The output laser is generated from the 10% port of the 90∶10 coupler.

The laser was developed and tested on an ultrastable optical stage at room temperature. A stable laser output was obtained when the pumping power reached 4 W. The central wavelength was 2048.76 nm, and the optical signal-to-noise ratio was 68 dB. The output spectrum was measured every 6 min for 60 min, and the spectrum obtained after ten consecutive scans [Fig. 8(a)]. To further quantify the stability of the laser, the power jitter and wavelength drift results over 60 min were analyzed with power fluctuation less than 0.15 dB and wavelength drift less than 0.02 nm [Fig.8(b)], indicating that the laser had good output stability at room temperature. The results of the single longitudinal mode of the output laser using the self-homodyne method show no obvious mode-hopping phenomenon over the three measurement ranges (Fig.9). To demonstrate the stability of the laser’s single longitudinal mode, ten sets of measurements were performed within 60 min, and no beat frequency signal generated by the longitudinal mode was captured [ Fig.9(a) inset]. When the SA was removed, a nonzero frequency rate peak was observed in the 0-500 MHz measurement range [Fig.9(d)]. The results show that the composite double-ring cavity effectively suppresses most longitudinal modes in the cavity; however, the remaining longitudinal modes must be further suppressed using saturable absorbers. To further characterize the linewidth characteristics of the TDFL, the frequency noise of the laser was measured using an unbalanced Michelson interferometer based on a 3×3 fiber coupler, and the linewidth of the laser was calculated using the β-separation line method. The laser linewidth was 9.17 kHz at 0.001 s and the relative intensity noise is below -129.69 dB/Hz at frequencies above 1 MHz.

A single longitudinal mode narrow linewidth TDFL based on a compound double-ring cavity with a saturable absorber operating in the 2050 nm wavelength band is reported, with its output stability and linewidth characteristics characterized in detail. The performance of the proposed filter was analyzed in detail, and it was confirmed that the structure suppresses dense multilongitudinal modes well and has the advantages of simple fabrication and high tolerance. In combination with the excellent single longitudinal mode selection capability of the unpumped thulium-doped fiber, the laser was guaranteed to be in a stable single longitudinal mode state. Experimental results demonstrate that the proposed laser has the advantages of a high optical signal-to-noise ratio (OSNR), high stability, and narrow linewidth, and can be more widely used in lidar and space optical communication systems by reducing fusion loss, good vibration isolation, and temperature compensation to achieve a superior laser output.