Single-frequency lasers with wavelengths above 2 μm have attracted significant interest because of their numerous applications in areas such as biomedicine, Doppler LiDAR, and space optical communication. Although the power of the fiber master oscillator power amplifier (MOPA) around 2 μm has already reached the kilowatt level, the output powers of single-longitudinal-mode (SLM) laser oscillators in this wavelength region are still limited to the hundred-milliwatt level due to the laser gain and frequency-selection approaches. When a piece of unpumped rear-earth doped fiber is found inside the cavity of a laser, the standing wave field inside it results in a dynamic grating because of the Kerr effect. Moreover, the absorption loss of the doped fiber is small under the oscillating longitudinal mode but considerably larger under the side modes. These processes may provide a strong frequency-selection effect, thus allow lasers with longer cavity lengths and resulting higher laser gain to operate in SLM. In this study, we demonstrate an efficient SLM fiber laser at 2050 nm. A piece of Tm³⁺/Ho³⁺-codoped fiber is used as gain fiber to provide a laser gain at over 2 μm wavelength, while a piece of Tm³⁺-doped fiber is inserted into the laser cavity as the saturable absorber (SA) for frequency selection. A linear cavity scheme is adopted, rather than the ring cavity usually used in SLM cavity lasers, based on the fiber SA approach to enhance longitudinal mode spacing for a higher power SLM output. A maximum SLM 2050-nm laser output power of 714 mW is obtained under the incident 1570-nm pump power of 3.5 W.

A schematic of the SLM laser is given in Fig. 1. The gain fiber used is a piece of 9-μm/125-μm Tm³⁺/Ho³⁺-codoped fiber with a length of 4.6 m. The pump light launched from a 1570-nm fiber laser is coupled into the core of the active fiber via the filter wavelength division multiplexer 1 (FWDM1). Two fiber Bragg gratings (FBGs) are used to make the linear laser cavity. The reflectivity and 3-dB bandwidth of the partially-reflective FBG with its pigtail angle cleaved at 8° are 39.7% and 0.075 nm, respectively. The Tm³⁺-doped SA fiber used has a core absorption coefficient of 0.5 dB/m at 2050 nm. Another FWDM2 is used to couple the residual pump out of the oscillator.

It is known that longer SA fibers have a stronger frequency-selection capability; however, their oscillation mode losses are greater, resulting in reduced laser power and efficiency. In the experiment, we use SA fibers with lengths of 1.5 m and 2.5 m. The laser wavelength determined by the FBGs is 2048.6 nm. Without the SA fiber in the cavity, the laser output power is 813 mW under the maximum 1570-nm pump power of 3.5 W, with a slope efficiency of 26.9%. When the 1.5-m and 2.5-m long SA fibers are added to the cavity, the laser output power under the same pump power decreases to 773 mW and 714 mW, with slope efficiencies of 26.1% and 25.1%, respectively, as shown in Fig. 2. The laser threshold also increases from 0.53 W without the SA to 0.62 W and 0.72 W when the two pieces of SA are used. With the 1.5-m long SA fiber, the laser is in SLM with a pump power of 2.8 W and lower (571-mW output power), which is confirmed using a scanning Fabry-Perot interferometer and the spectrum. For higher pump powers, the laser becomes multi-longitudinal-mode because of the insufficient frequency-selection capability. When the 2.5-m long SA fiber is used, the laser maintains stable SLM operation from the 0.72-W threshold pump power to the 3.5-W maximum pump power, with a maximum output power of 714 mW [Fig. 3(b)]. The optical spectrum recorded by the optical spectrum analyzer shows an optical signal-to-noise ratio (OSNR) of ~60 dB between the 2048.6-nm laser and the 1.9?2.0 μm amplifier spontaneous emission (ASE) peak, whereas the in-band OSNR is beyond 75 dB (Fig. 4). The 3-dB spectral linewidth of the SLM output is measured to be 17 kHz using the delayed self-heterodyne interferometer technique at the maximum power of 714 mW (Fig. 6).

In this study, we demonstrate a linear cavity SLM Tm³⁺/Ho³⁺-codoped fiber based on a Tm³⁺-doped SA fiber for frequency selection. The laser delivers a 714-mW SLM output at 2048.6 nm under a maximum 1570-nm pump power of 3.5 W, with a slope efficiency of 25.1% and an optical efficiency of 20.4%. The influence of the SA fiber length on the frequency-selection capability and the supported SLM output power is experimentally investigated. The results show that a linear cavity with an SA fiber inside can achieve a high-power SLM laser output.