High-power lasers within the 2-μm spectral range have been utilized in various applications in the fields of laser radar and satellite remote sensing, essentially serving as pump sources for optical parametric oscillators. Tm/Ho-doped fiber lasers are currently the primary methods for achieving high-power continuous-wave lasers or high-average-power pulsed lasers with high repetition rates owing to their homogeneous heat load distribution. However, photon dark effects, nonlinear effects, and transverse mode instability in fibers present as obstacles for further power/energy scaling, particularly for pico- or femtosecond-pulsed lasers. To tackle these challenges in the domain of high-power lasers, the introduction of gain media with novel structures is imperative and constitutes a popular area of research. Single-crystal fibers (SCFs) are long, thin crystalline rods, typically having a diameter of less than 1 mm and length of a few centimeters. Owing to their high thermal conductivities and low Brillouin gain coefficients, SCFs combine the advantages of both bulk crystals and glass fibers. Thus, they are promising candidates for high-power laser systems.

SCFs have been showcased as promising candidates for high-average or peak-power laser oscillators and amplifiers within the 1 μm spectral region. Nevertheless, research on the laser performance of SCFs in the mid-infrared 2 μm spectral range, particularly when doped with Tm³⁺ or Ho³⁺ ions, is limited. Recently, the first Tm∶YAG SCF laser, utilizing a 783-nm pump, was reported. Owing to high quantum defects, the use of near-infrared pumping results in a significant thermal load, thereby limiting its potential for power scalability. However, by directly pumping Tm³⁺ ions into the upper laser level (³H₆→³F₄), the issue of thermal loading can be effectively addressed, thus paving the way for further power scaling and enhanced laser efficiency.

The experimental setup is shown in Fig. 1. The pump source is a fiber-coupled laser diode (LD) at 1719 nm, with a maximum output power of 25 W, numerical aperture of 0.22, beam quality factor (M²) of approximately 100, and core diameter of 400 μm. The fiber output is focused into the SCF with a variable magnification ratio depending on the telescope system, which consists of a collimating lens (L1) and focusing lens (L2). The focal lengths L1 and L2 are 25.4 mm /30.0 mm and 30.0 mm/25.4 mm for the two pump-coupling schemes. Therefore, the pump beam waist radius is either 236 μm or 170 μm within the SCF. The single crystal fiber used in this experiment is a Tm∶YAG SCF with a diameter of 1 mm and a length of 40 mm (the atomic fraction of doped Tm is 3.5%). For better thermal effect management, undoped YAG caps with bonding lengths of 5 mm at each end of the crystal are used. In order to avoid parasitic laser oscillation, both end caps are coated with 2 μm band anti-reflection films. The Tm∶YAG SCF is mounted on a custom-made aluminum module (Fig. 1), and both ends of the SCF are sealed with glue, leaving a small protrusion of approximately 1 mm outside the module. This arrangement allows the entire Tm-doped section of the SCF to be directly water-cooled to approximately 8 °C. A planoconcave mirror with a radius of curvature of -200 mm serves as the input mirror (IM). Plane-wedged mirrors with transmittance values of 3%, 5%, 10%, and 15% are used as the output coupler (OC). A dichroic mirror (DM) functions as the beam splitter for the pump and laser beams. The propagation mode and intensity distribution of the pump light within the SCF are simulated and analyzed using the ray tracing method [Figs. 2(a) and (b)].

Initially, the laser performance under different pump-guiding conditions is investigated. As shown in Fig. 2(c), with a 170-μm pump beam waist, the peak slope efficiency achieved is 46.3%. Next, the Tm∶YAG SCF laser performance with different OCs and a pump beam waist of 170 μm is examined in detail. An optimal OC with transmittance (T_OC) of 5% yields a maximum output power of 7.85 W, correlating to slope efficiencies of 46.3% and 52.9% with respect to the incident pump power and absorbed pump power, respectively [Fig. 3(a)]. Figure 3(b) presents the optically measured spectra for different OCs, with the laser wavelength at T_OC=5% situated at 2017.7 nm. A red-shift in the wavelength from 2012.8 nm to 2017.7 nm is observed with decreasing OC transmission. Figure 4(a) shows the M² for various output powers. Within the measured range, the M² value gradually increases from 1.2 to 1.9; a typical beam quality measurement in the latter case is exhibited in Fig. 4(b). The degradation in beam quality can be attributed to the excitation of higher-order transverse modes because no component for mode limitation is used in the cavity. Figure 5 represents the calculated thermal lens focal lengths along the x-axis for different incident pump powers. The focal length at the peak pump power of 25 W, corresponding to 7.85 W output laser power, is estimated to be62 mm.

In conclusion, a 1.7-μm LD is used as the pump source for resonant pumping, and the continuous laser operation of Tm∶YAG SCF is realized by integrating mode matching and pump guiding. This approach generates an output power of 7.85 W at approximately 2.02 μm, which corresponds to a slope efficiency of 46.3%. In addition to pump guidance, we find that mode matching also plays a crucial role in laser performance of such a 1-mm diameter SCF. Nevertheless, a theoretical analysis of the thermal lens suggests that a higher pump intensity in the front segment of the SCF may trigger instability in the laser cavity. Certain specific designs (e.g., undoped end caps) can further bolster the pump guidance and alleviate the thermal stress of the SCF.