1. Introduction

High-power solid-state lasers in the 2 µm spectral range have a variety of applications, including laser radar, remote sensing, pumping of optical parametric oscillators (OPOs), laser welding and laser surgery, etc. Optical glass fibers are now the main approaches for achieving high power CW lasers or high-average-power pulsed lasers at high repetition rate due to their homogeneous heat load repartition [1,2]. However, the presence of various nonlinear effects, photon darkening and low damage threshold of the glass fiber limit further power/energy boosting, and in particular in the pico- and femtosecond regime. Chirped pulse amplification (CPA) technique can partially overcome these problems, but normally associated with complex design and additional losses [3,4]. To meet the challenges in the field of high power/energy lasers, gain media with novel structures are essential and always a hot research topic. Single-crystal fibers (SCFs) are long and thin crystalline rods with a diameter of less than 1 mm and a typical length of a few centimeters [5], thus possessing the spectroscopic and thermomechanical advantages of bulk crystals, as well as the large surface-to-volume ratio of optical fibers for efficient heat dissipation [6]. With free-space propagation of laser beams and wave-guiding of pump light, the SCFs can offer a longer gain region compared to conventional bulk materials and meanwhile conserving the high beam quality of the laser [7].

In comparison with the conventional silica fibers, YAG-SCF exhibits higher thermal conductivity (10.7 W/m·K^-1 VS 1.38 W/m·K^-1 for silica) and 10-times lower Brillioun gain coefficient (5 × 10⁻¹² m/W VS 50 × 10⁻¹² m/W), thus theoretically supporting a higher critical power [8–9]. Up to date, SCFs have been demonstrated to be promising candidates for high average/peak power laser oscillators or amplifiers in the 1 µm near-infrared spectral region [10–13]. For example, a maximum output power of 250 W has been demonstrated by employing an Yb:YAG SCF with 1 mm diameter and 40 mm in length [11]. In terms of laser amplification, 290 W average power, 829 fs pulses were produced in a simple and compact single-stage Yb:YAG-SCF amplifier without CPA process [12]. Moreover, 3.7 GW peak power was demonstrated by coherently combining beams produced from two individual Yb:YAG-SCF amplifiers [13]. All these results show the huge potential of SCFs for simultaneous handling of high-average and peak powers.

However, laser performances of the SCFs in the 2 µm mid-infrared spectral range by doping with Tm³⁺ or Ho³⁺ ions have been rarely studied. In comparison, Tm-doped SCFs can be pumped by commercially available AlGaAs diodes at ∼ 0.79 µm and exhibit a beneficial two-for-one cross-relaxation excitation process [14]. Most recently, we reported on the first Tm:YAG-SCF laser with waveguiding of the pump light at 783 nm, however, thermal-lens analysis showed the instability of the resonator cavity at a high incident pump power (100 W level) due to the high-pump absorption in the initial part of the SCF, thus limiting the power scalability [7]. Simple reduction of the Tm³⁺ concentration can mitigate the thermal load but result in a weak cross-relaxation effect [14]. Alternatively, pumping in the wings of the absorption line seems to be an effective way to alleviate such thermal issue by reducing the front-end absorption.

In this paper, we demonstrate a simple and compact high-power Tm:YAG-SCF laser wing-pumped by two laser diodes at 791 nm. The output power can be straightforwardly increased up to 63.3 W without thermal power roll-off, further power scaling was limited just by the available pump power. Compared to our previous report [7], the present work shows the power scalability of Tm-doped SCF laser in the 2 µm spectral range by using the wing-pumping method with commercially available laser diodes.

2. Experimental setup

The YAG-SCF doped with 3.5 at.% Tm³⁺ had a diameter of 1 mm and 40 mm in length. To facilitate thermal management, two 5-mm-thick undoped YAG end-caps were diffusion bonded to each end of the central doped section, thus giving a total length of 50 mm with polishing on the front end facet but anti-reflective coating on the rear end facet. Here, the Tm:YAG SCF with only polished on the entrance face was mainly to avoid the coating damage, however coated on the rear face to suppress the possible parasitic oscillation. In order to reduce the thermal load in the initial part of the SCF, wing-pumping scheme with two fiber coupled 791 nm AlGaAs laser diodes (200 µm fiber core, 0.22 NA, DILAS) were employed. The optical spectrum of the combined pump laser was recorded by using an optical spectrum analyzer (OSA, AQ6375, YOKOGAWA) and shown in Fig. 1(a). In comparison, the absorption spectrum of the Tm:YAG from 740 to 840 nm [15] is shown as the background, where the absorption peak of the ³H₆-³H₄ transition is located at 786 nm. Figure 1(b) shows the measured single-pass absorption of the Tm:YAG SCF at different incident powers, the absorption gradually decreased from 94.3% to ∼ 85.8% with increase of the pump power to 210 W. The lower absorption cross-sections with the weak ground-state bleaching can provide a longer and relatively symmetrical pump distribution over the entire SCF, and thus giving a longer region with effective gain. By using the ray-tracing analysis, spatial intensity distribution of pump light in the SCF was simulated. As shown in Fig. 1(c), the pump beam with ∼ 430 µm waist diameter and a M ²-factor of ∼ 360, was at first freely propagated in the front part (∼ 5 mm, i.e., undoped YAG end-cap) of the SCF and thereafter wave-guiding in the doped section. Here, the entire lateral surface of the SCF has been optically polished to enhance the light-guiding. The central axis along the SCF exhibited a higher power intensity than the edge parts, which is benefited for providing an effective utilization rate of pump light due to the “mode match” with the laser beam. In comparison, the intensity distribution for the 786 nm pump light was also simulated with the same beam parameters. As can be seen from Fig. 1(d), the pump intensity was mainly accumulated in the front part of the SCF due to the strong absorption, which will inevitably induce the severe thermal effects and thus limiting the power scaling.

 

Fig. 1. (a) Absorption spectrum of Tm:YAG crystal (yellow area) [15] and emission spectrum of LD at the maximum output power (red solid curve). (b) Single-pass absorption of the 50-mm-long Tm(3.5 at.%):YAG SCF at 791 nm, and the simulated normalized spatial intensity distributions of pump lights in the SCF by using ray-tracing analysis for the cases of 791 nm wing pumping (c) and peak absorption pumping at 786 nm (d). (e) Scheme of the wing-pumped Tm:YAG-SCF laser. LD, laser diode; L, lens; M1, input mirror; M2, output coupler; DM, dichroic mirror.

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The schematic of the Tm:YAG-SCF laser is shown in Fig. 1(e), two 791 nm pigtailed laser diodes combined by a (2 + 1) 1 combiner (400 µm cladding diameter, NA = 0.46, coupling efficiency of ∼ 97%) were employed as the pump source delivering a total power of ∼ 210 W. The pump beam was collimated by an achromatic lens L1 (f = 30 mm) and thereafter focused by an aspheric condenser lens L2 (f = 32 mm) into the SCF with a beam diameter of ∼ 430 µm, leading to a maximum power density of ∼ 145 kW/cm². To minimize the optical return loss, the output end of the fiber was cleaved to an angle of ∼ 8°. The theoretical beam quality (M ²-factor) of the delivered pump beam was calculated to be ∼ 360. The focus of the pump beam was ∼ 2 mm away from the input end of the SCF [see Fig. 1(c)]. A simple optical cavity composed of a flat input mirror (M1) and a plane-wedged output coupler (M2), was employed to study the laser performances. To mitigate the thermal load, the Tm:YAG SCF was mounted in a homemade aluminum module with both ends were glue-sealed, and left a protuberance of ∼ 1 mm outside the module. Hence the entire Tm-doped section of the SCF can be directly water-cooled. In the present work, a lowest cooling temperature of 6 °C was chose since a ∼ 5% power roll-off was experimentally observed at 20-°C cooling temperature.

3. Results and discussions

With a physical cavity length of 54 mm, laser performances of Tm:YAG SCF were characterized with different OC transmissions (T_OC= 3%, 5%, 10% and 15%). The output power with respect to the absorbed pump power (under lasing conditions) was shown in Fig. 2(a). With T_OC = 10%, a maximum output power of 63.3 W was achieved at an absorbed pump power of 205 W, corresponding to a slope efficiency of 34.2%. The efficiency is lower compared to our previous report with a same SCF where an optimized mode matching between the pump and laser beams in the initial part of the SCF has been employed [7], but here designed as pump guiding in the entire segment of doped SCF [see Fig. 1(c)] to improve the power scalability [7]. No power roll-off was observed, and the linear dependence clearly shows the potential of the wing-pumped Tm:YAG-SCF laser for further power scaling just by improving the available pump power. In addition to the long pump-guiding region and wing pumping scheme, such straightforward power scalability was also benefited from the fiber geometry structure of the SCF with undoped YAG end capes and direct water cooling. Figure 2(b) shows the measured optical spectra for different OCs, the laser wavelength with T_OC = 10% was located at 2014.6 nm. A wavelength red-shift from 2014.1 to 2021.9 nm was observed with decreasing the OC transmission. This red-shift, as a typical feature in the quasi-three- level laser system, can be attributed to the stronger reabsorption effect as lower population inversion is required for lasing with a lower OC transmission [16].

 

Fig. 2. CW output power (a) and the corresponding optical spectra (b) of the wing-pumped Tm:YAG-SCF laser with different OCs.

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The round-trip resonator loss of the wing-pumped Tm:YAG-SCF laser was calculated by a modified Caird analysis [17], $1/{\eta _s} = 1/{\eta _0}(1 + 2\gamma /{\gamma _{OC}})$ with $\gamma ={-} In(1 - L)$ and ${\gamma _{OC}} ={-} In(1 - {T_{OC}})$, where ${\eta _0}$ is the intrinsic slope efficiency, L is the internal loss per pass, and ${\gamma _{OC}}$ is the output coupling loss. As shown in Fig. 3, the best fit of the experimental data yielded a round-trip resonator loss of 2L = 1.36%. If ignore the passive losses caused by the cavity mirrors, the single-pass propagation loss of the 50-mm-long Tm:YAG SCF amounted to 0.0014 cm^-1 at the laser wavelength. This value is much smaller than the loss of the Ho:YAG SCF grown by micron pulling down method [18], indicating on one hand the high optical quality of the SCF fabricated by mechanical method [7] and on the other the small diffraction losses caused by the end facets of SCF in the lasing process.

 

Fig. 3. Caird plot for the high-power wing-pumped Tm:YAG SCF laser: inverse slope efficiency, $1/{\eta _s}$, with respect to the inverse output-coupling loss, $- 1/In(1 - {T_{oc}})$.

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To assess the beam quality of the Tm:YAG SCF laser, the beam propagation factors (M ²) were measured at different power levels by using two plano-convex spherical lenses and a mid-infrared CCD camera (WinCamD-IR-BB, Dataray Inc.). As shown in Fig. 4(a), the M ²-factors along the x- and y-axis exhibited a similar trend with the laser output power, which indicates a uniform heat distribution in the transverse cross-section of the SCF. With the absorbed pump power increased from 25 to ∼ 170 W (lower than the highest absorbed pump power of 205 W to protect the laser diodes from overheating during the long-time measurement), the M² values gradually increased from 1.7 to around 6. Typical measurement of the beam-quality in the latter case is shown in Fig. 4(b), the larger M² value was caused by the excitation of higher-order transverse modes owing to the high spatial pump intensity distribution along the guiding region in the Tm:YAG SCF [see Fig. 1(c)], and on the other, no additional elements, e.g., optical aperture, was used in the cavity to suppress the high-order modes. This was confirmed by comparing the beam profiles at 10 W and > 50 W output powers. As can be seen in Figs. 4(c) and 4(d), obvious high-order transverse modes were excited in a high-power level. So, further work will focus on optimizing the cavity design by enlarging the fundamental mode size and employing additional optical elements to suppress the high-order modes meanwhile maintaining the power scalability.

 

Fig. 4. Measured M ²-factors of the Tm:YAG SCF laser at different output powers with T_OC = 10% (a), and a typical M ² measurement at 170 W absorbed pump power (b). The far-field beam profiles recorded at the output powers of 10 W (c) and > 50 W (d). The inset in (b) shows the corresponding near-field beam profile.

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Finally, the focal lengths of the thermal lens at different pump powers were calculated by taking into account the measured M²-factors and beam sizes recorded at a distance of ∼ 35 cm away from the OC. Figure 5 shows the focal power, i.e., the reciprocal of focal length, along y-axe as a function of incident pump power. The experimental data agreed well with the fitting line using a modified thermal lens model [19]: $1/{f_{theory}} = \alpha {P_{\textrm{pump}}} + \beta$, where $\alpha$ and $\beta$ are constants. The similar features in both directions again indicated the uniform thermal load in the transverse cross-section of the SCF. From the good linear dependence, the thermal lens focal length was estimated to be 47 mm at the highest absorbed pump power of 205 W. According to the ABCD matrix theory [20], the resonator cavity becomes instability when the incident pump power increased up to ∼ 400 W. In comparison, the critical pump power was only ∼ 70 W in the case of peak absorption pump at 783 nm [7].

 

Fig. 5. Measured focal power of the thermal lens in SCF as a function of incident pump power.

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4. Conclusion

In summary, a simple and compact Tm:YAG SCF laser wing-pumped by 791 nm LDs has been experimentally demonstrated. Benefiting from wing-pump scheme, fiber-like-geometry structure with diffusion-bonded undoped YAG end caps, and unique pump-light propagation properties in the SCF, a maximum output power of 63.3 W with a slope efficiency of 34.2% was achieved at an absorbed pump power of 205 W, further power scaling was limited by the available pump power. In combination with the previous reports [5–7,21] for the laser operations of such fiber-like thin crystal rod with a typical diameter ≤ 1 mm and a length of few centimeters, we concluded that: 1), a longer free-space propagation region of pump light with optimized mode matching to the laser beam, can give a higher laser efficiency and high beam quality, however, not suitable for high power operation owing to the serious thermal effects in the initial part of the SCF as like in the traditional bulk lasers; 2), enhancement of pump guiding (extreme case is to make the pump focus outside the SCF) together with wing pumping result in a longer gain region and a much weaker thermal effects, thus enabling to obtain a high output power but normally accompanied by high-order transverse modes and relatively lower laser efficiency as like here; 3), thinner SCF diameter [22,23] or some special designs (e.g., undoped end caps in the present work) can further enhance the pump guiding and alleviate the thermal stress of the SCF, which is also a hot research topic now [24]. In terms of the laser amplifiers, such Tm-doped fiber-like thin crystal rods with pump guiding but free propagation of laser beam, should be promising candidates for amplification of ultrashort pulses towards high average/peak power with reduced nonlinear optical effects.