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inter-ion spacing, it benefits from higher Tm 3 + concentrations for efficient operation. Increasing doping concentration, however, also increases deleterious effects such as up-conversion, residual reabsorption, and concentration quenching, which can all play against high-power operation. Furthermore, the CR process generates additional heat due to the mismatch between the energy levels involved [ 21 – 24 ]. Although several efforts have aimed at further boosting efficiency at watt-level output powers, achieving such performance typically requires careful optimization and a more constrained resonator design. One effective approach is to use channel-waveguide laser geometries, which offer strong confinement and high pump absorption. For example, a channel-waveguide Tm laser [ 25 ] has demonstrated efficiencies as high as 80 % at 1.6 W of average output power.
Alternatively, in-band pumping at 1.65 – 1.85 lm, which directly excites Tm 3 + ions into the upper laser level manifold 3 F 4, is highly promising for further increasing power and efficiency. The low quantum defect associated with in-band pumping substantially reduces thermal load due to non-radiative transitions and eliminates the dependence on Tm – Tm cross-relaxation for high efficiency. Furthermore, the absorption peak corresponding to in-band pumping is significantly broader than its counterpart at 800 nm, which makes it less sensitive to pump wavelength and temperature variations [ 26, 27 ]. In most cases, Raman-shifted Er-fiber or short-wavelength Tm-fiber lasers are employed to achieve in-band pumping. While such an approach has been successfully applied to different crystal hosts and demonstrated watt-level operation [ 26, 28, 29 ], the cost and complexity of the pump scheme is an obstacle for the adoption of the technology beyond laboratory settings. Recently, high-brightness pump diode sources around 1.7 lm have become commercially available, making in-band diode pumping a simple and cost-effective alternative. Although multimode in band-pumping for Tm-doped gain media has already led to high average power and efficiency, this approach remains scarce and has been demonstrated with only a few different hosts [ 30, 31 ].
In the family of promising monoclinic double-tungstate host materials, Tm: KY( WO 4) 2( Tm: KYW) is particularly compelling. In addition to their commercial availability and robustness, Tm: KYW crystals have a large emission cross-section and broad gain bandwidth across 1.9 – 2.1 lm, which make them ideal for operation at high power, broad wavelength tunability, and generation of short pulses [ 17, 27, 32 ]. Tm: KYW exhibits suitable broad absorption bands for in-band diode pumping around 1.7 lm, with pronounced peaks at 1678 nm and 1716 nm [ 27, 32 ]. Although the magnitude of the absorption peaks in Tm: KYW around 1.7 lm is reduced by about a factor of 2.5 compared to conventional 800-nm pumping, it is largely compensated by the availability of high-power multimode pump diodes, and the high quantum-limited efficiency of 80 – 90 %. So far only one study reports on multimode diode in-band pumping with Tm: KYW [ 27 ], and the authors demonstrate only 86 mW of average output power with a moderate slope efficiency of 28 %.
In this work, we employ multimode in-band pumping in a Tm: KYW laser oscillator and demonstrate high average powers up to 4.55 W around 1940 nm with a slope efficiency of 83 %; approaching the quantum-defect limit. To the best of our knowledge, this constitutes both the highest efficiency and average power, as compared to previous Tm: KYW-based bulk systems and, more generally, even among Tm-doped double-tungstate crystal bulk lasers. In addition, the source offers smooth wavelength tunability from 1839 to 2100 nm and maintains excellent, near-diffraction-limited beam quality across the full operating range.
This simple, cost-effective and single-stage source is promising for many applications requiring high-power levels and broad tunability and motivates Kerr-lens mode locking in a purged cavity to exploit the Tm: KYW broad gain bandwidth for high power and efficiency sub-100-fs pulse generation near the central gain.
2 Experimental setup
The experimental setup is shown in Figure 1a. The laser is based on a 3-mm-long Tm: KYW gain medium with 4.6 at.% doping, cut along the N g. For thermal management, thecrystalwaswrappedinindiumfoilandmounted on a Peltier-cooled copper heatsink stabilized to around 12 ° C. Pumping is provided by a 15 W-rated fiber-coupled( NA = 0.22, 105 lm core diameter) diode laser at 1720 nm with roughly 10 nm FWHM spectral width. The pump light is unpolarized. Although the absorption around 1720 nm in the crystal is stronger for polarization along N m than along N p [ 27, 32 ], the high available pump power was sufficient to compensate for this difference. Therefore, for experimental convenience, no polarization-selection element is introduced in the pump scheme. N p, N m and N g denote the principal axes of the dielectric frame of the monoclinic Tm: KYW crystal, where N p is parallel to the crystallographic b-axis and N m and N g lie in the a – c plane. The diode is mounted on a water-cooled copper heat sink. To avoid crystal damage, we limit the incident pump power to 11.5 W. For experimental convenience, the absorbed pump fraction was measured in a temporary two-mirror resonator designed to reproduce the pump and laser mode sizes and intracavity power conditions of the final cavity, while allowing direct measurement of the transmitted pump. Under lasing operation, an absorbed-pump fraction of( 50 ± 1)% was obtained, consistent with reported values for in-bandpumped Tm: KYW [ 27, 29 ]. The pump scheme was singlepass. A pair of achromatic doublets( L1 and L2) with AR-AR coatings for the pump are used to image the fiber output into the crystal with a 1:1 ratio, producing a clean circular pump spot with a radius of 52.5 lm at the waist inside the crystal.
A Z-shaped hemispherical cavity with a total length of approximately 36 cm was implemented. The cavity directly starts with the entrance facet of the crystal. One facet of the Tm: KYW crystal carried a dichroic coating providing high pump transmission( T 96 % at 1.72 lm) while being highly reflective across the lasing range( HR, 1.85 – 2.1 lm). The second facet was AR-coated at both the pump and laser wavelengths. The crystal is followed by a concave mirror( CM, ROC-100 mm), and then a plane mirror. Both mirrors