J. Eur. Opt. Society-Rapid Publ. 2026, 22, 44 Ó The Author( s), published by EDP Sciences, 2026 https:// doi. org / 10.1051 / jeos / 2026040 Available online at: https:// jeos. edpsciences. org
Journal of the European Optical Society-Rapid Publications
RESEARCH ARTICLE
High-power, high-efficiency continuous-wave Tm: KYW laser with multimode in-band diode pumping
Firas Trawi *, Marin Hamrouni, Zekican Ertürk, Lukas W. Perner
, Valentin J. Wittwer, and Thomas Südmeyer Laboratoire Temps-Fréquence( LTF), Institut de Physique, Université de Neuchâtel, Avenue de Bellevaux 51, 2000 Neuchâtel, Switzerland
Received 25 February 2026 / Accepted 19 April 2026
Abstract. We report a high-power, high-efficiency continuous-wave Tm: KY( WO 4) 2( Tm: KYW) laser based on multimode in-band diode pumping at 1720 nm. In-band pumping reduces the quantum defect compared to conventional 800-nm pumping, enabling efficient high-power operation. We demonstrate up to 4.55 W of output power near 1.94 lm with a slope efficiency of 83 % with respect to absorbed pump power, approaching the quantum-defect limit. The laser provides smooth wavelength tunability from 1839 to 2100 nm and maintains near-diffraction-limited transverse beam quality across the full operating range. These results indicate that multimode in-band pumping of Tm: KYW is a simple and compact route to efficient, broadband, highpower sources in the 1.9 – 2.0 lm region, providing a practical basis for future high-power Q-switched and mode-locked systems.
Keywords: Continuous-wave lasers, High-power lasers, In-band multimode diode pumping, Purged operation, CW Tm: KYW laser, SWIR lasers.
1 Introduction
Solid-state lasers offering high-power and tunable wavelengths in the 2-lm band are of considerable interest for many applications. When combined with diode pumping, these lasers are efficient, simple and cost-effective shortwavelength infrared( SWIR) sources. The 2-lm window is especially compelling as it overlaps strong H2O rovibrational lines, enabling sensitive water-vapor spectroscopy and DIAL / LIDAR humidity profiling [ 1, 2 ], and it lies within an eye-safe atmospheric transmission band advantageous for free-space optical links and airborne sensing [ 3, 4 ]. It is also appealing for clinical applications such as endourology and soft-tissue surgery, where shallow optical penetration and efficient hemostasis are decisive [ 5, 6 ].
The most common gain media for high-power emission around 2 lm are based on Thulium( Tm), Holmium( Ho) and co-doped Tm / Ho ions. These three types of gain media feature different properties and their suitability depends on the application [ 7 ]. For instance, with a high emission crosssection around 2.1 lm and good thermal properties, Ho-doped crystals are ideal to deliver high average power beyond 2 lm [ 8 ]. However, these lasers are pumped around 1.9 lm and typically require a high-power single mode Tmbased laser as a primary pump source, increasing cost and
* Corresponding author: firas. trawi @ unine. ch complexity of the overall setup. In contrast, Tm-doped crystals feature a strong absorption peak around 800 nm corresponding to the 3 H 6! 3 H 4 transition of Tm 3 + ions, which is conveniently accessed by high-power off-the-shelf and mature AlGaAs diode lasers. Co-doping with Tm and Ho ions has been developed for combining the convenient pumping of Tm around 800 nm and the strong emission of Ho near 2.1 lm. While these crystals have demonstrated excellent results for broadband emission [ 9 ], these materials are prone to enhanced up-conversion processes [ 10 ] compared with singly Tm-doped crystals, which increases the thermal load, reduces efficiency, and constrains power scaling. For these reasons, Tm-based gain media remain ideal candidates for applications requiring simple and compact high-power sources.
Tm-based lasers using multimode diode pumping around 800 nm have been demonstrated many times with a large variety of host materials [ 11 – 20 ]. Despite the large quantum defect associated with the 3 H 6! 3 H 4 transition, this scheme relies on the cross-relaxation( CR) effect, known as a“ two-for-one” energy-transfer process, which populates the upper laser level 3 F 4, resulting in efficient laser operations [ 11, 17, 18, 20 ]. For instance, Ref. [ 17 ] reports on a 5-at.%-doped Tm: KYW bulk laser demonstrating up to 73 % of slope efficiency with respect to pump power, considerably larger than 41 % expected from the quantum defect alone. Since the CR probability depends on the
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