JEOS RP ISSN03 | Page 440

J. Eur. Opt. Society-Rapid Publ. 22, 44( 2026) 433

Figure 2b shows the normalized CW spectra at output coupling rates of 0.6 %, 2.0 %, and 7.5 % in dry-air operation. The laser emission shifts to shorter wavelengths as the output coupling increases, as is characteristic of quasi-threelevel operation. We observe smooth and continuous spectra, confirming the effective suppression of water-vapor absorption.

Figure 2c shows the wavelength tuning measurements. On the left y-axis we show the output power with output coupling rate of 0.6 %( red) and output coupling rate of 7.5 %( green) at various wavelengths. The right y-axis shows the calculated overall cavity transmission for both configurations( grey curves). For each output coupler( OC), the incident pump power was kept constant over the entire tuning range and set slightly above the lasing threshold measured without the birefringent filter( BRF). Specifically, for the 0.6 % OC, the incident pump power was fixed at 0.39 W( lasing threshold without BRF: 0.17 W), and for the 7.5 % OC, it was fixed at 0.68 W( lasing threshold without BRF: 0.31 W). Operating near threshold reduces gain saturation and thermal load, so the tuning behavior is primarily governed by the crystal gain cross section rather than thermal effects, while maintaining sufficient margin to prevent the laser from switching off over the entire wavelength scan.

At an output coupling rate of 0.6 %( red), a broad tuning range from around 1900 nm up to 2100 nm is obtained. The strongest output is observed in the 1980 – 2000 nm region. In the long-wavelength range, spectral tuning is limited to 2100 nm by the rapidly decreasing emission cross section [ 27, 32 ]. Below 1900 nm, spectral tuning is limited by the significant increase of cavity losses( grey solid line). At an output coupling rate of 7.5 %( green), the tuning range shifts towards a shorter wavelength and becomes narrower, from 1840 nm to 1980 nm, with a peak around 1940 nm. The change and the shift of the overall spectral shape is mainly due to the higher inversion level, caused by the increased output coupling rate. Below 1850 nm, spectral tuning is limited by the significant increase of the cavity losses( grey solid line). Above 1980 nm, wavelength tuning is limited by the finite modulation depth of the BRF filter. In that region, the modulation depth of the filter is smaller compared to the difference between the peak and the tail of the gain cross-section [ 27, 32 ].

The transverse beam profile and beam-propagation factors are shown Figures 2d and 2e. The beam profiles were acquired with a DataRay WinCamD-FIR2 camera( pixel size 17 17 lm 2), while M 2 measurements were performed using a DataRay BeamMap2-DD slit scanning beam profiler. In low-power operation( Fig. 2d), at P pump = 1W; P laser = 0.26 W, the estimated beam quality factors for the horizontal and vertical planes are M x 2 = 1.03 and M y 2 = 1.08. In high-power operation at P pump = 10W, yielding P laser = 3.9 W, the estimated beam quality factors for both planes slightly increase to M x 2 = 1.08andM y 2 = 1.21. The results confirm excellent beam quality with low astigmatism across the entire operation range, suggesting negligible thermal effects.

4 Conclusion

We have demonstrated multi-watt continuous-wave operation of a Tm: KYW laser in-band diode-pumped at 1.7 lm with a slope efficiency close to the quantum-defect limit in a dry-air purged environment. The laser delivers 4.55 W at 1.937 lm with an optical-to-optical slope efficiency of 83 %. To our knowledge, this result demonstrates the highest power and efficiency achieved by any CW inband-pumped Tm: KYW laser and even among Tm-doped double-tungstate crystal bulk systems. The laser is tunable from 1839 to 2100 nm and maintains excellent beam quality over the full power range, while maintaining a simple Z-shaped cavity and using a basic multimode pump diode. Further optimization and miniaturization may be possible by employing shorter and more highly doped Tm: KYW crystals, if supported by available crystal-growth methods and while preserving sufficient material quality, pump absorption, and crystal gain. This could help preserve the high pump absorption and crystal gain while improving the pump – laser mode overlap in the crystal. Efficient operation of 8 at. % [ 33 ] and15at.%[ 34 ] Tm-doped double tungstate lasers was already demonstrated in a waveguide and thin-disk geometry respectively. As a key message, this work shows that in-band pumping around 1700 nm based on a multimode diode, which became commercially available recently, is a simple and cost-effective effective solution for efficient high-power operation near 2 lm. Also, we believe that this platform will open a new route toward the development of powerful 2-lm mode-locked lasers.

Conflicts of interest The authors declare that they have no competing interests.

Data availability statement

Data underlying this paper can be obtained from the authors upon reasonable request.

Author contribution statement

F. T. and Z. E. performed the experimental work with assistance of M. H., V. J. W. and L. W. P. F. T. and M. H. analyzed the data and wrote the manuscript. T. S. supervised the project.

References

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