Highly Efficient, Compact Tm3+:RE2O3 (RE = Y, Lu, Sc) Sesquioxide Lasers Based on Thermal Guiding

Cubic sesquioxides, RE<sub>2</sub>O<sub>3</sub>, where RE = Y, Lu or Sc, are attractive host crystals for thulium (Tm<sup>3+</sup>) doping. A comparison of the spectroscopic properties of Tm<sup>3+</sup>:RE<sub>2 </sub>O<sub>3</sub> crystals in terms of transition cross sections and cross-relaxation (CR) efficiency required for efficient upconversion pumping is presented. Thermo-optic properties of Tm<sup>3+</sup>:RE<sub>2</sub>O<sub>3 </sub> crystals (thermal lensing, fractional heat loading, and thermo-optic coefficients) are described. The positive thermal lens, broadband emission, and efficient CR of the Tm:RE<sub>2</sub>O<sub>3</sub> crystals enable the development of compact, highly efficient and power-scalable lasers operating above 2 μm, based on thermal guiding. Nowadays, Tm:Lu<sub>2</sub>O<sub>3</sub> microchip lasers are capable of generating nearly 5 W of output power at ∼2.06 μm with a slope efficiency <inline-formula><tex-math notation="LaTeX">$ \eta $</tex-math> </inline-formula> of 67% and in a rod geometry—up to 47.5 W with <inline-formula> <tex-math notation="LaTeX">$ \eta $</tex-math></inline-formula> of 59%. For multiwatt output at even longer wavelengths around 2.15 μm, Tm:Sc<sub>2</sub>O<sub>3</sub> is an interesting candidate.

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