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

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