Off-resonance intracavity up-conversion pumping architecture enabling multi-watt operation of a 2.3-µm Tm:YLF laser

We present an intra-cavity upconversion pumping scheme for Thulium lasers operating on the ³H₄ → ³H₅ transition. It is based on detuning the pump wavelength from the resonance of excited-state absorption, ³F₄ → ³F_2,3, around 1 µm. This scheme is validated using a Tm:LiYF₄-based laser. The pump source consists of a diode-pumped Nd:YVO₄ laser emitting at 1.064 µm. This architecture permits for power scaling of 2.3-µm Tm-lasers. An output power of 2.7 W is generated in the continuous-wave regime for 62.3 W of intracavity pump power at 1.064 µm corresponding to 21 W of primary laser-diode power, making this kind of pumping competitive in terms of power, laser gain and efficiency with direct diode pumping at 0.78 µm. We also describe the heat management in this double-cavity laser benefiting from the intra-cavity pumping architecture allowing to share the thermal load between two gain crystals. The low absorption caused by the non-resonant ³F₄ → ³F_2,3 Tm³⁺ transition allows the Nd pump laser to reach higher intracavity power compensating for the low pump absorption efficiency in the Tm-crystal. The proposed off-resonance pumping scheme opens a new paradigm that holds great promise for high-power, high-gain 2.3 µm solid-state lasers based on thulium ions.