Highly efficient continuous-wave Tm³⁺:YLF cascade laser operating at telecommunication wavelengths

Tm³⁺-doped YLF crystals with different doping concentrations are grown and investigated for their suitability for continuous-wave laser operation on the ³H₄→³F₄ transition around 1.5 µm. Our spectroscopic investigations reveal the emission, absorption, and gain cross sections as well as the concentration-dependent lifetime of the relevant energy levels. Self-termination of the laser is circumvented in a cascade laser scheme ³H₄→³F₄→³H₆ for efficient depopulation of the long-living lower laser level ³F₄. In this way, simultaneous laser operation at 1509 nm and 1923 nm with slope efficiencies of 44% and 35% and pump-power-limited output powers of 590 mW and 670 mW are obtained under 781-nm Ti:sapphire laser pumping, respectively.