Cascade laser optimization for ³H₄ → ³H₅ and ³F₄ → ³H₆ sequent transitions in Tm³⁺-doped materials

We study a cascade laser scheme involving the ³H₄ → ³H₅ and ³F₄ → ³H₆ consecutive transitions in Tm³⁺-doped materials as a promising technique to favor laser emission at 2.3 µm. We examine the conditions in terms of the Tm³⁺ doping levels for which the cascade laser is beneficial or not. For this, Tm:LiYF₄ lasers based on crystals with several doping levels in the range of 2.5 - 6 at.% with and without cascade laser are studied. For low doping of 2.5 at.% Tm³⁺, adding the laser emission at 1.9 µm allows to double the output power at 2.3 µm, whereas for high doping of 6 at.%, allowing the laser to operate at 1.9 µm totally suppresses the laser emission at 2.3 µm. An analytical model is developed and confronted with experimental results to predict this doping-dependent phenomenon and forecast the potential benefits. This study of cascade laser emission on the ³H₄→ ³H₅ and ³F₄→ ³H₆ transitions versus the Tm³⁺ doping level is finally extended to other well-known Tm³⁺-doped laser materials.