Enhanced performance of a passively mode-locked Tm,Gd:CaF ₂ laser versus Tm:CaF ₂ around 1.9 μm

Mode-locked lasers operating in the 2 μm region are of great value for various spectroscopic applications and investigations of laser-matter interactions. However, ultrafast lasers that exhibit both a short pulse duration and a wide emission spectrum below 1.9 μm remain challenging. In this work, a passively mode-locked Tm:CaF₂ laser was demonstrated at approximately 1886.2 nm with a pulse duration of 3.7 ps and a maximum average output power of 171 mW, by using Gires-Tournois interferometer mirrors for intracavity dispersion compensation. In contrast, the introduction of Gd³⁺ co-doping enabled the first realization of a mode-locked Tm,Gd:CaF₂ laser, which generated significantly shorter pulses of 1.7 ps. Moreover, the Tm,Gd:CaF₂ laser exhibited a broadened tuning range of 142.2 nm compared to 127.6 nm obtained in the Tm:CaF₂ laser. These results confirm that co-doping Gd³⁺ ions enhances the performance of Tm-based CaF₂ lasers, making Tm,Gd:CaF₂ crystals a promising gain medium for a broadband mode-locked laser around 1.9 μm.