Broadly tunable Tm:BaF₂ and Tm:SrF₂ lasers

We report on the spectroscopic properties and first continuous-wave (CW) and broadly tunable operation of a Tm 3+ -doped barium fluoride (BaF 2 ) crystal, comparing it with another fluorite-type compound, Tm 3+ :SrF 2 . Tm 3+ ions in BaF 2 exhibit broader emission around 2 µm (bandwidth: 235 nm) at the expense of a lower stimulated-emission cross-section (1.36×10 −21 cm 2 at 1.86 µm) and a longer luminescence lifetime (12.03 ms) as compared with Tm:SrF 2 . The room-temperature CW Tm:BaF 2 laser generated 184 mW at 1.88 µm with a slope efficiency of 54.8% exceeding the Stokes limit and a low laser threshold of 84 mW. In the quasi-CW regime, its output was further scaled to 0.56 W with even higher slope efficiency of 67.2%. By using an MgF 2 -based Lyot filter, continuous wavelength tuning of this laser from 1833 to 2002 nm (tuning range: 169 nm) was demonstrated. Tm:BaF 2 is promising for ultrashort pulse generation from mode-locked lasers.