Comparative study of the spectroscopic and laser properties of Tm 3+ , Na + (Li + )-codoped Ca 3 Nb 1.5 Ga 3.5 O 12 -type disordered garnet crystals for mode-locked lasers

Tm3+,X+-codoped disordered calcium niobium garnet crystals, Ca3Nb2-xGa3+xO12 (CNGG), where X+ is an alkali cation, Na+ or Li+ (shortly Tm:CNNGG and Tm:CLNGG, respectively), are grown by the Czochralski method. A comparative study of their spectroscopic and laser properties is performed. Raman spectra are also studied. For Tm:CNNGG, the Judd-Ofelt parameters for the Tm3+ dopant ion are Ω2 = 1.847, Ω4 = 1.558 and Ω6 = 0.862 [10−20 cm2] and the radiative lifetime of the 3F4 state is 4.11 ms. The absorption, stimulated-emission (σSE) and gain cross-sections are determined for the 3H6 ↔ 3F4 Tm3+-ion transition. For Tm:CNNGG, the maximum σSE is 0.49 × 10−20 cm2 at 1867 nm. Both crystals exhibit flat and broad gain spectra extending beyond 2 µm. A compact diode-pumped Tm:CNNGG laser has generated 1.05 W at 2007.7 nm with a slope efficiency of 35%. Broadband tuning of Tm:CNNGG and Tm:CLNGG lasers (tuning ranges of 168 nm and 224 nm, respectively), was demonstrated. The Tm3+,X+-codoped CNGG crystals are the promising for sub-100 fs pulse generation at ~2 μm.