Tm,Ho:Ca(Gd,Lu)AlO4 crystals: Polarized spectroscopy and laser operation

Tm3+,Ho3+-codoped and compositionally “mixed” disordered Ca(Ga,Lu)AlO4 aluminate crystals are promising materials for ultrashort pulse generation above 2 μm. Their polarized spectroscopic properties are studied and the effect of Lu3+ codoping on the inhomogeneous spectral broadening is verified. The peak stimulated-emission cross-section for the 5I7 → 5I8 Ho3+ transition is 1.05 × 10−20 cm2 at 2000 nm for π-polarization. The Tm3+ ↔ Ho3+ energy-transfer parameters are P28 = 1.42 and P71 = 0.090 [10−22 cm3μs−1] indicating a direct transfer with a thermal equilibrium time of 4.35 ms. The effective gain spectra for the codoped crystal are derived. For σ-polarization, the gain bandwidth at ∼2.04 μm is about 100 nm; particularly for this light polarization, Lu3+ codoping induces a noticeable red-shift of the emission spectra further beyond 2 μm. Using absorption studies at 12 K, the Stark splitting of the Tm3+ and Ho3+ multiplets is resolved. A laser-pumped Tm,Ho:Ca(Ga,Lu)AlO4 laser generated a maximum output power of 763 mW at 2078.6 nm with a slope efficiency of 26.4% and π-polarized emission. A continuous wavelength tuning between 1887.7 and 2127 nm (tuning range: 239.3 nm) was achieved for σ-polarization. Diode-pumped operation of a-cut and c-cut crystals was also studied.