Thermal lensing, heat loading and power scaling of mid-infrared Er:CaF<sub>2</sub> lasers
Liza Basyrova, Pavel Loiko, Jean-Louis Doualan, Abdelmjid Benayad, Alain Braud, Bruno Viana, Patrice Camy
Abstract
Mid-infrared Er:CaF 2 laser operating on the 4 I 11/2 → 4 I 13/2 transition is developed. Its power scaling capabilities and thermo-optics (fractional heat loading and thermal lensing) are compared under pumping into the 4 I 11/2 and 4 I 9/2 states. Using a 4.5 at.% Er:CaF 2 crystal, a record-high continuous-wave output power of 0.83 W is achieved at 2800 nm with a slope efficiency of 31.6% and a laser threshold of 24 mW and the fractional heat loading is measured under lasing and non-lasing conditions, yielding the values of 52.0% and 71.7%, respectively (for pumping at 967.6 nm, into the 4 I 11/2 state). The thermal lens in Er:CaF 2 is negative (divergent) owing to the negative thermo-optic coefficient and large and negative contribution of the photo-elastic effect. The sensitivity factors of the thermal lens are M r = −4.84 and M θ = −5.15 [m −1 /(kW/cm 2 )] and the astigmatism degree is as low as 6%. When pumping into the higher lying 4 I 9/2 manifold, the thermal lens is enhanced owing to the additional heat generation from the multiphonon non-radiative path from this state, and the laser slope efficiency is deteriorated.