Two-micron cryogenically-cooled solid-state lasers: recent progress and future prospects

Efficient powerful laser sources in the two-micron regime are in demand for many applications in the areas of remote-sensing, defence, medicine, and materials interactions. Dramatic progress has been demonstrated in cw-power scaling of 2-micron fibre lasers; however, power-scaling in a pulsed mode of operation is limited by nonlinear effects and a relatively low damage-threshold-power. To fully capitalise on the potential advantage for high pulse-energies of the conventional 'bulk' 2-micron solid-state laser, extreme measures have to be taken to mitigate the three-level character and thermal effects in the laser medium resulting from heat generated during the pump cycle. Alleviation of these detrimental effects can be achieved by simply cooling the gain medium to cryogenic temperatures, benefitting from lower population in the terminal laser levels, and a large increase in the thermal conductivity, with a proportional decrease in the thermo-optic coefficient (dn/dT) and expansion coefficient. Combined these result in a massive reduction in thermo-optic aberrations. In this paper, we report on improved measurements of the spectroscopic properties of Ho:YAG at various temperatures between room and liquid nitrogen (LN) temperatures, utilising a multi-Watt Tm:fibre ASE source we have been able to properly identify the absorption features of interest with an accuracy better than 0.2nm. Results for other Ho-doped gain media will be discussed and the latest performance of a cooled 2-micron Ho:YAG laser in-band pumped by a narrow-linewidth Tm fibre laser presented