We present results for an efficient Ho:LuLiF<sub>4</sub> laser in-band pumped by a cladding-pumped Tm-doped silica fiber laser. The polarized, tunable Tm-doped silica fiber-laser operates at 1938 nm, ideally suited for in-band pumping the 40 mm long weakly-doped (0.25 at.%) Ho:LuLiF<sub>4</sub> crystal. Using a simple laser resonator a maximum output power of 5.1 W was achieved at a wavelength of 2066 nm for 8.0 W of absorbed pump power, when using an output coupling
mirror with 20% transmission, corresponding to a slope efficiency of 70%. At a higher cavity output coupling of 37%, the lasing wavelength shifted to a higher gain peak at 2053 nm, where a maximum output power of 5.4 W was obtained with a slope efficiency of 76%. Beam quality was measured to be M<sup>2</sup>~1.1 at the maximum output power for each resonator configuration. The spectroscopy, lifetime of the upper laser level, and the laser performance will be discussed in terms of future prospects for power scaling and further improvements in the laser efficiency.
Efficient powerful laser sources in the two-micron regime are in demand for many applications in the areas of
remote-sensing, defense, medicine, and materials interactions. Dramatic progress has been demonstrated in cw-power
scaling of 2-micron fiber lasers; however, power-scaling in a pulsed mode of operation is limited by nonlinear effects
and a relatively low damage-threshold-power. To fully capitalize 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 temperatures, utilizing a multi-Watt Tm-fiber 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-fiber laser presented.
Cryogenically-cooled diode-pumped lasers have received significant interest in recent years for their demonstrated
orders of magnitude improvement in output radiance using simple laser resonator configurations, with respect to their
room temperature counterparts. Here we present a technique that offers the potential for a further order-of-magnitude
radiance improvement utilising the in-band pumping hybrid-laser architecture, which employs high-power fiber lasers to
excite cryogenically-cooled bulk gain media. The ability to exploit the quasi-four-level nature of a two-level laser system
at very cold temperatures enables the operation of very low quantum defect transitions, thus providing reduction in the
required thermal dissipation per unit power for the in-band pumped Ho:YAG laser, compared to diode-pumped
Yb:YAG. Preliminary results will be discussed for a narrow linewidth Tm:fiber laser system operating in the 100W
regime, pumping a cryogenically cooled Ho:YAG gain element, and employing a simple cavity configuration. Low
quantum defect operation and power-scaling potential will be discussed.
Using a hybrid fiber-bulk laser scheme based on Er:YAG, we have achieved ~60 W and ~30 W of continuous-wave
output at 1645nm and 1617nm respectively, and Q-switched pulse energies up to ~30 mJ (limited by coating damage).
Investigation of various factors influencing laser performance has revealed that energy-transfer-upconversion can have a
very detrimental impact on efficiency, even in continuous-wave mode of operation. In this paper we report on the results
of this study, discuss various measures for reducing energy-transfer-upconversion and its effect on laser performance,
and consider the prospects for further increase in output power and pulse energy.