Saturation Effects and Thermal Balance in Laser-Cooled Solids

Abstract

The role of saturation of background absorption by unintended impurities has been investigated for the first time theoretically and experimentally in the context of laser cooling of crystalline solids and the operation of self-cooled or radiation-balanced lasers. In this thesis it is shown that when saturation of the background takes place at an intensity lower than the saturation intensity of dopant ions responsible for cooling, as in Yb3+:LiYF4, cooling efficiency can be improved by using elevated input intensity. With the same input power, the measured cooling efficiency in 10% Yb3+:LiYF4 crystal was doubled at 1020 nm when the beam radius was reduced by a factor of 10, by partially saturating the background impurity absorption. This is effective not only as a post-growth method of enhancing cooling efficiency but offers a new way of reaching lower temperatures than were previously possible with anti-Stokes fluorescent (ASF) refrigeration. Prior to this work, cooling efficiency was thought to be independent of intensity and crystal purification was considered as the only way to improve the cooling efficiency. It is further demonstrated that background saturation plays a key role in the operation of self-cooled or radiation-balanced lasers. Because radiation-balanced lasers operate under saturated conditions it is essential to include limitations imposed by impurity content of the gain medium. When this is done, quantitative predictions of the required conditions for radiation balance become possible. Laser operation can then be made more efficient and can be extended to new materials which may be relatively impure and uncommon but are well-suited to efficient operation under radiation-balanced conditions by virtue of a special combination of physical characteristics. Radiation-balanced laser operation is achieved in 3% Yb:YAG (1 × 1 × 10 mm3) and is found to result in exceptionally uniform temperature distributions using high resolution thermal imagery. Threshold levels, output power, and input power at the radiation balance point are all shown to be in close agreement with theory. This laser achieved 30.5% laser efficiency (agreed with predictions within 2%), which is the highest efficiency level of any self-cooled laser operated to date. Radiation-balanced lasing is also achieved for the first time in the tungstate crystal 2% Yb3+:KYW (0.9×1.2×10 cm3), despite having very high impurity content as the result of being an uncommon gain medium. The achieved laser efficiency was only 2.2%, as the result of non-optimal optical components, high Yb concentration, high impurity content, crystal polish, excitation wavelength, and output coupling. However, this is the first demonstration of an Yb3+ radiation-balanced laser in the tungstate host. Correction of these deficiencies should enable high quality Gaussian output at the 5 W level with 18% efficiency in existing commercial 1% Yb:KYW samples. Even better performance of this new self-cooled laser could result from improved crystal growth efforts.