Towards the power-scaling of sub-1 µm cryogenically cooled lasers.

Cante, Silvia (2021) Towards the power-scaling of sub-1 µm cryogenically cooled lasers. University of Southampton, Doctoral Thesis, 209pp.

Record type: Thesis (Doctoral)

Abstract

The general aim of the research presented in this thesis is the power-scaling of Nd-doped cryogenically-cooled solid-state lasers operating on the 9xx nm quasi-four-level transition. Capitalising on a lower quantum defect compared to the renowned 10xx nm transition, it has the potential for extremely high optical-to-optical efficiency. Moreover, similar to the well known 1 µm transition of Yb-doped crystals, the shorter 0.9 µm wavelength is obtained via a ’two-level’ energy scheme, which in Nd-doped crystals is the first meta-stable excited to ground state transition, ⁴F_3/2 → ⁴I_9/2. This relatively low gain 9xx nm transition competes with the higher gain 10xx nm transition and must also overcome reabsorption associated with population in the terminal ground-state level. Consequently, and due to the more complex energy level structure, other parasitic energy transfer processes such as Energy Transfer Upconversion and Cross Relaxation, which add to the thermal load, must also be addressed in developing power-scaled 9xx nm Nd-doped lasers. The cryogenically cooled laser architecture provides an excellent platform for such lasers, though their engineering requires a good knowledge of the parameters that would contribute to, or obstruct, efficient laser operation.
In setting out a treatise for developing these lasers, we first needed to undertake a study of the temperature-dependent spectroscopic properties of key Nd-doped materials. As such, we developed procedures that would be easily applicable to any kind of Nd-doped laser crystal. Firstly, we have characterised the ²H_9/2 + ⁴F_5/2 absorption cross section at temperatures ranging from Room Temperature to 450 K for Nd:YVO₄ and Nd:GdVO₄, and from Room Temperature to 77 K for Nd:YAG. Secondly, we have optimised a z-scan experiment for the characterisation of the macroscopic Energy Transfer Upconversion coefficient. This included a rigorous determination of the parameter dependencies and automation of the data collection. We have employed this setup to probe the dependence of this coefficient on elevated temperatures for Nd:YVO4 and Nd:GdVO4 over the same range explored with the absorption cross section measurements. Furthermore, we have verified its dependence on the doping-ion concentration of Nd:YAG, and therein characterised the Energy Transfer Upconversion in Nd:YAG at cryogenic temperatures. We have determined that although the ETU coefficient increases with decreasing temperature, the beneficial effects of cryogenic cooling dominate the laser performance, resulting in its overall enhancement.
Utilising previous studies for the absorption cross section dependence on sub-ambient temperatures, for in banding pumping Nd:YAG around 869 nm, we have engineered a novel Volume-Bragg-Grating locked at 869 nm diode-laser-array pump. This highly efficient pump-source was used to excite a cryogenically cooled Nd:YAG crystal, enabling record performance for 60 W 946 nm laser, which had a slope, and optical-to-optical, efficiency of 52%. With further improvements to the crystal mounting and pump conditioning, we have demonstrated an even better performance from this laser, with > 100 W of output power and 82% slope, and 74% optical-to-optical, efficiency. The beam quality of both lasers was found to degrade at high-power, however, these results still stand out as the state of-the-art in radiance in this wavelength regime.Through precise determination of the key spectroscopic parameters for cryogenically cooled Nd:YAG, we have gained useful insight into the limiting factors for this architecture and power scaling of the 946-nm Nd:YAG laser. These findings pave the way for future developments of these sub-micron lasers that will achieve further power-scaling of near-diffraction-limited beams with the excellent efficiency as demonstrated in this thesis.