• Thermal lensing in solid-state lasers

The end-pumped solid-state laser is a mature design architecture exploited for many scientific, industrial and medical laser applications, in the tens-of-watts power regime. Cost effective, compact, and relatively efficient, in recent decades their performance has capitalized on improving diode laser brightness and power. Further power-scaling however, is fundamentally limited by the thermo-optical properties of the gain medium and the induced optical distortions, primarily driven by the quantum defect between pump and the emission wavelengths. The basic design strategy for these lasers has been to try to mitigate the effects of the induced thermal lensing and aberrations, which are both theoretically and experimentally investigated in this work.
In the first part, we use the Kirchhoff transform to convert the non-linear heat equation into a solvable linear equation for a cylindrical radially isotropic gain element, considering its thermal conductivity as temperature-dependent. Analytical solutions for the temperature distribution along the length of a side- cooled end-pumped rod are presented for different pump distributions that can be used for practical configurations, such as near-diffraction-limited, to fibre- coupled diode-laser, pumps. Furthermore, this result provides novel analytical expressions for the thermal-lens strength associated with the pump-induced accumulated optical phase shift, which converge to well-known equations when a temperature-independent thermal conductivity is chosen.
In the second part it is shown how to experimentally investigate the thermal lens and aberrations in a diode-pumped Nd:YAG crystal, by means of a He-Ne laser. The experimental setup used and preliminary results obtained are discussed.