Investigation of thermal performance and thermal lensing effects in cryogenically cooled Fe: ZnSe lasers

A transient three-field numerical model has been developed to investigate the thermal performance and thermal lensing effects in cryogenically cooled Fe:ZnSe lasers. This model integrates fluid dynamics, solid mechanics and laser physics, utilizing the optical path difference (OPD) as a quantitative parameter to correlate the crystal's temperature and deformation with laser beam quality. This approach enables the numerical model to directly evaluate the impact of thermal management technologies on beam quality. In experiments involving Fe:ZnSe lasers, optimizations were made to the pin fin structure of the microchannel, significantly reducing thermal lensing and improving the beam quality factor (beta) from 4.4 to 1.9 at an output power of 105 W. The numerical model was validated against experimental data on beam quality and laser intensity distribution. Results show that the coefficients of the defocus term, primarily affected by peak-to-valley temperature variations along the radial direction of the laser's transmission path, improved from-0.47 to-0.13 following optimization. Additionally, temperature gradients were identified as the main contributor to thermal lensing, resulting in OPD values induced by temperature gradients (OPDt) on the order of 1 and those due to deformation (OPDd) around 10-4. To further mitigate thermal lensing, the impact of doping depth was investigated. The subsurface layer doping scheme demonstrated significant effectiveness, achieving beta of 2.1, 3.0, 3.5 for doping depth of 0.5 mm, 1.0 mm and 3.0 mm, respectively.