The divergence of laser diodes is asymmetric and must be collimated in the fast-axis and slow-axis to reach an adequate beam shape for most applications. The most common technical solution is a combination of a Fast Axis Lens (FAC) and a Slow Axis Lens (SAC). These optical components are usually made of glass in combination with an anti-reflective optical coating tuned for a specific wavelength range. During the last decade, high power lasers have become more and more powerful and the requirements for specific collimation optics continuously increased. The FAC beam shaping performance is dependent mostly on the lens design and achieved surface quality, while the thermal behavior of the FAC is dependent on the laser power and the optical absorption within the lens. The solution for a low absorption lens for a high power blue laser diode presented in this paper, is a fused silica FAC. It shows excellent thermal properties and reduces heat generation rate by a factor approximately corresponding to the extinction value ratio when compared to other high refractive glass solutions optimized for blue applications.
As power densities of laser diodes continuously increase, the effects of absorption losses in fast axis collimation lenses become exceedingly important. We report our analysis of two drivers of these absorption losses, coating absorption and glass bulk absorption, and how these absorption losses cause a thermal impact and have an influence on the performance of the laser beam quality emitted from a laser diode equipped with a fast axis collimation lens on a bottom tab. The presented results are derived from finite element method (FEM) simulations and the FEM model used is based on material data from data sheets and a heat transfer coefficient derived from cooling curves of components observed by a thermal infrared camera.
Methods for thermal modeling, analysis and optimization are presented. Modeling distinguishes between three basic types of lens mounts. For analysis also FEA-results are incorporated in the optical model. Optimization with a simple, glass substitution method and a paraxial method is presented.