The high-power diode-laser industry is dependent on highly qualified production output for fast axis collimation (FAC)
lenses in scalable quantities. Not only controlled production processes but also the optical qualification of the final FAC
product contribute to this goal. The product qualification approach chosen is based on diode laser collimation by active
alignment and automated positioning of the FAC in collimation for the fast axis and imaging in the slow axis, allowing
to evaluate the collimation performance with a specific and installed diode laser on individual emitter intensity profiles.
We present the automated setup with results for corresponding sensitivity and repeatability analysis for the measurement
of residual divergence.
KEYWORDS: Silica, Finite element methods, Absorption, Glasses, High power lasers, Semiconductor lasers, Temperature metrology, Refractive index, Collimators, Diodes
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.
Similar to the well-established high power laser diodes in the infrared wavelength range, the laser diodes in the blue wavelength range require tailored optics for beam shaping, to make the light usable for a variety of applications. High power laser diode arrays or single emitters require fast and slow axis optical collimation for further transport or photonics applications using high power laser radiation. With increasing requirements in higher brightness for slow axis collimation different engineering solutions exist. By using novel production technologies, e.g. precision molding, approaches that were considered too expensive for mass production become available to broad application fields. Here we report about the benefits of molded refractive, freeform slow axis collimation optics and compare them to the ubiquitous standard circular cylindrical, as well as acircular cylindrical slow axis collimation optics. By using refractive free form slow axis collimation optics it is possible to achieve significantly better brightness compared to circular cylindrical or acircular cylindrical slow axis collimation optics.
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