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.
High power diode laser arrays or single emitters require either fast or slow axis optical collimation, or both, for further transport or photonic application of high power laser radiation. With varying requirements and conditions for the slow axis collimation and the corresponding engineering solutions, a multitude of options exist from spherical refractive to freeform reflective in single lens elements or in array forms. Here we report on the benefits of reflective slow axis freeform optics and its implementation for the collimation with integrated right angle reflection for compact solutions and miniaturization of module packaging.
High power diode laser arrays need special micro-optic beamshaping to optimize for collimation and spot focusing. A
known and established solution makes use of cylindrical micro- optics for fast axis collimation and subsequent individual
twisting of each emitter beamlet by a micro-optic cylindrical telescope array in order to achieve a symmetric beam
parameter product. A crucial factor for the overall performance is the achievable transmission to collimation and the
fiber coupling efficiency. We present an optical analysis of vignetting effects on the transmission in the beam twisting
setup and show an optimization which can be achieved by advanced micro-optic production technologies.
Diode lasers are gaining importance, making their way to higher output powers along with improved BPP. The assembly of micro-optics for diode laser systems goes along with the highest requirements regarding assembly precision. Assembly costs for micro-optics are driven by the requirements regarding alignment in a submicron and the corresponding challenges induced by adhesive bonding. For micro-optic assembly tasks a major challenge in adhesive bonding at highest precision level is the fact, that the bonding process is irreversible. Accordingly, the first bonding attempt needs to be successful. Today’s UV-curing adhesives inherit shrinkage effects crucial for submicron tolerances of e.g. FACs. The impact of the shrinkage effects can be tackled by a suitable bonding area design, such as minimal adhesive gaps and an adapted shrinkage offset value for the specific assembly parameters. Compensating shrinkage effects is difficult, as the shrinkage of UV-curing adhesives is not constant between two different lots and varies even over the storage period even under ideal circumstances as first test results indicate. An up-to-date characterization of the adhesive appears necessary for maximum precision in optics assembly to reach highest output yields, minimal tolerances and ideal beamshaping results. Therefore, a measurement setup to precisely determine the up-to-date level of shrinkage has been setup. The goal is to provide necessary information on current shrinkage to the operator or assembly cell to adjust the compensation offset on a daily basis. Impacts of this information are expected to be an improved beam shaping result and a first-time-right production.
Micro optic components such as the fast axis collimator (FAC) are in widespread use in the high power diode laser industry. The requirement of high numerical aperture from diode laser and optical design and the pressure of cost to performance ratio have created convergence in the application towards the design of a plano-aspheric cylinder lens with high refractive index. The performance in power transmission and optical collimation of the FAC lens type are dependent on specific factors such as accuracy in form and refractive index, surface roughness and performance of the coating. We present a qualified method for measurement of the collimation efficiency and industrial analysis of the form error, of the surface roughness, of the AR coating as well as the result of a 1000h damp/heat test of the stability of the AR coating.
Broad area high power laser diodes emit in fast axis and slow axis direction very different beam profiles. In fast axis direction a nearly diffraction limited Gaussian beam, but in the slow axis direction a multi-mode beam profile. We present a top hat solution for individual fast axis and slow axis beam shaping using micro optical components for a standard high power laser diode bar with a wavelength of 810 nm. The compact design allows the individual beam shaping in a very small space. The quality of the beam shaping depends mainly on the precision of the micro optics and the advanced assembly process. These solutions enable the development of laser diode modules with individual and application specific beam shapes and small dimensions.
Single-mode-emitting high-power diode laser arrays (SM-HPDLA) are available industrially with more than 50 W
emission power per bar. Based on this platform an expandable prototype solution is realized for fiber coupling of a
stacked array with more than 100 W to an optical fiber with diameter of 200 micron and NA of 0.11. Advanced methods
of controlled assembly of micro-optics by infrared laser-soldering have been developed therefore. We present a compact
and scalable concept with scalability on 2 internal and 2 external factors. Internal factors are the increasing beam quality
and power stability of high-power single-mode-emitting arrays and the improved assembly accuracy for diode bar and
micro-optics. External factors are the interlaced coupling of stacked beam emission from the stacked array and the
further option to use optimized polarisation coupling with several diode laser stacks.