Design and Development of Multi-Emitter High Power Laser Diode Modules

Abstract

The growing demand for high-power laser diode modules for laser-based material processing machines has stimulated the development of a number of architectures that, taking advantage of multiple beam combination techniques, have allowed the realization of multi-emitter devices with unprecedented performance. However, these designs typically rely on roughly approximated relations, which have reached their limit of applications. Therefore, to further scale the output power and increase the brightness, new and more accurate models are necessary. From the market point-of-view, the deployment of high-power multi-emitter modules is limited by the cost per emitted watt, which is proportional to the number of required optical elements and package assembly time. Cost reduction, therefore, requires, again, accurate models to properly optimize the package layout, but also new assembly strategies and tools. The thesis analyzes in detail these two aspects - accurate models and assembly strategies and tools - and presents for both innovative solutions to help to push the technology beyond the current state-of-the-art. In particular, for what concerns the multi-emitter model, a new relation to predict the beam quality at the pigtail fiber input by taking into account the impact of lenses and the distance between two adjacent chips in spatial beam multiplexing has introduced. The model is based on the propagation and transformation of paraxial Gaussian beams and analyzes, not only the impact of the choices on the focal length of each collimating or focusing lens but also of the truncation caused by their finite aperture. Then, as the model requires the knowledge of the individual laser chip beam characteristics, specific benches for the measurement of the near and far field emissions have been developed. The proposed model has been validated in different working conditions and found to lead to an error lower than 6%. As for the multi-emitter assembly, an industrial grade procedure has been devised and a completely new approach based on back-propagation artificial neural network to automatically determine the optimal positioning of each optical element has been developed. The neural network is trained using ray tracing of Gaussian beams, starting from the emission characteristics of the laser chips. The new tool has been tested in practical cases with the most critical of all the components, the positioning of the fast axis collimator, obtaining a reduction of the assembly time of more than 50% with respect to current automatic assembly machines. Finally, the design model and the assembly procedure have been applied to the development of a prototype of a multi-emitter module that, by exploiting spatial, wavelength, and polarization multiplexing of a plurality of chips emitting about 10W each, delivers over 300W in a 105/0.15 fiber pigtail, figures that represent a remarkable improvement over the current state-of-the-art. This result has been very challenging because it required the combination of theoretical, experimental, and technological aspects, not limited to photonics, but including also measurement theory, precision mechanics and thermal management.