High power diode lasers (HPDL) have become very attractive as an intelligent tool for direct material processing as well as pump sources for miniaturized solid state lasers and fiber lasers since optical beam transformation systems generate the required beam circularization from the originally highly asymmetric brightness of the output beam. This presentation focuses on reliable microassembling processes to realize micro-optical beam transformation systems for high power diode laser bars and stacks for the efficient coupling of the beam into optical fibers at reasonable costs. The modular semi automatic micro assembly set up realized consists of a six axes alignment system with an accuracy in the submicron range for the three spatial axes as well as microrad for the three rotational axes, a semi-automatic glue dispenser as well as respective grippers and magazines which are necessary for the handling of the micro optical components. The established automation processes for the mounting of the fast axis collimation (FAC) lenses are of particular interest for the assembly of e.g. fiber coupled modules of diode laser stacks whereas more than 20 fast-axis collimations are necessary.

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The assembly of fast-axis collimating lenses (FAC-) with high power laser diode (HPLD) bars and stacks is discussed. FAC-lenses are cylindrical lenses of high numerical aperture (NA.> 0.5) and short focal length (200 .tm..1 mm) for the collimation of the strongly divergent radiation emitted perpendicular to the active region of a high power laser diode bar. These FAC-lenses are necessary for all kind of beam transformation systems using HPLD bars and stacks. The lenses have to be fixed at the heat sink ofthe laser diode bar with an accuracy of < 0.3 im and for special transformation systems with a defmite tilt of 1 ..2°and an accuracy of better than mrad. In most cases an active assembly of the whole beam transformation system is necessary. Here we propose another approach. . A master system is assembled in a traditional way. The diode bar and the fixed FAC-lens are removed from the system.. The intensity distribution behind the FAC-lens in connection with conventional cylindrical lens for slow axis imaging is measured and compared to a theoretical system design. . Stops are fabricated according to the intensity distribution and placed in a defmite distance to the laser diode bar.. FAC lenses are mounted in front of a single diode bar by maximizing the total power passing the stop system. With the help of this approach a first step towards a semi-automated assembly is done for fiber coupling of a laser diode bar into a 200 tm multimode optical fiber ofN.A.= 0.2. The system requires a slightly tilted FAC-lens of very short focal length (300 .tm) and hence a high accuracy of the lens assembly.

High-power quasi-CW laser bars are of great interest as pump sources of solid-state lasers generating high-energy ultrashort pulses for high energy projects. These applications require a continuous improvement of the laser diodes for reliable optical output powers and simultaneously high electrical-to-optical power efficiencies. An overview is presented of recent progress at JENOPTIK in the development of commercial quasi-CW laser bars emitting around 880 nm and 940 nm optimized for peak performance. At first, performances of 1.5 mm long laser bars with 75% fill-factor are presented. Both, 880 nm and 940 nm laser bars deliver reliable power of 500 W with wall-plug-efficiencies (WPE) >55% within narrow beam divergence angles of 11° and 45° in slow-axis and fast-axis directions, respectively. The reliability tests at 500 W powers under application quasi-CW conditions are ongoing. Moreover, laser bars emitting at 880 nm tested under 100 µs current pulse duration deliver 1 kW output power at 0.9 kA with only a small degradation of the slope efficiency. The applications of 940 nm laser bars require longer optical pulses and higher repetition rates (1-2 ms, ~10 Hz). In order to achieve output powers at the level of 1 kW under such long pulse duration, heating of the laser active region has to be minimized. Power-voltage-current characteristics of 4 mm long cavity bars with 50% fill-factor based on an optimized laser structure for strong carrier confinement and low resistivity were measured. We report an output power of 0.8 kW at 0.8 A with >60% conversion efficiency (52% WPE). By increasing the fill-factor of the bars a further improvement of the WPE at high currents is expected.