Assembly of fast-axis collimating lenses with high power laser diode bars

The assembly of fast-axis collimating lenses (FAC-) with high power laser diode (HPLD) bars and stacks is discussed. FAG-lenses are cylindrical lenses of high numerical aperture (N.A.> 0.5) and short focal length (200 mu m..1 mm) for the collimation of the strongly divergent radiation emitted perpendicular to the active region of a high power laser diode bar. These FAG-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 of the laser diode bar with an accuracy of < 0.3 mu m and for special transformation systems with a definite tilt of 1..2 degrees 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 FAG-lens are removed from the system. The intensity distribution behind the FAG-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 definite 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 mu m multimode optical fiber of N.A.=0.2. The system requires a slightly tilted FAG-lens of very short focal length (300 mu m) and hence a high accuracy of the lens assembly.