Beam-shaping technique for high brightness with high-power diode laser arrays

Fulltext: Because of the asymmetric emission characteristics of high-power diode laser bars, beam shaping is essential for coupling the light into a fibre. Preservation of the high brightness is the crucial point for minimum core diameter and NA. The radiation of two high-power diode laser bars is collimated in fast-axis direction by microlenses and polarisation multiplexed. A telecentric optics, mainly consisting of a concave and a convex mirror (Offner configuration) images the emitters on the retroreflecting mirrors. The surface of these mirrors consists of right angle v-grooves, whose pitch is matched to the emitter pitch of the diode laser. They are aligned at 45 degree in respect to the fast-axis direction of the incident beam. Therefore each image of an emitter is rotated by 90 degree when it is reflected from one side of a groove to the other. In the emerging beam after beam shaping the emitters are arranged in fast-axis direction. A polarisation beam splitter deflects the emer ging beam out of the incident-beam direction. The fast-axis is recollimated with a lenticular lens at the position, where maximum filling factor is obtained. A set of lenses couples the light into the fibre. As a result a spot size of less than 160 mu m to 80 mu m at NA 0.2 was achieved with a prototype setup. With the appropriate coatings a power transmission efficiency of more than 70 per cent is expected.