Monolithic bifocal zone-plate lenses for confocal collimation of laser diodes

An elliptical monolithic bifocal zone plate as a collimating lens, whose two focal lengths in the orthogonal directions match the different beam waist positions of the fast and slow axes' light from edge-emitting laser diodes, is proposed and experimentally demonstrated by employing femtosecond laser direct writing technology. The high-quality eight-level zone plate exhibits a diffraction efficiency of 92.9%, which is much higher than those ever reported. Shaped by the elliptical lens, the laser diodes' divergence angles are simultaneously reduced from 65 degrees (1134 mrad, fast axis) and 24 degrees (418 mrad, slow axis) to 7.7 and 136.5 mrad, respectively. (C) 2013 Optical Society of America