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° (1134 mrad, fast axis) and 24° (418 mrad, slow axis) to 7.7 and 136.5 mrad, respectively.