For use in combination with an optical source for generating a coherent beam along a main optical axis, an optical head device comprises a diffraction grating having a plurality of grating regions which are responsive to the coherent beam for directing a zeroth-order diffracted beam to a focussing lens...http://www.google.de/patents/US4945529?utm_source=gb-gplus-sharePatent US4945529 - Optical head comprising a diffraction grating for directing two or more diffracted beams to optical detectors
