An optical pickup head apparatus comprises a hologram element disposed in the flux of light reflected back from an optical disc, and first and second photodetectors respectively having linear element boundaries extending in a predetermined direction and disposed so as to receive two-focal-point images...http://www.google.de/patents/US4929823?utm_source=gb-gplus-sharePatent US4929823 - Optical pickup head with holographic servo signal detection using a spot size detection system
