Patent ID: 7072270

Claim:
A method for detecting optical data by applying light from a light source to a recording medium and detecting by a photo-detector the light which has been reflected by the recording medium and which carries data from the recording medium, the method comprising the steps of: diffracting the reflected light into ±1 diffraction orders and zeroth diffraction order so as to dispose the focal points of the ±1 diffraction orders to be offset from each other along the optical axis of the zeroth diffraction order with the focal point of the zeroth diffraction order being between the focal points of the ±1 diffraction orders; detecting the zeroth diffraction order by the photo-detector by increasing the diameter of the zeroth diffraction order in a direction substantially perpendicular to the direction of a track of the recording medium; and detecting the ±1 diffraction orders by the photo-detector, the ±1 diffraction orders forming circular light spots on the photo-detector.