Optical information processing unit

Claim 1

An optical information processing unit that functions by guiding light from a light source to an information recording medium, comprising: a photosensor for detecting reflected light from said information recording medium, said reflected light traveling in a light path between the information recording medium and the photosensor; and a diffraction element for diffracting the reflected light, wherein said diffraction element comprises a first diffraction grating and a second diffraction grating placed such that grating lines of the first diffraction grating are approximately rectangular to grating lines of the second diffraction grating, said first and second diffraction gratings each having a diameter larger than a diameter of flux of the reflected light, and said first and second diffraction gratings being spatially overlapped wherein a focusing error signal is detected from both of +1 order and -1 order diffracted lights of one of either diffracted light from the first diffraction grating or diffracted light from the second diffraction grating by a spot size detection method, focus positions of said +1 order and -1 order diffracted lights existing in front of and behind said photosensor, while a tracking error signal is detected from diffracted light from the one of either the first diffraction grating or the second diffraction grating from which the focusing error signal is not detected.