Patent ID: 7751286

Claim:
An optical pickup unit for recording information on and/or reproducing information from a multilayer information recording medium including a plurality of recording surfaces in each of which a guide groove serving as an information track on which the information is to be recorded is spirally formed, wherein a direction of a spiral of the guide groove alternates between the recording surfaces, the optical pickup unit comprising: a light source emitting a light beam; a diffraction element configured to split the light beam emitted from the light source into a main light beam and first and second sub light beams; an objective lens for focusing the main light beam and the first and second sub light beams onto one of the recording surfaces of the multilayer information recording medium; and a plurality of light receiving elements each divided into a plurality of light receiving parts shaped in any manner so as to receive the main light beam and the first and second sub light beams reflected from the one of the recording surfaces, wherein the focused first and second sub light beams are disposed at respective positions at least 3/2 tracks away from the focused main light beam on the one of the recording surfaces, so as to satisfy a condition: h ⁢ ⁢ sin ⁢ ⁢ θ ≥ ( n + 1 2 ) × 0.74 ⁢ ⁢ ( μ ⁢ ⁢ m ) , where h is a distance between the focused main light beam and one of the focused first and second sub light beams on the one of the recording surfaces, θ is an angle formed by the information track and a straight line connecting the focused main light beam and the one of the focused first and second sub light beams on the one of the recording surfaces, and n is an integer greater than or equal to one.