Patent ID: 7706236

Claim:
An optical pickup comprising: a light source which emits a light beam of a predetermined wavelength; a diffractive element which splits the light beam emitted from the light source into at least three light beams; an objective lens which separately converges the three light beams onto a signal recording surface of an optical disc; and a photodetector which has a light receiving surface that receives a returning light from the optical disc, wherein the diffractive element has first to fourth areas provided with a predetermined periodic structure in each of the areas, the second and third areas are arranged adjacently wherein phases of the periodic structures of the second and third areas are in anti-phase, the first area is arranged on the opposite side of the third area adjacently to the second area, and a phase of the periodic structure in the first area is in anti-phase with a phase of the periodic structure in the second area, the fourth area is arranged on the opposite side of the second area adjacently to the third area, and a phase of the periodic structure in the fourth area is in anti-phase with a phase of the periodic structure in the third area, and the objective lens and the diffractive element satisfy the relationship W 12 +W 13 =φ 1 ·α 1 , where W 12 is a width of the second area, W 13 is a width of the third area, φ 1 is a pupil diameter of the objective lens, and α 1 is a value between 0.95 and 1.05.