Patent ID: 8189435

Claim:
A spherical aberration detecting device, comprising: a transmission grating for dividing, in a tangential direction, a light beam emitted from a light source into a main beam, a first sub-beam and a second sub-beam; an optical element for focusing the three beams on an optical disk; a polarization hologram for diffracting a light beam reflected by the optical disk; and a photoreceiver for receiving a diffraction ray from the polarization hologram, wherein: the photoreceiver includes first through third light-receiving sections adjacently aligned for respectively receiving +1st order diffraction rays of the main beam, the first sub-beam and the second sub-beam generated in the polarization hologram, the first light-receiving section is divided into two regions by a border extending in a radial direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the main beam from the polarization hologram, so as to detect a spherical aberration, the second light-receiving section is divided into two regions by a border extending in the radial direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the first sub-beam from the polarization hologram, so as to detect a spherical aberration, the second light-receiving section includes two photodiodes having respective light-receiving regions which are identical in size and each of which corresponds to a half of the second light-receiving section, the third light-receiving section is divided into two regions by a border extending in the radial direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the second sub-beam from the polarization hologram, so as to detect a spherical aberration, the third light-receiving section includes two photodiodes having respective light-receiving regions which are identical in size and each of which corresponds to a half of the third light-receiving section, the second and third light-receiving sections are laid out such that an end of the second light-receiving section along the tangential direction and an opposite end of the third light-receiving section along the tangential direction are aligned on a straight line extending in the tangential direction across the first light-receiving section, the tangential direction and the radial direction are perpendicular to each other on a surface on which the first, second, and third light-receiving sections are disposed, and the first, second, and third light-receiving sections are separate light-receiving sections from each other.