Patent ID: 8174939

Claim:
An optical head, comprising: a light source for emitting a light; a diffraction element for splitting the light emitted from the light source into a main luminous flux that is not diffracted and a pair of auxiliary luminous fluxes that are diffracted and polarized; and an objective lens for focusing the main luminous flux and the pair of auxiliary luminous fluxes on an optical disc, wherein the diffraction element is divided in a direction corresponding to a radial direction of the optical disc into a first area, a second area adjacent to the first area and a third area adjacent to the second area; wherein the second area is further divided into a fourth area and a fifth area adjacent to the fourth area; wherein each divided area has a grating pattern of a periodic structure; wherein the periodic structure of the first area is displaced from that of the fourth area by a ¼ period, the periodic structure of the third area is displaced from that of the first area by a ½ period, and the periodic structure of the fifth area is displaced from that of the third area by a ¼ period and from that of the fourth area by a ½ period; wherein light diffracted by each of the first area, the third area, the fourth area and the fifth area has a different phase; wherein the phase of the light diffracted by the first area is shifted from the phase of the light diffracted by the fourth area by −π/2 radian, the phase of the light diffracted by the third area is shifted from the phase of the light diffracted by the fourth area by +π/2 radian, and the phase of the light diffracted by the fifth area is shifted from the phase of the light diffracted by the fourth area by π radian; wherein the diffraction element transmits the main luminous flux without application of a phase distribution; and wherein the pair of auxiliary luminous fluxes have phases opposite to each other, and are converted into a wavefront having a four-stage phase distribution including −π/2 radian by the first area, 0 radian by the fourth area, π radian by the fifth area, and +π/2 radian by the third area in the radial direction of the optical disc.