Patent ID: 8149677

Claim:
An optical pickup unit comprising a diffraction grating that divides light into at least three luminous fluxes, the optical pickup unit condensing the three luminous fluxes to apply at least three focusing spots, independent of each other, onto a signal side of a medium, the diffraction grating being divided into at least four regions, a first region, a second region, a third region, and a fourth region, wherein: the diffraction grating includes: one regional part having the first region and the second region adjacent to the first region, and the other regional part having the third region and the fourth region adjacent to the third region, the diffraction grating is divided into two equal parts, the one regional part and the other regional part, by a border between the second region and the third region adjacent to the second region, the second region and the third region are disposed between the first region and the fourth region, periodic structures of the first to fourth regions each are a periodic structure with a recess and raised-portion repeated in a same direction, the periodic structure of the second region is a periodic structure having a phase different by less than 180 degrees from that of the periodic structure of the first region; the periodic structure of the third region is a periodic structure having a phase different by less than or equal to 180 degrees from that of the periodic structure of the second region; the periodic structure of the fourth region is a periodic structure having a phase different by less than 180 degrees from that of the periodic structure of the third region, the periodic structure of the fourth region is a periodic structure having a phase differing by about 180 degrees from that of the periodic structure of the first region, and the second region is smaller in width than each of the first region and the fourth region, and the third region is smaller in width than each of the first region and the fourth region.