| 摘要 |
PURPOSE:To relax the degree of coincidence of the direction of the align of an information element and the direction of a lens, etc. for forming a read use beam, by positioning a track in which a position information element of the shortest period is recorded, in the center part of a track group. CONSTITUTION:On a rotary plate 21, six tracks No.1-6 are arranged like a concentric circle by the same track width. Among diffraction gratings G recorded in each track, that which is smallest period is formed in a position of the track No.4. The track No.4 is in the center part of a recording band constituted of the tracks No.1-6 together with the track No.3. A laser light beam from a laser diode 13 is radiated to a rotary plate 11 through a collimator lens 15, a beam splitter 16 and a condensing lens 17, and a reflected light therefrom is photodetected by a photodetecting device 14 through the condensing lens 17 and the beam splitter 16. |
