| 摘要 |
PURPOSE:To position a light beam accurately without contact, by coating a thin film-shaped magnetic recording layer on projecting parts between groovs of a round base board, which has plural concentraic circle-shaped or spiral grooves, also and coating a transparent protecting film on this magnetic recording layer. CONSTITUTION:In respect to a medium 21, the width of grooves 24 on a disc 41 is Tw, and the pitch of them is Tp, and the width of projecting parts 25 is Tw equal to that of grooves 24. The depth of grooves 24 is (d) corresponding to the 1/4 of the wavelength of a recording/reproducing light beam (laser beam). A thin film magnetic layer 42 is formed on the disc 41 by the sputtering method or the like, and a tansparent protecting film is formed on this layer 42. If a recording or reproducing laser beam spot 50 is deviated from an information track 51 as shown in figures (b) and (c), the deviation from the information track 51 is detected because a mutual interference light (diffracted light) 52 between reflection lights from parts of grooves having the 1/4 wavelength depth and the part of the information track is transmitted. Thus, the track following control is facilitated, and an optional track is accessed accurately at random. |
