High Density Recording on Conventionally Structured Magneto-Optical Disk by Magnetic Field Modulation
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ABSTRACT Two recent topics on high density recording are reviewed in this paper on conventionally structured MO disk by magnetic field modulation. One topic is the MO disk with a capacity of 650 MB/ • 64 mm. Using a light wavelength of 650 nm and an numerical aperture of an objective lens of 0.52, a track pitch of 0.95 jim and a bit length of 0.34 jim for (1,7) RLL coding has been applied to achieve 650 MB within a size of a Mini Disc(MD). Groove conditions such as the depth and groove duty in a 1.2-mm thick polycarbonate substrate are optimized to reduce the cross-talk from adjacent tracks. As a result, the recording power margin of ---20% is obtained. The disk tilt margin is +_ 0.82 deg and __0.63 deg in the radial and tangential direction, respectively. The other topics is land/groove recording using an optical phase shifter. A maximum carrier level and a minimum cross-talk are achieved simultaneously when an optical phase shifter is inserted into the optical path before an analyzer. The optimum phase shift is 40 deg for recording on land and -40 deg for recording in groove. Recording on 0.5 jim land/0.5 jim groove with 1.2-mm thick substrate is investigated with a bit length of 0.35 jim for (1,7) RLL code using a light wavelength of 693 nm and a numerical aperture of 0.55. MO recording using an optical phase shifter is promising for an areal density of 3.7 Gb/in 2 . Furthermore, when an optical phase shifter is applied to recording on 0.36 jim land/0.36 jim groove with a bit length of 0.258 jim, light wavelength of 642 nm, and NA of 0.7, a capability of 7.0 Gb/in2 using conventionally structured MO disk is shown.
INTRODUCTION Magneto-optical disk are expected to achieve removable storage devices with a huge capacity and high reliability. Especially, the magneto-optical recording by magnetic field modulation enables higher linear density than that by light intensity modulation, because a minute mark edge is not limited by an irradiated light profile but precisely determined by the switching of a magnetic field. Magnetic layers in magneto-optical disks can be roughly divided into two categories; conventionally structured layers and magnetically induced superresolution (MSR) layers[ 1,2]. New technologies such as magnetic amplify'ing magneto-optical system (MAMMOS) [3] and domain wall displacement detection (DWDD)[4] have been recently reported. They are considered to belong to MSR family, but have their own advantages in increasing the carrier level even for 0.1-jim mark. Conventionally structured MO can be compared to the conventional in-plane recording in hard disk drive technology. In contrast, the MSR family may be compared to per589 Mat. Res. Soc. Symp. Proc. Vol. 517 ©1998 Materials Research Society
pendicular recording. The MSR family has a great possibility in increasing areal density due to its higher resolving power than optical limit and due to crosstalk canceling effect. But it is not yet commercially available partly because of the complicated multilayer structure and partly because the laser powe
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