Yasushi Hozumi

Physical Characteristics and Format of a Two-Inch-Diameter Magneto-Optical Disk System with 3 GB Capacity

Recording at a high density four times that of a current digital versatile disk random access memory (DVD-RAM) has been achieved by applying the domain wall displacement detection (DWDD) technology using a conventional optical head with the same wavelength of 660 nm and numerical aperture (NA) of 0.60 as those the DVD head. It has been confirmed by using an alternating sampled servo, laser annealing of the inter track area and partial response maximum likelihood (PRML) that a recording density of 15 Gbit/inch2 with a track pitch of 540 nm and bit length of 80 nm has sufficient system margins. A newly developed 1-bit-distributed address format has achieved a redundancy of 23%. Furthermore, the edge shift compensation technique has reduced the recording magnetic field. These indicate that a capacity of 3 GB on a two-inch-diameter disk is now available for mobile use. If applied to a digital video camera, one hour or more of moving-picture-experts-group-2 (MPEG2) video contents can be recorded at a bit rate of 6 Mbps.

Non-bias magneto-optical recording

Without applying an external magnetic field, high-density recording beyond the optical resolution limit was realized with light intensity modulation, using Domain Tail Erasing (DTE) and Domain Wall Displacement Detection (DWDD). In order to obtain a sufficiently strong bias field to perform recording with a bias layer introduced into the DTE medium, the stray field distribution during the recording process was studied by simulation. A stray field most effectively worked on the recording layer when the Curie temperature of the bias layer was designed at a certain level below the recording temperature. Furthermore, by replacing a part of the bias layer with a bias assist layer, which had a higher Curie temperature and lower coercive force, an even stronger stray field was generated. According to the simulation, non-bias DTE medium was designed, and as a result a sufficiently low jitter value and dropout rate were obtained without bias magnets at a bit length of 80 nm and a track pitch of 0.74 μm, using the same optical parameters of a current DVD.

15-Gbit/in2 recording on a DWDD disc using a land/groove substrate with a red laser enabled by a side-wall annealing process

We developed a side-wall-annealing technique for land/groove substrates. By applying this technique to our Domain Wall Displacement Detection (DWDD) Magneto-Optical (MO) recording stack formed on a land/groove substrate, even with an NA of 0.6 and a wavelength of 660 nm, we realized a density of 15 Gbit/in2 with a sufficiently wide recording tolerance. This density corresponds to a capacity of 4.7 GB mm disc like MiniDisc.

Bit Length of 57 nm Recording on Domain Wall Displacement Detection Disk Using Turbo Codes with Blue Laser

Turbo codes with max log maximum a posteriori probability (max-log-MAP) algorithm were experimentally applied to a domain wall displacement detection (DWDD) disk using a blue laser. The max-log-MAP algorithm is an approximation of the maximum a posteriori probability (MAP) algorithm; however, it reduces the amount of calculation and consumption of memory. An effective bit length of 56.5 nm recording was achieved using a partial turbo method with a parity bit rate of 1/17 at a bit error rate (bER) of 4 ×10-5 using a blue laser, although a bit length of 68.5 nm was the limit using a red laser. According to an analysis of the number of errors, the bER of the turbo method was found to depend on the number of consecutive 2-bit errors that occur with the dropout of the 2T pattern, and a jitter value less than 16% was necessary to keep the bER below 4 ×10-5 for the turbo method.

A DWDD disc of 27-Gbit/in2 land-groove recording using a blue laser and side-wall-annealing technique

We realized acceptable read/write performances at a track pitch of 320 nm and a bit length of 75 nm, even with an NA of 0.65 and a wavelength of 405 nm, using side-wall-annealing. In addition to this result, we suggested the possibility of a higher linear density; that is, a jitter value of less than 15% was obtained both on land and in groove at a bit length of 60 nm, which corresponds to an areal density of 34 Gbit/in2.

System tolerance of 64-mm-diameter magneto-optical disks with a 4.7-Gigabyte capacity

We observed sufficiently wide detection tolerances in a magneto-optical (MO) drive system based on the domain-wall-displacement detection (DWDD) method. Those DWDD disks had a capacity of 4.7 Giga-byte on 64 mm diameter substrates optimized for land and groove (L/G) recording. Addressing was performed with the assistance of a one-side-wobbled groove. We investigated two types of DWDD disks i.e. the anneal-less type and the sidewall-annealed type.