Double-sided magneto-optical recording disk

Details Double-sided magneto-optical recording disk Double-sided magneto-optical recording disk

1998-09-04

2001-01-09

Dinh, Tan (Department: 2752)

Dynamic information storage or retrieval

Storage or retrieval by simultaneous application of diverse...

Reexamination Certificate

active

06172945

ABSTRACT:

FIELD OF THE INVENTION
The present invention is in the field of magneto-optical data storage discs. The present invention is more particularly in the field of magneto-optical storage discs for use with an overcoat incident recording technique.
BACKGROUND OF THE INVENTION
Typical magneto-optical (MO) disc drives record data by locally heating a portion of the disc. MO discs, or media, include a recording layer of a magnetic material. The coercivity of the heated portion of the media is lowered when it is heated by the laser beam. This allows the magnetic polarity in that area to be reversed by an applied magnetic field. In such disc drives, data is read from media by illuminating areas of the storage media with a linearly polarized laser beam. The Kerr rotation effect causes the plane of polarization of the illuminating beam to be rotated. The direction of rotation depends on the magnetic polarity in the illuminated area of the storage media. When the disc is read, the polarization rotation is determined with a pair of optical detectors and a polarization beam splitter to produce an output data signal. Limitations of MO disc drives include data access time and density with which data can be stored.
FIG. 1
is a diagram of prior MO recording system
100
typically used with 130 millimeter (mm) diameter MO media. System
100
is an example of a “substrate incident” recording system. In substrate incident systems, laser light is incident on a thick substrate, travels through the substrate, and is focused on a recording layer below the substrate. System
100
includes objective lens
102
for focusing a collimated beam of light on disc
116
. Disc
116
is an example of a typical two-sided MO disc. MO disc
116
includes substrates
104
and
114
forming outside layers on opposing sides of disc
116
. Substrates of
104
and
114
are made of materials such as plastic polycarbonate and are approximately 1.2 mm thick. Recording layer
106
is below substrate
104
, and recording layer
112
is below substrate
114
. Recording layers
106
and
112
can be made out of any one of a number of well-known materials, such as Tb—Fe—Co, a rare-earth transition-metal alloy. The laser light beam passing through objective lens
102
penetrates substrate
104
as shown and is incident on a focal point on the surface of recording layer
106
.
System
100
has several disadvantages. One of the disadvantages of system
100
is that it is necessary to apply energy to the recording layer to erase data prior to writing new data. This is because a large, stationary magnetic coil (not shown) having a large inductance is situated on the opposite side of disc
116
from objective lens
102
to assist in the writing process. Because the coil is held at a relatively great distance form the media surface and has a relatively large inductance, the magnetic field cannot be reversed at high frequencies. Therefore, it is necessary to erase data before writing new data. The necessity of erasing before rewriting slows the process of writing data to disc
116
.
Another disadvantage of system
100
is that the density of data stored on disc
116
is relatively low. A further disadvantage of system
100
is that only one side of disc
116
can be accessed at one time. This is because the relatively large coil occupies the space on the side of the disc opposite the objective lens. This space cannot therefore be used for another lens and actuator. In order to access a different side of disc
116
, disc
116
must be removed, turned over, and reinserted into system
100
. Disc
116
, however, provides good data security because relatively thick substrates
104
and
114
allow disc
116
to be handled without danger of data loss or difficulty in reading data because of contamination. The thickness of substrates
104
and
114
presents difficulties, however. Because the substrate material is applied so thickly, non-uniform heating and cooling characteristics at the outer radii cause non-uniform solidification during the molding process. Therefore, light passing through the substrate at the outer and inner radii is distorted such that birefringence occurs. The edges of the disc are thus unsuitable for data storage, and the data storage capacity of the disc is reduced. In addition, typically, with a thick substrate, some degree of light distortion is experienced throughout the disc.
Another disadvantage of system
100
is its susceptibility to tilt. Tilt is the deviation from 90 degrees of the angle between the axis of incident light and the plane of the recording layer of the media. Because incident light must pass through a relatively thick substrate before reaching a recording layer, the incident light becomes distorted with the occurrence of tilt. This effectively limits the possibility of reducing the spot size of the incident light, and consequentially effectively limits the possibility of increasing the storage density of the MO disc. It is desirable to decrease the spot size because a smaller spot size corresponds to a larger numerical aperture (NA) of the system. There are factors that place an upper limit on NA. One of the factors is tilt. Therefore, in a system such as system
100
, susceptibility to tilt limits the increase of NA and limits storage capacity of the disc.
FIG. 2
is a diagram of another prior MO recording system
200
. Collimated light beam
202
passes through objective lens
204
to disc
216
. Disc
216
includes substrate
206
that is typically 0.6-1.2 mm thick. Disc
216
further includes recording layer
208
between substrate
206
and protective layer
210
. In system
200
, the large, stationary coil of system
100
is replaced by a relatively small coil in flying magnetic recording head
214
. Flying height
212
is maintained by an air bearing created when disc
216
passes under flying magnetic recording head
214
. For writing to disc
216
, a magnetic field created by magnetic recording head
214
is used in conjunction with collimated light
202
which passes through objective lens
204
. The smaller coil of magnetic recording head
214
has less inductance than the large, stationary coil of system
100
. The reduced inductance allows direct overwrite of data on disc
216
by switching the magnetic field.
System
200
still possesses the disadvantage of relatively low storage densities. For example, because system
200
is a substrate incident system, the thick substrate
216
causes system
200
to have the same susceptibility to tilt as system
100
. In addition, disc
216
is a one-sided, rather than a two-sided disc, which reduces overall storage capacity.
System
200
also has the disadvantage of requiring mechanical coupling of light on one side of disc
216
with magnetic recording head
214
on the other side of disc
216
. Typically, this coupling is accomplished by mechanical linkages that pass from objective lens
202
to magnetic recording head
214
around the edge of disc
216
. The mechanical linkages cannot be allowed to interfere with the movement of objective lens
202
or with disc
216
.
FIG. 3
is a diagram of prior MO recording system
300
. System
300
is an example of an “air incident” design in which a lens is held very close to the media and laser light is incident on very thin protective layer
309
that is over recording layer
308
of disc
318
. Other prior art air incident designs may have no protective layer, wherein a surface of recording layer is the surface of the disc. System
300
employs flying magnetic recording head
316
, and a two-piece objective lens comprised of lens
314
and lens
312
. Prior art systems similar to system
300
use other lens designs, for example, single lens or three-piece objective lens designs. Lens
314
is held extremely close to disc
318
. Collimated light beam
302
passes through lens
312
and lens
314
. Lens
312
and lens
314
are integrated with slider
304
and magnetic recording head
316
. Flying height
306
for system
300
is typically less than the wavelength of the laser light used in readin

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