Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1998-11-12
2002-07-16
Huber, Paul W. (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S275400
Reexamination Certificate
active
06421313
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to encoding information, and relates more particularly to a system and method for encoding read-only information on storage media.
2. Description of the Background Art
Efficient, economic, and reliable storage of digital data is an important consideration of manufacturers, designers, and users of computing systems. In magneto-optical storage devices, digital data is typically stored in tracks located on rotating disks of magneto-optical (MO) storage media. Close positioning of the adjacent disk tracks maximizes the amount of stored data on a storage disk, thus providing significant economic benefits to system manufacturers and users. Therefore, system designers frequently seek new and improved methods of reducing track pitch to permit greater storage capacity on the storage media. As track pitch is reduced, differentiating between the tracks becomes of even greater importance for efficient and reliable storage of data.
Referring now to
FIG. 1
, a plan view of a front surface
112
of a magneto-optical storage medium
110
is shown. In MO storage devices, digital data is typically written into and read from a series of concentric or spiral tracks
114
located within a plurality of data sectors
177
on surface
112
of storage medium
110
. The digital data is read from and written to surface
112
of storage medium
110
by projecting a laser-generated light spot from a magneto-optical head onto a selected track
114
while storage medium
110
is rotating, and then sensing the polarization of light reflected back from storage medium
110
.
The head must be accurately positioned above track
114
of rotating storage medium
110
during a read/write operation on that track. Many factors, for example imperfections in track symmetry, may cause the head to be positioned slightly off the center of track
114
. Positional correction of the head is therefore required for acceptable performance during a read/write operation.
One prior art position correction technique utilizes a diffraction pattern to generate a position error signal from grooves that are positioned between tracks on the storage medium. Another correction technique utilizes a plurality of servo sectors
178
. Each servo sector
178
contains read-only information that indicates the position of the head on storage medium
110
. This read-only information may be in the form of position marks permanently embossed on surface
112
of storage medium
110
at manufacture. The position marks may be used to generate a position signal, which may then provide feedback to compensate for position errors by adjusting the position of the head.
Referring now to FIG.
2
(
a
), a diagram of position marks on sample storage media tracks within a servo sector
178
is shown. FIG.
2
(
a
) includes sample tracks
0
through
4
. Five tracks are presented for purposes of illustration, however storage medium
110
typically contains a significantly greater number of tracks. Furthermore, FIG.
2
(
a
) depicts track
0
through track
4
as straight, whereas in practice they are typically circular.
As shown in FIG.
2
(
a
), each track has three associated position marks which may be repeated at selected intervals along their corresponding track. The position marks are formed by depressions in surface
112
of storage medium
110
. The ellipses shown in FIG.
2
(
a
) represent the full-width-half-maximum dimensions of the depressions. The full-width-half-maximum dimensions of a depression are its dimensions at a plane located halfway between surface
112
and the bottom of the depression. When optical spot
220
(the full-width-half-maximum dimensions of the light spot from the head) travels over a position mark, the diffraction pattern is such that most of the light is not reflected back to the head. A resulting pulse occurs in a detected reflectivity signal that is based on the amount of light reflected back from storage medium
110
to the head.
Referring now to FIG.
2
(
b
), a drawing of a reflectivity waveform corresponding to position marks
210
,
212
, and
214
is shown. During a read/write operation on track
4
, the head is positioned over track
4
as storage medium
110
rotates at a selected rate of speed. The head initially encounters position mark
210
, which is a radial bar created by overlapping elliptical depressions. When optical spot
220
passes over position mark
210
, the amplitude of reflected light is reduced, generating negative-going sync pulse
230
at time
240
. Ideally, a position mark would cause the reflectivity signal to fall to zero as optical spot
220
passes directly over the mark. In practice, position mark
210
is detected when the reflectivity signal becomes small, as represented by sync pulse
230
.
Next, the head encounters position mark
212
, which is positioned at a specified perpendicular distance off-center from track
4
. Position mark
212
generates a negative-going pulse “A” at time
242
. The amplitude of pulse A is relatively less than the amplitude of sync pulse
230
because optical spot
220
does not pass directly over position mark
212
. Next, the head encounters position mark
214
, which is positioned at the same specified distance off-center of track
4
, but in the opposite direction of position mark
212
. Position mark
214
generates a negative-going pulse “B” at time
244
. The amplitude of pulse B is also relatively less than the amplitude of sync pulse
230
. A radial position error signal (PES) for the head may then be obtained by taking the difference of the peak reflectivity amplitudes of pulse A and pulse B.
In some designs for MO storage devices, optical spot
220
has a linear plane of polarization with a direction that cannot be controlled. The amount of light reflected back to the head as the head passes over a position mark may be affected by the unpredictable direction of the plane of polarization of optical spot
220
.
For example, optical spot
220
may have a plane of polarization that is parallel to position mark
210
. A diffraction pattern created by optical spot
220
passing over position mark
210
may be such that a significant amount of light is reflected back to the head. A resulting pulse in the reflectivity signal may not be large enough to indicate the presence of a position mark.
Undetected position marks create errors in the read-only information being read from servo sectors
178
. Errors in the read-only information read from servo sectors
178
cause the MO storage device to perform unreliably. Therefore, an improved system and method are needed to encode read-only information on storage media.
SUMMARY OF THE INVENTION
In accordance with the present invention, a system and method are disclosed to encode read-only information on storage media. One embodiment of the present invention is implemented in the context of a magneto-optical storage device. In the magneto-optical storage device, read-only information is encoded in servo sectors on surfaces of magneto-optical storage media.
One embodiment of the present invention includes a magneto-optical storage medium, a plurality of position marks disposed on a surface of the storage medium, and a light beam directed towards the position marks to produce a reflection of the light beam from the storage medium. The light beam is preferably a single-frequency laser beam.
The position marks are configured whereby the reflection of the laser beam is not responsive to a plane of polarization of the laser beam. In one embodiment, the position marks comprise substantially circular pits. The reflection from each of the substantially circular pits is not affected by the direction of the plane of polarization. The dimensions of the substantially circular pits depend on a wavelength of the light beam and a numerical aperture of a lens that directs the light beam towards the position marks. Each of the substantially circular pits has a depth of approximately one-quarter of the wavelength of the light beam.
The substanti
Huber Paul W.
Koerner Gregory J.
Seagate Technology LLC
Simon Nancy R.
Simon & Koerner LLP
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