MULTI-BIT CELLULAR REFLECTIVITY MODULATION FOR OPTICAL MEDIA

MODULATION DE LA REFLECTIVITE DE CELLULES DE STOCKAGE DE BITS POUR SUPPORT OPTIQUE

English Abstract

MULTI-BIT CELLULAR REFLECTIVITY
MODULATION FOR OPTICAL MEDIA
ABSTRACT OF THE DISCLOSURE
An optical storage device, comprising a disk
having a recording surface for reflecting laser light
incident thereon, and a plurality of cells representing
stored data. Each of the cells is characterized by one of
2N predetermined levels of effective reflectivity to the
incident laser light, where N>1 and wherein N represents the
number of bits stored per cell.

Claims

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
1. An optical storage device, comprising a disk
having a recording surface for reflecting laser light
incident thereon, and a plurality of cells representing
stored data, each of said cells being characterized by one
of 2N predetermined levels of effective reflectivity to said
laser light, where N>1 and wherein N represents the number
of bits stored per cell.
2. The optical storage device of claim 1, wherein a
first one of said cells is characterized by a first level of
effective reflectivity for reflecting a first predetermined
amount of said laser light, a second one of said cells is
characterized by a second level of effective reflectivity
for reflecting a second predetermined amount of said laser
light less than said first predetermined amount, a third one
of said cells is characterized by a third level of effective
reflectivity for reflecting a third predetermined amount of
said laser light less than said second predetermined amount,
and a fourth one of said cells is characterized by a fourth
level of effective reflectivity for reflecting a fourth
predetermined amount of said laser light less than said
third predetermined amount.
3. The optical storage device of claim 2, wherein
said first one of said cells comprises a flat portion of
said surface, and said second, third and fourth ones of said
cells comprise a plurality of pits in said recording surface
having respective depths of approximately 1/3(.lambda./4), 2/3(.lambda./4)
and .lambda./4, wherein .lambda. is the wavelength of said laser light.
4. The optical storage device of claim 2, wherein
said first one of said cells comprises a flat portion of
said surface, and said second, third and fourth ones of said
cells comprise a plurality of bubbles having respective
curvatures and size characteristics for defining said
predetermined levels of effective reflectivity.

5. The optical storage device of claim 3 or 4,
wherein the first one of said cells represents the bits 00,
the second one of said cells represents the bits 01, the
third one of said cells represents the bits 11, and the
fourth one of said cells represents the bits 10.
6. The optical storage device of claim 3 or 4,
wherein the first one of said cells represents no change in
said bits, the second one of said cells represents
complementing a least significant one of said bits, the
third one of said cells represents complementing both said
least significant one of said bits and a most significant
one of said bits, and the fourth one of said cells
represents complementing said most significant one of said
bits.
7. The optical storage device of claim 1, wherein for
each of said cells d/b = 0.6 for .lambda./4, where d is the
diameter of each of said cells and b is the diameter of said
laser light.
8. A method of encoding data into a WORM disk
utilizing a laser beam having a wavelength of .lambda., comprising
the steps of:
a) formatting said disc:
b) mapping two-bit data into four cell types
designated as cell type 1, cell type 2, cell type 3 and cell
type 4, wherein cell type 1 is characterized by a first
level of effective reflectivity for reflecting a first
predetermined amount of said laser beam, cell type 2 is
characterized by a second level of effective reflectivity
for reflecting a second predetermined amount of said laser
light less than said first predetermined amount, cell type 3
is characterized by a third level of effective reflectivity
for reflecting A third predetermined amount of said laser
light less than said second predetermined amount, and cell
type 4 is characterized by a fourth level of effective
reflectivity for reflecting a fourth predetermined amount of
said laser light less than said third predetermined amount;

c) setting said laser light at 1/3 of the power
required to create a cell having said fourth level of
effective reflectivity;
d) simultaneously rotating said disk a first
revolution and pulsing said laser light at predetermined
times for exposing cell type 2, cell type 3 and cell type 4
with said laser light at 1/3 of the power required to create
a cell having said fourth level of effective reflectivity;
e) rotating said disk an additional revolution and
reading cell type 2, cell type 3 and cell type 4 and
identifying any ones of cell type 2, cell type 3 or cell
type 4 having excessive threshold values;
f) simultaneously rotating said disk an additional
revolution and pulsing said laser light at predetermined
times for exposing predetermined ones of cell type 2, cell
type 3 and cell type 4 identified in step e) as having
excessive threshold values;
g) simultaneously rotating said disk an additional
revolution and pulsing said laser light at predetermined
times for exposing cell type 3 and cell type 4 with said
laser light at 1/3 of the power required to create a cell
having said fourth level of effective reflectivity;
h) rotating said disk an additional revolution and
reading cell type 3 and cell type 4 and identifying any ones
of said cell type 3 or cell type 4 having excessive
threshold values;
i) simultaneously rotating said disk an additional
revolution and pulsing said laser light at predetermined
times for exposing predetermined ones of cell type 3 and
cell type 4 identified in step h) as having excessive
threshold values;
j) simultaneously rotating said disk an additional
revolution and pulsing said laser light at predetermined
times for exposing cell type 4 with said laser light at 1/3
of the power required to create a cell having said fourth
level of effective reflectivity;
k) rotating said disk an additional revolution and
reading cell type 4 and identifying any ones of said cell
type 4 having excessive threshold values; and
l) simultaneously rotating said disk an additional
revolution and pulsing said laser light at predetermined

times for exposing predetermined ones of said cell type 4
identified in step k) as having excessive threshold values.
9. The method of claim 8, wherein said power required
to create a cell having said fourth level of effective
reflectivity is approximately 9 milliwatts.
10. A method of reading multi-bit per cell information
from an optical disk using a low power laser beam having a
wavelength of .lambda., wherein said information is encoded in said
disk as four cell types designated as cell type 1, cell type
2, cell type 3 and cell type 4, and wherein cell type 1 is
characterized by a first level of effective reflectivity for
reflecting a first predetermined amount of said laser beam,
cell type 2 is characterized by a second level of effective
reflectivity for reflecting a second predetermined amount of
said laser beam less than said first predetermined amount,
cell type 3 is characterized by a third level of effective
reflectivity for reflecting a third predetermined amount of
said laser beam less than said second predetermined amount,
and cell type 4 is characterized by a fourth level of
effective reflectivity for reflecting a fourth predetermined
amount of said laser beam less than said third predetermined
amount, comprising the steps of:
a) simultaneously rotating said disk and exposing
said disk to said low power laser beam causing modulation in
the amount of light returned from said disc; and
b) detecting said light returned from said disk
and converting said light returned by thresholding into any
one of four two-bit codes representing respective ones of
said cell types.
11. The method of claim 10, further comprising the
step of decompressing said light returned into a range of
from a base voltage representing minimal strength of said
returned light to a ceiling voltage representing maximum
strength of said returned light.
12. The method of claim 11, wherein said thresholding
is effected by means of determining in which one of four

voltage ranges said signal resides, and in response
assigning the corresponding one of said four two-bit codes.
13. The method of claim 12, wherein said four voltage
ranges and corresponding two-bit codes are as follows:
<IMG>
14. The method of claim 13, wherein said four voltage
ranges and corresponding changes to said two-bit codes are
as follows:
<IMG>
wherein LSB denotes a least significant bit of said two-bit
code and MSB denotes a most significant bit of said two-bit
code.

Description

CA9-92-005 208597~ ~ ~
MVLTI-BIT CELLULAR REFLECTIVITY ;~
MODULATION FOR OPTICAL MEDIA
Field of the Invention
This invention relates in general to optical disk :~
storage devices, and more particularly to multi-bit cellular
reflectivity modulation for CD-ROM and Write-Once-Read-Many -~
(WORM) drives. ~
Background of the Invention ~-
As computer power grows and computers deal more easily
with huge amounts of data, the need to store and retrieve
large amounts of data has become an industry need. The most
effective and compact method currently known for storing ~-~
digital data is via optical media. However, while optical
devices have hugely increased rates for data retrieval
dencity over magnetic media, the need still exists to
further increa~e data storage density on such media in order
to increa~e storage and data transfer rate~.
WORM and CD-ROM drives use pits to represent Gray level
changes of binary data on disk surfaces. The method for
creating the pits, and formatting of the media varies
between WORM and CD-ROM systems. However, in both prior art
~ystems, laser light incident on the media surface reflects
differently from a pit than from the flat surface of the
optlcal media. The pits are usually in the form of either a
bubble, or a depression on the disk surface.
The principal difference between WORM and CD-ROM disks
i8 that the WORM dis1c is made of material that can be
written to once. This means that the u~er must first format
the WORM disk, and then write data to be formatted on the
disk. By way of contrast, the CD-ROM optical media can only
be read from. Informhtion stored on a CD-ROM is first
copied to a ma~ter disk, at a production ~ite, and multiple
copie~ of the CD-ROM are then made from the macter disk, for
purchase by the end user. Thu~, as di~cu~ed above, the
principal difference between a CD-ROM and a WORM drive is
that the CD-ROM is not formatted for writing (i.e. instead,
it is pre-formatted with data) and cannot be written to,
whereas data may be written to a WORM disk one time for
subsequent read-back many times.

CA9-92-005 208~97~
A pre-formatted CD-ROM and a WORM disk on which data
has been recorded, are characterized by similar disk
structure. The disk is approximately 120 mm in diameter,
1.2 mm thick, and has a hole 15 mm in diameter across the
centre. The information is represented in a spiral of
either small pits or bubbles within the disk. That surface
is coated with a reflective layer, which is further coated
with a protective lac~uer. Traditionally, the pits have
been characterized 0.12 ~m in height or depth and
approximately 0.6 ~m wide. Successive turns of the spiral
patterns of pits or bubbles are arranged 1.6 ~m apart. This
spacing results in a track density of 16,000 cells per inch.
Between each pit or bubble, is a flat area referred to as a
land, such that the distance between successive pits in the
spiral is traditionally in the range of from 0.9 - 3.3 ~m.
Notwithstanding the differences in the recording of
data on CD-ROMs and WORM drives, the technique for reading
data from either a CD-ROM or WORM disk at a given cell is
the #ame.
In order to read data from either a WORM drive or
CD-ROM, a laser beam i9 focussed on the spiral tracks of
pits or bubbles and the light is reflected back through an
objective lens and measured. Light striking the pits or
bubbles is diffracted through a wide angle such that very
little of the light is returned to the lens. However, when
the laser light focusses on the flat land surface between
successive pits, most of the light is reflected back into
the objective lens. The modulated signal produced by the
combination of reflection, diffraction and interference of
light represent6 the information stored on the disk. The
reflected light iB received by a photodetector adjacent the
len~, which produce6 a current proportional to the light
intensity. Thus, the reflected llght signal changes each
tlme the la~er beam moves from a pit to a land, or vice
versa .
One of the primary objectives in all digital storage
devices, is to increase data density or capacity of the
storage device relative to the storage volume or area. It
is known in the prior art to increase data density by
decreasing the size of the pits or bubbles and the distance
between them. However, with a wavelength of approximately

CA9-92-005 3 2085974
:
0.6 microns for the read laser light, the limitations in
focussing of the beam on reduced sizes of the pits or
bubbles are significant. The well known Rayleigh Rule
quantifies the minimum resolution of lenses, while the
Heisenberg Uncertainty Principle dictates the uncertainty of
the position of an electron relative to its momentum. In
accordance with these laws, it has been determined that the
minimum possible resolution of focus in an optical system is
approximately ~/4~N, where N is the numerical aperture of
the lens. Therefore, it is virtually impossible to create
smaller optical cells than are currently available (eg. ~/2)
since in order to focus a laser beam on data cells which are
more densely distributed than in the current art without
resorting to quantum mechanical techniques.
Accordingly, various attempts have been made to encode
greater amounts of data into optical storage media.
For example, IBM0 Report RJ-3287 entitled Frequency
Domain Optical Storage, teaches a method for maximizing
optlcal recording storage denslty by storing information in
the frequency spectrum at each spatial spot.
U.S. Patent 4,963,464 (Setani) discloses an optical
medium having two or more bit streams of data stored
thereon, wherein one bit stream of data is distinguished
from another bit stream of data by the depth of the pits
utilized to represent the two data streams. Thus, by
interleaving pits of different depth in the spiral track,
two or more streams of data may be s.imultaneously stored.
However, according to the Setani Patent, each cell
represents a slngle bit of data.
U.S. Patent 5,060,223 (Segawa) describes an optical
information recording medium having specified pit and guide
groove shapes, where the pit depth varies from 0.46 ~/n to
0.58 ~/n in the preferred embodiment. Thus, the system of
Segawa provides guide grooves for tracking whlch are formed
ln a concentrlc or ~plral shape at a predetermlned pltch, as
well as pre-formatted pits formed approximately mld-way
between the guide grooves. As with the Setani Patent
discussed above, Segawa is limited in its teachings to
representing a single bit of data at each cell location.
Other prior art is known which considers the
consequence~ of varying pit depth in an optical storage
. .

CA9-92-005 4
--- 208~974
medium. For example, IBM Report RC-9860 entitled Two
Dimensional Modelling of Optical D.isk Read-Out, discloses a
model to examine the effects of data density, phase pit
depth, etc., on the quality of a read out signal.
IBM Technical Disclosure Bulletin, volume 34, number
lOb, 3/92, entitled Three Level Mask Method for Gray Scale
Printing, discloses ninety-nine "gray" levels of light
absorption and reflectance in a "super pixel" for
applications involving electro-photographic printing and
other processes.
U.S. Patent 4,852,076 (Ohta et al) describes an optical
information recording and reproducing disk employing a film
having thermally changeable optical characteristics, where
the depth and width of the pits are within =/- 10% normally,
and the material reflectivity of the disk is described as
being variable from 13% to 23% due to heat distortion in
fabrication of the disk.
U.S. Patent 4,551,828 (Chung), entitled "Quadrilayer
Optical DRAW Medium", discloses a thin "triggering" layer
used to enhance the pit formation in a DRAW medium, and for
causing pit depth variation and reflectance variations in
connection with same.
U.S. Patent 4,930,116 (Dil), entitled "Record Carrier
Containing Information in an Opti.cally Readable Information
Structure" concludes that the æhape of the pit walls has
llttle effect on the storage and retrieval of data from an
optical disk.
Summarv of the Inventl_n
It is known from the prior art that by varying the
laser power, or intensity, pits or bubbles of different
heights can be formed in an optical dislc. It is also known
that a pit or bubble of one height will reflect a different
amount of light back to the ob~ective lens of the optical
read head than a plt with a different height. The reason
that pits of dlfferent depth or height reflect different
amounts of light is because of scattering of the reflected
light and interference between the reflected light, which
has the same phase as the incident laser light, and the
light reflected from the pit which has a shifted phase
characteristic dependent on the pit depth.

CA9-92-005 5 2~8597~ j
Thus according to the present invention, it has been
discovered that multiple-bit data may be encoded on an
optical storage medium by correlating the depth of a pit or
bubble with a particular multiple bit sequence.
Various aspects of the invention are defined as
follows:
An optical storage device, comprising a disk having a
recording surface for reflecting laser light incident
thereon, and a plurality of cells representing stored data,
each of said cells being characterized by one of 2N
predetermined levels of effective reflectivity to said laser
light, where N'1 and wherein N represents the number of bits
stored per cell.
A method of encoding data into a WORM disk utilizing a
laser beam having a wavelength of ~, comprising the steps
of:
a) formatting said disc:
b) mapping two-bit data into four cell types designated
as cell type 1, cell type 2, cell type 3 and cell type 4,
wherein cell type 1 is characterized by a first level of
effective reflectivity for reflecting a first predetermined
amount of said laser beam, cell type 2. is characterized by a
second level of effective reflectivity for reflecting a
second predetermined amount of s~1d laser light less than
said first predetermined amount, cell type 3 is
characterized by a third level of effective reflectivity for
reflecting a third predetermined amount of said laser light
less than said second predetermined amount, and cell type 4
ls characterized by a fourth leve.l. of effective reflectivity
for reflecting a fourth predetermined amount of said laser
light less than said third predetermined amount;
c) setting said laser light at l/3 of the power
required to create a cell havi.ng said fourth level of
effectlve reflectivity;
d) simultaneotlsly rotating said disk a first revolution
and pulslng ~aid laser light at predetermined times for
exposing cell type 2, cell type 3 and cell type 4 with said
laser light at 1/3 of the power required to create a cell
having said fourth level of effective reflectivity;
e) rotating said disk an additional revolution and
reading cell type 2, cell type 3 and cell type 4 and

CA9-92-005 2~8~974
identifying any ones of cell type 2, cell type 3 or cell
type 4 having excessive threshold values;
f) simultaneously rotating said disk an additional
revolution and pulsing said laser light at predetermined
times for exposing predetermined ones of cell type 2, cell
type 3 and cell type 4 identified in step e) as having
excessive threshold values;
g) simultaneously rotating said disk an additional
revolution and pulsing said laser light at predetermined
times for exposing cell type 3 and cell type 4 with said
laser light at 1/3 of the power required to create a cell
having said fourth level of effective reflectivity;
h) rotating said disk an additional revolution and
reading cell type 3 and cell type 4 and identifying any ones
of said cell type 3 or cell type 4 having excessive
threshold values;
i) simultaneously rotating said disk an additional
revolution and pulsing said laser light at predetermined
tlmes for expo~ing predetermined ones of cell type 3 and
cell type 4 identified in step h) as having excessive
threshold values;
;) simultaneously rotating said disk an additional
revolution and pulsing said laser light at predetermined
times for exposing cell type 4 witl- said laser light at 1/3
of the power re~uired to create a cell having said fourth
level of effective reflectivity;
k) rotating said disk an additional revolution and
reading cell type 4 and identifyi.ng any ones of said cell
type 4 having excessive threshold values; and
1) simultaneously rotating said disk an additional
revolution and pulsing said laser light at predetermined
times for exposing predetermined ones of said cell type 4
identified in step k) as having excessive threshold values.
A method of reading multi-bit per cell information from
an optical di~k using a low power ].aser beam having a
wavelength of ~, wherein said information is encoded in said
disk as four cell types designated as cell type 1, cell type
2, cell type 3 and cell type 4, and wherein cell type 1 is
characterized by a first level of effective reflectivity for
reflecting a first predetermined amount of said laser beam,
cell type 2 is characterized by a second level of effective
.. . .

CA9-92-005 7
208~974
reflectivity for reflecting a second predetermined amount of
said laser light less than said fir6t predetermined amount,
cell type 3 is characterized by a third level of effective
reflectivity for reflecting a third predetermined amount of
said laser light less than said second predetermined amount,
and cell type 4 is characterized by a fourth level of
effective reflectivity for reflecting a fourth predetermined
amount of said laser light less than said third
predetermined amount;, comprising the steps of:
a) simultaneously rotating said disk and exposing said
disk to said low power laser beam causing modulation in the
amount of light returned from said disc; and
b) detecting said light returned from said disk and
converting said light returned by thresholding into any one
of four two-bit codes representing respective ones of said
cell types.
It is contemplated that the principles of the present
invention may be applied to all types of optical storage
~y~tem~, lncluding CD-ROM systems as well as WORM drives.
However, as discussed in greater detail below, the preferred
embodiment of the present invention is a WORM drive
implementation.
Brief DescriPtion of the Dra_ings
A description of the preferred embodiment is provided
in greater detail below, with reference to the following
drawings, in which~
Figure 1 is a cross sectional representation an optical
recording media structure with encoded pits characterized by
four cell ty~pe~, in accordance with a first embodiment of
the invention;
Figure 2 is a cross sectional representation of an
optical ~torage device with encoded bubbles of different
heights, according to the preferred embodiment o the
inventlon;
Figure 3 is a plot showing interference patterns for
cell~ of Figures 1 and 2;
Figure 4 is graph showing ranges in cell effective
reflectivity for the four cell types shown in the structures
of Figure 1 and 2;
,, , " "~ ,, ,,, "~ ", ", ,~, ~, ";

CA9-92-005 8
2085974
Figure 5 is a flow diagram showing the steps for
writing multi-bit data per cell information on an optical
disk, according the preferred embodiment;
Figure 6 is a flow diagram showing process steps for
reading multi-bit data per cell from an optical disk, in
accordance with the preferred embodiment;
Figure 7 is a graph showing peak read-out contrast
versus phase pit depth for the four cell types shown in
Figures 1 and 2 and
Eigure 8 is a plot showing interference patterns for
different cell types where the pits are deeper than ~/4;
Detailed Descri~tion of the Prefer_ed Embodimen_
As discussed above, according to the present invention,
a system is provided for creating data cells in an optical
storage device wherein each cell is characterized by a
multiple bit density. By increasing the bit value of each
cell, both the data density and the read rate are increased
by a factor of N, where N is the number of bits in each
cell.
In contrast with the known prior art, according to the
preferred embodiment, more than one bit of data is encoded
per cell by varying the effective reflectivity of each cell
such that specific ranges are achieved of reflective
co-efficient for each cell, between a predetermined minimum
and maxlmum. In the present specification, the term
effective reflectivity is used to indicate the cumulative
effects of reflectivity, scattering, interference and
diffraction. The term reflectivity alone and the term
material reflectivity represent only the amount of light
that i8 reflected assuming a flat surface. For example,
typical optical disks have a maximum value for reflectivity
of approximately 0.6, the minimum possible reflectivity is
approximately 0. Thus, by assigning different effective
reflectivities to respective cell types, various multiple
bit data se~uences may be associated wlth cell~ of different
effective reflectivity. As discussed in greater detail
below, the assigned cell types match a range of effective
reflectivity, not just a fixed value, to account for process
variations and inaccuracies. However, provided that the
cell effective reflectivity conforms to a predetermined
range between minimum and maximum, then accurate recording
:, ................ .... .

CA9-92-0~5 9
`' 208~97~ ,
and reproducing of multiple bit data is possible utilizing a
single cell.
Turning to Figure 1, an optical disk structure is shown
in accordance with the present invention (ie, either CD-ROM
or WORM). The optical disk comprises a label 1, overlying a
protective layer 2, and a signal recording surface 3
embedded in plastic polycarbonate layer 4.
According to an important aspect of the present
invention, multiple bit data values are assigned to
different pit depths ~or respective cells in the reflective
surface of the optical disk. Thus, for a two bit-per-cell
structure as shown in Figure 1, cell type 1 (identified by
reference numeral 8) is characterized by a depth of
approximately O, cell type 2 (identified by reference
numeral 6) is characterized by a cell depth of 1/3(~/4),
cell type 3 (identifled by reference numeral 5) i5
characterlzed by a cell depth of approximately 2/3(~/4) and
cell type 4 (identified by reference numeral 7) is
characterized by a cell depth of approximately ~/4, where ~
is the wavelength of the incident laser read beam.
As shown in Figure 2, bubble forming media can be
utilized, rather than pit forming technology, since bubble
forming technology i9 the current -trend in the market and is
generally considered to be more available and to be less
expensive. When pit forming is implemented, the present
invention uses exclusively thoæe media which are conducive
to pit melting techniques rather than bubble popping
techniques.
According to the preferred embodiment of Figure 2,
changes in the effective reflectivity result from a
scattering effect when the incident laser strikes the
bubbled surface. By way of contrast, with pit~, the
predomlnant effect i~ an interference between the incident
and reflected waves. Cell type 1 1~ represented in Figure 2
by reference numeral 8', cell type 2 is represented by
reference 6', cell type 3 is represented by reference
numeral 5', and cell type 4 is represented by cell type 7'.
In accordance with the principles of the present
invention, multi-bit values may be assigned to respective
ones of the cells. Thus, a Gray code encoding scheme may be
established in which cell type 1 represents the bits OO,
.:,:-. .,.. , .. . . : .... . .

CA9-92-005 10 2~8~97~ ,
cell type 2 represents the bits 01, cell type 3 represents
the bits 11, and cell type 4 represents the bits 10.
Alternatively, the various multi-bit cell types may be used
to indicate changes in value of multiple bit data. For
example, cell type 1 may represent no change in the
multi-bit data, cell type 2 may represent complementing the
lea~t significant bit only, cell type 3 may represent
complementing both bits of data, and cell type 4 may
represent complementing the most significant bit only.
For the two bit-per-cell structure of Figures 1 and 2,
the read rate and data density for the optical storage media
i8 increased by a factor of 2.
Apart from the varying cell depths, the remaining
characteri~tics of the optical disk structures illustrated
in Figurés 1 and 2, are consistent with the prior art. For
example, the cell sites are approximately 0.6 microns in
size, and approximately 0.9 microns apart from cell centre
to cell centre. The tracks are spaced approximately 1.6
microns apart, and standard prior art systems are utilized
for tracking of data.
Figure 3 is graph showing interference patterns for the
different cell types in accordance with the structure of
Figures 1 and 2, with an estimated 20% inefficiency caused
by diffraction effects and focussing errors for a surface
having effective reflectivity of 0.5.
With reference to Fig~lre 4, the effective cell
reflectivity for each cell type in the two bit-per-cell
embodiment of either of Figure 1 or Figure 2 i~ shown,
including maximum ranges in reflectivity permissible for
error free multiple bit decoding. Specifically, cell type l
iH characterized by an effective reflectivity of
approximately .500 + 0.0625, cell type 2 i8 characterized by
an effective reflectlvity of approximately 0.375 + 0.0625,
cell type 3 is characterized by an effective reflectivity of
approximately 0.250 i 0.0625, and cell type 4 i~
characterized by an effective reflectlvity of approximately
0.125 i 0.0625.
According to an aspect of the present invention, a
method is provided for encoding multiple bit data into a
WORM disk via an incremental approach for growing bubbles to
characterize cells of different depths. Although not shown,

CA9-92-005 1l
``~ 2085974
the WORM write laser is preferably a laser diode light
source with a 0.6 numerical aperture focussing lens. The
write laser preferably generates light of suitable laser
power dependent on the characteristics of the record medium,
in accordance with well known prior art.
Turning to the flow chart of Figure 5, the process for
writing multi-bit data per cell to a WORM disk is
represented. In step 1, the WORM disk is formatted using
standard techniques.
In step 2, software running on the computer to which
the WORM drive is attached, maps digital data to the various
cell types (1, 2, 3 and 4). i:
In step 3, the write laser power is set at 1/3 of the
power required in order to create a full bubble/pit on a .j~
standard 1 bit-per-cell WORM drive. The disk is caused to
rotate one revolution and the write laser is pulsed at the
appropriate times for exposing cell types 2, 3 and 4 with
1/3 la8er poWer.
In step 4, the write head reads all of the cells 2, 3
or 4 and identifies any cells with excessive threshold
values.
In step 5, the laser power i~ set at 1/8 of the power
required to create a full bubble/pit, and all cells
identified in step 4 as having excessive threshold value,
are expo~ed to the 1/8 laser power light.
In step 6, all type 3 and 4 cells are exposed to 1/3
power laser light. These cells are then read in order to
identify any of the type 3 or 4 cells having excessive
thre~hold value.
In step 8, a fourth write pass is undertaken in which
all of the type 3 or 4 cells identified with excessive
threshold value are re-exposed with laser power of 1/8.
A ~imllar procedure i8 undertaken in ~tep~ 9, 10 and 11
for creation of the type 4 cell~.
As indicated at step 12, following ~tep 11, the writing
of data on the WORM drive is complete.
Typical write powers for creating bubbles and pits are
in the order of 9 milliwatts. Typical ranges for the
frequency of the incident light are 700 nanometres to 900
nanometres. Thus, in accordance with the method discussed
above, p~ts are created having depths on the order of 0,
., . . . ."~ . . . . .. . . . .

CA9-92-005 12
2~8~97~
1/3(~/4), 2/3(~/4) and ~/4. Alternatively, as shown in
Figure 2, bubbles may be formed having similar levels of
effective reflectivity.
In the case of pits (Figure 1), the size of the pits
can be carefully chosen so that the amount of light
reflected to the photodetector in the read head from the
inside of the pit matches as closely as possible the amount
of light reflected back to the photodetector from outside of
the pit. Specifically, d/b = 0.6, for ~/4, where d is the
cell size (ie. recorded pit diameter) and b is the full
width to half maximum of the laser beam diameter. Assuming
optimal sizing of the pits, maximum interference occurs at
~/4, causing no light to be returned to the photodetector,
whereas a flat surface causes the superpositional
constructive interference to result in maximal light being
reflected to the photodetector. As discussed with reference
to Figure 4, pits with ~izes between O and ~/4 return an
amount of light power between the maximum and minimum
values .
Since the di~k is passed over 9 times during the write
cycle, the write cycle for encoding data according to the
present invention is slower than normal for a WORM process.
As di~cussed above, laser powers of 1/3 and 1/8 of the full
pit development power are utilized since there are 6 passes
where data is written to the disk, each pass further
updating the cell structure. Thus, there are 9 passes in
all to account for three read passes that verify data is
read correctly. Since the 6 write passes and the three read
passes allow for two write passes for each type of cell and
one read pass for each type of cell, since there are four
types of cell, one of which is flat, only three types of
cells actually need to be recorded in the media. By
allowing 6 write passes, a cell which is not properly formed
(ie. not characterized by the correct level of effective
reflectivity) after pa~s (3xn)-2, should be created after
pass 3xn. The read passes occur on passes (3xn)-1.
As discussed above, as an alternative to encoding
multi-bit data in pits or holes of a WORM disc, it is
contemplated that multi-bit data can be created in a CD-ROM
using masking technology, similar to that used in the
fabrication of CMOS VLSI circuits, or by using electron beam

CA9-92-0~5 13
2~85974
nano-lithography, such as used in the fabrication of GaAs
MESFET VLSI circuits.
In the case of CMOS VLSI design, it is well known
to create devices with minimum layer or aspect sizes which
are accurate to 0.05 microns (eg. the well known thin oxide
rule for polysilicon spacing). Likewise, the feature sizes
for devices created by electron beam nano-lithography are
approximately one tenth of the size of similar devices
fabricated for CMOS VLSI. Since the implementation of the
present invention requires dimensional accuracy on the order
of ~/4 (eg. 0.6 microns), it is clear that the creation of
pits of appropriate dimensions is attainable.
Turning to Figure 6, a flow diagram is provided for
illustrating the process of reading multi-bit per cell
information from CD-ROM, WORM or other optical media, in
accordance with the principles of the present invention.
The read process described with reference to Figure 6 is
valid for both bubble formed and pit formed optical media.
In step 1, a low power read laser beam is incident on
the surface of the spinning optical disk. As discussed
above, the surface contains cells, each cell depth
characteristic being used to represent multiple-bit data.
The cells are tracked using standard optical techniques
while the disk spins. The different vertical
characteristics of the cells ca~lse a modulation in the
amount of light returned to the read head.
In the preferred embodiment, there are two bits of data
encoded in each cell. In general, the number of cell types
on the optical disk is 2N, where N is the number of bits
contained in each cell. For the two bit per cell method,
there are four types of cells possible (including the flat
cell). Each cell type returns a specific amount of power to
the photodetector in the read head, within a predefined
range of tolerance.
In step 2, the returned light is detected by a
photodetector in the read head and is converted by
thresholding into a two-bit code, instead of a one bit code
as in the prior art. Such thresholding techniques are
standard electronic practices in the art. More
particularly, the detected signal is decompressed into a
five volt signal (such that maximum signal strength is
. .

CA9~92-005 14 208~974
represented by 5 volts and minimum signal strength is
represented by O volts). Decoding is effected by means of
determining the voltage range in which the signal resides
(e.g. 0-1.25 volts, 1.26-2.5 volts, 2.51-3.75 volts,
3.76-5.0 volts). Then, the appropriate two bit code is
assigned to the detected signal.
According to the preferred embodiment, the codes are
Gray coded to a base of 5 volts (although any suitable base
may be used (eg. 12 volts)), in order to enhance error
correction, as shown below with reference to Table 1.
TABLE 1
Voltage _ __ ___ _ _ Bit Code
0.00-1.25 00
1.26-2.50 01
2.51-3.75 11
3.76-5.00 10
Turnlng to Figure 7, experimental results are shown
Which indicate peak read-out contrast versus phase pit depth
where d/b = 0.6 and ~/b 1.5, where d denote~ the recorded
pit diameter, b denotes the beam diameter and s denotes the
distance between cells.
It has been found experimentally that defocussing
effects of the incident laser caused in a pit or bubble do
not significantly affect the effective reflectivity unle~s
the vertical displacement is in the order of twice the
wavelength. In the case of the present invention, the
variation caused by the pit or bllbble height is always less
than half the wavelength and therefore does not cause any
signal contrast degradation.
As discussed above, the principles of the present
lnvention are not limited to a two-bit scenario.
S~ecifically, 3, 4, 5, etc. bits-per-cell can be realized by
further dividing the ranges of height for the pits/bubbles
and assigning m~lltiple bit values thereto. The maximum
allowable bits-per-cell depends on the ability of current
technology to support the various ranges of effective
reflectivity within error margins.
Although the preferred embodiment involves the writing
of data on a WORM drive, to effect multiple bit encoding per
cell, CD-ROM mastering provides a more accurate pit forming
.

CA9-92-005 15
^-- 2085974
method and these techniques can be used to generate multiple
encoded cells for use in CD-ROM drives.
Various alternatives and modifications are possible to
the present invention. For example, as an alternative to
the write process discussed with reference to Figure 5, the
1/8 power laser strikes can be replaced by 1 or 2 passes
after the entire disk has been wrltten, with 1 pass before
each 1/8 power write pass. This would reduce the number of
passes from 9 to 5 or 7.
If the laser power is mod~llated during the write cycle
with sufficient accuracy and speed, then cells can be
created in one pass using 1/3, 2/3 and 3/3 laser power which
will be set according to the data intended at the striking
site (i.e. cell). This can then be followed by the read and
write cycles to ensure accuracy. According to this
alternative, the pos~ible number of passes may be reduced
from 9 to 3 or 5.
With reference to the process for reading pits as
di wu~sed in Figure 6, the pit depths were discussed as
being 0, 1/3(~/4), 2/3(~/4) and ~/4 for the preferred
embodiment of two bits per cell. It is contemplated that
depths of greater than ~/4 may be utilized. For example,
pit depths of 0, ~/4 + 1/3(~/4), ~/4 -~ 2/3(~/4) and ~/4 can
be u~ed. In thi~ regard, reference may had to Figure 8.
Moreover, the specific depths of the pits may not be limited
to the Gray code values disc~lssed in the preferred
embodiment. The only condition for the assigning of codes
to bit depths or heights, is that the read signal strength
returned by a specific cell type must be recognizable by the
reader after the thresholding operation. Therefore, any set
of pit reflectivities is acceptable provided that the reader
i~ able to identify them via their effective reflectivity
after thresholding of the output.
Furthermore, the application of the techniques of the
pre~ent invention to WORM drlve~ are not limited to the
material~ described in the preferred embodiment, but rather
any materials can be used that are ~uitable for DRAW medium,
or any materials that can be ~Ised in a one shot cell
creation scenario.
The technique is also applicable to dye-polymer and
phase change media. For example, a dye-polymer disk may be

CA9-92-005 16 208~974
exposed to varying durations of write and erase laser light
for producing intermediary levels of effective reflectivity.
Likewise, a phase change rewritable disk may be exposed to
varying levels of thermal energy for producing intermediate
states (ie. states between amorphous and crystalline) which
are characterized by intermediate levels of effective
reflectivity.
Finally, the speed of data writing, the laser powers,
laser types, etc., are all independent of the technique
advanced in the present application and can be substituted
and used by a person skilled in the art as desired.
All such modifications and variations are believed to
be within the sphere and scope of the invention as defined
by the claims appended hereto.

Representative Drawing