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BACKGROUND OF THE INVENTION
Cross Reference to Related Applications
The assignee herein is also the assignee of U.S. Pat. No. 4,864,536 and
entitled "Optical Memory Method and System," and U.S. Pat. No. 4,915,982
and entitled "Thin Film Photoluminescent Articles and Method of Making
Same." The disclosures of both of those patents are incorporated by
reference herein.
The present invention relates generally to mass storage devices for data
storage. More particularly, the present invention relates to a method of
and apparatus for mass data or information storage utilizing purely
photoelectronic processes for writing, reading, and erasing stored data.
Optical storage devices presently known generally permit two to three
orders of magnitude more data to be stored per disk than with magnetic
methods and apparatus. Because of the potential for much greater storage
of data and also because of the enormous projected market for such optical
memories, active development of optical storage devices is currently
occurring in several different directions. Such activities are directed
towards read-only, write-once-read many times (WORM) and erasable optical
memory systems. While read-only and WORM optical memories are already
available, erasable optical memory systems have encountered much greater
developmental difficulties than read-only WORM systems because the
qualities of the storage media required present problems of much greater
technical complexity.
Read-only optical memory devices for use as computer peripherals, such as
CD-ROMs, became commercially available with the advent of the digital
audio compact disk. Current disk data storage capacity for such units is
200-600 megabytes. Such disks are factory fabricated using a molding press
and metalizing operations and are suitable for low cost distribution of
large fixed database information.
WORM devices allow the user to encode his own data on the disk, however
only once. Data bits are stored at physical locations by irreversibly
"burning" the medium with a laser. Such permanent encoding can be read
back indefinitely, thus making WORM technology suitable for archival
storage of large quantities of information, including digitized images,
where random access to a large database is desirable.
It is the third category of optical disk storage devices, namely erasable
storage devices, that is believed to embody the greatest utility for mass
storage purposes. Such devices will be competitive with present magnetic
tape and disk mass storage, and will have a major impact on computer
technology in the years ahead. At present, the three most active
approaches now being pursued for erasable optical storage involve
magneto-optical material systems, dye polymers, and techniques that
produce crystal structure or phase transformation in the storage medium at
the spot being written to. All of these approaches require heat which
usually changes the physical or chemical structure of the materials in
performing the write or erase function. Thus, the time to write data to
such systems is dependent upon a certain "dwell" time during which the
spot to which data is being written must be heated or otherwise physically
transformed.
Another drawback with such approaches is that media performance is highly
sensitive to impurities, impurity diffusion, oxidation, and other
imperfections that propagate into defects and that only show up after
multiple switching cycles or at times later than the manufacturing and
testing of the devices. Of the three approaches discussed above, progress
has been greatest with magneto-optic materials. Laboratory results in this
area have reported millions of write/erase cycles. See, for example, H-P.
D. Shieh Ph.D. Thesis, Carnegie-Mellon University, Pittsburgh, Pa. (1987).
In order to utilize erasable optical media for mass storage, the optical
media must be fast enough to be marked at high data rates using low power
lasers. The media must also maintain almost error-free data at acceptable
computer industry standards for at least ten years, for example, no more
than one uncorrectable error in 10.sup.12 bits. Thus, finding the right
physical phenomenon to serve as the basis for erasablity in a high-speed,
high-resolution optical storage medium for use with an optical disk
storage system or other optical storage system has been very difficult.
Most of the effort in the optical disk area over the past ten years, as
described above, has gone into the use of magneto-optic materials.
However, the commercial realization of erasable magneto-optical storage
has not yet been achieved, nor are there yet any guarantees that it ever
will be. Unfortunately, the performance of the other approaches discussed
above generally is not comparable.
In order to overcome the problems of the prior art, and provide a basis for
a workable optical disk storage system, a new approach to the optical
storage materials problem which satisfies the optical media requirements
of density, speed and long cycle life has been developed. This development
utilizes the phenomenon of electron trapping in a class of new materials
which comprise an alkaline earth crystal typically doped with rare earth
elements. Thin crystalline films of such materials are formed on various
substrates, such as glass, polished sapphire or alumina, or other optical
quality substrates, in order to provide the disk storage medium.
Since the trapping phenomenon is a purely electronic process,
read/write/erase operations can be performed very fast. In addition, the
physical trapping phenomenon suggests that media life may be practically
limitless. Also, the effect of electron trapping yields a linear response
characteristic, which provides an analog dimension to the storage
capability. Thus, for example, the potential disk storage capacity of a
single 51/4 inch disk could be extended to several gigabytes. Obviously,
the density of stored information is extremely high.
The materials to be used as the media for the optical disk storage system
described herein are the subject of U.S. Pat. No. 4,915,982, which is a
continuation-in-part of U.S. Pat. Nos. 4,864,536 and 4,830,875. Other
materials useful as the storage media herein are disclosed in co-pending
U.S. Pat. Nos. 4,839,092 and 4,806,772; 4,879,186 and 4,842,960. The
assignee herein is the assignee in each of those applications. The
disclosure of each of those applications is incorporated by reference
herein.
The material described, for example, in U.S. Pat. No. 4,915,982,
demonstrates an extremely linear relationship between the intensity of the
write input light and the read output light resulting from a
fixed-intensity read command. Thus, this capability demonstrates a large
noise margin for binary storage, as well as an increased information
storage density when employed as an analog or multilevel digital memory
medium. Multilevel refers to the fact that by writing with a plurality of
intensities of the same laser beam, the linearity of the resulting
emissions upon being impinged by a read laser beam is such that
information can be stored and recognized at various "levels" of intensity,
for example, at 0.2, 0.4, 0.6, 0.8 and 1 intensity.
This particular media is in the form of a thin film and can be "charged"
and "discharged" with light by exciting ground state electrons to an
elevated energy level. Specifically, upon illumination by visible light,
electrons are raised to high energy trapping states, where they can remain
indefinitely. When later illuminated by infrared light, the electrons are
released from the traps, emitting a new visible light. Thus, with such
materials, digital or analog data is stored and retrieved by using low
energy lasers to trap and read the electrons at a particular location.
Such solid state photonic materials have electrons having bistable
equilibrium states; one with electrons in a ground state, and the other in
which electrons are "trapped" in a well-defined, specific, elevated energy
state. Electrons are raised to the higher energy state by the absorption
of visible light photons, thus filling available trap sites. An electron
in the elevated energy state can be released from its trap site by
inputting sufficient energy to the electron to permit it to escape from
the well. When that occurs, the electron falls back to its ground state
and emits a corresponding visible photon. The number of electrons in the
elevated energy state is proportional to the visible light intensity used
for recording. Thus, as a result of such characteristics, such materials
can, in effect, "store" light energy.
The purely photo-electronic mechanisms involved in such electron trapping
materials obviate the need for any thermal excursions and, therefore, the
number of electrons trapped in the material is inherently linear. Since
localized resolution of the "write" step depends only on the performance
of the addressing optics, an optical writing spot diameter of one micron
will allow at least 550 megabytes of storage on a 130 mm or 51/4" disk
coated with a single thin film material as disclosed herein. Multiple
layers of thin film materials provide for a like multiple of data storage.
For example, two layers of thin film materials will at least double the
data storage capacity to 1.1 gigabytes. With the use of encoding
techniques such as MFM, modified MFM, or record length limiting (RLL),
which techniques are commonly used with magnetic disk recording, the
storage capacity can be increased by up to a factor of 3 over the use of
FM or frequency modulation coding. The rise and fall times associated with
optical read and write pulses are in the nanosecond range. Thus, the read
and write data transfer rates have been found to be at least 200 megabits
per second for optical disk drive media utilizing electron trapping
materials.
Rotating disk memory systems require directions for the retrieval of the
stored information. One set of those directions informs the drive
mechanism where the requested information is or will be stored. The other
set provides alignment for the read/write mechanism during processing. The
alignment parameters include focusing, speed, tracks, and mark locations.
The writing of information is dependent upon the media used such as write
once, magneto-optic, dye polymer, or phase change, but in all cases,
involves a change in the reflection parameters in the spot written to. The
read method is based on detecting such reflectivity changes at the surface
of the disk.
The common method presently used for tracking with reflective surface
optical disks, such as the compact disc, is to rely on a grouped track as
the principal mode of aligning and focusing the read/write head in the
middle of the track. The speed information is either contained in the
repetitive pattern of marks or in a depth modulation of the group.
Yet another tracking method presently utilized is known as the "Sampled
Servo" system. That system relies on changes in the reflection of spots on
the disk surface. The spots are located in a manner which provides
information about the track location, the speed of the disk and the
adequacy of focusing.
The erasable optical disk memory systems disclosed herein, which rely upon
a thin film of electron trapping material as the media, do not rely on
reflection for readout. Rather, the emission of the media under infrared
stimulation can be utilized to retrieve pre-written tracking information
from the disk.
However, even with the advances made by the assignee herein set forth
above, the two-dimensional memory system disclosed in U.S. Pat. No.
5,007,037 has certain limitations. With the advent and continued
development of parallel processing computers, very fast response memory
systems having extremely high density storage capabilities are needed.
While there is much interest and development in two-dimensional erasable
optical memories, such an approach will eventually run into an optical
resolution limit. That is, a focused beam of light, even a laser, can only
be made so small, somewhat less than one micron in diameter. Due to that
limitation, only a limited number of bits stored per unit area can be
achieved. In order to overcome that physical shortcoming, the present
invention utilizes a three-dimensional optical memory storage system, that
is, a plurality of at least two layers of different electron trapping
materials, each of which responds to light of different wavelengths, in
order to greatly increase the storage capacity of, for example, an optical
disk memory system equipped with a disk prepared in such a manner.
As an alternative to utilizing "stacked" layers of different characteristic
electron trapping materials, a buffered stack of two-dimensional storage
planes could also be utilized. Electron trapping material characteristics
can be controlled separately, together, or in a defined sequence. Both
electron trapping media layers and optical layers can be utilized.
SUMMARY AND OBJECTS OF THE INVENTION
In view of the foregoing, it should be apparent that there exists a need in
the art for a method of and apparatus for operating and constructing a
three-dimensional erasable optical disk memory system in which electron
trapping material used as the storage media permits writing, reading, and
erasing essentially an unlimited number of times in which at least two
layers of electron trapping material, each having a sensitivity to a
different writing beam wavelength, are utilized. It is, therefore, a
primary object of this invention to provide a method of and apparatus for
operating and constructing a three-dimensional erasable optical disk drive
system which is characterized by a plurality of electron trapping media
layers such that the write, read, and erase functions can be accomplished
with high density, speed and without serious degradation over a large
number of erased functions.
More particularly, it is an object of this invention to provide a
three-dimensional erasable optical disk memory system for information
storage which is capable of storing orders of magnitude more data per disk
than inductive magnetic media systems or than even two-dimensional
erasable optical disk memory systems.
Still more particularly, it is an object of this invention to provide a
three-dimensional erasable optical disk drive memory system in which data
is stored as light energy and which is not dependent upon the reflective
properties of the disk for effecting storage or readout of stored
information.
It is another object of this invention to provide a three-dimensional
erasable optical disk memory system in which data is written to and read
from the disk in a parallel manner.
It is yet another object of this invention to provide a three-dimensional
erasable optical disk memory system in which one or more write laser beams
centered about different wavelengths of visible light are used to
simultaneously write data to one or more electron trapping media layers,
respectively.
It is still a further object of the present invention to provide a
three-dimensional erasable optical disk drive memory system in which the
data stored in each of the electron trapping media layers is released
simultaneously by the use of a single infrared reading light beam.
It is still a further object of this invention to provide a
three-dimensional erasable optical disk drive memory system in which the
data released from each of the plurality of electron trapping media layers
is centered about a predetermined wavelength of light and therefore can be
readily separated.
It is also another object of this invention to provide a three-dimensional
erasable optical disk drive memory system in which data may be stored as
light energy in either digital or analog form.
Briefly described, these and other objects of the invention are
accomplished in accordance with its apparatus aspects by providing a disk
containing at least two coatings of thin film electron trapping materials,
each having a different light sensitivity characteristic, which is rotated
in a manner similar to that of magnetic hard disk drives, also known as
Winchester disk drives. Data is read onto the disk in the form of a like
number of write lasers operating at a like number of different wavelengths
of visible light, preferably with wavelengths peaking between 450 and 600
nanometers. A read laser is utilized to irradiate the disk with near
infrared radiation having a wavelength with peaks between 700 and 1,450
nanometers, but preferably at about 1,000 nanometers. When stimulated by
the near infrared read radiation, any stored bits (representing, for
example, a binary one) will cause a predetermined radiation emission
characteristic that peaks in a predetermined light band, for example, at
about 620 nanometers (orange) and 495 nanometers (blue). Such detected
emissions correspond to a binary one recorded at those points. The absence
of such radiation emission characteristic corresponds to a binary zero
recorded at those points.
The optical disk memory storage system of the present invention also
includes an optical processing unit for transmitting both the read and the
plurality of write laser beams to the read/write head, as well as various
detector electronics and positioning electronics for positioning the head
over the disk. Data output and data input is handled by a standard
computer interface.
The disk is constructed of a substrate onto which the at least two layers
of electron trapping material are deposited as thin film media. The two or
more thin film media layers are separated by the use of deposited optical
coatings. As such, information can be written, read and erased by
multi-color optical signals. The time required for performing any of those
functions is on the order of about five nanoseconds. Using such electron
trapping media materials, a practically unlimited number of interrogations
can be made before the information is no longer accessible.
The electron trapping media layers are deposited utilizing such different
deposition techniques as electron-gun evaporation and sputtering. The
electron trapping layers may be placed directly on top of each other or
may be separated by passive optical layers. Although only two electron
trapping media layers are shown and described in the embodiments herein,
it is both possible and desirable to form a memory system having "n"
electron trapping media layers, as long as each layer is sensitive to a
different wavelength of visible light for writing purposes and produces a
separable wavelengths of output light in response to an infrared reading
laser beam.
With these and other objects, advantages and features of the invention that
may become hereinafter apparent, the nature of the invention may be more
clearly understood by reference to the following detailed description of
the invention, the appended claims and to the several drawings attached
herein.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a diagram showing the principles of operation of the thin film
storage media applied to the surface of the disk used with the present
invention;
FIG. 2 is a graph showing the trap-filling efficiency of a particular
electron trapping film suitable for use with the present invention as a
function of the wavelength of the exciting energy;
FIG. 3 is a graph showing the relative infrared sensitivity for the read
and erase functions for the same particular electron trapping material as
FIG. 2;
FIG. 4 is a graph showing the wavelengths of emission from the disk upon
infrared illumination of the disk for the same particular electron
trapping material shown in FIGS. 2 and 3;
FIG. 5 is a graph of the relative luminescence or read output as a function
of write energy for the same particular electron trapping material as
shown in FIGS. 2-4;
FIG. 6 is a pictorial diagram of the structure of an optical disk for use
with the present erasable optical disk memory system;
FIG. 7 is a schematic block diagram of the electronics and optics necessary
for reading, writing, and erasing data onto and from the disk;
FIG. 8 is a pictorial partially cutaway drawing of an optical disk storage
system which may be used with the present invention;
FIG. 9 consisting of FIG.(9A) and FIG.(9B) is a schematic block diagram of
the position servo tracking and velocity servo tracking circuitry for use
with the optical disk drive memory system of the present invention; and
FIG. 10 is a schematic block diagram of the optical processing unit and
actuator arm assembly for use with the optical disk drive memory system of
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now in detail to the drawings wherein like parts are designated
by like reference numerals throughout, there is illustrated in FIG. 1 an
explanation of the basis of the operation of the class of optical storage
media electron trapping materials used with the erasable optical disk
drive memory system of the present invention. As illustrated in FIG. 1,
the wide bandgap host material includes selected impurities which are
associated with energy levels E and T. The narrow E band is designated as
the communication band since electron interaction is allowed there. At
level T, which is referred to as the trapping level, the trapping sites
are non-communicating because they are of such concentration and
separation as not to allow electron interaction.
As indicated in FIG. 1, visible radiation, or charging light, excites
carriers so that they can fill the trapping sites. The trap depth in this
group of II-VI phosphors is about 1.2 electron volts, sufficiently above
the thermal energy range such that electrons cannot be dislodged by
thermal agitation. By controlling the trap density, the tunneling
interchange at trapping level T may be cut off. Under such conditions, the
trapped electrons cannot communicate with each other, and the possibility
of recombination is thus eliminated.
When the charging radiation terminates and the electron trapping material
is in its energized state, the traps are filled, the communication band E
is empty, and recombination from the trapping level T to the valence band
G is nonexistent. As a consequence, the electrons in the trapping level T
will remain or be "stored" there for many years.
If the charged electron trapping material is then exposed to infrared or
near infrared light, sufficient energy equal to the difference between the
energy of an electron in the communication band E minus the energy of an
electron at the trapping level T is provided which serves to move
electrons from the trapping level T to the communication band E. While in
the communication band E, the electrons may interact and then return to
the ground state or valence band G. However, as the electrons return from
the communication band E to the valence band G, a photon of energy E minus
G is emitted. By the selection of an appropriate doping rare earth
element, the wavelengths of light given off by the photon emitted when an
electron falls from the communication band E down to the valence band G
can be predetermined. The sensing of the occurrence of such an emission
serves to indicate whether a particular point being addressed on the disk
surface contains a bit (signifying a 1) or no bit (signifying a 0), or
vice versa.
In contrast to the physical changes which occur by the use of a writing
laser beam in known approaches to erasable (and non-erasable) optical
storage systems, the writing and erasing of a spot on the surface of
electron trapping materials requires only a change in the energy state of
the electrons at that spot. Since no heating is involved, latent, defect
induced read, write, and erasable forms of degradation do not occur.
Consequently, the number of switching cycles in the electron trapping
material is virtually unlimited, exceeding 10 million write/erase cycles
with no observed change in the thin film.
The filling of the traps at trapping level T requires that a threshold
energy be exceeded. A characteristic curve for one of the preferred
electron trapping materials for use as one storage media with the present
invention is shown in FIG. 2. That preferred material is disclosed in U.S.
Pat. No. 4,842,960. As shown in FIG. 2, the threshold energy level which
must be exceeded in order to fill the trapping level T begins at
wavelengths of visible light shorter than 600 nanometers.
After the traps contained in the trapping level T are filled, impingement
of near infrared radiation can cause electrons to be released. A graph
showing the relative infrared sensitivity versus the peaks of the infrared
radiation is shown in FIG. 3. In all instances, the graphs shown in FIGS.
2-5 are for the same material described in connection with FIG. 2. As
shown in FIG. 3, for the material discussed herein for use with the
preferred embodiment, the most efficient rate of release occurs at
wavelengths having peaks of just under 1 micron, or at about 1,000
nanometers.
As shown in FIG. 4, when one of the thin phosphor films disclosed herein
for use in the preferred embodiment is stimulated by infrared radiation
having peaks whose wavelengths are shown in FIG. 3, that phosphor film
displays an emission characteristic that peaks in the orange light band,
at about 620 nanometers. It should be noted, however, that FIGS. 2-5 are
only representative and that peak wavelengths can be altered by changing
the rare earth element dopants to produce other emission characteristics.
In addition, the spectral widths of the responses can also be altered
within certain limits.
The number of electrons trapped in the higher energy state at trapping
level T is proportional to the amount of writing energy incident on the
surface of the thin film electron trapping material. As shown in FIG. 5,
saturation is reached, for that thin film material, at a level of about 5
millijoules per square centimeter. At lower flux levels, linearity is
observed through orders of magnitude. In addition, the readout emission is
also linear with respect to the intensity of the write beam. This
linearity characteristic of the electron trapping material allows such
material, using the optical disk drive system disclosed herein, to record
and read back analog signals, such as video or analog data transactions,
using amplitude modulation, which is not possible with other optical disk
drive systems.
If the traps contained in the trapping level T are physically separated by
more than a tunneling distance, they do not communicate with each other
and self-discharge is eliminated. This condition corresponds to a highest
density of excited electrons on the order of about 10.sup.20 per cubic
centimeter. Each time an infrared signal is applied to the charged
electron trapping film characterized in FIGS. 2-5, emission of orange
light occurs. Of course, with each photon emitted, the number of electrons
remaining in traps in the trapping level T is reduced. Depending on the
sensitivity of the detector which, in this case for this material is at
620 nanometers, many readings can be made before the traps are depleted.
For the examples given herein, complete erasure of a bit storage spot 1
micrometer in diameter requires approximately 1 picojoule of infrared
energy. Of course, depletion can be avoided by reading at lower levels of
intensity. Periodic refreshing or immediate rewriting of data are both
possible.
As briefly described above, the ability of electron trapping materials to
"store" energy received in the form of light and to later release that
energy upon interrogation by another light beam, comprises the means of
digital switching used by the storage media of the present invention. Data
points on the storage media surfaces that have electrons trapped in the
elevated state can be interpreted as "on" or a binary one, while those
with no electrons in the elevated state are interpreted as "off" or binary
zero. Conversely, changed data points can be interpreted as "zeros" and
unchanged data points as "ones". It is also possible to "charge" the
entire disk with visible light and to write data using the infrared read
beam. In that case, the ones or zeros would be stored on the disk at
points which have been discharged and vice-versa.
Since at each such spot there are sufficient electrons that can switch
energy states, the present invention optically determines whether a spot
is a binary zero or one without altering the "switch" setting. That is
accomplished by interrogating the spot with an infrared beam of low
intensity in order to release a small fraction of the elevated energy
electrons, enough to determine whether or not the spot is a binary one or
binary zero. However, as a practical matter, not enough of the elevated
energy electrons are released to significantly deplete the inventory of
energized electrons stored at that spot. Utilizing the particular electron
trapping thin film materials discussed herein for media storage, switching
or write/erase speeds of 5 nanoseconds or less are achieved.
As previously discussed, the present invention is directed to the use of
multiple layers of differently doped electron trapping materials, one on
top of the other on a disk or other substrate. Since each layer produces a
different wavelength of output emission, it is possible to separate the
read beam signals on that basis.
While the present three-dimension optical memory system is described using
two differently doped media coatings, it is possible to use a plural | | |
