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BACKGROUND OF THE INVENTION
Cross Reference to Related Application
The assignee herein is also the assignee of U.S. patent application Ser.
No. 870,877, filed Jun. 5, 1986, and entitled "Optical Memory Method and
System," which issued as U.S. Pat. No. 4,864,536 on Sep. 5, 1989, and U.S.
patent application Ser. No. 184,263, filed Apr. 21, 1988, and entitled
Method of Making Thin Film Photoluminescent Articles, now U.S. Pat. No.
4,915,982. The disclosures of both of those applications 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 erasability in a high-speed,
high-resolution optical storage medium for use with an optical disk
storage system has been very difficult. Most of the effort in this 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 or alumina, 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. patent application Ser. No.
184,263, filed Apr. 21, 1988 now U.S. Pat. No. 4,915,982 which is a
continuation-in-part of U.S. patent application Ser. Nos. 870,877 and
870,809, both filed Jun. 5, 1986, now U.S. Pat. Nos. 4,864,536 and
4,830,875, respectively. Other materials useful as the storage media
herein are disclosed in co-pending U.S. patent application Ser. Nos.
034,332 and 034,334 both filed in Apr. 3, 1987, now U.S. Pat. Nos.
4,839,092 and 4,806,772, respectively; 147,215 filed Jan. 22, 1988 now
U.S. Pat. No. 4,842,960; and 078,829 filed Jul. 28, 1987, now U.S. Pat.
No. 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 in U.S. Pat. No. 184,263 demonstrates an extremely
linear relationship between the write input and the read output. Thus,
this capability demonstrates a large noise margin for binary storage, as
well as the possibility of increased information storage density when
employed as an analog or multi-level digital memory medium. Multi-level
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 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 bi-stable
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 the thin film material as disclosed herein. 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. This system relies on changes in the reflection of spots on
the disk surface. The spots are located in the manner which provide
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.
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 an
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. It is, therefore, a primary
object of this invention to provide a method of and apparatus for
operating and constructing an erasable optical disk drive system which is
characterized by an electron trapping media 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 an 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.
Still more particularly, it is an object of this invention to provide an
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.
Briefly described, these and other objects of the invention are
accomplished in accordance with its apparatus aspects by providing a disk
containing a coating of thin film electron trapping material 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 write laser operating at a wavelength of visible light, preferably with
wavelengths peaking between 450 and 600 nanometers and preferably at about
450 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 the orange light band, at about 620
nanometers. Such a detected emission corresponds to a binary one recorded
at that point. The absence of such radiation emission characteristic
corresponds to a binary zero recorded at that point.
The optical disk memory storage system of the present invention also
includes an optical processing unit for transmitting both the read and
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.
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 DRAWINGS
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 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
their infrared illumination 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 the optical disk used
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 cut-away drawing of the optical disk
storage system of the present invention;
FIGS. 9A and 9B are schematic block diagrams 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 EMBODIMENTS
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 ET. 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 do not allow electron interaction.
As indicated in FIG. 1, visible radiation, or charging light, excites
carriers so that they can fill the trapping sites. The trapped depth in
these group II-VI phosphors, of about 1.2 electron volts, is sufficiently
above the thermal energy range of the phosphors such that electrons cannot
be dislodged by thermal agitation. By controlling the trapped 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 electronically energized state, the traps are filled,
communication band E is empty, and recombination from trapping level T to
valence band G is nonexistent. As a consequence, the electrons in the
trapping level T will remain or be "stored" there for a time which
approaches infinity.
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 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 D 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
contained 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 the preferred electron
trapping material for use as the 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 the thin phosphor film disclosed herein for use in
the preferred embodiment is stimulated by infrared radiation having peaks
whose wavelengths are shown in FIG. 3, the 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. 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 the thin film material useful for the preferred
embodiment, at a level of about 5 milijoules per square centimeter. At
lower flux levels, linearity is observed through orders of magnitude. In
addition, the read-out 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, 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 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, if
necessary, by periodic refreshing of the stored data.
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 surface 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. Also, only a single
infrared beam would be necessary to both read and write data.
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.
It is also possible to coat multiple layers of differently doped electron
trapping materials, one on top of the other. Since each layer produces a
different wavelength of output emission, it is possible to separate the
read beam signals on that basis. Binary codes can additionally be used to
distinguish between layers.
In addition, the read, write and erase laser power requirements for the
present invention are low. It is sufficient for the write energy to be a
fraction of 1 picojoule per cubic micron, for example, about 0.5 picojoule
per cubic micron. The read energy is at femtojoules, for example, about 10
femtojoules per cubic micron. The erase energy is about 1 picojoule per
cubic micron. In watts, the write laser power is preferably about 0.1 mW,
the read laser power about 0.5 mW and the erase laser power is preferably
about 2.0 mW.
FIG. 6 is a side view of an optical disk showing the disk substrate 12 upon
which the electron trapping layer 16 is deposited. The disk substrate 12
is preferably made from an optical alumina material. However, it could
also be made from other materials such as optical glass, which can be
readily coated with a thin film material and withstand the temperatures
necessary to fuse the crystalline structure of the thin film material. The
first material deposited onto the disk substrate 12 is an interlayer
material 14 with a thickness of about several hundred Angstroms. The
purpose of this interlayer 14 is to define a crystalline surface structure
for the electron trapping layer. It also provides a chemical barrier to
prevent any leaching of the substrate material into the electron trapping
layer 16. Any suitable material, such as CaO, MoO.sub.3 or ZnS may be
used, however ZnS is preferred. Any suitable deposition process may be
utilized.
The second layer deposited on the disk 10, on top of the interlayer 14, is
the electron trapping layer 16. The thickness of the electron trapping
material 16 is preferably about 5 microns. The deposition processes
described in the pending U.S. patent applications referenced and
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