CA2037059A1 - Three-dimensional optical data storage structure - Google Patents

Three-dimensional optical data storage structure

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Publication number
CA2037059A1
CA2037059A1 CA 2037059 CA2037059A CA2037059A1 CA 2037059 A1 CA2037059 A1 CA 2037059A1 CA 2037059 CA2037059 CA 2037059 CA 2037059 A CA2037059 A CA 2037059A CA 2037059 A1 CA2037059 A1 CA 2037059A1
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Prior art keywords
data
recording
storage
medium
beam
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Abandoned
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CA 2037059
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French (fr)
Inventor
Kent J. Daniels
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Kent J. Daniels
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Abstract

ABSTRACT OF THE DISCLOSURE

A data storage medium is disclosed in which binary data may be stored in and read from a three-dimensional structure composed of a plurality of stacked recording layers. Logical "1"s are stored as data cells located at predetermined data locations within each recording layer; whereas logical "0"s are recorded as the absence of data cells at the aforesaid data locations. Various methods are taught for reading, writing and erasing data stored in the data storage medium.
Finally, an data recording and retrieval apparatus which uses the three-dimensional data storage medium of the invention is disclosed.

Description

2~3~15~

~HREE-DIMEN8IONAh C)PTICA~ DA~A ~TORAGE ~TRUCTUR~

FIELD OF THE INVENTION
This invention relates to media used for the storage of information, such as computer data, etc., wherein the data may be recorded, erased, and read from the media by optical means.
This invention also relates to data storage devices that use the above-mentioned data storage media.

BACKGROUND TO THE INVENTION
In the field of high-density data storage, it is known in the art to store data, such as computer data, music, or other information on storage media that may be "read" by optical means. One such method employs a laser to illuminate a micron size spot on a moving recording medium. The data may be ; recorded on the recording surface in any of the following forms:

1. regions of different height, whereby light is reflected back to a detecting means from a region ; of one height, but not from a region of the other height;

2. a series of pits, whereby light is reflected back to a detecting means ~rom unpitted areas, but is scattered by the pits, thus indicat~ng data; and 3. regions of differing magnetic polarity, whereby the plane of polarisation of polarised light is rotated, by the magneto-optic effect, through an angle which is dependent upon the direction of magnetic polarisation of the recording surface.
Thus data may be read by detecting the angle through which the plane of polarisation, of the polarised ~ 35 incident light beam, has been rotated.

; It should be understood that while the above descriptions relate to methods employing reflection of the incident beam from a recording surface, analogous methods are also known, ` 20370~9 whereby the light is transmitted through the recording surface.

The primary advantage of these methods is that they combine high density data storage with a reasonably high information access speed. This is due to the fact that the recording surface may be located on a rapidly moving carrier; for example a rotating disc. The rotation of the disc brings successive data bits to the optical head in rapid succession, making each data address readily available for reading (or writing).

In each of these methods, a bit of data may be stored on a micron-sized re~ion on the recording medium. However, in practice a data density of one bit per micron is very difficult to achieve. This is due to the fact that the regions in ~uestion generally do not have sharply defined edges. As a result, if two bits of the same value (for example, logical "1") are directly adjacent, it becomes very difficult to distinguish them. To avoid this problem, various schemes, such as 8 to 14 modulation, are used to ensure that at least one null bit is interspersed between each data bit.
These methods have the effect of greatly reducing the overall density of data storage.
second method of optical storage of data is known as holographic data storage. In this method, data is stored as a holographic image. The image may be reconstructed, and the data thereby read, by reproducing the reference beam originally employed to create the hologram. An interesting feature of holographic data storage results from the use of Fourier Transform holography, whereby several distinct holographic images (each containing a bit of data) may be recorded, on the same physical region of the holographic plate. The thereby superimposed holographic images (bits of data) may be accessed, as a group, by reproducing the reference beam. It will be apparent that this approach provides the possibility for creating optical data storage -~ 2~370~9 media with very high data densities. The potential for ultra high density data storage becomes more apparent in view of the fact that resolution (the size o~ the smallest ~eature that can be distinguished) of the holographic image, is a function o~ the wave-length of light used to form the hologram. It has been proposed, ~or example, to use X-rays to record holographic images of individual molecules. One might therefore foresee a holographic data storage medium, recorded using a high Ultra-violet light or X-rays, in which tens of bits of data are recorded on aach micron sized region o~ the recording medium.

However, holographic data storage suffers from a significant disadvantage. The data is stored as a pattern of interference fringes, similar to a diffraction grating. If the recording medium moves during the recording of data, the fringes become blurred and the information is obliterated. It will be apparent that the separation of interference fringes is approximately one wave-length, thus the amount of allowable movement of the recording medium, during recording, is less than one wave-length of the recording light. This restriction imposes severe restraints on the practical application of ultra-high data density holographic memory devices.

Implicit in the above described methods is the principle of recording information on a surface that, at least during reading of information, may be moving with respect to the reading head. However, "a recording surface" represents at most a two-dimensional data storage structure. For example, Fourier Transform holography, by distributing th~ information across the full width of the recorded region, yields a good representation of a two-dimensional data structure. However, in most commonly used embodiments (particularly when data is represented as small pits or magnetised regions), the recording surface is divided into individual tracks, thus yielding what is in effect a one-dimensional structure.
Furthermore, the recording surface is usually presumed to move with respect to the reading head. However, complex control ` 2~0~9 circuitry is necessary to maintain the recording surface at the required velocity with respect to the reading head, and renders the data storage device prone to mechanical failure.

SUMMARY OF THE INVENTION
_ It is therefore an object of the invention to provide an optical data storage medium wherein the data is stored in a three-dimensional storage structure.
It is a further object of the inve~tion to provide a 3-D
optical data storage medium which has the ~ollowing properties:

1. it must be possible to read from, and write to, any and all recording areas of the medium without disturbing other recording areas; and 2. there should, in principle at least, be no limit to the number of recording areas comprised within the three-dimensional data storage medium.

It is yet a further object of the invention to provide a three-dimensional data storage medium in which the data cells are stored in such a manner that they may be accessed in clusters or groups in order to permit the data access speed to be as high as possible.

It is another object of the invention to provide a data storage device that uses the 3-D data storage medium.

It is yet another object of the invention to provide a data storage device that uses a 3-D storage medium, wherein the 3-D storage medium is substantially motionless with respect to the read/write head, and which therefore has as few moving parts as possible.

2V37~9 The 3-D data storage medium of the invention is provided as comprising a plurality of parallel recording layers arranged one atop the other in order to produce a multi-layered structure. Each recording layer is delineated and separated by respective upper and lower transparent bounding layers, whereby the lower bounding layer of one recording layer also serves as the upper bounding layer of the next lower recording layer. On the top surface of the top-most bounding layer, and on the bottom surface of the bottom most bounding layer, there may be disposed transparent metallic electrodes such that an electric field may be applied to the recording layers interposed therebetween. Each respective recording layer comprises a recording material of which the principle characteristics are:
1. the material is substantially transparent;

2. the material is stable in any one of at least two transparent states characterised by having different refractive indices; and 3. the material may be caused to switch from one transparent state to the other by stimulating the material with a beam of light in the presence (or absence) of an electric or magnetic field.

The data storage device of the invention is provided as an apparatus for recording and reading information to and from a three-dimensional data storage medium. The apparatus comprises:
writing optical head means for directing at least one beam of light through the 3-D data storage medium so that sufficient optical energy may be absorbed by the recording material comprised within a predeterminately shaped reyion at a selected data location of a selected recording layer such that the energy absorbed by the recording material within the predeterminately shaped region is sufficient to permit the "` 2~370~9 formation of a data cell (containing a logical "1") at the selected data location;
reading optical head means for directing a light beam composed of parallel rays polarised in a predetermined direction through the data storage medium at a predetermined angle with respect to the recording planes so that data cells recorded therein may be detected by the pattern o light rays which emerge from the data storage medium:
photo-detecting means for detectiny the light emerging from the data storage medium;
signal processing means for detecting the signals produced by the photo-detecting means, and recognising the data from the pattern of light formed on the photo-detecting means;
15control maans for controlling the writing optical head means, the reading optical head means, the photo-detector means, and the signal processing means; and support means for supporting the data storage medium in an operative relation to the writing head means, the reading head means, and the photo-detector means.

The writing head means and the reading head means may be comprised within the same physical unit. The writing ; head means may produce a singIe beam of a selected frequency so that a particular recording layer of the data storage medium may be accessed for recording information according to the frequency of the beam. Alternatively, the writing optical head means may produce a plurality o~ writing beams which are adapted to intersect at a selected location on a selected recording layer of the 3-D data storage medium.

In combination, the 3-D data storage medium of the invention might have an appearance very similar to a transparent, or semi-transparent card. The card may be inserted into the data recording/reproducing device of another aspect of the invention, where it is held in an operative relation to the reading and writing optical head(s).

-` 2~70~
One class of material that meets the above recording material specification is known as Liquid Crystals. Liquid Crystals are materials that display the long-range molecular order which is characteristic o-f crystals in spite of the ~act that they are liquids. These materials demonstrate a marked change in their optical characteristics when heated and/or when subjected to electric fields. Liquid crystals display thres meso-phases between ~he solid and isotropic liquid states:
smectic, nematic, and cholesteric. It should be noted that while these three states are generally exhibited in order with increasing temperature, not all liquid crystal materials exhibit all phases.

Smectic A phase liquid crystal materials are highly viscous and composed of molecules aligned parallel to each other, and in layers. In this phase, the long axis of the molecules are ; arranged perpendicular to the layers. Smectic A phase liquid crystal materials may be arranged to have their molecules in two different states of molecular alignment with respect to the major bounding layers. In the homogenous state, the long axis of the molecules are arranged parallel to the major boundin~ layer~. In the homeotropic state, the long axis of the molecules are arranged perpendicular to the major bounding layers.
It has been found that in the homogenous state, the surface treatment of the major bounding layers determines the direction of alignment of the molecules. For example, Dow-Corning XZ-2-2300 or cetyl-trimethyl-ammonium-bromide may be used as a surfactant to encourage formation of a homogenous molecular orientation, while other surface treatments, such as oblique deposition of silicon oxide or the use of certain polymers such as polyvinyl alcohol rubbed in the desired alignment direction, may be used to encourage formation of homogenous molecular orientations with the molecules aligned in a specific direction. If the liquid crystal material is heated out of the smectic phase, and then allowed to cool, it will adopt the corresponding homogenous alignment.

2~370~

The homeotropic molecular orientation may be produced either by applying a strong electric field, which forces the molecules to align themselves with the electric field without leaving the smectic phase, or by heating the li~uid crystal out of the smectic phase, and then allowing it to cool in the presence of a weak electric field.

While both homogenous and homeotropic states are optically transparent, the differences in birefringence of the two molecular orientations result in a difference in the refractive indices of the two states to light polarised in a particular layer. Specifically, the index of refraction is higher when the molecules lie in the plane of polarisation and are oriented perpendicular to the direction of propagation of the liyht, than when the molecules are oriented perpendicular to the plane of polarisation, or lie in the plane of polarisation and are oriented parallel to the direction of propagation.
A variety of liquid crystal materials are available, and it is possible, to a point, to specify the desired temperature range in which the material is to be in ~he smectic phase.
For example, a typical temperature range may be between 0C
and 30C.

A further advantageous characteristic of smectic phase liquid crystal materials lies in the ~act that the two states may co-exist within the same body of material virtually indefinitely provided the material is not subjected to excessive heat or strong electric fields. This long-term stability has prompted significant research into the utilisation of smectic phase liquid crystal materials for data storage devices and other optical devices. See United States Patent No. 4,893,907 (Mallinson) and United States Patent NoO 4,240,712 (Thirant).

In addition, a liquid crystal may be stained with a dye, which may be selected to absorb a particular frequency of light 2~37~

while remaining substantially transparent to other frequencies. For example, a red dye is substantially transparent to red light, but strongly absorbs blue. Thus a laser (such as an argon laser) emitting a blue light may be used to record information in a red-dyed liquid crystal material, while a red light (such as from a helium-neon laser) would be readily transmitted through the li~uid crystal material without substantially heating the liquid crystal material and affecting the recorded information.
It should be noted that while liquid crystal materials are currently available which meet the requirements of the invention, and while the embodiments described herein employ such materials, the invention is not limited to Liquid crystal material. Indeed, any material which meet~ the above described performance specification may be used.

BRIEF DESCRIPTION OF THE DRAWINGS
The objects, ~eatures, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings wherein:

FIGURE 1 illustrates in perspective an enlarged portion of a data storage medium of the invention;
FIGURE 2 illustrates schematically the stepped splitting of a beam of light as it reads data cells in the data storage medium of the invention;
FIGURE 3 illustrates a principle of geometric optics central to the operation of the invention;
FIGURE 4 illustrates schematically an enlarged cross-section view of a basic structural unit from which the data storage medium of the invention is composed;
FIGURF 5 illustrates schematically an enlarged cross-section of a data storage medium of the invention comprising a plurality of recording layers;

~ O ~ 9 FIGURES 6A and ~B illustrate schematically two paths which may be followed by an incident ray of polarised light as it passes through a racording layer;
FIGURE 7 illustrates schematically a path followed by a ray of polarised light as it passes through a plurality of recording layers;
FIGURE 8 illustrates schematically the spatial relationships between adjacent data cells in each recording layer, and between adjacent recording layers.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 1 illustrates in perspective an enlarged portion of a data storage medium of the invention. A light beam 2 is shown to lay in the x-z plane and is incident on the upper surface of the storage medium 1 at an angle ~j such that it may be used to detect data stored on a plurality of data storage layers 3 separated from one another by respective bounding layers 4 as the light beam 2 passes down through the storage medium 1.
Each data storage layer 3 may contain data organised into a two-dimensional structure such as, for example, parallel tracks of data cells. It should be understood that the cross sectional views illustrated in Figures 2 through 8 are all in the plane of a light beam used to read the data. In the case shown in Figure 1, this corresponds to the x-z plane.

Referring to Figure 2, the top-most recording layer (level L1) is preliminarily fully loaded with bits of data (i.e., every available data location 6 has a data bit written into it). The level L1 serves as a target and verification level, rather than as data storage. A beam of light, which is composed of a plurality of parallel polarised rays, is caused to illuminate the side and top face of a level L1 cell. Part of the beam encounters the top of the data cell, and is split off from the remainder of the beam. This split-off beam then drops vertically out of the 3-D data structure 1 and is directed onto a photo-detector array 8, where it establishes `I ; .

~3~ 3 a target or datum point 9a. The remainder of the beam continues to penetrate down through successive recording layers of the 3-D data structure. Each time the beam encounters a data bit (representing, for example, logical 5 "1"), part of the beam encounters the side of the data cell, is offset slightly upwards, and continues down through successive layers with the same angle of incidence. However, the slight upward offset caused by the passage through the side of the data cell ensures that a portion of the beam also encounters the top of each data cell along the path of the beam. This portion of the beam is split off so that it drops vertically out of the 3-D data structure 1 and is directed to a spot 9b on the photo-detector array 8.

A result of the geometry described in the foregoing discussion is that a data cell on a given level below level Ll will illuminate a spot 9b on the photo-detector array which is separated from the datum point, 9a, by a geometrically fixed distance. This separation between the datum point 9a and a spot 9b is primarily determined by the angle of incidence 0;
of the beam, the level on which the data cell is located, and the separation distance D between the levels. Therefore, by arranging suitable optical imaging means (such as lenses etc) so as to project the "split-off" light rays onto the photo-detector array 8, it will be possible to detect the presenceor absence of a data cell in every data location along the path of the "read" beam simultaneously.

Referring now to Figure 3, a 3-D data storage medium of an embodiment of the invention comprises the following basic structural unit:

1. thin (less than 500 ~m thick), transparent upper an lower bounding layers 4 and 4a, composed, for example, of glass, siX-oX, or plastic;

2~3~

2. a thin (in the range of 5~m to 50~m), recording layer 3 composed of a suitable recording material, such as a liquid crystal material; and In order to facilitate reading of information from any given layer in the structure, the index of refraction of each of the bounding layers 4 are equal to each other, and furthermore are substantially equal to the index of refraction of the liquid crystal material in its low index state (with respect to a particular plane of polarisation of the reading light beam);

The surfaces of the bounding layers exposed to liquid crystal material may be suitably treated so as to encourage the formation of an homogenous molecular alignment. Under this arrangement, the plane of polarisation of the "read" beam would be selected such as to cause the homogenous alignment to be the low index state of the liquid crystal material.
Thus a logical "1" may be written into the homogenous liquid crystal material by heating a suitably shaped region of the liquid crystal in the presence of an electric field.
Subsequently, erasing the data cell (i.e., writing a logical "0") may be accomplished by reheating the region in the absence of the electric f ield.

Alternatively, the plane of polarisation might be selected such that the homogenous molecular orientation is the high index orientation, and the homeotropic alignment is the low inde~ state. The surfaces of the bounding layers are treated, for example, as described above, ~o as to encourage the formation of the homogenous orientation. The entire data storage medium (or a selected part thereof) might be preliminarily polarised to the homeotropic alignment using either a strong electric field alone or a weak electric field in combination with heating the recording layers. Thereafter, a logical "1" may be recorded by heating a suitably shaped region of a selected recording layer in the absence of an electric field, to thereby allow the li~uid crystal within the heated region to reassume a homogenous orientation.

--" 2~3~3~
Analogously, erasing a data cell (i.e., writing a logical "O") may be accomplished by reheating the data cell in the presence of an electric field to thereby re-polarise the liquid crystal material to the homeotropic alignment.

Using the above described basic structural unit, a composite structure i5 defined in which a plurality of structural units are stacked one on top of the other, as shown by the multi-layer composite structure illustrated in Figure 4, to create a multi-layer storage medium 1 with successive recording layers 31 ... 3~ separated by respective bounding layers 41 ...
4N~1. On the exterior surfaces of the upper-most and lower-most bounding layers, respective transparent metallic conductor films 5 and 5a are disposed which serve to provide means for applying an electric field to the recording layers 3 interposed therebetween. The entire multi-layer structure may be comprised within a suitable protective shielding (nok shown) to provide protection from physical damage or other forms of attack.
Within each recording layer 3, the liquid crystal material may be stained with a respective dye. By this means, writing to any given layer may be effected by selecting the appropriate frequency of light. The "writing" laser light may be focused into a narrow beam, and directed, normal to the recording layers 3, downward through successive recording layers 3. Each recording layer 3 is transparent to the writing beam, except the layer doped with the dye specific to that frequency of light. The dye absorbs the light and heats the liquid crystal material, allowing the presence or absence of a weak electric field between the transparent conductors 5 and 5a to effect a change in the molecular orientation of the liquid crystal material in the heated region. Thus writing may be accomplished at the selected location on the selected level.

Alternatively, writing to a particular location may be accomplished by a plurality of intersecting beams. For ``` 2~37~3~

example, three beams (one parallel to each orthogonal axis) may be caused to intersect at the point where the data cell is to be written into the recording layer. The power levels of the intersecting beams, and the energy absorption rate of the recording material may be suitably matched so that at the intersection point, the optical energy intensity is high enough to facilitate the writing (or erasure) of a data cell in the selected recording layer.

The following is a detailed discussion of the geometry required for proper operation of the 3-D optical data storage medium of the invention.

Figure 5 illustrates a principle of optical geometry which is central to the operation of the data storage medium of the invention. The reference 10 denotes a boundary surface between two transparent materials (or alternatively two different transparent states of the same material) wherein the index of refraction of the one material nl is less than the index of refraction of the other material n2. An incident ray of light 11 meets the boundary 10 at an incident angle of el and is refracted at the boundary 10 to an angle of e2. The an~les ~1 and ~2 are related by Snell~s law:

nl Sin~1 = n2 Sine2 [Equation 1]

Notice that if a ray of light penetrates the boundary 10 exactly normal to the layer of the boundary 10, both ~1 and e2 will be zeror and thus the variation in the indices of refraction will have no effect on the direction of the beam.
As ~2 increases, ~1 also ir.creases until e, equals 90, as illustrated by light ray 12 - 12a in Figure 3. In this case, ~2 iS often referred to as the critical angle ecrit~ and Snell's law may be reduced to:
sin~crit = n1/n2 [Equation 2 2 ~ 3 ~ ~ 5 ~

Figures 6A and 6B illustrate an enlarged cross-s~ction view of a particular recording layer 3a in the multi-level structure 1 shown in Figure 5 (designated as level La in Figure 4). Two incident light beams 13 and 14 are shown penetrating the recording layer 3a~ on level La~ and are seen to be refracted by respective regions which have been modified by the writing of bits of data (representing, for example, binary 1) into the r~cording layer 3a .

Referring to Figure 6A, the incident ray 13 penetrates the boundary region 15 from the major bounding layer 4a and passes into a region of liquid crystal material in its high index state (hereinafter referred to as high index liquid crystal).
As noted in Figure 6, the incident angle of ray 13 (with respect to an axis normal to the layer) is 8l. The ray is refracted according to Snell's law to angle er~ as it passes through the boundary region 15, and then is refracted again when the ray 13 crosses the boundary 16 between the high index liquid crystal and liquid crystal material in its low index state (hereinafter referred to as low index liquid crystal).
As illustrated in Figure 6A, by suitably selecting the angle of incidence el (or equivalently the angle ~j with respect to the layer), the angle of incidence at the boundary 16 can be made to be equal to ~crit- Recall that for a boundary between materials (or adjacent regions of the same material) with low and high indices of refraction of n1 and n2 respectively:
Sin~crit = n1/n2 [2]
3 0 which is equivalent to:
~ crit = Sin1 (n1/n2) and from Snell's law:
~ l = Sin1(n2-n1~1-Siner) furthermore:
~r = pi/2 ~ ~crit where pi = 3.1415927 = pi/2 - Sin~1 (nl/n2) and: ~j = pi/2 - ~l 2 ~ 3 ~

Thus:
e~ = Sin~1(n2-n~ Sin[pi/2 - Sin~1(n1/n2)]) and: [Equation 3]
~j = pi/2 - Sin-1(n2-n1~1~Sin[pi/2 - Sin~1(n1/n2)]) Under these conditions, the ray 13 will be refracted at the boundary 16, as shown in Figure 6A, and will exit normal to the recording layer. Clearly, once the ray is normal to the layer, all subsequent boundaries encountered by the ray will be either perpendicular or parallel to the ray, and thus the ray will pass down through successive recording layers substantially without further refraction.

As may be seen from Figure 6A, the ray 13 must encounter both boundary 15 and boundary 16 to become perpendicular to the layer. Thus there is a limiting relationship between the width w of the high index region and the thickness T of the recording layer 3~. Specifically:
Taner 2 w T

so that:
w ~ T Taner or:
w < T Tan[pi/2 - Sin1 (n1/n2)] [Equation 4]
So that the maximum allowable width of the high index region, wmax, is given by:

W~x = T Tan[pi/2 - Sin1(n1/n2)] [Equation 4a]

Referring now to Figure 6B, the incident ray 14 crosses boundary 17 between the upper bounding layer 4a and the low index liquid crystal material, at an incident angle of e; (as determined by Equation 3 above), without refracting by virtue of the indices of refraction being equal. The incident ray 14 then traverses a substantially planar boundary 18 at point 203~59 A and passes into a region of high index liquid crystal material. At this boundary the light beam is refracted, as illustrated in the Figure 6B, according to Snell's Law. When the ray 14 emerges from the high index liquid crystal, through a second substantially planar boundary 19 at point B, it again refracts according to Snell's Law and returns to iks original angle of incidence, aj. Notice, however, that while the emerging ray 14a retains its original direction, its path has been offset by a distance d.
For a high index liguid crystal region of width w; a light ray path (A - B) of length h through the high index region;
low and high refractive indices of n1 and n2 respectively; and incident and refractive angles ej and ar respectively, the following relations may be readily determined:

let ~2 = ~3i ~ 0r then d = h Sine2 from Snell's law:
er = Sin-1(n1-n2~1-Sine;) 30 since:h = w = w cOser Cos[Sin-l(n1-n2'~-sine;)~

Thus: [Equation 5]
d = _ w Sin[~j - Sin1(n1-n2~1-Sin~;)]
Cos[Sin-l(n1-n2-loSin~j)]

Figure 7 illustrates schematically a path followed by a ray of polarised light 20 as it passes through a plurality of recording layers 3. The angle of incidence, with respect to the layer, may advantageously be selected according to ~quation 3. Notice that each time the ray is incident of the side of a high index region 21 (a recorded bit, or binary 1), ~ ~7 3 Yt ~

the ray is offset according to Equation 4 and thus is caused to be incident on each successive high index region 21 at progressively higher points. As illustrated in Figure 7, the ray 20 will ultimately become incident on the top o~ a high index region 21 and will be refracted as illustrated in Figure 6A.

Figure 8 illustrates schematically the spatial relationships between adjacent data cells in each recording layer, and between adjacent recording layers. The top two recording layers (at levels L1 and ~) of a recording medium are shown in cross section. Three bounding layers (41~ 42, and 43) delineate two recording layers 31 and 32. Within each recording layer 3 there are a plurality of locations which are available to store data, and may be referred to as data locations 26. In the top-most recording layer (level L1), all of the data locations 26 have a "data cell" (ie. a region of high index of refraction) written into them, so that level L1 is used as a target and verification level rather than for data storage. All subsequent recording layers, however, are used for data storage, and thus the presence or absence of a "data cell" in any particular data location 26 on levels through ~ will be data-dependent.

As shown in Figure 8, each level is separated by a distance D. Within each recording layer, adjacent data locations 26 are of constant width w, and are separated by a low index region of width e. Thus each data location 26 occupies a region with a total width s = (w + e). In addition, each recording lay r 3 has a thickness, T, which is assumed to be constant throughout each layer.

In the top most recording layer (level L1), the width of the "read" beam 22 is at its maximum, and thus it is this level that determines the separation between data locations 26.
Each time the beam 22 encounters a data cell it is doubly refracted. A portion of the beam is split off by being incident on the top of the data cell, and drops vertically 2~3 ~ ~9 out of the data structure as described previously in connection with Figure 6A. Ths remainder of the beam is off-set by a distance d, and continues down through successive levels of the storage medium with the same angle of incidence ~j.

Consider a data cell, written in data location 26a on level ~1. A light beam 22, composed of parallel polarised rays (of which three, 23, 24 and 25 are illustrated) is made incident on data cell 26a at an angle of incidence of ej. Ray 23 encounters the top of the data cell 26a, and is split off as described above, becoming ray 23a. Note that, to avoid confusion, the path of ray 23 through the data cell 26a is not shown. Ray 25, which is a bounding ray of the beam 22, may just touch the upper left edge (as shown in figure 8) of the adjacent data cell. The ray then intersects the side of the data cell 26a at a distance ~t above the bounding layer 42~ is refracted, and then exits data cell 26a at the lower left edge of the cell. The thereby offset ray 25a then passes down through bounding layer 42 until it encounters data location 26b on level ~. If a data cell has been written into location 26b, then ray 25a will again be refracted and offset;
otherwise it will continue down through successive recording layers until it encounters a data cell. Ray 24 encounters the ` 25 side of data cell 26a just at the upper right edge (as illustrated in Figure 8), and is refracted in a manner similar to that of ray 25. However, notice that the offset ray 24a will encounter the next data cell (for example, at location 26b) on the top surface of the cell, thereby ensuring that a portion of the beam 22 is available for splitting off so that the data cell at 26b may be detected.

The above described ray paths are highly geometry dependent.
In particular, the projection of the ray 24 (shown as dashed line 24b) must connect the upper right edges of all of the data locations along the path of the beam 22. Thus 'ran ~j = D

~3~JQ'3 Now, it will be seen from Figure 8 that:
X = m s = m (e+w)m = integer So that the inter-level distance D will be given by:
D = X Tane or:
D = m (e+w) Tanej m = intege~Eguation 6]
Note that in Figure 8, the beam 22 is shown to be displaced by three data locations as it passes down through the two levels depicted. This corresponds to m = 3 in the example schematically illustrated in Figure 8. It will be clear, however, that this is strictly an illustrative example, and is not restrictive beyond the fact that m will be an integer ; value.
Furthermore, it will be seen that (also re~erring to Figure 6B):
~t = h Sin~r = w . Sin~r = w Taner Cos~r so that the minimum width, emjn, of the low index region between adjacent data locations 26 will be given by:
Tane; = T - ~t emin or: -35 emin = T ~t= T - tw Tan~r) Tanei Tane since:
~r = Sin1(nl-n21-Sin~j) then:
emin = T - (w TanrSin7(n1-n21 Sin~jLlL [Equation 7]
Tane;

Using the above detailed discussion of optical geometry and the interaction between individual rays of light and regions `"` 2 ~ 9 o~ high and low refractive index, it will now be possible to fully understand the operation of the 3-D data storage structure of the invention in which a plurality of recording layers are stacked as illustrated in Figures 2 and 4.

It will be apparent that the geometry of the data cell is also important to the proper functioning of the 3-D data storage medium of the invention. In particular, the data cell may be in the form of a rectangular parallelepiped, with its principle (longest) axis substantially orthogonal to the layer of the recording layer. The sides of the data cell through which the "read'l beam will pass must be substantially planar to prevent scattering of light. There is, however, no particular restriction on the shape of the sides o~ the data cell through which light does not pass, except that these sides must not interfere with the passage o~ light through the data cell, or the passage of light through adjacent data cells.

A detailed discussion o~ the data storage device of the invention is not provided here. However, the detailed discussion of characteristics and operation of the 3-D data storage medium of the invention contains enough information that necessary operational characteristics of the data storage device which uses the medium will be readily inferable by those skilled in the art.

It will be apparent that there exists many ways in which the cells of the invention may be applied. For example, the 3-D
data storage medium, and the data storage device which uses it, may be constructed so that the medium is held su~stantially motionless while the "read" beam scans across the medium (while maintaining a constant angle with respect to the medium). As an alternative, the medium may be movable so that the "read" beam scans across the medium as the medium movas under the reading optical head, in a manner similar to that used in conventional optical data storage methods.

~3~9 The detailed discussion above described staining each layer of liquid crystal material with a dye so that a laser beam tuned to a specific frequency may be used to write to a selected layer. However, a plurality of beams could be used.
In this case each beam would deliver a portion of the energy required to allow the liquid crystal material to change from one state to another. Only at the point of intersection of all of the beams would there be sufficient energy for a change in state to occur. Thus, writing to a specific point on a specific layer may be accomplished with beams of a single ~requency.

The embodiments discussed above and the figures used all illustrate a flat planar structure for the 3-D data storage medium. However, other shapes, such as discs or cylinders, are possible, particularly if the 3-D data storage medium is intended to be moving with respect to the optical head.
Furthermore, the 3-D data storage medium may be permanently installed within the data storage device of the invention, or may be removable.

Thus it will be seen that there are many possible embodiments of the data storage medium and device of the invention, all of which are to be considered to be included herein except as they depart from the scope of the appended claims.

Claims (9)

1. An information storage medium in which information may be recorded, read, and erased by optical means; wherein said data may be stored in a three-dimensional structure.
2. An information storage medium as claimed in claim 1, wherein said storage medium comprises a plurality of separate recording areas, each said recording area comprising a substantially transparent recording material.
3. An information storage medium as claimed in claim 2, wherein said plurality of recording areas are disposed with respect to each another so as to form a multi-layered structure of equidistant recording layers.
4. An information storage medium as claimed in claim 3, wherein said layers are bounded on both surfaces by substantially transparent bounding layers.
5. An information storage medium as claimed in claim 3, wherein said recording layers are planar.
6. An information storage medium as claimed in claim 3, wherein said recording layers are cylindrical.
7. An information storage medium as claimed in claim 1, wherein said recording material possesses the following properties:
the material is substantially transparent;
the material is stable in any one of at least two transparent states characterised by having different refractive indices to polarised light passing therethrough;
and the material may be caused to switch from one transparent state to the other by stimulating the material with a beam of light in the presence or absence of an electric field.
8. An information storage medium as claimed in claim 7, wherein said recording material is a liquid crystal material.
9. An information storage medium as claimed in claim 8, wherein said liquid crystal material is a smectic phase liquid crystal material having two transparent states in which the molecules may be oriented in respective perpendicular directions having different refractive indices to light polarised in a predetermined direction.

14. An information storage medium in which binary data may be recorded and read by optical means, wherein said information may be stored in a three-dimensional structure composed of a plurality of equidistant recording layers bounded on both surfaces by substantially transparent bounding planes, each said recording layer comprising a liquid crystal material having the following properties:
the material is substantially transparent;
the material is stable in any one of at least two transparent states characterised by having different refractive indices to polarised light passing therethrough;
and the material is caused to switch from a first transparent state to a second transparent state by stimulating the material with a beam of light in the presence of an electric field, and is caused to switch from said second transparent state to said first transparent state by stimulating the material with a beam of light in the absense of an electric field.

15. An information storage medium as claimed in claim 14, wherein said liquid crystal material is a smectic phase liquid crystal material having two transparent states in which the molecules may be oriented in respective perpendicular directions, each of said directions having different refractive indices to light polarised in a predetermined direction.

16. An information storage medium as claimed in claim 14, wherein said bounding planes have an index of refraction substantially equal to the index of refraction of said first transparent state of said recording material, said first transparent state being characterised as having a comparatively lower index of refraction to light polarised in a predetermined direction.

17. An information storage medium as claimed in claim 3, further comprising electrode means for applying an electric field substantially perpendicular to a plurality of recording layers, wherein said electrode means comprise at least two metallic electrodes disposed on respective opposite surfaces of said storage medium, said respective surfaces being parallel to said recording layers.

18. An information storage medium as claimed in claim 14, further comprising electrode means for applying an electric field substantially perpendicular to a plurality of recording layers, wherein said electrode means comprise at least two metallic electrodes disposed on respective opposite surfaces of said storage medium, said respective surfaces being parallel to said recording layers.

19. An information storage medium as claimed in claim 17 or 18, wherein said metallic electrodes are transparent.

20. An information storage medium as claimed in claim 17 or 18, wherein said metallic electrodes comprise:
a first metallic electrode, said first metallic electrode being optically reflective; and at least one second metallic electrode, said second metallic electrode being transparent.

21. An information storage medium as claimed in claim 14, wherein the logical l's of said binary data are stored by the presence of data cells at predetermined data locations within each of said recording layers, and wherein the logical 0's of said data are stored by the absence of said data cells at said predetermined data locations.

22. An information storage medium as claimed in claim 19, wherein said data cells comprise predeterminatly shaped regions of said recording material, and wherein said data cells are formed by causing recording material in said first transparent state to be switched to said second transparent state.

23. An information storage medium as claimed in claim 22, wherein said predeterminatly shaped region is a rectangular parallelepiped disposed with its long axis aligned perpendicular to said recording layer, said rectangular parallelepiped having:
a first pair of opposed faces, said first pair of faces being substantially planar and aligned perpendicular to the propagation plane of a read beam used to detect said data cell; and a second pair of opposed faces, said second pair of faces being aligned parallel to the propagation plane of a read beam used to detect said data cell, and wherein said second faces may be convex.

24. An information storage medium as claimed in claim 23, wherein said second pair of faces of respective adjacent data cells do not overlap each other.

25. An information storage medium as claimed in claim 23, wherein said second pair of faces of respective adjacent data cells at least partially overlap each other.

26. An information storage medium as claimed in claim 14, wherein each one of said plurality of recording layers is stained with a respective dye which absorbs light of a predetermined frequency, while remaining substantially transparent to light of a different frequency.

27. An information storage medium as claimed in claim 26, wherein said respective dye is selected so as to absorb a frequency of light unique to each respective layer.

28. An information storage medium as claimed in claimed 26, wherein said respective dye is selected so as to absorb light of the same frequency on every layer.

29. Apparatus for recording and reading information to and from a three-dimensional data storage medium, said apparatus comprising:
means for recording data in a three dimensional data structure;
means for reading data from a three dimensional data structure;
wherein said three dimensioanl data structure is a multi-layered structure composed of a plurality of equidistant recording layers.

30. An information storage device as claimed in claim 29, wherein:
said means for recording data comprises writing optical head means for directing at least one beam of light through the 3-D data storage medium so that sufficient optical energy may be absorbed by the recording material comprised within a predeterminatly shaped region at a selected data location of a selected recording layer, so as to permit the formation of a data cell at said selected data location; and said means for reading data comprises reading optical head means for directing a light beam composed of parallel rays polarised in a predetermined direction through the data storage medium at a predetermined angle with respect to the recording planes so that data cells recorded therein may be detected.

31. An information storage device as claimed in claim 30, further comprising:
photo detecting means for detecting the light passing out of the data storage medium;
signal processing means for detecting the signals produced by the photo-detecting means, and recognising the data from the pattern of light formed on the photo-detecting means;
control means for controlling the writing optical head means, the reading optical head means, the photo-detector means, and the signal processing means; and support means for supporting the data storage medium in an operative relation to the writing head means, the reading head means, and the photo-detector means.

32. An information storage medium as claimed in claim 31, further comprising voltage supply means, for supplying selected voltages to respective electrodes comprised within said data storage medium.

33. An information storage device as claimed in claim 31, wherein said writing head means and said reading head means are comprised within the same physical unit.

34. An information storage device as claimed in claim 31, wherein said writing head means generates a single light beam of a selectable frequency so that data may be stored in a selected recording layer of said data storage medium; said selected recording layer being selected according to the frequency of the beam and the absorption frequency of a dye comprised within the recording material of the selected recording layer.

35. An information storage device as claimed in claim 31, wherein said writing optical head means generates a plurality of writing beams which are adapted to intersect at a selected location on a selected recording layer of said data storage medium, whereby data may be recorded at the point of intersection of the beams.

36. An information storage device as claimed in claim 31, wherein said supporting means is adapted to hold said data storage medium substantially stationary.

37. An information storage device as claimed in claim 31, wherein said supporting means is adapted to hold said data storage medium such that said data storage medium is operably movable.

38. An information storage device as claimed in claim 31, wherein said predeterminately shaped region is a rectangular parallelepiped disposed with its long axis aligned perpendicular to said recording layer, said rectangular parallelepiped having:
a first pair of opposed faces, said first pair of faces being substantially planar and aligned perpendicular to the propagation plane of a read beam used to detect said data cell; and a second pair of opposed faces, said second pair of faces being aligned parallel to the propagation plane of a read beam used to detect said data cell.

39. An information storage device as claimed in claim 38, wherein said second pair of opposed faces of said data cells may be convex.

40. An information storage device as claimed in claim 39, wherein said second pair of faces of respective adjacent data cells do not overlap each other.

41. An information storage medium as claimed in claim 39, wherein said second pair of faces of respective adjacent data cells at least partially overlap each other.

42. A method of storing and erasing binary data within an information storage medium in which data may be stored in a three-dimensional data structure; said method of storing data comprising the step of causing a plurality of predeterminately shaped regions of a recording material to switch from a first transparent state to a second transparent state; and said method of erasing data comprising the step of causing said plurality of suitably shaped regions of said recording material to switch back from said second transparent state to said first transparent state.

43. A method of storing and erasing data as claimed in claim 42, wherein said information storage medium comprises a multi-layered structure comprising a plurality of equidistant recording layers.

44. A method of storing and erasing data as claimed in claim 42, wherein said first transparent state is characterised as having a comparatively low index of refraction, and wherein said second transparent state is characterised as having a comparatively high index of refraction.

45. A method of storing and erasing data as claimed in claim 42, wherein the logical l's of said binary data are stored by the presence of data cells at predetermined data locations within each of said recording layers, and wherein the logical 0's of said data are stored by the absence of said data cells at said predetermined data locations.

46. A method of storing and erasing data as claimed in claim 45, wherein said data cells comprise predeterminatly shaped regions of said recording material, and wherein said data cells are formed by causing recording material in said first transparent state to be switched to said second transparent state.

47. A method of storing and erasing data as claimed in claim 46, wherein said predeterminatly shaped region is a rectangular parallelepiped disposed with its long axis aligned perpendicular to said recording layer, said rectangular parallelepiped having:
a first pair of opposed faces, said first pair of faces being substantially planar and aligned perpendicular to the propagation plane of a read beam used to detect said data cell; and a second pair of opposed faces, said second pair of faces being aligned parallel to the propagation plane of a read beam used to detect said data cell, and wherein said second faces may be convex.

48. A method of storing and erasing data as claimed in claim 47, wherein said second pair of faces of respective adjacent data cells do not overlap each other.

49. A method of storing and erasing data as claimed in claim 47, wherein said second pair of faces of respective adjacent data cells at least partially overlap each other.

50. A method of storing and erasing data as claimed in claim 46, wherein each one of said plurality of recording layers is stained with a respective dye which absorbs light of a predetermined frequency, while remaining substantially transparent to light of a different frequency.

51. A method of storing and erasing data as claimed in claim 50, wherein said respective dye is selected so as to absorb a frequency of light unique to each respective layer.

52. A method of storing and erasing data as claimed in claim 50, wherein said respective dye is selected so as to absorb light of the same frequency on every layer.

53. A method of storing data as claimed in claim 51, wherein formation of a data cell comprises the steps of:

forming a laser beam composed of light of a selected frequency;
directing said laser beam into said data storage medium substantially perpendicular to said recording layers, whereby the recording material on a particular recording layer is heated by said beam, said particular recording layer being selected according to the frequency of the beam and the absorption frequency of said respective dye;
applying an electric field substantially perpendicular to said layers, such that the recording material heated by said laser beam is caused to switch from said first transparent state to said second transparent state; and removing said laser beam and allowing the recording material heated by said laser beam to cool under the influence of said electric field.

54. A method of erasing data as claimed in claim 51, wherein removal of a data cell comprises the steps of:
forming a laser beam composed of light of a selected frequency;
directing said laser beam into said data storage medium substantially perpendicular to said recording layers, whereby the recording material on a particular recording layer is heated by said beam, said particular recording layer being selected according to the frequency of the beam and the absorption frequency of said respective dye; and removing said laser beam so as to allow the recording material heated by said laser beam to return from said second transparent state to said first transparent state as it cools in the absence of an electric field.

56. A method of storing data as claimed in claim 52, further comprising the steps of:
forming a plurality of laser beams composed of light of a predetermined frequency;
launching said plurality of laser beams into said data storage medium at substantially perpendicular directions, such that said laser beams intersect a selected location within a selected recording layer;
applying an electric field substantially perpendicular to said layers, such that recording material heated by said laser beam is caused to switch from said first transparent state to said second transparent state; and removing said plurality of laser beams and allowing the recording material heated by said laser beam to cool under the influence of said electric field.

57. A method of erasing data as claimed in claim 52, further comprising the steps of:
forming a plurality of laser beams composed of light of a predetermined frequency;
launching said plurality of laser beams into said data storage medium at substantially perpendicular directions, such that said laser beams intersect a selected location within a selected recording layer;
removing said plurality of laser beams so as to allow the recording material heated by said laser beam to return from said second transparent state to said first transparent state as it cools without the influence of an electric field.

58. A method as claimed in either of claims 56 or 57, wherein the frequency of said plurality of laser beams is selected so as to be slightly absorbed by said dye comprised within said recording material.

59. A method as claimed in either of claims 56 or 57, wherein the power of each of said plurality of laser beams is selected such that the energy absorbed by the recording material along the length of a particular beam is insufficient to cause said recording material to switch from one transparent state to the other, except where said laser beams intersect.

60. A method of reading data stored in an information storage medium in which data may be stored in a three-dimensional data structure, said method comprising the step of launching a reading beam composed of light polarised in a predetermened direction into said information storage medium and detecting the pattern of rays emerging from said information storage medium.

61. A method of reading data as claimed in claim 60, wherein said information storage medium comprises a multi-layered structure comprising a plurality of equidistant recording layers.

62. A method of reading data as claimed in either of claims 60 or 61, wherein said first transparent state is characterised as having a comparatively low index of refraction, and wherein said second transparent state is characterised as having a comparatively high index of refraction.

63. A method of reading data as claimed in claim 61, wherein said suitably shaped regions consitute data cells comprised within a matrix of recording material in said first transparent state.

64. A method of reading data as claimed in claim 63, wherein each of said data cells is a rectangular parallelepiped disposed with its long axis aligned perpendicular to said recording layer, said rectangular parallelepiped having a first, a second, and a third pair of opposed faces, said first pair of faces being aligned perpendicular to the propagation plane of said read beam, and said third pair of faces being aligned substantially paralell to said recording layer.

65. A method of reading data as claimed in claim 64, wherein said second faces of said data cells may be convex.

66. A method of reading data as claimed in claim 65, wherein said second pair of faces of respective adjacent data cells do not overlap each other.

67. A method of reading data as claimed in claim 65, wherein said second pair of faces of respective adjacent data cells at least partially overlap each other.

68. A method of reading data as claimed in claim 64, wherein:
said first pair of opposed faces constitute first and second boundaries respectively; and said third pair of opposed faces constitute third and fourth boundaries respectively.

69. A method of reading data as claimed in claim 68, wherein said read beam is caused to be incident on and at least partially illuminate both of said first and third boundaries of said data cell.

70. A method of reading data as claimed in claim 69, wherein the angle of incidence of said read beam with respect to said recording layer is predetermined such that:
the portion of said read beam incident on said first boundary is refracted as it passes through said first boundary, traverses across said data cell, and is refracted again to return to its original angle as it passes through said second boundary; and the portion of said read beam incident on said third boundary is refracted as it passes through said third boundary, traverses across said data cell and becomes incident on said second boundary; whereby the angle of incidence of light on said second boundary is such that the light emerging from said data cell is substantially perpendicular to said recording layer.
CA 2037059 1991-02-26 1991-02-26 Three-dimensional optical data storage structure Abandoned CA2037059A1 (en)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5472759A (en) * 1993-12-16 1995-12-05 Martin Marietta Corporation Optical volume memory
US7660415B2 (en) * 2000-08-03 2010-02-09 Selinfreund Richard H Method and apparatus for controlling access to storage media

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5472759A (en) * 1993-12-16 1995-12-05 Martin Marietta Corporation Optical volume memory
US6045888A (en) * 1993-12-16 2000-04-04 Lockheed Martin Corporation Optical volume memory
US7660415B2 (en) * 2000-08-03 2010-02-09 Selinfreund Richard H Method and apparatus for controlling access to storage media

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