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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention in general relates to the field of optical recording systems and media and, in particular, to storage media comprising optically differentiated or discriminated data sites by which means a greater resolution and storage density is
attained.
2. Description of the Prior Art
Technical data relevant to the present application can be found in sources such as:
Optical Physics, Lipson and Lipson, Cambridge University Press, 1969.
Optical Materials, S. Musikant, Marcel Dekker, Inc., New York, 1985. P. 64-76.
Guerra, J. M., Phase Controlled Evanescent Field Systems and Methods for Optical Recording and Retrieval, Patent Allowed, Sept. 1997.
Conventional optical storage media commonly use a "land and groove" configuration in which alternating data tracks are separated in height by .lambda./6, where .lambda. is the illumination wavelength. The purpose of this height differentiation
between tracks is to cause destructive interference, or cancellation, of the ringing caused by the coherent illumination, thereby allowing spacing, or track pitch, that is closer to the resolution limit of an incoherently illuminated system. However,
the alternating height is usually binary, and does not allow super-resolution (i.e. track separation smaller than the resolution limit of the optical system).
Other examples of differentiated tracks or sites include the red, green, and blue color filter stripes or dot matrix such as found in color television monitors, and similar red, green, and blue color stripes in Polaroid's.TM. instant color slide
film and Polavision.TM. instant movie film. In both cases, the purpose of the color stripes was to cause selective local color change from white to black and any shade of color in between by combining one or all of the RGB color elements in various
intensity combinations. However, in neither example was the intent or effect to cause super-resolution, nor means for optical data storage in the case of the former.
U.S. Pat. No. 5,910,940 issued Jun. 8, 1999 "Optical Recording Systems and Media with Integral Near-Field Optics" issued to Guerra discloses optical storage media having an integral micro-optical structure to effect higher resolution. In
part, the higher resolution results from the larger system numerical aperture that is obtained by combining the micro-optics in the medium with the drive objective optics. This larger numerical aperture allows the higher spatial frequencies contained in
the evanescent, or "near-field," to contribute to the image, thereby increasing resolution and storage density.
SUMMARY OF THE INVENTION
In the present application, the active optical layer comprises micro-optical properties for the increase of resolution and information storage density, primarily for use in, but not limited to, a propagating light non-flying optical data storage
system. The micro-optical structures or domains in the optical media optically isolate, discriminate, and differentiate adjacent optical active sites or optical artifacts such that the detection limit, rather than the much larger resolution limit, of
the optical data storage system may be fully utilized for higher storage density.
Optical visibility and detection, rather than optical resolution, serve to increase the storage density of optical data systems. Optically-differentiated sites smaller than the resolution limit of an optical system are detected, or made visible,
by that system if the sites are separated by more than the resolution limit. Particles as small as 4 nanometers can be seen in normal dark-field microscopes, for example, if those particles are far enough apart. That is to say they must be separated at
least by a distance which is equal to or greater than the resolution limit "d," defined as the illumination wavelength .lambda. divided by the numerical aperture (N.A.) of the optical system:
where the numerical aperture is a product of the index n of refraction in which the object to be resolved is immersed, and the sine of the half angle .theta. subtended by the optical system at the object:
Equation (1) is the well-known resolution limit for a microscope as worked out first by Abbe in 1888.
The measured size of the particles will be much larger, on the order of the Airy disc for that system. Whether a particle is 1/10 or 1/2 the optical system resolution, the resulting Airy discs will be equivalent in diameter to each other and to
the resolution limit. However, the Airy disk for the smaller particle will be less bright. Given enough light for the required signal-to-noise, a particle much smaller than the resolution limit is visible, as long as it is isolated from the nearest
particles by at least the resolution limit. Bright-field microscopes, for example, also show particles smaller than their resolution limit, as do near-field microscopes as well.
As disclosed, data tracks comprise a width much smaller than the resolution limit of the optical read/write head objective, and the tracks are spaced closer than the resolution limit of the optics as well. However, each track is differentiated
by color, polarization, height, intensity, reflection, absorption, phase, refractive index, geometry (height or slope), or other parameter such that like tracks or sites are many tracks apart and separated by at least or more than the resolution limit.
For ease of illustration, color differentiation is described here, though other ways of differentiation, some mentioned above, may be preferable. Consider red, green, and blue alternating optical data tracks, where the track width is 1/6 and
track pitch is 1/3 the resolution limit of the objective. If a single unfiltered objective is used to read the data in white light, the tracks will not be resolved. However, adding a red filter (or providing red illumination) eliminates the green and
blue tracks, and the remaining red tracks, which are separated by the resolution limit, are resolved. Similarly, inserting green and blue filters will reveal the green and blue data tracks, respectively.
"Like" data sets light up as a single channel in whole-field illumination that is keyed to the isolating optical property of that data set, somewhat like a radio tuner being tuned to a specific channel. The more discriminating the tuner, the
higher the number of channels that may be fit into the receiver bandwidth, to further the analogy. If a detector array is used, and each pixel in the array is optically keyed to a discrete channel comprising an optically isolated data set, then a
plurality of channels can be read in parallel.
At present the burden of resolution in an optical data storage system is borne largely by the optical read/write head, such that higher data storage density requires shorter and shorter illumination wavelengths or larger and larger numerical
apertures. In the embodiments disclosed herein, the resolution burden is shifted in large degree to the medium itself, while decreasing the optical tracking and focus servo requirements on the optical drive. However, this shifting of burden to the
medium can be done at little additional economic cost because of mass production replication methods where the expensive precision is only in the master tool, and so the expense is amortized over the long product life of this tool.
The differentiated tracks may be read sequentially by a single head, where each convolved spiral is a "page," or in parallel either with multiple objectives or a single objective with a multi-channel filtered detector. If read in parallel,
multi-channel encoding is possible. For writing and erasing, although the illumination spot will illuminate multiple tracks, only the track with the same color, continuing this illustration, will be affected. In near-field optical recording, tracks may
be differentiated by physical height, or phase, or refractive index, such that the returning light is different for neighboring tracks. The resolution limit is defined for points of equal intensity. If there is a significant difference in
intensity, points closer than the resolution limit may be resolved.
While this invention brings advantages to near-field optical data storage, track differentiation has the greatest potential impact on more traditional optical data storage, such as magneto optical and phase change, where track density can be
increased by a factor of 400 times and more depending on signal-to-noise constraints. In addition, multi-channel encoding is made possible. Further, such an optically active medium with optically isolated active sites spaced more closely than the
resolution limit can be seen to have application in image printing, whether of a photographic system or a printing system.
Accordingly, this invention provides for optical storage density much greater than the resolution limit of an optical system by optically isolating and differentiating or discriminating between active optical sites, where the sites are made as
small as the detection limit of the optical system.
In accordance with a further feature of the invention, capability for whole-field parallel readout is provided.
In accordance with yet another feature of the invention, fast data transfer is effected.
In accordance with still another feature of the invention, there is provided removable media and optical systems with low numerical apertures and large working distances.
Additionally, there is provided surface volumetric storage and the capability for non-moving media.
Other features of the invention will be readily apparent when the following detailed description is read in connection with the drawings.
An optical storage media comprising optically differentiated data sites is disclosed, where the data sites are optically isolated or discriminated by additional optical elements in the optical system. The differentiated tracks may be read
sequentially by a single head or in parallel either with multiple objectives or a single objective with a multi-channel filtered detector. In near-field optical recording, tracks may be differentiated by physical height, or phase, or refractive index,
such that the returning light is different for neighboring tracks. Accordingly, there is achieved optical storage density much greater than the resolution limit of an optical system by optically isolating and differentiating the active optical sites,
said sites made as small as the detection limit of the optical system.
BRIEF DESCRIPTION OF THE DRAWINGS
The structure and operation of the invention, together with other objects and advantages thereof, may best be understood by reading the detailed description to follow in connection with the drawings in which unique reference numerals have been
used throughout for each part and wherein:
FIG. 1 is a diagrammatic elevational view of a conventional optical storage system comprising an objective lens and a medium used for data storage or retrieval;
FIG. 2 is a diagrammatic representation of two optical artifacts or data sites of essentially equal intensity incoherent illumination separated by a resolution limit dimension of d, as found in the medium of FIG. 1;
FIG. 3 shows the Airy disks associated with the optical artifacts of FIG. 2;
FIG. 4 shown the overlapping intensity profiles associated with the optical artifacts of FIG. 2;
FIG. 5 is a diagrammatic representation of two optical artifacts or data sites of different size;
FIG. 6 shows the Airy disks associated with the optical artifacts of FIG. 5;
FIG. 7 shown the overlapping intensity profiles associated with the optical artifacts of FIG. 5;
FIG. 8 is a diagrammatic representation of a plurality of dissimilar optical artifacts or data sites;
FIG. 9 shows the Airy disks associated with the optical artifacts of FIG. 8;
FIG. 10 shown the overlapping intensity profiles associated with the optical artifacts of FIG. 8;
FIG. 11 shows a plurality of data sites formed in the surface of an optical storage medium as detected by a writing spot;
FIG. 12 is a diagrammatical perspective view of an objective lens illuminating an optical storage medium in accordance with the present invention;
FIG. 13 is a schematic diagram of an optical data storage system comprising optical discrimination and differentiation elements in accordance with the present invention;
FIG. 14 illustrates differentiation of optical artifacts by means of stepped levels;
FIG. 15 illustrates optical differentiation by means of a prismatic surface;
FIG. 16 illustrates optical differentiation accomplished by means of vertically-dispersed layers formed in the active layer of the optical storage medium;
FIG. 17 illustrates optical isolation suitable for scatter or diffraction differentiation by means of data sites of varying width, depth, or slope;
FIG. 18 illustrates optical isolation or differentiation by means of discrete slope differences;
FIG. 19 illustrates optical isolation or differentiation by means of lenticulars or lenslets disposed over data sites;
FIG. 20 shows a geometric optical differentiation comprising an emulsion of spheres of different sizes;
FIG. 21 is a sectional diagram of an optical data storage system comprising an active layer and an optical differentiation layer;
FIG. 22 is a sectional diagram of an optical data storage system comprising an active layer and optical differentiating wedge elements; and,
FIG. 23 is a section drawing of an optical medium in which an active layer comprises optically isolated sites.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
There is shown in FIG. 1 a conventional optical storage system 10 comprising an objective lens 14, such as found in a Digital Versatile Disc (DVD) head, and a conventional recording medium 20, such as an optical or magneto-optical recording disc. Recording medium 20 typically stores data comprising optical artifacts 21 and 23, such as pits, formed at medium surface 25. Optical artifacts 21 and 23 are spaced no closer than a minimum distance of "d" to each other, as indicated, where d is
approximately the resolution limit of storage system 10. An illumination source 12 provides a reading/writing spot 27 at a medium surface 25. Reading/writing spot 27 may be reflected to a detector 18 by means of a partially-reflecting mirror 16, as is
well-known in the relevant art. Because optical artifacts spaced closer than the resolution limit will not be discerned, data storage density is limited by the resolution of optical storage system 10.
The theoretical basis for this limitation is most readily explained with reference to FIGS. 2 and 3 in which are shown optical artifacts optical artifacts 21 and 23 with corresponding Airy disks 31 and 33. (3). Optical artifacts 21 and 23 are
considered to be objects having equal intensity incoherent illumination. Airy disks 31 and 33 have diameters "A" which are dimensionally equivalent to the resolution limit of optical storage system 10, in FIG. 1. It should be understood that, although
higher-order diffraction rings exist, these are not shown for reasons of clarity. In optical storage systems employing coherent light sources, these higher orders add constructively to reduce resolution. Consequently coherently-illuminated optical data
storage systems typically require a track separation significantly larger than the theoretical resolution limit.
In FIG. 4, showing the intensity distribution 32 and 34 of Airy disks 31 and 33 respectively, the magnitude of a dip ".delta." between overlapping intensity distributions 32 and 34 determines whether the objects are resolved. The value for
.delta. varies depending on the resolution criteria used (e.g., Rayleigh or Sparrow criteria). To first order, if no dip is observed even in the second derivative, the objects are not resolved. Alternatively half-power beam width 35, or half width at
half maximum, can also be used to determine the resolution of an optical system when only one object of a known size is present. A more thorough discussion of resolution theory is provided in the cited reference by Lipson and Lipson.
FIGS. 5, 6 and 7 illustrate a condition wherein an object 41, of the same size as the resolution limit of optical system 10, is rendered as an Airy disk 42 having an intensity distribution of 43. In comparison, a smaller object 45 of a much
smaller size than the resolution limit also results in an Airy disk 46 having the same diameter as Airy disk 42. For smaller object 45, however, intensity distribution 47 is proportionately lower. Given an adequate signal-to-noise ratio, very small
objects can be detected by an optical system even when those objects are much smaller than the resolution limit of the detection system. As is understood by one skilled in the relevant art, intensity distributions 43 and 47 differ, increasing the
corresponding resolution, and allowing for even closer spacing of the corresponding objects. This condition is used to advantage in an embodiment discussed in greater detail below.
In FIG. 8 there are shown a first type of optical artifact 51 (indicated by circles), a second type of optical artifact 52 (indicated by squares), and a third type of optical artifact 53 (indicated by triangles). First optical artifact 51 is
spaced at a minimum distance of d (i.e., the resolution limit of the storage system) from an adjacent first optical artifact 51. Similarly, second optical artifact 52 is spaced at a minimum distance of d from an adjacent second optical artifact 52, and
third optical artifact 53 is spaced at a minimum distance of d from an adjacent third optical artifact 53. Optically dissimilar artifacts (e.g., optical artifacts 51 and 52) can be spaced at less than distance d from one another.
Corresponding Airy disks 61, 62, and 63 are shown in FIG. 9, and corresponding intensity distributions 71, 72, and 73 are shown in FIG. 10. In a conventional storage system, optical artifacts 51, 52, and 53 would remain unresolved if they were
optically similar artifacts. However, if optical artifacts 51, 52, and 53 can be optically differentiated from one another such that not all are concurrently visible, there is no effect on the resultant resolution. For example, optical artifacts 51 may
be linearly polarized sites oriented parallel to similarly polarized optical system illumination, and optical artifacts 52 and 53 may be polarized at other polarization angles to the illumination. In such a configuration, optical artifacts 52 and 53
would become optically differentiated or discriminative from optical artifacts 51, and would have no effect on the resolving of optical artifacts 51. As the polarization of the illumination is changed, optical artifacts 51 are no longer detected, and
either optical artifacts 52 or 53 may be detected instead. Each similar pair of optical artifacts is readily resolved if separated by at least the resolution limit d, and with optical discrimination or differentiation the resultant data density can be
much higher than that available in a conventional optical storage system. By way of comparison, if optically discriminative optical artifacts of 40 .ANG. particles are used in place of conventional 4000 .ANG. optical artifacts, it is possible to
attain an increase of two orders of magnitude in storage density for discrimination along a track width (i.e., y-density increase), to four orders of magnitude for discrimination along a track width and discrimination between tracks (i.e., x-y density
increase).
Referring to FIG. 11, there are shown a plurality of data sites comprising an array of optical artifacts 81 formed in the surface of an optical storage medium 80. Optical tracks 93 are comprised of similar optical artifacts 81c (designated by
the numeral 3) and are spaced a distance "d'" from one another, where d' is equal to or greater than d, the resolution of the optical storage system utilizing medium 80. The approximate relative size of this resolution limit is indicated by the size of
reading/writing spot 27. In the example provided, the optical storage system is detecting only optical track 93 by optically discriminating optical track 93 from optical tracks 91, 92, 94, 95, and 96. In this way, the size of each track, and the track
separation distance, can be made much smaller than the resolution limit of the optical storage system. Along the track direction, data is spaced at a distance "y" for conventional resolution, or may comprise optically dissimilarly optical artifacts as
indicated at 97.
Reading/writing spot 27 is larger than track separation, but only those tracks which are keyed to the optical properties of reading/writing spot 27 will react in either writing or reading data to medium 80. In an alternative embodiment, optical
track center-to-center separation d' is larger than resolution limit d so as to achieve a higher signal-to-noise (SNR) ratio and contrast. Similarly, an even larger d' can be used to allow for defocus and tracking errors, thus greatly reducing demands
on the optical storage system servos performing these functions.
In the example provided, the illumination of the optical system is keyed optically to optical artifacts 81c. Dissimilar optical artifacts within reading/writing spot 27 that are not optically similar to optical artifacts 81c are not detected and
thus do not affect the resolution of optical artifacts 81c. For clarity, similar optical artifacts are shown as being separated in the y-axis by at least the resolution limit d, but it will be understood that the same optical differentiation or
discrimination method can be applied in all axes, as indicated at 97.
FIG. 12 illustrates a perspective view of an objective lens 101 illuminating a medium 120 comprising optically-isolated predisposed optical artifacts 121 (X symbol), 123 (square symbol), 125 (triangle symbol), and 127 (circle symbol).
Reading/writing spot 27 illuminates optical artifacts 121, 123, 125, and 127, but is keyed, either at the source of illumination or at the detector, to discern only optical artifact 121, while optical artifacts 123, 125, and 127 are not discerned. As
discussed above, because optical artifacts 121 are separated by at least the resolution limit d, they are detected even though their dimensional size is smaller than the resolution limit.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
There is shown in FIG. 13, a schematic of an optical data storage system 100 in accordance with the present invention. Optical storage system 100 comprises an objective lens 130 providing a focal spot 131 for reading and writing, a detector 140,
a beam splitter 142, and a source of illumination 160, such as a laser diode, an LED, or other suitable tunable or fixed-frequency source of optical radiation. There may be a source discriminator 161 disposed between source 160 and a medium 150.
Alternatively, there may be a detection discriminator 141 disposed between medium 150 and detector 140. Discriminators 141 and 161 may comprise polarizers, phase shifters, mono-chromators, narrow-band wavelength filters, focal shifters, or aperture sets
for diffraction or scatter angle. Preferably, the degree of discrimination is variable over a predetermined range, either continuously or in discrete steps, such that the corresponding optically differentiated optical artifacts in medium 150 can be
selectively discriminated or isolated for purpose of reading or writing.
Medium 150 comprises an active layer 151 which is responsive to illumination source 160 such that a predetermined amount of illumination 160 impinging upon a portion of active layer 151 produces an optical artifact within a local region of active
layer 151. The optical artifact so produced may be a change in index of refraction of the material comprising active layer 151, or it may be a change in scattering coefficient, in polarization, in diffraction property, in refraction, or in absorption,
for example. Medium 150 may further comprise a differentiation layer 153 comprising a plurality of optical
differentiation elements 155 disposed between source 160 and active layer 151, as shown, and as described in greater detail below.
Detector 140 may comprise a single device, such as a CCD pixel, or may comprise a one- or two-dimensional array of such devices, where each pixel may correspond to one or more optical differentiation elements 155. In this way, an individual
pixel would receive differentiated illumination reflected from medium 150, such as optical information of a particular wavelength, phase, scatter or diffraction angle, for example. In this manner information from a plurality of tracks and data sites on
medium 150 can be read and processed in parallel for enhanced data transfer speed and for other functions, such as multi-tasking or multi-track encoding.
Medium 150 may also comprise a substrate 157 which may be a flexible or rigid material, as the particular application may require. Preferably, medium 150 is enclosed in a housing 159, comprising a slidable window 158, for protection against
contamination. Medium 150 may further be circular and rotated for data retrieval, or may be rectangular and scanned or traversed for data retrieval.
In a first embodiment of optical track discrimination, optical artifacts are differentiated geometrically, as shown schematically in FIG. 14. An active layer 177 is formed on a substrate 179 having stepped levels 171-175. Alternatively, this
configuration may be accomplished by an embossing process in the active layer material. The power level of illumination returned to detector 140 from stepped levels 171-175 will vary according to the degree of focus. Because conventional active layer
materials, such as phase-change materials, require a minimum incident power level to effect a detectable optical change (e.g., about 10 mW for a coherent focused laser diode source), thereby storing a data bit, only that stepped level present at the
proper focus will be affected. Shifting the focus to the next stepped level allows writing on that stepped level, and so on, without affecting the other stepped levels. The step height can be established by considering the focal depth of the optical
system, where a small focus depth is desirable for highest track differentiation and isolation. Similar stepped levels may be used to discriminate the optical sites by optical phase, where only one stepped level is at the correct constructive
interference height for the variable phase-shifted light.
In a second embodiment, shown in FIG. 15, the height level is continuously changed by replacing the stepped levels of FIG. 14 with a prismatic surface 190. Only a small area of a sloping prism surface 196 is in focus for either reading or
writing, and so reading/writing spot 198 must be refocused to see or write data on active layer 200 at each of the constant height planes indicated by the dashed lines at 194. The slope causes the power density of incident illumination to be of the
required writing level along only a narrow section, and is reduced below this power density level everywhere else on the prism facet by defocus and depth of field.
An added benefit is realized, in that active layer 200 is illuminated with the evanescent field, or near-field, prismatic surface 190 is illuminated from above so as cause total internal reflection at prism surface 196. Moreover, this internal
reflection condition increases contrast even with propagating light, because it is essentially dark-field, in that specular from ECD is not returned in this oblique geometry. A circular polarizer comprising a linear polarizer and a phase retarder (not
shown) reduces specular reflection from a planar medium surface in normal incidence illumination. The centroid of the power in a Gaussian beam shifts relative to the geometric beam center in total internal reflection (TIR), and this affects writing
location because of the placement of highest power density (i.e., the data track will be offset from the geometrical center of beam). Preferably, the source power level will be constant and single mode, because a power shift may also result in lateral
and vertical geometrical shift which causes running into other tracks on the sloped surface.
Such stepped levels and prismatic surfaces, as well as other geometric configurations described below, can be mastered by precision diamond machining on a diamond turning lathe, or may be formed by photo-lithographic means. Holographic
techniques have been described in the patent literature for forming such structures in three dimensions by using multiple laser beam interference. Substrates are then formed by compression/injection molding or embossing to these masters.
It is important to note that these geometrically-optically isolated structures, as well as all of the means for optical isolation and discrimination described herein, also have the advantage of whole-field parallel optical detection and writing.
For example, here the equal-level, equal-focus spots in all of the prisms can be seen at once and read out simultaneously by a detector array, for greatly enhanced data transfer rates. In such applications, a .lambda./6 criterion may be used to reduce
crosstalk in land and groove systems is applied to such sloped surfaces, and a 0.1 micron track spacing is achieved with a DVD laser source wavelength of 0.650 micron.
If the active layer comprises a phase-change material, the preferred mechanism for optical discrimination and detection is scatter. In this internal reflection mode, it is the backscatter that is detected rather than the forward. However, both
have the same high spatial frequency information, so the effective numerical aperture is quite high. In FIG. 16, a vertical optical isolation similar to those in FIGS. 14 and 15 is achieved either by means of an active layer 204 into which sites 206 are
written by, for example, ablation and void creation, or by writing into layers dispersed vertically within active layer 204, as indicated by dashed line planes 208. Both of these volumetric approaches to optical data storage are enhanced by the optical
isolation provided by shifting of the focus 210 of the illumination vertically, such that sites may be placed laterally at much closer spacing than the resolution limit of the optical system. Further optical isolation may be gained by varying the
optical index of the written sites within the volume, with, for example, different power levels during writing or different writing duration times at each site. Such an optical differentiation would be extremely sensitive when used with total internal
reflection and a critical angle "valve" technique (i.e., an aperture rejection device). Many more materials would become viable active optical layers in this case, with the result of lower cost media, where the substrate itself is the active optical
medium. Similarly, the size of the sites may be varied in order to cause optical isolation by scatter angle, optical resonance with wavelength tuning (e.g., selective scatter or "photonic bandpass crystals") or the material layers at planes 208
throughout the volume may be of different materials and have different absorption or polarization.
FIGS. 17, 18, and 19 depict geometric embodiments in which the optical isolation is in the form of differentiated scatter or diffraction angle from the optical artifacts, or differentiated scatter or diffraction efficiency (i.e., an intensity
detection). For example, in FIG. 17, the isolating optical surface 212 shows a number of ways of affecting the scatter or diffraction angle or efficiency, as with site width as shown at 214 and 216, site depth as in 215, or site slope, as in 217. In
particular, the scatter angle is indicated for 214 to be smaller than for site 216, such that an aperture placed at the detector (not shown) would reject one and accept the other. Preferably, detector array 141 is used (as in FIG. 13), where each pixel
in detector array 141 receives the diffracted or scattered light from a particular isolated track. In FIG. 18, an optical differentiation layer 220 is based on discrete slope differences between sites, as indicated at 222 and 224. As shown in FIG. 19,
lenticulars 231 and 233 of different curvature and focal length serve to discriminate between the optical sites. In the embodiments of FIGS. 17 and 18, the active optical layer may be deposited onto the isolation geometry by vacuum methods, for example,
or the isolation geometry and differentiation layer may be embossed or molded into the active optical layer.
FIG. 20 shows a geometric optical isolation comprising an emulsion of spheres 241 and 243 having different diameters, that are either deposited upon the active optical layer, or are themselves the active optical layer. Depending on the size
range of these spheres relative to the illumination, the optical isolation can be one of interference, as in the opalescent colors seen in opals, or scatter angle, or even optical wavelength-dependent resonance.
In FIG. 21., a section drawing of an optical data storage medium 250 is shown comprising an optical differentiation layer 237 adjacent to a homogeneous active layer 240. The illumination reading/writing spot 238 is shown such that it is keyed
optically, as indicated by the vertical line symbol, to an optically-similar optical differentiation element 234, while optically-dissimilar optical differentiation elements 230, 232, and 236 prevent or block the illumination from writing to or reading
from the underlying region of active layer 240. The region of active layer 240 below isolation element 234 can be written on or read from. Optical differentiation elements 234 may comprise polarizers of different rotation angle as in a magneto-optical
application, for example, or may comprise narrow bandpass filters, absorbers, or variations in refractive index.
In FIG. 22, there is shown an optical storage and retrieval medium 260 comprising a homogenous active layer 240, such as a phase-change material, with an adjacent optical differentiation layer 262 comprising wedges 233 and 235. Wedges 233 and
235 have a lateral dimension that is at least as large as the resolution limit of the optical system, as indicated by the focus spot 238. Wedges 233 and 235 may be used to continuously vary the phase along the length of the wedge, or may act as a
continuously varying interferometric filter. In the former case, the light entering wedge 233 or 235 is shifted in phase by an amount equal to twice the optical thickness of the wedge at the point of entry (i.e., the product of the physical height "h"
of the wedge at that point and the index of refraction n of the wedge material). An optical storage system utilizing medium 260 would comprise a polarizer and analyzer disposed between the source of optical radiation and the detector. Preferably, the
analyzer would be a variable unit, such that polarized light is accepted after wedge 233 or 235 has rotated the polarization angle (and converted from linear to circular), and optical radiation of other polarizations is not accepted.
In the interferometry configuration, wedges 233 and 235 act as local interference filters, where when the optical thickness "nh" at the optical entry point is .lambda./4, destructive interference occurs for that wavelength, while constructive
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