Patent Application
20090080317
This application claims priority from French patent application 07/06673, filed Sep. 24, 2007.
The invention relates to a device for storing data on a holographic storage medium and to a device for reading said data. The invention also relates to a device for copying a holographic data storage medium.
Holographic memories are seriously considered to potentially form the next generation of mass storage media. In practice, they present considerable advantages:
A general overview of the holographic data storage technology can be found on the sites of the main manufacturers in the field (www.aprilisinc.com and www.inphase-technologies.com) . . . .
As a general rule, to produce a hologram, two light beams that are mutually coherent (deriving from the same laser source), called object beam and reference beam, are superimposed within a photosensitive material. As the beams overlap, an interference figure or pattern is created and stored in the photosensitive volume. To read the hologram, it is necessary to illuminate it with a read beam identical to the reference beam (or its conjugate complex). The diffraction of this read beam by the hologram generates a replica of the object beam.
The theory of holography is explained in chapter 8 of the book by J. W. Goodman, “Introduction to Fourier Optics”, 1st edition, McGraw-Hill Book Company.
To store the information in a hologram, it is sufficient to code said information in the form of a spatial modulation (of phase and/or amplitude modulation) of the object beam, then use a reference beam whose properties are known. It will be understood that the reconstruction of the object beam makes it possible to simultaneously read all the coded data.
The spatial modulation of the object beam can be produced, in a manner known per se, by a spatial light modulator (SLM) with liquid crystals (references to SLMs of this type can be found on the manufacturer DisplayTech's website: www.displaytech.com) or with micro-mirrors (for example, the DLP micro-mirror matrix developed by Texas Instruments: www.dlp.com).
A large storage density is obtained thanks to multi-plexing techniques. In practice, it is possible to superimpose, in one and the same photosensitive volume, a plurality of distinct holograms, stored from different object and reference beams. When said photosensitive volume is illuminated by a read beam identical to one of the reference beams used to store the holograms, only the corresponding object beam is reproduced (affected by a background noise caused by the other holograms). It is this property that enables the holographic memories to achieve such high storage densities.
Several holographic multiplexing techniques are known from the prior art. Of particular note are phase coding and angle coding.
The phase-coding technique is described, for example, by the article by S. Yasuda et al. “Coaxial holographic data storage without recording the dc components”, Opt. Lett. 2006 31 (17) pp 2607-2609, or even by the reference: Z Karpati et al. “Comparison of coaxial holographic storage arrangements from the M number consumption point of view”, Jpn. J. Appl Phys 46 (2007) 3845-3849. It consists in using reference beams (and therefore read beams) spatially modulated in phase according to modulation configurations that are substantially orthogonal to each other.
The orthogonality of the phase coding makes it possible to avoid the read cross talk between the hologram coded by a reference beam and the diffraction of the read beam corresponding to this same reference beam by the other holograms. If [φ1m, φ2m, φ3m, . . . φnm] is used to denote the phase components of the reference beam m (φ=0 or π), then the orthogonality condition can be expressed by the following relation described by C. Denz et al. “Potentialities and limitations of hologram multiplexing by using the phase-encoding technique” Appl. Opt. 31 (1992) pp. 5700-5705:
This relation expresses the fact that two reference beams interfere destructively if they are different.
The spatial modulation of the reference and read beams is obtained thanks to spatial light modulators (SLM) with liquid crystals or micro-mirrors. These devices are costly, and while their presence can be tolerable in a data writing system (which, at least initially, can remain a professional or semi-professional item of equipment), it is far less so in the reading systems intended for the general public. Furthermore, the low operating frequency of the SLMs (a few hundreds of Hertz) strongly limits the data writing and reading speed.
The angle coding technique is known, for example, from U.S. Pat. No. 6,700,686. It consists in using reference beams (and therefore read beams) which are incident to the photosensitive material according to different angles. To do this, a data storage and/or reading device can use an optical system comprising a pivoting mirror, an acousto-optical modulator and a lens that moves translation-wise. In all cases, the result is relatively complex—and therefore costly—and slow systems. For example, if a pivoting mirror is used to determine the direction of the reference/read beam, its movement must be controlled finely.
It will be noted that angle multiplexing can be considered as a particular case of phase multiplexing, in which all the modulation configurations introduce a distinct linear phase shift of the light beam.
The other multiplexing and demultiplexing techniques present similar drawbacks: they require relatively complex and costly equipment, and do not make it possible to optimally exploit the potential of holographic storage regarding reading speed.
Furthermore, the replication of the “holographic discs” is slow and costly. In practice, the pressing or moulding techniques that have allowed for the emergence of the various generations of optical discs do not apply to holographic memories, because the data is stored in volume and no longer on a surface. Now, the depth of the media is not accessible on pressing.
All these drawbacks have greatly slowed down the commercial penetration of holographic data storage.
One aim of the invention is to resolve at least some of the problems posed by the prior art.
To achieve this aim, the invention relies on the coding of the reference and/or read beam by an optical storage medium-type object, presenting a structuring arrangement producing a succession of diffractive optical elements, each of said elements introducing a distinct modulation configuration of said beam. The movement of this object thus makes it possible, by scanning the diffractive optical elements, a sequential and repeatable modification of the wavefront of the reference or read beam. Preferably, the object is a disc that can be rotated in the path of the reference beam. The replication techniques commonly used in the optical storage field can be employed in order to allow a low-cost and mass-production fabrication of this object.
The use, for the multiplexing and the demultiplexing, of such an object—hereinafter called “coding disc”—makes it possible to produce devices for writing and reading data on holographic media that have a simple structure, present a reduced cost and can nevertheless operate at a very high speed (data rate). The combined use of a reading device and a writing device according to the invention makes it possible to replicate “holographic discs” in economically viable conditions.
More specifically, an object of the invention is a device for storing data on a holographic storage medium, comprising a plurality of data storage areas, said device comprising:
said means for spatially modulating said object and reference beams being synchronized to store a plurality of data blocks on said data storage area, each block being associated with a distinct modulation configuration of the reference beam, so as to multiplex said data blocks;
wherein said holographic storage medium is a disc that can be actuated rotationally about an axis and perpendicularly translationally about said axis, said data storage areas being arranged in a plurality of circular concentric or spiral tracks, the centre of which coincides with said axis.
In particular, said optical storage medium may be a disc suitable to be actuated rotationally about an axis and comprising a plurality of concentric circular tracks, the centre of which coincides with said axis, each of said tracks containing one and the same succession of diffractive optical elements, the device also comprising:
Another object of the invention is a device for reading data recorded on a holographic storage medium, comprising a plurality of data storage areas, said device comprising:
said means for spatially modulating said read beam being adapted to read a plurality of data blocks stored on said data storage area, each block being read using a distinct modulation configuration of the read beam, so as to demultiplex said data blocks;
in which said means of spatially modulating said read beam comprises an optical storage medium on which is implemented a succession of diffractive optical elements, each introducing one said distinct modulation configuration, and a first actuator for displacing said optical storage medium in order to sequentially bring said diffractive optical elements over the path of the read beam;
wherein said holographic storage medium is a disc that can be actuated rotationally about an axis and translationally perpendicularly to said axis, said data storage areas being arranged in a plurality of circular concentric or spiral tracks, the centre of which coincides with said axis.
Again, said optical storage medium may be a disc suitable to be actuated rotationally about an axis and comprising a plurality of concentric circular tracks, the centre of which coincides with said axis, each of said tracks containing one and the same succession of diffractive optical elements, the device also comprising:
A further object of the invention is a device for replicating a first holographic storage medium on a second holographic medium, said replication device comprising a device for storing data on said second holographic medium, as described above, in which the means of generating and spatially modulating the object beam or beams comprise a device for reading said first holographic storage medium as described above.
Other characteristics, details and advantages of the invention will become apparent from reading the description, given with reference to the appended drawings given by way of example and which represent, respectively:
a and 1b: the principle of storing and reading a hologram.
a and 6b: exemplary embodiments of a device for storing data on a holographic storage medium based on angular multiplexing, in co-propagative and contra-propagative storage configuration respectively.
a and 7b: exemplary embodiments of a device for reading data recorded on a holographic storage medium based on angular multiplexing, in co-propagative and contra-propagative data reading configuration respectively.
a and 8b: exemplary embodiments of a device for replicating a holographic medium obtained by angular multiplexing according to the invention: data written by co-propagative method and read by co-propagative method in
a and 11b: exemplary embodiments of a device for replicating a holographic medium obtained by phase multiplexing according to the invention; in
a-14e: steps of a method of fabricating coding discs for the reference beam by mastering and moulding replication.
b: an example of a diffractive optical element on the coding disc for phase multiplexing, according to a variant of the invention.
a-17c: fabrication steps for a coding disc by mastering and replication by modifying the surface roughness. And
a and 1b illustrate the general principle of the writing and reading of a hologram, which has already been described above. Two beams are super-imposed within a photosensitive material 3. The first, denoted 1, is called reference beam; the second, denoted 2, object beam. Each beam is characterized by a field being propagated in space. Where the beams overlap, a complex system of interferences is created. These interferences are stored in a volume of the holographic medium 3 to form the hologram 4.
For reading, this hologram is illuminated by a reference read beam 1b, the propagative field of which is identical to that of the storage beam 1 (or its conjugate complex, but this second case will be disregarded hereinafter). In this case, the hologram diffracts the reference beam so as to form a new beam 2b, the propagative field of which is, as a first approximation, identical to that of the object beam 2.
Beam 1 constitutes the reference. A moving mirror 7 generally makes it possible to modify the angle of the beam 1 in order to provide the multiplexing. In this case, an optic 8 is used in order to retain the overlapping of the beams 1 and 2 on the holographic medium 9 regardless of the angle imposed on the beam 1.
There are other solutions for changing angle, typical of which are the use of acoustic-optical modulators or the displacement of a lens.
As an example, the abovementioned document U.S. Pat. No. 6,700,686 describes in more detail an embodiment according to the concept of angle multiplexing.
Similarly, the reference beam 1 is also directed towards a second SLM 10 which can be identical or not to SLM 6. The objective of the modulator 10 is to modify the properties of the reference wavefront (the amplitude and/or phase; generally, the complex amplitude) sequentially and repeatably. Two focussing optics which can be combined into a single optic provide the focussing of the beams in the overlap zone.
The SLM 10 can be of the micro-mirror matrix type or can be a liquid crystal cell matrix.
Possibly, the coding of the reference beam and that of the object beam can be obtained from the same SLM as is demonstrated by Z. Karpati et al. in their publication “Comparison of coaxial holographic storage arrangements from the M number consumption point of view”, Jpn. J. Appl Phys 46 (2007) 3845-3849.
In both cases, the superimposition zone of the beams is positioned in a layer of photosensitive material 3 deposited on the storage medium 9. This medium is rotated in order to sequence the hologram writing zones.
The reference beam passes through the disc 11 in a structured zone 12. This zone contains a diffraction grating and has the effect of deflecting the incident beam by two angles α and ψ.
The object beam 2, the wavefront of which has previously been coded with the data to be stored, passes through the disc in a zone 12′ free of structuring and is not therefore subject to deflection.
An optic 13 is used to collect the beams 1 and 2 in order to make them converge towards the overlap zone in the photosensitive medium 3, on the hologram 4. To guarantee this good overlap, there can be various techniques. The figure represents that which uses a dual assembly 2f-2f and f-f. The structured zone 12 is located at a distance equal to two focal distances 2f of the optic 13, on the optical axis. All the beams obtained from this zone therefore converge in the image of the structured zone, at a distance 2f from the optic 13, on the optical axis.
Since the object beam is parallel to the optical axis of the system, if there is a desire to focus it on the hologram 3, a lens 14 must be used that has the same optical axis as the optic 13, but twice the focal length. This lens can be made as an insert of the preceding one, as shown in the figure, by moulding techniques.
Since the Fourier transform configuration is normally preferred, the SLM having coded the object wavefront is relayed by an optic that is not represented in the diagram so that its image is located in the zone 15, at the focal distance of the lens 14.
When the disc starts rotating, the beam 1 is subjected to a sequence of deflections by different angles α and ψ, but the overlapping of the object and reference beams remains on the hologram 4.
According to a preferred usage, the optic 13 can also comprise an opaque zone 13′ making it possible to filter the order 0 of the diffraction of the reference beam.
The beam 1 passes through a structured zone of the disc, on which are implemented diffractive optical elements 12, whereas the beam 2 passes through a free zone 12′ of the disc. This case is simpler than the preceding one because the optical elements 12 introduce a phase modulation which does not substantially modify the direction of propagation of the reference beam 1: the two beams 1 and 2 therefore remain parallel. The focussing optic 16 therefore has a single focal length, equal to the distance between the optic and the photo-sensitive medium. Since the Fourier transform configuration is generally preferred, the optical disc is also at focal distance from the optic 16 and, as in the preceding case, the object coding SLM is imaged by a relay optic that is not represented on the disc. Because of this, the hologram stores the interferences between the two Fourier transforms of the SLM and of the structured zone 12.
The spatial modulation of the reference beam can also be done by amplitude. More generally, the amplitude and the phase can both be modulated: the term complex amplitude modulation then applies.
According to a preferred embodiment, the coding disc 11 and the holographic medium 9 are made using similar materials and geometries (notably the thickness and the material of the substrate). Because of this, with thermal expansion phenomena, the coding variations of the reference beam follow the variations undergone by the hologram. Obviously, this is not possible when the modulation of the reference beam is produced by an SLM, as in the prior art.
A simple example is to consider an angle coding generated by a diffraction grating of pitch Λgrating. The first order diffraction angle of the reference beam 1 is given simply by the following diffraction grating equation (assuming normal incidence on the grating):
It is assumed that the object beam 2 is free of data. The hologram therefore consists of an interference figure, the pitch of which depends on the angle between the two beams 1 and 2. If the angle of incidence of the object beam on the medium is zero, the pitch of the interference figure is expressed by the equation:
If a change of temperature is reflected in an expansion by a factor ρ on the pitch of the interferences, the reciprocal angle α′ required to reconstruct the hologram becomes:
The diffraction grating for coding the reference beam has also been subjected to an expansion by a factor ρ, and the diffracted angle becomes α″:
For small angles it can therefore be seen that the effect of expansion on the grating offsets the angular variation induced by the expansion of the hologram:
α′=α″.
Whether the multiplexing of the data is done in phase mode or angle mode, and the propagation geometry is co- or contra-propagative, it is advantageous for the hologram-recording beam to be pulsed, and more particularly for the laser to be off during the transition between two diffractive optical elements 12. This makes it possible to avoid the overlap between two reference patterns for a given object coding. When the useful beam touches the boundary separating two coding zones, the beam is cut and the object coding refreshed. When the useful beam has fully penetrated into the next zone, the laser is lit.
The duration for which the laser is lit depends mainly on the sensitivity of the holographic material. The more sensitive the material is, the shorter the pulse can be. The shorter the pulse is, the faster the holographic disc can rotate.
Exemplary embodiments which in no way limit the general nature of the invention are given hereinafter.
a represents a device E for storing data on a disc-shaped holographic storage medium 9 (holographic disc), operating according to the co-propagative method. In this device, the incident laser beam, generated by a source S such as a laser diode, is split into two sub-beams by a splitting cube. The object beam 2 is formatted by a telescope to adapt its size to that of the SLM 6. Once reflected by the spatial modulator, the object beam 2 is once again formatted by an objective which conjugates the SLM on the coding disc 11.
The reference beam 1 is also directed towards the holographic disc 9, and an optic is used to make the beam converge on the disc and adapt its size on the holographic medium 9. After passing through the coding disc, the reference beam is deflected in at least one of the two angular directions α and ψ (see
The collection optic 13 makes it possible to converge the beams 1 and 2 within the overlap zone as is shown by
The coding disc 11 is rotated rapidly about an axis Z by an actuator that is not represented, so as to pass over the path of the reference beam 1 the diffractive optical elements 12, which are arranged along a circular track, the centre of which coincides with said axis Z.
A structuring 22 present on the disc 11 enables a detection system 17 to synchronize the writing of the data in the holographic disc 9. This detection system is described hereinbelow. It is omitted from the following figures for the sake of simplicity.
The holographic disc 9 is rotated slowly about an axis Z′ by a first actuator (not represented) to enable multiplexing of the holograms within one and the same data storage area 5. The disc 9 thus rotates by an angular value corresponding to the pitch of these volumes for each coding sequence on the disc 11. In a preferred example, the disc 9 turns by this elementary angular value on each complete rotation of the disc 11.
The holographic disc 9 is also driven by a translation movement in a direction perpendicular to the axis of rotation Z′, by a second actuator (also not represented) so that the insulation of the data storage areas 5 covers all the available surface of the disc. More specifically, the storage areas 5 are arranged in a plurality of concentric, circular or spiral tracks 50, 51, 52, the centre of which coincides with said axis.
It will be noted that the holographic storage medium 9 is not necessarily in disc form, but that is just a preferred embodiment.
b shows an arrangement very similar to the preceding one in the case of contra-propagative exposure, in which beams 1 and 2 are incident on the holographic disc 9 on two opposite sides of the latter. Unlike the preceding case, these two beams are focussed on the data storage areas of the disc 9 by two separate optics 13′, 14.
a and 7b represent the devices L for reading data corresponding to the storage, or writing, devices of
In
A synchronization system 17 is once again used to synchronize the matrix detector with the read coding.
a shows that the read head also comprises an SLM 6, which remains unused. In practice, in this way, one and the same optical head can be used equally to write and to read data.
b presents the case of the reading of a hologram written in contra-propagative mode. The reconstituted object beam 2b is therefore propagated in the reverse direction of the read beam 1b. The read arrangement is very similar to that of
A matrix detector 20′ is used to detect the data. The detector 20′ is also present in
The invention also provides a solution to the problem of replication of the holographic data storage media.
As explained above, the impossibility of using the pressing replication technique that has so contributed to the success of conventional optical discs prompts the consideration of replicating holographic discs by scanning, in which the holograms are replicated one by one. As
The speed with which the data is read and written is the main key to making this technology viable. It is therefore essential to be able to scan the holograms as quickly as possible. Now, the sequenced coding of the reference beam by a structured disc 11 indeed allows for a very fast scanning of the holographic disc 9.
If the coding disc 11 presents structuring zones 12 of size 630 μm arranged at a distance of 30 mm from the axis of rotation Z, there are 300 zones. If the disc rotates at 5000 revolutions per minute (rpm), the potential coding frequency is 25 kHz. A holographic disc with a holographic volume size of 600 Am contains approximately 23 000 available volumes. If each volume contains 300 holograms, it is therefore necessary, to read the disc entirely, to be able to have 7106 codings of the reference beam succeed each other.
At the coding frequency allowed by the rotation of the disc, it takes 4.6 minutes to replicate a disc. This is therefore far from the few seconds that pressing takes, but this time is more advantageous than in the case of a coding of the reference beam by an SLM. The coding frequency of the conventional SLMs is in practice a few hundred Hz and can rise to a few kHz in the case of the DLP products from Texas Instruments. In the case of the coding by revolving disc, the coding frequency will depend on the speed of rotation of the disc, and can therefore be very much higher.
To further reduce the production time, it may also be advantageous to expose several holograms at a time on different volumes at the same moment. The use of a revolving disc for the replication can therefore allow for a commercially advantageous solution.
a presents the case of the replication of a parent holographic disc 9A written in co-propagative mode to a replicated disc 9B also written in co-propagative mode. For this, two coding discs 11A and 11B are used. The two discs must be synchronized. The two beams 1A and 1B represented in the diagram originate from the same laser.
b shows an arrangement for replicating a parent holographic disc 9A written in contra-propagative mode to a replicated holographic disc 9B written in co-propagative mode. This arrangement has the advantage of being more compact than the preceding one and it is therefore possible to imagine combining the two discs 11A and 11B in a single piece in order to mechanically guarantee that their rotation will be synchronized.
Other configurations can, of course, be considered.
The comparison of
a describes the replication of a parent disc 9A written in contra-propagative mode in a replicated disc written in co-propagative mode in the case of phase coding.
b repeats the preceding figure in the case of multiple replication. As can be seen, the two reference laser beams are split into several sub-beams which each address a structured zone of the coding discs 11A and 11B. The holographic discs are moved horizontally during the writing over the distance separating the structured zones of the coding discs.
The discs 11A and 11B are in this case more complex and more voluminous than the preceding discs 11: in particular, they need to include a plurality of concentric circular tracks 120, 121, 122, the centre of which coincides with said axis, each of said tracks containing one and the same succession of diffractive optical elements 12. This does not however pose any problem because the replication equipment is generally technologically heavy. Its performance does not in effect relate to the compactness but to the bit rate. As an example, with 10 parallel replication heads, replication time for the preceding example changes to approximately 30 seconds.
By envisaging a reasonable distance between writing heads of 3 mm, 11 writing heads can be used to cover a total displacement of 36 mm (width of the writing zone on a disc of radius 60 mm).
Of course, multiple-head systems can be used each time a particularly rapid writing or reading of the data is desired, and not only in the context of replication.
The substrate 26, to be transformed into coding disc 11, revolves at the speed Vrot. The beam 24 is formatted by the writing head 25 which is driven by a translation movement at the speed Vtrans. The direction of Vtrans is radial, and perpendicular to the axis of rotation of Vrot. The laser spot exposure the substrate creates a modified zone in the sensitive layer 27, in the form of tracks separated by a pitch Λ. The value of the pitch sets the value of Vtrans for a given rotation speed:
With Λ in μm, Vtrans in μm/s and Vrot in rpm.
In the case of the spiral movement, the spot is displaced above the substrate with a speed called linear speed Vlin, an approximate expression of which is given by:
With R, the radius of the spot relative to the centre of rotation, in mm, and Vlin in m/s.
To retain a constant linear speed and therefore a uniform response from the material 27, the value of Vrot must change according to the position of the spot. The same applies with Vtrans.
The speed of rotation must in particular increase when the spot approaches the centre of rotation. Since this speed is bounded by a value Vrotmax, retaining the pitch at a given linear speed imposes a minimum radius Rmin:
If Rmax is used to denote the maximum radius of the spot, the exposure time T1 is deduced simply therefrom:
The scanning of the writing spot is therefore done on the basis of a spiral. It is on this spiral that the sampling grid for the structuring patterns of the disc 11 is based.
On a reduced zone of the substrate 26, the spiral appears like a succession of parallel lines. It is therefore possible to approximate an orthogonal grid of pitch dr in the radial direction of the disc and dθ in the direction of the linear speed of displacement of the spot.
The writing resolution is given by the smallest possible values of dr and dθ.
The linear resolution dθ is set by the capacity of the laser to be modulated at high frequency. Let f1 be the maximum modulation frequency of the laser.
Exposure laser sources that can be modulated at 500 MHz are available on the market (see the product LDM A350 from Omicron Laserage Laserprodukte GmbH, www.lasersystem.de). When associated with a movement of linear speed 5 m/s, a resolution dθ=10 nm is obtained.
The resolution in the radial domain is given by the accuracy of the translation movement. Accuracies of the order of a few nanometres are currently available, notably through the use of precision optical rules. The radial resolution sets the pitch of the spiral and therefore the exposure time. It is therefore essential to use the highest possible value of dr in order to reduce this time.
This structuring technology therefore offers great accuracy by comparison to the conventional SLM technologies. Structuring a zone of side 600 μm on a grid of 100 nm allows a sampling in 6000×6000 elements. This is therefore well above the definition of the current SLMs which are of the order of 1500×1500 pixels.
Once the material 27 has been exposed, the disc fabrication process follows the conventional lithographic processing steps.
In the case of a resin, the latter is developed and then a thick layer of nickel 28 is grown by galvano-plasty. This layer is then removed then used to reproduce the original disc by injection moulding a plastic material, for example polycarbonate 29 (
The coding discs 11 can thus be fabricated in large series by an extremely rapid and cost-effective moulding method. They are therefore, unlike SLMs, pivoting mirrors or acousto-optical modulator that they replace, low-cost devices, which have a negligible effect on the cost of the reading and writing devices incorporating them.
a and 17b present another method of fabricating a coding disc 11. A layer of material 30 is used which has the property of being degraded when exposed by laser. The platinum oxide, PtOx, is an example of material which, under the thermal effect of a focussed laser beam is broken down into platinum and gaseous oxygen. Degassing causes the surface to deteriorate. The substrate can thus be used as an amplitude mask to produce diffraction gratings in order to implement an angle multiplexing of the data. The deteriorated surface zones 31 will in effect diffract the incident beam and produce an effect similar to an absorbent zone. It is therefore a (real) amplitude coding; however, in the prior art, it is usually referred to, even in this case, as “phase coding”.
The laser degradation of the surface generates, on the surface of the original disc (the “master”) a random relief having amplitude of a few tens of nanometres. Such an original disc can then be replicated very rapidly and in large series by the moulding method of
hence:
The minimum thickness for a material index of 1.5 at a wavelength of 405 nm is 135 nm. These values can be achieved with the etching and replication technologies. These techniques also offer a good accuracy on h.
The competing phase-shift technologies based on micro-mirrors normally involve great difficulties in producing changes of thickness that are as low and in guaranteeing a good accuracy on these values. The solution with replicated coding disc is therefore particularly attractive for multiplexing with phase coding.
If the material requires a relatively lengthy exposure time, the rotation of the disc can pose a problem, both in the case of angle coding and in that of phase and/or amplitude coding (“complex amplitude”). In this case, the displacement of the spot in a pattern structured in two dimensions, as described in
To avoid this phenomenon, it may be advantageous to use a one-dimensional coding pattern in the radial direction of the disc, an example of which is represented in
A radial one-dimensional structuring can also be used for angle coding. In this case, the data is multiplexed only according to the angle α (see
As
The division into sections having variable orientations to offset the effect of the rotation of the disc can be applied also to phase coding (more generally, complex amplitude coding).
If the holographic material is very sensitive and can accept short writing pulses, then a two-dimensional coding scheme 12G, as represented in
| Number | Date | Country | Kind |
|---|---|---|---|
| 07/06673 | Sep 2007 | FR | national |
