WO2005027104A1

WO2005027104A1 - Storage medium, apparatus for retrieving data from such a storage medium

Info

Publication number: WO2005027104A1
Application number: PCT/IB2004/003009
Authority: WO
Inventors: Robert Frans Maria Hendriks
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-09-18
Filing date: 2004-09-10
Publication date: 2005-03-24
Also published as: AR045759A1; TW200522026A

Abstract

The invention relates to an apparatus for retrieving data from a storage medium having an information layer (IL), the information layer (IL) comprising at least one data object, the apparatus comprising: a radiation source for emitting an input radiation (R) toward the storage medium, a detector (DT) for detecting characteristics of a detection pattern resulting from interference in the intermediate field between the input radiation (R) and the data object, the detector (DT) being locatable at one or more heights (z) with respect to the information layer (IL), the data contained in said data object being derived from the characteristics of said detection pattern.

Description

"Storage medium, apparatus for retrieving data from such a storage medium"

FIELD OF THE INVENTION The invention relates to an optical data storage system intended to retrieve data from a data storage medium. The invention also relates to said optical data storage medium.

BACKGROUND OF THE INVENTION The use of optical storage media for the recording, dissemination and retrieval of data is well established. Recently, other types of storage media, such as solid state storage media (e.g. flash memory, etc.), have been introduced. However, such storage media remain relatively expensive to manufacture, and hence have not become frequently used for the purpose of the dissemination of data. Particularly in the read-only format, the cost of producing optical storage media is significantly less than the cost of solid state storage media. However, it would be desirable to reduce the size and complexity of optical data retrieval devices. One way to do this would be to provide for lens-less imaging; the omission of imaging optics can be used to significantly reduce the size and complexity of the data retrieval device. A data storage and retrieval method using lens-less imaging has been described in the paper "Compact read-only memory with lens-less phase-conjugate holograms," F. Zhao and K. Sayano, Optics Letters, Vol. 21, No. 16, August 15, 1996. However, this system uses a volumetric holographic material, for which a mass replication process would be relatively costly, due to the need to record the image using a holographic image recording stage.

OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention to provide a new apparatus for retrieving data from a data storage medium. In accordance with one aspect of the present invention, there is provided an apparatus intended to retrieve data from a storage medium having an information layer, the information layer comprising at least one data object, the apparatus comprising : a radiation source for emitting an input radiation toward the storage medium, a detector for detecting characteristics of a detection pattern resulting from interference in the intermediate field between the input radiation and the data object, the detector being locatable at one or more heights with respect to the information layer, the data contained in said data object being derived from the characteristics of said detection pattern.

This invention exploits the Talbot effect about self-imaging. Self-imaging, otherwise known as lens-less imaging, was discovered by H.F. Talbot, almost 200 years ago; see H.F. Talbot, Phil. Mag. 9, 401 (1836) and Mansuripur, Classical Optics and its Applications, Chapter 18. The effect is observed when a beam of light is reflected from (or transmitted through) a periodic pattern (also called object or data object). It is observed that this periodic pattern is reproduced at certain intervals above or below the original patterned surface. The Talbot effect is very general in its appearance. The pattern periodicities may be in one or two dimensions. The object may modulate both the amplitude and the phase of the light beam. The incident wave front may be plane or spherical (originating from a point source). The Talbot effect consists of the observation that when a grating (with period d) is illuminated coherently, an image appears behind the grating at distances N(2d²/λ), where N is an integer and λ the wavelength of the light; see Patorski, K. 1989, Prog. Optics, 27, 1-108. The image is repeated periodically, without lenses or other optical imaging elements. This diffractive effect is studied in gratings, but can also occur for non-periodic objects, providing the objects are constructed to be of a specific class of objects, which are self-imaging.

The near field is the region above (or below) an object where evanescent waves still play a role, which is roughly one wavelength away from the object. The far field is the region where the diffraction orders are well separated, and do not overlap (i.e. the region where the object distance can effectively be taken to be infinity). The intermediate field is the region where the evanescent waves have negligible influence, but the diffraction orders still overlap and still contain information about the object. To generally define the intermediate field region, the distance from the object is larger than the wavelength, but smaller than the lateral size of the object. The detection pattern corresponds to an intensity profile which is measured by the detector, the detector being placed in the intermediate field. This arrangement allows suppressing imaging optics between the information layer and the detector plane. As such, the size and complexity of the data retrieval device may be reduced significantly. The detector can be placed at a fixed height compared to the information layer, assuming that data object are self-imaged by the Talbot effect at this height. The detector can also be placed at different heights compared to the information layer, in particular in the case where the resolution of the detector is limited. Multiplexing by measuring the intensity profile at various heights is useful to increase the amount of data storage capacity. The information layer is scanned by the input radiation so that all data objects stored on the medium are read by the detector. The data are derived from the detected intensity distribution, for example, in the case of self-imaging objects, by conducting a transform on the intensity distribution detected by the detector; to provide a set of component spatial frequencies in the data object. According to the invention, data may be encoded in the data object by switching each of a permitted set of possible component spatial frequencies "on" (present) or "off (not present). The permitted set is determined by one or more constraints, in particular the requirement for self- imaging. Alternatively, data can be derived from the detected intensity distribution in other ways. For example, a model of the intensity distribution which arises from a data object at the detector may be calculated when constructing the data object, during the design stage of the manufacturing process. In this way, an object may be generated which generates a preferred intensity distribution at the detector which is relatively simple to process into useful data. The data may then be derived more directly (and in some embodiments, directly) from the individual readings at each detector, so as to reduce the amount of calculation that is required in the data readout device.

It is also an object of the invention to provide a new optical data storage medium intended to be read by the apparatus according to the invention described above. To this end, the storage medium comprises an information layer (IL), said information layer (IL) comprising at least one data object having a relief structure. This aspect of the invention provides a data storage medium which is read out, in common with the prior art volumetric holographic storage media described above (and distinct from conventional optical storage media such as optical discs), by means of interference effects in output radiation produced between the data object and an input radiation to produce useful data. However, in contrast to the prior art volumetric holographic storage media, the objects can be readily pre-calculated without the constraints for holographic imaging or a specific holographic image recording step, such that the objects can be manufactured in a relief structure which can be readily reproduced.

In a preferred embodiment, the relief structure is intended to produce an amplitude modulation pattern and/or a phase modulation pattern. An information layer with both amplitude and phase modulation has a potentially doubled information capacity as compared to an information layer where only the amplitude is modulated.

In a preferred embodiment, the data object is a non-periodic object having one or more predetermined constraints. It is possible to define the class of non-periodic objects with the constraint that the objects are self-imaged via the Talbot effect.

In a preferred embodiment, the data object corresponds to a self-imaging object. If the data object is selected to be self-imaging according to the Talbot effect, it is self-imaged at a predetermined and known distance from the data object.

In a preferred embodiment, the data object is a real and/or symmetrical object. By defining constraints such as selecting the object to be real and/or symmetrical, and by programming the reader apparatus according to these constraints, it can be ensured that a one-to-one relationship between an intensity distribution read at the detector and the data contained in the objects is derivable by appropriately processing the intensity distribution to derive the data there from. The data objects may be selected from a plurality of different groups of data objects, each of which is subject to different constraints, for example the group of self-imaging data objects and a different group of data objects subject to a different constraint. Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a data storage efficiency graph, Figs.2 and 3 show intensity distributions in a detector plane, Figs.4 to 8 are schematic illustrations of data retrieval arrangements according to various different embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Talbot effect Below is given a simple analysis, deriving the requirements for an object to be self- imaging; also published by Patorski, K. 1989, Prog. Optics, 27, 1-108. The object u(x,y,z) is in the y-plane, and is repeated in the z-direction. The following scalar wave equation is used : V²u + k₀ ²u = 0 (0.1)

_M(^,z)^v_B,(_V ^2»w (0.2) m

The latter equation indicates that the field behind the object is periodic, with period Δz. The object is :

For each value of the parameter m, the wave equation should be satisfied, and this defines the object v_m as follow:

where k_x and k_y are the spatial frequencies along axis x and y respectively.

The spatial frequencies k_x and k_y of a self-imaging object lie on concentric rings

(called Montgomery rings) having radius p_m , this radius being determined by the parameter m. The maximum value of p_m is (27t/λ), and evanescent waves do not play a role here. This requirement is related to the Talbot distance of a grating (i.e. the distance behind a grating at which the grating is re-imaged). A grating consists of one spatial frequency k_x defined by:

Δz m = n λ with Δz » λn (0.8)

Δz = n — ^~ (0.9)

For a grating period d, the repetition frequency corresponding to the Talbot distance is

Δz=2d²/λ. If a detector was placed at a distance Δz behind the grating the following frequencies would be imaged onto the detector : 4πk_n /c_v = n- (0.10) Δz Self-imaging and object reconstruction In exploiting the Talbot effect, it becomes possible to obtain information from measuring the intermediate field. First, a pure self-imaging will be considered, where the intensity distribution on the detector is the same as on the object, and subsequently will be described how it is possible to obtain information on the object outside the strict requirements for self-imaging. The requirement for self-imaging reduces the amount of data that can be stored. By considering other possible classes of object for which reconstruction of the data in the object is possible, the data storage can be made more efficient. For example, when the spatial resolution of the detector is sufficient (e.g. 4 pixels per bit) and the distance between the detector and the information carrier is known (with an accuracy of e.g. approximately a wavelength) a symmetrical object can be restored fully.

Self-imaging In one embodiment of the invention, a constraint placed on the class of object or objects in the information layer is that they are self-imaging. In this section, the requirements for an information layer to self-imaging on a detector at a distance z from the object are studied. For illustrative purposes, the object is assumed to be one-dimensional along axis x. However the object is in other embodiments of the invention two-dimensional in the plane (x,y).

The spatial frequency in the z direction for a certain spatial frequency in the x direction is:

where /t₀ = 2π/ is the wavenumber, λ is the wavelength of the light. At the detector, the spatial frequencies should have the same phase relation φ = k₂z as they have on the object (modulo 2π). This yields: φ = n2π

where n is n integer.

The number N of spatial frequencies that satisfy this relation is given by

N = -^Z (0.13) λ

By the change of variable m=N-n, Eq.(0.12) can be rewritten as 2π k = —^2Nm - ^■ m (0.14)

Now, the issue is that the spatial frequency bands have a width Δk-2 π/L, with L the length of the object. These bands cause a blurring of the image at the detector, due to the fact that the phase relation is distorted. The "dephasing" Δφ at the detector is given by: dk Aφ = z Ak dk (0.15) ■ zΔJc

Next it is required that the dephasing is smaller than a limiting value Δφ" k_x Aφ™ — < — - — (0.16) / , zΔ/c

Combined with Eq.(0.12) this indicates how many independent frequency components (i.e. bits) can be fitted on the object and detected on the detector :

It can be seen that even if the requirement on Aφ^m∞ is not too tight, the number of available independent frequency components is limited. As an example, a plot of N'λ/L as a function of z/L for Aφ^max = 2π 1 is shown in Fig.l. For illustrative purposes, the ratio N'λ/L is taken as the storage efficiency (if this value is 1, one bit per distance is stored on the data storage medium corresponding to the wavelength.) As can be seen from Fig.l, the maximum efficiency is 10%, using the self- imaging constraint.

Figs.2 and 3 illustrate the intermediate fields of self-imaging objects and the objects themselves at selected heights z above the object. In these figures, the solid line indicates the intensity distribution I in the plane of the object, whereas the dotted line indicates the resulting intensity distribution I in the plane of the detector.

In the examples for which the intensity distributions shown were produced, the following parameters were used : Wavelength : 4 pixels Object size : 400 wavelengths Detector distance : 100 wavelengths Self-imaging distance : 100 wavelengths Illumination : Plane wave on object Number of frequencies : 40 For illustrative purposes, the intensity trace shown is only 100 pixels long in the range

1900-2000, while in fact the object had a length of 1600 pixels. For each example the object is generated from a selection of the 40 available frequencies, being built out of sine-waves with varying amplitudes and the set of spatial frequencies given by Eq.(0.14) (Here m is smaller than the number of frequencies used). A look-up table making the relation between the binary data to be coded and the data object to be written on the storage medium may be used, the content of this table being previously defined with the constraint of self-imaging. As can be seen from Fig.2, when the detector height is at the correct Talbot distance (within ± 1-2 wavelengths), the object is correctly self-imaged at the detector plane. The solid line and the dotted line are almost superimposed. However, as can be seen from Fig.3, when the height is outside this range, the intensity distribution is very different. Hence it is important for self-imaging that the detector is at the correct Talbot distance (within ± 1-2 wavelengths).

Object restoration From the above, it can be seen that the requirement for self-imaging tends to limit the amount of data which can be stored. This requirement may be relaxed to just demand that the data within an object can be resolved from the intensity profile. If, hypothetically, it were possible to measure electric fields (instead of intensity) at the detector, the following procedure could be used : - Measure the electric field distribution E(x), - Fourier transform E(x) to find E(k_x) , Correct for the phase that has been accumulated during the propagation from the object to the detector: E'(k_x) - e^~'^kz{kχ)zE(k_x) , where k_z(k_x) is the z-component of the wave- vector with x-component k_x , - Back-transform to find the electric field distribution, E'(x) (and also the intensity profile) just after the object.

However, without additional information, the electric field profile cannot be deduced from an intensity measurement, because the phase information cannot be detected at the detector. This problem is solved in an embodiment of the invention by restricting the data object or objects held in the information layer to a class of objects, and using this information in the data retrieval device. In this way, the data held in the object or objects can be resolved. As an example, it can be required that the object (and the electric field amplitude E'(x) behind it) is real and symmetrical, discarding 50%) of the potential storage capacity.

This can be used as a priori knowledge in the data retrieval device that the Fourier transform

E k_x) is also real and symmetrical. As the phase accumulated during propagation from the object to the detector is known from the object-detector plane distance, the phase of the electric field at the detector (i.e. the phase of E'(k_x)) can be deteraiined. Using this a priori knowledge, the amplitude of E'(k_x) from a measurement of the intensity profile can be reconstructed.

Height multiplexing In the above described embodiments, a detector with a relatively high resolution is needed to resolve the data in the object or objects in the information layer. The detector is composed of an array of pixels, for example of the CMOS type. However, even with a detector with a low resolution, relatively high-resolution data may be obtained from the object. This can be done by measuring the intensity in the intermediate field at various distances from the object, and reconstructing more of the object from such measurements than is possible using measurements at a single height. The object does not necessarily need to be self-imaging. A look-up table making the relation between the binary data to be coded and the data object to be written on the storage medium may be used.

Replication of the information layer By use of the present invention, the objects may be computed by a data processing system, such as a computer and the objects may be added to the information layer using a replication process such as stamping from a master. In one embodiment, only the amplitude is modulated, while keeping the phase constant. For example, the class of real objects may be produced by placing a layer of amplitude modulating medium, matched in refractive index to the material from which the relief structure is made, in the information layer. Thus, the information layer may have the objects recorded therein by means of amplitude modulation alone. Alternative classes of objects can be produced using a phase modulating relief structure, either alone or in combination with an amplitude modulating relief structure. A combination of an amplitude and a phase modulation may be obtained by use of a relief structure comprising a first structure and a second structure which are aligned with respect to one another.

In the design stage of the manufacturing process of the optical storage medium, data objects according to the invention may be generated using data processing equipment (such as a computer), so as to select preferred characteristics of the objects. By generating the objects by calculation (rather then by means of a reproductive process such as holography), greater freedom of design of the data storage medium can be obtained. By limiting the type of objects calculated to a class by means of one or more given constraints which are selected to make the data storage medium more simple to produce. Such constraints may also define a set of objects which can individually be resolved with respect to each other at the distance from the object at which the detector is designed to measure the resulting intensity distributions.

Implementation of optical ROM card Fig.4 shows a schematic illustration of a data retrieval apparatus according to an embodiment of the invention. An illumination unit IU provides radiation R with a coherent wave front. As depicted, the illumination unit IU corresponds to a wave guide intended to expand an input laser beam LB, but other illumination principles are possible as well, as long a well defined wave front is created. The radiation R is applied to the surface of the information layer IL. The information layer IL is formed in an optical card, and contains an absorbing or diffractive pattern. In the simplest form, the data is a self-imaging pattern which is imaged onto the detector DT at the appropriate distance z. Then, the amount of data is limited by the pixel size of the detector, and the requirement on self-imaging. To increase data, readout multiplexing is possible by varying the spacing z between detector DT and the information layer IL. The optical card is illuminated by a well-defined wave front, and the (multi-element) detector DT, for example of the CMOS type, is placed in the intermediate field, where the diffraction orders still overlap. Information on the card is read by detecting the intensity profile in the intermediate field. An advantage is that no imaging optics are required.

The first option for detection is without multiplexing. In that case, all information is read from a single exposure of the detector. The structure can be either i) self-imaging or ii) of a class that can be reconstructed from the intensity profile. The advantage of i) is that there is no need for a significant computational load, the advantage of ii) is that the information density can be higher. The second option for detection is with multiplexing. In that case, information is reconstructed from a measurement of the intensity profile at various heights from the object. The major advantage here is that a detector with larger pixels can be used, as only the intensity modulation at lower frequencies has to be detected. The multiplexing can be performed by i) measuring the intensity profile at specific heights or ii) oscillating the detector in height above the object and detecting certain (temporal) frequency components.

Optical card with 1 -dimensional scanning Fig.5 depicts a first schematic illustration of a data retrieval apparatus according to a further embodiment of the invention. This embodiment comprises an optical card OC which is one-dimensional scanned along axis (x) by the radiation R. A detector DT is placed above the optical card for measuring the intensity profile of the output light beam generated by the optical card OC, when the source of radiation R illuminates the line L. From this intensity profile, the information of the optical card situated on the illuminated line L can be reconstructed. As illustrated, the detector DT can be sized so as to be smaller than the length of the optical card. In that case, the detector is moved simultaneously along axis (x) with the radiation R. Actuation means (not shown) such as step by step motors can be used for translating along axis (x) either the detector DT and the radiation source R. Alternatively, the detector DT can be as large as the length of the optical card so as to recover it completely. In that case, no actuation means are required for displacing the detector DT along axis (x) when the radiation R scans the optical card. If height multiplexing is used, the apparatus comprises actuation means (not shown) such as step by step motors for translating along axis (z) the detector DT until reaching a target height z.

Fig.6 depicts a second schematic illustration of a data retrieval apparatus according to a further embodiment of the invention. This embodiment comprises an optical card OC which is one-dimensional scanned along axis (x) by the radiation R. A detector DT is placed above the optical card for measuring the intensity profile of the output light beam generated by the optical card OC, when the source of radiation R illuminates the line L. From this intensity profile, the information of the optical card situated on the illuminated line L can be reconstructed. A two-dimensional detector DT is used to measure the intensity profile at various distances z from the object. As illustrated, the detector DT can be sized so as to be smaller than the length of the optical card. In that case, the detector is moved simultaneously along axis (x) with the radiation R. Actuation means (not shown) such as step by step motors can be used for translating along axis (x) either the detector DT and the radiation source R. Alternatively, the detector DT can be as large as the length of the optical card so as to recover it completely. In that case, no actuation means are required for displacing the detector DT along axis (x) when the radiation R scans the optical card. If height multiplexing is used, the apparatus comprises actuation means (not shown) such as step by step motors for translating along axis (z) the detector DT until reaching a target height z.

Optical disc with broad track Figs.7 and 8 are schematic illustrations of a data retrieval apparatus according to a further embodiment of the invention, in which the Talbot effect is used in an optical disc reader. The self-imaging structure of the data stored on the disc D is written in the radial direction d in a broad track. Above the disc is a unit U containing a laser light source LB, and a detector DT. The unit generates the radiation which is reflected on the surface of the disc D, and the reflected pattern of the disc is imaged on the detector DT. The track here are small as compared to optical cards, but wide when compared to conventional optical discs. The information that is written in the radial direction in the track is picked up by the detector.

Also here, a two-dimensional detector can be used, such that the intensity profile as a function of the distance form the disc can be measured, allowing multiplexing as described above. Fig.8 depicts a zoomed view of the unit U corresponding to the side facing the disc D.

This figure shows that the laser light source LB and the sensitive cells of the detector DT are placed next to each other.

Further embodiments The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

1. An apparatus for retrieving data from a storage medium having an information layer (IL), the information layer (IL) comprising at least one data object, the apparatus comprising : a radiation source for emitting an input radiation (R) toward the storage medium, a detector (DT) for detecting characteristics of a detection pattern resulting from interference in the intermediate field between the input radiation (R) and the data object, the detector (DT) being locatable at one or more heights (z) with respect to the information layer (IL), the data contained in said data object being derived from the characteristics of said detection pattern.

2. An apparatus as claimed in claim 1, wherein the detector (DT) is locatable at a height (z) corresponding to the Talbot distance.

3. An apparatus as claimed in claim 1 or 2, comprising actuation means for translating the input radiation (R) and/or the detector (DT) on the information layer (IL).

4. A storage medium comprising an information layer (IL), said information layer (IL) comprising at least one data object having a relief structure for generating an amplitude modulation pattern and/or a phase modulation pattern.

5. A storage medium as claimed in claim 4, wherein at least one data object is a non-periodic object of a class having one or more predetermined constraints.

6. A storage medium as claimed in claim 4 or 5, wherein said constraint is to be self-imaging.

7. A storage medium as claimed in claim 4, 5 or 6, wherein the data object is real and/or symmetrical.