DE102007063491A1 - Method for detecting / generating a plurality of holograms in a storage medium, device for detecting and system for generating a plurality of holograms in a storage medium - Google Patents

Abstract

The present invention relates to a method and an apparatus for detecting or generating a plurality of holograms. The present invention has for its object to provide a method and apparatus (system) for generating and detecting a plurality of holograms, which achieves a high storage density with relatively inexpensive optical components and a simplified control. The inventive method for generating a plurality of holograms in a storage medium comprises the following method steps: providing a storage medium with a storage layer (3) whose refractive index undergoes a change upon irradiation of electromagnetic radiation, focusing and counter-superimposing electromagnetic radiation of a light source (10) in such a way that an interference pattern is formed in a given spatial position (x, y, z) of the storage layer (3) due to the opposing superimposition and leads to a greater refractive index change in the focus in areas of constructive interference than in areas of destructive interference, and Refractive index change is a hologram with a plurality of layers with alternating refractive index is generated, wherein in the storage layer (3) successively a plurality of holograms in different lateral positions (x, y) and at different depths (z) of Storage layer (3) are generated, wherein the lateral extent of the ...

Description

The The present invention relates to a method for detection or for generating a plurality of holograms, a device for Detecting and a system for generating a plurality of holograms, In particular, the present invention relates to a micro-holographic Data memory, a device for reading mikroholographischer Interference grid and a method for generating and / or reading of micro-holographic interference gratings. Furthermore, a proposed a new method for multiplexing the micro holograms, a new dimension for micro-lographic data storage makes available.

The end DE 101 34 769 A1 and DE 10 2007 004 025.5 Known micro-holographic data storage methods are based on the generation of microscopic diffraction gratings in a storage layer. For the generation of gratings and the writing of information, two highly focused laser beams are oppositely superimposed within the photosensitive layer. The constructive interference of the two beams then leads to a light intensity modulation, which is converted in the memory material into a corresponding spatial variation of the refractive index. The grid detection and thus the data recovery done by only one of the two beams is radiated into the storage medium and reflected there on the written micro-grids. The diffraction of the one beam (reading beam) at the lattice planes of a micro-holographic grating leads to the reconstruction of the other beam used for writing the micro-grating (signal beam).

The Concept for high-capacity micro-holographic data storage is based on two premises: First, the area data density a single plane by physical localization of the micro grid dimensions be maximized. On the other hand, this "simple" data density increased several times using suitable multiplexing methods. In the o. G. Patent applications are two different multiplexing techniques described wavelength division multiplexing and depth multiplexing as a kind of spatial multiplexing. While the technological implementation of multiplexing in the spectral range requires additional laser, the main advantage lies the mikroholographischen volume storage against the conventional optical surface data storage (CD / DVD / BluRay) in the possibility of information in many, one below the other Layers of data (layers) instead of just a single store. One important advantage in the implementation of depth multiplexing or the multilayer storage is that the implementation This method does not significantly expand the optoelectronic Systems implies.

The Multiplexing of many data layers in a relatively "thick" Storage medium with a few hundred microns layer thickness is also possible thereby, that the refractive index modulation when writing micro-grids only in a small volume range the storage medium takes place. Due to the longitudinal intensity distribution A strongly focused light beam creates the micro-grids in discrete volume elements whose dimensions with the focal Area of the focused beam are comparable. This means, that they are both transversal in a depth plane and longitudinal in the depth of the storage medium a clearly limited extent exhibit. Using diffraction-limited laser beams for writing the micro-grating is the transverse extent approximately at the wavelength of light comparable during the longitudinal expansion or Depth of a microgrid approximately through the Rayleigh length of the light beam (according to Gauss) and also a few Wavelengths is great. The concept of multilayer Storage is based on this depth limitation of the micro-grids, which makes it possible to have many clearly separated data layers to deposit a total thickness of a few hundred micrometers distributed and without crosstalk (Cross Talk) read again.

Together with the premise, the storage density of individual data levels by maximizing the spatial localization of the modulation, this leads to the requirement, the laser beam spot in the focus area to represent as small as possible. Same request has the development of CD and DVD technology shaped and ultimately results in an operating regime at the optical resolution limit. Due to the conceptual similarity this leads in the technological realization of the micro-holographic process also the need to size the laser focus spot to minimize diffraction in the field of microgrid production. This is achieved by diffraction-limited laser beams from both sides is focused into the photosensitive layer. The three-dimensional Lattice localization is then directly from the focus size of the writing beam. When reading the generated Grids are detected without errors, d. H. the reading beam must be the same have diffraction limited focus size.

A diffraction-limited mapping is used when the performance of an optical system is only or at least significantly influenced by the physical effect of the diffraction, but not by Deficiencies in design or manufacture is limited. There are different ways to determine if an optical system is diffraction limited, for example, by calculating and measuring optical path length difference OPD, Strehl ratio, RMS radius, RMS OPD, standard deviation, and others. Due to the different criteria, it is possible that an optical system according to one of the aforementioned methods is classified as "diffraction-limited", but not as "diffraction-limited" according to another of the aforementioned methods.

According to the present invention, an image is considered to be "diffraction limited" if the RMS radius of the geometrically optical image is smaller than the Airy Disk radius The Airy Disk radius is defined as r AIRY = 0.61 λ N / A . where λ is the wavelength of the light imaged by the optical system (or the center wavelength of the coherent light source used) and NA is the numerical aperture. The numerical aperture (NA) is a dimensionless quantity and characterizes the angular range within which an optical system can image or actually image light. The numerical aperture is defined as follows: NA = n × sinσ = n '× sinσ' where n 'is the refractive index of the material in the image plane (in the case of the micro-holographic data memory, the refractive index of the polymer) and σ' is the image-side angle of the marginal ray optical axis is. In other words, the minimum possible focus radius (airy disk radius) or focus diameter is determined by the wavelength of the light to be focused and the marginal beam inclination of the focused laser beam realized by the optics used.

is the diameter of the irradiated laser beam larger as the aperture, so laser radiation is cut off at the aperture stop vignettiert, so is as edge beam in the sense of the present Invention is understood to be the one ray that is just not yet cut off or vignetted by the aperture stop of the system becomes. Is the diameter of the irradiated laser beam smaller as the aperture, so no laser radiation at the aperture stop cut off or vignettiert, so is called marginal ray in the sense of The present invention is understood to mean that beam whose Intensity 1% of the maximum intensity (Gaussian distribution intensity). Is the diameter of the irradiated Laser beam equal to the aperture, the edge beam contacted the laser radiation exactly the aperture diaphragm.

When RMS radius (or RMS spot radius) within the meaning of the present Invention becomes the root-mean-square (radial mean square) radial size designated. That is, the distance between each beam and a reference point (optical axis or center of gravity of all rays) is squared, then averaged over all rays and then the square root drawn. As already mentioned, becomes an optical system considered as "diffraction-limited" if the RMS radius is smaller as the Airy Disk radius is. The size of the RMS radius depends on the selection of the system Blasting off. So it is possible that an optical system after a certain beam selection as "diffraction-limited" and not as "diffraction-limited" after another ray selection is seen. For the purposes of the present application, the RMS Radius determined by the fact that over the aperture 5 rings a 10 beams (linear radial and related to the running angle evenly distributed). Here are the Radiation over the aperture as evenly distributed. When focusing parallel beams (as in this case) the 5 rings become equidistant to each other between marginal ray and optical axis (i.e., the first ring is located on the optical axis, the fifth ring is located in the radial distance of the marginal rays from the optical axis and second to fourth ring are equidistant in between distributed). The 10 rays per ring are circumferential, so each with a Angle of 36 ° distributed to each other. The rays of each Rings run on radial straight lines from the center to the outside, d. H. each 5 rays of each 10 rings can be connected by radial (star-shaped) lines.

It is therefore necessary in the prior art, when focusing in the disk occurring opening error to correct for aberration-free, diffraction-limited in the storage medium Laser focus spots to realize the light-induced emergence lead the micro grid. This requirement for a flawless, diffraction-limited optical imaging of the laser beam in the storage medium remains even when reading, since it is submicrometer-sized Structures is to be scanned with the reading beam. The detection process thus also takes place at the diffraction limit, the reading beam spot in focus should not be bigger its as the focal beam spots of the writing beams.

At a certain material thickness, ie when addressing only one depth plane (as in conventional CD and DVD systems), the correction of the spherical aberration known aperture error can already be taken into account in the design of the objective lens. With multilayer storage in many data planes distributed through the depth of the medium, the thickness to be corrected changes with the depth position of the respective data plane or with the position of the laser focus when writing and reading information in this data plane. In order to be able to address the entire layer thickness of the storage medium, it is therefore necessary manoeuvrable, the focusing optics, with which the rays are focused into the medium to adapt dynamically to the respective layer thickness. The layer thickness is defined here as the distance between the outer surface of the data carrier up to the current data layer. Preferably, the storage layer is formed by a photosensitive layer (photopolymer layer) having a thickness between 0.05 mm and 1.5 mm, which is surrounded by substrates having a thickness of between 0.1 mm and 1.2 mm on both sides. The layer thickness is then defined as the distance between the outer surface of the substrate facing the light source (laser) to the focus in the photosensitive layer (current data layer). Such dynamic correction of the aperture error makes comparatively complex optics with corresponding actuators necessary. For this purpose, lenses are used in the prior art, which are constructed as optical systems of multiple lenses and in which at least one lens is axially displaced to correct the aperture error. This disadvantageously leads to complex (expensive) optical components which on the one hand have to be positioned and readjusted by means of a complex control and on the other hand make the system more susceptible due to the increased number of movable optical components.

Of the The present invention is based on the object, a method and a device (system) for generating and detecting a Variety of holograms indicate that a high storage density in comparatively inexpensive optical components and a achieved simplified control. In addition, supposedly the inventive method and the invention Device a lower susceptibility to have mechanical influences.

These Tasks are according to the invention by a method according to independent claims 1 (Generation of holograms) and 15 (detection of holograms) as well by a device according to the independent Claims 19 (detection of holograms) and 24 (generation of holograms). Preferred embodiments of Invention are contained in the subclaims.

The inventive method for generating a Variety of holograms has the following process steps: Providing a storage medium with a storage layer whose Refractive index upon irradiation of electromagnetic radiation a change experiences, focusing and opposing overlaying of electromagnetic radiation of a light source such that in a given spatial position of the storage layer due to the opposite superposition, an interference pattern training and focus in areas of constructive interference a greater refractive index change than in areas destructive interference, and due to refractive index change a hologram with a variety of layers with alternating Refractive index is generated, wherein in the storage layer successively a variety of holograms in different lateral positions and generated at different depths of the storage layer, wherein the lateral extent of the foci of the holograms in different Depths of the storage layer according to the invention varies becomes.

The The idea of the invention is, under certain conditions to an adjusted correction (diffraction-limited focusing) Consistently high data density can be dispensed with. That is, it can both the storage density in individual data layers and the Number of layers remain high even though the system no longer operated on diffraction-limited laser beam spots. The essence lies in the contrary order of the both lenses or writing beams with a reflection unit for generating the directly returning jet. In this arrangement, a laser beam passes twice, namely in different propagation directions the focusing lens. At each pass the propagation is through a certain distance within the storage medium in the corresponding opening error result. According to the invention, a method for micro-holographic data storage, in which the resulting aberration of the laser beams is not corrected, but the spherical aberration used much more purposefully becomes.

Preferably the radiation gets into the different depths of the storage layer focused with a different aperture error. Preferably, the radiation is focused and counterposed in opposite directions, that the ratio between the largest lateral extent of a hologram and the smallest lateral Extension of a hologram at different depths of the storage layer along the optical axis for a given lateral Position greater than 1.1, more preferably greater 1.5, more preferably greater than 2, more preferably greater than 4, and more preferably greater than 7. The means that the foci in the data layers are significant size differences have since on a complex correction by means of several is dispensed along the optical axis sliding lenses. Therefore, in different data layers, i. H. in different Depth of focus (in preferred use of an aspherical Single lens, which is shifted according to the depth of focus) different aperture errors (spherical aberration) and thus result in different focus sizes.

Preferably the radiation focused and counterposed in opposite directions is that neighboring holograms within a perpendicular to the optical axis extending plane at least partially (preferred overlap by more than 50% of its lateral extent). It is also preferable that there are several adjacent holograms at least partially (preferably more than 50% of its lateral Extension) overlap. By the overlap adjacent holograms can increase the storage density despite larger Focus diameter (due to the accepted opening error) be as high or even higher than when diffraction limited Holograms (elaborate optics) next to each other (not overlapping) be used.

Preferably the intensities are superimposed on the opposite Radiation selected such that the extent of the holograms, at the refractive index difference of alternating layers at the edge Is 50% of the refractive index difference of alternating layers in the center, along the optical axis between 1 μm and 50 μm is.

Preferably is used for focusing a first focusing unit, which is formed, the light of a radiation source in the storage layer to focus, and a compensation unit is used, the is formed, which focused from the first focusing and after the first focus divergently propagating light to collimate, and further a back reflector for reflection of the collimated light is used in such a way that that of the back reflector reflected light by means of the optical compensation unit again is focused into the storage layer into a second focal point, wherein the first focus and the second focus are laterally and axially at least partially (preferably completely or to at least 90% of the lateral extent). Preferably become the refractive power of the first focusing unit and the refractive power the compensation unit in the staggered generation of holograms kept constant at different depths of the storage layer and the resulting aperture error. To correspond with it the lateral extent of a hologram to the depth of the hologram in the storage layer.

Preferably be the first focusing unit, the compensation unit and the back reflector to the thickness of the optical storage medium tuned so that the coming of the radiation source, on the first focusing unit striking light, after passing through the first focusing unit, the storage layer, the compensation unit, subsequent reflection at the back reflector, renewed passage by the compensation unit, the storage layer and the first one Focusing unit, congruent or substantially congruent is reflected back in itself. The clou is so, the optical components are not limited to diffraction-limited focusing in all data layers (variable) vote (high effort), but only on the total thickness of the storage layer (not variable) in such a way that the focused (out-of-focus) through which Memory layer passing, back-reflected and again in opposite directions light (back focus) congruent or substantially congruent with the beginning focussed light, ie out-of-focus and re-focus on top of each other lie. This is when using a plane mirror or retroreflector as a back reflector then achieved when the compensation unit that focused by the focusing unit and behind the focus again divergent propagating light collimated error-free, so parallelized. To Compensation unit and focusing unit on the Total thickness of the storage layer to be set (optimized) need however, at different focus depths (data layers) in theirs respective overall power can not be changed, ie it does not have to be several refractive surfaces relative be moved to each other. As a result, lower requirements to put the optical components.

wobei RMS_HIN der RMS Radius der fokussierten Strahlung und RMS_RÜCK der RMS Radius der gegenläufig überlagerten Strahlung im Fokus ist.Preferably, the focusing and the counter-overlapping with a numerical aperture greater than 0.2 (more preferably greater than 0.4, more preferably greater than 0.5, and even more preferably greater than or equal to 0.6). Preferably, the compensation unit and the focusing unit are adapted to the total thickness of the storage layer in such a way that the RMS radii of the oppositely superimposed light beams within the storage layer fulfill the following condition:

wherein RMS _HIN the RMS radius of the focused radiation and RMS _Re is the RMS radius of the counter superimposed radiation in focus.

Preferably, in a predetermined spatial position of the storage layer, a plurality of holograms with different aberrations are successively generated and superposed on each other. Preferably, for a given lateral position at a predetermined depth of the storage layer at least two Holograms generated one after the other and superposed on each other, wherein the RMS radius of the focused radiation when generating the two holograms by at least 10% (preferably by at least 100%) is different. Another aspect of the invention relates to a new multiplexing method for micro-holographic data storage. With the storage method according to the invention as described above, a novel multiplexing technique known as aberration multiplexing can be used. In this multiplex method, the dependence of the generated interference pattern on the local aberration of the two write beams is used as the unique identification of each hologram. If the aberration of the rays is changed, their specific field distribution and consequently the shape and distribution of the induced hologram lattice planes change as well. The spatial selectivity of such holograms when detecting opens up the possibility of writing many holograms (with different aberrations) in one and the same memory position and then being able to reconstruct them separately.

Preferably becomes a first between the focusing unit and the storage medium thickness-adjustable plane-parallel plate and between the Compensation unit and the storage medium a second variable thickness plane-parallel plate, wherein at a predetermined depth (z) the memory layer at least two holograms in succession generated and superimposed on each other, and where the sum the optical thickness of the storage medium and the first and second plane-parallel plates is kept constant and the optical thickness the first plane-parallel plate for each of the superimposed Holograms is varied. This allows (by means of the variable thickness plane parallel plates different aberrations in and written in the same memory location and then separated again be reconstructed without the total thickness of the storage layer change, matched to the compensation unit and focusing unit are.

The inventive method for detecting a multiplicity of holograms has the following method steps: generating a multiplicity of holograms in different spatial positions of the storage layer, irradiating and focusing radiation of a light source onto a hologram in a predetermined spatial position of the storage layer, detection of the hologram reconstructed Wavefront by means of a photosensitive detector, wherein the wavefront reconstructed by the hologram is focused on a confocal filter by means of a second focusing unit, wherein for the lateral extent d _{L of} the opening of the confocal filter the condition: 12:12 · λ N / A ≤ d L ≤ 3.6 · λ N / A . where λ is the wavelength of the radiation source and NA is the numerical aperture of the second focusing unit. The idea of the invention consists in being able to clearly discriminate the holograms that are partly overlapping due to the different aberrations during readout. The stored hologram now correctly reconstructs the corresponding other laser beam with correct illumination. To increase the positional selectivity of the stored holograms, a confocal filter is used in the detector branch which only allows a significant amount of light to pass through when the read-out hologram is illuminated as written (meeting the Bragg condition). It achieves nearly the same spatial resolution as achieved with "perfect" (diffraction-limited, non-overlapping) micro-holograms generated with fully aperture-corrected lenses.

Preferably During the focusing of the radiation, an opposite superimposition is superimposed the radiation avoided or at least suppressed. Preferably has the focused radiation used to generate a hologram the same or essentially the same aberrations we the focused radiation used to detect the hologram on. Preferably, the RMS radius differ for generating of the hologram used focused radiation and the RMS radius the focused radiation used to detect the hologram less than 2%.

The device according to the invention for detecting a multiplicity of holograms in a storage medium comprises: a radiation source, a recording medium for receiving a storage medium with a storage layer, the refractive index of which undergoes a change upon irradiation of electromagnetic radiation, a first focusing unit, which is configured to emit the light Focusing radiation source on a hologram of a storage layer held by the receiving means, wherein the first focusing unit and the focal point formed by the first focusing define a first optical axis, and the first focusing unit and the storage layer are movable relative to each other along the first optical axis, a Beam splitter, which is arranged on the first optical axis between the radiation source and the first focusing unit and formed such that the reconstructed by a hologram wavefront after passing through the first Fo kussiereinheit is deflected into a different optical axis from the first optical axis, and a photodetector, wherein between the beam splitter and the photodetector a confocal filter is arranged, and wherein for the lateral extent d _{L of} the opening of the confocal filter, the condition: 12:12 · λ N / A ≤ d L ≤ 3.6 · λ N / A where λ is the wavelength of the radiation source and NA is the numerical aperture of the radiation emitted by the radiation source and incident on the confocal filter radiation.

Preferably, a second optical focusing unit is provided between the beam splitter and the confocal filter, and the confocal filter is arranged at the focal point of the second optical focusing unit, wherein for the lateral extent d _{L of} the opening of the confocal filter the condition: 12:12 · λ N / A ≤ d L ≤ 3.6 · λ N / A where λ is the wavelength of the coherent radiation source and NA is the numerical aperture of the second focusing optical unit. Preferably, for the lateral (d _L ) extension of the confocal filter, the condition applies: 0.3 · λ N / A ≤ d L ≤ 1.2 · λ N / A , Preferably, the first optical focusing unit is formed by a single lens and / or the confocal filter is formed by the entrance facet of the photodetector. Preferably, the second focusing optical unit, the confocal filter opening and the photodetector define a second optical axis, and the confocal filter is movable along the second optical axis.

The Inventive system for generating a plurality of holograms in a storage medium, comprises: a radiation source, a storage medium with a storage layer whose refractive index upon irradiation of electromagnetic radiation undergoes a change, a first focusing unit configured to emit the light of Radiation source in a first focal point within the storage layer to focus, a compensation unit that is trained that focused by the first focusing unit and after the first focus to collimate divergently propagating light, a Back reflector for reflecting the collimated light so, that reflected by the reflector Rückre light by means of optical compensation unit back into the storage layer in Focusing a second focus, wherein the first focus and the second focus at least partially overlay, wherein the first focusing unit and the first focusing unit generated focal point define a first optical axis, and both the first focusing unit and the compensation unit along the first optical axis are designed to be movable, and wherein the refractive power of the first focusing unit and the refractive power of the Compensation unit in each case in all positions along the optical axis are constant or substantially constant.

definieren lässt, wobei f(h) die Pfeilhöhe oder die z-Koordinate entlang der optischen Achse, h der Abstand senkrecht zur optischen Achse, R der Scheitelradius, k die konische Konstante und A4, A6 usw. die asphärischen Parameter sind. Mindestens einer der asphärischen Parameter A4, A6 muss ungleich Null sein, damit eine asphärische Flächenform resultiert.Preferably, the first focusing unit and / or the compensation unit are formed by a single lens (particularly preferably by aspheric a single lens). As an aspheric lens in the context of the invention, a lens is understood, of the refractive surfaces of which at least one surface is non-spherical, that is not spherical. Preferably, all (both) refractive surfaces are non-spherical, ie aspherical. Preferably, the refractive surfaces are rotationally symmetrical. Specifically, an aspherical surface is understood to mean a rotationally symmetrical surface shape which is defined by a polynomial of the shape

where f (h) is the arrow height or z-coordinate along the optical axis, h is the distance perpendicular to the optical axis, R is the peak radius, k is the conic constant, and A4, A6, etc. are the aspherical parameters. At least one of the aspheric parameters A4, A6 must be non-zero in order for an aspherical surface shape to result.

Under A single lens becomes a lens with two outer ones understood rotationally symmetric, refractive surfaces, whose distance is not variable to each other. In between, it is preferable only an optical material with a refractive index greater 1 available. However, it is also possible that between the outer refracting surfaces more refractive surfaces (putty lens) are arranged. However, that is the distance of all refracting surfaces fixed, d. H. not variable.

Preferably, the distance between the first focusing unit and the compensation unit is constant or substantially constant along the optical axis at all focal depths. Preferably, the compensation unit and the first focusing unit are tuned to the thickness of the optical storage medium such that the radiation of the light source is completely collimated after passing through the first focusing unit, the storage medium and the compensation unit. Preferably, between the focussing unit and the storage medium, a first thickness-adjustable plane-parallel plate and between the compensation unit and the storage medium a two te thick-variable plane-parallel plate, wherein the sum of the thickness of the storage medium, the thickness of the first plane-parallel plate and the Thickness of the second plane-parallel plate is kept constant and the thickness of the first plane-parallel plate for superimposing different aberrated holograms is varied in the same position.

The Invention will be described in more detail below with reference to the figures described embodiments described in more detail. Show it:

1 an inventive device for detecting a plurality of holograms in a storage medium, in a schematic sectional representation,

1a an alternative embodiment of a device according to the invention for detecting a plurality of holograms in a storage medium, in a schematic sectional representation,

2 an inventive system for generating a plurality of holograms in a storage medium, in a schematic sectional representation,

3 an inventive system for generating and detecting a plurality of holograms in a storage medium, in a schematic sectional representation,

4 an inventive system for generating and detecting a plurality of holograms in a storage medium, with variable-thickness plane-parallel plates for aberration multiplexing, in a schematic sectional view,

5a the intensity of the Gaussian beam, focused with NA 0.75 with correction of the aperture error on focus depth in a schematic representation,

5b the intensity of the Gaussian beam, focused with NA 0.75 without correction of the aperture error on focus depth (200 μm focus offset of pre-corrected depth) in a schematic representation,

6a the wave field of the Gaussian beam according to 5a , focused with NA 0.75 with correction of the aperture error on focus depth in a schematic representation,

6b the wave field of the Gaussian beam according to 5b , focused with NA 0.75 without correction of the aperture error on focus depth (200 μm focus offset of pre-corrected depth) in a schematic representation,

7a Gaussian grating of two counter-rotating beams with correction of the aperture error on focus depth in a schematic representation,

7b Gaussian grating of two counter-rotating beams without correction of the aperture error on focus depth (200 μm focus offset of pre-corrected depth) in a schematic representation,

8a the illustration of aberration multiplexing using a Gaussian grid of two counter-rotating beams with a focus offset of +150 μm to the pre-corrected depth in a schematic representation,

8b the illustration of aberration multiplexing on the basis of wavefronts of the reading beam of a 8a generated grid in a schematic representation,

9a Illustration of Aberrationsmultiplexing using a Gaussian grating of two counter-rotating beams with a focus offset of +50 microns to pre-corrected depth in a schematic representation,

9b the illustration of aberration multiplexing on the basis of wavefronts of the reading beam of a 0a generated grid in a schematic representation,

10a the illustration of aberration multiplexing on the basis of a Gaussian lattice of two counter-rotating beams without focus offset (= pre-corrected depth) in a schematic representation,

10b the illustration of aberration multiplexing on the basis of wavefronts of the reading beam of a 10a generated grid in a schematic representation,

11a the illustration of aberration multiplexing using a Gaussian grid of two counter-rotating beams with a focus offset of -50 μm to the pre-corrected depth in a schematic representation,

11b the illustration of aberration multiplexing on the basis of wavefronts of the reading beam of a 11a generated grid in a schematic representation,

12a Illustration of Aberrationsmultiplexing using a Gaussian grating of two counter-rotating beams with a focus offset of -150 microns to pre-corrected depth in a schematic representation,

12b the illustration of aberration multiplexing on the basis of wavefronts of the reading beam of a 12a generated grid in a schematic representation,

13 an inventive system for generating a plurality of holograms in a storage medium, with a plane-parallel plate for aberration multiplexing, in a schematic sectional representation,

14 a system according to the invention for generating a plurality of holograms in a storage medium, with two thickness-variable plane-parallel plates for aberration multiplexing, in a schematic sectional view, and

15 a thickness-variable plane-parallel plate for Aberrationsmultiplexing, in a schematic sectional representation.

3 shows a holographic data memory according to the invention for generating and detecting a plurality of holograms in a storage medium, in a schematic sectional representation.

The holographic data memory has a diode laser 10 , a beam splitter 8th , a λ / 4 plate 9 , a focusing unit 1 , a compensation unit 2 , a back reflector 4 , another focusing unit 5 , a confocal filter 6 and a photodetector 7 on. Between the focusing unit 1 and the compensation unit 2 is the storage medium 3 arranged, which itself does not necessarily have to be part of the data memory. The combination of data storage and storage medium 3 is understood as a system for data storage. The beam splitter 8th and the λ / 4 plate 9 are not essential (merely advantageous) components of the holographic data memory according to the invention. Task of the beam splitter 8th (from element 10 about elements 8th . 9 about elements 1 . 3 . 2 . 4 and again elements 2 . 3 ) It is the writing beam path and the reading beam path (from element 10 about elements 8th . 9 about elements 1 . 3 and again elements 3 . 1 . 9 . 8th and elements 5 . 6 . 7 ) to separate. However, such a separation can also take place via other optical configurations.

To create holograms become the elements 5 . 6 . 7 not required. To read out holograms become the elements 2 and 4 not required. Accordingly, the devices of the 1 and 1a (no elements 2 and 4 ) Devices that are only suitable for reading holograms. And the 2 shows a device (no elements 5 . 6 . 7 ) which is only suitable for generating holograms. The devices of 3 and 4 can both generate and read holograms. The reference numerals in the 1 - 4 always show the same elements.

To create a hologram ( 2 - 4 ) becomes light of the laser diode 10 by means of the focusing unit 1 (preferably an aspherical single lens) in the storage layer 3 focuses (out of focus) and then diverges again. By means of the compensation unit 2 (preferably an aspherical single lens), the divergent light is again collimated, ie parallelized. Subsequently, the light hits the back reflector 4 and is completely reflected back in itself and then by means of the compensation unit 2 back into the storage layer 3 focused (back focus) and superimposed with the outward focus. This forms an interference pattern that leads to a refractive index modulation in the storage layer 3 leads. The grid thus generated (refractive index modulation) is a hologram.

For reading ( 1 . 1a . 3 . 4 ) becomes light of the laser diode 10 by means of the focusing unit 1 in the storage layer 3 focused. If a hologram (refractive index modulation) is present at the location of the focus and the Bragg condition is met, the focused light is reflected and can be used as a signal in the photodetector 7 be registered.

The optical beam path is corrected so that the lens 5 (Opening) error-free and preferably diffraction-limited (or almost diffraction-limited: RMS radius smaller than twice the Airy Disk radius) on the pinhole 6 focused (confocal filter). In particular, this can be done by having the subsystem consisting of 1 . 2 . 3 and 4 is built so that the laser beam after passing (ie from left to right through 1 . 3 and 2 , then reflected 4 and again through 2 . 3 and 1 in the opposite direction) is free of opening defects. This is z. B. achieved in that 1 and 2 in their sum the through 3 Correct the generated spherical aberration. That means from the laser 10 seen from the laser beam is in front 1 and behind 2 aberration free, albeit within the focus in 3 Spherical aberration is present and thus aberrationsbehaftete holograms are generated. In the material, the resulting field distribution is stored in a hologram. The beam path can thus be reconstructed later during readout and the light reflected at the hologram can thus be almost diffraction-limited the confocal filter 6 happen and with the detector 7 be registered.

However, it is also possible that the laser beam after passing through the subsystem 1 . 2 . 3 and 4 a remaining spherical aberration, which then only through 5 is corrected. To ensure a high resolution is only crucial that the total beam shaping at the pinhole of the confocal filter 6 diffraction-limited (or nearly diffraction-limited: RMS radius is less than twice the Airy-Disk radius). You store, so to speak, the wave fronts in one area (storage medium 3 ) of the entire beam path from the laser 10 to the detector 7 where there is a clear aberration. As the beam then but then on the way to the detector 7 is again corrected as completely as possible, one obtains virtually aberration-free holograms with almost the resolution actually aberrationfreier. To achieve the resolution is the confocal filter 6 is essential and ensures discrimination against holograms that partially overlap as a result of aberration. This also makes it possible to have a plurality of aberration states which can be separately read out from one another in the same volume element in the storage material 3 to create. Because the. Size of the filter panel 6 the resolution is affected, it is as small as possible to measure (smaller than Airy disk diameter), but so large that a sufficient amount of light for detecting the detector 7 reached. In confocal laser microscopy filter sizes between 0.25 × and 1 × Airy diameter are used. Below 0.25 Airy, there is virtually no improvement in the resolution more (diffraction limit).

The use of a polarizing beam splitter 8th and a lambda / 4 plate 9 is advantageous in that by reading the holograms almost all of the amount of light reflected on them to the detector 7 arrives. The polarizing beam splitter 8th and the lambda / 4 plate 9 are to be oriented so that the predominantly linearly polarized light of the laser 10 the way to the photodetector 7 takes.

To write a hologram, a device as in 2 - 4 used the components 5 . 6 . 7 are not required. To read a hologram is the reflector 4 eliminated or rendered ineffective - for example by means of a shutter; the Lens 2 is also unnecessary.

The consideration that can be waived under certain conditions to an adapted correction is based on this invention. It is a peculiarity of the optical structure and the corresponding beam path in a mikroholographischen storage system that allows a technological implementation of the method even without complex aberration correction and without any loss in memory performance. Both the storage density in individual data layers and the number of layers can remain high despite the fact that the system no longer operates with diffraction-limited laser beam spots. Although the individual lenses 1 . 2 be pre-optimized so that they focus diffraction-limited for a data layer, however, a diffraction-limited focus can not be achieved even in different data layers (at least not with single lenses).

The essence lies in the opposite arrangement of the two lenses 1 . 2 or write beams with a reflection unit 4 for generating the returning beam. In this arrangement, a laser beam passes twice, in different propagation directions, the focusing lens 1 . 2 , At each pass, the propagation through a certain distance within the storage medium 3 result in the corresponding opening error. According to the invention, a method for micro-holographic data storage is proposed, in which the resulting aberration of the laser beams is not corrected, but much more purposefully used.

The focusing lenses 1 . 2 only have to be designed so that in their sum they correct the aperture error caused by the total material thickness of the storage layer 3 is caused. There when corrected each of the two lenses 1 . 2 a part of through the disk 3 caused opening error. In other words, to focus the laser beam in the polymer (storage layer 3 ) a single lens 1 used, which has a noticeable opening error at different depths, but according to the invention a second Einzelellinse 2 behind the polymer (storage layer 3 ), wherein the second single lens 2 is formed such that by the polymer (storage layer 3 ) passing through (opening error-prone) divergent radiation collimated error-free. Subsequently, the radiation is reflected by a reflective element 4 over the second single lens 2 again focused back into that, whereby by the inventive design of the lenses 1 . 2 a (albeit open-flawed) very precise superposition of both foci can be achieved. Due to the superimposition, a standing wave is generated which has a corresponding refractive index change in the polymer (storage material 3 ), whereby a hologram (open-ended) can be generated in a defined manner.

The invention differs from other uncorrected systems in that according to the invention, although the focus with respect to the aperture error is not corrected or is not completely corrected, however, the radiation after passing through both individual lenses 1 . 2 error-free propagated, whereby the opening-defective foci can be superimposed congruent.

A Fixed correction of the spherical aberration is also found in conventional optical storage systems (CD / DVD / BluRay), because here too by an opening error-causing disk substrate is shown. Such a lens is generally possible perform as a single lens, which on the one hand the production costs reduced to a minimum and on the other hand to a very lightweight Lens leads. A low weight is essential for the fast movement through on fast voice coil actuators based servo system for maintaining the focus depth position and to track the data track in a fast rotating disc. With appropriate dimensioning of the substrate thickness of the holographic Disk, it's even possible to use exactly the same lenses as used in the current conventional storage systems become.

According to the invention the interference pattern generated with the two writing beams not more in the common aberration-free focus of the two beams stored in the form of a micro-grid. The data storage finds now in a depth position, in the result of the resulting spherical aberration no diffraction-limited beam focus spots available.

The resulting interference pattern in the area of the narrowest constriction of the two beams (minimal caustics) is in the storage medium 3 saved as a hologram. Decisive here is that in the special beam path with opposite writing beams, the position of the storage medium 3 between the lenses 1 . 2 almost no effect on the aperture error of the detector 7 has decoupled beam. The rays outside the area between the two focusing lenses 1 . 2 remain even with axial displacement of the storage medium 3 aberration-free, since the aperture error is always corrected by the complementary lens. All that needs to be done is to make sure that the aperture errors corrected by the lenses 1 . 2 in their sum the total storage media thickness 3 correspond (in other words are the first single lens 1 , the plane-parallel plate (the storage medium 3 including any existing substrates) as well as the second single lens 2 for parallel light aberration-free, ie parallel incident rays are parallel again after passage).

The stored hologram now correctly reconstructs the corresponding other laser beam with correct illumination. To increase the positional selectivity of the stored holograms becomes a confocal filter 6 used in the detector branch, which only allows a significant amount of light to pass if the hologram is illuminated to read out as it was written (meeting the Bragg condition). It achieves nearly the same spatial resolution as we achieve with "perfect" micro-holograms generated with fully aperture-corrected lenses.

The main difference to conventional micro-gratings lies in the size and shape of the three-dimensional lattice structure. This is directly given by the field distribution, ie wavefronts of the writing beams and, as already explained limited in diffraction-limited micro-gratings on a few, localized in the focal area of the superimposed beams lattice planes. If the aperture errors are not corrected, this diffraction-limited field distribution is distorted and deformed. As a result, the aberrated holograms arise in a much larger volume range, ie, the refractive index modulation is effectively more evenly distributed across larger volume elements of the memory material 3 , Another important effect is the aberration-specific deformation of the wave fronts, which is clearly due to the local focus position of the writing rays in the depth of the medium 3 is defined. The optical resolution of the memory ver driving is not lost, although the hologram structure is very large compared to diffraction-limited micro-grids. This large aberration-distorted structure is exactly bound to a focus area of the write beam that is only a few hundred nanometers wide and changes location-dependently with each displacement in the transverse and longitudinal direction.

In this is a significant advantage of the storage method according to the present invention - it is according to the invention a much larger volume range for storage used by hologram production at unchanged high optical resolution and spatial selectivity. An efficient hologram reconstruction only comes about when the wavefronts of the opening error deformed reading beam overlap exactly with the aberration hologram grid planes. The physical meaning can be easily explained by the Number and spatial extent of the light-induced lattice planes interpret. Aberration-prone and distorted holograms feature effectively over a greater number of lattice levels and a larger modulation volume. Accordingly can be used with such holograms at the same exposure energy even achieve higher diffraction efficiencies than with "real" aberration-free micro holograms. This can be on the higher number of effectively stored lattice planes due. The effective refractive index modulation Although takes square with the falling light intensity but the entire diffraction signal is cumulatively caused by reflection at all modulated lattice levels.

By the higher diffraction efficiency makes the holograms easier detectable so that the error rate due to increased signal-to-noise Ratio can be reduced. In addition, let at sufficiently high diffraction efficiency needed for writing Reduce laser energy. With lower exposure energies per writing process becomes the limited dynamic range of the storage medium less stressed and thus more efficient for multiplexing techniques be used so that with the inventive Method can be achieved higher multiplex rates.

As a result the higher effective number of lattice planes results also a higher wavelength selectivity the generated holograms. This in turn has the advantage that when using wavelength division multiplexing with lower worked spectral distances of the individual wavelength channels can be. This will make it possible to get a bigger one Number of wavelength channels to use and accordingly to increase the data density. At a low Multiplexing rates of wavelength division multiplexing can be the requirements for the chromatic imaging performance of the lenses reduce, since the necessary correction for smaller Spectral ranges easier to realize. For very narrow wavelength ranges can even be completely to dispense with chromatic corrections.

Another aspect of the invention relates to a new multiplex method for micro-holographic data storage ( 4 . 13 - 15 ). With the storage method according to the invention as described above, a novel multiplexing technique known as aberration multiplexing can be used. In this multiplex method, the dependence of the generated interference pattern on the local aberration of the two write beams is used as the unique identification of each hologram. If the aberration of the rays is changed, their specific field distribution and consequently the shape and distribution of the induced hologram lattice planes change as well. The spatial selectivity of such holograms when detecting opens up the possibility of writing many holograms in one and the same memory position and then being able to reconstruct them separately.

As mentioned, both lenses need 1 . 2 together always correct the same total opening error, the ratio of the individual correction parts of the two lenses 1 . 2 but can be changed. This change leads to the creation of holograms that differ in their shape. Any particular setting of the lenses 1 . 2 leads to a clearly defined local field distribution of the laser beams, which can be represented as aberrated wavefronts. If such wavefronts are superimposed on the two writing beams, a complex spatial modulation of the refractive index, which is greatly degenerated by the aberration in comparison to normal micro-gratings according to the prior art, results. This modulation represents a hologram that is now clearly defined by the aberration of the wavefronts.

According to the invention Many such holograms overlap in one and the same volume element or spatially multiplex. When reading the data is then only the particular hologram detected, the exactly to Aberration of the reading beam fits. The required selectivity the hologram corresponds to the classic Bragg selectivity thick volume holograms. Only when the wave fronts of the reading beam Aberrationsrichtig with the lattice planes of a hologram overlay, the Bragg condition is met. This then results in a high efficiency of diffraction of the reading beam on the hologram, which continues to correspond to a high level of the read signal.

With this technique for multiplexing opens up the fifth Dimension for micro-logical data storage, because next to three spatial dimensions and the spectrum of light as the fourth Dimension now also the wavefront aberration space for the multiplexing is available.

8th - 12 demonstrate the Aberrationsmultiplexing: Left are each the Gaussian grid of two counter-rotating beams shown, right, the wavefronts of the reading beam (not to scale). 8th : Storage layer 3 is too close to the first lens 1 (Δz = -150 μm) 9 : Storage layer 3 is shifted in the direction of the corrected position / corrected layer thickness (Δz = -50 μm). 10 : Picture optimally corrected. 11 : same displacement of the substrate as in 9 but from the first lens 1 path. 12 : same displacement of the substrate as in 8th but from the first lens 1 path.

The Aberrationsmultiplexing can be practically by a controlled change by the two lenses 1 . 2 implemented generated opening error. The dynamic change of the aperture errors of the two beams can be achieved for example by correction plates 11 . 12 ( 4 . 13 - 15 ), each between lens 1 . 2 and disc 3 be inserted. An inserted thicker correction plate 11 . 12 on one side of the disc 3 is compensated by a correspondingly thinner on the other side of the disk, so that the total optical thickness remains constant ( 13 . 14 ).

Height Multiplexing rates in aberration multiplexing can be relatively easy to achieve in the between each lens and the storage medium such a stepless, externally electronically controlled correction element is arranged.

13 shows a system according to the invention for generating a plurality of holograms in a storage medium, with a plane-parallel plate for Aberrationsmultiplexing, in a schematic sectional view. A simple correction tile 11 is in the beam path between the lens 1 and the storage medium 3 alternately arranged from both sides of the medium to superimpose two data layers with differently aberrated holograms and to record them in one and the same depth position. 14 shows a system according to the invention for generating a plurality of holograms in a storage medium, with two thickness-variable plane-parallel plates for aberration multiplexing high multiplex rate: Between each lens 1 . 2 and the storage medium 3 becomes an optical thickness-variable correction component 11 arranged to vary the aberration of the beams stepless and electronically controllable.

As in 15 illustrates, the thickness variable component 11 ( 12 ) is preferably realized as a compressible component. A preferred implementation is a liquid 13 between two thin plane plates 14 to arrange and the two flat plates 14 form mutually movable, so that the thickness of the component 11 . 12 can be varied. The thickness-variable component 11 . 12 can be fixed in the optical system. The change in thickness of the compressible component is realized by suitable electromechanical actuators (electromagnetic, piezo, electrostatic). The task solution is that the two thin plane plates 14 are designed to move each other. Through the elastic areas 15 Can the between the plane plates 14 located liquid 13 escape or re-flow. Alternatively, a solid, compressible material such as rubber or gelatin may be used, the thickness of which is then varied.

An important advantage of this solution is that in addition to the lenses 1 . 2 only one more optical element is present in the beam path. It is particularly advantageous that the thickness of such an element 11 . 12 by applying a current or an electric field (electromagnetic, piezo, electrostatic) can be varied. As a result, the number of mechanically moved components is reduced to a minimum, thus increasing the reliability of such a system. Furthermore, very fast switching times can be achieved because of the control via the electric field. The path length of the compression is on the order of magnitude in the range of the focal depth range, ie in the range of 50-500 micrometers. The variation of the thickness can be achieved, for example, by magnetized area (in the edge area of the plane plates 14 can be arranged) and electrical contacts. Is about the electrical contacts (each on one of the plane plates 14 are arranged - z. As ITO contacts) applied an electric field, the plane plates 14 on each other and the excess part of the liquid 13 in the elastic areas 15 pressed. The thickness can be controlled by the strength of the electric field. If the field strength is reduced, then the moves in the elastic areas 15 located liquid 13 back into the area between the plane plates 14 and the thickness increases. Of course, a number of other means of varying the thickness are possible.

The variable-thickness component adopts the setting of the degree of aberration in multiplexing. Preferably, the sum of the thicknesses of both correction elements 11 ( 14 ) respectively. 11 . 12 ( 4 ) constant.

With the method of the invention is the practical Realization of a micro-lographic storage system greatly simplified. Furthermore, the storage capacity can be increased. Aberrated holograms can also improve the security of the data from unauthorized access, since they can only be read with laser beams of correct aberration. Therefore, the inventive method for the holographic security technology for the identification of Objects and persons or for the protection of stored content limited access suitable.

1

Focusing / single lens

2

Compensation unit / single lens

3

storage layer

4

back reflector

5

Focusing / single lens

6

confocal filter

7

photodetector

8th

Beam splitter (in combination with 9 polarizing)

9

λ / 4 plate

10

Radiation source / diode laser

11

thick variable, plane parallel plate

12

thick variable, plane parallel plate

13

liquid

14

displaceable glass plate

15

elastic Area

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - DE 10134769 A1 [0002]
• - DE 102007004025 [0002]

Claims (28)

A method for generating a plurality of holograms in a storage medium, comprising the following method steps: - providing a storage medium with a storage layer ( 3 ), whose refractive index undergoes a change upon irradiation of electromagnetic radiation, - focusing and opposing superimposition of electromagnetic radiation of a light source ( 10 ) such that in a predetermined spatial position (x, y, z) of the storage layer ( 3 ) forms an interference pattern due to the opposite superposition and in the focus leads in areas of constructive interference to a greater refractive index change than in areas of destructive interference, and due to the refractive index change, a hologram is generated with a plurality of layers with alternating refractive index, wherein - in the memory layer ( 3 ) successively a plurality of holograms in different lateral positions (x, y) and in different depths (z) of the storage layer ( 3 ), characterized in that the lateral extent of the foci of the holograms at different depths (z) of the memory layer ( 3 ) is varied. A method according to claim 1, characterized in that the radiation in the different depths (z) of the storage layer ( 3 ) is focused with a different aperture error. A method according to claim 1 or 2, characterized in that the radiation is focused and counterposed in opposite directions such that the ratio between the largest lateral extent of a hologram and the smallest lateral extent of a hologram at different depths (z) of the memory layer ( 3 ) is greater than 2 along the optical axis for a given lateral position (x, y). Method according to one of the preceding claims, characterized in that the radiation is focused and is superimposed in opposite ways, that neighboring Holograms within a perpendicular to the optical axis (z) extending Partially overlap plane (x, y). Method according to claim 5, characterized in that that adjacent holograms within a perpendicular to the optical Axis (z) extending plane (x, y) to more than 50% of its lateral Overlap extent. Method according to one of the preceding claims, characterized in that the intensities of the counterposed superimposed Radiation be chosen such that the extension of the Holograms, where the refractive index difference of alternating layers at the edge 50% of the refractive index difference of alternating layers in the center is, along the optical axis between 1 micron and 50 microns. Method according to one of the preceding claims, characterized in that for focusing a first focusing unit ( 1 ), which is designed to emit light from a radiation source ( 10 ) in the storage layer ( 3 ) and a compensation unit ( 2 ), which is formed by the first focusing unit ( 1 ) and to collimate divergently propagating light after the first focal point, and furthermore a back reflector ( 4 ) is used for the reflection of the collimated light in such a way that that of the back reflector ( 4 ) reflected light by means of the optical compensation unit ( 2 ) back into the storage layer ( 3 ) is focused in a second focal point, wherein the first focal point and the second focal point laterally and axially at least partially overlap. A method according to claim 7, characterized in that the refractive power of the first focusing unit ( 1 ) and the refractive power of the compensation unit ( 2 In the case of staggered generation of holograms at different depths (z) of the storage layer (FIG. 3 ) is kept constant and the resulting aperture error and thus the lateral extent of a hologram to the depth (z) of the hologram in the memory layer ( 3 ) correspond. Method according to one of claims 7 and 8, characterized in that the first focusing unit ( 1 ), the compensation unit ( 2 ) and the back reflector ( 4 ) are tuned to the thickness of the optical storage medium such that the of the radiation source ( 10 ), to the first focusing unit ( 1 ) passing light, after passing through the first focusing unit ( 1 ), the storage layer ( 3 ), the compensation unit ( 2 ), subsequent reflection at the back reflector ( 4 ), re-passage through the compensation Ness ( 2 ), the storage layer ( 3 ) and the first focusing unit ( 1 ), congruent or substantially congruent in itself. Method according to one of the preceding claims, characterized in that the focusing and the opposite superposition with a numerical aperture greater than 0.5. Method according to one of the preceding claims, characterized in that the RMS radii of the oppositely superimposed light beams within the storage layer ( 3 ) satisfy the following condition:

wherein RMS _HIN the RMS radius of the focused radiation and RMS _Re is the RMS radius of the counter superimposed radiation in focus. Method according to one of the preceding claims, characterized in that in a predetermined spatial position (x, y, z) of the storage layer ( 3 ) several holograms with different aberration are generated one after another and superimposed on each other. Method according to one of the preceding claims, characterized in that for a given lateral position (x, y) at a predetermined depth (z) of the storage layer ( 3 ) at least two holograms are successively generated and superimposed on each other, whereby the RMS radius of the focused radiation differs by at least 10% when generating the two holograms. Method according to one of claims 5-13, characterized in that between the focusing unit ( 1 ) and the storage medium, a first variable-thickness plane-parallel plate ( 11 ) and between the compensation unit ( 2 ) and the storage medium, a second variable-thickness plane-parallel plate ( 12 ) is arranged, wherein at a predetermined depth (z) of the storage layer ( 3 ) at least two holograms are successively generated and superimposed on each other, and wherein the sum of the optical thickness of the storage medium and the first and second plane-parallel plates ( 11 . 12 ) is kept constant and the optical thickness of the first plane-parallel plate ( 11 ) is varied for each of the superimposed holograms. Method for detecting a plurality of holograms in a storage medium, comprising the following method steps: irradiation and focusing of radiation from a light source ( 10 ) on a hologram in a predetermined spatial position (x, y, z) of the storage layer ( 3 ), - Detection of the wavefront reconstructed by the hologram by means of a photosensitive detector ( 7 ), characterized in that the wavefront reconstructed by the hologram is detected by means of a second focusing unit ( 5 ) to a confocal filter ( 6 ), wherein for the lateral extent d _{L of} the opening of the confocal filter (FIG. 6 ) the condition: 12:12 · λ N / A ≤ d L ≤ 3.6 · λ N / A . where λ is the wavelength of the radiation source ( 10 ) and NA the numerical aperture of the second focusing unit ( 5 ). Method according to claim 15, characterized in that that during the focusing of the radiation, an opposite superimposing the radiation is avoided or at least suppressed. Method according to claim 15 or 16, characterized in that the focused radiation used to generate a hologram the same or essentially the same aberrations we the focused radiation used to detect the hologram having. Method according to claim 17, characterized in that that the RMS radius used to generate the hologram focused radiation and the RMS radius of detecting the Holograms used focused radiation less than 2% differ. Apparatus for detecting a plurality of holograms in a storage medium, comprising: a radiation source ( 10 ), a receiving means for receiving a storage medium with a storage layer ( 3 ) whose refractive index undergoes a change upon irradiation of electromagnetic radiation, a first focusing unit ( 1 ), which is formed, the light of the radiation source ( 10 ) on a hologram of a storage layer held by the receiving means ( 3 ), wherein the first focusing unit ( 1 ) and the first focussing unit ( 1 ) define a first optical axis, and the first focusing unit ( 1 ) and the storage layer ( 3 ) are movable relative to each other along the first optical axis, a beam splitter ( 8th ) located on the first optical axis between the radiation source ( 10 ) and the first focusing unit ( 1 ) and is designed such that the wavefront reconstructed by a hologram, after passing through the first focusing unit ( 1 ) is deflected into an optical axis different from the first optical axis, and a photodetector ( 7 ), characterized in that between the beam splitter ( 8th ) and the photodetector ( 7 ) a confocal filter ( 6 ), wherein for the lateral extent d _{L of} the opening of the confocal filter ( 6 ) the condition: 12:12 · λ N / A ≤ d L ≤ 3.6 · λ N / A . where λ is the wavelength of the radiation source ( 10 ) and NA the numerical aperture of the radiation source ( 10 ) and onto the confocal filter ( 6 ) is incident radiation. Apparatus according to claim 19, characterized in that between the beam splitter ( 8th ) and the confocal filter ( 6 ) a second optical focusing unit ( 5 ), and the confocal filter ( 6 ) in the focal point of the second optical focusing unit ( 5 ), wherein for the lateral extent d _{L of} the opening of the confocal filter ( 6 ) the condition: 12:12 · λ N / A ≤ d L ≤ 3.6 · λ N / A . where λ is the wavelength of the coherent radiation source ( 10 ) and NA the numerical aperture of the second optical focusing unit ( 5 ). Apparatus according to claim 19 or 20, characterized in that for the lateral (d _L ) extension of the confocal filter ( 6 ) the condition: 0.3 · λ N / A ≤ d L ≤ 1.2 · λ N / A . applies. Device according to one of claims 19-21, characterized in that the first optical focusing unit ( 1 ) is formed by a single lens and / or the confocal filter ( 6 ) through the entrance facet of the photodetector ( 7 ) is trained. Device according to one of claims 19-22, characterized in that the second optical focusing unit ( 5 ), the opening of the confocal filter ( 6 ) and the photodetector ( 7 ) define a second optical axis, and the confocal filter ( 6 ) is designed to be movable along the second optical axis. A system for generating a plurality of holograms in a storage medium, comprising: a radiation source ( 10 ), a storage medium with a storage layer ( 3 ) whose refractive index undergoes a change upon irradiation of electromagnetic radiation, a first focusing unit ( 1 ), which is formed, the light of the radiation source ( 10 ) into a first focal point within the storage layer ( 3 ), a compensation unit ( 2 ), which is formed by the first focusing unit ( 1 ) and to collimate divergently propagating light after the first focal point, a back reflector ( 4 ) for the reflection of the collimated light in such a way that that of the back reflector ( 4 ) reflected light by means of the optical compensation unit ( 2 ) back into the storage layer ( 3 ) is focused in a second focal point, wherein the first focus and the second focus at least partially overlap, the first focusing unit ( 1 ) and the first focussing unit ( 1 ) define a first optical axis, and both the first focusing unit ( 1 ) as well as the compensation unit ( 2 ) are movably formed along the first optical axis, characterized in that the refractive power of the first focusing unit ( 1 ) and the refractive power of the compensation unit ( 2 ) are constant or substantially constant in all positions along the optical axis (z). System according to claim 24, characterized in that the first focusing unit ( 1 ) and / or the compensation unit ( 2 ) are formed by a single lens. System according to claim 24 or 25, characterized in that the distance between the first focusing unit ( 1 ) and the compensation unit ( 2 ) is constant or substantially constant along the optical axis (z) at all focal depths (z). System according to one of claims 24-26, characterized in that the compensation unit ( 2 ) and the first focusing unit ( 1 ) is tuned to the thickness of the optical storage medium such that the radiation of the light source ( 10 ) after passing through the first focusing unit ( 1 ), the storage medium and the compensation unit ( 2 ) is collimated. System according to one of claims 24-27, characterized in that between the focusing unit ( 1 ) and the storage medium, a first variable-thickness plane-parallel plate ( 11 ) and between the compensation unit ( 2 ) and the storage medium, a second variable-thickness plane-parallel plate ( 12 ), wherein the sum of the thickness of the storage medium, the thickness of the first plane-parallel plate ( 11 ) and the thickness of the second plane-parallel plate ( 12 ) is kept constant and the thickness of the first plane-parallel plate ( 11 ) is varied to superimpose different aberrated holograms in the same position.