DE102007063494A1 - Method and device for generating holograms - Google Patents

Abstract

The present invention relates to an apparatus and a method for generating a plurality of holograms, in particular the present invention relates to a mikroholographischen data memory and a method for the production of mikroholographische interference gratings. The object of the present invention is to provide on the basis of a commercially available, lightweight aspherical lens a mikroholographischen data memory and a method for producing mikroholographische interference gratings, which implement a compensation of the spherical aberration and thereby allow a complete elimination of the occurring aperture errors. For this purpose, the radiation of the light source (10) by means of a first aspheric lens (1) and a second aspherical lens (2) is focused in the storage layer (12) and after passing through the storage layer (12) by means of a third aspherical lens (3) and a fourth aspherical lens (4) collimated and focused after reflection on a reflector (5) weider in the memory layer (12) and counterposed to form an interference pattern, wherein for changing the depth (z) of the focus in the memory layer (12) second aspherical lens (2) and the third aspheric lens (3) are moved relative to the storage layer (12), and the first aspherical lens (1) relative to the second aspherical lens (2) and the third aspherical lens (3) relative to the fourth .. ,

Description

The The present invention relates to a method and an apparatus for generating a plurality of holograms, in particular the present invention is a micro-logical data storage and a method for the production of micro-holographic interference gratings, in particular the generation of micro-holographic gratings with diffraction-limited imaged laser beams within a storage medium.

Regarding The storage density is crucial, the spatial extent of the grid on as possible to locate small volume elements. This is achieved by the laser beam from both sides diffraction limited in the photosensitive Layer is focused. The three-dimensional lattice localization is then directly from the focus size of the write beam dependent. When reading the generated grids are detected error-free be, d. H. the reading beam must have the same diffraction-limited focus size.

From a diffraction-limited figure is spoken when the performance an optical system only or at least authoritative by the physical effect of the diffraction, but not by defects in the Design or production is limited. There are different Methods determine if an optical system diffraction-limited is, for example, by calculation and measurement of the optical path length difference OPD, the Strehl ratio, of the RMS radius, the RMS OPD, the standard deviation and others. Due to the different criteria, it is possible that an optical system according to one of the aforementioned methods as "diffraction-limited", but after a other of the above methods is not classified as "diffraction-limited".

According to the present invention, an image is considered to be \ "diffraction limited \" if the RMS radius of the geometrically optical map is less than the Airy Disk radius. "The Airy Disk radius is defined as r _AIRY = 0.61 λ / NA, where λ 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 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 Is the angle of the marginal ray to the optical axis, in other words, the wavelength of the light to be focused and that used by the Optics realized edge beam tilt of the focused laser beam due to the diffraction minimally possible focus radius (Airy Disk radius) or fixed focus diameter.

is the diameter of the irradiated laser beam is greater than 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 is smaller than the Aperture, so no 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 mean that beam whose intensity is 1% of the intensity in the maximum (Gaussian distribution the intensity) equivalent.

is the diameter of the incident laser beam is the same as the aperture, the marginal ray of the laser radiation contacts exactly the aperture diaphragm.

As RMS radius (or RMS spot radius) according to the present invention, the root-mean-square (root mean square) Radial size is called. That is, the distance between each ray and a reference point (optical axis or centroid of all rays) is squared, then averaged over all rays, and then the square root is drawn. As mentioned earlier, if the RMS radius is smaller than the Airy Disk radius, then an optical system is considered to be "diffraction-limited." The size of the RMS radius depends on the selection of the rays passing through the system in that, according to the present application, the RMS radius is determined by the fact that via the aperture 5 rings a 10 rays (linearly radial In the case of focusing parallel beams (as in the present case), the five rings are arranged equidistantly between the edge beam and the optical axis (ie the first ring is located) on the optical axis, the fifth ring is at the radial distance of the marginal rays from the optical axis, and second to fourth rings are equidistantly distributed therebetween). The 10 rays per ring are circumferential, so each distributed at an angle of 36 ° to each other. The rays of the individual rings run on radial straight lines from the center point to the outside, ie in each case 5 rays of the individual 10 rings can be connected by radial (star-shaped) lines.

The Depth multiplexing (multilayer) is used to calculate the total capacity of the storage medium multiple area capacity of one to maximize each data level. To data in multiple data layers Writing and reading out is a focus of the laser beam in different depths areas of a storage layer (along the optical axis, d. H. along the beam direction) is necessary. According to the state of the art, this is done by focusing on adapted to a specific depth range of the storage layer aspherical Single lens used.

at Focusing the beam to a different depth, however, leads to an aperture error, which depends on the depth of focus and by spherical Aberration of the incident beam in the layer will be described. Due to the opening error when focusing in a depth other than the lens corrected depth are the focus sizes in different Focus depths are no longer diffraction limited reduced to the minimum spot area. This effect happens spatially and leads both to a widening of focus in the plane as well as to a longitudinal widening in depth. This will generate the Microgrids enlarged both transversally and longitudinally.

In a memory system (containing a plurality of micro-holograms as Writes bits to the storage layer and reads them out if necessary) lead differently size Laser spots at different depths, d. H. Data levels on the one hand to the problem of increased Expenses for the driver and control electronics. The by the spherical aberration caused enlargement of the Above all, laser spots result in a reduced storage capacity. In addition, will changed by compared to an optimally focused beam Wavefront deformation of the resulting lattice structure caused.

The mentioned problem does not occur with CD and DVD, since there only in a data layer, or in two closely adjacent layers with a constant focus depth must be focused, which easily with a single of the layer thickness adapted (aspheric) Lens can be realized.

It However, it could be proved that a single lens is not the necessary (diffraction limited) imaging performance for one Mikroroholographischen data storage over a depth range (along the optical axis) of more than 100 microns can realize.

The Object of the present invention is a mikroholographischen Data storage and a method for generating micro-holographic To provide interference gratings that compensate for the spherical Aberration implement and thereby a complete elimination the occurring opening error enable. Such compensation results in the technological system implementation in a higher one memory with little equipment effort.

These Task is achieved by a method according to the independent claim 1 and a device according to the independent claim 10 solved. Preferred embodiments of the invention are contained in the subclaims.

The inventive method comprises the following steps: providing a storage medium having a storage layer, the refractive index of which undergoes a change upon irradiation of electromagnetic radiation, focusing and opposing superimposition of electromagnetic radiation of a monochromatic light source such that in a predetermined spatial position of the storage layer due to In opposite superimposition forms an interference pattern and leads in focus in areas of constructive interference to a greater refractive index change than in areas of destructive interference, and due to the change in refractive index, a hologram is generated with a plurality of layers with alternating refractive index, wherein in the storage layer offset in time a plurality of holograms be generated in different lateral positions and at different depths of the storage layer, wherein the radiation of the light source by means of a first Focused aspherical lens and a second aspherical lens in the storage layer and collimated after passing through the storage layer by means of a third aspheric lens and a fourth aspherical lens and refocused after reflection on a reflector in the storage layer and is superposed in opposite directions to form an interference pattern, the Change the depth of focus in the storage layer the second aspherical lens and the third aspherical Lin be moved relative to the storage layer and the first aspheric lens relatively second aspherical lens and the third aspherical lens is moved relative to the fourth aspherical lens. A monochromatic radiation source is understood to mean a radiation source whose half-width at the center wavelength is less than 10 nm.

With In other words, a holographic data storage is proposed wherein the storage layer (preferably one of two substrates surrounded disk-shaped Polymer layer) in relation to the optical system (first to fourth Lens) both laterally displaceable and along the optical axis (beam direction) is designed to be displaceable. Preferably, the lateral relative movement between storage layer and optical system by rotating the disk-shaped storage layer and the longitudinal relative movement between storage layer and realized optical system by movement of the first lens to fourth lens. Thereby can focused into arbitrary positions within storage layer (reflected back and again focused and thus out-of-focus and re-focus) in opposite directions and thus microholograms representing one bit are generated become. If there is no micro hologram at one position, then there is no bit written there. The minimum achievable size of the micro holograms is by the wavelength of the light and the numerical aperture of the optics used (first Lens to fourth lens), provided it is one diffraction-limited image before.

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.In order to be able to realize diffraction-limited imaging at different depths (along the optical axis), the spherical aberration is eliminated according to the invention by using two aspherical individual lenses each (first lens to fourth aspherical 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 conical constant, and A ₄ , A ₆ , etc. are the aspherical parameters. At least one of the aspherical parameters A ₄ , A ₆ must be non-zero in order for an aspherical surface shape to result.

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

The The idea is to use the first and second aspherical single lens for a predetermined distance to a certain substrate depth (= depth within the memory layer) and when controlling other depths within the storage layer only the distance between the Lenses and the distance of the lenses to the substrate (meaning it to the storage layer, which is surrounded by two thin substrates) to vary, but without the lenses themselves (or their refractive powers) too change.

It was found that each with only two aspherical Single lenses a diffraction-limited focusing / collimation with a numerical aperture of 0.6 (and greater) over a variable substrate depth (= Depth within the memory layer) of 200 microns (and more) can be realized can. This imaging performance can even with actually existing Component tolerances of 0.2 ° tilt and 40 μm Decentering (all Lenses, the storage medium and the reflector) can be achieved. in principle would be expected that a significantly larger number of lenses for generating diffraction-limited foci in different Depth is required, as is the case with zoom lenses The case is, although these are usually not the diffraction limit reach, d. H. do not represent diffraction-limited.

According to the invention, holograms are produced at different depths along the optical axis, the outer holograms being more than 50 μm (preferably more than 100 μm and more preferably more than 200 microns) from each other (along the optical axis) are spaced apart.

One particular advantage of the optical system proposed here in that to the second and third aspherical lens only one additional (first and fourth) lens (in front of and behind the polymer) is needed. Therefore, the system remains compact and to compensate for the Discrotation resulting error signals (tracking error signal and Focus Error Signal) fully adopted the standard technologies of CD and DVD development lines Because the weight of the lenses does not exceed the weight of a commercial one DVD lens is.

The nominal imaging performance of the system can be limited as diffraction be designated.

to Creating a hologram must follow the focused light reflected back and again focused and superimposed in opposite directions. this will through the third and fourth aspherical Single lens and the reflector realized. The reflector is preferably designed as a plane mirror or as a retroreflector.

Also the third and fourth aspherical Single lens become collimation for a given distance to a certain substrate depth (= depth within the storage layer) optimized and when controlling other depths within the storage layer Only the distance between the lenses and the distance of the Lenses to the substrate varies, but without the lenses themselves (or. their powers) to change.

Preferably becomes the displacement of the first aspherical lens, the second aspherical Lens, the third aspherical Lens and / or the fourth aspherical Lens along the optical axis respectively by means of actuators or Realized engines. Preferably, for focusing and collimation only the first aspherical Lens, the second aspherical Lens, the third aspherical Lens and the fourth aspherical Lens, d. H. no further optical components used.

Preferably is the distance between the storage layer and the second aspherical Lens to increase reduces the depth of focus in the storage layer. As depth the focus in the storage layer becomes the distance of the focus along the optical axis of the outer, the Light source (laser) facing side of the storage layer understood (If present, surrounding the storage layer substrates as part of the Storage media (disk) remain in the determination of the depth of focus disregarded).

Farther is preferably the distance between the storage layer and the third aspherical lens to increase the depth of focus in the storage layer is increased. Furthermore, it is preferable the distance between the first aspherical lens and the second aspherical Lens to increase reduces the depth of focus in the storage layer. Farther is preferably the distance between the third aspherical Lens and the fourth aspherical Lens to increase the depth of focus in the storage layer is increased. The position of the reflector (along the optical axis) needs adjustment / variation the depth of focus can not be varied, d. H. the reflector is preferred fixed or fixed.

Preferably is used as a light source, a laser with a central wavelength in the range 300nm-430 nm and a half width of less than 10 nm used. Preferably is used as a light source, a semiconductor laser with a central wavelength in the range 400 nm-430 nm and re the intensity used with a half width of less than 3 nm.

Preferably become the beam diameter of the light source, the surface shape the first aspherical Lens, de second aspherical Lens, the third aspherical Lens and the fourth aspherical Lens so chosen that the focusing of the electromagnetic radiation of the monochromatic Light source in the memory layer with a numerical aperture greater than 0.5 (more preferably greater than or equal to 0.6). Preferably, the beam diameter of the light source, the surface shape the first aspherical Lens, de second aspherical Lens, the third aspherical Lens and the fourth aspherical Lens so chosen that the focusing of the electromagnetic radiation of the monochromatic Light source is diffraction limited in the storage layer. A Calculation and optimization of the surface shapes of the lenses can (also under consideration predefinable tolerances) with appropriate optics software such as ZEMAX done.

The device according to the invention for generating a multiplicity of holograms has a monochromatic radiation source, a flat storage medium which has a material which undergoes a refractive index change upon irradiation of electromagnetic radiation, a focussing unit which has at least one Focusing part of the light of the radiation source into the storage layer, means for collimating at least part of the light passing through the storage layer, and a reflector for reflecting back the collimated light, the reflector and the means for collimation being arranged such that the reflected light is focused back into the storage layer, and the focusing unit and the means for collimation for generating holograms at different depths of the storage layer relative to the storage layer slidably formed, wherein the focusing unit by a first aspheric lens and a second aspherical lens and the means for collimation by a third aspherical lens and a fourth aspheric lens are formed, and the second aspherical lens and the third aspherical lens are adapted to be moved to change the depth of focus in the storage layer relative to the storage layer; first aspheric lens is adapted to be moved for changing the depth of focus in the storage layer relative to the second aspherical lens and the third aspherical lens is adapted to be moved to change the depth of focus in the storage layer relative to the fourth aspheric lens.

Preferably the device is adapted such that the distance between the storage layer and the second aspherical lens for increasing the Depth of focus in the storage layer is reduced and / or the distance between the storage layer and the third aspherical Lens to increase the depth of focus in the storage layer is increased and / or the distance between the first aspherical Lens and the second aspherical Lens to increase the depth of focus in the storage layer is reduced and / or the distance between the third aspherical lens and the fourth aspherical Lens to increase the depth of focus in the storage layer is increased.

Preferably are means for displacing the first aspherical lens, the second aspherical lens, the third aspherical Lens and / or the fourth aspherical Lens along the optical axis corresponding to a predetermined Position coordinate of the focus provided in the storage layer. Preferably, the means for shifting the first aspheric Lens, the second aspherical Lens, the third aspherical Lens and / or the fourth aspherical Lens along the optical axis respectively by an actuator or formed by a motor.

Preferably are the refractive indices (materials) of all components related to one wavelength constant, d. H. it is preferable to always use the same lenses (no lens exchange).

The Invention will be described below with reference to in the figures embodiments explained in more detail. It demonstrate:

1 a device according to the invention for generating holograms (holographic data memory), which realizes depth multiplexing with correction of the spherical aberration, in a schematic sectional representation,

2 a schematic representation of the distances between the lenses as a function of the depth of focus in the storage layer, and

3 a schematic representation of the distances between the lenses and the substrate (and thus the storage layer) as a function of the depth of focus in the storage layer.

1 shows a device according to the invention for generating holograms (holographic data storage), which realizes depth multiplexing with correction of the spherical aberration, in a schematic sectional representation.

The holographic data memory according to the invention has a laser diode 10 , a first aspheric single lens 1 , a second aspherical single lens 2 , a means for receiving a flat storage medium 12 (For example, a holding means for a disk-shaped storage medium 12 Not shown in the figure), a third aspherical single lens 3 , a fourth aspherical single lens 4 and a reflector 18 on.

The information is preferably bit by bit in the form of micro-heterologous interference gratings in a thin photosensitive layer 12 saved. The grating is generated in spatially limited volume elements by laser beams of the light source 10 , which are focused to the optical diffraction limit, ie diffraction-limited. The interference pattern generated by the superimposition of the incident and reflected beams is converted into a corresponding local modulation of the refractive index of the photosensitive material in the storage medium 12 transfer. The storage medium 12 is realized, for example, in the form of a disk by applying the photosensitive layer on an optical disk substrate and the data therein be encoded in a spiral track. The beam propagation during writing and reading takes place collinearly and on an axis perpendicular to the storage medium 12 runs.

When recording becomes the storage layer 12 moved by the discrete perpendicular to the laser beam axis, so that the lattice are dynamically induced as strips of any length. The read-out signal is produced by diffraction of a laser beam at the inscribed gratings under Bragg condition. The three-dimensional, strongly localized lattice structure makes it possible to exploit the entire volume of the storage medium 12 in that the data is in several discrete (preferably mutually equidistant) planes within the storage layer 12 be filed. The storage layer is regular 12 by a photopolymer layer having a thickness between 0.05 mm and 0.5 mm, which is surrounded by substrates on both sides with a thickness between 0.1 mm and 1.2 mm.

The lateral position of the bits is determined by discrotation of the storage layer 12 varied. The longitudinal position of the bits (= focal depth of the laser 10 ) is caused by displacement of the lenses 1 - 4 realized along the optical axis, the lenses 1 and 2 for focusing and the lenses 3 and 4 used for collimation, and all lenses in relation to the disk 12 is arranged displaceably along the optical axis z and when changing from one data plane to another data plane corresponding to the depth position of the data plane within the storage layer 12 be moved. The almost linear displacement of the lenses 1 - 4 can for an embodiment the 2 and 3 be removed. The corresponding travel paths of the lenses 1 - 4 can be calculated in advance and stored in the system. If a hologram is now to be generated in a certain depth of focus, the stored coordinates for the lenses 1 - 4 adjusted accordingly (for example by means of actuators). If an already existing hologram is to be read out in a certain depth of focus, the lenses will also be read out 1 and 2 positioned according to the deposited coordinates. The lenses 3 and 4 However, they are not positioned according to the deposited coordinates (or alternatively, the beam path of the lenses 3 and 4 interrupted, for example by a shutter), so that although a focus, but no collimation and opposite overlay is realized. If the Bragg condition is fulfilled, ie a hologram is present and is irradiated with the correct wavelength (with which the grid = hologram was written), a reflection results, which can be registered as a signal.

According to the invention, a diffraction-limited focusing in different layer depths (= focus depths) by means of only two aspherical lenses can thus be achieved 1 . 2 in front of the storage layer 12 and only two aspherical lenses 3 . 4 (+ Reflector 18 ) behind the storage layer 12 will be realized.

Preferably, the lenses 1 and 4 as well as the lenses 2 and 3 preferably of the same design, ie same thickness, same material, same focal length, same aspheric coefficients (surface shape), etc. The first lens 1 and the fourth lens 4 However, then are preferably used mirror-inverted in the beam path. The same applies analogously to the second lens 2 and the third lens 3 ie the second lens 2 and the third lens 3 are preferably used mirror-inverted in the beam path.

The first lens 1 and second lens 2 are pre-corrected for an initial depth of focus with respect to the spherical aberration, corresponding to the third lens 3 and the fourth lens 4 also pre-corrected for the initial focus depth. With variation of the depth of focus (change of the data level) the lenses become 1 - 4 readjusted immediately (within a time of less than 100 milliseconds, preferably less than 20 milliseconds) by means of a corresponding device (actuators, motors, etc.).

Of the micro-holographic data storage is one of the most promising Technologies with regard to the next Generation of data storage and has considerable potential for replacement the DVD. The above solution allows a significant increase in storage density, although with other solutions as well can be achieved (such as microscope lenses, movable Wedges), however, are all alternative options commercially not useful (too expensive, too vulnerable or too slow).

1

aspherical lens

2

aspherical lens

3

aspherical lens

4

aspherical lens

5

reflector

10

Radiation source / semiconductor laser

12

storage layer

Claims (15)

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 ( 12 ), whose refractive index undergoes a change upon irradiation of electromagnetic radiation, - focusing and oppositely superimposing electromagnetic radiation of a monochromatic light source ( 10 ) in such a way that an interference pattern is formed in a predetermined spatial position (x, y, z) of the storage layer due to the opposite superimposition and leads to a greater refractive index change in the focus in areas of constructive interference than in areas of destructive interference, and a hologram due to the refractive index change is produced with a multiplicity of layers of alternating refractive index, wherein - a plurality of holograms in different lateral positions (x, y) and at different depths (z) of the storage layer are generated in the storage layer with a time offset, characterized in that the radiation of the Light source ( 10 ) by means of a first aspherical lens ( 1 ) and a second aspherical lens ( 2 ) in the storage layer ( 12 ) and after passing through the storage layer ( 12 ) by means of a third aspherical lens ( 3 ) and a fourth aspherical lens ( 4 ) and after reflection on a reflector ( 5 ) back into the storage layer ( 12 ) and counteracted to form an interference pattern, wherein for changing the depth (z) of the focus in the memory layer ( 12 ) the second aspherical lens ( 2 ) and the third aspherical lens ( 3 ) relative to the storage layer ( 12 ) and the first aspherical lens ( 1 ) relatively second aspherical lens ( 2 ) and the third aspherical lens ( 3 ) relative to the fourth aspherical lens ( 4 ) is moved. Method according to claim 1, characterized in that the displacement of the first aspherical lens ( 1 ), the second aspherical lens ( 2 ), the third aspherical lens ( 3 ) and / or the fourth aspherical lens ( 4 ) along the optical axis (z) by means of actuators and / or motors. Method according to one of the preceding claims, characterized in that for focusing and collimation only the first aspherical lens ( 1 ), the second aspherical lens ( 2 ), the third aspherical lens ( 3 ) and the fourth aspherical lens ( 4 ) be used. Method according to one of the preceding claims, characterized in that the distance between the storage layer ( 12 ) and the second aspherical lens ( 2 ) for increasing the depth (z) of the focus in the storage layer ( 12 ) and / or the distance between the storage layer ( 12 ) and the third aspherical lens ( 3 ) for increasing the depth (z) of the focus in the storage layer ( 12 ) and / or the distance between the first aspherical lens ( 1 ) and the second aspherical lens ( 2 ) for increasing the depth (z) of the focus in the storage layer ( 12 ) and / or the distance between the third aspherical lens ( 3 ) and the fourth aspherical lens ( 4 ) for increasing the depth (z) of the focus in the storage layer ( 12 ) is increased. Method according to one of the preceding claims, characterized in that the light source is a laser with a central wavelength in the range 380nm-430 nm and a half width of less than 10 nm is used. Method according to claim 5, characterized in that that as a light source, a semiconductor laser having a central wavelength in the range 400 nm-430 nm and re the intensity is used with a half width of less than 3 nm. Method according to one of the preceding claims, characterized in that the beam diameter of the light source ( 10 ) is selected and the first aspherical lens ( 1 ), the second aspherical lens ( 2 ), the third aspherical lens ( 3 ) and the fourth aspherical lens ( 4 ) are chosen such that the Focusing the electromagnetic radiation of the monochromatic light source ( 10 ) into the storage layer having a numerical aperture of greater than 0.5. Method according to claim 7, characterized in that the beam diameter of the light source ( 10 ) is selected and the first aspherical lens ( 1 ), the second aspherical lens ( 2 ), the third aspherical lens ( 3 ) and the fourth aspherical lens ( 4 ) are selected such that the focusing of the electromagnetic radiation of the monochromatic light source ( 10 ) in the storage layer ( 12 ) with a numerical aperture greater than or equal to 0.6. Method according to one of the preceding claims, characterized in that the beam diameter of the light source ( 10 ) is selected and the first aspherical lens ( 1 ), the second aspherical lens ( 2 ), the third aspherical lens ( 3 ) and the fourth aspherical lens ( 4 ) are selected such that the focusing of the electromagnetic radiation of the monochromatic light source ( 10 ) in the storage layer ( 12 ) is diffraction-limited. Apparatus for generating a plurality of holograms, comprising: a monochromatic radiation source ( 10 ), means for receiving a planar storage medium ( 12 ), which comprises a material which undergoes a refractive index change upon irradiation of electromagnetic radiation, a focussing unit which transmits at least part of the light of the radiation source ( 10 ) in the storage layer ( 12 ), means for collimating at least part of the memory layer (16) 12 ) passing light, a reflector ( 18 ) for the return reflection of the collimated light, wherein the reflector ( 18 ) and the means for collimation are arranged such that the reflected light is again in the storage layer ( 12 ), wherein the focusing unit and the means for collimation for generating holograms in different depths of the memory layer ( 12 ) relative to the storage layer ( 12 ) are formed displaceable, characterized in that the focusing unit by a first aspherical lens ( 1 ) and a second aspherical lens ( 2 ) and the means for collimation by a third aspherical lens ( 3 ) and a fourth aspherical lens ( 4 ) are formed, wherein the second aspherical lens ( 2 ) and the third aspherical lens ( 3 ) are adapted to change the depth (z) of the focus in the memory layer ( 12 ) relative to the storage layer ( 12 ) and the first aspherical lens ( 1 ) is adapted to change the depth (z) of the focus in the storage layer ( 12 ) relatively second aspherical lens ( 2 ) and the third aspherical lens ( 3 ) is adapted to change the depth (z) of the focus in the storage layer ( 12 ) relative to the fourth aspherical lens ( 4 ) to be moved. Apparatus according to claim 10, characterized in that the device is adapted such that the distance between the storage layer ( 12 ) and the second aspherical lens ( 2 ) for increasing the depth (z) of the focus in the storage layer ( 12 ) and / or the distance between the storage layer ( 12 ) and the third aspherical lens ( 3 ) for increasing the depth (z) of the focus in the storage layer ( 12 ) and / or the distance between the first aspherical lens ( 1 ) and the second aspherical lens ( 2 ) for increasing the depth (z) of the focus in the storage layer ( 12 ) and / or the distance between the third aspherical lens ( 3 ) and the fourth aspherical lens ( 4 ) for increasing the depth (z) of the focus in the storage layer ( 12 ) is increased. Device according to one of claims 10 or 11, characterized in that means for displacing the first aspherical lens ( 1 ), the second aspherical lens ( 2 ), the third aspherical lens ( 3 ) and / or the fourth aspherical lens ( 4 ) are provided along the optical axis (z). Device according to Claim 12, characterized in that the means for displacing the first aspherical lens ( 1 ), the second aspherical lens ( 2 ), the third aspherical lens ( 3 ) and / or the fourth aspherical lens ( 4 ) is formed along the optical axis (z) respectively by an actuator or by a motor. Device according to one of claims 10-13, characterized in that the refractive surfaces of the first aspherical lens ( 1 ), the second aspherical lens ( 2 ), the third aspherical lens ( 3 ) and / or the fourth aspherical lens ( 4 ) are rotationally symmetrical. Device according to one of claims 10-14, characterized in that the monochromatic Radiation source ( 10 ) is a laser diode with a central wavelength between 400 nm and 430 nm.