DE4208328C2 - Method and device for erasable storage of information - Google Patents

Description

Subject of the invention

The invention relates to a method for erasable Storage of information using digital Storage systems, with the proviso that in a photorefractive storage medium information bits as holographically inscribed grids are introduced that the storage process is reversible and that the stored digital information with compact disc players State of the art technology can be accessed.

State of the art

The digital storage of smallest information units (BIT) is u. a. with liquid crystalline polymers (LCP) possible. Such LCP carrier systems are for example in EP-PS 231 856 (= US 4,886,718), in which a Process for reversible, optical data storage under Use of LCPs is claimed. Here, means linearly polarized laser light over an optically not linear effect due to the positive orientation of the mesogenic groups in a film information in the form of a optically anisotropic phase object stored. EP-OS 231 858 (= US 4,896,292) describes a device for reversible optical data storage using of LCPs with mesogenic side chains as storage medium, set up for storing information by means of selective variation of the order of the LCPs using a  Heat source. Here, the macroscopically oriented LCP film for storing the information with a Heat source selective, local in the isotropic, liquid Condition heated up and after switching off the heat source the local information in the glassy state of the polymer fixed.

The invention set out in DE-OS 36 23 395 (= US 5,024,784) relates to a device for reversible optical Information storage with an LCP storage medium, wherein the storage medium from a macroscopically oriented film of a liquid crystalline polymer which is photochromic Contains, consists and where the Information storage through local disorientation of the Molecules induced by photoisomerization using a selectively acting light source.

DE-OS 38 10 722 (= US 5 023 859) again describes one Reversible optical device Information storage using polymers as Storage medium, the device being a film contains an amorphous polymer as a storage medium and so is aligned to by means of a local variation of the Molecular order to store the information.

In document EP-A 0 455 539 is the method for writing and reading binary information of a disc. However, the backward compatibility on the commercial CD technology not in EP-A 0 455 539 described.

The reversible properties of amorphous and liquid crystalline polymers are in the Publications DE-A 38 10 722 and DE-A 36 23 395 described; the photorefrak tive polymers are not in connection with reversible data storage described.

A serious obstacle, that of practical Introduction of LCP information carrier systems stands in the way is the lack of compatibility with the standard playback and recording devices, especially the Compact Disk (CD) technology. As far as the - however constantly evolving - overlooks the scene, there are currently no backwards integrable, erasable optical memory available. Apparently pray also the magneto-optical ones developed to date Storage systems this backward integration does not.

Task and solution

The object was therefore to provide a method and a device for erasable digital storage systems which are compatible with the existing CD technology. The object defined in this way can be achieved with the aid of the storage method according to the invention. The present invention relates to a method for the erasable storage of information by means of a digital storage system, information bits being introduced as holographically inscribed grids in a storage medium and the direction of oscillation of the linearly polarized laser light being rotated relative to the direction of oscillation for inscription for erasing. The information written with the specially developed write / read device, which can be used to write, read and erase, can be read with any CD player. The storage medium, which has a sandwich structure, consists of a carrier material ( FIGS. 1, 1) and a photorefractive material ( FIGS. 1, 2). In a further embodiment of the invention ( FIG. 2), a reflective layer ( 3 ) can be located on one side between the carrier material ( 1 ) and the photorefractive layer ( 2 ).

The write / erase cycle consists of writing a holographic grating in the photorefractive layer ( 2 ) or erasing a grating that has already been written. In the case of storage medium A ( FIG. 1), the incident reading beam ( 5 ) is reflected by the reflection grating ( 4 ) located in the photorefractive layer ( 2 ) when the bit is set, and the reading beam penetrates the photorefractive bit when the bit is not set Layer ( 2 ) ( Fig. 3). In the case of storage medium B ( FIG. 2), the incident reading beam ( 5 ) is deflected away by the grating ( 4 ) in the photorefractive layer ( 2 ) when the bit is set and is reflected back by the reflecting layer ( 3 ) when the bit is not set ( Fig. 4). This storage method is compatible with the existing CD technology, since the diffraction efficiencies of the reflection gratings ( 4 ) reach the "bit on" or "bit off" signal level of the CD technology.

Implementation of the invention The photorefractive layer

The polymers which can be used according to the invention for the photorefractive layer ( 2 ) can have a liquid-crystalline or an amorphous structure. The polymer chains consist partially or completely of suitable photosensitive molecular building blocks.

Macroscopically oriented films made of liquid-crystalline polymers which contain photochromic groups are used, for example, as described in DE-OS 36 23 395 (= US Pat. No. 5,024,784). The holographic lattice is inscribed by local disorientation of the molecules, induced by photoisomerization of the photochromic groups using a selectively acting light source. These polymer systems are referred to below as S ₁ . The thickness of the photorefractive layer is advantageously between 2 and 1000 μm.

Dyes are advantageously in the storage medium as photochromic groups. The dye molecules can be covalently bonded constituents of the liquid-crystalline polymer or they can be added to the storage medium and distributed therein. The glass transition temperature Tg of the liquid crystalline polymer is above room temperature. The glass transition temperature Tg can be determined according to A. Turi "Thermal Characterization of Polymeric Materials", pages 169 ff, Academic Press, New York 1981. The information can be read out by illuminating the polymer film with monochromatic coherent light. Various orientations of the liquid crystalline polymer film are possible for storing the information:

• a) The mesogenic groups are aligned uniformly parallel to the surface normal of the polymer film layer (= photo refractive layer ( 2 )). This can be done by applying an alternating electric field to the substrate material ( 1 ) coated with (transparent) electrodes, the electric field being parallel to the normal of the photorefractive layer, by applying a magnetic field or by surface treatment.
• b) The mesogenic groups are tilted parallel to the photorefractive layer and oriented parallel to a macroscopically predetermined direction. This can be done either by coating the substrate material ( 1 ) using a suitable material such as polyimide and by structuring this coating in the direction of the desired preferred orientation, or by suitable oblique vapor deposition of the substrates with silicon oxide. The required orientation can also be generated by suitable shearing or stretching.

In both cases a) and b) the orientation takes place in the liquid crystalline state. The orientation is through Cooling frozen in the glass state. The deletion of the Information is provided by heating the sample in the anisotropic or isotropic range above the Glass temperature Tg.

Furthermore, films made of amorphous polymers are used, for example, in which the information is stored by local variation of the molecular order using polarized light, as described in DE-OS 38 10 722 (= US Pat. No. 5,023,859). The information is written in by generating a local geometric disturbance at the location of the photochromic groups using a polarized light beam, which leads to a reorientation of the uplift of these groups. The polymer systems mentioned above are referred to below as S ₂ . The thickness of the photorefractive layer formed from S ₂ is advantageously between 2 and 1000 μm. The polymers making up the storage medium S ₂ generally obey the principle that the polymer chains are partially or completely constructed from suitable photosensitive molecular building blocks and are amorphous. The amorphous state here means the absence of crystalline order in significant proportions, in particular the absence of the liquid-crystalline state. For the definition of the liquid crystalline state cf. Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd. Ed. Vol. 14, pages 395 to 427, John Wiley and Sons, New York, 1981. The photosensitive molecular building blocks can be covalently bonded constituents of the amorphous polymer or can be admixed with the amorphous polymer and distributed therein.

Information can be stored in S ₂ systems both in the tough elastic state and in the glass state.

In the case of a macroscopically isotropic initial state, this leads to anisotropic areas that show strong birefringence. In the case of a macroscopically anisotropic initial state, this leads to regions with a changed preferred direction of the molecular groups and thus to changes in the local birefringence. In both cases, a phase object results. After switching off the light, the changed orientation status freezes. In principle, the stored information in S ₂ systems can be deleted by increasing the temperature above Tg to beyond the temperature range of the viscoplastic state.

The photochromic groups used in the systems S ₁ and S ₂ are photosensitive units selected from the group consisting of azobenzene, bisazobenzene, trisazobenzene, azoxybenzene, stilbene, spiropyran and / or substituted derivatives of the aforementioned compounds. For a more detailed molecular structure of the photosensitive groups, reference is made to DE-OS 36 23 395 (= US 5 024 784) and DE-OS 38 10 722 (= US 5 023 859).

The substrate material

The photorefractive layer ( 2 ) is generally arranged between two plane-parallel substrate layers ( 1 ). The transparent substrate layer ( 1 ) is either firm or flexible. The thickness of the substrate layer is between 10 and 2000 microns, preferably between 50 and 1000 microns.

Transparent materials are preferably used as substrate materials Glasses or transparent amorphous plastics, such as for example polymethyl methacrylate (PMMA) or Polycarbonate, application. Here, the substrate material in a preferred form a low birefringence.

The memory cell

As indicated schematically in FIGS. 1 and 2, the memory cell consists of two plane-parallel, transparent plates made of substrate material ( 1 ), between which there is the photorefractive layer ( 2 ) and optionally also a reflective layer ( 3 ). The reflective layer ( 3 ) is deposited on the substrate material ( 1 ) by vapor deposition with metals or metal oxides, for example by deposition from a high-frequency plasma, or by wet mirroring. The latter is described for example for amorphous plastic materials, such as PMMA, in DE-OS 33 41 536 (= US 4,663,199).

The distance between the two plane-parallel substrate plates ( 1 ) is generally below 1 mm, preferably about 10 µm. The desired distance between the two substrate plates is fixed by suitable spacers of the appropriate dimensions, preferably made of polyimide plastic. The substrate plates are fixed to one another with the aid of a temperature-stable adhesive, for example a silicone adhesive, in such a way that a cell-like empty interior is formed, each with only one inlet and outlet of a few mm in width. After the adhesive seams between the spacers and the substrate plates have dried on, the memory cell is filled on a heatable device with the liquid-crystalline polymer in the isotropic state or with the amorphous polymer in the molten state. Due to capillary action, the still free cell space is completely filled with the polymer melt. The advantage of this procedure over the use of a still partially open cell is, among other things, that the inclusion of air bubbles is reliably prevented (cf. DE-OS 36 23 395 = US 5 024 784).

The geometry of the memory cell is arbitrary, preferably round. The base area is usually a few Square centimeters to a few square decimeters.

The read / write device combination

Before the writing process, the memory cell is exposed to monochromatic, polarized light until the dye molecules have a preferred orientation. In the case of a round memory cell ( 6 ) as an analogue to a CD, this is preferably done by means of a radially arranged exposure slit ( 7 ) as shown in FIG. 5. Furthermore, the orientation of the mesogenic groups and the dye molecules can, for example, be brought about by additional structured surfaces which are used, for example, in conventional display technology to orient the liquid-crystalline phases. The advantage here is that the total information of the memory cell can be deleted by annealing without the memory cell having to be exposed before the next writing process.

After exposure, the dye molecules ( 8 ) have a radial preferred orientation, which is particularly pronounced with the LCP storage media ( FIG. 6).

In a further embodiment of the invention, the exposure takes place in a track ( 9 ), the preferably round storage cell rotating ( FIG. 7). The principle of storing information in S ₁ or S ₂ systems has been described above.

In Fig. 8 the enrollment device and the enrollment process is shown schematically. The linearly polarized laser beam ( 10 ) is divided into two mutually coherent partial beams by the beam splitter ( 11 ). These partial beams are bundled by focusing units ( 13 ) and overlaid at an angle of 180 degrees. The interference grating ( 14 ) generated in this way is written into the storage layer by reorienting the photochromic groups in the photorefractive layer and the associated change in the refractive index. The partial beams can act on the storage layer in pulses, the intensity of which depends on the laser power and the duration of which lies in the range between nano- and milliseconds, via the shutter ( 12 ). This enables a quick registration of digital information.

The reading process, which is shown in Fig. 9, is carried out with a partial beam of reduced output intensity of the laser ( 10 ). The light intensity backscattered by the inscribed grating ( 14 ) or by a reflecting layer (cf. FIG. 4) is detected by the photodiode ( 15 ) and recognized as a "set" information bit.

In the absence of a grating and nonexistent reflective Layer no information bit is recognized.

The deletion process is shown schematically in FIG . Individual bits of information can be selectively removed from the storage medium by deleting the individual bit grids ( 14 ). In this case, the direction of oscillation of the linearly polarized laser beam ( 10 ) is rotated by a Kerr cell ( 16 ). The beam is then focused ( 13 ) on the relevant bit grating ( 14 ) and quenches the grating by reorienting all of the dye molecules in the exposed area.

An overall overview of the write-read-erase combination according to the invention is given in FIG. 11. It is constructed from a laser ( 1 ) which emits linearly polarized light. For this purpose, a continuous wave laser suitable for holography, such as, for example, by H. Marwitz et al. In "Practice of holography" (page 53 ff, Expert Verlag, Ehningen bei Böblingen 1990). The laser is divided into two coherent partial beams via the beam splitter ( 11 ), preferably dielectric-coated beam splitters, as described, for example, by J. Collier et al. described in "Optical Holography" (page 167 ff, Academic Press, INC., London 1971). The shutters ( 12 ) separate the partial beams into radiation pulses, the intensity of which depends on the laser power and the duration of which lies in the range between nano- and milliseconds. Kerr cells are preferably used as shutter elements, as described, for example, in Bergmann, Schafer "Textbook of Experimental Physics" (page 624 ff, W. de Gruyter-Verlag, Berlin, New York, 1987). The laser partial beam pulses are passed via the two focusing units ( 13 ) into the storage layer via monomodal optical waveguides ( 17 ) and generate the interference grating ( 14 ) there by reorienting the photochromic groups in the photorefractive layer. For the optical waveguide ( 17 ) are preferably single-mode optical waveguides, as described for example by H. Marwitz et al. in "Practice of holography" (page 37 ff, Expert Verlag, Ehningen bei Böblingen 1990). The focusing units ( 4 ) preferably consist of index gradient lenses, the aperture of which should be adapted to the aperture of the optical waveguide used (H. Marwitz, loc. Cit. Page 37).

The light intensity detected during the reading process by the inscribed grating ( 14 ) or by the reflecting layer ( Fig. 4) by backscattered light is detected by the photodiode ( 15 ). Here, photodiodes are used that can detect the wavelength of the laser light, preferably sensitive.

The rotation of the oscillation direction of the linearly polarized laser light required in the quenching process ( FIG. 10) is brought about by a Kerr cell ( 16 ), the same cells as are described for the shutter elements ( 12 ) being able to be used here.

Advantageous effects of the invention

A disk with a photorefractive layer described with the write-read-erase device combination according to the invention can be played in a conventional CD player of the prior art, since in this device the information about the intensity of the back-reflected light is also read out. When using LCPs, whose refractive index depends on the direction of polarization of the incident light, it must be taken into account here that the commercially available CD players have unpolarized lasers as light sources. The diffraction efficiency of the interference grating ( 14 , FIG. 7) must be correspondingly higher in these cases, since only the portions of the light with a certain direction of polarization are reflected by the grating.

Individual bits of information can be selectively removed from the storage medium by deleting the individual interference grids. In the case of LCPs, the direction of oscillation of the linearly polarized beam is rotated by the Kerr cell ( 16 ), the beam being focused on the relevant interference grating ( 14 ) and erasing the grating by reorienting the dye molecules in the photorefractive memory matrix.

The method for information storage according to the invention with the specially developed read-write-delete device compatible with the prior art CD technology and thus regarding the stored information rückwärtsintregrierbar.

The following examples serve to explain the Invention.  

EXAMPLES example 1 Preparation of the liquid crystalline storage layer

A dye-liquid-crystal copolymer containing 70% by weight of an azobenzene comonomer of the following schematic formula serves as the organic polymeric carrier T:

x = 0.7; y = 0.3
Mw = 7200; U (inconsistency) = 0.38
Tg = 27 degrees C, (λmax = 360 nm)
Tn, i = 127 degrees C.

The preparation according to the spin or pull method takes place in thin layer on preoriented slides (glass with a polyimide layer). After coating is briefly over Tn, i (127 degrees C) heated up and slowly (1-10 min) Cooled to 100 degrees C and annealed for 10 minutes. The result is a liquid crystalline polymer film with monodomain structure.  

Example 2 information storage

The orientation of the dye molecules can be checked via their dichroic absorption behavior in the UV / VIS spectrophotometer experiment. In Fig. 12, the absorbance of linearly polarized light of wavelength 355 nm is plotted as a function of the angle Φ between polarization and preferred direction. The dyes are oriented in the direction of the preferred axis with an order parameter of 0.7.

The following is intended to show how you can use linear polarized light this via surface effects induced preferential axis of the dye molecules can adjust.

The corresponding experimental setup is shown schematically in FIG. 13.

A linearly polarized laser beam ( 18 ) with a wavelength of 514.5 nm is expanded via the lens combination 19 . The sample ( 20 ) is exposed to 200 mW / cm ^{2 for} 4 min, the direction of polarization of the light being parallel to the preferred axis of the polymer. After the exposure, the sample is first stored protected from light for 1 week. It then appears that the dye molecules are oriented perpendicular to the original direction after exposure with an orientation parameter of 0.4 ( FIG. 14).

A macroscopic change in birefringence due to irradiation of the sample with linearly polarized light can be demonstrated with the experiment shown schematically in FIG. 15.

Structures between 500 and 2 µm with a polarization direction of 45 degrees to the preferred direction are inscribed using a test mask ( 23 ). The wavelength of the Ar ion laser ( 21 ) is 514.5 nm; Intensity = 300 mW / cm ² , exposure time = 4 min.

In exposed areas, the change in birefringence due to the polarized laser light will hardly be observed in the polarizing microscope. Fig. 16 shows a polarization microscope image of a sample described with a preferred direction parallel to the analyzer.

In Fig. 17, the relative intensities of the exposed areas (25) are shown relative to the unexposed matrix (26). From the shift of the intensity maxima by 45 degrees it follows that the birefringence axis was rotated by 45 degrees by irradiating the sample with linearly polarized light.

FIG. 18 shows an experimental setup with which the optical axis of the memory polymer system can be switched between two orientations in a targeted manner, statements about the speed and number of cycles of the switching process being obtained.

At the beginning of the experiment, the direction of polarization of the detection laser ( 30 ) is set with a wavelength λ ₁ = 633 nm parallel to the optical axis of the sample. The transmission direction of the analyzer ( 34 ) is perpendicular to the direction of polarization of the detection laser and thus prevents the light from passing through. The photodiode ( 35 ) measures no incidence of light. The write laser ( 27 ), which has a wavelength of λ ₂ = 514.5 nm, now controls a short light pulse of 20 ms onto the sample ( 32 ) via the shutter ( 28 ). The filter ( 33 ) prevents the light from the writing light from entering the photodiode ( 35 ). The direction of polarization of this light is set to an angle of 45 degrees to the preferred direction of the sample using a λ / ₂ plate ( 29 ). The optical axis of the sample rotates through this light pulse to an angle of -45 degrees in relation to the starting position. This - new - position of the optical axis changes the direction of polarization of the detection light as it passes through the sample ( 22 ) and light falls on the photodiode ( 35 ).

In the next step, the λ / ₂ plate ( 29 ) is rotated so that the polarization direction of the next light pulse is perpendicular to the original preferred axis. The optical axis of the sample ( 32 ) is thereby rotated into the original position, the polarization direction of the detection light is no longer changed and no light falls on the photodiode ( 35 ). In Fig. 19 the results are shown of the optical axis to the experiment of switching.

Holographic experiments prove that phase gratings in the liquid crystalline polymer can be registered.  

The inscription takes place, for example, using a Michelson interferometer structure, a line grating with a grating constant of 50 μm being obtained with appropriate adjustment. This line grating with a relatively large grating constant can be detected in the polarizing microscope ( FIG. 20). A diffraction efficiency of up to 70% is measured when changing to higher layer thicknesses (e.g. 10 µm in display cells) and to smaller grating constants (0.3 to 0.8 µm). This means that up to 70% of the incident light can be diffracted at the grating and used to detect an information bit.

Claims (5)

1. A method for the erasable storage of information by means of digital storage systems, characterized in that information bits are introduced as a holographically inscribed grid in a photorefractive storage medium, that the storage process is reversible, that the stored digital information with playback devices of the compact disk technology of the prior art the technology can be called up and the direction of oscillation of the linearly polarized laser light is rotated with respect to the direction of oscillation for writing to erase. 2. The method according to claim 1, characterized in that the photore fractive storage medium as a layer between transparent substrate material rialien located. 3. The method according to claims 1 and 2, characterized in that the photorefractive layer has photosensitive molecular building blocks. 4. The method according to claims 1 to 3, characterized in that the photorefractive storage medium a liquid crystalline polymer matrix having. 5. Device for a method according to claims 1 to 4, characterized characterized that information bits are written with the same device, can be read and deleted.