OPTICAL DIGITAL DATA STORAGE IN ADHESIVE TAPES

Stefan Stadler, Ph.D., tesa AG, Hamburg, Germany Annouschka Blazejewski, tesa AG, Hamburg, Germany J6m Leiber, Ph.D., tesa AG, Hamburg, Germany Matthias Gerspach, Universit~it Mannheim, Germany Steffen Noehte, Ph.D., European Media Lab, Heidelberg, Germany Christoph Dietrich, Ph.D., European Media Lab, Heidelberg, Germany

Abstract

A new technology enables us to record digital optical information (bit-by-bit or as holograms) in common adhesive tapes with the help of a 50,000 dpi laser lithograph. Among laser irradiation the adhesive tape changes its optical properties, e.g. its reflectivity and its refractive index. One roll of adhesive tape could hold up to 10 Gbyte of data. For the reading and writing process it is not necessary to unwind the tape. The different layers are adressed by moving the laser focus. The holograms, which can store the equivalent of 30 written pages in 1 mm 2, are extremely forgery-proof and can be easily individualized. These properties make them ideal candidates for logistics applications and security systems to prevent counterfeiting.

Introduction

Optical Data Storage The increasing flood of information in our modern communication society requires data storage media with very high data densities and fast access times. Optical storage devices have achieved a strong position on the very competitive storage market in the past few years. In the meantime, most information and data are stored optically. This is thanks to the success of the data compact disc (CD-ROM). There is currently no other medium with which information can be duplicated so quickly and cheaply as on a CD. In a modern production facility CDs are printed, produced in seconds and at a price of less than one D- Mark per CD. At present, this technology is the one with the lowest environmental burden for the duplication of information. The entire information of a mail-order catalogue of more than 800 pages can be stored on one CD with a storage capacity of 690 Mbytes. The imprinted information is secure from electromagnetic interference. The second wave of success for the CD came approx, three years ago as the recording devices and the writable CDs (CD-R) became substantially cheaper for the user. Today one blank CD costs < 1 US$. The user and the entertainment industry, however, are demanding new media with even higher storage capacity. The film industry, for example, wants storage media that can store 120 minutes of digital film with multiple sound tracks for different synchronised languages and additional information. The successor of the CD is the DVD (Digital Versatile Disc) with a storage capacity of 2.8 to 17 gigabyte. The 17 Gbyte DVD has the size of a CD; the higher storage density is achieved by the use of a short- wave laser (658 nm instead of 830 nm) which enables a finer pit structure and therefore a higher storage density (see figure 1). This is due to the so called diffraction limit, which leads to the fact, that lasers with shorter wavelengths can be focused to smaller spot sizes than lasers with longer wavelengths. At the same time both sides of the disc are written, in two layers, to give a total of four layers. Despite their success the DVD and CD-ROM are not suited for very small electronic devices, such as digital video cameras, palmtop computers or mobile telephones.

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Here a multilayered spiral-shaped data carrier in the form of a common PSA-tape roll (we call it tesa ROM ~) could be the solution. With a height of 12 mm, a diameter of 30 mm and 5 - 10 layers such a roll could hold 2,5 to 5 GByte if a data density of 4,1 Bits/pm 2 (current single layer DVD bit density) is achieved. This is sufficient to fulfill the requirements for video storage (~ 20 minutes to 2 hours play, depending on the compression algorithms used). Beside the small size other advantages of the tesa R OM ~ include the low cost, the high velocity and the low power consumption (see below).

Optical Holography Another approach to achieve extremely high data densities is holographic data storage. IBM in Almaden/USA, Bayer in Leverkusen, CALTEC, the University of Stanford, the Taiwanese Research centre, the group of Prof. Tschudi in Darmstadt are amongst the groups researching in this field. Holograms are structures which deflect light (e.g. from a laser beam) in a very special way so that the previously recorded information (e.g. a picture) is reconstructed. This only works if the ,,sub-structures" in the hologram are very small (preferred in the region of the wavelength of visible light) so that the light waves can be diffracted on these microstructures. Holograms are therefore often referred to as ,,diffraction gratings". The hologram itself does not resemble the original picture (which also is the reconstructed information) at all. A hologram, for instance, which consists of parallel lines (a so called line grating) yields a number of points (as the ,,picture") when irradiated with a laser beam. As the diffraction grating becomes more complex, any information one could imagine can be achieved from the hologram (see figure 2). A unique feature of holograms is, that (theoretically) the whole information is stored in every single point of the hologram. That means that every part of the hologram is capable of reconstructing the complete image. If one cut the right hologram in figure 2 in two pieces and illuminated them with a laser each of the fragments would be able to reconstruct the complete dog, although the brightness of the reconstructed image would be a little bit lower than before. This property makes holograms very interesting for data storage considering the problems CD-ROMs or DVDs have with dust, dirt or scratches. The normal way to produce holograms is by recording the interference pattern of two laser beams on a photosensitive material, e.g. a silver halide emulsion. One of the beams is scattered at the object to be recorded (object beam), the other one serves as a reference so that not only the amplitude (as it is the case with photographs) but also the phase of the object wave is recorded. Therefore the information about the object is ,,complete" which normally leads to the 3-dimensionality found with most holograms. In general a setup is used, where the object and reference beams are not in line I but form an angle (off-axis holograms)). To get the information out of the hologram one has to illuminate it with a light beam which comes from the same direction and has the same wavelength as the reference beam that was used during the recording process (see figure 3). , If three-dimensional storage media~are used (e.g. photonic crystals), this restriction makes it possible to store lots of different holograms in the same volume simply by changing the angles between object and reference beams (angle multiplexing 4) or the wavelength of the laser (wavelength multiplexingS). It is also possible to change the transversal distribution of the phase of the reference beam (phase coding6). With these multiplexing techniques it is possible to store several thousands of holograms in 1 cm 3. If machine readable data have to be stored the preferred format is a so called data page. This is a pattern of black and white pixels which correspond to the 1 s and 0s (,,bits") of the binary data system. The data pages can be generated with the help of a liquid crystal spatial light modulator (LC-SLM). Figure 4 shows a typical setup.With a CCD-chip as a detector these data pages can be read and transformed to a data string (the data format which computers need to work with).

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Since the single bits do not have to be ,,collected" by scanning a laser beam over a surface but are read with a single ,,shot" of the laser beam, reading speed and acquisition times are dramatically faster than in common optical storage technologies (CD, DVD, magneto-optical discs).

Holographic storage materials If all the advantages (data density, speed, resistance to dirt/scratches) are considered the question arises why holography is so far not used widely for data storage purposes. The reason is the lack of an affordable holographic material. Most experiments so far used photorefractive inorganic single crystals7 (e.g. LiNbO3). These crystals, however, are very expensive and hard to handle (easily damaged). In addition they are very light sensitive, so that normal daylight could erase the stored information. Recently enormous efforts have been carried out to develop cheap photopolymeric materials, which utilize either the photorefractive effect 8 or a reorientation of liquid crystalline states to achieve a change in the refractive index by laser irradiation. Among the best known is the so-called ,,Holo-CD" which is developed by Bayer 9 and Sony (in the b~ginning IBM was involved too). These materials are substantially less expensive than inorganic crystals but still too costly for the mass storage market. Since they are reversible (and therefore re-writable) they have similiar problems with long-term durability of the stored data. Most of these materials consist of liquid crystalline side chain polymers which have special dyes (e.g. azo dyes) covalently bound to the polymer backbone. Among laser irradiation these dyes repeatedly carry out cis-trans isomerizations and thus reorient the surrounding liquid crystalline polymer matrix. In our search for new polymeric (holographic) storage materialswe found a solution that seems astonishigly simple: Common adhesive tapes (see below).

Holograms as Security Features Counterfeiting of products (e.g. consumer products, pharmaceutical products and replacement parts) is a worldwide problem. Every year world economy suffers losses in the range of 300 billion $ due to forged goods, and this does not include compensation demands (e.g. in the case of inferior replacement parts or ineffective drugs). Unlike ten years ago not only expensive brands are forged but almost anything one could think of. Therefore the market for security systems for the protection of goods is highly interesting and glowing massively each year. The widely used so-called white light or rainbow holograms (to be found for example on virtually every credit card) are no real protection anymore these days, since so-called dot matrix printers for less than 250,000 US$ are capable of producing high quality holograms in a very short time. Therefore the industry is desperately looking for new security systems, as a new alliance in this field by US consumer products giants (Procter&Gamble, Johnson&Johnson, Eastman Kodak and others) shows. With our new technology we have the possibility to produce microholograms within adhesive tapes or self-adhesive labels which are extremely forgery-proof and can store the equivalent of 30 written pages in 1 mm 2.

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Material Properties Every data storage medium has to be able to occupy two different states which represent the l s and 0s in the binary system. In the case of the tesa ROA~ one of the two states is the ,,ground"-state, i.e. the unaltered standard tape. The other state is reached by relaxation of the polymer chains among laser irradiation. Thus, optical properties of the tape (especially reflectivity and refractive index) are changed.

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The material used in our experiments is a specially modified tesa Multifilm ® whose backing consists of BOPP (biaxially oriented polypropylene). This material is stretched in two directions after extrusion so that the Gibbs free energy is high (mainly due to the low entropy). This state is unfavorable but frozen at room temperature, since the mobility of the polymer chains is not high enough. However, if heat is applied, the polymer chains get into motion and can relax to their favored state (see figure 5). In our case the heat is supplied indirectly in the form of laser light which is transformed into thermal energy within the material. The required laser power is surprisingly low, several mW are sufficient. This is in the range of common laser pointers. Since the laser light is applied in the form of very short pulses and due to the poor heat conductivity of polymers the relaxation process is limited to a very small area. The diameter of the written ,,pits" is typically in the range of 0,5 to 1 gm (more or less the ,,practical" diffraction limit for visible lasers) which in the end leads to a very high data density (see also figure 6). The change in refractive index in BOPP depends in a characteristic way upon the intensity (quotient of laser power and illuminated area) of the writing laser beam (see figure 7). There is a certain threshold that has to be reached to induce any material change. If the intensity is not high enough the BOPP remains unaltered, if the intensity is too high the material is damaged. In between lies a region where the index of refraction changes almost linearly with the laser intensity. This curve shape has several consequences which are stressed out in the following section and without whose the tesa R O M ° could not work.

The tesa R O M ® The type of illumination described in the previous section is called phase illumination; the material remains transparent but the information can be read out exactly as in a CD player. If a reading laser beam is scanned over the surface the intensity of the back reflected light is lower when a ,,pit" (i.e. a relaxed area) is hit compared to the unchanged foil. These different intensities are monitored with a photo diode and correspond to the ones and zeros of the binary data system just as it is the case with the common CD-ROM (figure 8). For the reading and writing process the tape does not have to be unwound. The information is well protected in the inner volume of the roll. In our current laboratory setup of the tesa R O M ~ drive (figure 9 left) the data are written and read from the outside. The tesa roll is rotated and the different layers are addressed by moving the focus of the laser beam(s). At the moment we reach reading times which are in the range of the fastest currently available CD-ROM drives. In the final device it might even be unneccessary to rotate the tesa R O M ~. Instead a mirror could be rotated inside the tesa Film ~ roll and the writing and reading beams are guided to the storage layers via that mirror (see fig. 9 right). Since the dimensions of such a mirror are very small and it weighs only 0,5 - 1 g, it can be rotated very fast so that the reading and writing speed of the tesa R O M ~ could be much higher than that of common 50 x CD-ROM drives, since there are no problems with balancing. The path of the read head is short, which should result in fast access times. For all this to work however, two important requirements have to be fulfilled. Firstly, the strength of illumination must be adjustable. A phase change that is too strong, even if it is only a phase change in the material, would scatter the light of the laser through the first layers so much that insufficient intensity would remain for the layer to be read. If the phase change is too weak, the signal will also be too weak. The material must therefore be able to display ,,grey values" (the grey values also enable us to store more than 1 bit of information in one point so that the overall storage capacity could be much higher than the several GBytes mentioned earlier, but this still has to be demonstrated). Secondly, the material must have a threshold value (that this is the case has already been mentioned) because otherwise, as it is usual for surface illumination for data storage, the same amount of energy will be

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deposited in the layers above and below as in the targeted layer, which would result in erasing the previously written layers.

The tesa Microhologram Diffraction gratings can either be realized by amplitude modulation (that means the hologram consists of areas with different transmission) or by phase modulation (different areas of the hologram have different optical pathlengths). Phase modulation can be achieved either by locally changing the topography or the index of refraction. Almost all common holograms are phase holograms of the former type since the embossing process (which is normally used to produce them) induces a surface structure. Our microholograms are mainly phase holograms of the latter type, since we use the same material effect as with the tesa ROM ~. However, there also are contributions from amplitude modulation. In contrast to classical two-beam-intereference holography we have a different approach to the generation of holograms" digital computer generated holography (DCGH). Digital holograms have fundamental advantages. Optimised calculations enable holograms with much higher diffraction efficiency and without distortion to be produced by illumination. Here the holographic pattern which corresponds to the desired image is calculated by a specially designed computer program we developed under the name ,,DIGIHOL". It is used to calculate various kinds of holograms and simulate reconstruction. Digital holograms resemble the digital bit pattern on a CD. However, the hologram points have to be in the order of magnitude of one wavelength, i.e. less than 1 gin. To illuminate the digital holograms, we had to design and build a suitable device called a lithograph. This lithograph is essentially a highly accurate laser writer with a resolution of 50,000 dpi. This is equivalent to approx. 20,000 pages on one DIN A4 sheet with normal laser writer resolution. The laser lithograph utilizes in principle the same material relaxation effects mentioned above. The whole process of calculating, writing and reconstructing the hologram is visualized in figure 10. The lithograph can write 1 million points in 1 second (this is 1 MPixel/second). This is fast enough for most industrial applications. The storage capacity currently is in the range of several kBytes per mm 2 which equals 30 pages plain text. For machine-readable data the information is encoded in the form of data pages (figure 11). The holograms we are calculating and writing have a special feature" They are translational invariant, i.e. in contrast to normal holgrams the position of the reconstructed image is stationary when the hologram is moved relatively to the laser beam (normally it is more likely that the laser beam is moved). This is crucial if the microhologram has to be read with a handheld device (similiar to the common handheld barcode readers): If the reconstructed data changed their position when the reading device is slightly moved (consider e.g. the person who has to operate it is trembling a little bit) it would be impossible to detect them. During the reading process the image (i.e. normally a data page) is reconstructed in transmission or reflexion (depending on the application) from the hologram by illumination with the light of a red diode laser and detected with a CCD camera module. A computer algorithm forms a data string out of the data page. The data string then can be processed by the appropriate application software (figure 12).

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Summary and Conclusion

In our search for new data storage materials we found that modified pressure sensitive adhesive tapes are excellent candidates. They can serve as raw materials for two different types of data storage: the tesa ROM ~, i.e. more ,,conventional" bit-by-bit storage, but in a new geometry (multi-layer spiral-formed) and holographic storage (in the form of microholograms). The material effect we utilize for the storage of data is the relaxation of the polymer chains among laser irradiation which causes a dramatic change in the optical properties. With our self-developed laser lithograph we can change the material properties

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with a resolution of 50,000 dpi. The material is capable of recording true grey values which can multiply the storage capacity of the data roll (which already is in the range of many kBytes per mm2). The microhologram approach is not only interesting for pure data storage purposes but also for protection from counterfeiting due to the fact that they can be fully individualized and it is impossible to forge them. While the development of the tesa ROM ° will still last 4-5 years we are confident that we will be able to introduce the microhologram in less than two years. This project is a cooperation between the tesa AG, Hamburg, Germany, the EML GmbH, Heidelberg, Germany and the University of Mannheim, Germany.

References

Gabor, D. :Nature 161 (1948) 777 Leith, E., Upatnieks, J.: J. Opt. Soc. Am. 52 (1962) 1123 Denisjuk, J. N.' DANSSSR 144 (1962) 1275 Huignard, J.P., D'Auria, L., Spitz, E." IEEE Trans. Magn. 9 (1973) 83 Rakuljic, G., Leyva, V., Yariv, A. : Opt. Lett. 17 (1992) 1471 Denz, C., Pauliat, G., Roosen, G., Tschudi, T.: Opt. Commun. 85 (1991) 171 Chen, F.S., La Macchia, J.T., Frazer, D.B.: Appl. Phys. Lett. 13 (1968) 223 Meerholz, K. et al.: Nature 371 (1994) 367 Bayer Research Magazine, October 1998, pp.36

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