OPTICAL DATA STORAGE METHOD AND MATERIALS THEREFOR

Abstract: A method of optical data storage uses photo-induced dispersion change (PIDC) material -such as a photochromic fulgide- which is reversibly transformable upon absorption of radiation at a wavelength lambda w to a thermally stable, changed form of a different refractive index. Writing is conducted at a wavelength lambda w to transform the PIDC material in localised regions to the changed form to produce a difference in refractive index delta n between each region and the PIDC material immediately surrounding. Reading is carried out at a wavelength lambda r through a path length L of the PIDC material, with delta n and L being selected such that there is a phase difference of substantially n lambda r/2 between radiation passing through a region and the PIDC immediately surrounding that region. Reading, writing and erasing can be conducted from one or both sides of the PIDC material which is conveniently provided on a substrate disc.

Description: - I - Optical data storage method and materials therefor. This invention relates to the optical storage of data which may for example be of audio, video, or computer form. It is well understood that optical storage-of data provides a number of significant advantages over magnetic and other data storage techniques. Optical techniques currently permit significantly increased data storage densities and offer much improved lifetimes. In circumstances where the data is written once and never changed, optical storage already has a strong commercial presence. In the field of audio, for example, the so-called compact disc (CD) has had considerable success and can usefully be discussed as an example. There is a large measure of standardisation between CDs of different manufacturers and it is thus sensible to speak of a single CD technology. Indeed, there are wide-spread plans to extend the existing CD standard to the storage of non-audio data producing a so-called CD-ROM. A conventional CD comprises a disc of polycarbonate having a spiral or concentric circular array of small pits moulded into one surface of the disc. These pits are typically 0.12 μm deep. The moulded surface of the disc is coated with a reflective metallic layer and the disc is "read" by directing a laser beam at the reflective layer through the polycarbonate. In general terms, the reflective light is monitored to distinguish between pits and the land surfaces between pits. This binary system is indicative of the data stored, although the skilled man will understand that the encoded binary, information includes timing signals, error correction codes, and the like. It is important to note that the high-storage capacities of CDs (typically 500 Mbytes) is only achieved by careful design of the pits formed in the polycarbonate and thus in the reflective layer. It is arranged that for an intended reading laser wavelength of A» the depth of each pit corresponds to a distance Λ/^- it follows that the phase difference between laser light passing through a pit and being reflected, as compared with laser light passing through the immediately adjacent land and being reflected, will be - Destructive interference will occur and the pit edge can thus be detected with considerable accuracy. In one proposal (W0-A-86/00458) , the pits of a CD are replaced by "holes" etched in a photoresist layer and then filled with - for example - a polyester resin or a fluid. The refractive indices of the photoresist and the polyester resin or fluid are selected to provide the same destructive interference effect utilised in CD's. This proposal was intended to permit easier fabrication than with conventional CD's but is not believed to have proved practicable. It should be mentioned that CD is not the only form of read only optical disc that has been produced and many more are .under development. So-called video discs are, for example, commercially available. These store video information in essentially analogue form. It has, moreover, been proposed to store digital data by a holographic technique in which an image is first produced which is representative of the digital information and a hologram of that image then constructed in holographic film or other suitable optical media. There are of course a large number of applications, particularly in the computer field, where the end-user requires to write data for subsequent long-term storage or where data requires frequent updating. These applications are largely served presently by magnetic data storage techniques. Optical technology has, however, been developed which offers the former facility under the acronym WORM (write once read many times). WORM systems have, for example, been produced in which a reflective layer is "burnt" by a contrσlled laser beam to produce pits which can be read subsequently in a similar manner to the CD. Currently, WORM devices are expensive and have a lower signal-to- noise ratio than the conventional CD disc. Optical technologies which permit erasure and rewriting of data are under development by many manufacturers but there remain considerable problems to be overcome. It is believed that in very many cases the problems stem from the fact that the technologies are thermally actuated. Work has been done in exploring the data storage possibilities of materials known as photochromies. These may broadly be defined as materials which, on absorption of radiation, undergo a change between two states having different radiation absorption characteristics. A class of compounds known as fulgides and fulgimides has been identified as offering potentially useful photochromies and reference is directed in this regard to British patent nos-. 1 464 603, 1 600615, 2002752 and 2051 813 and PROCEEDINGS OF SPIE - THE INTERNATIONAL SOCIETY FOR OPTICAL ENGINEERING, Volume 420, June 6-10 1983 pp 186-193. Photochromic fulgides have been developed which exhibit pronounced absorption shifts and which are thermally stable. It can also conveniently be arranged that the wavelengths required to transform the photochromic from one optical form to the other, coincide with wavelengths producible with lasers. It has been suggested that photochromic fulgides might be used as a replacement for silver halide film in holographic data storage techniques as briefly mentioned above. The advantages of being able to rewrite data are felt to outweigh the disadvantages, the most important of which is the lack of optical gain.— Whilst one photon can lead to the formation of an entire grain of silver in silver halide film, one photon can only cause the transformation of one molecule in photochromic material. It has been recognised that the two optical forms of photochromic fulgides differ not only in their absorption but also in their refractive index. A significant refractive index differential between the two optical forms exists outside the two absorption bands so that it is possible to construct a hologram having a large phase contribution which can be reconstructed at a wavelength which is not absorbed by the photochromic in its changed form. It is, however, a problem with this approach that the holographic reconstruction will be significantly degraded having regard to the different constructing and reconstructing wavelengths. This possibility has been suggested as a means of overcoming the gradual restoration to the unchanged form which would occur if the photochromic material were read at or near the wavelength of maximum absorption. It is one object of this invention to provide an improved method of optical data storage which offers large storage capacities with low error rates. It is a further object to provide an improved method of optical data storage which permits erasure and rewriting of data. This invention makes use of the change in refractive index between the two forms in photochromic materials and for convenience it is proposed to refer to such a property in general terms as photo-induced dispersion change. According to the present invention, there is provided a method of optical data storage using photo-induced dispersion change (PIDC) material which is reversibly transformable upon absorption of radiation at a wavelength A to a thermally stable, changed form of a different refractive index, comprising the writing step of directing a radiation beam of wavelength A <-- selected localised regions of the PIDC material to transform at least some PIDC material in said regions to the changed form to produce a difference in refractive index δn between each region and the PIDC material immediately surrounding, the arrangement of said regions in the PIDC material being representative of said data; and a reading step of directing a beam of radiation of wavelength A successively at said regions and monitoring radiation passing through a path length L of the PIDC material, wherein on and L are selected such that there is a phase difference of substantially n A /2 between radiation passing through any of said regions and the PIDC immediately surrounding that region, where n is an odd integer. It will be recognised that the invention provides in this way for destructive interference at each region "edge". - The regions can thus be detected with accuracy and high storage densities become possible. It is important to note that in the method according to this invention, data is recorded directly in digital form, that is to say there is no intermediate stage of image formation, as in holographic data storage techniques. Moreover, the reading wavelength becomes in the method according to this invention - independent of the writing wavelength. It is possible to read data at a wavelength A r which is not absorbed so that a potentially infinite number of read operations can be performed without corrupting the stored data. The change in refractive index δn between the two optical forms of the photo induced dispersion change (PIDC) material will be known for a particular reading wavelength and the path length of the reading beam in the PIDC material can be chosen accordingly. In the most usual arrangement, the PIDC material will be used in reflection, that is to say light will be transmitted through the PIDC material both before and after reflection at a boundary of the material. The reflecting boundary itself contains no data storage indicia such as the pits in CD technology. The reflective boundary need not be a metallic layer and can, for example, comprise the interface between PIDC material and a material of different refractive index, such as a dielectric coating. The reflecting boundary can be arranged to be partly transmitting or wholly transmitting at certain wavelengths. This opens up the very important possibility of reading and writing the PIDC material from opposite sides. Erasure of the stored data can be achieved by irradiating the regions of said changed form with appropriate wavelength light to reverse the transformation. Erasure can be carried out in bulk, or more usually, for selected regions. The wavelengths used for writing ( , reading ( r) and erasure ( λS) will be broadly governed by the PIDC material and may as a matter of practicality be selected to match available laser diode wavelengths. It will also be convenient to employ laser diode pumped rare earth solid state lasers. It will of course be understood that the writing step could be prefaced by an activation step in which the material in its entirety is transformed to the changed state. The regions which are written will then be partially or wholly returned to the original form. Since each incident photon in the writing laser beam will on absorption cause one molecule of the PIDC material to be transformed, the flux of incident writing radiation can usefully be controlled to provide in each localised region of the PIDC material a desired molecular ratio between the two optical forms. '" In this way it is believed possible to produce an effective refractive index shift δn which is intermediate the refractive index change obtained by total transformation between the two optical forms. In one. important example of this invention this approach could be adopted to vary δn during the writing stage to compensate for sensed variations in the path length L which will typically be determined by the thickness of the PIDC material. Thus the optical density would, in one instance, be monitored via the measured transmission of the writing beam, optical density serving as an indication of thickness. In dependence upon sensed variations in thickness, the pulse duration of the writing beam is increased or reduced as appropriate. ' In this way the manufacturing tolerances upon the thickness of the PIDC material could safely be reduced. In another example, control over δn will enable the thickness of the PIDC method to be independently optimised against other criteria. It is one object of this invention to provide a method of optically recording data which is compatable with current CD technology in that, for example, a disc recorded by the method according to this invention can in certain cases be read on an existing CD player. It would be proposed, for example, to provide a disc that was mechanically equivalent to the CD and which was capable of being read in the near infra red. Using the option mentioned above of reading or writing from opposite sides it would be possible to write in UV without the problems of absorption in the conventionally employed polycarbonate substrate. In the further description of this invention it will be helpful to refer to the accompanying drawings, in which-:-- Figure 1 is " a graph showing absorption spectra and refractive index differential for a PIDC material suitable for use in this invention; Figure 2 is a diagram illustrating optical data storage media according to this invention; and Figures 3,4 and 5 are diagrams illustrating alternative forms of data storage media according to this invention1 An example of PIDC material which can be used in a method according to this invention is 7,7a~DHBF. The structure of this compound and its photochro e are set out below:- where x = oxygen and R = cyclopropyl. The compound is transformed to the coloured photochrome on absorption of UV wavelengths up to around 400 nm with the reverse transformation occuring at visible wavelengths substantially in the range 400 to 600 nm. The absorption spectra of the two forms are shown in Figure 1. In the same figure, there is shown the refractive index differential between the two forms. Referring now to Figure 2, there is diagrammatically illustrated an optical disc having a substrate 10 carrying -a layer 12 of PIDC material applied by spin coating, film deposition or other suitable technique. The PIDC material comprises a solution of the 7,7a-DHBF compound in a polymethylmethacrylate or other appropriate transparent polymer matrix. Covering the PIDC layer is a dielectric coating 14 which is selected in a manner which will be self evident to those skilled in the art to be reflective at a wavelength of around 630 nm and to transmit wavelengths of around 500 nm. A suitable protective layer 16 is applied over the dielectric coating. The PIDC material is "activated" by irradiation with UV at 350 to 390 nm and, in a preferred example, at 366 nm. This serves to convert the PIDC material to the more coloured photochrome. Data is written to the disc using a laser operating at 514 n and in a direction through the dielectric coating. As is depicted diagrammatically in Figure 2, the effect of the .writing beam is to bleach or convert to the less coloured form some or all of the PIDC material lying within the beam cross-section. If the flux of the writing beam is sufficient, all of the PIDC material within the region or dot 18 will be bleached. In this way, there is provided a refractive index change δn between the region 18 and the area immediately surrounding, the value of δn being governed by the concentration of PIDC material in the matrix. Typically, changes of one to three percent in the refractive index are achieved. The quantum efficiency for bleaching of the 7,7a-DHBF compound has been measured (in toluene solution) at wavelength 546nm to be around 13% • Reference is directed in this context to "Special Publication No. 60 : Proceedings of Symposium Organised by the Fine Chemicals and Medicinals Group of the Industrial- Division of the Royal Society of Chemistry - Editor P. Bamfield - 1986 - pp 120-135"- I is expected that the quantum efficiency for bleaching will increase with reducing wavelength and a writing wavelength of nearer ^ 500nm is believed therefore to be preferable. Data is read from the disc using a laser at 633 nm through the polycarbonate substrate. It will be recognised that the reading laser beam is reflected by the dielectric coating and has a physical path length L in the PIDC material which is equal to twice the thickness d. From Figure 1 it can be noted that the PIDC material has no significant absorption at the reading wavelength. At the edge of a written dot, there will be a phase difference in the reflective beams which is determined by the reading wavelength r, the physical path length L and the refractive index differential δn, the latter being of course a function of wavelength. In accordance with the present invention, these parameters are selected and arranged such that there is a phase difference of approximately /\/2 between beams reflected either side of the dot edge. Destructive interference will take place giving an easily detected change in amplitude of the reflected beam. It will be understood that a phase difference of nA/2 can suitably be employed, if this is more convenient. The arrangement by which reading and writing are performed from opposite sides of the media has the advantage that the substrate need be transmissive at only one of the wavelengths. Compatability with existing read-only technology can, if thought desirable, be more easily achieved. To assist in registration, the write head would preferably be combined with a control read head. Erasing can be conducted from either side. In circumstances where the transmissivity of the substrate presents no difficulties, or where it is otherwise appropriate, reading, writing and erasing can all be conducted through the substrate. Alternatively, reading, writing and erasing can all be conducted from the opposite sides. The wavelengths required for reading, writing and erasing (and where appropriate, control read) are conveniently generated by diode lasers or diode laser pumped rare earth solid state lasers. In those cases where, say, read and erase beams are generated in a common head, it may prove convenient to use different harmonic frequencies of a single laser. For example the second (355 nm) and third (532 nm) harmonics of a Nd:Yag laser. To achieve data storage densities equivalent to current CDs, it would be preferable to write dots of sub-micron dimension. The transverse spread in intensity of the writing laser beam is approximately Gaussian and according to a further aspect of this invention steps are taken such that the transverse dimension of the written dot is reduced relative to the overall beam width. According to this aspect of the invention, the protective layer in Figure 2 contains a saturable absorber, such as for example Rhodamine 6G. The saturable absorber has energy states which will strongly absorb light at the writing wavelength until a power level is reached at which there are equal populations in the two states. Thereafter absorption is clamped. It can be shown that the saturation power P of the absorber is given by:- P = -InTo Qhv 2στ where: Q = cross-sectional area of beam To = transmittance under low flux conditions σ = absorption cross-section τ = lifetime of excited state with respect to refilling of ground state. Choosing typical values for these parameters, a saturation power level can be calculated at around 70 mW. The absorption of the writing beam in the absorber is governed by where x is the distance of travel and N 1 , N_ are the populations of the excited and ground states respectively. It can be clearly seen that at equal populations there is no net absorption. With the intensity of the beam falling off with radius R according to the formula:- the saturation power can be chosen so that the wings of the Guassian are significantly attenuated. If it were arranged that absorption occured at intensities up to 0.9 I , the effective beam radius would o be reduced to 0.23 w . In this manner a sub-micron writing resolution should be achievable. In another example, using the same PIDC material, the following read/write/erase strategy could be employed: Write at 355 nm Read at 633 nm Erase at 3 nm In other words, there is no pre-colouring step; the PIDC is coloured to write and bleached to erase. With this arrangement it would be possible to use other dye laser molecules as the saturable absorber. Alternative PIDC materials can have the same general formula (1) with X = S or NPh in place of oxygen, or R = methyl in place of cyclopropyl. Still further alternative PIDC materials will occur to the skilled man and reference in this context is directed to IEE proceedings Volume 130 Part I No. 5 October 1983, "Organic Fatigue-Resistant Photochromic Imaging Materials". Two further alternative strategies using other suitable PIDC materials are as follows: I II Activate with blue Write at 400 nm Write at 660 nm Read at 780 nm Read at 78O nm Erase at 660 nm Activate to Erase These have the advantage of reading in the near infra red in common with conventional CDs. By way of further example, reference is directed to Figure 3 which illustrates an alternative structure according to this invention. The arrangement shown in Figure 3 employs the strategy I and has a layer of PIDC material 30 which colours at around 400nm and bleaches at around 660nm. A suitable PIDC compound is that having the structural formula (1) with X = NPh. Between the substrate 32 of polycarbonate and the PIDC layer 30, there is interposed a dielectric layer 3 which has transmission windows around 66θnm and 78θnm and is outside these windows reflective. At the opposite side of the PIDC layer is a further dielectric layer 36 which transmits up to around 400nm but is reflective at higher wavelengths. Reading is carried out at 78θnm. For the purposes of writing data, it is necessary to provide means for indexing. In current WORM technology, discs are provided with mechanical grooves which serve for tracking. It would be possible to use similar techniques in optical discs according to this invention and reference is directed in this context to Figure 4. This shows an upper polycarbonate disc layer 20 which is formed with tracking and focusing grooves 22 and is coated with a dielectric layer 24. This transmits at 400 nm and reflects (70 to 80%) at 780 nm. A lower disc layer 26 of polycarbonate and other suitable polymer is provided which is opaque at wavelengths beneath 750 nm. This carries the PIDC layer 28. In a suggested manufacturing technique, the upper disc layer 20 is grooved and then the dielectric layer 24 vacuum deposited. The PIDC layer 28 is spin coated on to the lower disc layer with the two disc layers then being bonded together. Care is taken to ensure that the bond is of optical quality. As shown diagrammatically in Figure 4, the disc is written and erased from above at approximately 400 nm. A combined Direct Read During Write and Erase (DRDWE) head is provided. This has the considerable advantage that segments can be verified immediately after writing and both immediately before and after erasure. This is made possible by the fact that, in accordance with the invention, a readable indicia is formed practically instantaeously after writing " . Reading is at 78O nm in a conventional CD read head. An opaque cover is provided to protect against sunlight. It can be arranged that the cover is removed manually before insertion .of the disc into the reader/writer; alternatively a mechanism could be provided for automatic retraction of a suitably designed cover. In addition to the tracking grooves, segment marker codes are written to the disc. According to another aspect of the invention, grooves are provided in the PIDC material itself. Reference is now directed to Figure 5» It will be seen that the PIDC material 50 is grooved in such a manner that when the material has been activated, the phase retardation in the valley 52 is A /4 and in the ridge 5 A /2. It follows that the valleys are non-reflective and the peaks are highly reflective so that the grooves are readily detectable for tracking purposes. Dots are written in the valleys in the manner described previously. That is to say the PIDC material is bleached to provide a refractive index differential between the dot region and the surrounding valley region such that destructive interference occurs. It will be seen that in this example an aluminium reflecting layer 56 is employed and reading/writing/erasure all take place from the same side of the disc. If necessary, a saturable absorber could be added over the PIDC material. However, because of the lower storage densities dictated by the groove structure, the saturable absorber may not be required. In alternative arrangements, grooves will not be required and tracking will rely on sensing an initially written spiral. This may offer significant advantages in manufacture over conventional groove techniques. It will be understood that indexing marks can be left on the disc alongside subsequently written data. " Alternatively, the indexing marks are erased as each segment is written. The erasure procedure will then ensure that sufficient indexing marks are left at all times. It should be understood that this invention has been described by way of examples only and a wide variety of modifications are possible without departing from the scope of the invention. Thus, for example, other PIDC materials can be employed beyond those specifically mentioned. Whilst reference has been made to optical discs, it should be recognised that the invention is not limited to one structural form and wafers, cards and three dimensional arrays represent other possibilities.

Claims: CLAIMS 1. A method of optical data storage using photo-induced dispersion change (PIDC) material which is reversibly transformable upon absorption of radiation at a wavelength Aw to a thermally stable, changed form of a different refractive index, comprising the writing step of directing a radiation beam of wavelength A a selected localised regions of the PIDC material to transform at least some PIDC material in said regions to the changed form to produce a difference in refractive index δn between each region and the PIDC material immediately surrounding, the arrangement of said regions in the PIDC material being representative of said data; and a reading step of directing a beam of radiation of wavelength A successively at said regions and monitoring radiation passing through a path length L of the PIDC material, wherein δn and L are selected such that there is a phase difference of substantially n A /2 between radiation passing through any of said regions and the PIDC immediately surrounding that region, where n is an odd integer. 2. A method of optical data storage according to Claim 1, further comprising an erasing step of directing radiation of wavelength A at said regions to reverse said transformation. 3. A method of optical data storage according to Claim 1 or Claim 2, wherein the PIDC material has substantially no absorption at the wavelength Λ • 4. A method of optical data storage according to any one of the preceding claims with the PIDC material being supported on a substrate which is substantially transparent at the wavelength , wherein said reading step comprises directing a beam of radiation of wavelength A through said substrate and wherein said writing step comprises directing a radiation beam of wavelengthA at the PIDC material without first passing through the substrate. 5. A method of optical data storage according -to Claim 4, wherein there is provided at a side of the PIDC material remote from the substrate a layer which is substantially reflective at the wavelength Ai and substantially transmissive at a wavelength AW- 6. A method of optical data storage according to any one of the preceding claims, wherein the said writing step further comprises controlling the intensity and duration of the writing radiation to transform a proportion only of the PIDC material in each said region. 7. A method of optical data storage according to Claim 5, wherein the writing step further comprises continuous monitoring of the path length L and controlling the intensity and duration of the wriring radiation in depenedence upon the monitored path length. 8. A method of optical data storage according to any one of the preceding claims, with said radiation beam of wavelengthA having an intensity reducing in the beam cross section away from the beam axis, wherein said writing step further comprises directing the radiation beam at wavelength A at the PIDC material through a saturable absorber adapted to absorb radiation away from the beam axis. 9. A data storage element adapted to be read optically at a wavelength A comprising a body of photo-induced dispersion change (PIDC) material which is reversibly transformable upon absorption of radiation at a wavelength Λ different from A_ to a thermally stable, changed form having a substantially different refractive index at Λ , there being selected localised regions of said body wherein at least some PIDC material has through absorption of radiation at wavelength A been transformed to said changed form z o produce a w difference in refractive index δn at wavelengthA between the region and the PIDC material immediately surrounding the region, the arrangement of said regions in the PIDC material being representative of the stored data, the element being adapted to provide a physical path length L in the PIDC material for a radiation beam of wavelength A , wherein n and L are selected such that there is substantially maximal interference between radiation passing through a said region and the PIDC material immediately surrounding. 10. A data storage element according to Claim 6, wherein said body of PIDC material comprises a layer of PIDC material supported upon a substrate. 11. A data storage element according to Claim 9 or Claim 10, further comprising a body of saturable absorber adapted to absorb radiation of wavelength A up to a threshold power level. 12. A data storage element according to any one of Claims 9 to 11, in the form of disc. 13. A data storage element according to Claim 12, further comprising circular or spiral optically readable tracks. 14. A data storage element according to any one of Claims to 13 further comprising a body of material which is substantially reflective of the wavelength^ and substantially transmissive of the wavelength •

Inventors: WALLACE STEPHEN CHARLES, WALLACE, STEPHEN, CHARLES
Applicants: TRAQSON LTD, TRAQSON LIMITED
Application year: 1988
Main classification: G11B7
IPC codes: B41M5, G11B7