REPRODUCTION OF OPTICAL DATA STORAGE MEDIA

Abstract: An optical disc (10) is reproduced holographically by holding together the disc and a pre-formed blank of photographic film (16) and illuminating the disc with a collimated laser beam passing through the film material. A transparent spacer plate (14) may be positioned between the disc and the film and a suction effect employed to ensure intimate contact.

Description: N O This invention relates to the reproduction of optically readable data storage media. It is well understood that optically readable data storage media offer theoretical advantages over alternative techniques such as magnetic tape or discs and conventional phonograph records. A principal advantage of the optical technology is the higher storage densities that can be achieved, although there are other advantages in particular applications, such as resistance to wear or damage. It is a fact that, to date, despite the theoretical advantages of optical data storage, practical difficulties and constraints have in many fields prevented optical techniques from supplanting existing methods. One area in which optical techniques have enjoyed commercial success is with so-called compact discs. These replace conventional phonograph records and, in common with those records, are read-only. The compact disc conventionally takes the form of a disc of plastics material which is injection moulded or pressed in a mass-production process which replicates an original which has been produced by the action of a modulated laser beam upon light sensitive material. The plastics disc is metallised and variations in reflectivity of the metallised disc can then be detected in reading apparatus to reconstruct the information contained within the original modulated laser beam. The skilled man will appreciate that this brief description represents a considerable simplification of a complicated manufacturing process which can only produce discs economically if very large numbers are required. For this reason, the production of compact discs is largely concentrated in the phonograph industry although the application to computer read only memories has been appreciated and so-called CD-ROM's have been proposed. There are two other categories of applications for optical data storage, these being video discs and digital optical discs. This is a fast developing field, however, and the distinction between the two categories may not remain clear. Video discs have been produced by a number of companies and, in technology,they are broadly similar to compact discs, with the information content being video rather than audio. What has been said above in relation to the costs of manufacturing compact discs applies with the same force to video discs. The technique is again read only and because of competition from video tape recorders, which of course offer both recording and play back facilities, video discs have been commercially successful only in those areas in which they offer special advantages. One such area is in the provision of educational or instructional subject matter where the ability to operate in an interactive mode is especially useful. Another area is electronic publishing where the high storage density is valuable and the read only constraint not important. In both these areas, the cost penalty that attaches to small production runs can be a limiting factor to the application of the technique. Many further applications which involve small numbers of reproductions are removed from consideration altogether by the economic factor. Digital optical discs have been proposed by a number of companies and a considerable amount of research work is currently been undertaken. Apart from very limited applications where read only operation is sufficient, it is necessary to provide the user with the capability of writing information to the disc. Two approaches have evolved; "write- once" optical discs allow the user to write information on a virgin disc with no possibility thereafter of modifying that information, whilst "erasable/rewritable" discs provide a large number of rewriting cycles. A number of disadvantages have emerged with the various proposals for "write once" and "erasable/rewritable" optical discs. In a practical application, it would be necessary to provide at least one and preferably several back-up copies of the original disc by repeating the writing operation. This consumes computer time. The disc constructions that have been proposed are, moreover, relatively expensive so that the need for back-ups can significantly increase the operating costs of the system. This increase is most easily seen with write-once back-ups which are obviously not reusable. The erasable/rewritable discs tend to be more expensive so that, although the back-ups can be reused, the need to hold an increased inventory of discs represents a significant additional cost. In the development of new disc technologies, high storage density and durability are desiderata which sometimes conflict. In other words, a technology which is capable of very high storage density may be impractical for the reason that it cannot withstand normal wear. As a corollary, special handling precautions may be required with certain discs if an acceptable storage density is to be achieved. It is an object of this invention, in one aspect, to provide a method of reproducing data storage media which is simple and relatively inexpensive and which provides a reproduction which is durable, resistant to damage and demanding of minimal special handling precautions. It is a further object of this invention to provide a method of reproducing data storage media which is capable of high storage densities whilst retaining the above mentioned advantages. - . - It is still a further object of this invention to provide a method of reproducing data storage media which replicates any optically detectable tracking information provided on said media. It has been recognised in the prior art that in many cases where a laser is used to write information on photo-sensitive material, holographic techniques can offer advantages. Reference is drawn in this regard to for example EP-A 0090.5 US-A-3,842,197; GB-A-1 351 202 and FR-A-2.217.760. In these various prior proposals, the writing and reading processes incorporate holography so that increased storage densities can often be achieved.There is, moreover, a resistance to damage or an error tolerance which arises from the fact that the recorded image is not a binary indicia but an interference fringe pattern spread over an area which may be large compared with the size of a scratch, foreign body or other possible source of mis-reading. Whilst the application of holography may in this way enhance a particular optical technology, this very often involves significant increases in cost. There have also been considerable difficulties in referencing the stored data for reading purposes. Attention is drawn also to French .published patent application number 236878O which discloses a method of replicating an optical data storage medium which is transparent and in which data is encoded by variations in the phase of transmitted light. It is proposed to illuminate the medium with coherent light to establish in a plane beyond the medium a fringe pattern arising from interference between light passing normally through the medium and light defracted at each indicia. This fringe pattern is recorded in photo-sensitive material to produce an interferogram. This is not capable of producing a directly viewable image, but the image can be recreated with the appropriate optical system if care is taken to separate the various orders of defraction. The recording technique of this proposal is applicable only to one specific type of optical data storage medium. It is of no use, for example with data storage medium in which the indicia are distinguishable by reflectivity, this category of discs representing the majority of those currently in use. Moreover, the prior document presupposes the existance of conventional equipment for producing interferograms. It will be understood, for example, that the production of an interferogram or a hologram requires that during the exposure time there should be no relative movement between the object and the photographic emulsion or other photo sensitive material (or that any such movement should be small compared to the wavelength of the light being used.) Because of this requirement, holograms are conventionally produced on optical tables which by their substantial mounting and vibration proof suspensions, eliminate troublesome vibrations. A further difficulty arises from the fact that unless the data storage medium to be copied has a rigid substrate (a photographic emulsion coated on glass for example) thermal stress or creep in the medium on a scale which is microscopic but nonetheless significant compared with the wave length of light is likely to destroy the interferogram. The technique could not be applied for example, to photosensitive material with a thin plastics substrate. It is an object of this invention to provide an improved method of reproducing data storage media and particularly such storage media bearing a plurality of indicia distinguishable through reflection of light. Accordingly, the present invention consists in one aspect in a method of reproducing a data storage medium bearing a plurality of indicia distinguishable through reflection of light, comprising the steps of positioning the medium adjacent photo-sensitive material and illuminating the medium with coherent light such that interference between the incident light and light reflected from the medium is recorded in the photo-sensitive material to provide a holographic reproduction of the data storage medium. Advantageously, the photo-sensitive material is normally transparent and the medium is illuminated with coherent light through the photo-sensitive material. Preferably, the medium and photo-sensitive material form an integral rigid structure. Suitably, the medium and photo-sensitive material are held in contact with respective opposite and parallel faces of a transparent plate, preferably through a suction effect. The method according to this invention provides a simple reproduction technique which can be applied with a wide variety of constructions of data storage medium having indicia distinguishable through reflection of light. The photo-sensitive material may take the form of photographic emulsion which, even for high quality grades is signficantly less expensive than optical discs blanks. With the medium and film held as an integral structure, the problems of vibration and thermally generated movement are considerably reduced and it becomes possible to provide reproducing apparatus for desk top use. By the novel use of the photo-sensitive material in the light path, the need for a separate beam splitter is avoided. This not only simplifies the reproducing apparatus but also produces a hologram which can be read in a manner closely similar to that used with conventional optical discs. An important advantage of the present invention lies in the fact that the hologram reproduces not only the data stored in the data storage medium but also the tracking marks that are typically provided to enable reading of that medium. Whatever form of tracking marks are provided on the medium for optical detection in conventional reading of the medium, will be reproduced in the hologram and will appear in the reconstructed image. The image of the tracking marks will then serve the identical purpose in reading of the hologram as do the original marks in reading of the medium itself. It is possible to envisage an arrangement, particularly where the rate of data change is small compared with the frequency of reading of the data, in which relatively few erasable/rewritable discs are employed for data capture with holographic copies according to this invention serving as the working copies. This has economic advantages, particularly where a number of working copies are rquired to be used concurrently, and would also allow the use of high density storage discs which might not be sufficiently stable or durable themselves for use as working copies. The invention will now be described by way of example with reference to the accompanying drawings in which:- Figure 1 represents diagrammatically apparatus according to this invention for reproducing an optical data storage disc; Figure 2 illustrates a method of reading a reproduction formed with the apparatus of Figure 1; and Figures 3 and . illustrate alternative reading methods. Referring to Figure 1, the data storage medium to be reproduced is shown at 10. This medium is in the form of a disc having on the surface 10a) a spiral track, the reflectivity of which is modulated in any suitable fashion. The disc 10 has a central aperture 10b) designed for mounting the disc in conventional reading apparatus and this aperture 10b) is used to locate the disc on a mandrel 12. A transparent circular plate 14 of optical glass is disposed on the mandrel 12 coaxially with the disc 10. This plate 14 has two precisely parallel opposite faces one of which engages the surface 10a) of the disc and the other of which engages the emulsion bearing surface of a disc-shaped blank of photographic film 16. This film is preformed with a central aperture and is again located on the mandrel 12. In order firmly to hold the disc 10, plate 1-4 and film 16 together, a peripheral cuff 20 is provided which effects an external seal between the disc and the film. An outlet 22 is provided to enable the air within the cuff to be withdrawn so that the various elements are forced tightly together by air pressure. In this way, an integral, rigid structure is provided which considerably reduces the liklihood of relative movement between the disc and film, whether as a result of vibration or factors such as thermal stress. In order to produce a holographic reproduction of the disc, a collimated beam of laser light is produced by means of laser 24 and a beam expander 26. This beam is incident upon the film 16 which in the undeveloped state is transparent. A major proportion of the beam is transmitted through the plate 14 with the optical properties of that plate being selected to minimize scattering and reflection at the boundaries. Light is reflected at the surface 10a) of the disc and this reflected light interferes with the incident light to produce a hologram in the film 16. In more detail, each indicia or pixel on the disc 10 may be regarded as a secondary source which when illuminated by the plane incident wave provides a spherical wave interfering with the incident wave in the plane of the photographic emulsion to produce a fringe pattern. Broadly speaking, the extent of that fringe pattern in the plane parallel to the optical axis is equal to the separation between the disc and the emulsion. For reasons which will be described hereinafter, this distance is desirable kept small but subject to the constraint that sufficient fringes should be produced to provide a true hologram of the pixel. It is believed that a thickness for the plate 14 of approximately 1mm is to be preferred. It will be understood that a volume hologram is produced with the fringes extending through the thickness of the film. Referring now to Figure 2, there is shown in diagramatic form an arrangement of reading apparatus according to this invention. The hologram 16 is mounted on a disc carrier 28 having a central drive pin 29 which passes through the aperture in the hologram. A small ram 30 is provided to drive the disc carrier, with the hologram in position, into engagement with an optical glass plate 32 carried on the hollowed shaft 3- of a drive motor 36. A tracking assembly 40 produces a pencil laser beam 42 which is normally incident upon the hologram through a beam splitter 44. Since this beam is both normally incident and collimated, the optical geometry of the recording process is reproduced, albeit that only one small part of the disc is illuminated at any one time. As will be appreciated, an image of the original pixel is produced which is spaced from the hologram by a distance equivalent to the thickness of the plate 14. In order to be able to use, as so far as possible, reading apparatus which is capable also of reading the original disc, this distance should be kept as small as possible and within the focusing correction of the laser system. A detector or detector array 48 is provided with imaging optics 7 and is capable of scanning, through the beam splitter 44, the image of the pixel. A suitable arrangement is provided for moving the tracking system radially. A conventional reader of optical discs will typically employ a focussed laser beam for maximum resolution in reading. To produce reading apparatus with the minimum alteration, it is proposed in a different embodiment to employ a composite light source as shown in Figure 3- The tracking assembly 50 is arranged, suitably through the use of fibre optics, to produce a pencil beam 2 of white light which illuminates the appropriate area 5^ of the hologram to recreate the image of one pixel 56- It will be understood that this is possible in white light because the hologram is a volume rather than a 2D hologram. There is further produced a focussed laser beam 8 which with detector 60 and detector optics 62 is capable in broadly conventional fashion of reading the recreated image. A further embodiment of this invention is illustrated in Figure 4. This arrangement employs a diode laser 70 and has the advantages of being compact and reliable. A lens 72 is provided and this is preferably a graded refractive index lens to compensate for the beam divergence typically exhibited by a diode laser. The angle of incidence relative to the disc is in the range 20 to 30 and it is considered that this will improve the signal-to-noise ratio as compared with normal incidence, if such improvement is necessary. Separate illviminating means (which may be white light) can be provided to first recreate the image; this may be the entire image of the disc or purely that part containing the pixel which is to be read. A two dimensional detector array 7 is provided with a detector lens 76. The use of an array of detectors should improve tracking. 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 forms of photo-sensitive material can be employed, such as a photochromic material or a photoresist in combination with a suitable etchable layer. A photochromic material, for example, has the advantage over photographic emulsion that there is no shrinkage or change in refractive index arising from processing. The term "photo-sensitive" is also intended to include those plastics materials which are sensitive to the local heat generated by an incident laser beam. The described approach of exposing the entire film or other photo-sensitive material simultaneously can be varied by the use of a narrower beam which is scanned over the disc surface. A pulsed laser could be employed. If spurious reflections from beyond the disc which is to be reproduced, present difficulties, a matt black coating can be applied to the reverse face of the disc. Although it has been mentioned that the medium to be copied has indicia arranged in a spiral track, other arrangements are possible such as concentric circles. The advantage has been described of using a transparent plate to ensure that the optimum spacing is provided between the medium and the photo-sensitive material whilst still enabling the formation of an integral rigid structure. It will be apparent to the skilled man that commercially produced compact discs - for example - bear a transparent protective coating. If the medium to be copied takes such a form, the protective coating may itself serve as the transparent spacer plate. It should also be noted that there are many alternative ways of creating a suction effect to hold together the medium and the photo-sensitive material; indeed, other ways of creating a rigid integral structure may be used which do not employ a suction effect. Although the description has been confined to collimated beams - these being felt to be advantageous - it will be appropriate in certain circumstances to employ other arrangements such as a divergent beam. Whilst this invention has particular application to the reproduction of discs read in reflection, certain aspects of the invention may usefully be applied in alternative arrangements.lt is also the case that discs produced substantially as described above might in certain circumstances be read in transmission rather than reflection. Similarly whilst the description has been confined to media in disc form, the invention is not to be regarded as so limited.

Claims: CLAIMS 1. A method of reproducing a data storage medium bearing a plurality of indicia distinguishable through reflection of light, comprising the steps of positioning the medium adjacent photo-sensitive material and illuminating the medium with coherent light such that interference between the incident light and light refelected from the medium is recorded in the photo-sensitive material to produce a holographic reproduction of the data storage medium. 2. A method according to Claim 1, wherein the data storage medium is illuminated through the photo-sensitive material. 3- A method according to Claim 2, wherein the data storage medium and the photo-sensitive material are held together to form a rigid integral structure. 4. A method according to Claim 3. wherein the data storage medium and the photo-sensitive material are held together by a suction effect. 5. A method according to Claim 3 t wherein the data storage medium and the photo-sensitive material abut respective opposite parallel faces of a transparent plate. 6. A method according to Claim 1, wherein the medium is illuminated through a normally incident laser beam. 7. A method according to Claim 6, wherein the cross-sectional area of the laser beam is substantially coincident with that of the data storage medium. 8. A method of reproducing a data storage medium bearing a plurality of optically readable indicia, comprising the steps of positioning the medium adjacent photo-sensitive material; holding the medium and the photo-sensitive material together to form a rigid integral structure and illuminating the medium with coherent light to produce in the photo-sensitive material a holographic reproduction of the data storage medium. 9. Apparatus for reproducing a data storage medium bearing a plurality of indicia distinguishable through reflectivity of light, comprising means for locating the data storage adjacent a sheet of photo-sensitive material and means for illuminating the medium with coherent light such that interference between the incident light and light reflected from the medium is recorded in the photo-sensitive material to produce a holographic reproduction of the data storage medium. 10. Apparatus according to Claim ' , wherein said means for locating comprises means for holding together the data storage medium and the sheet of photo-sensitive material to form an integral structure. 11. A disc bearing a holographic reproduction of a data storage medium and produced by a method according to Claim 1.

Inventors: HAUGHTON ANDREW EDWARD, HAUGHTON, ANDREW, EDWARD
Applicants: HOLODISC LTD, HOLODISC LIMITED
Application year: 1986
Main classification: G11B7
IPC codes: G03H1, G11B7