EP0316394A1 - Procede de stockage de donnees optiques et materiaux destines a cet usage - Google Patents

Procede de stockage de donnees optiques et materiaux destines a cet usage

Info

Publication number
EP0316394A1
EP0316394A1 EP88904589A EP88904589A EP0316394A1 EP 0316394 A1 EP0316394 A1 EP 0316394A1 EP 88904589 A EP88904589 A EP 88904589A EP 88904589 A EP88904589 A EP 88904589A EP 0316394 A1 EP0316394 A1 EP 0316394A1
Authority
EP
European Patent Office
Prior art keywords
pidc
wavelength
radiation
data storage
optical data
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP88904589A
Other languages
German (de)
English (en)
Inventor
Stephen Charles Wallace
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
TRAQSON Ltd
Original Assignee
TRAQSON Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by TRAQSON Ltd filed Critical TRAQSON Ltd
Publication of EP0316394A1 publication Critical patent/EP0316394A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
    • G11B7/004—Recording, reproducing or erasing methods; Read, write or erase circuits therefor
    • G11B7/0055—Erasing
    • G11B7/00552—Erasing involving colour change media
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
    • G11B7/24—Record carriers characterised by shape, structure or physical properties, or by the selection of the material
    • G11B7/241—Record carriers characterised by shape, structure or physical properties, or by the selection of the material characterised by the selection of the material
    • G11B7/242—Record carriers characterised by shape, structure or physical properties, or by the selection of the material characterised by the selection of the material of recording layers
    • G11B7/244—Record carriers characterised by shape, structure or physical properties, or by the selection of the material characterised by the selection of the material of recording layers comprising organic materials only

Definitions

  • This invention relates to the optical storage of data which may for example be of audio, video, or computer form.
  • 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.
  • optical storage already has a strong commercial presence.
  • CD compact disc
  • 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 ⁇ m 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.
  • 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.
  • each pit corresponds to a distance ⁇ / ⁇ - 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.
  • 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.
  • CD is not the only form of read only optical disc that has been produced and many more are .under development.
  • 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.
  • 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.
  • the wavelengths required to transform the photochromic from one optical form to the other coincide with wavelengths producible with lasers.
  • 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.
  • 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 ⁇ n 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
  • 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.
  • 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 ⁇ n 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.
  • 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 can be carried out in bulk, or more usually, for selected regions.
  • 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 ⁇ n which is intermediate the refractive index change obtained by total transformation between the two optical forms. In one.
  • this approach could be adopted to vary ⁇ n 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.
  • 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.
  • 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.
  • control over ⁇ n will enable the thickness of the PIDC method to be independently optimised against other criteria.
  • 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.
  • FIGS 3,4 and 5 are diagrams illustrating alternative forms of data storage media according to this invention1
  • 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:-
  • 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.
  • FIG. 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.
  • 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 ⁇ n between the region 18 and the area immediately surrounding, the value of ⁇ n 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 ⁇ n, 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.
  • 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.
  • 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 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.
  • 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.
  • 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:-
  • a saturation power level can be calculated at around 70 mW.
  • the absorption of the writing beam in the absorber is governed by
  • 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.
  • the following read/write/erase strategy could be employed:
  • the PIDC is coloured to write and bleached to erase.
  • Figure 3 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 dielectric layer 3 which has transmission windows around 66 ⁇ nm and 78 ⁇ nm and is outside these windows reflective.
  • a further dielectric layer 36 which transmits up to around 400nm but is reflective at higher wavelengths. Reading is carried out at 78 ⁇ nm.
  • FIG. 4 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.
  • 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.
  • 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.
  • segment marker codes are written to the disc.
  • grooves are provided in the PIDC material itself.
  • an aluminium reflecting layer 56 is employed and reading/writing/erasure all take place from the same side of the disc.
  • a saturable absorber could be added over the PIDC material.
  • the saturable absorber may not be required.
  • 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.
  • 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.

Landscapes

  • Optical Recording Or Reproduction (AREA)

Abstract

Un procédé servant à stocker des données optiques utilise un matériau de changement de dispersion photo-induit (PIDC), tel qu'un fulgide photochrome, qui peut se transformer de façon réversible par absorption d'un rayonnement à une longueur d'ondes lambdaw et prendre une forme modifiée thermiquement stable ayant un indice de réfraction différent. Les opérations d'écriture sont effectuées à une longueur d'ondes lambdaw, ce qui permet de transformer le matériau PIDC dans des régions localisées pour qu'il prenne la forme modifiée, produisant ainsi une différence de l'indice de réfraction deltan entre chaque région et le matériau PIDC immédiatement adjacent. Les opérations de lecture sont effectuées à une longueur d'ondes lambdar le long d'un chemin de longueur L sur un matériau PIDC, deltan et L étant choisis de sorte qu'il y ait une différence de phase de nlambdar/2 entre le rayonnement traversant une région et le matériau PIDC immédiatement adjacent à cette région. Les opérations d'écriture, de lecture et d'effacement peuvent être effectuées à partir de l'un ou des deux côtés du matériau PIDC qui est placé de façon adéquate sur un substrat en forme de disque.
EP88904589A 1987-05-29 1988-05-27 Procede de stockage de donnees optiques et materiaux destines a cet usage Withdrawn EP0316394A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB878712687A GB8712687D0 (en) 1987-05-29 1987-05-29 Optical data storage
GB8712687 1987-05-29

Publications (1)

Publication Number Publication Date
EP0316394A1 true EP0316394A1 (fr) 1989-05-24

Family

ID=10618136

Family Applications (1)

Application Number Title Priority Date Filing Date
EP88904589A Withdrawn EP0316394A1 (fr) 1987-05-29 1988-05-27 Procede de stockage de donnees optiques et materiaux destines a cet usage

Country Status (7)

Country Link
EP (1) EP0316394A1 (fr)
JP (1) JPH01503501A (fr)
AU (1) AU1801488A (fr)
DK (1) DK35089A (fr)
FI (1) FI890425A0 (fr)
GB (1) GB8712687D0 (fr)
WO (1) WO1988009549A1 (fr)

Families Citing this family (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4950579A (en) * 1988-07-08 1990-08-21 Minnesota Mining And Manufacturing Company Optical disc recording medium having a microstructure-derived inhomogeneity or anisotropy
US5249175A (en) * 1988-09-09 1993-09-28 Matsushita Electric Industrial Co., Ltd. Optical information recording medium and information recording and reproducing method therefor
DE68927731T2 (de) * 1988-09-09 1997-07-31 Matsushita Electric Ind Co Ltd Optischer Datenaufzeichnungsträger sowie zugehöriges Aufzeichnungs- und Wiedergabeverfahren

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB1600615A (en) * 1977-10-28 1981-10-21 Plessey Co Ltd Photochromic compounds
NL8401995A (nl) * 1984-06-22 1986-01-16 Docdata Bv Optische informatiedrager met mozaiekstructuur.

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO8809549A1 *

Also Published As

Publication number Publication date
JPH01503501A (ja) 1989-11-22
FI890425A (fi) 1989-01-27
DK35089D0 (da) 1989-01-27
WO1988009549A1 (fr) 1988-12-01
GB8712687D0 (en) 1987-07-01
DK35089A (da) 1989-01-27
AU1801488A (en) 1988-12-21
FI890425A0 (fi) 1989-01-27

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