WO1996016402A1 - Support de memorisation numerique fonde sur le principe de fabry-perot - Google Patents

Support de memorisation numerique fonde sur le principe de fabry-perot Download PDF

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Publication number
WO1996016402A1
WO1996016402A1 PCT/EP1995/004605 EP9504605W WO9616402A1 WO 1996016402 A1 WO1996016402 A1 WO 1996016402A1 EP 9504605 W EP9504605 W EP 9504605W WO 9616402 A1 WO9616402 A1 WO 9616402A1
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WO
WIPO (PCT)
Prior art keywords
storage medium
digital storage
medium according
liquid crystalline
layer
Prior art date
Application number
PCT/EP1995/004605
Other languages
English (en)
Inventor
Robert Jan Van Wijk
Original Assignee
Akzo Nobel N.V.
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 Akzo Nobel N.V. filed Critical Akzo Nobel N.V.
Publication of WO1996016402A1 publication Critical patent/WO1996016402A1/fr

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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
    • G11B7/24—Record carriers characterised by shape, structure or physical properties, or by the selection of the material

Definitions

  • the present invention is in the field of digital storage media, such as compact discs (CDs) and digital tapes or cards, more particularly on so-called WORM media (write-once-read-many-times compact discs or tapes) and rewritable CDs and tapes. These types of media allow the information being written by the consumer.
  • digital storage media such as compact discs (CDs) and digital tapes or cards
  • WORM media write-once-read-many-times compact discs or tapes
  • rewritable CDs and tapes rewritable CDs and tapes.
  • the information is stored in pits which are embossed in the disc.
  • the reading is based on pit-edge interference: As the laser focus is wider than the pit (0.6 ⁇ m) , interference occurs between the laser light which falls within the pit and laser light which falls outside the pit (land). This results in a modulation in reflection, which is used for reading the information.
  • the conventional read-only CDs are only suitable for large production, as the production steps (for obtaining a written disc) are rather complicated and therefore only cost effective in mass production. There is a need for CDs and digital tapes or cards which can be produced in small quantities, or which can even be written by the consumer himself.
  • a CD comprising a liquid crystalline polymer (LCP) layer which is coated on a reflective layer.
  • LCP liquid crystalline polymer
  • the writing is done by locally changing the phase of the LCP by means of laser irradiation. This results in a change of refractive index in the pit.
  • the reading is again based on pit-edge interference: When irradiating with the reading laser the light travels through the LCP layer and reflects at the reflective layer. As the refractive index within a pit differs from the refractive index outside the pit (land), the optical pathlenght within the pit differs from that of the land. The laser light which falls within the the pit interferes with the light which falls on the land.
  • the resulting reflection modulation is used for reading the information.
  • This kind of CD is described in, for instance, CA-A1-2,014,698.
  • EP-A1-0608924 a storage medium is described which comprises a homeotropically aligned LCP layer having a dichroic dye dispersed therein.
  • the reading principle is now based on difference in absorption: In the homeotropic phase the dichroic dye is alligned along with the mesogenic groups of the LCP, which is perpendicular to the surface of the medium. In this state there is only low absorption of the incident light by the dichroic dye molecules, thus a high reflectance. After local irradiation of the LCP, the mesogenic groups, and thus the dye molecules, are orientated randomly, which results in a higher absorption of the incident light by the dye molecules, thus a lower reflectance.
  • the present invention provides a digital medium having an enhanced change in reflection.
  • the digital storage medium according to the invention comprises a grooved substrate (1) provided with a thin reflective layer (2) being partially transparant, which is provided with a layer (3), which comprises liquid crystalline material having a thickness (d) between 100 and 1200 nm, which is provided with a thick reflective layer (4) having a reflectance above 50%.
  • an extra thin reflective layer is present, resulting in the liquid crystalline material layer (3) being sandwiched between two reflective layers.
  • a Fabry-Perot etalon is created.
  • the Fabry-Perot phenomenon is used to obtain a difference in reflection between the written and unwritten state in the digital storage medium.
  • the difference in reflection between the written and unwritten areas in digital media based on difference in absorption and digital media based on pit-edge interference may also be enhanced by introducing the Fabry-Perot phenomenon.
  • a Fabry-Perot etalon typically consists of two parallel, reflecting layers that are placed at some distance (d) from each other.
  • FIG. 1 The dependence of the reflectance of a Fabry-Perot etalon on the wavelength is shown in figure 1, wherein schematically a CD is depicted with a substrate (1) having a refractive index of 1.58 and a thickness of 1.2 mm, a thin reflective layer (2) having a refractive index of 0.08 + i 4.60 and a thickness of 30 nm, a liquid crystalline material layer (3) having a refractive index of 1.67 and a thickness of 5 ⁇ m, and a thick reflective layer (4) having a refractive index of 0.08 + i 4.60 and a thickness of 200 nm.
  • the reflectance is high. At regular intervals, however, the reflectance changes abruptly to a low value.
  • This negative peak in reflectance is hereinafter referred to as reflection dip.
  • This resonance-like behaviour occurs when light cycling back and forth between mirrors interferes constructively with itself. This is the case when the requirements of equation 1 are fulfilled. If the requirements of equation 1 are fulfilled the reflectance is low.
  • ⁇ stands for the phase shift of the laser light on reflection by the mirrors
  • n stands for either n u or n w
  • d stands for the layer thickness of the liquid crystal line material
  • ⁇ stands for the wavelength of the laser light used for reading
  • m is an integer from 1-5.
  • the phase shift ⁇ depends on the wavelenght of the laser light, the mirror thickness and the indices of refraction of the mirrors and the adjacent media. It is prefered that the recording medium according to the invention either is withgin the grooves in the high reflective area of a detuned Fabry-Perot with reflectivity higher than 70 % in the unwritten state and in the reflectance dip of a tuned Fabry-Perot with the reflectivity being below 28% of the reflectivity of the unwritten state or vice versa.
  • the writing laser can be guided along a predetermined path. For this reason a substrate with a spiral shaped track, a grooved substrate, is necessary.
  • the tracking can take place in the digital storage medium according to the invention by employing the difference in reflected amplitude and/or phase of the thin reflective layer/substrate interface within and outside the groove resulting in diffraction. Said groove is also used for tracking with the reading laser.
  • the position of the reflection dip is determined by equation 1.
  • the width and the depth of the refelection dip is influenced by the thickness of the thin reflective layer (2) and the absorption coefficient of the liquid crystalline material layer (3). These influences can be determined with the help of a computer program based on a 2 X 2 matrix formalism for wave propagation in isotropic stratified media developed by Abeles such as described in M. Born, E. Wolf, Principles of Optics, 4 th ed., Pergamon Press (1970), p.51.
  • the formalism by Abeles can be extended to 4 X $ matrices in order to incorporate anisotropic media (such as liquid crystalline material) as described in J. Opt. Soc. Am. 60 (1970), p. 830.
  • the digital storage media according to the invention will have a detuned Fabry-Perot in its unwritten state (high reflectance), and a tuned Fabry-Perot in the written areas, the pits (low reflectance), but it is also possible to start with a tuned Fabry-Perot which is locally detuned. The latter embodiment will be described later.
  • the recorded information is stored in a spiral track in which regions of low reflectance (pits) are alternated by regions with the background reflectance (land) having a reflectance higher than 70 %.
  • the pit length varies from 0.9 to 3.3 ⁇ m in 0.3 ⁇ steps.
  • the reflectance In the longest pits (1IT signal) the reflectance must drop to below 40% of the background reflectance.
  • the readout laser in a conventional CD player has a wavelength between 780 and 830 nm, in general 780 ⁇ 10 nm.
  • a CD according to the invention should have a reflectance in the unwritten state of 70 % and the reflectance in the longest pit should be below 40 % of the background reflectance, i.e. 28, when using a conventional readout laser for CD players.
  • the present invention provides for CDs having parameters which can be set to make the CD compatible with the conventional read-only CD.
  • the digital storage medium according to the invention comprises a grooved substrate (1).
  • said substrate is much thicker than the liquid crystalline layer and its mirrors (varying from 1.0 to 1.5 mm).
  • the medium is read through the substrate. Therefore, the substrate should be optically transparent for the laser light used for reading and writing.
  • laser light is used with a wavelength of 780 nm.
  • Suitable substrates which are optically transparent at this wavelength and have sufficient thermal stability and resistance to humidity are polycarbonate, amorphous polyolefin, and glass.
  • polycarbonate has poor resistance to solvents which are used to apply liquid crystalline material onto the substrate (provided with the thin reflective layer (2) by spin coating.
  • Amorphous polyolefins appear to be resistant to the spin coating solvents and have equal thermal stability and resistance to humidity compared with polycarbonate. Therefore, the use of amorphous polyolefins is prefered.
  • the substrate may be provided with a anti- reflection structure on the side not covered with the thin reflective layer.
  • the thin reflective layer (2) is preferably a metal layer such as gold or aluminum which is applied in the substrate by, for instance, chemical vapour deposition or sputtering.
  • the layer should be thin enough to be partially transparent for the laser light. If aluminum or gold is used, the thickness of the thin reflective layer may vary from 1 to 40 nm.
  • the thin reflective layer is preferrably made of aluminum as this gives the highest reflection at these small thicknesses.
  • the thin metal layer may advantageously be used as the counter- electrode for homeotropically aligning the liquid crystalline material with poling using an electric field. In combination with the absorption coefficient of the liquid crystalline layer (3), the thickness of the thin reflective layer influences the dip in reflection in the tuned Fabry-Perot, as is explained above.
  • the liquid crystalline material may have a ne atic, smectic, chiral smectic or cholesteric liquid crystalline phase and may be aligned uniform planair or homeotropically. It is preferred that the lquid crystalline material is homeotropically aligned in its unwritten state because in this case the refractive index of the liquid crystalline material is independent of the polarisation of the incident light.
  • surfactants may be, int. al., silanes, higher alcohols, and the like, e.g., n-dodecanol and Liquicoat ® PA, ex Merck.
  • the electric field may be generated by corona poling (using a sharp needle, sharp knife or a thin wire as electrode).
  • a counter-electrode on the other side of the liquid-crystalline layer (e.g., an ITO-layer, a metal layer, or a conductive polymer layer), so that the poling field will be positioned over the liquid-crystalline layer.
  • the liquid-crystalline layer may be provided with a conductive layer on either side, and an electric field applied thereto.
  • Uniform planar orientation can likewise be obtained by surface treatment, or by shear.
  • Suitable liquid crystalline materials which may be used for layer (3) is high molecular weight material (1000-250000) such as liquid crystalline polymers and liquid crystalline glasses.
  • high molecular weight material 1000-250000
  • side chain polyesters side chain polyurethanes, and side chain polyethers are prefered, for their polability, their thermal stability (Tg), and suitable viscosity.
  • Tg thermal stability
  • suitable viscosity for further information on liquid crystalline side-chain polyesters reference may be had in EP-A1-0478052 which is incorporated herein by reference.
  • Information on liquid crystalline side-chain polyurethanes may be obtained from EP-A1-0350 112 which is also incorporated by reference.
  • International application No. PCT/EP 95/03176 for information on liquid crystalline side-chain polyethers reference may be had in International application No. PCT/EP 95/03176.
  • Liquid crystalline glasses are also very suitable for use in digital storage media according to the invention as they are thermally stable, readily polable, and have a low viscosity above Tg.
  • thermally stable, readily polable, and have a low viscosity above Tg For information on liquid crystalline glasses reference may be had in Ineternational patent application No. PCT/EP 95/02981.
  • the absorption coefficient of the liquid crystalline material layer can be set by incorporating dyes in the liquid crystalline material.
  • the absorption coefficient of the liquid crystalline material layer is determined by the absorption of the liquid crystalline material (which is normaly neglectible in the wavelenght area of 750-800 nm) , the extinction coefficient of the dye used and the concentration of the dye.
  • the absorption coefficent of the liquid crystalline material in combination with the thickness of the thin reflective layer (2) influences the depth and width of the dip in reflectivity. In genereal up to 30 wt% dye may be present in the liquid-crystalline material.
  • the Fabry-Perot phenomenon can also be used to enhance the contrast between written and unwritten parts on the digital storage medium of media in which the contrast is based on difference in media based on pit-edge interference.
  • a digital storage medium is prepared containing a liquid crystalline layer which comprises a dichroic dye, it is possible to read the information via the difference in absorption, which results in a difference in reflection.
  • Said difference in reflection may be enhanced by using a digital storage medium according to the invention having two reflective layers, with for instance a high reflectivity in the unwritten state and a low refelctivity in the written state owing to the Fabry-Perot phemonenon.
  • the digital storage media according to the invention having homeotropically alligned liquid crystalline material which comprises a dichroic dye are prefered, because these CDs have a higher contrast than the CDs based on pit-edge interference.
  • CDs with homeotropically alligned liquid crystalline material and dichroic dye both phenomena are active simultaneously, and it is impossible to tell the contribution of each phenomenon to the contrast.
  • the absorption coefficient in combination with the thickness of the thin reflective layer influences the dip in reflectivity in the tuned Fabry-Perot.
  • the absorption coefficient is determined by the dye concentration and its extinction coefficient in the liquid crystalline layer. This can be used to determine the parameters for CDs according to the invention which are compatible with the conventional read-only CDs.
  • the thick reflective layer is preferrably a metal layer such as gold or aluminum which is applied in the liquid crystalline layer by, for instance chemical vapour deposition or sputtering.
  • This thick layer should not be transparent for the laser light and therefore should have a thickness of at least 40 nm.
  • aluminum is cheaper than gold, and the reflectivity of an aluminum layer with a thickness above 70 nm is as sufficiently high, the use of aluminum for the thick reflective layer is prefered.
  • the liquid crystalline material fulfills the requirements of constructive interference for a Fabry-perot etalon in its unwritten state.
  • the substrate contains a spiral track (groove) to allow radial tracking of the laser focus during writing.
  • a CD according to the invention comprises a substrate (1), a thin reflective layer (2), a liquid crystalline material layer (3) which is provided with a groove (6), and a thick reflective layer (4) is depicted. Said CD is irradiated with laser beam (5) .

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  • Optical Record Carriers And Manufacture Thereof (AREA)

Abstract

L'invention s'applique au domaine des supports de mémorisation numériques, et plus particulièrement les supports de type WORM (disques compacts non effaçable à lectures illimitées) et les supports réenregistrables. Selon l'invention, le support de mémorisation numérique comprend un substrat rainuré (1) pourvu d'une mince couche réfléchissante (2) partiellement transparente, recouverte d'une couche (3) comprenant un matériau à cristaux liquides d'une épaisseur (d) comprise entre 100 et 1200 nm, qui est elle aussi recouverte d'une couche réfléchissante épaisse (4) dont le pouvoir réflecteur est supérieur à 50 %. Lorsqu'un disque compact réalisé selon l'invention est utilisé, on fait appel au phénomène de Fabry-Pérot pour obtenir ou renforcer une différence de réflexion entre l'état enregistré et l'état non enregistré.
PCT/EP1995/004605 1994-11-22 1995-11-21 Support de memorisation numerique fonde sur le principe de fabry-perot WO1996016402A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
EP94203398 1994-11-22
EP94203398.6 1994-11-22

Publications (1)

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WO1996016402A1 true WO1996016402A1 (fr) 1996-05-30

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1998010416A1 (fr) * 1996-09-02 1998-03-12 Akzo Nobel N.V. Support d'enregistrement optique comprenant une couche tampon reticulee
EP0836180A2 (fr) * 1996-10-10 1998-04-15 Samsung Electronics Co., Ltd. Support d'enregistrement optique ayant une couche d'enregistrement métallique
NL1006904C2 (nl) * 1997-09-01 1999-03-02 Od & Me Bv Werkwijze en inrichting voor het vervaardigen van een optisch beschrijfbaar medium en daaruit vervaardigde producten.
US5925433A (en) * 1995-12-01 1999-07-20 Akzo Nobel N.V. Optical recording medium based on Fabry-Perot principle
EP1014119A1 (fr) * 1997-08-11 2000-06-28 Optiva, Inc. Polariseur dichroique

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0235748A2 (fr) * 1986-03-04 1987-09-09 Sel Semiconductor Energy Laboratory Co., Ltd. Mémoire à disque à cristaux liquides
EP0271900A2 (fr) * 1986-12-17 1988-06-22 Canon Kabushiki Kaisha Méthode et appareil d'enregistrement optique
EP0278446A2 (fr) * 1987-02-11 1988-08-17 BASF Aktiengesellschaft Méthode d'écriture et de lecture optique par laser
JPH0329117A (ja) * 1989-06-26 1991-02-07 Canon Inc 情報記録方法
EP0461619A2 (fr) * 1990-06-12 1991-12-18 Canon Kabushiki Kaisha Milieu d'emmagasinage d'information et méthode d'enregistrement/conservation d'information utilisant le milieu

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0235748A2 (fr) * 1986-03-04 1987-09-09 Sel Semiconductor Energy Laboratory Co., Ltd. Mémoire à disque à cristaux liquides
EP0271900A2 (fr) * 1986-12-17 1988-06-22 Canon Kabushiki Kaisha Méthode et appareil d'enregistrement optique
EP0278446A2 (fr) * 1987-02-11 1988-08-17 BASF Aktiengesellschaft Méthode d'écriture et de lecture optique par laser
JPH0329117A (ja) * 1989-06-26 1991-02-07 Canon Inc 情報記録方法
EP0461619A2 (fr) * 1990-06-12 1991-12-18 Canon Kabushiki Kaisha Milieu d'emmagasinage d'information et méthode d'enregistrement/conservation d'information utilisant le milieu

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 015, no. 162 (P - 1194) 23 April 1991 (1991-04-23) *

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5925433A (en) * 1995-12-01 1999-07-20 Akzo Nobel N.V. Optical recording medium based on Fabry-Perot principle
WO1998010416A1 (fr) * 1996-09-02 1998-03-12 Akzo Nobel N.V. Support d'enregistrement optique comprenant une couche tampon reticulee
US6139933A (en) * 1996-09-02 2000-10-31 Akzo Nobel N. V. Optical recording medium comprising a cross-linked buffer layer
EP0836180A2 (fr) * 1996-10-10 1998-04-15 Samsung Electronics Co., Ltd. Support d'enregistrement optique ayant une couche d'enregistrement métallique
EP0836180A3 (fr) * 1996-10-10 1999-06-30 Samsung Electronics Co., Ltd. Support d'enregistrement optique ayant une couche d'enregistrement métallique
EP1014119A1 (fr) * 1997-08-11 2000-06-28 Optiva, Inc. Polariseur dichroique
EP1014119A4 (fr) * 1997-08-11 2001-12-12 Optiva Inc Polariseur dichroique
US6942925B1 (en) 1997-08-11 2005-09-13 Optiva, Inc. Dichroic polarizer
NL1006904C2 (nl) * 1997-09-01 1999-03-02 Od & Me Bv Werkwijze en inrichting voor het vervaardigen van een optisch beschrijfbaar medium en daaruit vervaardigde producten.
WO1999012160A1 (fr) * 1997-09-01 1999-03-11 Odme International B.V. Procede et dispositif servant a fabriquer un support optique d'enregistrement et produits fabriques a partir de ce support

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Publication number Publication date
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