FR2482756A1 - DISC CONTAINING INFORMATION AND METHOD FOR REVERSIVELY RECORDING AND DELETING INFORMATION THEREOF - Google Patents

Abstract

REVERSIBLE INFORMATION RECORDING DISC CHARACTERIZED IN THAT THE MATERIAL CONSTITUTING THE TOP LAYER 24 IS EFFECTIVE TO INHIBIT THE FORMATION OF AN IRREVERSIBLE RECORDING IN THE ABSORBENT LAYER 22 WHEN THE INFORMATION DISK IS EXPOSED TO A BEAM OF LUMINOUS OF RECORDING OR DELETION AND, IN THAT THE INFORMATION TRACK INCLUDES A SERIES OF REGIONS 42-44 IN THE ABSORBENT LAYER 22, WHICH MAY BE REVERSIBLY SWITCHED TO A STATE WITH DIFFERENT OPTICAL PROPERTIES TO MODIFY THE POUCHANT DISC IN THESE REGIONS, VARIATIONS IN THE LENGTH OF THESE REGIONS ALONG THE TRACK AND FOR SPACING BETWEEN SUCCESSIVE REGIONS REPRESENTATIVE OF THE RECORDED INFORMATION.

Description

This invention relates to a disqce - éver tb1e "-itena - es n-r tions, ce

  disc with an erasable information track,

  while maintaining the ability to re-record the environment.

  US Patent No. 4,097,895 describes an ablative optical recording medium for use in an optical recording system. This medium comprises a reflective layer which is covered with an absorbent layer, the thickness of the absorbent layer being chosen so as to reduce the reflectivity of the recording medium. A beam of light

  modulated focused, directed on the recording medium, vaporizes or eli-

  mine the absorbent layer while leaving an opening in this layer,

  leaving the reflective layer uncovered. When reading the

  registration, the difference in reflective power between the non-

  disturbed recording media and portions thereof in the

  what there is an opening or a deformation is detected optically and

  transformed into an electrical signal representative of the recorded information.

  In U.S. Patent Application No. 054,437, filed July 3

  1979 by Bell, was described a triblate ablative optical recording medium

  layers comprising a spacer layer interposed between the reflecting and absorbing layers of the optical recording medium according to US Patent No. 4,097,895. The relationships between the thickness of the layer

  absorbent and that of the spacer layer and the relationships between the

  The optical aunts (the refractive index and the extinction coefficient) of the reflecting, spacing and absorbent layers are chosen so as to reduce the optical reflecting power of the recording medium. The energy absorbed from a modulated and focused beam of light removes or melts the

  absorbent layer, producing an opening in this layer, thus leaving

  if exposed the underlying reflective layer through the layer

  spacing. Alternatively, the absorbent layer can deform without reacting

  reading of an opening, which causes an irreversible modification of the

  reflective power. This recording medium allows the use of a dQ-

  many more materials, in a medium with low reflecting power, than that provided by the recording medium according to the American patent

no 4,097,895.

  U.S. Patent No. 4,222,071 describes a recording medium

24 827 56

  improved three-layer optics, comprising an absorbent layer of a material with a low melting temperature, such as tellurium, which exhibits a recording capacity of high quality and with a signal-to-noise ratio

  high, with the formation of openings in the absorbent layer.

  U.S. Patent No. 4,101,907 describes an overcoating structure for recording media according to the U.S. patents.

  Nos. 4,097,895 and No. 4,222,071, which includes a thin barrier layer

  covering the absorbent layer, and a thin overcoat layer which covers the barrier layer. This barrier layer is thermally insulating and chemically non-reactive, reducing both the effect of the heat generated in the absorbent layer on the overcoat layer, and the effect of the solvents present during the deposition of the overcoat on the layer. absorbent. The thick overcoat layer contains dust

  which are deposited on the structure distant from the focal plane of the recording lens

  in order to reduce the influence of the presence of dust on the

  recording and playback signal.

  Blom, in "Applied Physics Letters" 35, 81 (1979) describes the combination

  origin of a three-layer tellurium-based disc according to US Patent No. 4,222,071, with a thin barrier layer covering the absorbent layer,

  and he calculated the thermal efficiency of this structure for the formation of

vertures in the absorbent layer.

  In all of the recording environments described above, information

  tion is irreversibly recorded by forming wells or openings

  tures, or by irreversibly altering or modifying it in any other way,

the absorbent layer.

  The Applicant has discovered that a track of information can be

  recorded and erased, reversibly, on a useful domain of light energies

  incident, in a recording medium comprising a layer

  upper covering the absorbent layer. The information track includes

  takes a series of regions discovered in the absorbent layer, the optical properties of which have been reversibly changed compared to those of the undiscovered regions of the absorbent layer. This change in optical properties produces a change in the reflecting power of the information recording medium, which change is detected. The upper cover layer prevents any irreversible recording in the absorbent layer when the recording disc is exposed to

  a recording or erasing light beam.

  In addition to the reversible information disc, the characteristics of which have been specified above, the invention relates to a recording method,

  erasure and re-recording of information in an environment of

information recording.

  Other features and advantages of this invention will emerge

  of the description given below, with reference to the appended drawings, which in it-

  polish examples of embodiments devoid of any limiting character.

  In the drawings - Figure 1 is a schematic sectional view of a first embodiment of a reversible information recording medium according to the present invention; - Figure 2 is a schematic sectional view of a second embodiment of a reversible information recording medium according to the present invention; Figure 3 is a schematic sectional view of a third embodiment of a reversible information recording medium

  - Figure 4 is a schematic sectional view of a recording disc

  registration according to the present invention, comprising information recorded

  very reversibly; and, - Figure 5 is a schematic view of a recording, reading and erasing device to be used with the recording disc

according to this invention.

  Referring to Figure 1, it can be seen that the reversible disc 1 contains

  nant the information may have a monolayer structure which comprises a substrate 12, an absorbent layer 22, placed above the substrate 12,

  and an upper layer 24, disposed over the absorbent layer 22.

  This last layer absorbs light at the wavelengths of the recording, reading and erasing beams. The upper layer 24 is

  essentially transparent to the wavelengths of the recording beams

  The top and bottom layers are made of materials that are effective in inhibiting the formation of an irreversible change in the absorbent layer upon exposure to

  recording or erasing light beam.

  Referring next to FIG. 2, it can be seen that the reversible disc 2 containing the information can be a two-layer structure which consists of: a substrate 12, a layer 18 reflecting the light, arranged above the substrate, this layer 18 reflecting a substantial part of

  the incident light it receives, at the wavelengths of the light beams

  recording and playback nodes; an absorbent layer 22, arranged

  on the reflective layer, in a material which absorbs light in the long

  wavelength of the recording, reading and erasing light beams

  is lying; and an upper layer 24, deposited on the absorbent layer 22,

  this layer 24 being made of a material which is essentially trans-

  related to the wavelengths of the recording, read- ing light beams

  ture and erasure. The reflective layers 18 and upper 24 are effective in preventing the formation of an irreversible change

  in the absorbent layer, upon exposure to a light beam from

registration or erasure.

  In the embodiment shown in Figure 3, a reversible information recording medium 3 comprises a substrate 12, having

  a main surface 14; a non-conforming substitution layer 16,

  placed above the main surface 14 of the substrate 12; a reflective layer

  chissant 18 above the layer 16; a spacer layer 20, disposed on the reflective layer 18; an absorbent layer 22, above the spacer layer 20; an upper layer 24, disposed on the absorbent layer 22, and a thick overcoat 26, covering the upper layer 24, this overcoat 26 being essentially transparent to the lengths

  recording, playback and erasing waves. Layers of space-

  and higher are effective in preventing the formation of a change.

  irreversible in the absorbent layer when exposed to a beam

  bright recording or erasing.

  An information track, after recording, comprises a series of regions, in the absorbent layer, which have been reversibly switched to a state having optical properties different from those of the layer

  not recorded, so that the reflecting power of the recording medium

  48 o6 information recording, at the wavelength of the reading, is changed in these regions. The information is coded as variations of the

  length and / or spacing of the regions thus switched.

  In the embodiment shown in Figure 4, the layers constituting the components of the information disc 4 were identified in the same way as those constituting the information recording medium 3, shown in Figure 3. L information is recorded in the form of a track produced in the absorbent layer 22, this track comprising a series of undisturbed regions 42 and a series of disturbed regions 44, the optical properties of which, at the wavelength of the reading , have been modified by exposure to the recording light beam. This modification of the local optical properties of the absorbent layer is accompanied by a

  change of the local reflecting power of the disc containing the information.

  Variations in the reflectivity of the information disc are detected

  optically and transformed into an electrical signal representative of the information

registered mation.

  The substrate 12 may consist of glass or of a plastic material which is effective in inhibiting the formation of an irreversible modification of the absorbent layer, for example a polyvinyl chloride or a polymethyl methacrylate, this substrate being typically produced in the form of a disc. As a variant, the substrate 12 can be made of a material, such as aluminum, which reflects light at the wavelength of the recording,

  this material thus combining the functions of the substrate 12 and the reflective layer

  light-passing 18. It is enough that the substrate is thick enough to

support the rest of the structure.

  Since any roughness on the surface 14 of the substrate 12, the size of the diameter of the focused light beam, would produce noise in the signal channel, after reading, a substitution layer can be deposited on the surface 14 16, in the form of a non-conforming coating of a plastic material, such as an epoxy or acrylic resin,

  this deposition being carried out before forming the light reflecting layer 18.

  This layer 16 may have a microscopically smooth surface, which

this source of noise.

  Preferably, the reflective layer 18 reflects a substantial fraction of the incident light at the recording and reading wavelengths, and is typically formed from a metal such as aluminum.

  or gold, which have a high reflectivity at these wavelengths.

  Preferably, the reflective layer 18 reflects at least 50% of the light.

  incidental. The reflective layer 18, the thickness of which is typically about 30 to 60 nanometers, can be deposited on the surface 14 of the

  substrate or on the surface of layer 16, using evaporation techniques

  vacuum poration. Alternatively, a dielectric reflector can be used

mono or multi-layer.

  The spacer layer 20 is made of a material such as silicon dioxide, silicon monoxide, titanium dioxide or aluminum oxide, which are optically non-diffusing, and whose properties are

  such as to prevent irreversible recording in the ab-

  sorbent, when exposed to a recording light beam or

  of erasure. Materials can be deposited on the reflective layer

  health 18 using electron beam evaporation techniques.

  Alternatively, organic materials can also be used which can

  form a smooth coating, essentially free of defects, and present

  both the inhibition properties sought. These materials can be de-

  placed on the reflective layer using evaporation techniques,

  of centrigufation or of deposit by luminescent discharge.

  The absorbent layer 22 is made of a material which is capable of reversibly switching from its original state to a second state having different optical properties, for the wavelength of the reading. In this case, "reversible" designates the ability of the material to return to its original state, with its original optical properties, by exposure to an erasing light beam or by exposure to heat. It is not necessary

  that the optical properties at recording and eff wavelengths

  facement change, it suffices simply that the absorbent layer, in the

  second state, absorbs light at the erasing wavelength. The change-

  optical properties may include a change in the refractive index

  fraction, of the extinction coefficient, or a combination of these two factors, or a modification of higher order optical constants, for example the magneto-optical or electro-optical coefficients which may come from a domain inversion. Changing optical properties can cause detectable change in the amount of reflected light

  from the recording of the information.

  Among the classes of materials which may undergo reversible changes in their optical properties, upon exposure to a recording or erasing light beam, mention may be made of photochromic materials, such as doped CaF2 and CaTiO3, and organic compounds of magneto-optical materials, such as MnBi and PtCo, and materials that undergo a crystal lattice phase transition, where the degree of crystallinity of the material changes. Examples of the latter type of material include: tellurium, selenium, chalcogenide based alloys

  tellurium or selenium, arsenic trisulphide and arsenic triselenide.

  These materials can be deposited by appropriate techniques, by

example by vacuum evaporation.

  After exposure to the atmosphere, some of these materials oxidize, leaving an absorbent layer which is thinner than the layer originally deposited. This effect can be compensated for by depositing a thicker layer than desired, the subsequent oxidation reducing the effective thickness to the desired value. In the monolayer structure, the thickness of the absorbent layer is chosen so as to ensure a balance between the absorption and the reflection of the recording and reading light beam. Typically, the thickness of the absorbent layer is between 10 and 100 nanometers approximately. In the two-layer structure, the thickness of the absorbent layer

  is chosen so as to reduce the reflecting power of the recording medium

  the wavelengths of the light beams for recording and

  reading, and preferably to minimize the reflective power of these

  wavefighters. Typically, this thickness is between about

  ron 5 and 100 manometers. For example, for a structure comprising a

  reflective aluminum layer and an absorbent layer containing approx.

  In 90% selenium, the thickness of the absorbent layer is around 20 nanometers, when the reflectivity is reduced to a minimum

  for a wavelength of 488 nanometers.

  In the three-layer structure, the ratio of the thickness of the absorbent layer to those of the spacer and upper layers, as well as the

  relationships between the optical constants of the reflective and space layers

  absorbent, superior and overcoating, are chosen so as to reduce the reflecting power of the unexposed recording medium, for the recording wavelength. Preferably, the recording and playback wavelengths are the same, and the reflecting power

  is reduced to a minimum, which corresponds to the anti-reflection condition. In-

  following, a modification of the optical properties of the absorbent layer, to

  the wavelength of the reading leads to an increase in the reflectivity

  chissant from the recording medium.

  The optimal values of the thicknesses of the spacer and absorbent layers can be calculated using, for example, the matrix method, as set out in the publication "Optical Properties of Thin Solid Films" by O.S. Heavens, Dover Publications, Inc., New York, 1965,

page 69.

  Alternatively, useful values of the thicknesses of the spacer and absorbent layers can be obtained by depositing the spacer layer as

  indicated above, and then depositing the absorbent layer while controlling

  the reflectivity of the recording medium at the recording wavelength. The spacer layer has a thickness of at least nanometers, which thickness can range up to about 500 nanometers, and is typically about 10 to 150 nanometers. Typical values

  of the thickness of the absorbent layer range from about 1 nanometer to 60 nanometers

about meters.

  The upper layer 24 can be made of a material which is

  chemically non-reactive with respect to the absorbent layer 22 and with respect to

  wearing the overcoat 26, and the properties of which are such that this layer inhibits irreversible recording in the absorbent layer,

  when exposed to a recording and erasing light beam.

  The Applicant believes that the mechanical rigidity of the upper layer 24

  and material below the absorbent layer is the characteristic

  most important to prevent the formation of an opening, or

  other deformations in the absorbent layer. Among the pre-

  ferers to constitute this layer, mention may be made of silicon dioxide, silicon monoxide, titanium dioxide and aluminum oxide, which can

  be deposited using electron beam deposition techniques.

  Other materials, such as organic materials which have the required properties, may also be useful for this application. The thickness of the upper layer 24 is chosen, in the first place, so as to prevent the formation of an irreversible recording in the absorbent layer. Typically, the thickness of this layer is included

between 100 and 1000 nanometers.

  An overcoat 26, preferably having a thickness of between 0.05 and 1 millimeter, can be applied to the top layer 24, in order to eliminate or reduce signal defects caused by surface dust which will precipitate, leave of the ambient atmosphere, on the recording medium. Dust particles which are located on the upper surface of the overcoat layer are distant from the focal plane of the

  optical system, in order to considerably reduce their effect on the recording

  and reading the information from the disc. A useful material for such an application is a silicone, acrylic or epoxy resin. Alternatively, the thickness of the top layer may be such that the top layer

  also plays the role of thick over-coating.

  An information track can be formed in the recording environment.

  of information, thereby forming an information recording, by exposing the recording medium described above to a beam of light

  modulated recording of sufficient intensity, and for a period allowing

  both to modify the optical properties of the absorbent layer. In turn,

  this change in the optical properties of the exposed regions produces a change

  fication of the local reflecting power of the recording medium, for the reading wavelength. To record information at video speeds, the recording process must be initiated in a time period of the order of 10 to 30 nanoseconds. The erasure process, i.e. returning the recording medium to its initial state, can take more than

  time. For example, the time to initiate the crystal phase transition-

  amorphous in tellurium and similar materials, is limited by the rate at which the material is heated substantially to its melting point, and 1 0

2 482756

  experience shows that this transition occurs with exposure times

  tion located in the nanosecond domain. The time required for the tran

  Amorphous-crystalline reverse sition is relatively lower, since it is limited by crystal nucleation and the growth rate of the crystal phase. The speed of crystallization can be increased by heating the material to a temperature above its glass transition temperature, the duration of recrystallization being typically of the order of

microseconds to milliseconds.

  The information can be recorded in the information disc by reducing the degree of crystallinity of the absorbent layer by exposure to the recording light beam. The exposed region need not become completely amorphous, since partial exposure to

  the amorphous state is sufficient to obtain a sufficient change in the

  optical aunts of the absorbent layer. It is clear that the reverse method of recording information, by increasing the degree of crystallinity, is possible since only a change in the degree of crystallinity is necessary. The erasure will then consist in a reduction in the degree of crystallinity, or in a return to the amorphous state of the absorbent layer. This approach may be applicable when using low data rates, or when the rate of growth of crystallization, upon heating, is

  comparable to the recording speed.

  In the prior art, it was believed that the role of the

  barrier was to protect a thick layer of over-coating from dom-

  mages of thermal origin resulting from the fusion of a material with high melting point, such as titanium or rhodium. With absorbent material to

  low melting temperature, such as tellurium or other chalco-

  genures, the barrier layer is not necessary, since the cha-

  their generation, during the melting process, does little or no damage to the overcoat layer. The surprising result obtained by the

  combination of an upper layer and a recording medium

  formations using tellurium, selenium or a chalcogenide alloy, is

  that, in the resulting structure, the information can be recorded and e-

  faceted using a relatively wide range of beam energies from

  recording light without irrevocably recording information

  sensitive, for example by making openings, or other deformations

  manentes, in the absorbent layer.

  The role of the upper layer in the present recording process

  First, is to prevent the formation of an opening or other permanent deformation in the absorbent layer, during the period of time during which the absorbent layer melts. The inhibitory function of the upper layer is limited in that, with increasing energy or recording power, one finally reaches a point where an opening or other deformation is formed, which makes the process

irreversible.

  Figure 5 shows schematically a recording system

  trement, reading and optical erasure to be used with the information disc according to the present invention. The system includes a recording light source 62 which may be a laser or a light emitting diode. To the recording light source is coupled an input signal source 64 to modulate the light source 62. The beam of modulated light emitted by the source 62 is collected and shaped so

  to adapt to objective 68 lenses using recording optics

  68. The modulated beam then penetrates appropriate beam control means S70, then the objective lenses 68 where it is focused on the recording medium or the information disc 74. The relative movement between the recording medium 74 and objective lenses 68 can

  be obtained by means of a drive mechanism 76 and means of transmission

  radial relationship (not shown). A servo-focus 78 detects the spacing between the objective lenses 68 and the recording medium 74 and adjusts this spacing so as to maintain the focus of the light beam on the recording medium. As the recording medium 74 moves through the modulated recording beam, a series of alternating exposed and unexposed regions is formed in the recording medium,

  the exposed regions having different optical properties. An information track contained in the recording medium 74 can

  be read using a reading light source and optics (not shown) which can be separated or the same as the optics and sources of the recording and erasing medium. The reading light source produces a beam of continuous wave light which is

  focused on the recording medium 74, modulated by the reflecting power

  variable health of the information track, collected by the objective lenses and which propagates through beam control means in the recording playback optics 80 where it is shaped so as to be able to be detected by the photodetector 82 and transformed into an electrical signal

to the output signal terminal 84.

  The light coming from the restitution optics 80 is also

  coupled in a servo-guide 86 which generates a proportional error signal

  tional to the difference between the position of the light beam on the medium

  information record 74 and an information track that is entered therein

  registered. This error signal is coupled to the beam control means in order to carry out the radial correction of the position of the light beam.

  to keep the beam of light focused on the information track

mation.

  The light beam emitted by an erasing light source 88 is collected and shaped by erasing optics 90 and then coupled in the optical path, by means of appropriate beam control means 70. The beam passes through the objective 68 lenses

  oh he's focused on the information track contained in the media environment

  registration 74, which must be deleted. The information track is exposed to an erasing light beam of sufficient power and duration to bring the absorbent layer of the regions exposed substantially back to its state

  of origin. Erasure can be done in a single exposure, in slow motion

  weaving the speed of the plate by action on the drive mechanism

  76 if necessary, or by repeated exposures of the track.

  Note that the light sources for recording, reading and erasing can be formed by the same source which can be

  vary the intensity and modulation characteristics for partial use

culier desired.

  A reversible information track recording method consists in: a - recording an information track by exposing the information recording medium, described above, to a beam of

  mnodulated recording light, so that regions of the

  absorbent che are reversibly switched to a second state having different optical properties in order to modify the reflecting power

  of the information recording environment in the regions thus

  tees, to form an information track in the information recording medium;

  b - to erase the information track by exposing the regions corm-

  mutated to a light beam so that the regions so switched

  are reversibly switched to a state presenting appreciably the

original optical properties;

  c - to expose the information recording medium in the

  gions thus erased to a beam of light modulated in such a way that the

  regions of the absorbent layer are switched reversibly to a se-

  cond state with different optical properties, thus forming a new

  new information trail in the information recording environment.

  For materials for which the erasing process is thermal in nature, the entire information record can be erased by exposing the absorbent layer to a beam of erasing light or by removing the disc from the device and heating it with an external source such as an oven. If the erasure process involves recrystallization of the regions of the recording medium, then the layer

  absorbent can be heated to a temperature between its temperature

  glass transition and its melting point to accelerate the process

over erasure.

* The examples which follow which of course have no limiting character.

illustrate the invention.

Example 1

  A tri-layer information recording medium made according to the principles of the present invention included a chloride chloride substrate.

  polyvinyl coated with a layer of acrylic resin (acrylic resin fa-

  Brick by the American firm S. C. Johnson, Inc., Racine Wisconsin) having a thickness between 10 and 25 mm, of a reflective layer

  aluminum about 80 nanometers thick, a layer of space

  62 nanometers thick silicon dioxide layer

  tellurium absorbent 5.5 nanometers thick and with an upper layer

  silicon dioxide of 167 nanometers thick. The power

  reflective of this recording medium for the recording wavelength

  recording, reading and erasing of 488 nanometers being of the order of 10%. This recording medium has been tested in an optical system having a focused light beam spot dimension of the order of 0.4 to 0.6.

  micrometers. The power of the recording light beam, inci-

  tooth on the recording medium being about 7 milliwatts. For rea

  read the erasure we used an incident light beam power of

  around 3 milliwatts for a period of approximately 30 seconds. At these ni-

  power calves 52 cycles of recording, reading and erasing were performed with little or no reduction in the quality of information

  video recorded with a signal-to-noise ratio of 45 decibels (cr8te-

  at peak / square root of noise in a bandwidth of 4.5 MHz).

Example 2

  A two-layer information recording medium produced according to the principles of the present invention comprised a polyvinyl chloride substrate, covered with the same layer of acrylic resin as in the example.

  1, a reflective layer of aluminum with a thickness of about 80 nan

  nometres, of an absorbent layer of Se90 Te5 As5 with a thickness of gold-

  20 nanometer dre and top coat of light-cured epoxy

  ultra-violet (Polyrad UV59 manufactured by the American firm Polymer In-

  industries, Stamford, Connecticut), having a thickness of the order of 80 to 120 micrometers. The reflective power of this recording medium

  was several percent at 488 nanometers.

  Using the optical system described above, five read, erase and re-record cycles were carried out with an incident recording power of the order of 16 milliwatts, a reading power of approximately 0.3 milliwatts and an erasing power of approximately 2 milliwatts. The signal-to-noise ratio, as defined in Example 1, was

  about 40 dB for each recording.

  It remains to be understood that this invention is not limited to the exemplary embodiments described or shown here but that it encompasses

all variants.

2482756

Claims (16)

  1 - Reversible information recording disc which   takes a substrate (12); an absorbent layer (22), above the substrate, of a material which absorbs light at wavelengths of a beam   bright recording, playback and erase, which can be reversed   significantly switched to a state with different optical properties and   which is provided with an information track and an upper layer (24)   placed above the absorbent layer (22), this upper layer being made of a material which is substantially transparent to the wavelengths of the   recording, playback and erasing beams, this disc being charac-   characterized in that the material constituting the upper layer (Z4) is effective in inhibiting the formation of an irreversible recording in the absorbent layer (22) upon exposure of the information disc to a beam of recording light or and in that the information track comprises a series of regions (42-44) in the absorbent layer (2Z), which can be reversibly switched to a state having different optical properties in order to modify the reflecting power of the disc in these regions, variations in the length of these regions along the track and for spacing between successive regions being representative recorded information.   2 - Reversible information recording disc which comprises a substrate (12); a diaper. absorbent (22), above the substrate, of a material which absorbs light at the wavelengths of a recording, reading and erasing light beam, which can be reversibly switched to a state having a different degree crystallinity so that   the reflective power of the disc is modified in the regions thus   mutated and which includes an information track and, an upper layer (24) of a material substantially transparent to the wavelength of the beams   recording, reading and erasing light, this disc being characteristic   terized in that the top layer (24) is effective in preventing   mation of an irreversible recording in the absorbent layer (22) during   exposure of the disc to a recording or erasing light beam   surround and in that the information track in the absorbent layer (22> com-   takes a series of regions (42-44) having a different degree of crystallinity   from that of the rest of the layer so that the reflective power of the   be changed in these regions, variations in the length of these   regions along the track and / or the spacing of the access regions being   representative of the information recorded.   3 - Disc according to one of claims 1 or 2, characterized in   that it includes a reflective layer (18), interposed between the layer   absorbent (22) and the substrate and in that the thickness of this layer reflects   is chosen, compared to the optical constant of the reflected layers   sliding (18), absorbent (22) and upper (24), so as to reduce the   reflecting power of an unexposed portion of the disc.   4 - Disc according to claim 3, characterized in that it comprises   carries a spacer layer (20) which is substantially transparent to   wavelengths of recording and reading light beams.   - Disc according to claim 4, characterized in that the thickness of the absorbent layer (22) is chosen, with respect to the thicknesses of the spacer (20) and upper (24) layers and with respect to the constants   optics of the reflecting, spacing, absorbing and upper layers   higher, so as to reduce the reflecting power of a portion not exposed disc.   6 - Disc according to claim 3, characterized in that the cou-   reflective che (18) is made of a material chosen from the group which includes aluminum and gold.   7 - Disc according to claim 4, characterized in that the thicknesses of the spacing layers (20), absorbent (22) and upper Z5 (24) are such that the reflecting power of the unexposed portions of the disc is minimized.   8 - Disc according to claim 4, characterized in that the thick-   The spacing layer (20) is on the order of 10 nm to 500 nm approx.   ron and that of the absorbent layer (22) is between 1 and 60 nano- meters.   9 - Disc according to claim 8, characterized in that the cou-   spacer (20) is made of a material selected from the group which includes silicon dioxide, silicon monoxide, oxide   aluminum and titanium dioxide.   - Disc according to one of claims 1 or 2, characterized in   that the absorbent layer (22) consists of a material chosen from   the group which includes tellurium, tellurium-based alloys, sele-   niumn, selenium-based alloys, arsenic tri-sulfide and tri-sele- arsenic niure.   11 - Disc according to one of claims 1 or 2, characterized in   that the thickness of the upper layer (24) is between 100 and 1000 nanometers approximately.   12 - Disc according to claim 11, characterized in that the upper layer (24) consists of a material chosen from the group   which includes silicon dioxide, silicon oxide, aluminum oxide   minium and titanium dioxide, 13 - Disc according to claim Z, characterized in that the degree of crystallinity of the regions of the absorbent layer (22) exposed to the recording light beam is less than that of the unexposed regions of this layer.   14 - Disc according to one of claims 1 or 2, characterized in   that it has an overcoat layer (26) over the layer upper (24).   - Disc according to claim 14, characterized in that the thick-   sor of this over-coating layer (26) is between 0.05 and 1.0 millimeters.   16 - Method for reversibly recording an information track   mation in an information recording medium that includes a sub-   strat (12); an absorbent layer (22) above the substrate, made of a material   which absorbs light at the wavelength of the recording light beam   tremrnent, reading and erasing and which can be reversibly switched to a second state having different optical properties, and an upper layer (24), above the absorbent layer, this material being substantially transparent to the length of wave of recording, reading and erasing light beams and being effective in inhibiting the formation of an irreversible change in the absorbent layer upon exposure of recording medium to the recording and erasing light beam , this process being characterized in that it consists in   - record an information track by exposing the recording environment   information recording to a modulated recording light beam   so that the regions of the absorbent layer (22) are switched   versibly to a second state having different optical properties so that the reflectivity of the optical recording medium in the regions thus switched is modified, thereby forming an information track in the optical recording medium; - clear the information track by exposing the switched regions to a light beam so that these regions in the second state are reversibly switched to a state having substantially the original optical properties - exposing the optical recording medium in the regions so   erased to a beam of light modulated so that the regions of the cou-   absorbent che (22) are reversibly switched to a second state having diff erent optical properties thereby forming a new   information track in the information recording medium.   f 17 - Method according to claim 16, characterized in that   the exposure of the information recording medium causes a modification   tion of the degree of crystallinity of the absorbent layer in the regions as well exposed s.   18 - A method according to claim 17, characterized in that the erasure of the information track causes a change in the degree of crystallinity of the absorbent layer in the erased region to bring it back in its unexposed state.   19 - Method for reversibly erasing recorded information   recorded in a disc which comprises a substrate (12), an absorbent layer   (22), above the substrate, made of a material which absorbs light at long   wavelengths of the recording, reading and erasing light beams   0 cement, this material can be reversibly switched to a state   with a different degree of crystallinity so that the power reflects   disk health is changed in the switched regions, this material further comprising an information track, and an upper layer (24),   above the absorbent layer, of a material which is substantially transparent   at the wavelength of the light beams for recording, reading   erase and erasure and which is effective in inhibiting the formation of a recording   irreversible recording in the absorbent layer, during the exposure of the disc to the recording or erasing light beam, this process being characterized in that the absorbent layer is heated to a temperature range where said regions of the absorbent layer containing the   track return to their initial degree of crystallinity.   - Method according to claim 19, characterized in that the temperature range is between the transition temperature of the   glass and the melting temperature of the absorbent layer.   21 - Method according to one of claims 16 or 19, characterized   in that the recording medium comprises a reflective layer (18),   which reflects light at the wavelength of the light beams   recording and reading, this reflective layer being interposed between   the absorbent layer (22) and the substrate (12).   22 - Method according to claim 21, characterized in that the information recording medium comprises a spacer layer (20), substantially transparent at the wavelength of the light beams   recording and reading, this layer being interposed between the layers   reflective (18) and absorbent (22) panels. vs:-