FR2490858A1 - OPTICAL RECORDING MEDIUM WITH VARIABLE SENSITIVITY AND DISC CONTAINING INFORMATION, MADE BY USING SUCH MEDIUM - Google Patents

Abstract

OPTICAL RECORDING MEDIUM CHARACTERIZED IN THAT IT INCLUDES A SUBSTRATE 12 HAVING A MAIN SURFACE 14 PROVIDED WITH A PLURALITY OF INDENTATIONS 32, AND AN ABSORBENT LAYER 36 OVERCOMING THE MAIN SURFACE 14 AND FILLING SAID INDENTATIONS 32; THIS LIGHT ABSORBING LAYER AT THE WAVELENGTH OF A LIGHT BEAM FOR RECORDING AND READING, IN ORDER TO FORM FIRST AND SECOND REGIONS IN THE RECORDING MEDIUM, SUCH AS THE SENSITIVITY TO THE RECORDING , IN THE FIRST REGIONS, IS GREATER THAN THE SENSITIVITY TO REGISTRATION OF SAID MIDDLE IN THE SECOND REGIONS.

Description

1 2490858

  The present invention relates to an optimum recording medium.

  tick, as well as to a disc containing information having regions of different recording sensitivities, so that the information can be recorded in the regions of high sensitivity without recording occurring in the region of low sensitivity. U.S. Patent No. 4,097,895 describes an ablative bilayer optical recording medium which includes a reflective layer covered with an absorbent layer, wherein the thickness of the absorbent layer is

  chosen such as the reflecting power (or reflectivity) of the recording medium

  is reduced. The absorbent layer is melted or removed by abla-

  when recording, which changes the reflectivity of the mid

  place of registration. When reading the recording, the difference in reflective power between disturbed and undisturbed parts of the

  optical recording medium is detected optically, and it is trans-

  formed into an electrical signal representative of the recorded information.

  U.S. Patent No. 4,216,501 describes a three-layer optical recording medium comprising a transparent spacer layer interposed between the reflecting and absorbing layers of the recording medium.

  optics described in US Patent No. 4,097,895. The relationship between thick-

  The absorbent layer and the optical constants (the refractive index and the extinction coefficient) of the reflective, spacing and absorbent layers are such that a reduction in the reflecting power of the recording medium is obtained. During the course of the recording process, the absorbent layer is melted or is removed by ablation, which causes an opening to be made in this layer, resulting in a modification of the reflecting power of the recording medium. As a variant, the absorbent layer can deform without forming an opening or without modifying the optical properties of the absorbent layer, which modifies the power

reflect healthily.

  In each case, the optical recording medium has, from point to point, a substantially uniform recording sensitivity. By

  therefore, the shape and dimensions of a change in the absorbent layer

  bante, in the form, for example, of an opening in the absorbent layer

  bante, depend. dt Idforme and dimensions of the light beam from

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  registration. U.S. Patent No. 4,176,377 describes a video disc configuration in which the surface is sensitive to recording along a spiral track, and not sensitive to recording in the regions between adjacent tracks. This result can be obtained by using a high power unmodulated laser to eliminate, by thermal tracing, the areas of

  unwanted surface of a disc blank) uniformly sensitive. On the way

  The disc thus traced can be used as an original for the manufacture

  tion, by photolithographic means, of discs comprising spiral tracks sensitive to recording. This additional treatment is undesirable, since the sensitive surface must present a high degree of perfection, in order to

  faithfully reproduce the information recorded there.

  Given the drawbacks of the current solutions examined above

  above, the need arises to be able to have a medium and an optical recording disc comprising a variable recording sensitivity

  locally, so that the change recorded in the absorption layer

  bante is independent of the shape and dimensions of the recording light beam, and which also requires no further processing after

  the formation of the absorbent layer.

  Consequently, this invention relates to a recording medium

  an optical ment which comprises a substrate comprising a plurality of indentations on one of its main surfaces, and an absorbent layer surmounting said substrate and filling the indentations by forming first and

  second regions in the optical recording medium. The sensitivity of

  recording of the optical recording medium, in the first regions, is significantly greater than the recording sensitivity of the medium

  optical recording in the second regions.

  This invention also relates to an information disc comprising

  as an information track, which is recorded as a segment

  rie of zones, in the first regions, which possess op-

  ticks different from those of other first regions and second re-

gions.

  Other characteristics and advantages of this invention stand out

  from the description given below with reference to the accompanying drawings, which

  illustrate various embodiments thereof, devoid of any limiting character. In the drawings: - Figure 1 is a schematic perspective view of an optical recording medium according to the invention; - Figure 2 is a view of a cross section of part of the first embodiment of the medium according to the invention;

  - Figure 3 is a schematic view of a cross section of a

  tie of a second embodiment of an optical recording medium according to the invention;

  - Figure 4 is a schematic view of a cross section of a

  part of a third embodiment of the invention;

  - Figure 5 is a schematic view of a cross section of a

  part of an information disc according to the invention;

  - Figure 6 is a schematic view of a cross section of a

  tie of a second embodiment of a disc according to the invention; and,

  - Figure 7 is a schematic view of a cross section of a

  part of an optical recording medium comprising indentations of different

thickness annuities.

  Referring to Figure 1, it can be seen that the optical recording medium 10 comprises a substrate 12 having a main surface 14, which

  is covered with an absorbent layer 16. The recording medium op-

  tick 10 has first regions 18 which have an ab-

  sorption, at the wavelengths of a recording light beam, higher than that of the second regions 19 which constitute the rest of the optical recording medium. For purposes of illustration, the first regions 18 have been represented in the form of grooves 20, or, alternatively, in the form of a network or in a chess configuration 22. As shown

  will see in the description which follows, it is obvious that the first regions

  18 can be produced and arranged in any other form in which

  the user wishes to record the information in the recording medium.

  Given the high absorption power in these regions, a beam

  recording light, on the optical recording medium 10, recording

  preferably saves information in the form of a modification of the

  optical properties of the recording medium in the first regions 18,

4 to

  with respect to those of the second regions 20, scanned by said beam.

  This modification may take the form of an irreversible recording, for example by making an opening in the absorbent optical layer 16, or alternatively, it may constitute a reversible recording, for example by a reversible change in the degree of crystallinity of the absorbing layer. 16. We now refer to Figure 2, which represents a first

  embodiment of an optical recording medium according to the invention

  tion, which comprises a substrate 12 having a main surface 14, a plurality of

  a set of indentations 32 extending in the substrate 12 over a certain distance from the main surface 14, a plurality of intervals 34 separating the indentations, and an absorbent layer 36 which fills the indentations 32, and which can also cover the intervals 34 between the indentations. The absorbent layer is then made up of two parts: a first part

  40, which fills the indentations 32 and which is thicker than a second part

  tie 42, which covers the intervals 34 between the indentations. The upper surface

  44 of the light absorbing layer 36 is flat, and can be removed

  covered with a covering layer 46.

  We now refer to Figure 3, on which the references

  assigned to the elements common to the second embodiment 50, shown

  felt in this FIG. 3, and in the first example of embodiment 30 of the invention

  tion, are the same. The second embodiment 50 differs from the first 30

  in that a reflective layer 52 overcomes the main surface 14

  bearing the indentations 32 and the intervals or intermediate parts 34 separating them, and in that it comprises an absorbent layer 54 which fills the

  indentations 32 and which can also cover the part of the reflective layer

  chissant 52 covering the intervals 34 between the indentations 32. The absorbent layer is then made up of two parts: a first part 56 which fills the indentations and which is thicker than a second part 58, which covers the intervals between the indentations. The flat upper surface 60 of the light absorbing layer 54 can be covered with a

layer 46.

  We now refer to Figure 4, on which the references

  assigned to the elements common to the third embodiment 70,

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  presented in this Figure 4, and in the second embodiment 50, are the same. This third embodiment of the invention 70 differs from the second 50

  in that a spacer layer 72 fills the indentations 32 of the sur-

  main face 14 and can also cover the reflective layer 52 above the gaps 34. The spacer layer therefore consists of a first part 74, in the region of the indentations of the main surface 14, and a second part 76, which is thinner than the first part 74 and which covers the gaps 34 between the indentations 32. An absorbent layer 78, of substantially uniform thickness, covers the surface 80 of the spacer layer 72. The third embodiment of the recording medium according to this invention is further distinguished from the second embodiment by the presence of a covering layer 82 which covers the surface 84 of the light absorbing layer 78. A covering layer 46 can come

  cover the cover layer 82, or, in the absence of this cover layer

  verture, it can cover the light absorbing layer 78.

  The substrate 12 can be made of a plastic material, for example polyvinyl chloride or (poly) methyl methacrylate, and, typically, this substrate can be in the form of a disc. The indentations can be formed in a main surface of the substrate 12

  by duplication (making replicas) of a model containing these indenta-

  - tions, by stamping the surface 14. Alternatively, the indentations can be made during compression molding of the disc. Typically,

  the depth of the indentations is between 5 nanometers and 200 nano-

about meters.

  Indentation of the main surface of the substrate 12 denotes

  both wells or the like, in this surface, and roughness exceeding

  health of the disc surface. The role of these wells and these asperities in re-

  lief is identical: provide optically sensitive coatings of varying thickness. In the first example of embodiment 30, illustrated by FIG. 2, the depth of the indentations 32 is chosen so that the thickness of the absorbent layer 36 which fills the indentations 32 substantially represents the thickness (or is slightly less) which is necessary to ensure a balance between absorption and reflection, in order to obtain a more sensitive

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  high. If the depth is less than said thickness, any thickness

  more than the indentation will bring the thickness to the desired value.

  Typically, the depth of the indentations 32 is between 5 and

  nanometers approximately, for this first example of embodiment 30.

  In the second and third embodiments shown in Figures 3 and 4, the depth of the indentations is chosen so

  that the thickness of the layer which fills the indentations corresponds appreciably-

  ment to the thickness required to minimize the local reflecting power of the optical recording medium, or that it is substantially less than this thickness. If the depth is less than this thickness, any thickness

  additional above the indentations brings the thickness to the desired value.

  Typically, the depth of the indentations 32 is between about

  and 80 nanometers, for the bilayer structure of the second example of realization

  tion 50, shown in Figure 3, and is between about 30 and 200 nanometers for the three-layer structure of the third embodiment

  , illustrated by Figure 4. As the following description will show,

  preferred thickness of absorbent layer, or spacer layers

  and absorbent, depends on the properties of the material constituting the layers.

  The reflective layer 52 of Figure 3 preferably reflects

  a relatively large fraction, preferably at least 50%, of the lu-

  incident light at the wavelengths of the recording and reading light beams, and is typically formed from a metal such as aluminum

  or gold, which, for such wavelengths, have a reflecting power

  high health. The thickness of the reflective layer 52 is typically comprised

  between 30 and 60 nanometers, and this layer can be deposited on the surface

  face 14 of the substrate comprising the indentations, for example by a vacuum evaporation technique. Alternatively, a dielectric reflector can be used

mono- or multilayer.

  The absorbent layer 36 of the first embodiment 30 (Fig. 2) and the absorbent layer 54 of the second embodiment 50 (Fig. 3) are made of a material which absorbs light at wavelengths

  recording and playback light beams, which can be

  placed on the surface 14 or on the reflective layer 52 so that the

  indentations 32 of the main surface 14 of the substrate are filled preferably

  basically. Once these indentations are filled, the excess material can form a thin layer above the indentations 32 and the intervals 34 between the indentations 32, in order to form a continuous upper surface and

  smooth of the absorbent layer. According to another embodiment of the in-

  vention, the absorbent layer can consist of an organic dye in

  solution in a solvent, which is then deposited by centrifugal techniques

  fugation. A suitable dye, contained in a binder, is a 4.8% by weight solution of 4-phenylazo-1-naphthylamine in a photoresist "Shipley 1350 B", which is diluted in a ratio of 25/1 with 2-methoxyethyl acetate, and which is deposited by centrifugation. This material is suitable for a length

  wave of a recording or reading light source of about 480nm.

  The overcoating layer 46 is made of a material which is substantially transparent, for the wavelengths of the light beams.

  recording and playback. A thick overcoating layer

  a size of between 0.05 and 1 mm is particularly suitable.

  The dust particles which deposit on the upper surface of this overcoat layer are then moved away from this surface, from the focal point of the optical recording and reading system, in such a way

  that the effects of such dust particles are reduced in proportion

considerable.

  A material particularly suitable for producing a coating layer suitable for the particular application to the present invention consists of a silicone-based resin. This silicone resin can be deposited using any known deposition technique of

  those skilled in the art, such as, for example, centrifugation methods.

  The thickness of the absorbent layer 54 of the second exemplary embodiment

  tion 50 (Fig. 3) is related to the optical constants of the reflective and absorbent layers, and to those of the overcoat layer, if present, so that the reflectivity of the optical recording medium in the first regions is reduced , and preferably reduced to a minimum, which corresponds to the antireflection conditions. Optimal values for the thickness of the absorbent layer can be calculated using, for example, the matrix method, as discussed in the publication

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  American "Optical Properties of Thin Solid Films", by O.S. Heavens, Dover Publications, Inc., New York, 1965, page 69. The thickness of the layer

  absorbent with reduced reflectivity is typically

  taken between 20 and 100 nanometers approximately.

  The thickness of the absorbent layer 54 then varies over the surface of the optical recording medium: it is thicker in the regions of the

  indentations 32 only in the regions where the intervals 34 are located. The first

  minimum reflectance, for recording wavelengths

  trement and reading, occurs on indentations. As soon as the indentations are filled, the thickness of the second part 58, above the intervals 34, increases at the same time as the thickness of the first part 56 located

  above the indentations 32. When the thickness of the second part increases

  lies, the reflective power of this part decreases, while the re-

  flectivity of the first parts begins to increase, after passing through a minimum. Therefore, by controlling the thickness of the coating above

  of the second parts, the regions of low reflecting power and of sensitivity

  high bed can be located either above the indentations 32 or

  above the intermediate parts 34. Preferably, the thickness of the section

  This part is reduced to a minimum, and the parts located above the indentations 32 constitute the regions of low reflecting power and of

high sensitivity.

  In the third embodiment shown at 70 in Figure 4, it is the thickness of the spacer layer 72 which varies. This spacer layer 72 is made of a material which is essentially transparent to the wavelengths of the recording and reading light beams, and which can be deposited on the light reflecting layer 52 in such a way that the indentations 32 , in the surface of the substrate, are preferably filled, thus forming a first part 74 of the spacer layer. After filling the indentations, the thickness of the first part 74 and of a second part 76, located above the intermediate parts or intervals 34, increases. A suitable material for producing the spacer layer is constituted by poly-4-methylstyrene dissolved in toluene, and deposited using the centrifugation techniques.

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  The light absorbing layer 78, which covers the space layer-

  ment 72, is made of a material that absorbs light at lengths

  of the recording and reading light beams. Among the ma-

  suitable materials for this layer, there may be mentioned in particular: titanium, rhodium, tellurium, selenium, alloys based on tellurium or selenium, arsenic trisulfide and arsenic triselenium. These materials

  can be deposited by vacuum evaporation techniques. After expo-

  sition to the atmosphere, some of these materials oxidize, leaving an absorbent layer which is actually thinner than the layer originally deposited. This effect can be compensated for by depositing a layer which is more

  thick as desired, so that subsequent oxidation will reduce the thickness

  effective value at the desired value.

  The thicknesses of the spacer and absorption layers 72 and 78, respectively, are chosen such that the reflectivity of the first and second parts of the optical recording medium is reduced, and is calculated taking into account the optical properties reflective, spacing and absorbent layers, as well as those of

  cover and overlay layers, if present, and also

  the wavelengths of the recording and playback light beams

  ture. Typically, the thickness of the spacer layer, to obtain

  a reduced reflecting power, is between approximately 20 and 200 nano-

  meters, and the thickness of the absorbent layer is between 3 and 50 nanometers approximately. The variation of the sensitivity on the surface of the information disc is then obtained by the variations in thickness of the spacer layer, this layer being thicker in the region of the indentations 32 than in the region of the intermediate parts or intervals 34. As in the case of the absorbent layer of the second embodiment shown

  in Figure 3, the first minimum of the reflecting power must be

  duire on indentations. As soon as the indentations are completed by the

  spacer layer, the thickness of the second part 76 of the insulation layer

  placement, above the intervals 34, increases with the thickness

  sor of the first part 74, in the indentations. When the thickness of

  the second part increases, the local reflecting power of the recording medium

  optical registration in this region decreases, while the power reflected

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  health of the first part begins to increase, after going through the

  minimum. As a result, by controlling the thickness of the space layer-

  above the second part, the regions presenting a reflecting power

  lower pitch and higher sensitivity can be located either

  above the indentations 32, or above the intervals 34. It is preferable that the thickness of the second part of the spacer layer be reduced to a minimum, so that the regions of the first parts 74, above the indentations 32, become regions of lower reflective power

and higher sensitivity.

  The cover layer 82 can cover the upper surface 84 of the light absorbing layer 78. The role of the cover layer is to prevent any thermal damage to the overcoat layer 46,

  if a high melting point material, such as titanium or rhodium, is used

  used to form the absorbent layer 78. If, however, a material with a low melting temperature, such as tellurium, selenium, or an alloy containing one of these materials, is used to make the absorbent layer 78, it 'is not necessary to provide the' cover layer to prevent

  thermal damage to the overcoat layer 46.

  However, if the cover layer 82 and the spacer layer

  72 are made of materials that are effective in preventing the formation of

  tion of openings or other permanent deformations of the absorbent layer

  bante 78, a reversible recording, that is to say a recording which can be erased, then re-recorded on the same support, can be formed in the absorbent layer 78. Among the materials suitable for carrying out the

  absorbent layer 78, in this example, mention may be made in particular of

  tellurium, selenium, tellurium or selenium-based alloys, arsenic trisulphide and arsenic triselenide. As suitable materials

  to achieve the cover layer, and which are effective in preventing

  expensive the formation of a reversible record we can cite the dioxide of

  silicon, silicon monoxide, titanium dioxide and tellurium dioxide.

  These materials can be deposited using evaporation techniques

  by electron beam. Typically, the thickness of a layer of

  groove capable of preventing the formation of openings or other deformations

  in the absorbent layer is between 50 and 500 nanometers approximately.

  It should be noted that, if it is desired to record information in a reversible manner, the overcoat and cover layers 46 and 82 must also be transparent at the wavelength of the erasing light beam, and layer 78 must absorb light at the wavelength of the erasing light beam. The indentations of the disc surface can be arranged according to

  a large number of different formats, such as those illustrated in Figure 1.

  These indentations can have the shape of a groove, circular or spi-

  rale, provided with continuous indentations, provided in the surface of the disc, so that the optical recording medium is thicker above the groove than in the intervals or intermediate parts situated between the grooves. Then, if the optical recording medium is more sensitive in the region above the groove than in the region above the intermediate parts, the light beam which covers both the groove and the intermediate parts surrounding this groove will record preferentially

  information in the train path rather than in the intervals around it.

  Alternatively, a series of indentations may be arranged in circular or spiral tracks, spaced from each other, with alternation of indentations and intervals along the track. In this case, the indentations may be of equal lengths on each track, or, alternatively, they may be of equal angular length, the indentations having the longest lengths being located at the greatest distances from the center of the

  disk. As a result, the information recorded on the disc will be recorded.

  recorded and read in the first case at a variable frequency, but with a constant dimension of recorded element, whereas, in the second case,

linen-

  training can be recorded and read at a constant speed, but with a

  variable registered item size. If the recording medium op-

  tick is more sensitive above the indentations than above the intervals or intermediate parts separating them, a light beam, which covers the

  once an indentation and the intermediate parts along the track, record

  preferentially treats information only in the region of indenta-

  tion. Another variant consists of a configuration in the form of a chess game, in which an indentation, along a track, has

  neighboring intermediate parts both along the track and perpendicular-

lie to it.

  Reference is now made to FIG. 5, on which a two-layer information disc 90 has been represented, on which a track containing information has been recorded. The identification of the elements common to the information disc 90 and to the recording medium 50 represented in FIG. 3 is the same. The information disc 90 has been shown with indentations arranged in series along a track, these indentations being separated by intervals or intermediate parts. The information is recorded in the form of a series of openings 92 in the absorbent layer 54, the presence

  or the absence of an opening 92 and the spacing between the openings provided-

  being an indication of the information recorded. Alternatively, the openings

  can be formed above intervals 34.

  Referring now to Figure 6, there is shown a three-layer information disc 100 having an information track recorded in its surface. The identification of the elements of the disc 100 and of the corresponding elements of the three-layer recording medium 70 represented on the

  Figure 4 is the same. The information is recorded in the form of a segment.

  lines 102, in the absorbent layer 78, which have different optical properties from those of the rest of the absorbent layer. These areas

  may have the form of an irreversible deformation of the ab-

  sorbent, or a reversible rmwdification of the optical properties of the absorbent layer, resulting for example from a change in degree of crystallinity of the absorbent layer 78. The presence or absence of a modification of the optical properties of the absorbent layer causes a change in the reflectivity of the recording medium, the length as well as the spacing

  between these areas providing an indication of the information recorded.

  It will be noted that, if the recording light beam covers the regions of lower sensitivity, and the energy density of the recording beam is sufficiently large, the modification of the absorbent layer, which provides an indication of the information registered, may

  also occur in the second regions of lower sensitivity.

  The optical recording medium according to the present invention has different recording sensitivity regions, due to

13 2490858

  varying thicknesses of the layers. These thickness differences are caused by the presence of indentations in the surface of the substrate. During the manufacturing of the substrate, information can be pre-recorded, in

  the optical information medium, by the formation of indentations having a dis-

  particular spatial contribution, or by varying the depth of the indentations. For example, the depth of a part of a spiral or of a circular groove can be modulated by forming a third region having a different reflecting power from those of the first and second regions. This information can be used, for example, to ensure identification of

  the track, to indicate the start or end of a track, or to provide data

  synchronization. For other indentation arrangements, similar information encoding schemes can be used. Reference is now made to FIG. 7, in which the identification of the elements common to the optical recording medium 110 and to the optical recording medium 50, represented in FIG. 3, is identical. The indentations 112 and 114 extend over different distances in the substrate 12, thus forming the

third region.

  When a recording, reading or erasing system is operating, the light beam is centered on the track containing, or

  to contain, the information. Regarding the recording medium -

  optically described above, the light beam is also centered on

  the first regions of high sensitivity, during the implementation of the program

  transfer of registration. If the reflecting power is different between the pre-

  first and second regions, this difference in reflectivity can be used to deliver a signal proportional to the movement of the light beam

  transversely to the track. This signal can be used to correct the posi-

* tion of the light beam on the track.

  Of course, this invention is not limited to the various

  examples of embodiments described and represented here, but that it

includes all variants.

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Claims (22)

  1 - Optical recording medium, characterized in that it comprises   a substrate (12) having a main surface (14) provided with a plurality of inden-   tations (32), and an absorbent layer (36) surmounting the main surface (14) and filling said indentations (32), this layer absorbing light at the wavelength of a recording and reading light beam, to   to form first (18) and second (19) regions in the recording medium   registration, in such a way that the sensitivity to the recording of said medium,   in the first regions (18), greater than the sensitivity to recording   trement of said medium in the second regions (19).   2 - Optical recording medium according to claim 1, character-   laughed in that it comprises a reflecting layer (52) which is interposed between the substrate (12) and the absorbing layer (54), this layer reflecting a substantial portion of the light incident on this layer, for the wavelengths reading and recording light beams, the thickness of this absorbent layer (54) being chosen so as to reduce   the reflective power of the recording medium in the first re- regions (18).   3 - optical recording medium according to claim 2, charac-   terized in that it comprises a spacer layer (72) interposed between the reflective layer (52) and the absorbent layer (78), and which fills said indentations (32), the thicknesses of this spacer layer (72 ) and some   absorbent layer (78) being chosen so as to reduce the reflectivity   bending of the recording medium in the first regions (18).   4 - optical recording medium according to one of claims   previous, characterized in that the indentations (32) extend in the substrate (12), from the main surface (14), over a distance which is   between 5 nanometers and 200 nanometers approximately.   - Optical recording medium according to claim 1, charac-   terized in that said substrate (12) consists of a plastic material.   6 - optical recording medium according to claim 2, charac-   terized in that the thickness of the absorbent layer (54) in the first   regions (18) is chosen so as to minimize the reflecting power.   7 - optical recording medium according to claim 3, charac- 2490858   terized in that the thicknesses of the spacer layer (72) and of the layer   absorbent (78) of the first regions (18) are chosen so as to minimize be the reflecting power.   8 - optical recording medium according to claim 1, charac-   terized in that it comprises a layer of a thick coating (46) deposited over   above the absorbent layer (36).   9 - optical recording medium according to claim 3, charac-   terized in that it comprises a covering layer (82) covering the absorbent layer (78), and a thick covering layer (46) surmounting the layer cover (82).   - Optical recording medium according to one of claims   1 or 2, characterized in that the absorbent layer (36, 54) consists of a organic material.   Il - Optical recording medium according to one of claims   3 or 9, characterized in that the absorbent layer (78) consists of a material chosen from the group which comprises titanium, rhodium, tellurium, selenium, tellurium-based alloys, alloys based on selenium,   arsenic trisulfide and arsenic triselenium.   12 - optical recording medium according to claim 1, charac-   terized in that the indentations (32) are made in the form of a groove   (20) in a main surface (14) of the substrate (12).   13 - optical recording medium according to claim 1, charac-   terized in that the indentations (32) are produced in the form of series   alternating first (18) and second (19) regions.   14 - optical recording medium according to claim 1, charac-   terized in that part of the indentations (114) extend in the substrate (12) over a thickness which is different from that of the remaining indentations   (112), in order to form a third region having a different reflecting power   rent from those of the first (18) and second- (19) regions.   15 - Information disc provided with a track containing the recorded information, characterized in that it comprises: a substrate (12) comprising a main surface (14) provided with a plurality of indentations (32); an absorbent layer (54), deposited on the main surface (14) and filling the indentations, this layer (54) absorbing light for the wavelength 16 2490858   a recording and playback light beam, in order to form in the disc first regions (18) and second regions (19) so that the recording sensitivity, in the first regions (18), is more greater than the recording sensitivity in the second regions (19), and in that said track comprises a series of first regions (92) having optical properties different from those of the other first regions   (18) and those of the second regions (19), the length and spacing of the   said first regions (92) having different optical properties being   an indication of the information recorded.   16 - Information disc according to claim 15, characterized in that it comprises a reflective layer (52) interposed between the substrate (12) and the absorbent layer (54), this layer (52) reflecting a part   of the incident light, at the wavelengths of the beams   light recording and reading lights, the thickness of said absorbent layer (54) being chosen so as to reduce the reflecting power of the   in the early regions (18).   17 - Information disc according to claim 16, characterized in   that it includes a spacer layer (72) interposed between the reflective layer   flexing (52) and the absorbent layer (78), this spacer layer   creasing the indentations (32), and in that the thicknesses of the layer of es   pacement (72) and the absorbent layer (78) are such that the power   bending of the disc in the first regions (18) is reduced.   18 - Information disc according to any one of claims   to 17, characterized in that said indentations (32) extend in the substrate (12), from the main surface (14), over a distance comprised   between about 5 nanometers and 200 nanometers.   19 - Information disc according to claim 15, characterized in   that said substrate (12) consists of a plastic material.   - Information disc according to claim 16, characterized in that the thickness of the absorbent layer (54), in the first regions   (18), is chosen so as to minimize the reflecting power.   21 - Information disc according to one of claims 15 or 16,   characterized in that it comprises a thick coating layer (46) at the   above the absorbent layer (54). 17 2490858   22 - Information disc according to claim 17, characterized in   that the thicknesses of the spacer layer (72) and the absorbent layer   bante (78) of the first regions (18) are chosen so as to minimize reflective power.   23 - Information disc according to claim 17, characterized in   that it comprises a cover layer (82) covering the absorbent layer   bante (78), and a thick covering layer (46) surmounting the layer of cover (82).   24 - Information disc according to one of claims 17 or 23,   characterized in that the absorbent layer (78) consists of a material   chosen from the group which includes titanium, rhodium, tellurium, selec-   nium, tellurium-based alloys, selenium-based alloys, tri-   arsenic sulfide and arsenic triselenium.   - Information disc according to claim 15, characterized in that the indentations (32) are produced in the form of a groove (20) in   a main surface (14) of the substrate (12).   26 - Information disc according to claim 15, characterized in that the indentations (32) are produced in the form of alternating series   first (18) and second (19) regions.   27 - Information disc according to claim 15, characterized in that a part of the indentations (114) extend in the substrate (12) over thicknesses which are different from those of the remaining indentations (112), so as to form a third region which has optical properties   different from those of the first (18) and second (19) regions.