CH636212A5 - Medium carrying recorded information - Google Patents

Description

The present invention relates to a medium carrying information recorded in a form suitable for reading by an incident light beam, medium of the type on which the recording is carried out by means of a frequency modulation signal which contains information presenting in the form of a carrier signal which exhibits frequency variations over time, from a central frequency.

Systems have been developed for recording video frequency signals on discs, tapes or other media. Such systems have used, inter alia, optical recording on a photosensitive support, recording by electron beam on thermoplastic surfaces, and still other systems allow recording of video information that can be carried out instantaneously.

The state of the art can be broadly divided into systems using photographic surfaces, systems using electron beam sensitive surfaces, magnetic recording systems and, as in the present invention, systems in which a beam of radiated energy causes an irreversible modification of a surface, thus carrying out a writing of the information.

Photographic systems are described in U.S. Patent No. 3,234,326 (PC Goldmark et al.) Which relates to recording on a continuous medium such as a tape or film, or the U.S. Patent 'America N ° 3361873 (WR Johnson) which concerns the photographic recording of video information on a disc spinning along a spiral path.

U.S. Patent No. 3283310 (WC Hughes et al.) Illustrates the technique of recording information on a surface of a thermoplastic film using an electron beam writing apparatus as described in U.S. Patent No. 3,120,991.

Still other systems have used an electron beam to record information on a particular recording medium. Such a system is described in US Patent No. 3,350,503 (D.G. Gregg). A variant using an electron beam on a photosensitive support such as a photographic film is taught by United States patent No. 3444317 (R.F. Dubbe et al.).

In recent years, other methods have been proposed for recording at high density, methods based either on the elimination of material, or on the vaporization of material by laser beam bombardment. These processes were briefly discussed in the review "Electronics" of March 3, 1969, p. 110. In addition, a laser thermal micro-image recorder has been described in some detail in an article published in the journal "Wescon Technical Papers", vol. 12.1968, section 16/1, p. 1. The authors, MM. CO. Carison and H.D. Ives, refer to journal articles; "Science" of December 23, 1966, vol. 54, N ° 3756, pp. 1550-1551; "Proceedings of the Fall Joint Computer Conference", 1966, pp. 711-716; "Bell Systems Technical Journal", March 1968, pp. 385.405.

These publications disclose a recording technique which uses thin metal film coating on a substrate. The thin film of metal, under the effect of heat, quickly melts and forms small droplets inside a saved task. A highly concentrated spot of laser radiation can apply enough heat in a short enough time that an appropriately modulated laser beam falling on a moving surface can produce holes in the metal surface which, when re-read, may allow the recorded information to be reproduced.

As underlined in the article indicated above by MM. Carison and Ives, the size of the stain or hole recorded may be much smaller than the diameter of the incident laser beam. By a suitable choice of metallic film material, film thickness, laser divergence and spot power, a suitable system can be designed to record video frequencies with reasonably high resolution. However, the quality of the video signal reproduced from such recordings has not been found to be good due to a low signal-to-noise ratio resulting from the direct recording of the video signal on the recording medium. In addition, the use of a modulated laser beam to selectively produce a pattern of holes on the metal surface of a video disc does not contribute to an accurate representation of an analog video signal of variable amplitude. Also, reading the information recorded in the form of a track of holes or spots on the surface of a disc does not allow it to faithfully recreate an analog video signal from the track recorded on the disc. The lack of fidelity in the reproduced video signal can be attributed in the first place to the inability of the laser beam modulator to write precise analog signals into the coating of the disc. The poor writing quality is also partly due to the lack of control of the optical modulator.

The invention claimed in this patent aims to increase the quality of the reproduced signal and, more particularly, to increase the signal to noise ratio of the signal recorded on the recording medium.

According to the invention, the support carries the information in the form of a linear series of first marks and second marks, discrete and alternating, placed in a spiral track on the surface; each of the first surface marks reflects light so as to give a first level of reflectance of light under the effect of the incident light beam; each of the second marks gives a second level of reflectance of the light under the effect of the incident light beam; the first level of light reflectance is greater than the second level of light reflectance; each of the second marks has a constant dimension in the radial direction of the shaped recording medium

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disk and each of the first and second marks has a variable length to represent the frequency modulated signal; the marks are established in a heat-sensitive coating which is formed on the recording medium; each of the spiral tracks is separated from the next spiral track by a guard region; the first marks and the guard region are formed by part of the coating; the second marks are formed by openings created in the coating, and the heat-sensitive coating is formed on a transparent substrate.

This form of signal coding makes it possible to obtain a much better signal-to-noise ratio than that resulting from the application of the techniques of the prior art.

The apparatus which can be used to record a frequency modulated signal on the information-carrying medium such as, for example, a video disc, comprises a movable writing beam and a video disc which is mounted on a tray turning. The turntable is driven by a movement control device which rotates the disc so that each of its points precisely describes a circle, with a constant speed of rotation. A translational drive device communicates to the writing beam a translational movement at very constant and very low speed, along a radius of the rotating disc. The rotation of the disc is synchronized with the translation of the writing beam, so as to create a spiral track of predetermined pitch. In a preferred embodiment, the spacing between the axes of the adjacent tracks of the spiral is 2 p. The marks that are formed have a width of 1 p., Which leaves a guard band between the tracks by 1 | x between the marks of the adjacent tracks. If desired, the marks can be arranged in concentric circles, performing the translational movement step by step, instead of performing it at constant speed, as just indicated.

Advantageously, a microscope objective is placed at a constant height above the video disc, on an air bearing. This lens is used to focus the writing beam on the photosensitive surface of the video disc. It is necessary that the height is constant, because of the shallow depth of field of the lens. A dry type microscope objective, with a numerical aperture of 0.65, is used to focus the writing laser beam on a point with a diameter of 1 µ on the surface of the photosensitive coating. Since the coating rotates at a relatively high speed, the length of the mark which forms on the photosensitive coating depends on the duration for which the intensity of the light beam at the focus exceeds the value necessary to form such a mark.

The source of the writing beam consists of an ion laser with argon in linear polarization. A Pockels cell is used to rotate the plane of polarization of the writing beam relative to its fixed plane of linear polarization. A linear polarizer attenuates the writing beam which has been rotated the plane of polarization, and this attenuation is proportional to the difference between the axis of polarization of the light of the writing beam, and the axis of linear polarizer. The combination of a Pockels cell and a linear polarizer modulates the writing beam with the video information to be recorded. This modulation follows the variations which are defined by the control signals which come from a driving circuit of the Pockels cell.

The video signal to be recorded is applied to a frequency modulator. The frequency modulator output signal is a rectangular wave, the frequency of which is proportional to the level of the video signal. The duration of each cycle of the rectangular wave varies, which is characteristic of a frequency modulated signal. The rectangular wave typically has a higher voltage level and a lower voltage level. The Pockels cell driver circuit amplifies the upper and lower voltage levels of the rectangular wave, and uses them to drive the Pockels cell. The Pockels cell changes the angle of polarization of the light passing through it, in response to the instantaneous voltage level

of the control signal which comes from the drive circuit of this cell.

In a first operating mode, in the presence of a first voltage level of the rectangular control signal which is applied to the driving circuit of the Pockels cell, the light beam passes without attenuation through the combination of the Pockels cell and of the linear polarizer, and has a first intensity at the output which is sufficient to form a first mark in a photosensitive coating. When the control signal changes, and presents its second voltage level, the Pockels cell rotates the plane of polarization of the light which constitutes the writing beam, to give it a new angle of polarization. Because of this variation in polarization of the light which forms the writing beam, there appears to be a shift between the angle of polarization of the light which emerges from the Pockels cell, and the preferred angle of polarization of the linear polarizer. In this case, the linear polarizer acts as an attenuator, and lets in less light. This reduces the light intensity of the writing beam to a value lower than that which is necessary to form the first mark in the photosensitive coating.

A Pockels cell stabilization circuit detects a fraction of the writing beam, so as to maintain at a predetermined level the average power of the modulated writing beam, despite variations in the transfer characteristic of the Pockels cell , which result from small temperature variations. The stabilization circuit includes a level adjustment circuit which selectively adjusts the power level to form marks in different photosensitive coatings, which will be defined later. Different types of video disc can be used with this method and recording apparatus. Each disk has a specific configuration. In a first configuration, the video disc comprises a glass substrate whose upper surface carries a thin metallic coating which constitutes a photosensitive coating. In this configuration, the writing beam forms openings of variable length, defining tracks, in the metallic coating.

The intensity of the writing beam is adjusted so as to form, for example, an opening during each positive half-cycle of the frequency modulated signal which is to be recorded, and not to form an opening during the negative half-cycle. . Under these conditions, the first and second marks which represent the recorded information consist of a linear series of openings separated by an interposed part of the coating of the surface.

In this first configuration, part of the glass substrate is exposed in each opening. The exposed part of the glass substrate appears as a region of diffuse reflection of light, for an incident reading beam. The interposed part of the metallic coating which remains between the bare parts of the substrate appears as a region of specular reflection. We call specular reflection a reflection in which a significant part of the reflected light returns by taking the same path as the incident light beam (in the case of a normal incidence), that is to say that there is a 180 ° reversal of the path of the reflected beam, relative to the incident beam. Diffuse reflection is called a reflection in which there is no significant fraction of the incident beam reflected in the direction of this beam.

In a second configuration, the video disc comprises a glass substrate, the upper surface of which carries a thin layer of photosensitive resin, constituting the photosensitive coating. In this configuration, the writing beam forms, in the photosensitive resin, exposed and unexposed regions of variable length, which define tracks. The intensity of the writing beam is adjusted so as to form, for example, a region of photosensitive resin exposed during the positive half-cycles of the frequency modulated signal to be recorded, leaving a region of photosensitive resin not exposed during negative half cycles. Thus, the first and second marks which represent the recorded information are in the form of a linear sequence of

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exposed and unexposed parts, respectively, of the surface coating.

Reading of the recorded information can be carried out by means of a reading device advantageously comprising a reading laser which produces a collimated and polarized light beam, and which has a preferential angle of polarization. An optical reading device directs and projects the laser beam so that it reaches the marks on the surface of the video disc. The video disc is used to record on its surface a frequency modulated signal, which is in the form of a linear series of regions. These regions have alternately a specular reflection and a diffuse reflection of light. The optical reading device concentrates the reading beam into a light point with a diameter of about 1 n, and directs this concentrated point onto the linear series of regions. The intensity of the reading beam is adjusted so that the optical reading device collects a reflected reading beam which has a sufficiently high level.

A motion controller rotates the video disc at a uniform speed sufficient to reconstruct the frequency of the frequency modulated signal that was originally recorded. The frequency of a characteristic signal modulated in frequency and recorded in this way varies between 2 and 10 MHz. The rotational speed of the video disc is preferably set at about 1800 rpm, so as to convert the frequency modulated signal which is spatially recorded into an electrical signal in real time. The movement control device comprises a translational drive device which produces a translation of the reading beam, at a very low and very constant speed, along the radius of the rotating disc, so that this beam reaches the linear sequence of reflecting regions and diffusing regions which are on the disc.

The reflected reading beam collected by the optical reading device is directed to a light detection circuit which converts the reflected and intensity modulated light beam into a frequency modulated electric signal which corresponds to the reflected and intensity modulated light beam.

A polarization selective beam splitter is positioned in the path of the read beam between the read laser source and the video disc. Once the reading beam has passed through the polarization selectivity beam splitting element, this beam is in linear polarization in the preferential plane. A quarter wave plate is interposed between the output of the polarization selectivity beam splitter, and the video disc. The quarter wave plate converts the linear polarization of the light from the reading beam into circular polarization. The reflected light retains its circular polarization until it crosses the quarter-wave plate a second time. During this second passage in the quarter-wave plate, the reflected light is reconverted from the circular polarization into a linear polarization having rotated by 90 ° relative to the preferential plane which is established by the beam splitting element with selectivity of polarization, as indicated previously.

The polarization selectivity beam splitter is sensitive to 90 ° offset of the reflected light beam, and in response it deflects the reflected beam towards the light detection circuit, and it prevents the reflected light beam from returning to the laser reading source.

A divergent lens is used in the optical reading device to spread the practically parallel light beam coming from the reading laser source, so that this beam covers at least the entry opening of the objective.

It is also possible, in the optical reading device, to place an optical filter in the path of the reflected reading beam, in order to eliminate all wavelengths of light other than those of the light produced by the source. reading laser.

In a recording device, only the write function is used to record the frequency modulated information on a video disc. In a video disc reproducing apparatus,

only the read function is used to reconstruct the frequency modulated information which is recorded on the surface of the video disc. In a third mode of operation, the reading and writing functions are combined in a single device. In this combined apparatus, the reading apparatus is used to control the accuracy of the information which is recorded by the recording apparatus.

To implement the control function, the read beam from the helium-neon (He-Ne) type read laser is also introduced into the path of the write beam. The optical reading device is adjusted so as to direct the reading beam through the microscope objective, with a small angle relative to the writing beam. This angle is chosen so that the reading beam illuminates an area belonging to the track being written by the writing beam, but offset by approximately 4 to 6 | t downstream of the writing point. More precisely, the reading beam is projected onto the information track which has just been formed by the writing beam. Sufficient time is allowed for the brand carrying the information to be formed on the video disc. In this way, the reading beam is projected onto alternating regions which have different reflection characteristics. In one embodiment of the reading device, the reading beam reaches the parts of the metal which have not been heated by the writing beam, and also reaches the glass substrate which has been exposed in the openings which have just been formed by the writing light point. The regions with different reflection characteristics transform an incident reading beam of constant intensity into an intensity modulated reflected reading beam.

In this control mode, a wavelength different from that of the write laser beam is chosen for the read laser beam. An optical filter with wavelength selectivity is placed in the path of the reflected light beam, making sure that the reading laser beam is included in the pass band of the filter. Any energy of the writing laser beam which follows the path of the reflected reading beam is stopped by the filter, and therefore cannot disturb the reading operation.

The control mode is used when recording video information to the video disc to facilitate control over the quality of the signal being recorded. The output signals from the reading beam path are displayed on an oscilloscope and / or a television monitor. Visual examination of this displayed signal indicates whether the marks are formed with the preferred duty cycle. The preferred duty cycle is obtained when, on average, the length of a region with specular reflection, which represents half a cycle of the frequency modulated signal, is equal to the length of the next region, with diffuse reflection, which represents the consecutive half cycle of the frequency modulated signal.

The control mode, or read mode after recording, is also used in an error control mode, in particular in the case of the recording of information of digital type. The input video information is delayed by a duration equal to the value of the accumulated delays, from the frequency modulation of the input video information signal, during the recording operation, to the frequency demodulation of the recovered reflected signal which comes from the detection circuit, including the delay of displacement of the point on the recording medium, between the recording point of the input video information signal and the point of incidence of the reading light beam. The information retrieved is then compared with the delayed input information, to verify the agreement. The existence of too many discrepancies can serve as the basis either for a new control and a new adjustment of the device, or for the rejection of the disc.

The playback apparatus can be used with a current television receiver, by adding an HF modulator to modulate the video signal to an appropriate carrier frequency, corresponding to one of the channels of a current television receiver. The current television receiver can then process this signal in the same way as those from a normal transmitter.

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The characteristics and advantages of the invention will be better understood on reading the following description of embodiments, and with reference to the accompanying drawings in which:

fig. 1 is a block diagram of the recording apparatus; fig. 2 is a section of a video disc before it is the subject of a writing operation using the recording apparatus of FIG. 1;

fig. 3 is a partial representation, from above, of a video disc, after writing using the recording apparatus of FIG. l;

fig. 4 represents a video signal which is used in the recording apparatus of FIG. 1;

fig. 5 shows a frequency modulated signal which is used in the recording apparatus of FIG. 1;

fig. 6 is a graph which shows the intensity of the writing laser beam which is used in the recording apparatus of FIG. l;

fig. 7 is a graph which shows the modulated writing beam produced by the apparatus of FIG. 1;

fig. 8 is a section along line 8-8 of the disc shown in FIG. 3;

fig. 9 is a detailed block diagram of a motion control device of an appropriate type;

fig. 10 is a block diagram of a reading apparatus;

fig. 11 is a block diagram showing the combination of a reading apparatus and a recording apparatus;

fig. 12 is a schematic representation which shows the reading and recording beams passing through a single objective, as in the block diagram of FIG. 1, and fig. 13 is a diagram of an appropriate stabilization circuit for the recording apparatus of FIG. 1.

A given element is designated by the same reference number in the different figures. The terms recording and writing will be used without distinction, as well as the terms reproduction and reading.

Fig. 1 shows the apparatus for recording video information in the form of a frequency modulated signal, on an information recording medium 10. An information signal source 12 provides an information signal to record. This signal, present on a line 14, is a frequency modulated signal, and the information to be recorded is represented by the variations in the carrier frequency of the signal. Fig. 5 shows a typical example of a frequency modulated signal. The information signal source 12 uses a video signal circuit 16 to apply an information signal to a line 18, the information of which is represented by a voltage which varies over time. Fig. 4 shows a characteristic example of a signal consisting of a voltage which varies over time. A frequency modulator 20 is driven by the video signal circuit 16, and it converts the voltage signal varying over time to give the frequency modulated signal which is present on line 14, as shown in fig. 5.

The information recording medium 10 is mounted on a turntable 21. In the representation of FIG. 2, the support 10 bears no mark, and it comprises a substrate 22 which has a first surface 24, covered by a photosensitive coating 26. A movement control device 28 communicates a uniform movement to the recording medium 10, with respect to to a writing beam 29 ′ which is generated by a light source 30. The movement control device 28 is shown in FIG. 9, and will be described in more detail in connection with this figure. The movement control device 28 comprises a rotational drive circuit 32 which communicates a uniform rotational movement to the information recording medium 10, and a translational drive circuit 34, which is synchronized with the circuit d 'rotational drive 32, so as to radially move the concentrated light beam 29' on the coating 26. The movement control device 28

further includes an electrical synchronization circuit 36, which maintains a constant relationship between the rotational movement that the rotational drive circuit 32 communicates to the recording medium 10, and the translational movement that the translational drive circuit 34 communicates with the light beam 29.

The light source 30 provides a light beam 29 whose intensity is sufficient to produce an interaction or a modification of the coating 26, while the latter is in motion, and is positioned on the information recording medium 10, in motion . In addition, the intensity of the light beam 29 ′ is sufficient to produce in the coating 26 permanent marks which represent the information to be recorded. A suitable light source 30 includes a writing laser which produces a collimated writing beam, consisting of monochromatic polarized light.

We will now return to fig. 2 which shows a section through a first configuration of an information medium, or video disc, 10, of the appropriate type. A suitable glass substrate 22 has a first flat and smooth surface 24. The photosensitive coating 26 is formed on the surface 24.

In one of the embodiments described, the coating 26 consists of a thin and opaque metallized layer, which has suitable physical properties to allow localized heating under the action of the writing light beam 29 which comes from the source or writing laser 30. During operation, heating produces a localized melting of the coating 26, which is accompanied by a migration of the molten material towards the periphery of the molten region. After cooling, this leaves a permanent opening, like that shown at 37 in FIGS. 3 and 8, in the thin metallic coating 26. The opening 37 constitutes a type of mark used to represent information. In this embodiment, the openings 37 positioned one after the other are separated by a portion 38 of coating 26 intact. Part 38 is the second type of mark used to represent information. We will describe in more detail, in relation to FIGS. 5 to 8, how marks 37 and 38 represent the frequency modulated signal.

A mobile optical device 40 and an optical beam direction device 41 jointly define an optical path for the light beam 29 which emerges from the light source 30. These optical devices project the light beam 29 to form a point 42 on the coating 26 that carries the recording medium 10. The optical path is also represented by the line designated by the references 29 and 29 '.

A light intensity modulation device, 44, is interposed in the optical path 29, between the light source 30 and the coating 26. In its most general operating mode, the light intensity modulation device modulates intensity of the light beam 29, with the information to be recorded. The light intensity modulation device 44 operates under the control of an amplified version of the frequency modulated signal which is shown in FIG. 5. This frequency modulated signal alternates the device 44 between its high light transmission state and its low light transmission state, during each cycle of the frequency modulated signal. This rapid passage between the transmission states modulates the light beam 29 with the frequency modulated signal which must be recorded.

The light beam 29 is modulated by passing through the light intensity modulation device 44. The modulated light beam, which is now represented by the reference 29 ', is then projected onto the coating 26 by the optical devices 40 and 41. When the modulated light beam 29 'reaches the coating 26, it forms on this coating a mark which is representative of the frequency modulated signal which must be recorded.

The light intensity modulation device, 44, comprises an electrically controlled sub-assembly 46, which, under the dependence of the frequency modulator 20, gives the intensity of the

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light beam 29 'a value greater than a predetermined intensity, for which the concentrated beam 29' modifies the coating 26 carried by the information recording medium 10. In addition, under the control of the frequency modulator 20, the sub- electrically controlled assembly 46 gives the intensity of the light beam a value lower than the predetermined intensity, for which the concentrated beam 29 'does not modify the coating 26. The modifications which are formed in the coating 26 are representative of the modulated signal in frequency to record. When the coating 26 carried by the information recording medium 10 consists of a layer of photosensitive resin, the modifications are in the form of exposed and unexposed photosensitive resin elements, of size similar to that indicated above for the respective brands 37 and 38.

When the coating 26 worn by the information recording medium 10 is a metallic coating, the electrically controlled sub-assembly 46 gives the intensity of the writing beam 29 ′ a value greater than a first predetermined intensity, for which the concentrated beam 29 'melts the metallic coating, without vaporizing it, and it also gives the intensity of the writing beam a value lower than the predetermined intensity, for which the concentrated beam 29' does not melt the metal surface.

The light intensity modulation device 44 includes a stabilization circuit 48 which provides a reaction signal which is used for temperature stabilization of the operating level of the electrically controlled sub-assembly 46. This makes it possible to operate this subset between a predetermined higher light intensity and a predetermined lower light intensity. The light intensity modulation device 44 comprises a light detection circuit which detects at least part of the light beam, designated by the reference 29 ", which emerges from the electrically controlled sub-assembly 46, to give an electric signal which represents the average intensity of the beam 29 ′. The reaction signal is applied to the electrically controlled sub-assembly 46 by the lines 50a and 50b, to stabilize its level of operation.

The light detection circuit produces an electrical reaction signal which represents the average intensity of the modulated light beam 29 '. In this way, the light intensity modulation device 44 is stabilized so as to emit the light beam with a constant average power level. The stabilization circuit 48 also includes a level adjustment element which makes it possible to selectively adjust the average power level of the light beam 29 ′, to bring it to a predetermined value, in order to obtain the preferred duty cycle, in a coating 26 which may consist of metal, a photosensitive resin or any other substance.

The mobile optical device 40 comprises an objective 52 and a pneumatic bearing 54, intended to support the objective 52 above the coating 26. The laser beam 29 'which the laser source 30 generates consists of practically parallel light rays. In the absence of the lens 66, these practically parallel light rays have practically no natural tendency to diverge.

On the other hand, the inlet opening 56 of the objective 52 has a diameter greater than that of the light beam 29 '. A plano-convex divergent lens 66, positioned in the light beam 29 ′, is used to spread the light beam 29 ′, practically parallel, so that it fills at least the entry opening 56 of the objective 52.

The optical beam-directing device 41 further comprises a number of mirrors 58, 60, 62 and 64, intended to fold the light beams 29 ′ and 29 "in the desired manner. The mirror 60 is shown in the form of a flat mirror, and it is used to obtain strictly circular tracks, rather than the spiral tracks which correspond to the preferred embodiment. The spiral tracks require only one fixed mirror.

As indicated above, the light source 30 gives a polarized laser beam 29. The control sub-assembly

electric 46 rotates the plane of polarization of the laser beam 29, under the control of the frequency modulated signal. The electrically controlled subassembly can include a Pockels cell 68, a linear polarizer 70, and a Pockels cell driver 72. The Pockels cell driver 72 mainly consists of a linear amplifier , which is attacked by the frequency modulated signal which is present on line 14. The output of the Pockels cell driver 72 applies drive signals to the Pockels cell 68 so as to rotate the plane of polarization of the laser beam 29. The linear polarizer 70 is oriented in a predetermined manner with respect to the plane of origin of the polarization of the laser beam 29 which leaves the laser source 30.

Fig. 7 shows that the maximum light transmission axis of the linear polarizer 70 is perpendicular to the direction of polarization of the light coming out of the source 30. As a result of this configuration, the light coming out of the polarizer 70 is reduced to a minimum when the Pockels cell 68 does not add any polarization rotation to the writing beam 29. The light which leaves the polarizer 70 is maximum when the Pockels cell adds a rotation of 90 ° to the writing beam 29. The positioning which just described for the linear polarizer is a matter of choice. By aligning the axis of maximum light transmission of the polarizer 70 and the direction of polarization of the light coming out of the laser source 30, we would obtain transmission states opposite to those indicated above, the maximum transmission corresponding to a 0 ° rotation and the minimum transmission corresponding to 90 ° rotation. However, the recording apparatus would operate in essentially the same way. The linear polarizer 70 attenuates the intensity of the beam 29 which is offset from its natural angle of polarization. It is this attenuation effect of the linear polarizer 70 which gives a modulated laser beam 29 ′ corresponding to the frequency modulated signal. A Glan prism can be used for the linear polarizer 70.

The Pockels cell drive circuit 72 is alternately coupled to the Pockels cell 68. The reaction stabilization circuit 48 is continuously coupled to the Pockels cell 68.

We will now consider all of FIGS. 4 to 7, which represent certain electrical and optical signals which appear in the embodiment of FIG. 1. Fig. 4 shows a video signal which is generated by the video signal circuit 16. Such a video signal can typically be generated by a television camera or a video recorder reproducing a signal which has been generated by a television camera, and recorded previously . The video signal can also come from a mobile trace analyzer. The information signal which is shown in fig. 4 is typically a signal with a peak-to-peak amplitude of 1 V, and the information contained in this signal, in the form of a voltage which varies over time, is represented by a line 73. The maximum instantaneous rate of change of a characteristic video signal is limited by the bandwidth of 4.5 MHz. This video signal is of the type which can be directly represented on a television monitor.

The video signal which is shown in fig. 4 is applied to the frequency modulator 20, as shown in FIG. 1. The modulator 20 generates the frequency modulated signal 74 which is shown in FIG. 5. The signal in fig. 5 contains the same information as that of FIG. 4, but the shape of the signal is different. The information signal which is shown in fig. 5 is a frequency modulated signal in which the information is represented by a carrier signal whose frequency varies over time around a central frequency.

By examining fig. 4 and 5, it can be seen that the region of lower amplitude, generally designated by the reference 75, of the video signal 73 of FIG. 4, corresponds to the lower frequency part of the frequency modulated signal 74 of FIG. 5. A brace 76 generally designates a cycle of the lower frequency part of the frequency modulated signal 74. A region of higher amplitude of the video signal 73, generally designated by the ref 5

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rence 77, corresponds to the upper frequency part of the frequency modulated signal 74. A brace 78 designates a complete cycle of the upper frequency part of the frequency modulated signal 74. A region of intermediate amplitude of the video signal 73, designated by generally by the reference 79, corresponds to the intermediate frequency part of the frequency modulated signal 74. A brace 79a designates a cycle of the intermediate frequency part of the frequency modulated signal, which corresponds to the region of intermediate amplitude 79.

Examination of figs. 4 and 5 shows that the frequency modulator 20, shown in FIG. 1, converts the voltage signal varying over time, represented in FIG. 4, in a frequency modulated signal, shown in FIG. 5.

Fig. 6 represents the intensity of the writing beam 29 generated by the writing laser 30. It can be seen that the intensity of the writing beam 29 corresponds to a constant level which is represented by the line 80. After the initial operations setting, this intensity remains constant.

Fig. 7 shows the intensity of the writing beam 29 ′, after it has passed through the light intensity modulation device 44. The intensity modulated writing beam has several peaks 92 which represent the high transmission state of the light from the light intensity modulation device 44, and several recesses 94 which represent the low light transmission state of the light intensity modulation device 44. The line 80 which represents the maximum intensity of the laser 30 is superimposed on the signal 29 'to show that there is an attenuation of the light intensity in the device 44. This attenuation is indicated by a line 96 which shows the difference between the intensity of the light beam 29' which is generated by the laser 30, and the maximum intensity 92 of the light beam 29 ′ which is modulated by the device 44.

Examination of figs. 6 and 7 will bring out better this modulation of the intensity of the writing beam 29 to form a writing beam modulated in intensity 29 '. Fig. 6 shows the unmodulated beam 29, which has a constant intensity represented by the line 80. FIG. 7 shows the modulated beam 29 ′, which has maximum intensity levels 92, and minimum intensity levels 94.

By considering lines 98, 100 and 102, we can compare the intensity modulation of the writing beam 29, and the effect of rotation of the Pockels cell 68. The intersection between lines 98 and 29 'indicates the intensity of the beam 29 ′ which leaves the linear polarizer 70, when the Pockels cell 68 does not add any rotation to the angle of polarization of the light passing through it. The intersection of lines 100 and 29 ′ indicates the intensity of the beam 29 ′ which leaves the linear polarizer 70 when the Pockels cell 68 adds a rotation of 45 ° to the angle of polarization of the light passing through it. The intersection of lines 102 and 29 ′ indicates the intensity of the beam 29 ′ which leaves the linear polarizer 70 when the Pockels cell 68 adds a rotation of 90 ° to the angle of polarization of the light passing through it.

It will be easier to understand the formation of an opening, like the opening 37 in FIGS. 3 and 8, by the intensity modulated beam 29 ′ of FIG. 7, by comparing the two figs. 7 and 8.

The line 100 is drawn in a middle position between the intensity level 92 which represents the state of high light transmission of the device 44, and the intensity level 94 which represents the state of low light transmission of the device 44. device 44. Line 100 represents the intensity generated by device 44 when the Pockels cell 68 rotates the angle of polarization of the writing beam 29 passing through it by 45 °. In addition, line 100 represents the threshold intensity of the modulated beam 29 ′, necessary to form a mark in the photosensitive coating 26. This threshold is reached by rotating the direction of polarization of the writing beam 29 by 45 °.

By comparing figs. 7 and 8, it can be seen that an opening 37 is formed when the Pockels cell 68 rotates the direction of polarization of the writing beam 29 which crosses it through an angle ranging from 45 to 90 °, then returning to 45 °. No opening is formed when the Pockels cell 68 rotates the direction of polarization of the writing beam 29 which passes through it by an angle ranging from 45 to 0 ", and returning to 45 °.

We will now refer again to FIG. 3, which is a top view of the video disc shown in section in FIG. 8. Examination of fig. 3 makes it easier to understand how the linear sequence of reflecting and diffusing regions 38 and 37 is formed on the video disc 10. The disc 10 is preferably rotated at the speed of 1800 rpm, and marks 37 and 38 are formed on the photo-sensitive coating 26 as shown in FIG. 8. The movement control device 28, shown in FIG. 1, forms the openings 37 according to circular tracks. Reference 104 designates part of an indoor track, and reference 105 designates part of an outdoor track. A dotted line 106 represents the center line of track 105, and a dotted line 107 represents the center line of track 104. The length of a line 108 represents the distance between the center lines 106 and 107 of the adjacent tracks 105 and 104. The distance between the center lines of two adjacent runways is typically 2 | x. The length of a line 20 109 indicates the width of an opening 37. The opening has a characteristic width of 1 | t. The distance between the adjacent openings is represented by the length of a line 110. This distance between adjacent tracks, corresponding to what is called the inter-track region, has a characteristic value of 1 | X. The length of an opening is represented by a line 112, and typically varies between 1.0 and 1.5 | x. All these dimensions depend on many variable parameters of the recording device. For example, these dimensions may vary depending on the frequency range generated by the frequency modulator 20, the size of the light point 42 formed by the optical writing devices 40 and 41, and the speed of rotation that 'we chose for disc 10.

We will now refer to FIG. 9 which is a more detailed block diagram of the movement control device 28, which is also shown in FIG. 1. The rotation drive circuit 32 comprises a rotation control circuit 130, associated with an axis 132. The axis 132 is one piece with the turntable 21. The axis 132 is driven by a printed circuit motor 134. The rotational movement produced by the printed circuit motor 134 is controlled by the rotation control circuit 130, which performs a phase locking of the rotation of the turntable 21, on a signal which is generated by a color subcarrier quartz oscillator 136, belonging to a synchronization circuit 36. The synchronization circuit 36 further comprises a first divider 138 and a second divider 140. The first divider 138 reduces to the reference rotation frequency the color subcarrier frequency generated by the oscillator 136. The axis 132 is coupled to a tachometer 143 which generates a signal whose frequency indicates the exact speed of rotation of the assembly formed by axis 1 32 and the turntable 21. The tacho-50 meter signal is present on a line 142, and the rotation reference signal which comes from the first divider 138 is present on a line 144. The tachometer signal present on line 142 is applied to the rotation control circuit 130, and the rotation reference signal present on line 144 is also applied to the rotation control circuit 55. The rotation control circuit 130 performs a phase comparison between these two input signals. When the phase of the tachometer signal has an advance over the phase of the rotation reference signal, the rotation speed is too high, and the rotation control circuit 130 60 generates a signal which is applied to the motor 134 by a line. 146, in order to slow down the rotation speed and bring the tachometer signal into phase agreement with the rotation reference signal. When the comparison made by the rotation control circuit 130 shows that the phase of the tachometer signal delays 65 with respect to the phase of the rotation reference signal, the rotation speed is too low, and the control circuit of rotation 130 generates a signal which is applied to the motor 134 by a line 148 to increase the speed of rotation and bring the phase of the

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tachometer signal in agreement with that of the rotation reference signal.

The second divider 140 reduces the frequency of the color subcarrier generated by the oscillator 136 to a translation reference frequency which is used to advance the translation drive circuit 34 over a fixed distance, at each revolution. complete of the disc 10. In the preferred embodiment, the translational drive circuit 34 advances by a distance of 2 p., for each revolution of the disc 10.

The color subcarrier quartz oscillator 136 and its two dividers 138 and 140 constitute an electrical synchronization circuit which maintains a constant relationship between the rotational movement of the disc, produced by the rotational drive device 32, and the translational movement between the writing beam 29 and the covering 26, which is produced by the translational drive device 34.

The mobile optical devices which are represented in FIGS. 1, 10 and 11 are mounted on a platform designated by the reference 142. This mobile platform is driven radially by the translational drive device 34, which advances it by 2.0 (i, per revolution of the axis 132. This translational movement has a radial direction with respect to the rotating disc 10. This radial advance for each revolution of the axis 132 corresponds to what is called the step of the recording. uniformity of the pitch of the completed recordings depends on the regularity of the advance of the optical devices which are mounted on the platform 142, care must be taken to run in a return screw 143 which belongs to the drive device in translation 34, preload a translational drive nut 144 which is screwed onto the return screw 143, and establish a coupling as rigid as possible, represented by a bar 146, between the nut 144 and the platform 142.

We will now refer to FIG. 10 which represents a reading apparatus which is used to reproduce the frequency modulated signal which is recorded on the information recording medium 10, in the form of a linear series of marks 37 and 38, in the manner previously described. A read laser 152 generates a read beam 150, which is a polarized and collimated light beam. A support member, such as the turntable 21, is used to hold the information recording medium 10 in a substantially predetermined position.

A fixed optical reading device 154 and a mobile optical device 156 define an optical reading path along which the reading light beam 150 propagates between the laser 152 and the information recording medium 10. In addition, it is possible to use one or the other of these optical devices to concentrate the light beam 150 on the reflecting regions 38 and the diffusing regions 37 which are arranged alternately in successive positions, on the information recording medium 10. uses the mobile optical device 156 to collect the light which is reflected by the reflecting regions 38 and the diffusing regions 37. The movement control device 28 establishes a relative movement between the reading beam 150, on the one hand, and the regions reflective 38 and diffusing regions 37, on the other hand.

The optical devices 154 and 156 also define the optical path traversed by the beam which is reflected from the coating 26. The path of the reflected beam is designated by the reference 150 '. This reflected beam path 150 ′ comprises part of the path of the initial reading beam 150. In the parts in which the reflected beam 150 ′ coincides with the reading beam 150, the two references 150 and 150 ′ are used. A light detection element 158 is positioned in the path of the reflected light beam 150 ', and it generates a frequency modulated electrical signal which corresponds to the reflected light it receives. The frequency modulated electrical signal generated by the light detection element 158 is applied to a line 160, and the information it contains is represented by a carrier frequency which varies over time, corresponding to the information recorded. The output signal of the light detection circuit 158 is applied to a discriminator 162 by an amplifier 164. The discriminator 162 is sensitive to the output signal of the light detection circuit 158, and it converts the frequency modulated electrical signal into a voltage signal, varying over time, which represents the information recorded. The voltage signal varying over time is also called video signal, and is applied to a line 165. This voltage signal varying over time contains information which is represented by a voltage which varies over time, and it can be displayed on a conventional television monitor 166 and / or on an oscilloscope 168.

The optical reading devices 154 and 156 further include a polarization selectivity beam splitter 170, which functions as a beam polarizer for the incident beam 150, and a selective beam splitter for the reflected beam 150 '. The optical reading devices also comprise a quarter-wave plate 172. The beam polarizer 170 removes from the reading beam 150 all the wavelengths whose directions of polarization are not aligned with the axis of polarization of this polarizer. The axis of polarization of the reading beam 150 being fixed in a particular orientation by the polarizer 170, the quarter-wave plate 172 converts the linear polarization into circular polarization. The polarizer 170 and the quarter wave plate 172 are arranged in the path of the reading light beam 150. The polarizer 170 is located between the source 152 of the reading beam 150 and the quarter wave plate 172. The quarter plate d 'wave 172 is also in the path of the reflected reading beam 150'. Thus, the quarter-wave plate 172 not only has the effect of converting the linear polarization of the reading beam into a circular polarization, during the propagation of the beam between the reading laser 152 and the information recording medium 10, but also to convert the light reflected in circular polarization into light in linear polarization which is offset by 90 ° relative to the preferred direction which is defined by the source 152 and the polarizer 170. The beam 150 ′ which has undergone a rotation of its polarization direction is selectively directed towards the light detection element 158, which transforms it into a corresponding electrical signal. It will be noted that the polarizer 170 reduces the intensity of the incident light beam 150 which passes through it. This reduction in intensity is compensated for by adjusting the initial intensity of the reading beam 150 to a sufficient value.

The quarter-wave plate 172 gives the reflected beam 150 ′ a total rotation of 90 ° relative to the incident beam 150, during the transition from linear polarization to circular polarization, and from return to linear polarization. As previously mentioned, the polarizer 170 also constitutes a beam splitting cube which is in the path of the reflected beam 150 '. As the plane of polarization of the reflected reading beam 150 ′ is offset by 90 °, due to its double passage through the quarter wave plate 172, the part of the polarizer 170 which constitutes the beam splitting cube directs the beam of reflected reading 150 ′ to the light detection circuit 158. The light detection element 158 may consist of a photodiode. Element 158 is capable of converting the frequency modulated and reflected light beam 150 ′ into an electrical signal, the information of which is represented by frequency variations over time, around a carrier frequency. The optical devices 154 and 156 further comprise the objective 52 which is supported by a pneumatic bearing 54, above the covering 26 which the information recording medium 10 carries.

As indicated above, the reading beam 150 is formed by practically parallel light rays. The entrance opening 56 of the objective 52 has a diameter greater than that of the reading beam 150 which is generated by the laser source 152. A divergent plano-convex lens 174 is placed between the laser source 152 and the opening entry 56 of the lens 52, to spread the practically parallel light rays which form the reading beam 150, to give a light beam 150 whose diameter is sufficient to cover at least the entry opening 56 of the goal 52.

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The optical devices 154 and 156 further comprise a number of fixed plane mirrors 176 and 178, for folding the reading light beam 150 and the reflected light beam 150 ', along a path which is determined so as to reach the previously mentioned items.

An optional optical filter 180 is placed in the path of the reflected beam 150 ', and eliminates all wavelengths other than that of the incident beam. The use of filter 180 improves the quality of the image shown on the television monitor 166. This filter is essential when using the playback device with the recording device, as will be described below. more detailed in relation to fig. 11. As part of this reading operation after recording, part of the writing beam 29 propagates along the path of the reflected reading beam 150 ′. The filter stops this part of the writing beam, and lets the reflected beam 150 ′ pass, without reduction in intensity.

An optional converging lens 182 is placed in the path of the reflected beam 150 ', so as to project the reflected beam onto the active surface of the light detection element 158. This converging lens reduces the diameter of the reflected beam 150', and concentrates the light intensity of the reflected beam on the active surface of the light detection element 158.

Amplifier 164 amplifies the output signal from light detection element 158 and raises the amplitude of the frequency modulated electrical signal which is generated by light detection element 158, so as to correspond to the signal level input required for demodulator 162.

We will now consider again the electrical and optical signals which are represented in FIGS. 4 to 7. These signals are also generated in the reading device, shown in FIG. 10, during the reproduction of the frequency modulated signal which has been recorded in the coating 26 carried by the disc 10. In FIG. 6, line 80 represents the constant intensity of a writing laser beam which is generated by a laser source. The read laser 152 generates a read beam 150 of constant intensity, but of lower level.

Fig. 7 represents a writing laser beam which is intensity modulated. The reflected reading beam 150 ′ is modulated in intensity because it meets the reflecting regions 38 and the diffusing regions 37 carried by the disc 10. The reflected reading beam 150 ′ is not a perfectly square signal such as that which is shown in fig. 7. On the contrary, the corners of this signal are rounded, due to the finite size of the reading light point.

Fig. 5 shows a frequency modulated electrical signal which contains information which is represented by a carrier signal whose frequency varies over time, around the central frequency. The output signal from the light detection element 158 is of the same type. Fig. 4 represents a video signal which contains information in the form of a voltage varying over time. The output signal from demodulator 162 is of the same type.

The movement control device 28 which is shown in FIG. 10 operates in the same manner as the motion control device 28 of FIG. 1. In the reading device, the movement control device 28 produces a rotation movement of the disc, under the control of the rotation drive device 32. The device 28 also produces a translational movement, intended to move radially the mobile optical reading device, on the surface of the recording medium.

The device 28 further comprises a synchronization circuit which maintains a constant relationship between the rotational movement and the translational movement, so that the read beam 150 meets the information tracks carried by the disc 10. References 104 and 105, in fig. 3, designate parts of characteristic information tracks.

We will now refer to FIG. 11 which is a block diagram showing the combination of the writing apparatus of FIG. 1 and the reading apparatus of FIG. 10. The elements shown in fig. 11 operate in a manner identical to that which has been

described above, and we will not repeat here the description of this detailed operation. We will limit ourselves to a brief description to avoid repetition and the risk of confusion.

The path of the unmodulated write beam is shown at 29, and the path of the modulated beam is shown at 29 '. A first optical device defines the path of the modulated beam 29 ′, between the output of the linear polarizer 70 and the coating 26. The fixed optical writing device 41 comprises the mirror 58. The mobile optical writing device 40 comprises the diverging lens 66, a partial transmission mirror 200, a plane mirror 60 and the objective 52. The modulated writing beam 29 ′ is projected at a writing point 42, on the photosensitive coating, and reacts with this coating to form a brand, as previously described. The path of the reading beam is shown at 150. The optical reading devices define a second optical path for the reading beam 150, between the reading laser 152 and the information recording medium 10. The fixed optical device of reading 154 includes the mirror 176. The mobile optical reading device 156 comprises the divergent lens 174, the polarization shift element 172, a second fixed mirror 202, the selective transmission mirror 200, the plane mirror 60 and the lens 52 The reading beam 150 is projected at a reading point 157, which is located downstream of the recording point 42, as will be described more fully in relation to FIG. 12. The mirror 200 is a dichroic mirror which lets in the light which corresponds to the wavelength of the writing beam 29 ', and which reflects the light which corresponds to the wavelength of the reading beam 150'.

The intensity of the writing beam 29 'is greater than that of the reading beam 150. While the writing beam 29' must modify the photosensitive coating 26 so that it retains marks representative of the video signal to be recorded, the the intensity of the reading beam 150 need only be sufficient to illuminate the marks formed in the coating 26, and to give a reflected light beam 150 ′ of sufficient intensity to cause a good signal to appear after collection by the optical reading and conversion device of the intensity modulated reflected beam 150 ′ into an electric signal modulated in frequency, by the light detection circuit 158.

The fixed mirror 58 which is in the optical writing path and the two fixed mirrors 176 and 202 which are in the optical reading path are used to direct the writing beam 29 'towards the objective 56, according to a angle defined with respect to the reading beam 150. This angle between the two incident beams produces a separation between the writing light point 42 and the reading light point 157, when these beams are projected onto the covering 26.

It has been found that a spacing of 4 to 6 n is sufficient. This distance corresponds to an angle that is too small to appear clearly in FIG. 12. This angle is therefore exaggerated in FIG. 12 simply for clarity of the figure.

The read beam 150 'is demodulated in a discriminator 162, and is shown on a conventional television monitor 166 and on an oscilloscope 168. The television monitor 166 shows the image quality of the recording, while the oscilloscope 168 shows the video signal in more detail. This playback after recording function allows you to instantly control the quality of the video signal that is recorded during a recording operation. If the quality of the recorded signal is not good, we know it immediately, and we can correct the writing operation, or we can reject the information recording medium 10 which records the signal d poor quality video information.

In the read after recording mode, the write laser 30 and the read laser 152 operate simultaneously. The dichroic mirror 200 is used to incorporate the reading beam 150 into the writing beam 29 '. In this reading mode after recording, a different wavelength is chosen for the writing beam 29 and for the reading beam 150. The optical filter 180 is used to stop any fraction of a writing beam

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which followed the path of the reflected reading beam. Thus, the optical filter 180 transmits the reflected reading beam 150 'and stops any part of the writing laser beam 29' which follows the path of the reflected reading beam 150 '.

In the comparison or control mode, use is made of the operation of reading after recording, as it has been described in relation to FIG. 11. In this control mode, a comparison circuit 204 compares the output signal of the demodulator 162 with the original video information signal which comes from the source 18.

More precisely, the video output signal from the discriminator 162 is applied to a comparator 204, via a line 206. The other input signal from comparator 204 comes from the video signal source 16, by via line 18, an additional line 208, and a delay line 210. The delay line 210 communicates to the input video information signal a delay which is equal to the sum of the delays from the frequency modulation of the input video information signal, until the frequency demodulation of the electrical signal reproduced by the detection circuit 158. This delay also includes the travel time between the point of the recording medium 10 at which the input video information signal is recorded by the writing light point 42 and the point of incidence of the reading light point 157.

To obtain a correct value delay, it is preferable to use, for the delay line 210, a variable delay line which is adjusted to give optimum operation.

The output video signal of the discriminator 162 is theoretically identical in all respects to the input video signal on lines 18 and 208. All the differences which one notices represent errors which can be produced by imperfections of the surface. disc, or malfunctions of the write circuits. This identity, essential in the case of recording digital information, is less critical when recording information of another type.

The comparator 204 output signal can be counted using a counter (not shown) to determine the actual number of errors present on any disk. When the number of errors counted exceeds a number chosen in advance, the recording operation is terminated. If necessary, a new disc can be recorded. Any disc with an excessive number of errors can then be reprocessed.

In fig. 11, the comparator 204 compares the output signals which are present on the lines 208 and 206. According to a more direct variant, the comparator 204 can compare the output signal of the frequency modulator 20, and that of the amplifier 164 which is shown in fig. 10.

We will now refer to FIG. 12 which represents, in an exaggerated form, the slightly different optical paths of the intensity modulated writing beam 29 ′ which comes from the writing laser 30, and of the unmodulated reading beam 150, which comes from the reading laser 152. The information recording medium 10 moves in the direction indicated by arrow 217. It can be seen that an unexposed coating 26 'approaches the writing beam 29',

while a linear series of openings 37 leaves the intersection of the writing beam 29 'with the coating 26. The writing beam 29' coincides with the optical axis of the microscope objective 52. The axis of the reading beam 150, designated by the reference 212, makes a certain angle with the axis of the writing beam 29 ', designated by the reference 214. This angle is indicated by a double arrow 216. Because of this slight difference between the optical paths of the writing beam 29 ′ and of the reading beam 150 in the objective 52, the writing light point 42 falls a certain distance in front of the reading light point 157. The advance of the point d writing 42 with respect to reading point 157 is equal to the length of line 218. The length of line 218 is equal to the product of the angle between the beams and the focal distance of objective 52. The resulting delay between writing and reading allows solidification of the molten metal of the coating 26, so that the recording be read in its final solidified state. If the reading were carried out too early, while the metal was still molten, the reflection from the edge of the opening would prevent obtaining a good quality signal for display on the monitor 166.

We will now refer to FIG. 13 which shows a block diagram of a Pockels cell stabilization circuit 48, which can be used in the apparatus of FIG. 1. It is known that a Pockels cell 68 rotates the plane of polarization of the applied writing light beam 29, as a function of an applied voltage, such as that which is shown in FIG. 7.

For a particular type of Pockels 68 cell, a voltage variation of the order of 100 V causes the plane of polarization of the light passing through the cell to rotate by 90 °. The Pockels cell driver amplifies the output signal from the information signal source 12, to give a peak-to-peak output excursion of 100 V. This gives an appropriate drive signal for the Pockels cell 68. The Pockels cell driving circuit 72 generates a signal which has the form shown in FIG. 5, and a peak-to-peak voltage excursion of 100 V.

The Pockels cell must be operated with an average rotation of 45 ° so that the intensity of the modulated light beam very faithfully reproduces the electrical attack signal. A bias voltage must be applied to the Pockels cell to maintain the cell at this average operating point. In practice, the electrical bias voltage which corresponds to an operating point giving a rotation of 45 ° varies continuously. This continuously varying bias voltage is generated using a feedback or feedback loop. This feedback loop involves the comparison between the average value of the light which is transmitted and an adjustable reference value, and the application of the reference signal to the Pockels cell, via a continuously operating amplifier. . This configuration stabilizes the operating point. The reference value can be adjusted to correspond to the average transmission corresponding to the operating point with a rotation of 45 °, and the control loop provides the correction bias voltages which maintain the Pockels cell at this average rotation. 45 °.

The stabilization circuit 48 includes a light detection element 225. This element can be constituted by a silicon diode. The diode 225 detects a fraction 29 "of the writing beam 29 'which comes from the optical modulator 44, and which passes through the partial reflection mirror 58, as shown in Fig. 1. The silicon diode 225 operates in a way very similar to a solar cell, and constitutes a source of electrical energy when it is illuminated by incident radiation. One of the terminals of the silicon diode 225 is connected to the common reference potential point 226, by a conductor 227. The other terminal of the diode 225 is connected to an input of a differential amplifier 228 by a conductor 230. A load resistor 232 is connected between the terminals of the silicon diode 225, to give the diode a response linear.

The other input of the differential amplifier 228 is connected by a conductor 238 to the cursor 234 of a potentiometer 236. One end of the potentiometer 236 is connected to the reference potential point 226 by a conductor 240. A power source 242 is connected to the other end of the potentiometer 236, which makes it possible to adjust the differential amplifier 228 so that it applies to conductors 244 and 246 a feedback signal allowing the average power level of the beam to be adjusted to a predetermined value modulated laser 29 '.

The output terminals of the differential amplifier 228 are connected by respective resistors 248 and 250, and the output conductors 244 and 246 at the input terminals of the Pockels cell 68 which is shown in FIG. 1. The Pockels cell drive circuit 72 is alternately coupled to the Pockels cell 68, by capacitors 252 and 254 while the differential amplifier 228 is continuously coupled to the Pockels cell 68.

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The circuit which has just been described operates in the following manner. The circuit being energized, the fraction 29 "of the light of the writing beam 29 'which reaches the silicon diode 225 causes a differential voltage to appear on an input of the differential amplifier 228. The potentiometer 236 is initially adjusted so as to that the average transmission of the Pockels cell corresponds to a rotation of 45 ° Then, if the average level of the light intensity which reaches the silicon diode 225 increases or decreases, the differential amplifier 228 generates a correction voltage. The correction voltage which is applied to the Pockels cell 68 has an appropriate polarity and level to bring the average intensity level back to the predetermined level which has been chosen by adjusting the input voltage which is applied to the another input of the differential amplifier via line 238, this adjustment being effected by moving the cursor 234 along the potentiometer 236.

The cursor 234 of the potentiometer 236 constitutes the means which makes it possible to select the average level of the intensity of the light generated by the writing laser 30. The best results are obtained when the length of an opening 37 is exactly equal to the length of the next space 38, as indicated above. It is the adjustment of potentiometer 236 which makes it possible to achieve this equality of length. When the length of an opening is equal to the length of the adjacent space, a 50% duty cycle is obtained. This cyclical relationship can be detected by examining, on the television monitor 166 and / or the oscilloscope 168, the display of the information which has just been written, as indicated above. Acceptable results are obtained for commercial use when the length of an opening 37 varies between 40 and 60% of the combined length of an opening and the space following it. In other words, we measure the length of an opening and the space that follows it. The opening can then have a length of between 40 and 60% of the total length.

We will now refer to FIG. 8 which shows a section of an information track shown in FIG. 3, in which a region of specular light reflection 38 is placed between two regions of diffuse light reflection 37. In the section which is shown in FIG. 8, the incident reading or writing beam moves relative to the recording medium 10 in the direction which is represented by the arrow 217. This means that a reading beam first reaches the region of specular reflection 38a , then reaches the region with diffuse reflection 37a. In this configuration, the positive half-cycle of the signal to be recorded is represented by the specular reflection region 38a and the negative half-cycle of the signal to be recorded is represented by the specular reflection region 37a. The duty cycle of the signal which is shown in fig. 8 is 50%, insofar as the length of the region of specular reflection 38a, designated by a brace 260, is equal to the length of the region of diffuse reflection 37a, designated by a brace 262. This cyclic ratio is obtained preferred by the combination of the following adjustments: adjustment of the absolute intensity of the writing beam 29; adjustment of the power of the writing laser 30; setting the potentiometer 236 of the stabilization circuit 48 to a level for which an opening is formed from a 45 ° rotation of the polarization angle of the writing beam 29.

We will now return to the opening formation operation which is shown in FIGS. 7 and 8, to note that the melting of the thin metallic coating 26 occurs when the power in the light point exceeds a threshold which is characteristic of the composition and the thickness of the metal layer, and of the properties of the substrate. The power of the light point is modulated by the light intensity modulation device 44. It is made so that the transitions between the two powers are short, to precisely define the ends of the holes, despite variations in the threshold of fusion. These variations in the melting threshold may result from variations in the thickness of the metal coating and / or the use of a different material for the information recording layer.

It takes an average power of the order of 200 mW for the light point to form an opening in a thin metallic coating 26 with a thickness of between 20 and 30 nm. Because the carrier frequency of the frequency modulated signal is about 8 MHz, 8 x 106 holes of variable length are cut per second, and the energy per hole is 2.5 x 10-9J.

In a first embodiment of a video disc 10, part of the glass substrate is exposed in each opening. The exposed part of the glass substrate appears as a region with diffuse reflection, for an incident reading beam. The part of metallic coating which remains between the successive openings appears as a region with high specular reflection for an incident reading beam.

When forming the first and second marks using a coating of photosensitive resin, the intensity of the writing beam 29 'is adjusted to a level such that a rotation of 45 ° of the plane of polarization produces a light beam 29' having a threshold intensity for exposing and / or reacting to the photosensitive resin coating 26, while the photosensitive resin coating is in motion and is positioned on the information recording medium 10. The combination of the Pockels 68 cell and Glan 70 prism form an element of modulation of the light intensity which, starting from the established rotation conditions of 45 °, gives a state with lower light transmission, associated with a rotation angle close to 0 °, and a state of strong light transmission, associated with a rotation angle close to 90 °. When the intensity of the writing light beam 29 'rises above the initially set level, or predetermined starting intensity, and increases as it approaches the state of strongest light transmission, the light beam d 'incident writing 29' exposes the photosensitive resin which it illuminates. This exposure continues after the intensity of the writing beam has reached the conditions corresponding to the state of maximum light transmission, and begins to decrease again towards the predetermined initial intensity which is associated with a rotation of 45 ° of the plane of polarization of the light which comes from the writing laser 30. When the rotation takes a value lower than 45 °, the intensity of the writing beam 29 'which leaves the prism of Glan 70 becomes lower than l threshold intensity above which the concentrated writing beam no longer exposes the photosensitive resin which it illuminates. The absence of exposure of the illuminated photosensitive resin continues after the intensity of the writing beam has reached the conditions corresponding to the minimum state of light transmission, and begins to increase again in the direction of the initial intensity. predetermined which is associated with a rotation of 45 ° of the plane of polarization of the light which leaves the writing laser 30.

The Pockels 72 cell driver typically consists of a high gain, high voltage amplifier which provides an output signal having a voltage excursion of 100 V. This signal is intended to satisfy the requirements for attack of the Pockels cell 68. This typically means that the mid-voltage value of the output signal of the Pockels cell drive circuit 72 corresponds to a control voltage sufficient to produce a rotation of 45 ° in the Pockels cell 68, so that approximately half of the total light available at the output of the laser 30 comes out of the linear polarizer 70. When the output signal of the driver 72 becomes positive, the fraction of the light of the laser which is transmitted increases. When the output signal of the driving circuit 72 becomes negative, the fraction of the light of the laser which is transmitted decreases.

In the first embodiment, which uses a metallic coating 26, the output power of the laser 30 is adjusted so as to give an intensity which begins to melt the metallic coating 26, positioned on the disc 10, when the output signal of the driver 72 is zero, and the operating point of the Pockels cell corresponds to a rotation of 45 °. Thus, when the output signal of the driving circuit 72 becomes positive, the fusion takes place

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continues. In addition, when the output of the driver 72 becomes negative, the fusion ceases.

In a second embodiment, which uses the photosensitive resin coating 26, the output power of the laser 30 is adjusted so as to produce an intensity which both illuminates and exposes the photosensitive resin coating 26 when the signal output of the driver 72 is at mid-voltage. Thus, when the output signal from the driving circuit 72 becomes positive, the writing beam continues to illuminate and expose the photosensitive resin. In addition, when the output signal of the driving circuit 72 becomes negative, the photoresist continues to be lit, but the energy of the writing beam is insufficient to expose the lit region. term expose in its technical sense, which denotes the physical phenomenon which occurs in an exposed photosensitive resin. The exposed photosensitive resin can be developed, and the developed resin can be removed by conventional techniques. The photosensitive resin which is illuminated by a light of insufficient intensity to achieve an exposure cannot be developed and removed.

In the first and second embodiments which have just been described, the absolute power level 80 is increased or decreased, which is represented by the line 80 in FIG. 6, to obtain this effect, by adjusting the power supply to the writing laser 30. We use

also the potentiometer 236, in association with this adjustment of the absolute power level of the writing laser 30, to form marks in the coating 26 when the plane of polarization of the beam 29 rotates by more than 45 °, as described previously.

In a device operating only in reproduction, such as that of FIG. 10, the optical filter 180 is optional and is usually not necessary. Its use in a device operating only in reproduction introduces a slight attenuation in the path of the reflected light, which requires a slight increase in the intensity of the reading laser 152, to obtain at the level of the detector 158 the same intensity as in a device operating only in reproduction and not using the filter 180.

The converging lens 182 is optional. In a suitably produced reproducing apparatus, the reflected reading beam 150 ′ has practically the same diameter as the useful surface of the photodetector 158. If this is not the case, a converging lens 182 is used to concentrate the reflected reading beam 150 'on the less extensive useful surface of the photodetector 158 that has been chosen.

Of course, various modifications can be made by those skilled in the art to the devices or methods which have just been described only by way of nonlimiting examples, without departing from the scope of the invention.

6 sheets of drawings

Claims (3)

636,212 1. Support carrying information recorded in a form suitable for reading by an incident light beam, support on which the recording is carried out by means of a signal in frequency modulation which contains information presented in the form of a carrier signal which exhibits frequency variations over time, from a central frequency, characterized in that it carries the information in the form of a linear series of first marks (38) and second marks (37), discrete and alternating, placed in a spiral track on the surface; each of the first surface marks reflects light so as to give a first level of reflectance of light under the effect of the incident light beam; each of the second marks gives a second level of reflectance of the light under the effect of the incident light beam; the first level of light reflectance is greater than the second level of light reflectance; each of the second marks has a constant dimension in the radial direction of the disc-shaped recording medium and each of the first marks and second marks has a variable length to represent the frequency modulated signal; the marks are established in a heat-sensitive coating (26) which is formed on the recording medium; each of the spiral tracks is separated from the next spiral track by a guard region; the first marks (38) and the guard region (110) are formed by a part of the coating; the second marks are formed by openings created in the coating; and the heat-sensitive coating is formed on a transparent substrate (22). 2. Information carrier according to claim 1, characterized in that the heat-sensitive coating (26) is a metallic coating. 3. Information carrier according to claim 1, characterized in that the substrate (22) is made of glass.