NO783283L - PROCEDURE FOR READING AN INFORMATION SIGNAL STORED ON A REGISTRATION ELEMENT, AND OPTICAL SYSTEM FOR RECOVERY OF SUCH A SIGNAL - Google Patents

Description

The invention relates to recording or writing a frequency-modulated electrical signal on an information-bearing surface of a video disc element in the form of a linear series of first and second characters which are arranged in a track-like manner on the surface.

The device for writing a frequency-modulated signal on a video disc element comprises a movable writing beam and a video disc element which is mounted on a turntable or disc counter lark. The turntable is driven by a motion control assembly which rotates the disc precisely in a circle at a constant rotational speed, and a translational drive assembly to translate the writing beam at a very constant and very low speed along a radius of the rotating disc. The rotational drive of the disk is synchronized with the translational drive of the writing beam to form a spiral track of predetermined pitch. In a preferred embodiment, the distance between adjacent tracks of the spiral is two ym from center to center. The characters are formed with a width of one ym. This leaves a gap or guard area of one ym between characters in adjacent slots. If desired, the characters can be formed as concentric circles by translating in incremental steps instead of translating at a constant rate as just described.

In a preferred embodiment, a microscope objective lens is positioned at a constant height above the video disc element on an air bearing. This objective lens is used to focus the writing beam on the photosensitive surface of the video disc element. The constant height is necessary because of the short focal length of the objective lens. A 0.65 NA dry microscope objective lens is used to focus the writing laser beam onto a one µm diameter spot on the photosensitive coating. Because the coating rotates at a relatively high speed, the length of the characters formed in the photosensitive coating depends on the length of time that the spot intensity exceeds that necessary to form such a character.

A linearly polarized argon ion laser is used as the source for the writing beam. 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 weakens the rotated writing beam to a degree that is proportional to the difference in polarization between the light in the writing beam and the axis of the linear polarizer. The combination of a Pockels cell and a linear polarizer modulates the write beam with the video information to be stored. This modulation follows the pattern provided by control signals provided by a Pockels cell driver.

The video signal to be recorded is supplied to a frequency modulator circuit. The output signal from the modulator circuit is a square wave whose frequency is proportional to the video signal. The duration of each cycle of the square waveform is variable as is characteristic of a frequency modulated signal. As is characteristic of a square wave, it has an upper voltage level and a lower voltage level. The square wave's upper and lower voltage levels are amplified by a Pockels cell driver and used to control the Pockels cell. The Pockels cell. changes the polarization angle of the light passing through it in response to the instantaneous voltage level of the control signal supplied by the Pockels cell driver.

In a first mode of operation responsive to the one voltage level of the quadrature control signal applied to a Pockels cell driver, the light beam passes unhindered through the Pockels cell and linear polarizer combination with a first intensity sufficient to form a first character in a photosensitive coating. When the control signal changes so that it represents its second voltage level, the Pockels cell turns the polarization of the light that makes up the writing beam to a new polarization angle. Due to this change in polarization of the light forming the writing beam, a certain adaptation occurs between the polarization angle of the light coming from the Pockels cell and the preferred polarization angle of the linear polarizer. In this situation, the linear polarizer acts as an attenuator and less light passes through the linear polarizer. This reduces the light intensity of the writing beam below the intensity required to form such a first character in the photosensitive coating.

A portion of the write beam is sensed by a Pockels cell stabilization circuit to maintain the average power of the modulated write beam at a predetermined level despite changes in the Pockels cell transfer characteristics produced by small temperature variations. The stabilization circuit comprises a level adjustment circuit for selectively adjusting the power level to form a character in different light-sensitive coatings as described later.

Different types of video disc elements can be used

in connection with this writing method and writing device. Each such element has a different configuration. In a first configuration, the video disc element comprises a glass substrate, with an upper surface bearing a thin metal coating as a photosensitive coating. In this configuration, the writing beam forms openings of variable length in a track-like manner in the metal coating.

The intensity of the writing beam is adjusted or set so that an opening is formed, for example, during each positive half-period of the frequency-modulated signal to be stored, and no opening is formed during the negative half-period. The first and second characters which are representative of the stored information are consequently a linear series of openings which are separated by an intermediate portion of the surface coating.

In this first configuration, a portion of the glass substrate is exposed in each opening. The uncovered part of the glass substrate appears as an area of non-specular light reflectivity for an incident reading beam. The intermediate part of the metal coating that remains between the openings appears as an area of specular light reflectivity, and with the help of this device of specular reflectivity, a significant part of the reflected light returns along the path of the incident light beam, i.e. by 180° reversal in paths between the incident and reflected ray paths. Non-specular reflectivity means that no significant part of the incident beam is reflected along the path of the incident beam.

In a second configuration, the video disc element comprises a glass substrate with a surface bearing a thin layer of light-resistant material ("photeresist") as a light-sensitive coating. In this configuration, the writing beam forms areas of variable length of exposed and unexposed photoresist material in a track-like manner in the photoresist coating. The intensity of the writing beam is set so that an area of exposed or uncovered photoresist material is formed, for example, during positive half-periods of the frequency-modulated signal to be stored, and an area of unexposed photoresist material remains during the negative half-periods. The first and second characters which are representative of the stored information are consequently a linear series of exposed and unexposed parts of the surface coating.

A preferred embodiment of a reading device uses a reading laser to produce a polarized, collimated light beam with a preferred polarization angle. An optical reading system directs and images the laser beam so that it falls on the characters carried by the surface of the video disc element. The video disc element is used for storing a frequency-modulated signal on the element's surface in the form of a linear series of areas. The areas are alternately specular light-reflective and non-specular light-reflective. An optical system for reading focuses the reading beam towards a point of light which is approx. 1 ym in diameter, and directs the focused point so that it hits the linear array of areas. The intensity of the reading beam is set so that a sufficiently strong reflected reading beam signal is picked up by the optical reading system.

A motion control assembly rotates the video plastic element at a uniform speed sufficient to reconstruct the frequency of the originally stored, frequency-modulated signal. A typical frequency modulated signal stored in this way varies in frequency between 2 MHz and 10 MHz. The rotation speed of the video disc element is preferably set to approx. 1800 revolutions per

minute to change the spatially stored, frequency modulated signal into an electrical signal in real time. The motion control assembly includes a translation drive assembly to translate the read beam at a very constant and very low speed along the radius of the rotating disk so that the beam falls on the linear array of light-reflecting and light-scattering areas on the disk.

The reflected reading beam captured by the optical reading system is directed to a light-sensing circuit to change the intensity-modulated reflected light beam into a frequency-modulated

electrical signal corresponding to the intensity-modulated, reflected light beam.

A polarization-selective, beam-splitting element is placed in the read beam path between the read laser source and the video disc element. After the reading beam passes through the polarization-selective beam-splitting element, the reading light beam is linearly polarized in the preferred plane. A quarter-wave plate is placed between the output of the polarization-selective, beam-splitting element and the video plate element. The quarter-wave plate changes the light in the reading beam from linear polarization to circular polarization. The reflected light retains its circular polarization until it passes through the concave wave plate a second time. During this second passage through the quarter-wave plate, the reflected light is changed by circular polarization back to linearly polarized light which is rotated 90° from the preferred plane established by the polarization-selective, beam-splitting element as described above.

The polarization selective beam splitting element responds to this 90° shift in the reflected light beam to divert or redirect the reflected beam to the photosensitive circuit and prevent the reflected light beam from re-entering the reading laser source.

A spreading lens is used in the optical reading system to spread the essentially parallel light beam from the reading laser source, so that it at least fills the entrance opening of the objective lens.

In a second embodiment of the optical reading system, an optical filter is placed in the reflected reading beam path to filter out all other wavelengths of light than the light wavelength generated by the reading laser source.

In a recording device, the reading function is used alone to write the frequency-modulated information into a video disc element. In a video disc player, the reading function is used alone to recover the frequency-modulated information stored on the surface of the video disc element. In a third mode of operation, the reading and writing functions are combined in a single machine. In this combined device, the reading device is used to check the accuracy of the information entered by the writing

the facility.

To realize the monitoring function, the read beam from the helium-neon (He-Ne) read laser is added to the write beam path. The reading optics are adjusted to direct the reading beam through the microscope's objective lens at a small angle in relation to the writing beam. The angle is chosen so that the reading beam illuminates an area on the same track that is written by the writing beam, but at a point located approx. 4 - 6 ym downstream from the writing point. More precisely, the read beam is mapped onto the information track that was just formed by the write beam. Sufficient time has been allowed for the information characters to be formed on the video disc element. In this way, the reading beam is imaged in alternating areas with different reflectivity. In one embodiment of the reading device, the reading beam hits the parts of the metal that have not been heated by the writing beam, and also hits the glass substrate that has been exposed or uncovered in the openings that have just been formed by the writing point.

The areas of different reflectivity act to change an incident reading beam of constant intensity into an intensity-modulated, reflected reading beam.

In this monitoring mode of operation, the reading laser beam is selected to operate at a wavelength different from the wavelength of the writing laser beam. A wavelength-selective optical filter is placed in the path of the reflected light beam and has a passband that includes the reading laser beam. Any reading laser beam energy that follows the reflection path of the reading beam is blocked by the filter and therefore cannot interfere with the reading process.

The monitoring operation mode is used at the time of reading the video information onto the video disc element as an aid for checking the quality of the signal being recorded. The output signals from the read path are displayed on an oscilloscope and/or a television monitor. The visual inspection of this display signal indicates whether the characters are formed with the preferred relative pulse length (pulse ratio). The preferred pulse ratio is achieved when the length of a specular reflective region, representing one half period of a frequency modulated signal, is on average the same as the next succeeding region of non-specular reflectivity, representing the next succeeding half period

of a frequency modulated signal.

The monitoring mode of operation or the read-after-write mode is also utilized in an error-controlling mode, particularly if information of a digital type is written. The input video information is delayed by an interval equal to the cumulative values of the time delay beginning with the frequency modulation of the completed video information signal during the writing process, and continuing through the frequency demodulation of the recovered reflected signal from the sense circuit, and which includes the walking or running time delay for the point of the storage element moving from the point of storage of the supplied video information signal to the point of impact of the reading light beam. The recovered information is then compared with the delayed input information to check its accuracy. The existence of far too many differences will be a basis for either re-checking and adjusting the device or discarding the plate.

The reading device is suitable for use with a conventional television set by adding an intermediate frequency modulator to add the video signal to a suitable carrier frequency tuned to one of the channels of a conventional television set. The television set then processes this signal in the same way as those received from a normal transmitter station.

The invention will be described in more detail below with reference to the drawings, where fig. 1 shows a block diagram of the reading device, fig. 2 shows a cross-sectional view of a video disc element before writing on it using the one in fig.

1 showed a writing device, fig. 3 shows an incomplete plan of a video disc element after writing has taken place using the writing device according to fig. 1, fig. 4 shows a waveform of a video signal used in the writing device of fig. 1, fig. 5 shows a waveform of a frequency-modulated signal used in the writing device of fig. 1, fig. 6 is a diagram showing the intensity of the writing laser used for the writing device of fig. 1, fig. 7 is a diagram showing the modulated writing beam as modified by the writing device of FIG. 1, fig. 8 shows a radial cross-sectional view along the line 8 - 8 in fig. 3, fig. 9 shows a detailed block diagram of a suitable motion control assembly, fig. 10 is a block diagram showing a reading device, fig. 11 is a block diagram showing a combination of a reading and writing device, fig. 12 is a schematic representation of

ling showing the read and write beam passing through a single objective lens as used in the block diagram of fig. 1,

and fig. 13 shows a circuit diagram of a suitable stabilization circuit for use in the writing device shown in fig. 1.

In the different figures, the same reference number is used to indicate the same element. The terms registration and storage are used synonymously with the term writing. The term recycling is used synonymously with the term reading.

The device for storing video information in the form of a frequency-modulated signal on an information storage element 10 is shown in fig. 1. An information signal source circuit 12 is used to provide an information signal to be registered. This information signal, which is present on a wire 14, is a frequency modulated signal if. information content is in the form of a carrier frequency with frequency changes in time that represent the information to be registered. Fig. 5 shows a typical example of a frequency-modulated signal. The information signal source circuit 12 uses a video signal circuit 16 to provide an information signal on a wire 18 whose information content is in the form of a voltage that varies with time: Fig. 4 shows a typical example of

a voltage signal that varies with time. A frequency modulator circuit 20 responds to the video signal circuit 16 to transform the time varying voltage signal into the frequency modulated signal on the line 14 as shown in fig. 5.

The information storage element 10 is mounted on a turntable or turntable 21. The element 10 is shown in fig. 2

without any signs formed thereon and comprises a substrate 22 with a first surface 24 and a light-sensitive coating 26 covering the first surface 24. A motion control assembly 28 imparts smooth motion to the storage element 10 in relation to a writing beam 29' produced by a light source 30. The motion control assembly 28 is shown and described in more detail in connection with fig. 9. The motion control assembly 28 comprises a rotation drive circuit 32 for providing smooth rotational motion to the information storage element 10, and a translation drive circuit 34 which is synchronized with the rotation drive circuit 32 to move the focused light beam

29' radially above the coating 26. The movement control assembly 28 further comprises an electrical synchronization assembly 36 for maintaining a constant relationship between the rotational movement which is transmitted to the element 10 by the rotation drive circuit 32, and the translational movement which is transmitted to the light beam 29 by the translation drive circuit 34.

The light source 30 provides a light beam 29 which has sufficient intensity to interact with or change the coating 26 when the coating is in motion and placed on the movable information storage element 10. The intensity of the light beam 29' is also sufficient to produce permanent characters in the coating 26 which are representative of the information to be registered. A suitable light source 30 consists of a writing laser for producing a collimated writing beam of polarized, monochromatic light.

Referring again to fig. 2, there is shown a cross-sectional view of a first embodiment of a suitable video disc element 10. A suitable substrate 22 is made of glass and has a smooth, flat, planar first surface 24. The photosensitive coating 26 is formed on the surface 24.

In one of the embodiments shown, the coating 26 is a thin, opaque, metallized layer having suitable physical properties to allow local heating in response to the direct action of the writing light beam 29 from the writing laser 30. In operation, the heating causes local melting of the coating 26 accompanied by retraction of the molten material towards the perimeter of the molten area. When solidified, this leaves a permanent opening, e.g. as shown at 37 in fig. 3 and 8, in the thin metal coating 26. The opening 37 is one type of character used to represent information. In this embodiment, successively arranged openings 37 are separated by a portion 38 of the undisturbed coating 26. The portion 38 is the second type of sign used to represent information. A more detailed description regarding the process by which characters 37 and 38 represent the frequency modulated signal is given in connection with fig. 5-8.

A movable optical assembly 40 and a beam guiding optical assembly 41 jointly define an optical path for the light beam 29 coming from the light source 30. The optical assemblies image the writing beam 29 at a point 42 on the coating 26 carried by the storage element 10. The optical path is also represented by the line designated by the reference numbers 29 and 29'.

A light intensity modulating assembly 44 is placed in

the optical path 29 between the light source 30 and the coating 26. In its widest working mode, the light intensity modulating assembly intensity modulates the light beam 29 with the information to be stored. The light intensity modulating assembly 44 operates under the control of an amplified form of the frequency modulated signal shown in fig. 5. This frequency modulated signal causes the assembly 44 to change between its highest light transmitting state and its lowest light transmitting state during each period of the frequency modulated signal. This rapid switching between transmission states modulates the light beam 29 with the frequency-modulated signal to be stored.

The light beam 29 is modulated as it passes through the light intensity modulating assembly 44. Then the modulated light beam, now represented by the reference numeral 29', is imaged on the coating 26 by means of the optical assemblies 40 and 41. When the modulated light beam 29' hits the coating 26, the character formed in the coating which represents the frequency-modulated signal to be stored.

The light intensity modulating assembly 44 contains an electrically controllable sub-assembly 46 which responds to the frequency modulator 20 to vary the intensity of the light beam 29' above a predetermined intensity at which the focused beam 29' changes the coating 26 carried by the information storage element 10. The electrical controllable subassembly 46 also responds to the frequency modulator 20 to vary the intensity of the light beam below a predetermined intensity at which the focused beam 29' fails to change the coating 26. The changes formed in the coating 26 are representative of the frequency modulated signal to be stored. When a photoresist or photoresist layer forms the coating 26 which is carried by the information storage element 10, the changes are in the form of exposed and uncovered elements which are analogous to the size described above in connection with the characters 37 and 38 respectively.

When the coating 26 carried by the information storage element 10 is a metal coating, the electrically controllable subassembly 46 varies the intensity of the writing beam 29' above a first predetermined intensity at which the focused beam 29' melts the metal coating without vaporizing it, and further varies the intensity of the writing beam below a predetermined intensity at which the focused beam 29' fails to melt the metal surface.

The light intensity modulating assembly 44 comprises a stabilization circuit 48 for providing a feedback signal which is used for temperature stabilization of the operating level of the electrically controllable subassembly 46 so that it works between a predetermined highest light intensity level and a predetermined lower light intensity level. The light intensity modulating assembly 44 includes a light-sensing circuit for sensing at least a portion of the light beam, indicated at 29", which comes from the electrically controllable subassembly 46, to produce an electrical feedback signal representative of the average intensity of the beam 29' The feedback signal is coupled to the electrically controllable subassembly 46 via leads 50a and 50b to stabilize its operating level.

The light-sensing device produces an electrical feedback signal which is representative of the average intensity of the modulated light beam 29'. In this way, the light intensity modulating assembly 44 is stabilized so that it emits the light beam with an essentially constant, average power level. The stabilization circuit 48 also comprises a level setting device for selectively setting the average power level of the light beam 29' to a predetermined value, in order to achieve the preferred pulse ratio in either a metal coating 26 or a photoresist coating 26, or any other material used as the coating 26.

The movable optical assembly 40 comprises an objective lens 52 and a hydrodynamic air bearing 54 for supporting the lens 52 above the coating 26. The laser beam 29' produced by the laser source 30 is formed by essentially parallel light beams. In the absence of the lens 66, these largely parallel light rays have essentially no natural tendency to diverge. The objective lens 52 then has an entrance opening 56 which is larger in diameter than the diameter of the light beam 29'. A plano-convex spreading lens 66 which is placed in the light beam 29' is used to spread the essentially parallel light beam 29' so that it at least fills the entrance opening 56 of the objective lens 52.

The beam guiding optical assembly 41 further comprises

a number of mirror elements 58, 60, 62 and 64 for deflecting the light rays 29' and 29" in the desired manner. The mirror 60 is shown as a plane mirror and is used to form completely circular tracks instead of the preferred spiral tracks. Spiral tracks require only a fixed mirror.

As described above, the light source 30 produces a

j

polarized laser beam 29. The electrically controllable subassembly 46 rotates the polarization plane of this laser beam 29 under control of the frequency modulated signal. A suitable electrically controllable subassembly comprises a Pockels cell 68, a linear polarizer 70 and a Pockels cell driver 72. The Pockels cell driver 72 is essentially a linear amplifier and responds to the frequency modulated signal on the line 14. The output of the Pockels cell driver 72 provides drive signals to the Pockels cell 68 to rotate the laser beam 29 plane of polarization. The linear polarizer 70 is oriented in a predetermined relation to the original polarization plane of the laser beam 29 coming from the laser source 30.

As can be seen from fig. 7, the axis of maximum light transmission for the linear polarizer 70 is placed at right angles to the polarization angle of the light coming from the source 30.

Because of this arrangement, minimal light emerges from the polarizer 70 with zero degrees of rotation added to the writing beam 29 by the Pockels cell 68. Maximum light emerges from the polarizer 70 with 90 degrees of rotation added to the writing beam 29 by the Pockels cell 68. This arrangement of the linear polarizer as described is a matter of choice. By aligning the axis of maximum light transmission for the polarizer 70 with the polarization angle of the light coming from the laser source 30, the maximum and minimum states would be opposite to those described when subjected to zero degrees and 90 degrees rotation. However, the writing device would essentially work in the same way. The linear polarizer 70 acts so that it weakens the intensity of the beam 29 which is turned away from its natural angle of polarization. It is this weakening or dampening effect of the linear polarizer 70 which forms a modulated laser beam 29' which corresponds to the frequency modulated signal. A Glan prism is suitable for use as a linear polarizer 70.

The Pockels cell driver 72 is alternating current coupled to the Pockels cell 68. The stabilizing feedback circuit 48

is DC coupled to the Pockels cell 68.

As reference is now made collectively to fig. 4-7, these figures show selected waveforms of electrical and optical signals present in the embodiment shown in fig. 1. A video signal generated by the video signal source circuit 16 is shown in FIG. 4. A typical device for generating such a video signal is a television camera or a video tape player which plays a previously recorded signal generated by a television camera. A light spot scanner is an additional source for such a video signal. The information signal shown in fig. 4, is typically a signal with a maximum voltage variation of 1 volt and has its information content in the form of a time-varying voltage represented by a line 73. The maximum instantaneous rate of change for a typical video signal is limited by the bandwidth of 4.5 MHz. This video signal is of the type that can be displayed directly on a television monitor.

That in fig. 4 shown video signal is supplied to the frequency modulator 20 which is shown in fig. 1. The modulator 20 generates the frequency modulated waveform 74 shown in FIG. 5. The information content of the one in fig. 5 shown waveform is the same as the information content of the waveform shown in fig. 4, but the shape is different. That in fig. 5 shown information signal is a frequency modulated signal which has its information content in the form of a carrier wave signal with frequency changes in time around a center frequency.

By considering fig. 4 and 5 it can be realized that the area with the lowest amplitude, generally denoted by 75, of the one in fig. 4 shown video waveform 73 corresponds to the part with the lowest frequency of the frequency modulated signal 74 which is shown in fig. 5.

Such a period of the lowest frequency portion of the frequency modulated signal 74 is generally shown at 76. A region of the video waveform 73 which has a higher amplitude and is generally denoted by 77 corresponds to the higher frequency portions of the frequency modulated signal 74. A complete period of the higher frequency portion of the frequency modulated signal 74 is represented by a parenthesis 78. A region of intermediate amplitude of the video waveform 73, which region is generally denoted by 79, corresponds to the intermediate frequency portions of the frequency modulated signal 74. A only period of the intermediate frequency part of the frequency modulated signal, which represents the intermediate amplitude range 79, is represented by a parenthetical sign 79a.

By considering fig. 4 and 5, it can be seen that the frequency modulator 20 in fig. 1 converts the time-varying voltage signal shown in fig. 4, to a frequency modulated signal as shown in fig. 5. Fig. 6 illustrates the intensity of the writing beam 29 generated by the writing laser 30. The intensity of the writing beam 29 is shown to be at a constant level represented by a line 80. After initial setup or assembly procedures, this intensity remains unchanged. Fig. 7 illustrates the intensity of the writing beam .29' after its passage through the light intensity modulating assembly 44. The intensity modulated writing beam is shown to have a number of upper peaks 92 which represent the high light transmission condition of the light intensity modulating assembly 44, and a number of troughs 94 which represents the low light transmission condition for the light intensity modulating assembly 44. The line 80 representing the maximum intensity for the laser 30,

is superimposed on the waveform 29' to show that a certain loss of light intensity occurs in the assembly 44. This loss is indicated by a line 96 which shows the difference in the intensity of the light beam 29' produced by the laser 30, and the maximum intensity 92

for the light beam 29' which is modulated by the assembly 44.

This intensity modulation of the writing beam 29 to form an intensity modulated writing beam 29' is best illustrated by a consideration of fig. 6 and 7. Fig. 6 shows the unmodulated beam 29 having a constant intensity represented by line 80. Fig. 7 shows the modulated beam 29' having maximum intensity levels shown at 92 and minimum intensity levels shown at 94.

The intensity modulation of the writing beam 29 is compared with the turning effect of the Pockels cell 68 by reference to lines 98, 100 and 102. The intersection between the line 98 and the line 29' shows the intensity of the beam 29' coming from the linear polarizer 70 when the Pockels cell 68 does not adds some rotation to the polarization angle of the light passing through it. The point of intersection between the line 100 and the line 29' shows the intensity of the beam 29' coming from the linear polarizer 70

when the Pockels cell 68 adds a 45° rotation to the polarization angle of the light passing through it. The intersection of line 102 and line 29' shows the intensity of the writing beam 29' coming from the linear polarizer 70 when the Pockels cell 68 adds a 90° rotation to the polarization angle of the light passing through it.

The formation of an opening, such as the opening 37 shown

on fig. 3 and 8, by means of the intensity modulated beam 29' shown in fig. 7, can best be understood by a comparison between fig. 7 and 8.

The line 100 is drawn midway between the intensity 92 representing the higher light transmission state of the assembly 44, and the intensity 94 representing the lower light transmission state of the assembly 44. The line 100 represents the intensity generated by the assembly 44 when the Pockels cell 68 rotates polaris the ion angle of the writing beam 29 passing through it, an angle of 4 5°. The line 100 also represents the threshold intensity of the modulated beam 29' required to form a character in the photosensitive coating 26. This threshold is reached by rotating the polarization angle of the writing beam 29 by an angle of 45°.

In a comparison between fig. 7 and 8 it can be seen that an opening 37 is formed while the Pockels cell 68 turns the polarization angle of the writing beam 29 passing through it, between the angle of 45° and 90°, and back to 45°. No opening is formed while the Pockels cell 68 rotates the polarization angle of the writing beam 29 passing through it, between the angle of 45° and 0° and back to 45°.

Referring again to fig. 3, it is there shown

is a top view of the video disc element shown in FIG

radial section view in fig. 8. A consideration of fig. 3 is useful for understanding the manner in which the linear arrays of light-reflecting and light-scattering areas 38 and 37 are formed on the video disc element 10. The disc element 10 is rotated at a preferred rotational speed of 1800 rpm. minute, and characters 37 and 38 are formed in the light-sensitive coating 26 as shown in fig. 8. Motion control assembly 28, which is shown in fig. 1, form the openings 37 in a circular, track-like manner. Reference numeral 104 denotes a section of an inner track, and reference numeral 105 denotes a section of an outer track. A dotted line 106 represents the center line of the track 105, and a dotted line 107 represents the center line of the track 104. The length of a line 108 represents the distance between the center lines 106 and 107 of adjacent tracks 105 and 104. A typical distance between the center lines of the adjacent track is 2 ym. The width of an opening 37 is indicated by the length of a line 109. A typical width of an opening is 1 ym". The distance between adjacent openings is indicated by the length of a line 110. This distance between adjacent tracks is known as the inter-track area and typically has a length of 1 um. The length of an opening is represented by a line 112 and typically varies between 1.0 and 1.5 um. All of these dimensions depend on many variables in the writing device. For example, these dimensions may vary depending on frequency range generated by the frequency modulator 20, the size of the spot or spot 42 formed by the writing optics systems 41 and 42, and the selected rotational speed of the disk 10.

In fig. 9 shows a more detailed block diagram of the motion control assembly 28 shown in FIG. 1. The rotation drive circuit 32 comprises a spindle servo circuit 130 and a spindle shaft 132. The spindle shaft 132 is integrally connected to the turntable 21. The spindle shaft 132 is driven by a motor 134 of pressure circuit type. The rotational movement provided by the printed circuit motor 134 is controlled by the spindle servo circuit 130 which phase-locks the rotational speed of the turntable 21 to a signal generated by a color auxiliary carrier wave crystal oscillator 136 which forms part of the synchronization assembly 36. The synchronization assembly 36 further comprises a first dividing circuit 138 and a second dividing circuit 140. The first dividing circuit 138 reduces the color auxiliary carrier frequency generated in the oscillator circuit 136 down to a rotation reference frequency. Spindle shaft 132 contains a tachometer 143 for generating a frequency signal indicating the exact rotational speed of the combination of shaft 132 and turntable 21. The tachometer signal is available over a wire 142, and the rotation reference signal from the first dividing circuit 138 is available on a wire 144. The tachometer signal on the wire 142 is supplied to the spindle servo circuit 130, and the rotation reference signal on line 144 is also supplied to the spindle servo circuit 130. The spindle servo circuit 130 phase compares these two input signals. When the phase of the tachometer signal is ahead of the phase of the rotation reference signal, the rotation speed is too high and a signal is generated in the spindle servo circuit 130 for supply to the motor 134 over a line 146 to reduce the rotation speed and bring the tachometer signal into phase agreement with the rotation reference signal. When the phase of the tachometer signal lags behind the phase of the rotation reference signal, as compared in the spindle servo circuit 130, the rotational speed is too low and a signal is generated in the spindle servo circuit 130 for supply to the motor 134 over a line 148 to increase the rotation speed and bring the phase of the tachometer signal into phase for the rotation reference signal. The second dividing circuit 140 reduces the color auxiliary carrier frequency generated by the oscillator 136 down to a translation reference frequency for advancing the translation drive circuit 34 a fixed distance for each completed revolution of the element 10. In the preferred embodiment, the distance that the translation drive circuit 34 is advanced for each revolution of the element 10, a distance of 2 ym. ;The color auxiliary carrier wave crystal oscillator 136 with its two sub-circuits 139 and 140 functions as an electrical synchronizing circuit for maintaining a constant relationship between the rotational movement of the plate as provided by the rotary drive assembly 32, and the translational movement between the writing beam 29 and the coating 26 provided by the translation drive assembly 34. ; The movable optical assemblies shown in fig. 1, ;10 and 11, is mounted on a platform shown at 142. This movable platform is driven radially by the translation drive circuit 34 which advances the platform 142 2.0 ym per rotation of the spindle shaft 132. This translational movement is radial in relation to the rotating plate 10. This radial advance per rotation of spindle shaft 132 is identified as the pitch for registration. Since the uniformity of the pitch for the completed registration depends on the stable advancement of the optical assemblies mounted on the platform 142, care is taken to build a lead screw 143 into the translation drive circuit 34, preload a translation drive nut 144 which is engaged with the lead screw , and make the connection between the nut 144 and the platform 142 as rigid as possible, as represented by a rod 146. Referring to fig. 10, there is shown a reading device which is used for recovery of the frequency-modulated signal which is stored on the information storage element 10 as a line-shaped series of characters 37 and 38 as described above. A reading beam 150 is generated by a reading laser 152 which produces a polarized, collimated light beam 150. A support element, such as the turntable or turntable 21, is used to hold the information storage element 10 in a largely predetermined position. A stationary optical reading assembly 154 and a movable optical assembly 156 define an optical path over which the reading light beam 150 travels between the laser source 152 and the information storage element 10. One or the other of the optical assemblies can also be used to focus the light beam 150 on the alternately arranged, light-reflecting areas 38 and the light-scattering areas 37 which are carried in successive positions on the information storage element 10. The movable optical assembly 156 is used to collect the reflection from the light-reflecting areas 38 and the light-scattering areas 37. The motion control assembly 28 provides relative movement between the reading beam 150 and the alternating areas of light reflection 38 and light scattering 37. The optical assemblies 154 and 156 also define the optical path covered by a beam reflected from the coating 26. The path of the reflected beam is denoted by the reference number 150'. This path 150' for reflected light includes part of the original reading beam path 150. In the parts where the reflected beam 150' coincides with the reading beam 150, both reference numbers 150 and 150' are used. A light- ;150' to a corresponding electrical signal. It should be noted that the element 170 .' reduces the intensity of the incident light beam 150 when it passes through it. This fall in intensity is compensated for by setting the original intensity of the reading beam 150 at a level which is sufficient to offset this reduction. The quarter-wave plate 172 provides a total rotation of 90° of the reflected beam 150' relative to the incident beam 150 during the change from linear polarization to circular polarization and back to linear polarization. As previously mentioned, the element 170 is also a beam-splitting cube in the path 150' of the reflected reading beam. When the plane of polarization of the reflected reading beam 15' is shifted 90° as a result of the beam's double passage through the quarter-wave plate 172, the beam-splitting cube portion of the element 170 directs the reflected reading beam 150' to the light-sensing circuit 158. A suitable element for function as a light-sensing element 158, is a photodiode. Each such element 158 is capable of changing the reflected, frequency-modulated light beam 150' into an electrical signal which has its information content in the form of a carrier frequency with frequency variations in time that vary from the carrier frequency. The optical assemblies 154 and 156 further comprise the objective lens 52 which is supported by a hydrodynamic air bearing element 54 which supports the lens 52 above the coating 26 which is carried by the information storage element 10. As described above, the reading beam or rather the reading beam bundle 150 is formed with substantially parallel light beams . The objective lens 52 has an entrance opening 56 which has a larger diameter than the diameter of the reading beam 150 as it is generated by the laser source 152. A plano-convex spreading lens 174 is arranged between the laser source 152 and the entrance opening 56 of the objective lens 52 to spread the essentially parallel light rays - which form the reading beam bundle 150, to a beam of light 150 with a diameter sufficient to at least fill the entrance opening 56 of the objective lens 52. The optical assemblies 154 and 156 further comprise a number of stationary, planar mirrors 176 and 178 for deflection of the beam of reading light 150 and the reflected beam of light 150 ' along a path which is calculated to hit the elements mentioned above. Sensing element 158 is placed in the path 150' of the reflected light beam and is used to generate a frequency-modulated electrical signal that corresponds to the reflections that hit the element. The frequency-modulated electrical signal generated by the light-sensing element 158 is present on a line 160 and has its information content in the form of a carrier frequency with frequency changes in time corresponding to the stored information. The output signal from the light-sensing circuit 158 is fed to a discriminator circuit 162 via an amplifier 164. The discriminator circuit 162 responds to the output signal from the light-sensing circuit 158 and is used to change the frequency-modulated electrical signal into a time-dependent voltage signal that represents the stored information. The time-dependent voltage signal is also identified as a video signal and is present on a wire 165. This time-dependent voltage signal has its information content in the form of a voltage that varies with time and is suitable for display on a regular television monitor 166 and/or an oscilloscope 168. ; The optical assemblies 154 and 156 further comprise a polarization ion selective, beam splitting element 170 which functions as a beam polarizer for the incident beam 150, and which functions as a selective beam splitter for the reflected beam 150'. The optical assemblies further comprise a quarter-wave plate 172. The beam polarizer 170 filters out from the reading beam 150 any light waves that are not aligned with the polarization axis of the beam polarizer 170. With the polarization axis of the reading beam 150 fixed in a particular orientation by means of the element 170, the quarter-wave plate 172 changes the plane of polarization from linear to circular. The element 170 and the quarter-wave plate 172 are placed in the reading light beam path 150. The element 170 is located between the source 152 for the reading beam 150 and the quarter-wave plate 172. The quarter-wave plate 172 is also located in the path 150' for the reflected reading beam. Therefore, the quarter-wave plate 172 not only changes the polarization of the read beam from linear to circular during the beam's travel from the read laser 152 to the information storage element 10, but the quarter-wave plate 172 further changes the circularly polarized reflected light back to linearly polarized light which is rotated 90° relative to the preferred direction which is fixed by the source 152 and the element 170. This rotated beam 150' is selectively directed to the light-sensing element 158 which modifies the reflected light beam; An optional optical filter 180 is placed in the path 150' of the reflected beam and filters out all other waves -lengths than the wavelength of the incident beam. The use of this filter 180 improves the quality of the image displayed on the television monitor 166. This filter 180 is essential when the reading system is used together with the writing system as described in more detail in connection with fig. 11. In this read-after-write mode of operation, part of the write beam 29 travels along the path 150' of the reflected read beam. The filter stops this part of the writing beam and lets through the entire intensity of the reflected beam 150'. An optional converging lens 182 is placed in the path 150' of the reflected beam to image the reflected beam on the active area of the photosensitive element 158. This converging lens 182 reduces the diameter of the reflected beam bundle 150' and concentrates the light intensity of the reflected beam onto the active area of the light-sensing element 158. ;The amplifier 164 amplifies the output signal of the light-sensing element 158 and raises the amplitude of the frequency-modulated electrical signal generated by the light-sensing element 158 to match an input signal requirement of the demodulator 162. ;Whereas on again, reference is made to the electrical and optical waveforms shown in fig. 4-7, these waveforms are also generated by the one in fig. 10 showed a reading device during the recovery of the frequency-modulated signal stored in the coating 26 carried by the disk 10. Fig. 6 shows a laser source which generates a writing laser beam which has a constant intensity represented by the line 80. The reading laser 152 generates a reading beam 150 which has constant intensity, but at a lower level. Fig. 7 shows an intensity modulated writing laser beam. The reflected reading beam 150' is intensity modulated when it hits the light-reflecting and light-scattering areas 38 and 37 which are carried on the plate element 10. The reflected reading beam 150' will not be a perfect square wave as shown in fig. 7. Instead, the right-angled edges are rounded due to the finite size of the reading point. Fig. 5 shows a frequency-modulated electrical signal which has its information content in the form of a carrier wave signal with frequency changes in time that vary around the center frequency. The output signal from the light-sensing element 158 constitutes the same type of signal. Fig. 4 shows a video signal which has its information content in the form of a time-varying voltage. The output signal from the demodulator 162 constitutes the same type of signal. The one in fig. 10 movement control assembly 28 works in the same way as the movement control assembly 28 shown in fig. 1. In the reading device, the motion control assembly 28 produces a rotational movement of the disk element under the control of a rotation drive assembly 32. The assembly 28 further produces a translational movement to move the movable optical assembly 156 radially across the surface of the storage element. The assembly 28 further comprises a synchronization circuit for maintaining a constant relationship between the rotational movement and the translational movement, so that the reading beam 150 hits the information tracks carried by the plate element 10. Parts of typical information tracks are shown at 104 and 105 in fig. 3. On fig. 11 shows a block diagram illustrating the combination of the writing device shown in fig. 1, and the reading device shown in fig. 10. Those in fig. 11 shown elements work in a way that is identical to that described above, and this detailed way of working shall not be repeated here. Only a brief description should be given to avoid repetition and confusion. ;The unmodulated write beam path is shown at 29 and the modulated beam path is shown at 29'. A first optical assembly defines the modulated beam path 29' between the output of the linear polarizer 70 and the coating 26. The fixed optical assembly 41 for writing comprises the mirror 58. The movable optical assembly 40 for writing comprises the diffusing lens 66, a partially transmitting mirror 200, a planar mirror 60 and the objective lens 52. The modulated writing beam 29' is imaged against a writing point 42 on the photosensitive coating and interacts with the coating to form characters as described above. ;The read beam path is shown. at 150. The reading optical assemblies define a second optical path for the reading beam 150 between the reading laser 152 and the information storage element 10. The fixed optical reading assembly 154 comprises the mirror 176. The movable optical reading assembly 156 comprises the spreading lens 174, the polarization changing device 172, a second fixed mirror 202, the selectively transmitting mirror 200, the planar mirror 60 and the lens 52. The reading beam 150 is imaged on a reading spot 157 at a point which is separated on the downstream side of the reading spot 42, as described in more detail in connection with fig. 12. The mirror 200 is a dichroic mirror which is transmitting at the wavelength of the writing beam 29' and which is reflective at the wavelength of the reading beam 150'. ;The intensity of the write beam 29' is higher than the intensity of the read beam 150. While the write beam 29' must modify the light-sensitive coating 26 to retain characters representative of the video signal to be stored, the intensity of the read beam 150 need only be sufficient to illuminate the characters formed in the coating 26, and produce a reflected light beam 150' of sufficient intensity to provide a good signal after collection by the optical reading assembly and conversion from an intensity-modulating, reflected beam 150' to a frequency-modulated electrical signal by using the light-sensitive circuit 158. ;The fixed mirror 58 in the optical writing path and the two fixed mirrors 176 and 202 in the optical reading path are used to direct the writing beam 29' towards the objective lens 56 with a controlled angle in relation to the reading beam 150. This angle between the two incident beams provides a distance between the write spot 43 and the read spot 157 when they are imaged on the coating 26. During operation, a sufficient distance has been found to be 4 - 6 ym. This distance corresponds to an angle which is far too small to be shown clearly in Fig. 12. Consequently, this angle is exaggerated in Fig. 12 for illustration purposes only. The reading beam 150' is demodulated in a discriminator circuit 162 and displayed on a conventional television monitor 166 and an oscilloscope 168. The television monitor 166 displays the image quality of the recording, and the oscilloscope 168 displays the video signal in more detail. This read-after-write feature allows the quality of the video signal stored during a write operation to be checked or observed instantaneously. In the event that the quality of the stored signal is bad, this is known ;immediately, and the write operation can be corrected or the information storage element that stores the video information signal with poor quality can be discarded. ;In the read-after-write mode of operation, the write laser 30 and the read laser 152 work at the same time. A dichroic mirror 200 is used to combine the read beam 150 into the write beam 29'. In this mode of operation; with reading after writing, the wavelength of the write beam 29 is selected so that it is different from the wavelength of the read beam 150. An optical filter 180 is used to block any part of a write beam that has followed the path of the reflected read beam. The optical filter 180 consequently passes through the reflected reading beam 150' and filters out any part of the writing laser beam 29' which follows the path 150' of the reflected reading beam. In the comparison operating mode, the read-after-write operation is practiced as described in connection with fig. 11. when operating in this monitoring mode of operation, a comparator circuit 204 compares the output signal from the demodulator 162 with the original video information signal provided by the source 18. More specifically, the video output signal from the discriminator 162 is supplied to a comparator 204 over a wire 206. The second input signal to the comparator 204 is taken from the video source 16 via the line 18, a further line 208 and a delay line 210. The delay line 210 imparts a time delay to the applied video decoding signal equal to the accumulated values of the delay beginning with the frequency modulation of the input video information signal, and extends over the frequency demodulation of the recovered electrical signal from the photosensitive circuit 158. This delay also includes the time delay from the point on the storage element 10 in which the supplied video information signal is stored on the information storage element by means of write patch ken 42, and continues to the point of impact for the read spot 157. The correct degree of delay is best generated by designing the delay circuit 210 as a variable delay circuit which is set for optimal operation. ;Ideally, the video output signal from the discriminator ;162 is identical in all respects to the video input signal on wires 18 and 208. Any differences noted represent errors which may be caused by imperfections in the surface of the disc or malfunctions in the write circuit. Although this application is essential when recording digital information, it is less critical when other information is recorded. The output signal from the comparator circuit 204 can be counted in a counter (not shown) to establish the actual number of errors present on any disk. When the errors counted exceed the predetermined selected number, the write operation is terminated. If necessary, a new disc can be written. Any disc with an excessive number of errors can then be reprocessed. ; On fig. 11, the comparator 204 compares the output signals available on the lines 208 and 206. An alternative and more direct connection of the comparator 204 is to compare the output signals from the frequency modulator 20 and the amplifier 164 which is shown in connection with fig. 10. Referring now to fig. 12, this figure shows in somewhat exaggerated form the slightly deviating optical paths of the intensity-modulated writing beam bundle 29' from the writing laser 30 and the unmodulated reading beam bundle 150 from the reading laser 152. The information storage element 10 moves in the direction indicated by an arrow 217. This shows an uncovered coating 26' approaching the writing beam beam 29', and a linear array of apertures 37 leaving the intersection of the writing beam beam 29' and the coating 26. The writing beam beam 29' coincides with the optical axis of the microscope -the objective lens 52. The central axis of the reading beam beam 150 shown at 212 forms an angle with the central axis of the writing beam beam 29' shown at 214. This angle is represented by the double-headed arrow 216. As a result of this slight difference in the optical paths of the writing beam beam 29' and the reading beam beam 150 through the lens 52, the writing spot 42 falls a distance in front of the reading spot 157. The writing spot 42 lies in front of the reading spot 157 a distance which is equal to the length of a line 218. The length of the line 218 is equal to the aforementioned angle times the focal length of the objective lens 52. The resulting delay between writing and reading allows the molten metal coating 26 to solidify so that the record is read in its final, solidified state. If it was read too early while the metal was still molten, the reflection from the edges of the opening would fail to provide a high quality signal for display on the minotaur 166. In fig. 13 is an idealized diagram of a Pockels cell stabilization circuit 48 suitable for use in the device of FIG. 1. As is known, a Pockels cell 68 rotates or rotates the plane of polarization of the applied writing light beam 29 as a function of an applied voltage, as shown in connection with fig. 7. ;Depending on the individual Pockels cell 68, a voltage change of the order of 100 volts causes the cell to rotate the plane of polarization of light passing through it a full 90°. The Pockels cell driver 72 has the task of amplifying the output signal from the information signal source 12 to an output voltage with a maximum voltage variation of 100 volts. This provides an appropriate input drive signal to the Pockels cell 68. The Pockels cell driver 72 generates a waveform having the form shown in FIG. 5 and has a peak-to-peak voltage swing of 100 volts. The Pockels cell should be driven with an average rotation of 45° to bring the modulated light beam intensity to reproduce the electrical drive signal as accurately as possible. A bias voltage must be provided to the Pockels cell to maintain the cell at this average operating point. The electrical bias that corresponds to a working point with a rotation of 45° varies in practice continuously. This continuously varying bias voltage is generated using a servo feedback loop. This feedback loop comprises comparison of the mean value of the transmitted light with an adjustable reference value, and supplying the difference signal to the Pockels cell by means of a direct current amplifier. This arrangement stabilizes the working point. The reference value can be set to correspond to the mean transfer corresponding to the operating point of 45°, and the servo feedback loop provides corrective biases to hold the Pockels cell at this mean 45° rotation. The stabilization circuit 48 contains a light-sensing device 225. A silicon diode acts as a suitable light-sensing device. The diode 225 senses a portion 29" of the writing beam beam 29' coming from the optical modulator 44 and passing through the partially reflective mirror 58 as shown in Fig. 1. The silicon diode 225 acts much like a solar cell and is a source of electrical energy when illuminated by incident radiation. One output line of the silicon diode 225 is connected to a common reference potential 226 via a line 227. The other output line from the diode 225 is connected to one input of a differential amplifier 228 via a line 230. The output leads from the silicon diode 225 are shunted by a load resistor 232 which enables linear response mode. The second input to the differential amplifier 228 is connected to an adjustable arm 234 of a potentiometer 236 via a lead 238. One end of the potentiometer 236 is connected to the reference potential 226 via a wire 240. A power source 242 is connected to the other end of the potentiometer 236, which enables in setting the differential amplifier 228 to generate a feedback signal on lines 244 and 246 for setting the average energy level of the modulated laser beam beam 29' to a predetermined value. The output terminals of the differential amplifier 228 are connected via respective resistor elements 248 and 250 and output leads 244 and 246 to the input terminals of the Pockels cell 68 shown in FIG. 1. The Pockels cell driver 72 is AC coupled to the Pockels cell 68 via capacitive elements 252 or 254, while the differential amplifier 228 is DC coupled to the Pockels cell 68. ;During operation, the system is energized. The part 29" of the light from the writing beam bundle 29' that hits the silicon diode 225 generates a differential voltage on one input of the differential amplifier 228. Initially, the potentiometer 236 is set so that the average transfer through the Pockels cell corresponds to a rotation of 45°. average intensity level impinging on the silicon cell 225 then either increases or decreases, a corrective voltage will be generated by the differential amplifier 228. The corrective voltage applied to the Pockels cell 68 is of a polarity and magnitude suitable to restore the average intensity level to the predetermined level selected by setting the input voltage to the second input to the differential amplifier across line 238, by moving the movable arm 234 along the potentiometer 236. The adjustable arm 234 on the potentiometer 236 constitutes the means for selecting the mean intensity level of the light generated of write the elaser 30. Optimum results are achieved when the length of an opening 37 is exactly equal to the length of the subsequent space 38 as described above. The setting of the potentiometer 236 is the means of achieving this length equality. ;When the length of an opening is equal to the length of its next adjacent room, a fifty-fifty pulse ratio is achieved. Such a pulse ratio can be detected by examining the display of the information just written on the television monitor and/or the oscilloscope 166 or 168.., as previously described. Commercially acceptable results occur when the length of an opening 37 varies between 40 and 60% of the combined length of an opening and its next, successively located space. In other words, the length of an opening and the next, successively placed room is measured. The opening can then have a length that falls within the range from 40 to 60% of the total length. Referring to fig. 8, there is shown a radial cross-section of an information track along the line 8 - 8 in fig. 3 where a specular light-reflecting area 38 is placed between two non-specular light-reflecting areas 37. In the radial cross-sectional view shown in fig. 8, the incident reading or writing beam bundles are moved relative to the element 10 in the direction represented by arrow 217. This means that a reading beam bundle first hits the specular light-reflecting area 38a, after which it hits the non-specular light-reflecting area 37a. In this configuration, the positive half-period of the signal to be recorded is represented by a specular light-reflecting area 38a, and the negative half-period of the signal to be recorded is represented by the non-specular light-reflecting area 37a. The pulse ratio for the signal shown in fig. 8, is a 50% pulse ratio to the extent that the length of the specular light-reflecting area 38a represented by the parenthetical character 260 is the same length as the length of the non-specular light-reflecting region 37a represented by the parenthetical character 262. This preferred pulse ratio is created by the combination of setting the absolute intensity of the writing beam beam 29, setting the energy level of the reading laser 30 and setting the potentiometer 236 in the stabilization circuit 38 at a level where an opening is formed at the beginning of a 45° rotation of the polarization angle of the writing beam beam 29. ;While referring again to the opening forming process which is illustrated in fig. 7 and 8, melting of a thin metal coating 26 occurs when the energy in the light spot exceeds a threshold which is characteristic of the composition and thickness of the metal film and the properties of the substrate. The spot energy is modulated by the light intensity modulating assembly 44. The on-off transitions are kept short to make the location of the hole ends accurate despite variations in the melting threshold. Such variations in the melting threshold can occur due to variations in the thickness of the metal coating and/or the use of a different material as an information-storing layer. The average power in the spot required to form an opening in a thin metal coating 26 having a thickness between 200 and 300 Angstroms is of the order of 200 milliwatts. ;As the FM carrier frequency is approx. 8 MHz, 8 x IO<6> holes of variable length are cut per second and the energy per holes are 2.5 and 10~<9>Joule. In this first embodiment of a video disc element 10, a portion of the glass substrate is exposed in each opening. The uncovered portion of the glass substrate appears as an area of non-specular light reflectivity for an incident reading beam beam. The part of the metal coating that remains between successively placed openings appears as an area of high light reflectivity for an incident reading beam bundle. ;When the formation of the first and second characters is carried out using a coating of photoresist material, the intensity of the writing beam beam 29' is set at such a level that a 45° rotation of the plane of polarization generates a light beam beam 29' ;of threshold intensity, to reveal and/ or cooperate with the photoresist coating 26 while the photoresist coating is in motion and placed on the movable information storage element 10. The combination of the Pockels cell 68 and the Glan prism 70 constitutes a light intensity modulating element which operates from the 45° setup condition to a lower light transmission state associated with an operating state with a rotation of nearly zero degrees, to a higher light transmission state associated with an operating state with a rotation of nearly 90°. When the intensity of the writing light beam 29' increases above the initially set level or the predetermined starting intensity, and increases towards the higher light transmission state, the incident reading light beam 29' exposes the photoresist material illuminated by the beam. This detection continues after the intensity of the writing beam reaches the maximum light transmission condition and starts back down towards the original, predetermined intensity associated with the 45° rotation of the plane of polarization for the light coming from the writing laser 30<*>. When the rotation falls below the value of 45°, the intensity of the writing beam beam 29' emanating from the Glan prism 70 falls below the threshold intensity at which the focused writing beam beam fails to reveal the photoresist material thus illuminated. This inability to detect the photoiesite material illuminated by the beam continues after the writing beam intensity reaches the minimum light transmission state and starts back up towards the original, predetermined intensity associated with a 45° rotation of the plane of polarization of the light coming from the writing laser 30.

The Pockels cell driver circuit 72 is typically a high gain, high voltage amplifier with an output signal that provides an output voltage swing of 100 volts". This signal is intended to match the driving conditions of the Pockels cell 68. As a typical example, this means that the middle voltage value of the output signal from the Pockels cell driver 72 provides a sufficiently large voltage to drive the Pockels cell 68 over a 45° rotation, so that approximately half of the total available light from the laser 30 emerges from the linear polarizer 70 .When the output signal from the driver 72 goes in the positive direction, more light passes from the laser.When the output signal from the driver 72 goes in the negative direction, less light passes from the laser.

In the first embodiment which used a metal coating 26, the output signal from the laser 30 is set so that an intensity is produced that begins to melt the metal layer coating 26 which is placed on the plate 10, when the output signal from the driver 72 is zero and the operating point of the Pockels cell is 45 °. If the output signal from the driver 72 goes in a positive direction, the melting therefore continues. When the output signal from the driver 72 goes in a negative direction, the melting stops.

In a second embodiment where a photoresist coating 26 is used, the output signal from the laser 30 is set so that an intensity is produced which both illuminates and uncovers the photoresist coating 26 when the output signal from the driver 72 generates its middle voltage value. Consequently, when the output signal from the driver 72 goes in the positive direction, the illumination and exposure of the photoresist material illuminated by the writing beam beam continues.

When the output signal from the driver goes in a negative direction, the illumination continues, but the energy in the writing beam bundle is insufficient to uncover the illuminated area. The term "uncover" is used here in its technical sense which describes the physical phenomenon accompanying exposed photoresist material. Exposed photoresist material can be easily developed, and the developed photoresist removed using conventional methods. Photoresist illuminated by light of insufficient intensity to expose the photoresist material cannot be developed and removed.

In both the first and second embodiments just described, the absolute energy level illustrated by line 80 in FIG. 6, adjusted upwards and downwards to achieve this effect by adjusting the power supply of the writing laser 30. In combination with this setting of the absolute power level for the writing laser 30, the potentiometer 236 is also used to cause signs to form in the coating 26 when the beam bundle - 29. - is rotated approx. 45° as described above.

In that in fig. 10, the read-only system shown, the optical filter 180 is optional and is generally not required. Its use in the read-only system introduces a slight attenuation in the reflected path and thus requires a slight increase in the intensity of the read laser 152 to ensure the same intensity at the detector 158 compared to a read-only system which

does not use a filter 180.

Converging lens 182 is also optional. In a suitably arranged reading system, the reflected reading beam bundle 150' has essentially the same diameter as the photodetector 158 working area. If this is not the case, a converging lens 182 is used to concentrate the reflected reading beam bundle 150' on the smaller working area of the selected photodetector 158.

Claims (14)

1. Optical system for recovering a frequency-modulated signal that is stored on a surface of an information storage element in the form of a linear series of areas, the areas being alternately specular light-reflective and non-specular light-reflective, characterized in that it comprises a device for producing a polarized collimated light beam, a support device for holding the information storage element, an optical device for defining an optical path between the light beam generating device and the information storage element held on the support device, and for focusing the light beam beam onto the alternately located specular light reflective areas and non-specular light reflective areas areas, the optical device also being used for collecting reflections from the light-reflecting areas, a device for providing relative movement between the light beam bundle and the alternating areas to generate reflections from the brighter deflecting areas that represent the stored frequency-modulated signal, and a light-sensing device that reacts to the reflections to generate a frequency-modulated electrical signal that corresponds to the said reflections, the latter frequency-modulated signal having its information content in the form of a carrier wave signal with frequency changes in time that corresponds to the stored information. 2. System according to claim 1, characterized in that it comprises an information storage element with a surface, in that the element has a linear series of areas that are arranged in a track-like manner on the said surface, in that the areas are alternately specular light-reflecting and non-specular light-reflecting, and the sequence of alternating areas represents a frequency-modulated signal that has its information content in the form of a carrier wave signal with frequency changes in time that vary from a centef frequency. 3.. System according to claim 1, characterized in that it comprises a device that reacts to the light-sensing device's output signal to change the frequency-modulated electrical signal into a time-dependent voltage signal that represents the stored information, the time-dependent voltage signal having its information content in the form of a time-varying voltage signal and is suitable for display using an ordinary television monitor. 4. System according to claim 2, characterized in that it comprises a beam polarizer and a polarization shifting device for shifting the polarization path of the beam, the polarizer and the shifting device being placed in the path of the light beam with the polarizer located between the light beam source and the shifting device, in that the displacing device rotates the beam bundle during incident and reflected passages through the displacing device to and from the storage element, the polarizer substantially reducing the intensity of the reflected beam bundle that passes through the displacing device towards the light beam bundle producing device. 5. System according to claim 4, characterized in that the displacing device is a quarter-wave plate for turning the light beam bundle 90° by cumulative incident and reflected passages through the plate. 6. System according to claim 4, characterized in that the beam polarizer is a polarizing, beam-splitting cube which is designed to direct the reflected beam that is guided through the displacing device towards the light-sensing device. 7. System according to claim 1, characterized in that the optical device comprises an objective lens and a hydrodynamic air bearing device for supporting the lens over the surface of the information storage element. 8. System according to claim 1, characterized in that the collimated light beam bundle has essentially parallel light beams, that the objective lens has an entrance opening that has a larger diameter than the diameter of the light beam bundle provided by the light source, and that the optical device further comprises mirror devices for deflection of the light beam bundle provided by the light source, and a spreading lens for spreading the essentially parallel light beam bundle from the light source so that it at least fills the entrance opening of the objective lens. 9. System according to claim 1, characterized in that the storage element is the plate, that the device for providing relative movement comprises a rotation drive device for producing uniform rotational movement of the plate, and that the device further comprises a translation drive device which is synchronized with the rotation drive device to move the focused light beam bundle radially over the plate-shaped storage element's surface, and an electrical synchronizing device for maintaining a constant connection between the rotational movement and the translational movement. 10. Method for reading an information signal that is stored on a recording element, the information signal being in the form of alternating areas that are specular light-reflective and non-specular light-reflective, characterized in that it comprises the steps of providing an information storage element with an information-bearing surface, the surface has a linear series of areas that are arranged in a track-like manner on the surface, the areas being alternately specular light-reflective and non-specular light-reflective, the sequence of alternating areas representing a frequency-modulated signal that has its information content in the form of a carrier wave signal with frequency changes in time varying from a center frequency, that a polarized, collimated beam of light is imaged on the series of areas, that relative motion is provided between the imaged beam of light and the alternating areas to generate reflections from the light-reflecting areas representing d a stored frequency-modulated signal, and that the reflections are sensed and a frequency-modulated electrical signal corresponding to the said reflections is generated, the frequency-modulated electrical signal having its information content in the form of a carrier wave signal which has frequency changes in time from a center frequency. 11. Method according to claim 10, characterized in that the frequency-modulated electrical signal is demodulated to produce a time-dependent voltage signal that represents the stored information, the time-dependent voltage signal having its information content in the form of a time-varying voltage and is suitable for display using a normal television monitor . 12. Method according to claim 10, characterized in that it comprises the step of displacing the plane of polarization for the polarized beam bundle by turning the beam bundle during incident and reflected passages to and from the storage element, so that the intensity of the reflected beam bundle which is led to a light beam bundle source that is used for generation of the light beam beam at the imaging stage, is significantly reduced. 13. Method according to claim 12, characterized in that the displacement step comprises turning the light beam bundle 90° by cumulative incident and reflected passages of the second light beam bundle to and from the surface of the storage element. 14. Method according to claim 10, characterized in that the storage element is the plate and that the step of providing relative movement comprises producing uniform rotational movement of the plate, and synchronizing the rotational movement with movement of the storage element to effect relative movement of the imaged light beam bundle radially across the plate-shaped storage element surface to maintain a constant connection between the rotational movement and the translational movement.