GB2033132A - Recording and playback - Google Patents

Abstract

This invention relates to the writing on and reading from, a video disc record of frequency modulated video information signals, for example, for a television record. The video information is formed into a frequency modulated signal which is used to vary the intensity of a laser writing beam which forms a track on the metallised surface of a video disc, consisting of non- reflective lengths of the track, corresponding to marks of the frequency modulated waveform, and reflective lengths of the track corresponding to the spaces of the frequency modulated waveform. The track can be read by use of a reading light beam and collecting light reflected from the reflective lengths as the beam moves over the track. There are feedback means for maintaining the average intensity of the writing light beam, and means for monitoring the record after it has been formed by comparing it with the original waveform to be recorded.

Description

SPECIFICATION Method and apparatus relating to the recording on and the reading from a disc or other record of electrical signals The present invention relates the recording or writing of a frequency modulated electrical signal, for example representing video information, upon a record for example, a disc, the recording being in the form of a track carried on the record, and consisting of a series of first and second indicia.

An object of the invention is to provide methods and apparatus of that kind enabling the information to be recorded and recovered with great accuracy, and the invention may be considered to have a number of aspects.

According to one aspect, the invention consists of an information storage member for storing a frequency modulated signal and having a surface carrying a lineal series of indicia positioned in track-line fashion upon said surface; the indicia representing the frequency modulated signal having its informational content in the form of a carrier signal having frequency changes with time varying from a centre frequency. According to a second aspect, the invention consists of apparatus for storing information in the form of a frequency modulated signal upon an information storage member, comprising means for imparting uniform motion to said storage member, a light source for providing a light beam for interacting with a coating on the storage member for altering said coating to retain indicia representative of said information; optical means for defining an optical path between said light source and said coating on said storage member, and for focusing said light beam upon said coating; and light intensity modulating means positioned in said optical path and arranged to operate between a higher light transmitting state and a lower light transmitting state responsive to said frequency modulated signal.

According to a third aspect, the invention consists of a method for recording on an information storage member information carried by a frequency modulated electrical signal having a carrier signal with frequency changes over time, comprising the steps of controlling the intensity of the transmission of a laser light beam upon a light sensitive surface of an information storage member, using said frequency modulated signal as a control signal; and moving the information storage member relative to said light beam while focusing said light beam upon said light sensitive surface, said controlling step including said transmitted light beam for irreversibly altering said light sensitive surface of said information storage member under the control of one portion but not a second portion of said frequency modulated signal.

According to a fourth aspect, the invention consists of an optical system and method for reading an information signal stored on a record member, the information signal being in the form of alternate regions which are alternately specular light reflective and non-specular light reflective, the sequence of alternate regions representing a frequency modulated signal having its informational content in the form of a carrier signal having frequency changes with time varying from a centre frequency; in which system and method a polarised collimated beam of light is focused upon said series of regions; and the beam of light moves in relation to the alternate regions to generate reflections from said light reflection regions representing said stored frequency modulated signal and a frequency modulated electrical signal is generated from the reflections, said frequency modulated electrical signal having its informational content in the form of a carrier signal having frequency changes in time from a centre frequency.

According to a fifth aspect, the invention consists of a method and apparatus for monitoring the storage of video information upon an information storage member in which the information in the form of a voltage varying with time is converted to a frequency modulated signal with a carrier signal having frequency changes in time corresponding to said voltage variations with time, and the frequency modulated signal is stored upon a light sensitive member in the form of alternate regions of specular light reflectivity and non-specular light reflectivity; light from a light beam is reflected from said regions while relative movement occurs between the light beam and the member, the frequency modulated signal is recreated from the reflections, an output video information signal in the form of a voltage varying with time suitable for display on a standard television monitor is obtained from the frequency modulated signal, and the output video information signal is compared with the original video information voltage.

According to a sixth aspect, the invention consists of a method and apparatus for recording a modulated electrical signal representing video information on a recording surface having a surface layer for retaining indicia representative of said information to be recorded; in which a laser light write beam has its intensity controlled in response to the modulated electrical signal and is used to form indicia of a first type in the surface layer when at a predetermined intensity, and in which the intensity is stabilised in relation to the predetermined intensity by sensing at least a portion of the laser write beam after intensity control for use as an electrical feedback signal.

One form of apparatus for writing a frequency modulated signal upon a video disc member includes a movable writing beam and a video disc member mounted on a turntable. The turntable is driven by a motion control assembly which rotates the disc precisely in a circle at a constant rate of rotation and a translational drive assembly for translating the writing beam at a very constant, and very low velocity along a radius of the rotating disc.

The rotational drive of the disc is synchronized with the translational drive of the writing beam to create a spiral track of predetermined pitch. In a preferred embodiment the spacing between adjacent tracks of the spiral is two microns, centre to centre. The indicia is formed having a width of one micron. This leaves an intertrack or guard area of one micron between indicia in adjacent tracks. If desired, the indicia can be formed as concentric circles by translating in incremental steps rather than by translating at a constant velocity as just described.

In the preferred embodiment, a microscope objective lens is positioned at a constant height above the video disc member on an air bearing. This objective lens is employed for focusing the write beam upon the light sensitive surface of the video disc member.

The constant height is necessary because of the shallow focal depth of the objective lens. A 0.65 NA dry microscope objective lens is employed to focus the write laser beam to a spot one micron in diameter upon the light sensitive coating. Because the coating is rotating at a relatively high rate, the length of the indicia formed in the light sensitive coating depends upon the length of time the spot intensity exceeds that needed to form such an indicia.

A linearly polarized argon ion laser is used as the source of the writing beam. A Pockels cell is used to rotate the plane of polarization of the writing beam with respect to its fixed plane of linear polarization. A linear polarizer attenuates the rotated writing beam in an amount 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 celi and linear polarizer modulates the writing beam with the video information to be stored. This modulation follows the pattern provided by control signals furnished by a Pockels cell driver.

The video signal to be recorded is applied to a frequency modulator circuit. The output from the modulator circuit is a rectangular wave whose frequency is proportional to the video signal. The duration of each cycle of the rectangular waveform is variable as is characteristic of a frequency modulated signal. As is characteristic of a rectangular wave, it has an upper voltage level and a lower voltage level. The upper and lower voltage levels of the rectangular wave are amplified by a Pockels cell driver and used to control the Pockels cell. The Pockels cell changes the angle of polarization of the light passing therethrough 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 one voltage level of the rectangular-shaped control sig nal applied to a Pockels cell driver, the light beam passes unhindered through the Pockels cell linear polarizer combination at a first intensity sufficient to form a first indicia in a light responsive coating.

When the control signal changes to represent its second voltage level, the Pockels cell rotates the polarization of the light which forms the writing beam to a new angle of polarization. Due to this change in polarization of the light forming the writing beam, a mismatch occurs between the angle of polarization of the light issuing from the Pockels cell and the preferred angle of polarization 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 first indicia in the light responsive coating.

A portion of the writing beam is sensed by a Pockels cell stabilizing circuit for maintaining the average power of the modulated writing beam at a predetermined level in spite of changes in the Pockels cell transfer characteristic produced by small temperature variations. The stabilizing circuit includes a level adjusting circuit for selectively adjusting the power level to form indicia in different light sensitive coatings as identified hereinafter.

Different types of video disc members can be used with this writing process and apparatus. Each such member has a different configuration. In a first configuration, the video disc member includes a glass substrate having an upper surface carrying a thin metal coating as a light responsive coating. In this configuration, the write beam forms variable length apertures in a track-line fashion in the metal coating.

The intensity of the write beam is adjusted such that an aperture is formed, for example, during each positive half cycle of the frequency modulated signal to be stored, and no aperture is formed during the negative half cycle. Accordingly, the first and second indicia representative of the stored information is a lineal series of apertures separated by an intervening portion of the surface coating.

In this first configuration, a portion of the glass substrate is exposed in each aperture. The exposed portion of the glass substrate appears as a region of non-specular light reflectivity to an impinging reading beam. The intervening portion of the metal coating remaining between specular reflectivity means a significant portion of the reflected light returns along the path of the incident light beam, ie., a 1800 reversal in paths between the incident and reflected beam paths. Non-specular reflectivity means that no significant portion of the incident beam is reflected along the path of the incident beam.

In a second configuration, the video disc member includes a glass substrate having an upper surface carrying a thin layer of photoresist as the light responsive coating. In this configuration, the write beam forms variable length regions of exposed and unexposed photoresist material in a track-like fashion in the photoresist coating. The intensity of the write beam is adjusted such that a region of exposed photoresist material is formed, for example, during positive half cycles of the frequency modulated signal to be stored and a region of unexposed photoresist material is left during the negative half cycles. Accordingly, the first and second indicia representative of the stored information is a lineal series of exposed and unexposed portions of the surface coating, respectively.

A preferred embodiment of a reading apparatus is described employing a read laser for producing a polarized collimated beam of light having a prefer red angle of polarization. A read optical system directs and images the laser beam to impinge upon the indicia carried upon the surface of the video disc member. The video disc member is employed for storing a frequency modulated signal on its surface in the form of a lineal series of regions. The regions are alternately specular light reflective and nonspecular light reflective. A read optical system focuses the read beam to a spot of light approximately one micron in diameter and directs the focused spot to impinge upon the lineal series of regions. The intensity of the read beam is adjusted such that a sufficiently strong reflected read beam signal is gathered by the read optical system.

A motion control assembly rotates the video disc member at a uniform rate of speed sufficient to reconstruct the frequency of the originally stored frequency modulated signal. Atypical frequency modulated signal stored in this matter varies in frequency between two megacycles and ten megacylces. The rotational rate of the video disc member is preferentially set at about 1800 rpm to change the spatially stored frequency modulated signal into a real time electrical signal. The motion control assembly includes a translational drive assembly for translating the reading beam at a very constant, and very low velocity along the radius of the rotating disc so as to impinge upon the lineal series of light reflective and light scattering regions contained thereon.

The reflected read beam gathered by the read optical system is directed to a light sensing circuit for changing the intensity modulated reflected light beam to a frequency modulated electrical signal corresponding to the intensity modulated reflected light beam.

A polarization selective beam splitting element is positioned in the read beam path intermediate the read laser source and the video disc member. After the read beam passes through the polarization selective beam splitting element the read light beam is linearly polarized in the preferred plane. A quarterwave plate is positioned intermediate the output of the polarization selective beam splitting element and the video disc member. The quarterwave plate changes the light in the read beam from linear polarization to circular polarization. The reflected light retains its circular polarization until it passes through the quarterwave plate a second time.During this second pass through the quarterwave plate the reflected light is changed by circular polarization back into linear polarized light rotated ninety degrees from the preferred plane established by the polarization selective beam splitting element as described hereinabove.

The polarization selective beam splitting element is responsive to this ninety degree shift in the reflected light beam for diverting the reflected beam to the light sensing circuit and prevents the reflected light beam from reentering the read laser source.

A diverging lens is employed in the read optical system for spreading the substantially parallel light beam from the read laser source to at least fill the entrance aperture of the objective lens.

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

In a recording apparatus, the write function alone is employed for writing the frequency modulated information onto a video disc member. In a video disc player, the read function alone is employed for recovering the frequency modulated information stored on the surface of the video disc member. In a third mode of operation, the read and write functions are combined in a single machine. In this combined apparatus, the read apparatus is employed for checking the accuracy of the information being written by the write apparatus.

To implement the monitoring function, the read beam from the Helium-Neon (He-Ne) read laser is added into the writing beam path. The read optics are adjusted to direct the read beam through the microscope objective lens at a slight angle with respect to the writing beam. The angle is chosen so that the reading beam illuminates an area on the same track being written by the write beam, but at a point that is approximately four - six microns downstream from the writing spot. More specifically, the read beam is imaged upon the information track that was just formed by the write beam.

Sufficient time has been allowed for the information indicia to be formed on the video disc member. In this manner, the read beam is imaged upon alternate regions of different reflectivity. In one form of the read apparatus, the read beam impinges upon the portions of the metal not heated by the write beam and also impinges upon the glass substrate exposed in the apertures just formed by the writing spot. The regions of different reflectivity function to change an impinging read beam of constant intensity into an intensity modulated reflected read beam.

In this monitoring mode of operation, the read laser beam is selected to operate at a wavelength different from that of the write laser beam. A wavelength selective optical filter is placed in the reflected light beam path having a band pass which includes the reading laser beam. Any write laser beam energy which follows the read reflected path is excluded by the filter and therefore cannot interfere with the reading process.

The monitoring mode of operation is employed at the time of writing the video information onto the video disc member as an aid in 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 displayed signal indicates whether the indicia are being formed with the preferred duty cycle. The preferred duty cycle is achieved when on the average the length of a specular reflective region, which represents one half cycle of a frequency modulated signal, is the same as the next succeeding region of non-specular reflectivity, which represents the next consecutive half cycle of a frequency modulated signal.

The read after write or monitoring mode of operation is also utilized in an error checking mode, especially if digital type information is being written.

The input video information is delayed for an interval equal to the accumulative values of the time delay beginning with the frequency modulation of the input video information signal during the write process and continuing through the frequency demodulation of the recovered reflected signal from the sensing circuit, and including the delay of travel time of the point on the storage member moving from the point of storing the input video information signal to the point of impingement of the read light beam. The recovered information is then compared with the delayed input information for accuracy. The existence of too many dissimilarities would be a basis for either rechecking and realigning the apparatus or rejecting the disc.

The read apparatus is suitable for use with a standard home television receiver by adding an RF modulator for adding the video signal to a suitable carrier frequency matched to one of the channels of a standard home television receiver. The standard television receiver then handles this signal in the same manner as are received from a standard transmitting station.

Brief description of the drawings Figure 7 is a block diagram of the write apparatus; Figure 2 is a cross-sectional view of a video disc member prior to writing thereon using the write apparatus shown in Figure 1; Figure 3 is a partial top view of a video disc member after writing has taken place using the write apparatus shown in Figure 1; Figure 4 is a waveform of a video signature employed in the write apparatus shown in Figure 1; Figure 5 is a waveform of a frequency modulated signal used in the write apparatus shown in Figure 1; Figure 6 is a graph showing the intensity of the write laser used in the write apparatus shown in Figure 1; Figure 7 is a graph showing the modulated write beam as changed by the write apparatus shown in Figure 1; Figure 8 is a radial cross-sectional view taken along the line 8-8 of the disc shown in Figure 3;; Figure 9 is a detailed block diagram of a suitable motion control assembly; Figure 10 is a block diagram showing a read apparatus; Figure 11 is a block diagram showing the combination of a read and write apparatus; Figure 12 is a schematic representation showing the read and write beams passing through a single objective lens as used in the block diagram of Figure land Figure 13 is a schematic diagram of a suitable stabilizing circuit for use in the write apparatus shown in Figure 1.

Detailed description of invention The same numeral is used to identify the same element in the several views. The terms recording and storing are used interchangeably for the term writing. The term retrieving is used interchangeably for the term reading.

The apparatus for storing video information in the form of a frequency modulated signal upon an information storage member 10 is shown with reference to Figure 1. An information signal source circuit 12 is employed for providing an information signal to be recorded. This information signal present on a line 14 is a frequency modulated signal having its informational content in the form of a carrier frequency having frequency changes in time representing said information to be recorded. Figure 5 shows a typical example of a frequency modulated signal. The information signal source circuit 12 employs a video signal circuit 16 for providing an information signal on a line 18 having its informational content in the form of a voltage varying with time format. Figure 4 shows a typical example of a voltage varying with time signal.A frequency modulator circuit 20 is responsive to the video signal circuit 16 for converting the voltage varying with time signal to the frequency modulated signal on the line 14 as shown in Figure 5.

The information storage member 10 is mounted upon a turntable 21. The member 10 is shown in Figure 2 with no indicia formed thereon and includes a substrate 22 having a first surface 24 and a light responsive coating 26 covering the first surface 24. A motion control assembly 28 imparts uniform motion to the storage member 10 relative to a write beam 29' generated by a light source 30. The motion control assembly 28 is shown and described in greater detail with reference to Figure 9. The motion control assembly 28 includes a rotational drive circuit 32 for providing uniform rotational motion to the information storage member 10 and translational drive circuit 34 synchronized with the rotational drive circuit 32 for moving the focused light beam 29' radially across the coating 26.The motion control assembly 28 further includes an electrical synchronizing assembly 36 for maintaining a constant relationship between the rotational motion imparted to the member 10 by the rotational drive circuit 32 and the translational motion imparted to the light beam 29 by the translational drive circuit 34.

The light source 30 provides a beam of light 29 which is of sufficient intensity for interacting with or altering the coating 26 while the coating 26 is in motion and positioned upon the moving information storage member 10. Additionally, the intensity of the light beam 29' is sufficient for producing permanent indicia in the coating 26 representative of the information to be recorded. A suitable light source 30 comprises a writing laser for producing a collimated writing beam of polarized monochromatic light.

Referring again to Figure 2, there is shown a cross-sectional view of a first configuration of a suitable video disc member 10. A suitable substrate 22 is made of glass and has a smooth, flat, planar first surface 24. The light responsive coating 26 is formed upon the surface 24.

In one of the disclosed embodiments, the coating 26 is a thin, opaque metallized layer having suitable physical properties to permit localized heating responsive to the impingement of the write light beam 29 from the writing laser 30. In operation, the heating causes localized melting of the coating 26 accompanied by withdrawal of the molten material towards the perimeter of the melted area. Upon freezing, this leaves a permanent aperture such as at 37, shown in Figures 3 and 8, in the thin metal coating 26. The aperture 37 is one type of indicia employed for representing information. In this embodiment, successively positioned apertures 37 are separated by a portion 38 of the undisturbed coating 26. The portion 38 is the second type of indicia employed for representing information.A more detailed description concerning the process by which the indicia 37 and 38 represent the frequency modulated signal is given with reference to Figures 5 through 8.

A movable optical assembly 40 and a beam steering optical assembly 41 collectively define an optical path for the light beam 29 issuing from the light source 30. The optical assemblies image the read beam 29 into a spot 42 upon the coating 26 carried by the storage member 10. The optical path is also represented by the line identified by the numerals 29 and 29'.

A light intensity modulating assembly 44 is positioned in the optical path 29 between the light source 30 and the coating 26. In its broadest mode of operation, 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 Figure 5. This frequency modulated signal causes the assembly 44 to change between its higher light transmitting state and its lower light transmitting state during each cycle of the frequency modulated signal. This rapid change between transmitting 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.

Thereafter, the modulated light beam, now represented by the numeral 29', is imaged upon the coating 26 by the optical assemblies 40 and 41. As the modulated light beam 29' impinges upon the coating 26, indicia are formed in said coating 26 representative of the frequency modulated signal to be stored.

The light intensity modulating assembly 44 includes an electrically controllable subassembly 46 which is responsive to the frequency modulator 20 for varying the intensity of the light beam 29' above a predetermined intensity at which the focused beam 29' alters the coating 26 carried by the information storage member 10. Additionally, the electrically controllable subassembly 46 is responsive to the frequency modulator 20 for varying the intensity of the light beam below a predetermined intensity at which the focused beam 29' fails to alter the coating 26. The alterations formed in the coating 26 are representative of the frequency modulated signal to be stored.When a photoresist layer forms the coating 26 carried by the information storage member 10, the alterations are in the form of exposed and unexposed photoresist areas analogous to the previously described apertures and portions 37 and 38, respectively.

When the coating 26 carried by the information storage member 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 the predetermined intensity at which the focused beam 29' fails to melt the metal surface.

The light intensity modulating assembly 44 includes a stabilizing circuit 48 for providing a feedback signal employed for temperature stabilizing the operating level of the electrical controllable subassembly 46to operate between a predetermined higher light intensity and 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", issuing 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 connected to the electrically controllable subassembly 46 over the lines 50a and 50b to stabilize its operating level.

The light sensing means produces an electrical feedback signal which is representative of the average intensity of the modulated light beam 29'. In this manner, the light intensity modulating assembly 44 is stabilized to issue the light beam at a substantially constant average power level. The stabilizing circuit 48 also includes level adjustment means for selectively adjusting the average power level of the light beam 29' to a predetermined value to achieve the preferred duty cycle 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 includes an objective lens 52 and a hydrodynamic air bearing 54 for supporting the lens 52 above the coating 26. The laser beam 29' generated by the laser source 30 is formed of substantially parallel light rays. In the absence of the lens 66, these substantially parallel light rays have substantially no natural tendency to diverge. Then the objective lens 52 has an entrance aperture 56 larger in diameter than the diameter of the light beam 29'. A planar convex diverging lens 66 positioned in the light beam 29' is employed for spreading the substantially parallel light beam 29' to at least fill the entrance aperture 56 of the objective lens 52.

The beam steering optical assembly 41 further includes a number of mirror members 58, 60, 62 and 64 for folding the light beams 29' and 29" as desired.

The mirror 60 is shown as a planar mirror and is employed for making strictly circular tracks rather than the preferred spiral tracks. Spiral tracks require only a fixed mirror.

As previously described, the light source 30 produces a polarized laser beam 29. The electrically controllable subassembly 46 rotates the plane of polarization of this laser beam 29 under the control of the frequency modulated signal. A suitable electrically controllable subassembly includes 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 is responsive to the frequency modulated signal on the line 14. The output from the Pockels cell driver 72 provides driving signals to the Pockels cell 68 for rotating the plane of polarization of the laser beam 29. The linear polarizer 70 is orientated in a predetermined relationship with respect to the original plane of polarization of the laser beam 29 issuing from the laser source 30.

As seen with reference to Figure 7, the maximum lighttransmitting axis of the linear polarizer 70 is positioned at right angle with the angle of polarization of the light issuing from the source 30. Because of this arrangement, minimum light exits the polarizer 70 with zero degree rotation added to the write beam 29 by the Pockels cell 68. Maximum light exits the polarizer 70 with ninety degree rotation added to the write beam 29 by the Pockels cell 68. This positioning of the linear polarizer as described is a matter of choice. By aligning the maximum light transmitting axis of the polarizer 70 with the angle of polarization of the light issuing from the laser source 30, the maximum and minimum states would be opposite from that described when subjected to zero degrees and ninety degree rotation.However, the write apparatus would essentially operate the same.

The linear polarizer 70 functions to attenuate the intensity of the beam 29 which is rotated away from its natural polarization angle. It is this attenuating action by the linear polarizer 70 which forms a modulated laser beam 29' corresponding to the frequency modulated signal. A Glanprism is suitable for use as a linear polarizer 70.

The Pockels cell driver 72 is AC coupled to the Pockels cell 68. The stabilizing feedback circuit 48 is DC coupled to the Pockels cell 68.

Referring collectively to Figures 4 through 7, there are shown selective waveforms of electrical and optical signals which are present in the embodiment shown with reference to Figure 1. A video signal generated by the video signal source circuit 16 is shown in Figure 4. A typical device for generating such a video signal is a television camera or a video tape recorder playing back a previously recorded signal generated by a television camera. A flying spot scanner is a still further source of such a video signal. The information signal shown in Figure 4 is typically a one volt peak-to-peak signal having its informational content in the form of a voltage varying with time format is represented by a line 73.

The maximum instantaneous rate of change of a typical video signal is limited by the 4.5 megacycles bandwidth. This video signal is of the type which is directly displayable on a television monitor.

The video signal shown in Figure 4 is applied to the frequency modulator 20 as shown in Figure 1.

The modulator 20 generates the frequency modulated waveform 74 shown in Figure 5. The informational content of the waveform shown in Figure 5 is the same as the informational content of the waveform shown in Figure 4, but the form is different. The informational signal shown in Figure 5 is a frequency modulated signal having its informational content in the form of a carrier signal having frequency changes in time about a center frequency.

By inspection, it can be seen that the lower amplitude region, generally indicated by a numeral 75, of the video waveform 73 shown in Figure 4, corresponds to the lower frequency portion of the frequency modulated signal 74 shown in Figure 5.

One such cycle of the lower frequency portion of the frequency modulated signal 74 is indicated generally by a bracket 76. A higher amplitude region, indicated generally by the numeral 77 of the video waveform 73, corresponds to the higher frequency portions of the frequency modulated signal 74. One complete cycle of the higher frequency portion of the frequency modulated signal 74 is represented by a bracket 78. An intermediate amplitude region, generally indicated with a numeral 79 of the video waveform 73, corresponds to the intermediate frequency#por- tions of the frequency modulated signal 74. A single cycle of the higher frequency portion of the frequency modulated signal representing the intermediate amplitude region 79 is indicated by a bracket 79a.

By an inspection of Figures 4 and 5, it can be seen that the frequency modulator 20, shown in Figure 1, converts the voltage varying with time signal shown in Figure 4, to a frequency modulated signal as shown in Figure 5.

Figure 6 illustrates the intensity of the writing beam 29 generated by the write laser 30. The intensity of the write beam 29 is shown to be at a constant level represented by the line 80. After initial setup procedures, this intensity remains unchanged.

Figure 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 having a plurality of upper peaks 92 representing the higher light transmitting state of the light intensity modulating assembly 44, and having a plurality of valleys 94 representing the low light transmitting state of the light intensity modulating assembly 44. The line 80 representing the maximum intensity of the laser 30 is superimposed with the waveform 29' to show that some loss in light intensity occurs in the assembly 44. This loss is indicated by a line 96 showing the difference in the intensity of the light beam 29' generated by the laser 30 and the maximum intensity 92 of the light beam 29' 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 an inspection of Figures 6 and 7.

Figure 6 shows the unmodulated beam 29 having a constant intensity represented by the line 80. Figure 7 shows the modulated beam 29' having maximum levels of intensity indicated at 92 and minimum levels of intensity indicated at 94.

The intensity modulation of the writing beam 29 is compared to the rotational effect of the Pockels cell 68 by reference to lines 98, 100 and 102. The intersection of the line 98 with the line 29' shows the intensity of the beam 29' issuing from the linear polarizer 70 when the Pockels cell 68 adds no rotation to the angle of polarization of the light passing therethrough. The intersection of the line 100 with the line 29' shows the intensity of the beam 29' issuing from the linear polarizer 70 when the Pockels cell 68 adds a forty-five degree rotation to the angle of polarization of the light passing therethrough. The intersection of the line 102 with the line 29' shows the intensity of the read beam 29' issuing from the linear polarizer 70 when the Pockels cell 68 adds a ninety degree rotation to the angle of polarization of the light passing therethrough.

The formation of an aperture, such as 37 shown in Figures 3 and 8, by the intensity modulated beam 29', shown in Figure 7 can best be understood by a comparison between the two Figures 7 and 8.

The line 100 is drawn midpoint between the intensity 92 representing the higher light transmitting state of the assembly 44 and the intensity 94 representing the lower transmitting state of the assembly 44. The line 100 represents the intensity generated by the assembly 44 when the Pockels cell 68 rotates the angle of polarization of the write beam 29 passing therethrough through an angle of fortyfive degrees. Additionally, the line 100 represents the threshold intensity of the modulated beam 29' required to form an indicia in the light responsive coating 26. This threshold is reached upon rotation of the angle of polarization of the write beam 29 through an angle of forty-five degrees.

By a comparison between Figures 7 and 8, it can be seen that an aperture 37 is formed while the Pockels cell 68 is rotating the angle of polarization of the write beam 29 passing therethrough between the angle of forty-five degrees and ninety degrees and back to forty-five degrees. No aperture is formed while the Pockels cell 68 is rotating the angle of polarization of the write beam 29 passing therethrough between the angle of forty-five degrees and ninety degrees and back to forty-five degrees.

Referring again to Figure 3, there is shown a top view of the video disc member shown in radial cross-sectional view in Figure 8. An inspection of this Figure 3 is helpful in understanding the manner in which the lineal series of light reflecting and light scattering regions 38 and 37 are formed upon the video disc member 10. The disc member 10 is rotated at a preferred rotational rate of 1800 rpm and the indicia 37 and 38 are formed in the light responsive coating 26 as shown with reference to Figure 8. The motion control assembly 28, shown with reference to Figure 1, forms the apertures 37 in circular track-like fashion. A numeral 104 is employed to identify a section of an inner track, and a numeral 105 is employed to identify a section of an outer track.A dashed line 106 represents the center line of the track 105 and a dashed 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. Two microns is a typical distance between center lines of adjacent tracks. The width of an aperture 37 is indicated by the length of a line 109. A typical width of an aperture is one micron. The distance between ajdacent apertures is represented by the length of a line 110. This distance between adjacent tracks is known as the intertrack region and typically is one micron in length. The length of an aperture is represented by a line 112 and typically varies between 1.0 and 1.5 microns. All of these dimensions depend upon many variables in the write apparatus.For example, these dimensions vary depending upon the frequency range generated by the frequency modulator 20, the size of the spot 42 formed by the write optical systems 41 and 42 and the rotational speed selected for the disc 10.

Referring to Figure 9, there can be seen a more detailed block diagram of the motion control assembly 28 shown with reference to Figure 1. The rotational drive circuit 32 includes a spindle servo circuit 130 and a spindle shaft 132. The spindle shaft 132 is integrally joined to the turntable 21. The spindle shaft 132 is driven by a printed circuit type motor 134. The rotational motion 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 subcarrier crystal oscillator 136 which forms a portion of the synchronization assembly 36. The synchronization assembly 36 further includes a first divider circuit 138 and a second divider circuit 140.

The first divider circuit 138 reduces the color subcarrierfrequency generated in the oscillator circuit 136 down to a rotational reference frequency. The spindle shaft 132 contains a tachometer 143 for generating a frequency signal indicating the exact rotational speed of the shaft 132 and turntable 21 combination.

The tachometer signal is available over a line 142 and the rotational reference signal from the first divider circuit 138 is available on a line 144. The tachometer signal on line 142 is applied to the spindle servo circuit 130 and the rotational reference signal on the line 144 is also applied to the spindle servo circuit 130. The spindle servo circuit 130 phase compares these two input signals. When the phase of the tachometer signal leads the phase of the rotational reference signal, the rate of rotation is too high and a signal is generated in the spindle servo circuit 130 for application to the motor 134 over a line 146 to slow the rotational speed and bring the tachometer signal into phase agreement with the rotational reference signal.When the phase of the tachometer signal lags the phase of the rotational reference signal as compared in the spindle servo circuit 130, the rate of rotation is too slow and a signal is generated in the spindle servo circuit 130 for application to the motor 134 over a line 148 to increase the rotational speed and bring the phase of the tachometer signal into agreement with the phase of the rotational reference signal.

The second divider circuit 140 reduces the color subcarrier frequency generated by the oscillator 136 down to a translational reference frequency for advancing the translational drive circuit 34 a fixed distance for each complete revolution of the member 10. In the preferred embodiment, the distance advanced by the translational drive circuit 34 for each revolution of the member 10 is a distance of two microns.

The color subcarrier crystal oscillator 136 with its two divider circuits 138 and 140 functions as an electrical synchronizing circuit for maintaining a constant relationship between the rotational motion of the disc as provided by the rotational motion of the disc as provided by the rotational drive assembly 32 and the translational motion between the write beam 29 and the coating 26 is provided by the translation drive assembly 34.

The movable optical assemblies illustrated in Figures 1,10 and 11 are mounted on a platform indicated at 142'. This movable platform is driven radially by the translational drive 34 which advances the platform 142' 2.0 microns per revolution of the spindle shaft 132. This translational movement is radial with respect to the rotating disc 10. This radial advancement per revolution of the spindle shaft 132 is identified as the pitch of the recording. Since the pitch uniformity of the finished recording depends on the steady advance of the optical assemblies mounted on the platform 142', care is taken to lap a lead screw 143 in the translation drive 34, preload a translation drive nut 144' which engages the lead screw 143 and make the connection between the nut 144' and the platform 142' as stiff as possible as represented by a bar 146'.

Referring to Figure 10, there is shown a read apparatus which is employed for retrieving the frequency modulated signal stored on the information storage member 10 as a lineal series of indicia 37 and 38 previously described. A reading beam 150 is generated by a read laser 152 which produces a polarized, collimated beam 150 of light. A support member, such as the turntable 21, is employed for holding the information storage member 10 in a substantially predetermined position.

A stationary read optical assembly 154 and a movable optical assembly 156 define a read optical path over which the read light beam 150 travels between the laser source 152 and the information storage member 10. Additionally, either of the optical assemblies can be employed to focus the light beam 150 upon the alternately positioned light reflective regions 38 and the light scattering regions 37 carried in successive positions upon the information storage member 10. The movable optical assembly 156 is employed for collecting the reflections from the light reflective regions 38 and the light scattering regions 37. The motion control assembly 28 provides relative motion between the read beam 150 and the alternate regions of light reflectivity 38 and light scattering 37.

The optical assemblies 154 and 156 also define the optical path travelled by the beam reflected from the coating 26. The path of the reflected beam is identified by the numeral 150'. This reflected light path 150' includes a portion of the initial read beam path 150. In those portions where the reflected beam 150' coincides with the read beam 150, both numerals 150 and 150' are used. A light sensing element 158 is positioned in the reflected light beam path 150' and is employed for generating a frequency modulated electrical signal corresponding to the reflections impinging thereupon. The frequency modulated electrical signal generated by the light sensing element 158 is present on a line 160 and has its informational content in the form of a carrier frequency having frequency changes in time corresponding to the stored information.The output of the light sensing circuit 158 is applied to a discriminator circuit 162 by an amplifier 164. The discriminator circuit 162 is responsive to the output of the light sensing circuit 158 and is employed for changing the frequency modulated electrical signal into a time dependent voltage signal representing the stored information. The time dependent voltage signal is also identified as a video signal and it is present on a line 165. This time dependent voltage signal has its informational content in the form of a voltage varying with time format and is suitable for display over a standard television monitor 166 and/or an oscilloscope 168.

The read optical assemblies 154 and 156 further include a polarization selective beam splitting member 170 which functions as a beam polarizer to the incident beam 150 and which functions as a selective beam splitter to the reflected beam 150'. The read optical assemblies further include a quarterwave plate 172. The beam polarizer 170 filters out from the read beam 150 any light waves which are not aligned with the axis of polarization of the beam polarizer 170. With the axis of polarization of the read beam 150 fixed in a particular orientation by the member 170, the quarterwave plate 172 changes the plane of polarization from linear to circular. The member 170 and the quarterwave plate 172 are disposed in the read light beam path 150. The member 170 is located between the source 152 of the read beam 150 and the quarterwave plate 172.The quarterwave plate 172 is also located in the reflected read beam path 150'. Therefore, not only does the quarterwave plate 172 change the read beam polarization from linear to circular during its travel from the read laser 152 to the information storage member 10, but the quarterwave plate 172 further changes the circularly polarized reflected light back into linearly polarized light which is rotated ninety degrees with respect to the preferred direction fixed by the source 152 and the member 170. This rotated beam 150' is selectively directed to the light sensing element 158 which changes the reflected light beam 150' into a corresponding electrical signal. It is to be noted that the member 170 reduces the intensity of the incident light beam 150 as it passes therethrough.This drop in intensity is compensated for by setting the initial intensity of the read beam 150 to a level sufficient to offset this reduction.

The quarter wave plate 172 gives a total rotation of ninety degrees to the reflected beam 150' with respect to the incident beam 150 during the change from linear polarization to circular polarization and back to linear polarization. As previously mentioned, the member 170 is also a beam splitting cube in the reflected read beam path 150'. As the plane of polarization of the reflected read beam 150' is shifted ninety degrees due to its double passage through the quarterwave plate 172, the beam splitting cube portion of the member 170 directs the reflected read beam 150' to the light sensing circuit 158. A suitable element for functioning in the capacity of 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 having its information content in the form of a carrier frequency having frequency variations in time varying from the carrier frequency. The optical assemblies 154 and 156 further comprise the objective lens 52 supported by a hydrodynamic air bearing member 54 which supports the lens 52 above the coating 16 carried by the information storage member 10.

As previously described, the read beam 150 is formed with substantially parallel light rays. The objective lens 52 has an entrance aperture 56 larger in diameter than the diameter of the read beam 150 as it is generated by the laser source 152. A planar convex diverging lens 174 is provided intermediate the laser source 152 and the entrance aperture 56 of the objective lens 52 for spreading the substantially parallel light rays forming the reading beam 150 into a light beam 150 having a diameter sufficient to at least fill the entrance aperture 56 of the objective lens 52. The optical assemblies 154 and 156 further include a number of stationary, planar-shaped mirrors 176 and 178 for folding the read light beam 150 and the reflected light beam 150' along a path calculated to impinge upon the previously mentioned elements.

An optional optical filter 180 is positioned in the reflected beam path 150' and filters out all wavelengths other than that of the incident beam.

The use of this filter 180 improves picture quality as displayed over the television monitor 166. This filter 180 is essential when the read system is used with the write system as discussed in greater detail with reference to Figure 11. In this read after write environment, a portion of the write beam 29 travels along the reflected read beam path 150'. The filter stops this portion of the write beam and passes the full intensity of the reflected beam 150'.

An optional converging lens 182 is positioned in the reflected beam path 150' for imaging the reflected beam onto the active area of the light sensing element 158. This converging lens 182 reduces the diameter of the reflected beam 150' and concentrates the light intensity of the reflected beam upon the active area of the light sensing element 158.

The amplifier 164 amplifies the output 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.

Referring again to the electrical and optical waveforms shown in Figures 4 through 7, these waveforms are also generated by the read apparatus, shown in Figure 10 during the retrieval of the frequency modulated signal stored in the coating 26 carried by the disc 10. Figure 6 shows a laser source generating a write laser beam having a constant intensity represented by the line 80. The read laser 152 generates a read beam 150 having a constant intensity but at a lower level.

Figure 7 shows an intensity modulated write laser beam. The reflected read beam 150' is intensity modulated by the act of impinging upon the light reflective and light scattering regions 38 and 37 carried on the disc member 10. The reflected read beam 150' will not be a perfect squarewave as shown in Figure 7. Rather, the square edges are rounded by the finite size of the read spot.

Figure 5 shows a frequency modulated electrical signal having its informational content in the form of a carrier signal having frequency changes in time varying about the center frequency. The output of the light sensing element 158 is the same type of signal. Figure 4 shows a video signal having its informational content in the form of a voltage varying with time format. The output of the demod ulator 162 is the same type of signal.

The motion control assembly 28 shown in Figure 10 operates in the same manner as the motion control assembly 28 shown in Figure 1. In the read appai;#tus, the motion control assembly 28 produces a rotational motion to the disc member under the control of a rotational drive assembly 32. The assembly 28 further produces a translational motion for moving the movable read optical assembly 156 radially across the surface of the storage member.

The assembly 28 further includes a synchronizing circuit for maintaining a constant relationship between the rotational motion and the translational motion so that the read beam 150 impinges upon the information tracks carried by the disc member 10.

Portions of typical information tracks are shown as 104 and 105 in Figure 3.

Referring to Figure 11, there is shown a block diagram illustrating the combination of the write apparatus shown in Figure 1, and the read apparatus shown in Figure 10. The elements shown in Figure 11 operate in an identical manner as previously described and this detailed operation is not repeated here. Only a brief description is 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, write optical assembly 41 includes the mirror 58. The movable, write optical assembly 40 includes the diverging lens 66, a partially transmissive mirror 200, a planar mirror 60 and the objective lens 52. The modulated write beam 29' is imaged to a write spot 42 upon the light responsive coating and interacts with the coating to form indicia as previously described.

The read beam path is shown at 150. The read optical assemblies define a second optical path for the read beam 150 between the read laser 152 and the information storage record carrier 10. The fixed, read optical assembly 154 includes the mirror 176.

The movable, read optical assembly 156 includes the diverging lens 174, the polarization shifting means 172, a second fixed mirror 202, the selectively transmissive mirror 200, the planar mirror 60 and the lens 52. The read beam 150 is imaged to a read spot 157 at a point spaced downstream from the write spot 42, as is more completely described with reference to Figure 12. The mirror 200 is a dichroic mirror which is transmissive at the wavelength of the write beam 29' and which is reflective at the wavelength of the read 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 alter the light responsive coating 26 to retain indicia representative of the video signal to be stored, the intensity of the read beam 150 should only be sufficient to illuminate the indicia formed in the coating 26 and provide a reflected light beam 150' of sufficient intensity to provide a good signal after collection by the read optical assembly and conversion from an intensity modulated reflected beam 150' to a frequency modulated electrical signal by the light sensing circuit 158.

The fixed mirror 58 in the write optical path and the two fixed mirrors 176 and 202 in the read optical path are employed for directing the write beam 29' toward the objective lens 56 at a controlled angle with respect to the read beam 150. This angle between the two incident beams provides a spacing between the write spot 42 and read spot 157 as they are each respectively imaged upon the coating 26.

In operation, a sufficient spacing has been found to be four to six microns. This distance corresponds to an angle too small to show clearly in Figure 12.

Accordingly, this angle is exaggerated in Figure 12 for purpose of illustration only.

The read beam 150' is demodulated in a discriminator circuit 162 and displayed on a standard television monitor 166 and an oscilloscope 168. The television monitor 166 shows the pictorial quality of the recording and the oscilloscope 168 shows the video signal in more detail. This read after write function allows the quality of the video signal being stored during a write operation to be instantaneously monitored. In the event that the quality of the stored signal is poor, it is known immediately and the write procedure can be corrected or the information storage member 10 storing the poor quality video information signal can be discarded.

In the read after write mode of operation, the write laser 30 and the read laser 152 are operating at the same time. A dichroic mirror 200 is employed for combining the read beam 150 into the write beam 29'. In this read after write mode of operation, the wavelength of the write beam 29 is chosen to be different from the wavelength of the read beam 150.

An optical filter 180 is employed for blocking any portion of a write beam which has followed the reflected read beam path. Accordingly, the optical filter 180 passes the reflected read beam 150' and filters out any part of the write laser beam 29' following the reflected read beam path 150'.

In the comparison mode of operation, the read after write operation is practiced as described with reference to Figure 11. When operating in this monitoring mode of operation, a comparator circuit 204 compares the output of the demodulator 162 with the original video information signal provided by the source 18.

More specifically, the video output of the discriminator 162 is applied to a comparator 204 over a line 206. The other input of the comparator 204 is taken from the video source 16 over the line 18, an additional line 208 and through a delay line 210. The delay line 210 imparts a time delay to the input video information signal equal to the accumulated values of the delay beginning with the frequency modulation of the input video information signal and extending through the frequency demodulation of the recovered electrical signal from the sensing circuit 158. This delay also includes the delay of travel time from the point on said storage member 10 at which the input video information signal is stored upon the information storage member by the write spot 42 and continuing to the point of impingement of the read spot 157.

The correct amount of delay is best generated by making the delay circuit 210 a variable delay circuit which is adjusted for optimum operation.

Ideally, the video output signal of the discriminator 162 is identical in all respects to the video input signal on the lines 18 and 208. Any differences noted represent errors which might be caused by imperfections in the disc's surface or malfunctions of the writing circuits. This application, while essential if recording digital information, is less critical when other information is recorded.

The output signal from the comparator circuit 204 may be counted, in a counter (not shown), for establishing the actual number of errors present on any disc. When the errors counted exceed the predetermined selected number, the writing operation is terminated. If necessary, a new disc can be written. Any disc with excessive errors can then be reprocessed.

In Figure 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 of the frequency modulator 20 and the amplifier 164 shown with reference to Figure 10.

Turning next to Figure 12, there is shown in somewhat exaggerated form, the slightly differing optical paths of the intensity modulated write beam 29' from the writing laser 30 and the unmodulated read beam 150 from the reading laser 152. The information storage member 10 is moving in the direction indicated by an arrow 217. This shows an unexposed coating 26' approaching the write beam 29' and a lineal series of apertures 37 leaving the intersection of the write beam 29' and the coating 26.

The writing beam 29' coincides with the optical axis of the microscope objective lens 52. The central axis of the reading beam 150 shown as 212 makes an angle with the central axis of the write beam 29' shown as 214. The angle is represented by a double headed arrow 216. Due to this slight difference in optical paths of the write beam 29' and read beam 150 through the lens 52, the write spot 42 falls a distance ahead of the read spot 157. The write spot 42 falls a distance ahead of the read spot 157. The write spot 42 leads the read spots 157 by a distance equal to the length of a line 218. The length of the line 218 is equal to the 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 recording is read in its final solidified state.If it were read too soon while the metal was still molten, the reflection from the edge of the aperture would fail to provide a high quality signal for display on the monitor 166.

Referring to Figure 13, there is shown an idealized diagram of a Pockels cell stabilizing circuit 48 suitable for use in the apparatus of Figure 1. As is known, a Pockels cell 68 rotates the plane of polarization of the applied write light beam 29 as a function of an applied voltage as illustrated with reference to Figure 7.

Depending upon the individual Pockels cell 68, a voltage change of the order of 100 volts causes the cell to rotate the plane of polarization of the light passing therethrough a full ninety degrees. The Pockels cell driver functions to amplify the output from the information signal source 12 to a peak-topeak output swing of 100 volts. This provides a proper input driving signal to the Pockels cell 68. The Pockels cell driver 72 generates a waveform having the shape shown in Figure 5 and having a peak-topeak voltage swing of 100 volts.

The Pockels cell should be operated at an average rotation of forty-five degrees in order to make the modulated light beam intensity most faithfully reproduce the electrical drive signal. A bias voltage must be provided to the Pockels cell for keeping the Pockels cell at this average operating point. In practice, the electrical bias voltage corresponding to a forty-five degree rotation operating point varies continuously. This continuously changing bias voltage is generated through the use of a servo feedback loop. This feedback loop includes the comparison of the average value of the transmitted light to an adjustable reference value and applying the difference signal to the Pockels cell by means of a DC amplifier. This arrangement stabilizes the operating point.The reference value can be adjusted to correspond to the average transmission corresponding to the forty-five degree operating point and the servo feedback loop provides corrective bias voltages to maintain the Pockels cell at this average rotation of forty-five degrees.

The stabilizing circuit 48 includes a light sensing means 225. A silicon diode operates as a suitable light sensing means. The diode 225 senses a portion 29" of the writing beam 29' issuing from the optical modulator 44 and passing through the partially reflective mirror 58 as shown in Figure 1. The silicon diode 225 functions in much the same fashion as a solar cell and is a source of electrical energy when illuminated by incident radiation. One output lead of the silicon diode 225 is connected to common reference potential 226 by a line 227. The other output lead of the diode 225 is connected to one input of a differential amplifier 228 by a line 230. The output leads of the silicon cell 225 are shunted by a load resistor 232 which enables a linear response mode.

The other input to the differential amplifier 228 is connected to an adjustable arm 234 of a potentiometer 236 by a line 238. One end of the potentiometer 236 is connected to reference potential 226 by a line 240. A source of power 242 is coupled to the other end of the potentiometer 236 which enables the adjustment of the differential amplifier 228 to generate a feedback signal on the lines 244 and 246 for adjusting the average power level of the modulated laser beam 29' to a predetermined value.

The output terminals of the differential amplifier 228 are, respectively, connected through resistive elements 248 and 250 and output lines 244 and 246 to the input terminals of the Pockels cell 68 shown in Figure 1. The Pockels cell driver 72 is AC coupled to the Pockels cell 68 by way of capacitive elements 252 and 254, respectively, while the differential amplifier 228 is DC coupled to the Pockels cell 68.

In operation, the system is energized. The portion 29" of the light from the writing beam 29' impinging on the silicon diode 225 generates a differential voltage at one input to the differential amplifier 228.

Initially, the potentiometer 236 is adjusted so that the average transmission through the Pockels cell cor responds to forty-five degree of rotation. Thereafter, if the average level of intensity impinging on the silicon cell 225 either increases or decreases, a correcting voltage will be generated by the differential amplifier 228. The correcting voltage applied to the Pockels cell 68 is of a polarity and magnitude adequate to restore the average level of intensity to the predetermined level selected by adjustment of the input voltage to the other input of the differential amplifier over the line 238, by movement of the movable arms 234 along the potentiometer 236.

The adjustable arm 234 of potentiometer 236 is the means for selecting the average level of intensity of the light generated by the write laser 30. Optimum results are achieved when the length of an aperture 37 exactly equals the length of the next succeeding space 38 as previously described. The adjustment of potentiometer 236 is the means for achieving this equality of length. When the length of an aperture equals the length of its next adjacent space, a duty cycle of fifty-fifty is achieved. Such duty cycle is detectable by examining the display of the just written information on the TV monitor and/or oscilloscope 166 and 168, respectively, as previously described. Commercially acceptable results occur when the length of an aperture 37 varies between forty and sixty percent of the combined length of an aperture and its next successively positioned space.

In other words, the length of an aperture and the next successively positioned space is measured. The aperture can then be a length falling within the range of forty and sixty percent of the total length.

Referring to Figure 8, there is shown a radial cross-section of an information track shown with reference to Figure 3 in which a specular light reflective region 38 is positioned intermediate a pair of non-specular light reflective regions 37. In the radial cross-sectional view shown in Figure 8, the impinging read or write beam is moving relative to the member 10 in the direction represented by the arrow 217. This means that a reading beam impinges first upon the specular light reflective region 38a followed by its impingement upon the non-specular light reflective region 37a. In this configuration, the positive half cycle of the signal to be recorded is represented by a specular light reflective region 38a and the negative half cycle of the signal to be recorded is represented by the non-specular light reflective region 37a.The duty cycle of the signal shown with reference to Figure 8 is a fifty percent duty cycle insofar as the length of the specular light reflected region 38a as represented by a bracket 260, is equal in length to the length of the non-specular light reflective region 37a as represented by the bracket 262. This preferred duty cycle is set up by the combination of adjusting the absolute intensity of the write beam 29, by adjusting the power supply of the write laser 30 and by adjusting the potentiometer 236 in the stabilizing circuit 48 to a level wherein an aperture is formed beginning with a forty-five degree rotation of the angle of polarization in the write beam 29.

Referring again to the aperture forming process illustrated with reference to Figures 7 and 8, melting of a thin metal coating 26 occurs when the power in the light spot exceeds a threshold characteristic of the composition and thickness of the metal film and the properties of the substrate. The spot power is modulated by the light intensity modulating assembly 44. The on-off transitions are kept short to make the location of the hole ends precise in spite of 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 the information storing layer.

The average power in the spot required to form an aperture in a thin metal coating 26 having a thickness between 200 and 300 Angstroms is of the order of 200 milliwatts. Since the FM carrierfrequency is about 8 MHz, 8 x 106 holes of variable length are cut per second and the energy per hole is 2.5 and 10-9 joul.

In this first embodiment of a video disc member 10, a portion of the glass substrate is exposed in each aperture. The exposed portion of the glass substrate appears as a region of non-specular light reflectivity to an impinging reading beam. The portion of the metal coating remaining between successively positioned apertures appears as a region of high light reflectivityto an impinging reading beam.

When the forming of first and second indicia is being undertaken using a coating of photoresist, the intensity of the write beam 29' is adjusted to a level such that a forty-five degree rotation of the plane of polarization generates a light beam 29' of threshold intensity for exposing and/or interacting with the photoresist coating 26 while the photoresist coating is in motion and positioned upon the moving information storage member 10. The Pockels cell 68 and Glan-prism 70 combination comprises a light intensity modulating member which operates from the forty-five degree setup condition to a lower light transmitting state associated with a near zero degree state of operation, to a higher light transmitting state, associated with a near ninety degree state of operation.When the intensity of the write light beam 29' increases above the initially adjusted level of predetermined start intensity, and increases towards the higher light transmitting state, the incident write light beam 29' exposes the photoresist illuminated thereby. This exposure continues after the intensity of the write beam reaches the maximum light transmitting state and starts back down towards the initial predetermined intensity associated with a forty-five degree rotation of the plane of polarization of the light issuing from the write laser 30. As the rotation drops below the forty-five degree value, the intensity of the write beam 29' exiting the Glanprism 70 drops below the threshold intensity at which the focused write beam fails to expose the photoresist illuminated thereby.This failure to ex pose the photoresist illuminated thereby continues after the intensity of the write beam reaches the minimum light transmitting state and starts back up towards the initial predetermined intensity associated with a forty-five degree rotation of the plane of polarization of the light issuing from the write laser 30.

The Pockels cell driver circuit 72 is typically a high gain and high voltage amplifier having an output signal providing an output voltage swing of 100 volts. This signal is intended to match the driving requirements of the Pockels cell 68. Typically, this means that the mid-voltage value of the output of the Pockels cell driver 72 provides a sufficient control voltage for driving the Pockels cell 68 through forty-five degree so that about one half of the total available light from the laser 30 issues from the linear polarizer 70. As the output signal from the driver 72 goes positive, more light from the laser is passed. As the output signal from the driver 72 goes negative, less light from the laser is passed.

In the first embodiment using a metal coating 26, the output from the laser 30 is adjusted so as to produce an intensity which begins to melt the metal layer coating 26, positioned on the disc 10, when the output from the driver 72 is zero and the operating point of the Pockels cell is forty-five degrees. Accordingly, as the output from the driver 72 goes positive, melting continues. Also, when the output from the driver 72 goes negative, melting stops.

In a second embodiment using the photoresist coating 26, the output from the laser 30 is adjusted so as to produce an intensity which both illuminates and exposes the photoresist coating 26 when the output from the driver 72 is generating its midvoltage value. Accordingly, as the output from the driver 72 goes positive, the illumination and exposure of the photoresist illuminated by the write beam continues. Also, when the output from the driver 72 goes negative, the illumination continues but the energy in the write beam is insufficient to expose the illuminated region. The term expose is herein being used for its technical meaning which describes that physical phenomenon which accompanies exposed photoresist. Exposed photoresist is capable of being developed and the developed photoresist is removed by standard procedures.Photoresist which is illuminated by light, insufficient in intensity to expose the photoresist, cannot be developed and removed.

In both the first and second embodiments just described, the absolute power level 80 illustrated by the line 80 in Figure 6 is adjusted upward and downward to achieve this effect by adjusting the power supply of the write laser 30. In combination with this adjustment of the absolute power level of the write laser 30, the'potentiometer 236 is also used to cause indicia to be formed in the coating 26 when the beam 29 is rotated above forty-five degrees as previously described.

In a read only system as shown in Figure 10, the optical filter 180 is optional and usually is not required. Its use in a read only system introduces a slight attenuation in the reflected path thus requiring a slight increase in the intensity of the read laser 152 to ensure the same intensity at the detector 158 when compared to a read only system which does not use a filter 180.

The converging lens 182 is optional. In a properly arranged read system the reflected read beam 150' has essentially the same diameter as the working area of the photodetector 158. If this is not the case, a converging lens 182 is employed for concentrating the reflected read beam 150' upon the smaller working area of the photodetector 158 selected.

Claims (112)

1. An information storage member for storing a frequency modulated signal having an informational content in the form of a carrier signal having frequency changes with time varying from a center frequency, said member comprising: an information storage member having an upper surface, said surface carrying a lineal series of indicia positioned in track-like fashion upon said surface; said indicia representing a frequency modulated signal having its informational content in the form of a carrier signal having frequency changes with time varying from a center frequency.

2. The information storage member as claimed in Claim 1, wherein said lineal series of indicia includes alternately specular light reflective and non-specular light reflective regions.

3. The information storage member as claimed in Claim 1, comprising: a glass disc substrate, one surface of said substrate defining said upper surface; and a light sensitive coating carried by said surface; said coating altered to carry said lineal series of spaced indicia positioned in track-like fashion.

4. The information storage member as claimed in Claim 2, wherein said alternately specular light reflective and non-specular light reflective regions are positioned in a spiral path.

5. The information storage member as claimed in Claim 2, wherein each specular light reflective region is a metal coating.

6. Apparatus for storing information in the form of a frequency modulated signal upon an information storage member, comprising: first means for providing an information signal to be recorded, said signal having its informationl content in the form of a carrier signal having frequency changes in time representing said information to be recorded; an information storage member including a substrate having a first surface and a light responsive coating covering said first surface for retaining indicia representative of said information signal; means for imparting uniform motion to said storage member; a light source for providing a light beam, said light beam being of sufficient intensity for interacting with said coating while said coating is in motion and positioned upon said moving information storage member and for altering said coating to retain indicia representative of said information, optical means for defining an optical path between said light source and said coating on said storage member, and for focusing said light beam upon said coating; and light intensity modulating means positioned in said optical path between said light source and said coating on said storage member, said light intensity modulating means operating over a range between a higher light transmitting state and a lower light transmitting state for intensity modulating said light beam with said information to be stored; wherein said light intensity modulating means is responsive to said frequency modulated signal and changes between its higher light transmitting state and its lower light transmitting state during each cycle of said frequency modulated signal for modulating said light beam with the frequency modulated signal to be stored; and wherein said light passing through said light intensity modulating means and focused upon said coating by said optical means forms indicia in said coating representative of said frequency modulated signal to be stored.

7. The apparatus as claimed in Claim 6, wherein said first means comprises: means for providing an initial information signal having its informational .content in the form of a voltage varying with time format; and frequency modulator means, responsive to said means providing an initial information signal for converting said voltage varying with time signal to said frequency modulated signal.

8. The apparatus as claimed in Claim 7, wherein said light intensity modulating means includes electrically controllable means responsive to said frequency modulator means for varying the intensity of said light beam above a predetermined intensity at which the focused beam alters said coating and below said predetermined intensity at which the focused beam fails to alter said coating, said alteration being representative of said frequency modulated signal.

9. The apparatus as claimed in Claim 8, wherein: said initial information signal has its informational content in the form of a voltage varying with time signal suitable for display on a standard television monitor; said light source comprises a writing laser for producing a collimated writing beam of polarized monochromatic light; said substrate defines a smooth flat rigid disc, said first surface being a planar surface; said coating is a thin opaque metal coating having suitable physical properties to permit localized heating responsive to the impingement of light from said writing laser, said heating causing localized melting accompanied by withdrawal of the molten material toward the perimeter of the melted area, leaving upon freezing a permanent aperture in the thin metal coating; and said electrically controllable means is responsive to said frequency modulator means for varying the intensity of said writing beam above said first predetermined intensity at which the focused beam melts said metal coating without vaporizing it and below said predetermined intensity at which the focused beam fails to melt said metal surface.

10. The apparatus as claimed in Claim 6, wherein: said storage member is disc-shaped; said means for imparting relative motion comprises rotational drive means for producing uniform rotational motion of said disc; and wherein said apparatus further comprises: translational drive means synchronized with said rotational drive means for relatively moving said focused light beam radially across said first surface of said disc-shaped storage member; and electrical synchronizing means for maintaining a constant relationship between said rotational motion and said translational motion.

11. The apparatus as claimed in Claim 8, wherein said light intensity modulating means further includes feedback apparatus for stabilizing the operating level of said electrically controllable means to operate between a predetermined lower light intensity, said light intensity modulating means including light-sensing means for sensing at least a portion of the light beam issuing from said electrically controllable means to produce an electrical feedback signal representative of the intensity of the beam and applying thefeedbacksignal to said electrically controllable means to stabilize its operating level.

12. The apparatus as claimed in Claim 11, wherein said light-sensing means produces an electrical feedback signal which is representative of the average intensity of the light beam, the operating level of said light intensity modulating means being stabilized to issue the light beam at a substantially constant average power level.

13. The apparatus as claimed in Claim 9, wherein said optical means includes: an objective lens; and hydrodynamic air-bearing means for supporting said lens above said first surface of said information storage member.

14. The apparatus as claimed in Claim 13, wherein said collimated beam of light has substantially parallel light rays; said objective lens has an entrance aperture larger in diameter than the diameter of said light beam as provided by said light source; and said optical means further includes mirror means for folding said light beam path provided by said light source, and a diverging lens for spreading the substantially parallel light beam from said light source to at least fill said entrance aperture of said objective lens.

15. The apparatus as claimed in Claim 8, wherein said light source produces a polarized laser beam, and said electrically controllable means includes: means for rotating the plane of polarization of said laser beam from said source under control of said frequency modulated signal; and a linear polarizer the output of which is an intensity modulated laser beam corresponding to said frequency modulated signal.

16. The apparatus as claimed in Claim 12, wherein said feedback apparatus includes level adjustment means for selectively adjusting the average power level of said light beam to a predetermined value.

17. The apparatus as claimed in Claim 12, wherein: said electrically controllable means comprises a Pockels cell driver and a Pockels cell device, said Pockels cell driver responding to said frequency modulated signal to provide corresponding driving signals to said Pockels cell device; and wherein said Pockels cell driver is A.C. coupled to said Pockels cell, and said stabilizing feedback apparatus is D.C.

coupled to said Pockels cell.

18. A method for recording information on an information storage member using a laser beam, comprising the steps of: providing a frequency modulated electrical signal to be recorded, said frequency modulated electrical signal having a carrier signal with frequency changes over time corresponding to said information to be stored; controlling the intensity of the transmission of a laser light beam upon a light sensitive surface of an information storage member, using said frequency modulated signal as a control signal; and moving the information storage member at a constant rate relative to said light beam while focusing said light beam upon said light sensitive surface of said information storage member; said controlling step including using said transmitted light beam for irreversibly altering said light sensitive surface of said information storage member under the control of one portion of said frequency modulated signal, as said member moves at a constant rate, and lowering the intensity of the transmitted light beam to said light sensitive surface of said information storage member under the control of a second portion of said frequency varying signal, as said member moves at a constant rate.

19. The method as claimed in C'aim 18, wherein said step of providing a frequency modulated electrical signal includes: providing an initial electrical signal having its informational content in the form of a voltage varying with time format; and changing said voltage varying with time signal to a frequency modulated electrical signal having its informational content in the form of a carrier signal having frequency changes with time corresponding to said voltage variations with time.

20. The method as claimed in Claim 19, wherein said light beam is held stationary and said storage member is moved at said constant rate relative to the stationary beam.

21. The method as claimed in Claim 19, wherein said controlling step includes using said frequency modulated signal for varying the intensity of said light beam above a predetermined intensity at which the focused beam alters said light sensitive surface and below said predetermined intensity at which the focused beam fails to alter said light sensitive surface, said alteration being representative of said frequency modulated signal.

22. The method as claimed in Claim 21, wherein: said initial information signal has its informational content in the form of a voltage varying with time signal suitable for display on a standard television monitor; said controlling step includes producing a modulated collimated writing laser beam of polarized monochromatic light for impinging upon said light sensitive surface of the storage member, said surface being a thin planar opaque metal coating having suitable physical properties to permit localized heating responsive to the impingement of light from said writing laser, said heating causing localized melting accompanied by withdrawal of the molten material toward the perimeter of the melted area leaving upon freezing a permanent aperture in the thin metallized coating; and said controlling step includes using said frequency modulated signal for varying the intensity of said writing beam above said predetermined intensity at which the focused beam melts said metal coating without vaporizing it and below said predetermined intensity at which the focused beam fails to melt said metal surface.

23. The method as claimed in Claim 20, wherein the storage member is disc-shaped and said storage member moving step includes: producing uniform rotational motion of said disc; and synchronizing said rotational motion with movement of said storage member for effecting relative movement of said focused light beam radially across said surface of said disc-shaped storage member to maintain a constant relationship between said rotational motion and said translational motion.

24. The method as claimed in Claim 22, wherein said controlling step futher includes stabilizing the level of modulation of said light beam to operate between a predetermined higher light intensity and a predetermined lower light intensity; sensing at least a portion of the laser writing beam after modulation of the beam to produce an electrical feedback signal representative of the intensity of the beam; and utilizing the feedback signal in said controlling step to effect stabilization of the level of modulation of said writing beam.

25. The method as claimed in Claim 24, wherein said step of sensing at least a portion of said writing beam produces an electrical feedback signal which is representative of the average intensity of the modulated writing beam, the operating level of light beam modulation being stabilized to issue the modulated writing beam at a substantially constant average power level.

26. The method as claimed in Claim 22, wherein said controlling step includes: rotating the plane of polarization of said laser beam under control of said frequency modulated signal; and linearly polarizing the rotating beam to produce a modulated laser beam corresponding to said frequency modulated signal.

27. The method as claimed in Claim 24, wherein said controlling step includes selectively adjusting the average power level of said modulated writing beam to a predetermined value.

28. The method as claimed in Claim 24, wherein said controlling step comprises: amplifying said frequency modulated signal to provide corresponding driving signals to a Pockels cell device; A.C. coupling the amplified frequency modulated signal to the Pockels cell; and D.C.

coupling said feedback signal to said Pockels cell.

29. An optical system for retrieving a frequency modulated signal stored on a surface of an information storage member in the form of a lineal series of regions, said regions being alternately specular light reflective and non-specular light reflective; said system comprising: means for producing a polarized collimated beam of light; support means for holding the information storage member; optical means for defining an optical path between said light beam producing means and said information storage member held upon said support means, and for focusing said light beam upon the alternately positioned specular light reflective areas and non-specular light reflective areas, and said optical means being further employed for collecting reflections from said light reflective areas; means for providing relative motion between said beam of light and said alternate regions for generating reflections from said light reflective region representing the stored frequency modulated signal; and light-sensing means responsive to said reflections for generating a frequency modulated electrical signal corresponding to said reflections, said last mentioned frequency modulated signal having its informational content in the form of a carrier signal having frequency changes in time corresponding to the stored information.

30. The system as claimed in Claim 29, including an information storage member having a surface, said member having a lineal series of regions positioned in track-like fashion upon said surface, said regions being alternately specular light reflective and non-specular light reflective, and the sequence of alternate areas representing a frequency modulated signal having its informational content in the form of a carrier signal having frequency changes with time varying from a center frequency.

31. The system as claimed in Claim 29, including means responsive to the output of said light-sensing means for changing said frequency modulated electrical signal into a time dependent voltage signal representing said stored information, said time dependent voltage signal having its informational content in the form of a voltage varying with time format and being suitable for display by a standard television monitor.

32. The system as claimed in Claim 30, further comprising: a beam polarizer, and a polarization shifting means for shifting the plane of polarization of said beam, said polarizer and shifting means disposed in the path of said light beam with the beam polarizer located between said light beam source and said shifting means, said shifting means rotating said beam during incident and reflected passages through the shifting means to and from the storage member, said polarizer substantially reducing the intensity of the reflected beam passed through said shifting means toward said light beam producing means.

33. The system as claimed in Claim 32, wherein said shifting means is a quarterwave plate for rotating said light beam ninety degrees by accumulative incident and reflected passages through said plate.

34. The system as claimed in Claim 32, wherein said beam polarizer is a polarizing beam splitting cube adapted to direct said reflected beam passed through said shifting means toward said lightsensing means.

35. The system as claimed in Claim 29, wherein said optical means includes: an objective lens; and hydrodynamic air-bearing means for supporting said lens above said surface of said information storage member.

36. The system as claimed in Claim 29, wherein said collimated beam of light has substantially parallel light rays; said objective lens has an entrance aperture larger in diameter than the diameter of said light beam as provided by said light source; and said optical means further includes mirror means for folding said light beam path provided by said light source, and a diverging lens for spreading the substantially parallel light beam from said light source to at least fill said entrance aperture of said objective lens.

37. The system as claimed in Claim 29, wherein: said storage member is disc-shaped; said means for providing relative motion comprises rotational drive means for producing uniform rotational motion of said disc; and wherein said apparatus further comprises: translational drive means synchronized with said rotational drive means for moving said focused light beam radially across said surface of said discshaped storage member; and electrical synchronizing means for maintaining a constant relationship between said rotational motion and said translational motion.

38. A method for reading an information signal stored on a record member, the information signal being in the form of alternate areas of specular light reflective and non-specular light reflective, comprising the steps of: providing an information storage member having an information bearing surface, said surface having a lineal series of regions positioned in track-like fashion upon said surface, said regions being alternately specular light reflective and non-specular light reflective, the sequence of alternate regions representing a frequency modulated signal having its informational content in the form of a carrier signal having frequency changes with time varying from a center frequency; imaging a polarized collimated beam of light upon said series of regions; providing relative motion between said imaged beam of light and said alternate regions for generating reflections from said light reflective regions representing said stored frequency modulated signal; and sensing said reflections and generating a frequency modulated electrical signal corresponding to said reflections, said frequency modulated electrical signal having its informational content in the form of a carrier signal having frequency changes in time from a center frequency.

39. The method as claimed in Claim 38, including the step of demodulating said frequency modulated electrical signal to produce a time dependent voltage signal representing said stored information, said time dependent voltage signal having its informational content in the form of a voltage varying with time format and being suitable for display by a standard television monitor.

40. The method as claimed in Claim 38, further comprising the step of shifting the plane of polarization of said polarized beam by rotating said beam during incident and reflected passages to and from the storage member, thereby substantially reducing the intensity of the reflected beam passed to a light beam source utilized in producing the light beam in said imaging step.

41. The method as claimed in Claim 40, wherein said shifting step includes rotating said light beam ninety degrees by accumulative incident and reflected passages of said second light beam to and from said storage member surface.

42. The method as claimed in Claim 38, wherein the storage member is disc-shaped and said step of providing relative motion includes: producing uniform rotational motion of said disc; and synchronizing said rotational motion with movement of said storage member for effecting relative movement of said imaged light beam radially across said surface of said disc-shaped storage member to maintain a constant relationship between said rotational motion and said translational motion.

43. Information handling apparatus, comprising: first means for providing a video information signal to be recorded, said signal having its informational content in the form of a carrier signal having frequency changes in time representing said information to be recorded; a record carrier including a substrate having a first surface and a light responsive coating covering said first surface for retaining indicia representative of said video signals; a first light source for providing a light beam, said first light beam being of sufficient intensity for interacting with said coating, and for altering said coating to retain indicia representative of said video information; means for imparting relative motion to said record carrier with respect to said light beam; first optical means for defining a first optical path between said first light source and said record carrier including said coating, and for focusing said first light beam upon said coating; light intensity modulating means positioned in said optical path between said light source and said coating on said record carrier, said light intensity modulating means operating over a range between a higher light transmitting state and a lower light transmitting state for intensity modulating said light beam with said information to be stored; said light intensity modulating means being responsive to said frequency modulated signal and changing between its higher light transmitting state and its lower light transmitting state during each cycle of said frequency modulated signal for intensity modulating said light beam with the frequency modulated electrical signal to be stored; said light passing through said light intensity modulating means and focused upon said coating by said optical means for altering said coating to retain indicia representative of said video information; a second light source for providing a second light beam; a second optical means for defining an optical path between said second light source and said record carrier, including a portion of said first optical path, for focusing said second light beam upon said coating; said second light beam being of sufficient intensity for illuminating selected portions of said coating on said carrier, said second light beam reflected from certain ones of said illuminated portions, said light beam being scattered from certain others of said illuminated portions; said second optical means being further employed for collecting said reflections from said certain ones of said illuminated portions; and sensing means responsive to said reflected light for generating a frequency modulated electrical signal corresponding to said reflections, and said last mentioned frequency modulated signal having its informational content in the form of a carrier signal having frequency changes in time corresponding to the stored video information.

44. The apparatus as claimed in Claim 43, where in said first means comprises: means for providing an initial information signal having its informational content in the form of a voltage varying with time format; and frequency modulator means, responsive to said means providing an initial information signal, for converting said voltage varying with time signal to said frequency modulated signal.

45. The apparatus as claimed in Claim 44, including demodulator means responsive to the output of said sensing means for changing said frequency modulated electrical signal into a time dependent voltage signal representing said stored video information, said time dependent voltage signal having its informational content in the form of a voltage varying with time format and being suitable for display by a standard television monitor.

The apparatus as claimed in Claim 45, including means for comparing said voltage varying with time video information signal from said demodulator means with said initial information signal.

47. The apparatus as claimed in Claim 46, wherein said second light beam is focused upon said record carrier at a point on said carrier downstream of the point of impingement of said first light beam, and wherein said apparatus further comprises delay means in the signal path of said initial information signal, said delay means imparting a time delay of said initial information signal equal to the accumulated values of the delay from frequency modulation of said initial information signal through frequency demodulation of said signal from said sensing means and including the delay of travel time of the point on said carrier moving from the point of impingement of said first beam to the point of impingement of said second beam.

48. The apparatus as claimed in Claim 44, wherein said light intensity modulating means includes electrically controllable means responsive to said frequency modulator means for varying the intensity of said first light beam above a predetermined intensity at which the focused first beam alters said coating and below said predetermined intensity at which the focused first beam fails to alter said coating, said alteration being representative of said frequency modulated signal.

49. The apparatus as claimed in Claim 48, wherein: said initial information signal has its informational content in the form of a voltage varying with time signal suitable for display on a standard television monitor; said first light source comprises a writing laser for producing a collimated writing beam of polarized monochromatic light, said substrate defines a smooth flat rigid disc, said first surface being a planar surface; said coating is a thin opaque metal coating having suitable physical properties to permit localized heating responsive to the impingement of light from said writing laser, said heating causing localized melting accompanied by withdrawal of the molten material toward the perimeter of the melted area, leaving upon freezing a permanent aperture in the thin metal coating; and said electrically controll able means is responsive to said frequency modulator means for varying the intensity of said writing beam above said first predetermined intensity at which the focused beam melts said metal coating without vaporizing it and below said predetermined intensity at which the focused beam fails to melt said metal coating.

50. The apparatus as claimed in Claim 43, wherein: said record carrier is disc-shaped; said means for imparting relative motion comprises rotational drive means for producing uniform rotational motion of said disc; and wherein said apparatus further comprises: translational drive means synchronized with said rotational drive means for relatively moving said first and second focused light beams radially across said first surface of said disc-shaped record carrier; and electrical synchronizing means for maintaining a constant relationship between said rotational motion and said translational motion.

51. The apparatus as claimed in Claim 48, wherein said light intensity modulating means further includes feedback apparatus for stabilizing the operating level of said electrically controllable means to operate between a predetermined higher light intensity and a predetermined lower light intensity, said light intensity modulating means including lightsensing means for sensing at least a portion of the modulated first light beam issuing from said electrically controllable means to produce an electrical feedback signal representative of the intensity of the modulated first beam and applying the feedback signal to said electrically controllable means to stabilize its operating level.

52. The apparatus as claimed in Claim 51, wherein said light-sensing means produces an electrical feedback signal which is representative of the average intensity of the first light beam, the operating level of said light intensity modulating means being stabilized to issue the modulated first light beam at a substantially constant average power level.

53. The apparatus as claimed in Claim 49, wherein said first and second optical means include: an objective lens; and hydrodynamic air-bearing means for supporting said lens above said first surface of said record carrier.

54. The apparatus as claimed in Claim 53, wherein said collimated first beam of light has substantially parallel light rays; said objective lens has an entrance aperture larger in diameter than the diameter of said first light beam as provided by said first light source; and said first optical means further includes mirror means for folding said first light beam path provided by said first light source, and a diverging lens for spreading the substantially para llel light beam from said first light source to at least fill said entrance aperture of said objective lens.

55. The apparatus as claimed in Claim 48, where in said first light source produces a polarized laser beam, and said electrically controllable means in cludes: means for rotating the plane of polarization of said first laser beam from said first source under control of said initial frequency modulated signal; and a linear polarizer employed as an attenuator of said rotated first laser beam for producing the output of an intensity modulated laser beam corresponding to said initial frequency modulated signal.

56. The apparatus as claimed in Claim 52, wherein said feedback apparatus includes level adjustment means for selectively adjusting the average power level of said light beam to a perdetermined value.

57. The apparatus as claimed in Claim 52, wherein: said electrically controllable means comprises a Pockels cell driver and a Pockels cell device, said Pockels cell driver responding to said initial frequency modulated signal to provide corresponding driving signals to said Pockels cell driver is AC coupled to said Pockels cell, and said stabilizing feedback apparatus is DC coupled to said Pockels cell.

58. The apparatus as claimed in Claim 43, further comprising: a beam polarizer, and a polarization shifting means for shifting the plane of polarization of said second beam, said polarizer and shifting means disposed in the path of said second light beam with the beam polarizer located between said second light beam source and said shifting means, said shifting means rotating said second beam during accumulative incident and reflected passages through the shifting means to and from the record carrier, said polarizer substantially reducing the intensity of the reflected second beam passed through said shifting means toward said second light beam producing means.

59. The apparatus as claimed in Claim 58, wherein said shifting means is a quarterwave plate for rotating said light beam ninety degrees by accumulative incident and reflected passages through said plate.

60. The apparatus as claimed in Claim 58, wherein said beam polarizer is a polarizing beam splitting cube adapted to direct said reflected beam passed through said shifting means toward said lightsensing means.

61. The apparatus as claimed in Claim 54, wherein said second beam of light is collimated and has substantially parallel light rays; the entrance aperture of said objective lens being larger in diameter than the diameter of said second light beam as provided by said second light source; and said second optical means further includes mirror means for folding said second light beam path provided by said second light source, and a second diverging lens for spreading the substantially parallel light beam from said second light source to at least fill said entrance aperture of said objective lens.

62. The apparatus as claimed in Claim 61, wherein said first and second optical means include beam directing means for directing said first beam toward said objective lens at an angle with respect to said second beam, thereby spacing the beams as they emerge from said objective lens and impinge upon said record carrier.

63. The apparatus as claimed in Claim 43, wherein: said first light source produces an argon ion laser beam; said second light source produces a Helium Neon laser beam; and wherein said second optical means includes a filter, opaque to an argon ion beam, in the path of said second beam reflected from said record carrier toward said sensing means.

64. A method for storing information on and retrieving information from an information storage member using a pair of laser beams, comprising the steps of: providing a frequency modulated electrical signal to be recorded, said frequency modulated electrical signal having a carrier signal with frequency changes overtime corresponding to said information to be stored; controlling the intensity of the transmission of a first light beam upon a light sensitive surface of an information storage member, using said frequency modulated signal as a control signal; moving the information storage member at a constant rate relative to said first light beam while focusing said first light beam upon said light sensitive surface of said information storage member; said controlling step including using said transmitted first light beam for irreversably altering said light sensitive surface of said information storage member under the control of one portion of said frequency modulated signal, as said member moves at a constant rate, and lowering the intensity of the transmitted first light beam to said light sensitive surface of said information storage member under the control of a second portion of said frequency varying modulated signal, as said member moves at a constant rate, said controlling step thereby producing in the light sensitive surface of the storage member a lineal series of regions stored in track-like fashion upon said surface, said regions being alternately specular light reflective and non-specular light reflective, the sequence of alternate regions representing said frequency modulated signal; imaging a second light beam of polarized collimated light upon said series of regions, said moving step providing relative motion betweeen said second imaged beam of light and said alternate regions for generating reflections from said light reflective regions representing said stored frequency modulated signal; and sensing said reflections and generating a frequency modulated electrical signal corresponding to said reflections, said frequency modulated electrical signal having its informational content in the form of a carrier signal having frequency changes in time from a center frequency.

65. The method as claimed in Claim 64, wherein said step of providing a frequency modulated electrical signal includes: providing an initial electrical signal having its informational content in the form of a voltage varying with time format; and changing said voltage varying with time signal to said frequency modulated electrical signal having its informational content in the form of a carrier signal having frequency changes with time corresponding to said voltage variations with time.

66. The method as claimed in Claim 65, wherein said first and second light beams are held stationary and said storage member is moved at said constant rate relative to the stationary first and second beams.

67. The method as claimed in Claim 65, wherein said controlling step includes using said frequency modulated signal for varying the intensity of said first light beam above a predetermined intensity at which the focused beam alters said light sensitive surface and below a predetermined intensity at which the focused beam fails to alter said light sensitive surface, said alteration being representative of said frequency modulated signal.

68. The method as claimed in Claim 67, wherein: said initial information signal has its informational content in the form of a voltage varying with time signal suitable for display on a standard television monitor; said controlling step includes producing a modulated collimated writing laser beam of polarized monochromatic light for impinging upon said light sensitive surface of the storage member, said surface being a thin planar opaque metal coating having suitable physical properties to permit localized heating responsive to the impingement of light from said writing laser, said heating causing localized melting accompanied by withdrawal of the molten material toward the perimeter of the melted area, leaving upon freezing a permanent aperture in the thin metal coating; and said controlling step includes using said frequency modulated signal for varying the intensity of said writing beam above said predetermined intensity at which the focused beam melts said metal coating without vaporizing it and below said predetermined intensity at which the focused beam fails to melt said metal coating

69. The method as claimed in Claim 66, where.n the storage member is disc-shaped and said storage member moving step includes: producing uniform rotational motion of said disc; and synchronizing said rotational motion with movement of said storage member for effecting relative movement of said first and second light beams radially across said surface of said disc-shaped storage member to maintain a constant relationship between said rotational motion and said translational motion.

70. The method as claimed in Claim 68, wherein said controlling step further includes stabilizing the level of modulation of said first light beam to operate between a predetermined higher light intensity and a predetermined lower light intensity; sensing at least a portion of the laser writing beam after modulation of the writing beam to produce an electrical feedback signal representative of the intensity of the writing beam; and utilizing the feedback signal in said controlling step to effect stabilization of the level of modulation of said writing beam.

71. The method as claimed in Claim 70, wherein said step of sensing at least a portion of said writing beam produces an electrical feedback signal which is representative of the average intensity of the modulated writing beam, the operating level of light beam modulation being stabilized to issue the modulated writing beam at a substantially constant average power level.

72. The method as claimed in Claim 68, wherein said controlling step includes: rotating the plane of polarization of said writing laser beam under control of said frequency modulated signal; and using a linear polarizing element as an attenuator of said rotated beam for producing an intensity modulated laser beam corresponding to said frequency modulated signal.

73. The method as claimed in Claim 70, wherein said controlling step includes selectively adjusting the average power level of said modulated writing beam to a predetermined value.

74. The method as claimed in Claim 70, wherein said controlling step comprises: amplifying said frequency modulated signal to provide corresponding driving signals to a Pockels cell device; AC coupling the amplified frequency modulated signal to the Pockels cell; and DC coupling said feedback signal to said Pockels cell.

75. The method as claimed in Claim 65, including the step of demodulating said frequency modulated electrical signal produced in said sensing step to produce a time dependent voltage signal representing said stored information, said time dependent voltage signal having its informational content in the form of a voltage varying with time format and being suitable for display by a standard television monitor.

76. The method as claimed in Claim 64, further comprising the step of shifting the plane of polarization of said polarized second beam by rotating said second beam during incident and reflected passages to and from the storage member, thereby substantially reducing the intensity of the reflected second beam passed to a light beam source utilized in producing the second light beam in said imaging step.

77. The method as claimed in Claim 66, wherein said shifting step includes rotating said second light beam ninety degrees by accumulative incident and reflected passages of said second light beam to said storage member surface.

78. The method as claimed in Claim 75, including the step of comprising the time dependent voltage signal produced in said demodulating step with said initial information signal.

79. The method as claimed in Claim 78, wherein said second light beam is focused upon said storage member at a point on said storage member downstream of the point of impingement of said first light beam, and wherein said method further comprises delaying said initial information signal, said delaying step imparting a time delay of said initial information signal equal to the accumulated values of the delay from frequency modulation of said initial information signal through frequency demodulation of said signal produced in said sensing step and including the delay of travel time of the point on said storage member moving from the point of impingement of said first beam to the point of impingement of said second beam.

80. The method as claimed in Claim 64, including providing an objective lens adjacent said storage member for receiving both of said first and second beams and for directing the beams toward said storage member, said method including the step of directing said first beam toward the objective lens at an angle with respect to said second beam, thereby spacing the beams as they emerge from said objective lens and impinge upon said storage member.

81. The method as claimed in Claim 64, wherein: said first light beam is an argon ion laser beam; said second light beam is a Helium-Neon laser beam; and wherein said method includes optically blocking any portion of the argon ion beam in the path of reflection from said storage member, prior to sensing said reflected second beam in said sensing step.

82. Apparatus for monitoring the storage of video information upon an information storage member comprising: means for providing an input video information signal to be recorded in the form of a voltage varying with time; modulator means for converting said voltage varying with time signal to a frequency modulated signal with a carrier signal having frequency changes in time corresponding to said voltage variations with time; means for storing said frequency modulated signal upon a light sensitive member in the form of alternate regions of specular light reflectivity and non-specular light reflectivity; a light source for providing a light beam to illuminate said alternate regions; means for imparting relative motion between said illuminating beam and said alternate regions for generating reflections from said light reflective regions; sensing means responsive to said light reflected from said regions for recreating a frequency modulated signal corresponding to said reflections representative of said video information; demodulator means responsive to the output of said sensing means for producing a voltage varying with time output video information signal to be dispiayed, suitable for display on a standard television monitor; and a comparator for comparing said output video information signal with said input video information signal.

83. The apparatus as claimed in Claim 82, wherein said apparatus further comprises delay means in the signal path of said input video information signal, said delay means imparting a time delay to said input video information signal equal to the accumulated values of the delay from frequency modulation of said input video information signal through frequency demodulation of said signal from said sensing means, and including the delay of travel time of the point on said storage member moving from the point of storing said input video information signal by said storing means to the point of impingement of said light beam.

84. The apparatus as claimed in Claim 83, wherein said means for storing includes a writing light beam source producing a writing light beam, and intensity modulating means having electrically controllable means responsive to said modulator means for varying the intensity of said writing light beam above a predetermined intensity at which the writing light beam alters said light sensitive member and below said predetermined intensity at which the writing light beam fails to alter said light sensitive member, said alteration being representative of said frequency modulated signal.

85. The apparatus as claimed in Claim 82, wherein: said storage member is disc-shaped; said means for imparting relative motion comprises rotational drive means for producing uniform rotational motion of said disc; and wherein said apparatus further comprises: translational drive means synchronized with said rotational drive means for relatively moving said light beam radially across the surface of said discshaped storage member; and electrical synchronizing means for maintaining a constant relationship between said rotational motion and said translational motion.

86. The apparatus as claimed in Claim 84, wherein said light intensity modulating means further includes feedback apparatus for stabilizing the operating level of said electrically controllable means to operate between a predetermined higher light intensity and a predetermined lower light intensity, said light intensity modulating means including lightsensing means for sensing at least a portion of the writing beam issuing from said electrically controllable means to produce an electrical feedback signal representative of the intensity of the writing beam and applying the feedback signal to said electrically controllable means to stabilize its operating level.

87. The apparatus as claimed in Claim 86, wherein said light-sensing means produces an electrical feedback signal which is representative of the average intensity of the writing light beam, the operating level of said light intensity modulating means being stabilized to issue the writing light beam at a substantially constant average power level.

88. A method for monitoring the storage of video information upon an information storage member comprising the steps of: providing an input video information signal to be recorded in the form of a voltage varying with time; converting said voltage varying with time signal to a frequency modulated signal with a carrier signal having frequency changes in time corresponding to said voltage variations with time; storing said frequency modulated signal upon a light sensitive member in the form of alternate regions of specular light reflectivity and non-specular light reflectivity; illuminating said alternate regions with a reading laser beam generating a beam of polarized monochromatic light; providing relative motion between said illuminating beam and said alternate regions for generating reflections from said light reflective regions; sensing said light reflected from said regions and recreating a frequency modulated signal corresponding to said reflections representative of said video information; demodulating said recreated frequency modulated signal for producing an output video information signal to be displayed suitable for display on a standard television monitor, said video information signal to be displayed being in the form of a voltage varying with time; and comparing said output video information signal with said input video information signal.

89. The method as claimed in Claim 88, including the step of delaying said input video information signal to impart a time delay to said input video information signal equal to the accumulated values of the delay from frequency modulation of said input video information signal through frequency demodulation of said signal in said demodulating step, said delay including the delay of travel time of the point on said storage member moving from the point of storing said input video information signal in said storing step to the point of impingement and reflection of said reading laser beam.

90. The method as claimed in Claim 88, wherein said step of storing includes: providing a writing laser beam; modulating said writing laser beam using said frequency modulated signal for varying the intensity of said writing laser beam above a predetermined intensity at which the beam alters said light sensitive surface and below said predetermined intensity at which the beam fails to alter said light sensitive surface, said alteration being representative of said frequency modulated signal.

91. The method as claimed in Claim 90, wherein the storage member is disc-shaped and said step of providing relative motion includes: producing uniform rotational motion of said disc; and synchronizing said rotational motion with movement of said storage member for effecting relative movement of said writing and reading laser beams radially across the surface of said disc-shaped storage member to maintain a constant relationship between said rotational motion and said translational motion.

92. The method as claimed in Claim 90, including the step of stabilizing the level of modulation of said writing beam to operate between a predetermined higher light intensity and a predetermined lower light intensity; sensing at least a portion of said writing laser beam after modulation of the writing beam to produce an electrical feedback signal representative of the intensity of the writing beam; and utilizing the feedback signal in said storing step to effect stabilization of the level of modulation of said writing beam.

93. The method as claimed in Claim 92, wherein said step of sensing at least a portion of said writing beam produces an electrical feedback signal which is representative of the average intensity of the modulated writing beam, the operating level of light beam modulation being stabilized to issue the modulated writing beam at a substantially constant average power level.

94. Apparatus for use in a system for recording a modulated electrical signal representing video information on a recording surface having a thin metal surface layer for retaining indicia representative of said information to be recorded, said apparatus comprising: a source of a relatively high intensity laser light write beam; write means for imaging the write beam to a small spot on the surface layer; controlling means responsive to the modulated electrical signal for controlling the intensity of the laser write beam impinging on the recording surface, said controlling means including an optical modulator positioned in the path of said laser write beam between said source and the recording surface and responsive to the modulated electrical signal for modulating the intensity of the laser write beam above said predetermined first intensity at which the beam forms an indicia of a first type in the surface layer and below said predetermined intensity at which the beam does not form an indicia of said first type in the surface layer; said controlling means further including feedback apparatus for stabilizing the operating level of the optical modulator to operate above and below said predetermined intensity, said apparatus including light-sensing means for sensing at least a portion of the laser write beam issuing from the optical modulator to produce an electrical feedback signal representative of the intensity of the beam and applying the feedback signal to the optical modulator to stabilize the operating level of the modulator.

95. The apparatus as claimed in Claim 94, wherein said light-sensing means produces an electrical feedback signal which is representative of the average intensity of the write beam, the operating level of said modulator being stabilized to issue the write beam at a substantially constant average power level.

96. The apparatus as claimed in Claim 94, including: means for providing an initial video information signal having its informational content in the form of a voltage varying with time format; and frequency modulator means responsive to said means providing an initial information signal for converting said voltage varying with time signal to a frequency modulated signal, said optical modulator responsive to said frequency modulated signal to optically modulate said laser write beam with corresponding frequency modulated information.

97. The apparatus as claimed in Claim 94, wherein: said laser write beam source produces a collimated writing beam of polarized monochromatic light; said recording surface is defined by a smooth flat rigid disc having a planar upper surface; said surface layer is a thin opaque metal coating having suitable physical properties to permit localized heating responsive to the impingement of light from said write laser, said heating causing localized melting accompanied by withdrawal of the molten material toward the perimeter of the melted area, leaving upon freezing a permanent aperture in the thin metal coating; and said controlling means is responsive to said modulated signal for varying the intensity of said laser write beam above said first predetermined intensity at which the write beam melts said metal coating without vaporizing it and below said predetermined intensity at which the laser write beam fails to melt said metal coating.

98. The apparatus as claimed in Claim 94, wherein said laser light source produces a polarized laser beam, and said optical modulator comprises: means for rotating the plane of polarization of said laser beam from said source under control of said modulated signal; and a linear polarizer the output of which is a modulated laser beam corresponding to said modulated signal.

99. The apparatus as claimed in Claim 98, wherein said feedback apparatus includes level adjustment means for selectively adjusting the average power level of said modulated laser beam to a predetermined value.

100. The apparatus as claimed in Claim 94, wherein: said controlling means comprises a Pockels cell driver and a Pockels cell device, said Pockels cell driver responding to said modulated signal to provide corresponding driving signals to said Pockels cell device; and wherein said Pockels cell driver is A.C. coupled to said Pockels cell, and said stabilizing feedback apparatus is D.C. coupled to said Pockels cell.

101. An improvement in a method for recording a modulated electrical signal representing video information on a recording surface having a surface layer for retaining indicia representative of said information to be recorded, said method comprising the steps of: providing a source of relatively high intensity laser light to form a write beam; imaging the write beam to a small spot on said surface layer; controlling in response to the modulated electrical signal, the intensity of the laser write beam impinging on the recording surface, said controlling step including optically modulating said laser write beam above said predetermined first intensity at which the beam forms an indicia of a first type in the surface layer and below said predetermined intensity at which the beam does not form and indicia of said first type in the thin metal surface layer, said controlling step further including stabilizing the level of optical modulator to operate above and below said predetermined intensity; sensing at least a portion of the laser write beam after optical modulation of the beam to produce an electrical feedback signal representative of the intensity of the beam; and utilizing the feedback signal in said controlling step to effect stabilization of the level of modulation of said write beam.

102. The method as claimed in Claim 101, wherein said sensing step produces an electrical feedback signal which is representative of the average intensity of the write beam, the operating level of optical modulation being stabilized to issue the write beam at a substantially constant average power level.

103. The method as claimed in Claim 102, including the step of developing said modulated electrical signal by: providing an initial electrical signal having its informational content in the form of a voltage varying with time format; and changing said voltage varying with time signal to a frequency modulated electrical signal having its informational content in the form of a carrier signal having frequency changes with time corresponding to said voltage variations with time.

104. The method as claimed in Claim 101, wherein: said controlling step includes producing a modulated collimated laser write beam of polarized monochromatic light for impinging upon said recording surface, said surface being a thin planar opaque metal coating on a smooth flat substrate, said coating having suitable physical properties to permit localized heating responsive to the impingement of light from said laser write beam, said heating causing localized melting accompanied by withdrawal of the molten material toward the perimeter of the melted area, leaving upon freezing a permanent aperture in the thin metal coating; and said controlling step includes using said modulated signal for varying the intensity of said laser write beam above said predetermined intensity at which the laser write beam melts said metal coating without vaporizing it and below said predetermined intensity at which the laser write beam fails to melt said metal coating.

105. The method as claimed in Claim 101, wherein said controlling step includes: rotating the plane of polarization of said laser write beam under control of said modulated signal; and using a linear polarizing element as an attenuator of said rotated beam for producing an intensity modulated laser beam corresponding to said modulated signal.

106. The method as claimed in Claim 105, wherein said controlling step includes selectively adjusting the average power level of said modulated laser write beam to a predetermined value.

107. The method as claimed in Claim 102, wherein said controlling step comprises: amplifying said modulated signal to provide corresponding driving signals to a Pockels cell device; A.C. coupling the amplified modulated signal to the Pockels cell; and D.C. coupling said feedback signal to said Pockels cell.

108. An information storage member substantially as herein specifically described with reference to the accompanying drawings,

109. Apparatus for storing information arranged substantially as herein specifically described, with reference to the accompanying drawings.

110. A method of recording information on an information storage member, performed substantially as herein specifically described with reference to the accompanying drawings.

111. A system for retrieving a signal stored on the surface of an information storage member, operating substantially as herein specifically described with reference to the accompanying drawings.

112. A method of reading an information signal stored on a record, performed substantially as herein specifically described with reference to the accompanying drawings.