CA1153468A - Mastering machine - Google Patents

Abstract

ABSTRACT

This invention relates to an apparatus and process for writing video information in the form of a frequency modulated signal upon a videodisc member and for recovering the video information from the video record. The writing apparatus includes a laser as a source of a laser write beam and a write optical system for directing the stationary laser write beam to a moving videodisc member. The intensity of the focused spot is changeable under the control of a light intensity modulating assembly. This light intensity modulator changes the intensity of the write laser beam with respect to a predetermined threshold intensity sufficient to form a first type of indicia in the coating. A read optical system directs a reading laser beam to impinge upon the successively positioned specular light reflective and non-specular light reflective regions carried by the videodisc member. The reflected light beam is intensity modulated by the specular light reflective and non-specular light reflective regions. The read optical system also directs the reflected light beam to a light sensing circuit for changing the intensity modulated reflected light beam to a corresponding frequency modulated electrical signal which is representative of the video signal stored on the videodisc member.

Description

~5~ 8 MASTERING MACHIME

TECHNICAL FIEL~
The present invention relates to the wrlting of a frequency modulated electrical signal upon an information bearing surface of a video disc member in the form o~ a llneal series of flrst and second indicia positioned in track-like ~ashion upon such surface.
EACKGROUND OF THE PRIOR ART
The apparatus for writing a ~requency 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 15 of rotation and a translational drive assembly for trans-lating the writing beam at a very constant, and very low velocity along a radius of the rotating disc. The rota tional drive of the disc is synchronized with the trans-lational drlve of the writlng beam to create a spiral track of predetermined pitch. In a pre~erred embodimentJ
the spacing between adjacent tracks of the spiral is two microns, center to center. The indicia is ~ormed having a width of one micron. This leaves an intertrack or guard area of one micron between indicia in ad~acent tracks.
If desired, the indicia can be formed as concentric circles by translating in incremental steps rather than by trans-lating at a constant velocity as just described.
In the preferred embodiment, a microscope ob~ec-tive lens is positioned at a constant height above the 1~5~

video disc member on an air bearing. This ob~ective lens i~ employed for f ocusing the wr~te beam upon the light sensit~ve sur~ace Gf the video disc member. The constant height is necessary because of the shallow focal depth of the objective lens. A o.65 NA dry microscope ob~ective lens is employed to focus the write laser beam ko a spot one micron in diameter upon the light sensitive coating.
~ecause the coating is rotating at a relatively high rate, `the length of the indicia formed in the light sensitlve coating depends upon the length of time the spot intensity exceeds that needed to form such an indicia.
A linearly polarized ion laser is used as the source of the writing beam. A Pockels cell is used to rotate the plane of polarizatlon of the wrlting beam with respect to ~ts fixed plane o~ linear polarization. A
linear polarizer attenuates the rotated writlng beam in an amount p~oportional to the difrerence in polarization between the light in the writlng beam and the axis of the linear polarizer. The co~bination of a Pockels cell and linear polarizer modulates the writing beam wlth the video i~f~ormation 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 modu-lator circuit is a rectangular wave whose frequenc~ is proportional to `the video signal. The duration of each cycle of the rectangular waveform is variable as is characteristic of a f'requency 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 o~ the rectangular wave are ampli-fied by a Pockels cell driver and used to control the Pockels cell. The Pockels cell changes the angle of pola~
ization o~ the light passing therethrough in response to the instantaneous voltage level of the control signal supplied ~y the Pockels cell driver.
In a first mode of operation responsive to one voltage level of the rectangular-shaped control signal applied tc a Pockels cell driver, the light beam passes unhindered through the Pockels cell linear polarizer combination at a first intensity sufficient to ~orm a firs~ indicia in a light responslve coating. When the control slgnal changes to represent its second voltage levelJ the Pockels cell rotates the polarization of the light which forms the writing beam to a new angle o~ pola~
ization. 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 llnear polarizer acts as an attenuator and less light passes through the linear polarizer. ~his reduces the light intensity of the writing beam below the intensity required to form such first indlcia 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 pre-determined level in spite of changes in the Pockels celltransfer characteristic produced by small temperature variations. The stabilizing circuit includes a level ad~usting circuit for selectively adjusting the power level to form indicia in different light sensitive coat-ings as identified hereinafter.
Circuitry is described for achieving a predeter-mined duty c~cle modulation of the indicia ~ormed during the wri~ing process. The preferred duty cycle lies within the range of 60/40 to ~0/60, with a preferred value of 50/50. The output of the linear polarizer is adjusted such that the hal~ power point from the polarizer equals the threshold power level of the material forming the ln~ormation storage layer. This is achieved in part by ~atching the ~5 rotation of the Pockels Cell with the half power output level of the linear polarizer.
Different types of video disc members can be used with this wrlting proce~s and apparatus. Each such member has a different configuration. In a first configuration, the video disc member includes a glass substrate having 3~

an upper surfaAe carrying a th~n metal coatlng as a light responsive coating. In this con~iguration, the write beam forms variable length apertures in a track-like fashion in the metal coating.
The intensity of the write beam is ad~usted such that an aperture is formed, for example, during each positive half cy^le 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 serles of apertures separated by an intervenlng 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 beam. The intervening portion of the metal coating remaining between specular reflectivity means a si~nificant portion of the reflected light returns along the path of the light beam, ie., a 180 reversal in paths between the incident and reflected beam paths. Non-specular reflectivity means that no significant portion of the incident beam is re-flected along the path of the incident beam.
In a second configuration, the video disc member i~cludes a glass substrate having an upper surface carry-ing a thin layer of photoresist as the light responsive coating. In this configuration~ the write beam forms variable length regions of exposed and unexposed photo-resist material in a track-like fashion in the photo-resist coating. The intensity of the write beam isadjusted such that a region of exposed photoresist ma~erial ls 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 nega-tive half cycles. Accordingly, the first and secondlndicia 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 ~5~6~3 describe~ employlng a read laser for producing a polar-i~ed collimated beam of light having a preferred angle of polarizaticn, h read optical system directs and images the laser ~eam 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 sig-nal on its surface in the form of a lineal series of regions. The regions are alternately specular light reflective and non-specular 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 ad~usted such that a sufficiently strong reflected read beam signal is gathered 15 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 fre-quency modulated signal. A typical frequency modulated signal stored in this matter varies in frequency between two megacycles and ten megacycles. The rotational rate o~ the video disc member is preferentially set at about 1800 rpm to change the spatially stored frequency modu-lated signal into a real time electrical signal. The motion control assembly includes a translational drive assembly for translating the reading beam at a very con-stant, 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 contalned thereon.
The reflected read beam gathered by the read optical system is directed to a light sensing circuit ~or 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 ~53468 spl~tting element the real ligh~ beam is llnearly polar~
i~ed in the preferred plane. h quarterwave plate is positioned in~ermediate ~he 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 po~arization to circular polariza-tion. The reflected light reta~ns its circular polariza-tion until it passes through the quarterwave plate a second time. During this second pass through the quarterwave plate the reflected light is changed b~ circular polariza-tion back into linear polarized light rotated ninety degrees from the preferred plane established by the polar~
ization selective beam splitting element as described hereinabove.
1~ The polarization selective ~eam splitting element is responsive to this ninety degree shift in the reflected light beam for divert~ng the reflected beam to the light sensing circuit and prevents the reflected ~ight 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 ob~ective 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 llght other than the wavelength of light generated by the read laser source.
In a recording apparatus, the write ~unction 30 alone is employed for writing the frequency modulated information onto a video disc member. In a video disc player, the read ~unction alone is employed for recover~
lng the frequency modulated information stored on the sur~ace of the video disc member. In a third mode of 35 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 ~3~6~
, beam from the Helium-l~eon (He~l~e) re~d laser is added into the writing beam path. ~he read optics are ad~usted ~o direc~ the read beam ~hrough the mi.croscope objective lens at a ligh~ angle with respect to the wri~ing beam.
The angle is chosen so tha~ ~he reading beam illuminates an area on the same track being written by the wrlte beamJ
but at a poin~ that is approximately four ~o 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 im-pinged upon alternate regions of different reflectivity.
In one form of the read apparatus, the read beam impinges upon the port~ons of the metal not heated by the write beam and also impinges upon the glass substrate exposed ln 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 modu-lated reflected read beam.
In this monitoring mode of operation, the readlaser beam is selected to operate at a wavelength differ-ent 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. m e monitoring mode o~` 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 ~requency modulated signal, is the same as the next succeeding region of 53~
.

n~n-specular re~lectivity~ which represents tne next consecutive hal~ cycle of a frequencJ modulated signal.
The read after write or monitoring mode of oper-ati~n is als~ utilized ~n an error checking mode, especi-ally if digital type information is bein~ written. Theinput video informa~ion is delayed for an interval equal ~o the accumulative values of the time delay beglnn.ing with the frequency modulation ~f the input video informa-tion signal during the write process and continuing through 10 the ~requency demodulation of ~he recovered re~lected signal from the sensing circuit, and including the delay of travel tlme o~ the point on the storage member moving ~rom the point of storing the inpu~ vide~ information signal to the point of impingement of the read li~ht 15 beam. The recovered in~ormation is then compared with the delayed input in~ormation for accuracy. The ex~stence o~ to~ many dissimilarities would be a basis for either re~hecking and realigning the apparatus or rejectlng the disc.
~he read apparatus is suitable ~or use with a standard home television receiver by adding an RF modu-lator for adding the video signal to a sultable carrier frequency matched to one of the channels o~ a standard home television receiver. The standard televlsion re-ceiver then handles this signal in the same manner as are received ~rom a standard transmitting station.
In accordance with one aspect of the invention there is provided an apparatus for reoording an electrical information signal upon an information storage member, comprising:
first means for providing an information signal to be recorded and said signal having its informational content in the form of a variable amplitude, cyclical signal alternating between a first higher amplitude and a second lower amplitude; an information storage member having a substrate and a thin, light-sensitive surface layer overlying said substrate; means for moving said storage member in a prescribed fashion; a laser light source for providing a write beam of light having ~.

-8a-sufficient intensity to interact with said light-sensitive layer and produce indicia representative of the information signal; said thin surface layer having a substantially uniform threshold power level; said 5 threshold power level being of the type wherein said layer reacts to said laser write beam having an intensity greater than said threshold power level and said layer does not react to said laser write beam having an intensity less than said threshold power level; an 10 optical modulator positioned in said laser write beam intermediate said thin surface layer and responsive to the electrical signal for modulating the intensity of the laser write beam to include a predetermined first intensity at which said beam forms an indicia in said L~ thin surface layer and a predetermined second intensity at which said beam does not form an indicia in said thin surface layer; and feedback apparatus for stabilizing the operating level of said optical modulator to issue said write beam at a predetermined average power level 23 equal to the threshold power level of said layer.
Ihere is also provided apparatus for storins an elec~rical signal upon an info~mation storage m~mber, o~prising: first means for providing an information signal to be recorded; said signal having its informational content in the form of a variable amplitude, cyclical signal alternating between a first higher amplitude and a second lower amplitude; 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;
said coating having a threshold power level above which said indicia are formed; means for imparting uniform motion to said storage member; a laser light source for providing a write light beam, and said write beam being of sufficient intensity for interacting with said coating while said coating is in motion and said coating is positioned upon said moving information ~3~6~
-8b-storage member, and said light beam being of sufficient intensity for altering said coating to retain indicia representative of said information signal; said intensity of said light beam having a predetermined value relative to said threshold power level of said coating; optical means for defining an optical path between said light source and said storage member including said coating, and said optical means being further employed for imaging said light beam to a spot upon said coating;
1~ light intensity modulating means positioned in said optical path between said light source and said coating, and said light intensity modulating means operating over a range between a maximum light transmitting state and a minimum light transmitting state for intensity 15 modulating said light beam with said information to be stored; said light intensity modulating means being responsive to said information signal and changing between its maximum light transmitting state and its minimum light transmitting state during each cycle of 20 said information signal for modulating said light beam with the information signal to be stored; said light intensity modulating means has an intermediate light transmitting state at which the instantaneous power in said modulated light beam equals one half that of said 25 modulated light beam transmitted at said maximum state and said intermediate light transmitting state also equals the threshold power level of said coating; and said light passing through said light intensity modulating means and imaged upon said coating by said optical means begins to form indicia in said coating representative of said information signal to be stored when said intermediate light transmitting state is exceeded.
In accor~'~nce with another aspect of the i~vention there is provided a method f~r recording an information signal on a rotatable recording disc, comprising the steps of:
providing an electrical signal to be recorded, and said signal having its informational content in the form of ~L53~61~
-8c-a variable amplitude, cyclical signal alternating between a first higher amplitude and a second lower amplitude; providing a rotatable recording disc having a substrate and a thin, light-sensitive coating overlying the substrate; rotating the recording disc in a prescribed fashion; providing a beam of light having sufficient intensity to interact with said light-sensitive coating and produce indicia representative of the information signal; directing the beam of light along an optical path to impinge on said light-sensitive coating;
controllably moving the point of impingement of the beam of light on said recording disc radially in a prescribed fashion, such that the beam of light impinges on the rotating disc in a succession of substantially circular and concentric recording tracks; modulating the intensity of the beam of light in accordance with the information signal to be recorded, the intensity varying between a prescribed maximum intensity and a prescribed minimum intensity during each cycle of the signal, the prescribed maximum intensity corresponds to the first higher amplitude portion of the information signal and is greater than a predetermined r~cording threshold of said light-sensitive coating~ and the prescribed minimum intensity corresponds to the second lower amplitide portion of the informative signal and is less than the predetermined recording threshold; and stabilizing the average intensity of the intensity-modulated beam of light at a prescribed level corresponding both to one-half the prescribed maximum intensity and to the predetermined recording threshold of said light-sensitive coating, the indicia being arranged in a succession of substantially circular and concentric recording tracks.
There is further provided a method for recording information on an information storage member using a laser beam, comprising the steps of: providing an electrical signal to be recorded, and said signal having its informational content in the ..~

~3~6~
-8d-form of a variable amplitude, cyclical signal alternating between a first higher amplitude and a second lower amplitude; using said electrical signal as a control signal for controlling the application of a light beam upon a light sensitive surface of an information storage member; adjusting the average intensity of said light beam to equal the threshold power level of said light sensitive surface; moving the information storage member at a constant rate while focusing said stationary light beam to a spot upon the light sensitive surface of said in~ormation storage member; using said focused light spot to irreversibly alter the characteristics of said light sensitive surface of said information storage member as said member moves at a constant rate and under the control of said higher amplitude portion of said information signal; and blocking the transmission of said focused light beam to said light sensitive surface o~ said information storage member as said member moves at a constant rate and under the control of said lower amplitude portion of said information signal.
BRIEF DESCRIPTIOI~ OF THE DRAWINGS
FIGURE 1 ls a block diagram of the write appara-tus;
FIGURE 2 is a cross-sectional view of a video disc member pr~or to writlng there on using the write apparatus shown in Figure 1;
FIGURE 3 is a partial top view of a video disc member a~ter writing has taken place using the Write apparatus shown in Figure l;

p ~ .

~53~
g FIGURE 4 is a waveform of a video si~nature employed in the ~nrite apparatus shown in Pigure 1, FIGURE 5 is a wave~orm of a frequency modulated signal used in the wrlte apparatus shown in Flgure l;
5FIGURE 6 is a graA~h shol~ing the intensity of the ~nlte laser used in the write apparatus shown in Figure l;
FIGURE 7 is a graph sho~ing the modulated wrlte beam as changed by the write apparatus shown in Figure l;
FIGU~E 8 is a radial cross sectional view taken along the line 8-8 of the disc shown in higure 3 FIGURE 9 is a detailed block diagram of a suit-able motion control assembly;
FIGURE 10 ls a block diagram showing a read ap-paratusj 15FIGURE 11 is a block diagran showing the combi~
nation of a read and write appara~us;
FIG~RE 12 is a schematic representation showlng the read and wrlte beams passing through a single ob~ec-tlve lens as used in the block diagram o~ Flgure ls - FIGURE 13 is a schema~ic diagran o~ a suitable stabilizing circuit for use in the wrlte apparatus shown in Figure 1.
FIGU~E 14 shows various waveforms used in illus-trating the operatlon of a masterlng machine~

2~FIGURE 15 shows a cross-sectlonal schematlc view o~ one form o~ a video disc~
FIGURE 16 shows a photoresist coded stora~e me~-berj FIGU~E 17 shows certain portlorls removed from the photoreslst coded storage me~ber o~ Figurè 16~
FIGURE 18 shows the transfer characterlstic of a Pockels cell used herein, FIGURE lg shows the trans~er characteristic of a Glan prism used herein;
35FIGURE 20 shows a light ir~ensity waveforn~

~5346;~3 --10~
FIGU.~E 21 shows in con~unction with Figure 20 aseries of waveforms useful in explainin~ the duty cycle of recor~ing~
FIGUI~E 22 shoh~s an additional waveform used in illustrating the operation of a mastering ~achine;
FIGURE 23 is a block diagram of a Pockels cell bias servo system~
FIGURE 24 is a diagram o~ a second harmonlc detec-tor used lr, Figure 23; and 10FIGURE 25 is a diagram of a hi~h voltage ampli~ier used in ~igure 23.

Ihe same numeral is used to ldentify the same ele-ment in the several views. m e terms record~ng and stor-ing are used interchangeably for the term ~nriting. The term retrieving is used interchangeably for the term read-ing.
The apparatus for storlng video information in theform of a frequency modulate~ signal upon an info~mation storage ne~ber 10 is shown ~ith reference to Figure 1. An information signal source circuit 12 is employed ~or provld-ing an information signal to be recorded. qhis informa--tion slgnal present on a line 14 is a frequency modulated signal having its information21 content in the ~orm of a carrier frequency having frequency changes ln time repre-senting said informatlon to be recorded. Figure 5 shows a typical example of a frequency modulated signal. Ihe information signal source circuit 12 employs a video sig-nal circuit 16 for providing an infor~mation signal on a line 18 havin~ its informational content in the ~orn of a voltage varying wlth time format. Figure 4 shows a typical example of a voltage varying with tine signal. A

~53~
I I
frequency modulator circuit 20 is responsive to the video signal circuit 1~ ror convertin~ the voltage var~ing with time si~nal to vhe frequency modulated signal on the line 14 as shown in Figure 5.
The in'ormation storage member 10 is ~oun~ed 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 25 covering the first surface 24. A motion control assembly 28 imparts uni~orm mo~ion to the storage member 10 relative to a write beam 29~ generated by a l~ght 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 informa-tion 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 in-cludes an electrical synchronizing assembly 36 for main-taining a constant relationship between the rotational motion imparted to the member 10-s by the rotational drive circuit 32 and the translational motion imparted to the light beam 29 b~T the translational drive circuit 34.
The llght 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 su~ficient for producing permanent lndicia 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 mono-chromat-lc light.
Referring again to Figure 2, there is shown a cross-sectional view of a first configuration of a suit~
able video disc member 10. A suitable substrate 22 is made of glass and has a smooth~ flat~ planar first surface 24. The li~ht responsive coating 2~ is formed upon the ~L~53~

surface 24.
In ore of the disclosed embodiments~ the coating 26 is a thin, opaque metallized layer having suitable physical properties to permit locali~ed heating responsive to the impingement of the write light beam 29 from the wri~ing laser 30. In operation, the heating causes local-ized melting of the coating 26 accompanied by withdrawal of the mol~en material towards the perimeter ~ the melted area. Upon freezing, this leaves a permanent aperture such as at 37T ~hown in Figures 3 znd 8, in the thin metal coa~
ing 26. IThe aperture 37 is one type of ~ndicia employed for representing informat~on. In this embodiment, succes-sively positioned apertures 37 are separated by a portion 38 o~ the undisturbed coating 26. The portion 38 is the second type of indicia employed for representing informa-tion. 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 lO.
The optical path is also represented by the line identi~iedby the numerals 29 and 29'.
A light intensity modulating assembly 4~ is posi-tioned in the optical path 29 between the light source 30 and the coating 2S. In its broadest mode ~ operation, the light intensity modulating assembly intensity modu-lates the light beam 29 with the information to be stored.
The light intensity modulating assembly 44 operates under the control o an amplified form of the frequency modu-lated 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.
I~.is rapid change between transmitting states modulates the light beam 29 with the frequency modulated signal to ~53~68 be stored.
Ihe light beam 29 is modulated as it passes through the light intensity modulating assembly 44. Therea~ter, the modulatecl light beam~ now represented by the numeral 29', is imaged upon the coat~ng 26 by the optical assem-blies 40 and ~1. As the modulated light beam 29' impinges upon the coating 26~ indicia is formed in said coating 26 representative o~ the ~requency modulated signal to be stored.
The light intensity modulating assembly 44 in-cludes 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 sotrage member 10. Addi-tionally, the electrically controllable subassembly 46 is responsive to the ~requency modulator 20 ~or varying the intensity of the light beam below a predetermined intensity at which the ~ocused beam 29' fails to alter the coating 26. The alterations formed in the coating 26 are repre-sentative o~ the frequency modulated signal to be storedO
.~hen a photoresist layer forms the coating 26 carried by the in~ormation storage member 10~ the alterations are in the ~orm of exposed and unexposed photoresist members analogous to the size as previously descrlbed with respect to indicia 37 and 38, respectively.
When the coating 25 carried by the information storage member 10 is a metal coating, the electrically controllable subassembly 46 varies the intensity o~ the writing beam 29' above a ~irst predetermined intensity at which the ~ocused beam 29' melts the metal coating without vaporizing it and ~urther varies t'ne intensity o~ the writing beam below the predetermined intensity at which the ~ocused beam 29' fails to melt the metal sur~ace.
The light intensity modulating assembly 'l4 in-cludes a stabilizing clrcuit 48 ~or provic1ing a feeclback signal employed ~or temperature stabilizing the operating level o~ the electrical controllable subassembl~J 4~ to operate between a predetermined higher light intensity and ~l~53~

predetermined lower light intensity level. The light intensity modulating assembly LL4 includes a light sensing circuit ~or sensing at le2st a portion o~ the light beam, indicated a~ 29 !~ issuing from the electrically controllable subassembly 46 to produce an electrical feedback signal representative of t~le average intensity of the beam 29~.
The feedback signal is connected to the electrically con-trollable subassembly 46 over the lines 50a and 50b to stabili~e its operating level.
The light sensing means produces an electrical feedback signal which is representative of the average intensity of the modulated ligh~ beam 29~. In this manner, the light intensity modulating assembly 44 is stabilized to issue the light beam at a substantially constant aver-age power level. The stabilizing circuit 48 also includes level adjus~ment means for selectively adjusting the average power level of the light beam 29' to a predeter-mined value to achieve the preferred duty cycle in either a metal coating 25 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 suppor~ing 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 i~ the light beam 29' is ~mployed 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 in-cludes a number of mirror members 58; 60~ 62 and 54 forfolding the light beams 29' and 29" as desired. The mirror 60 is shown as a movable mirror and is employed for making strictly circular tracks rather than the preferred spiral tracks. Sprial tracks require only a fixed mirror.

'........................ l~ 34~8 As previously descrcribed, the light source 30 produces a polarized laser beam 29. The electricall~J con-trollable subassembly ~6 rotates the plane of polarization of this laser beam 29 under the control of the ~requency modulated slgnal, A suitable electrically controllable subassembl~ includes a Pockels cell 689 a linear polarizer 70 and a Pockels cell driver 72, The Pockels cell driver 72 is essen,tiall~J a linear amplifier and is responsive to the frequency modulated signal on the line 14, The output from the ~ockels 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 orienta-ted in a predetermine 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 light transmitting axis of the linear polarizer 70 is positioned at right angle with ~he angle of polarization of the light issuing ~rom ~he source 30, ~ecause of this arrangement, minimum light exits the polarizer 70 with zero degree rotation added to the write beam 29 b~ the Pockels cell 68, Maximum light exits the polarizer 70 with ninety degree rotation added to the write beam 29 by the Pockels cell 58, This positioning of the linear polarizer as described is a matter of choice. By aligning the maximum light transmitting axis of the pola~zer 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 o~ the beam 29 which is rotated away from its natural polarization angle, It is this attenuating action b~ the linear polarizer 70 which forms a modulated laser beam 29' cor-responding to the frequenc~ modulated signal, A Glan-prism 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 ~53~

DC coupled to ~he Pockels cell ~8.
Referring ~ollectively to Figures 4 through 7g there are shown selective waveforms of electrical and optical signals which are present ~n the embodiment shown with reference to Figure 1. A video signal generated by the video signal source circuit 15 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 ol such a video signal. The information slgnal shown in Figure 4 is typically a one volt peak-to-peak signal having its informational content in the form Or a voltage varying with time ~orma~ is represented by a line 73. The ma~imum instantaneous rate of change of a typical video signal is limited by the 4.5 megacycles bandwidth. This video signal is o~ the type which is directly displayable on a televi~ion mon tor.
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 ~igure 5. The informational content o~ the waveform shown in Figure 5 is the same as the information-al content of the waveform shown in Figure 4, but the L orm is di~ferent. 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 amplltude region, generally indicated by a numeral 75, of the video wave~orm 73 shown in Figure 4, corresponds to the lower ~requency 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 7~. A higher amplitude region, indicated generally by the numeral 77 o~ the video wave~orm 73, corresponds to the higher frequency.portions of the frequency modulated signal 74. One complete cycle ol the higher ~requency portion o~ the frequency modulated ~'~ 5 3 ~ 6 signal 74 is represented by a bracket 78. An intermediate amplitude region, generally indicated with a numeral 79 of t~e video waveform 73, corresponds to the intermediate frequency portions 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 13 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, ~his 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 repre-senting the higher light transmitting state o~ the llght intensitywmodulating 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 intensit~J 92 of the light beam 29' modulated by the assembly 44.
This intensity modulation of the writing beam 29 to form an intensit~J 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 intensi~
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.

S;~6~3 The intensity modulation of the writing beam 29 is compared to the rotational effect of the Pockels cell ~8 by reference to lines 98, 100 and 102. The intersec-tion of the line 98 with the line 29' shows ~he intensity 5 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 inte~
sectlon of the line 100 with the line 29 ' shows the inten-sity of the beam 29 l issuing ~rom the linear polarizer 70 10 when the Pockels cell 68 adds a forty-five degree rotation to the angle of' polarization of the light passing there-through. 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 ~8 adds a 15 ninety degree rotation to the angle of polarization of the light passing therethrough.
The formation of an aperture, such as 37 shown in ~igures 3 and 8; b~ the ~ntensity modulated beam 29 '~
shown in Figure 7 can best be understood by a comparison 20 between the two Figures 7 and 8.
The-line 100 is drawn midpoint between the inten-sity 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 25 represents the intensity generated b~J the assen~bl~r 44 when the Pockels cell 68 rotates the angle of polarizatlon of the write beam 29 passing therethrough through an angle of forty-five degrees. Additionally, the line 100 repre-sents the threshold intensity of the modulated beam 29 ' 30 required to form an indicia in the light responsive coat-ing 26. Thls threshold is reached upon rotatio n of the angle of polarization of the write b eam 29 through an angle of forty-five degrees.
By a comparison between Figures 7 and 8, it can 35 be seen that an aperture 37 is forrned while the Pockels cell ~8 is rotating the angle of polarization of the write beam 29 passing therethrough between the angle of forty-five degrees and ninet~J degrees and back to forty-five degrees. No aperture is formed while the Pockels cell ~8 B

is rotating the ang~e of~ polarizatlon of the write beam 29 passing t`nerethrough between the angle of fort~-five degrees and ninety degrees and back to forty-five degrees.
No aperture is formed while the Pockels cell 68 ls rotating the angle of polarization of the write beam 29 passing t'nerethrough between the angle o~ forty-five degrees and zero degrees and back to forty-five degrees.
Re,ferrlng again to Figure 3g there is shown a top view of the video disc member shown in radial cross-sectional ~iew in Figure 8. An inspection of this Figure3 is helpful in understanding the manner in which the lineal series of light reflecting and light scatter-regions 38 and 37 are formed upon the video disc member 10. The disc member 10 ls rotated at a preferred rota-tional rate of 1800 rpm and the indicia 37 and 38 areformed in the light responsive coating 26 as shown with reference to Figure 8. The motion control assembly 28, shown with reference to Figure 19 forms the apertures 37 in ci~cular track-like fashion. A numeral 104 is employed to identify a section of an inner trackg and a numeral 105 is employed to identi~y a section o~ an outer track. A
dashed line 106 represents the center line o~ 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 10~ and 107 of ad~acent tracks 105 and 104. Two microns is a typical distance between center lines of adJacent tracks. The widt,h 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 ad~acent 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 fre~
quency modulator 20g the size of the spot 42 formed by the write optical systems 41 and 42 and the rotational - ~5~

speed selected ~or the disc 10.
Referring to Figure 9, there can be seen a more detailed block diagram of the motion control assembl~J ~8 shown with reference to Figure 1. The rotational drive circuit 32 includes a spindle ser~o circuit 130 and a spindle sh~ft 132. The spindle shaft 132 is integrally ~oined to the turntable 21. ~he spindle shaft 132 is driven by a printed circuit type motor 134. The rota-tional 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 b~ a color subcarrier crystal oscilla~or 136 which forms a portion of the s~nchronization assembl~-36. The synchronization assembl~ 35 further includes a ~irst divider circuit 138 and a second divider circuit 140, The first divider circuit 138 reduces the color subcarrier frequency generated in the oscillator circuit 136 down to a rotational reference frequency. The spindle shaft 132 contains a tachometer 143 for generatlng a t~O ~requenc~ signal indicating the exact rotational speed of the sha~t-132 and turntable 21 combination. The tach-ometer signal is available over a line 142 and the rota-tional reference signal from t'ne ~irst 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.
T.~en 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. lt~hen the phase of the tachometer signal lags the phase OL 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 ~53~6~3 148 to increase the rotational ~peed and bring the phase cf the tachometer s~gnal into agreement with the phase o~
~he rotational reference signal.
The second divider circuit 140 reduces the color subcarrier ~requency generated by the oscillator ~135 down to a translational reference ~requency for advancing the transla~ional drive circuit 34 a fixed distance for each complete revolution of the member 10. In the pre-~erred embodiment, the distance advanced by the trans-lational drive circuit 34 for each revolution of the member10 is a distance of two microns.
The color subcarrier crystal oscillator 136 with its two divider circuits 138 and 140 ~unctions as an electrical synchronizing circuit ~or maintaining a con-15 stant relationship between the rotational motion o~ 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 ls 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 o~ the spindle shaft 132. This 25 translational movement is radial with respect to the rotating disc 10. This radial advancement per revolution of the spindle shaft 132 is identi~ied as the pitch of the recording. Since the pitch uniformity of the ~inished recordir.g depends on the steady advance of the optical

3 assemblies mounted on the platform 142, care is taken to lap a lead screw 143 in the translation drive 34, pre-load a translation drive nut 144 which engages the lead screw 143 and make the connection between the nut 144 and the platform 142 as stif~ as possible as represented by a bar 146.
Re~erring to Figure lOg there is shown a read apparatlus which is employed for retrieving the ~requency modulated signal stored on the in~ormation storage member 10 as a lineal serles of indicia 37 amd 38 previously described. h reading beam 150 i~ generated by a read laser 152 whicll produces a polarized, collimated beam 150 of light. ~ support memberg such as the turntable 21, is employed for holding the information storage member 10 in a substantially predetermined position.
~ stationar~ read optical assembly 154 and a mova ble 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. Addi-tionally, either of the optical assemblies can be employedto focus the light beam 150 upon the alternately posi-tioned light reflective regions 38 and the light scatter-ing regions 37 carried in successive positions upon the informatibn storage member 10. The movable optical assem-bly 156 is employed for collecting the reflections fromthe light reflective regions 3S and the l-ight scattering regions 37. The motion control assembly 28 provides relative motion between the read beam 150 and the alter-nate 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 gener-3 atlng a frequency modulated electrical signal correspond-ing to the reflections impinging thereupon. The frequency modulated electrical signal generated by the light sensing elemenk 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 16~. The discriminator circuit 162 is responsive to the output of the light sensing circuit 15~ and is employed for changing the frequency modulated electrlcal signal into a time dependent voltage signal representing the stored information. The time dependent voltage s~gnal is also identi~ie~ as a video si~nal and it is present on a line 165. This time dependent voltage signal has its inLormational content in the form of a voltage varylng with time format and is sultable for display over a standard televlsion 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'. ~he read optical assemblies further include a ~uarterwave plate 172. The beam polar-izer 170 f~lters out from the read beam 150 any spurious light waves which are not aligned with the axis of polar-ization of the beam polarizer 170. ~ith the axis of polarization of the read beam 150 fixed in a particular orlentation 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 locat~
between the source 152 of the read beam 150 and the quarter-25 wave plate 172. The quarterwave plate 172 is also locatedin the reflected read beam path 150~. Therefore) not only does the quarterwave plate 172 change the read beam polar-lzation from linear to circular during lts travel from the read laser 152 to the information storage member 10, but the quarterwave plate 172 further changes the cir-cularly polarized reflected light back into line~rly polarized light which is rotated ninety degrees with respect to the preferred direction fixed by the source 152 and t.he member 170. This rotated beam 150' is selec-tively directed to the llght sensing element 158 whichchanges the reflected light beam 150' into a correspond-ing 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 ~.~.53~
-2~-compensated for by setting the initial intensity of the read beam 15C to a level sufficien~ ts offset this re-duction. The quarter~ave plate 172 glves a to~al rota-tion of ninety degrees to the reflected beam 150l with respect to the incident beam 150 during the change from linear polari~ation to circular polarization and back to linear polarization. As previously mentioned, the member 170 is also a beam splitting cube in the re~lected read beam path 150'. As the plane o~ polarizàtion of the re-10 ~lected read beam 150' is shifted ninety degrees due toits 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 15 light sensing element 158 is a photodiode. Each such element 158 is capable of changing the reflected fre-quenc~- modulated light beam 150' into an electrical signal having its information content in the form of a carrier ~requency having frequency variations in time varying from 20 the carrier frequency. The op~ical assemblies 154 and 156 ~urther comprise the objective lens 52 supported by a hydrodynamic air bearing member 54 which supports the lens 52 above the coating 26 carried b~ the information storage member 10.
` As previously described~ the read beam 150 is formed with substantially parallel light rays. The objec-tive 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 3 174 is provided intermediate the laser source 152 and the entrance aperture 56 o~ the ob~ective lens 52 for spreading the substantially parallel light rays formlng the reading beam 150 into a light beam 150 having a diameter suf~icient to at least fill the entrance aperture 56 of the objec~ive lens 52. The optical assemblies 154 and 156 further include a number o~ stationary or ad~ust-able mirrors 176 and 178 for ~olding the read light beam 150 and the reflected light beam 150' along a path cal-culated to impinge upon the previously mentioned elements.

5 ~ ~ ~ 8 ~ n optional optical fLlter 180 is positloned in the reflected beam path 150' and filters out all wave-lengths other than tha~ of the incldent beam. The use of this filter 180 improves pic~ure 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 re~erence to F ~ ure 11. In this read a~ter wrlte environment, a por-tlon of the write beam 29 travels along the re~lected read beam path 150'. The filter stops this portion of the write beam and passes the full intensity of the re~lected beam 150'.
An optional converging lens 182 is positioned in the reflected beam path 150' for ~maging the reflected beam onto the active area of the light sensing element 158. This converging lens 182 reduces the diameter of the reflect.ed beam 150' and concentrates the light inten-sity of the reflected beam upon the active area o~ the light sensing element 158.
The amplifier 1~4 ampli~ies the output o~ 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 3 shows a laser source generating a write laser beam hav~ng a constant intensity represented by the line 80i.. The read laser 152 generates a read beam 150 having a constant in-tensity but at a lower level.
Figure 7 shows an intensity modulated write laser beam. The reflected read beam 150' is lntensity 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 ln Figure 7. Rather3 the ~L3l5346~

square edges are rounded by the f~nite size of the read spot.
Figure 5 shows a frequency modulated electrical signal having its informational content ln the form of a carrier signal having frequency changes in time varying about the center rrequency. The output of the light sensing element 158 is the same type of s~gnal. Figure 4 shows a video signal having its informational content in the form of a voltage vary~ng with time ~ormat. The output of the demodulator 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 apparatus, the mo~ion control assembly 28 produces a rotational motlon to the disc member under the control of a rotational drive assem-bly 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.
me assembly 28 further includes a synchronizlng 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 in-formation tracks are shown as 104 and 105 in Figure 3.
Referring to Flgure 11, there is shown a block diagram illustratlng the combination of the write apparatus shown in Figure 1, and the read apparatus shown in Figure 10. The elements shown ~n Figure 11 operate in an identical manner as previously described and this de-30 tailed operation is not repeated here. Onl~ a brief description ls 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 firæt optical assembly defines the modulated beam path 29' 35 between the output of the linear polarizer 70 and the coating 26. ~he fixed, write optical assembly 41 includes the mirror 58. The movableJ write optical assembly 40 includes the diverging lens 66, a partially transmissive mirror 200, a movable mirror 60 and the ob~ective lens 52.

3~ a~3~L6~

The ~odulated wr~te bea~ 29' ls imaged to a write spot 42 upon the light responsive coating and interacts with the coating to form i-ndicia as previously described.
The read beam path is shown at 150. The read optical assem~lies de~ine a second opt~cal path for the read beam 150 between the read laser 152 and the informa-tion storage record carrier 10. The fixed, read optical assembly 154 includes the mirror 176. The movable, read optical assembly 155 includes the diverging lens 174, the polarization shifting means 172, a second fixed mirror 202, the partially transmissive mirror 200, the movable mlrror 50 and the lens 52. The read beam 150 is imaged to a read spot 157 at a point spaced downstream from the write spot ~2/ as is more completely described with refer-ence to F~gure 12 The mirror 200 is a dichroic mirror which is transmissive at the wavelength o.~ the write beam 29' and which is reflective at the wavelength of the read beam 150'.
The intensi~y 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 khe coat-lng 26 and provide a re~lected 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 150l to a frequency modulated 30 electrical signal by the light sensing circuit 158.
The lixed 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 35 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 -2&-to be four to six microns. This distance corresponds to an angle too small to show clearly in Figure 12. Accord-ingly, this angle is exaggerated in Figure 12 for purpose o~ illustration only.
The read beam 150' is demodulated in a discrim-i~ator circuit 162 and displayed on a standard television monitor 166 and an oscllloscope 168. The television monitor 166 shows the pictorial quality of the recording and the oscilloscope 168 shows the video signal in rnore 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 o~ the stored signal is poor~ it is known immedi-ately and the write procedure can be corrected or the in~ormation storage member 10 storing the poor quality video in~ormation 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 ~or combining the read beam 150 into the write beam 29'. In this read after write mode of operation, the wavel~ngth of the write beam 29 is chosen to be different from the wavelength o~ the read beam 150. ~n optical filter 180 is employed for blockin~ any portion of a write beam which 25 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' ~ollow-ing the reflected read beam path 150l.
In the comparison mode of operation, the read 30 a~ter write operation is practiced as described with re~e~
ence to Figure 11. ~lhen operating ln this monitorlng mode o~ operation, a comparator circuit 204 compæres the output of the demodulator 162 with the original video information signal provided by the source 18.
More specifically, the video output of khe dls-criminator 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 ~g S39~

imparts a time delay to the inp~t video information signal equal to the accumulated values of the delay beginning with the frequency modulation of the input video informa-tion signal and extending through the frequency demodula-tion of the recovered electrical signal from the sensingcircuit 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 informa-tion 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 c~rcuit 210 a variable delay circuit which is ad~usted ~or optimum operation.
Ideally~ the video output signal of the discrim-inator 162 is identical in all respects to the video lnputsignal on the lines 18 and 208. Any di~ferences noted represent errors which mlght be caused by imperfections in the disc's surface or malfunctions of the writing cir-cuits. This application, while essential if recording digital informationg is less cri~ical when other informa-tion 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 20~ compares the output signals available on the lines 208 and 206. An alternatlve 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 ls 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.

~;3~6~

This shows an une.~posed coating 26 approaching the wrlte beam 29 ' and a lineal series of apertures 37 leav-lng the intersection of~ the wr:ite beam 29 l and the coating 26.
The writing beam 29 ' coincides with the optical axis of the 5 microscope ob~ective lens 52. The central axis of the reading beam 150 shown as 212 makes an angle with the central a,sis of the write beam 29 ' shown as 214. The angle is represented by a double headed arrow 216. Due to this slight dif`ference in optical paths of the write beam 29 ' 10 and read beam 150 through the lens 52~ the write spot 42 ~alls 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 llne 218 is equal to the angle times the focal length of the ob~ective 15 lens 52. The resulting delay between writing and reading allows the molten metal coating 26 to so~idify so that the recordin~ 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 20 provide a high quality signal for display on the monitor 166, Referring to Figure 13, there is shown an ideal-ized diagram of a Pockels cell stabilizing circuit 48 suitable for use in the apparatus of Figure 1. As ls 25 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 Pockel~ cell 68 a voltage change of the order of 100 volts causes the 3 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 'che in-formation signal source 12 to a peak-to-peak output swing of 100 volts. This provides a proper input driving signal 35 to the Pockels cell ~8. The Pockels cell driver 72 gener-ates a waveform having the shape shown in Figure 5 and having a peak-to-peals voltage swing of 100 volts.
The Poclsels cell should be operated at an average rotation of forty-five degrees in order to make t~e ~a5;3~8 modulated light beam intensity most faith~ull~ reproduce ~he electrical drive signal. A bias voltage must be pro-vided to ~he Pockels cell for keeping the Pockels cell at this average operating point. In practice, the electrical bias voltage corresponding to a ~orty-five de~ree rotation operating point varies continuously. This contlnuously changing bias vol~age is generated through the use of a servo feedback loop. This feedback loop includes ~he comparison of the average value of the transmitted light to an adjustable reference value and applying the differ-ence signal ~o ~he Pockels cell by means of a DC amplifier.
This arrangement stabilizes the operating point. The reference value can be ad~usted to correspond to the average transmission corresponding to the forty-five degree operating point and the servo feedback loop provldes cor-rective bias voltages to maintain the Pockels cell at this average rotation of forty-five degrees.
The stabilizing circuit 48 includes a light sens-ing means 225. A silicon diode operates as a suitable light sensing means. The diode 225 senses a portion 29"
of the writing beam 29' issuin~ from the optical modulator 44 and passing through the partially reflective mirror 58 as shown in Figure 1. The silicon dfode 225 functions in much the same ~ashion as a solar cell and is a source o~
electrical energy when ~lluminated b~J lncident~radiation.
One output lead of the sillcon diode 225 ls connected to common reference potential 226 by a line 227. The other output lead of the diode 225 is connected to one input o~
a differential amplifier 228 by a line 230. The output leads of the silicon cell 225 are shunted by a load resis-tor 232 which enables a linear response mode.
The other input to the differential ampllfier 228 is connected to an adjustable arm 234 of a potentiometer 236 by a line.238. One end of the pokentlometer 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 ad~stment of the differential amplifier 228 to generate a feedback signal on the lines 244 and 246 for ad~usting the average power ~ ll5;3~

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 ou~pu~ lines 244 and 246 ~o the input terminals o~ the Poci~els cell 58 in Figure l. The Pockels cell driver 72 is AC coupled to the Pockels cell 68 by way of capacitive elements 252 and 254J respectively, while the differential amplifier 228 is DC coupled to the Pockels cell 68.
In operation, the system is energized. The por-tion 29'1 of the light from the writing beam 29' impinging on the silicon diode 225 generates a differential volbage at one input to the differential amplifier 228. Initia11Y, the potentio~eter 236 is ad~usted so that the average transmission through the Pockels cell corresponds to forty-~ive degree of rotation. Thereafter, if the average level of intensity impinging on the silicon cell 225 either in-creases or decreases, a correcting voltage will be gener-20 ated by the differential amplifier 228. The correctingvoltage ap~lied to the Pockels cell 68 1S of a polarity and magnitude adequatç to restore the average level of intensity to thè predetermined level selected by ad~ust~ent of the input voltage to the other input of the differen-tial amplifier over the line 238, by movement of themovable arms 234 along the potentiometer 236.
The adjustable arms 234 0~ potentiometer 236 is the means for selecting the average level of intensity of the light generated by the write laser 30. Optimum re-sults are aohieved when the length of an aperture 37 ex-actly 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 spaceg a duty cycle of fifty-flfty is achieved. Such duty cycle is detectable by examlning the display of the just written information on the TV monitor and/or oscillosoope 166 and 168, respectively, as pre-viously described. Commercially acceptable results occur ~534~6f~

when the length of an aperture 37 varies between forty and s~xty percenv of the combined length of an aperture and its ne~t successively positioned space. In other words, the length of an aperture and the next successively posi~
tioned space is measured. The aperture can then be a length ~alling within the range of forty and sixty percent o~ the total length.
Re~erring to Figure 8, there is shown a radial cross-section of an in~ormation track shown with re~erence to Figure 3 in which a specular light re~lective region 38 is positioned intermediate a pair o~ 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 b~ its impingement upon the non-specular ligh~ reflective region 37a. In this con~igur-ation, the positive half cycle o~ the signal to be re-corded is represented by a specular light reflectiveregion 38a and the negative half cycle of the signal to be recorded is represented by the non-specular light re~lective region 37a. The duty cycle of the slgnal shown with ~.è~erence to Figure 8 is a fifty percent duty cycle insofar as the length o~ the specular light re-flected 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 pre~erred duty cycle set up by the comblnation o~
ad~usting the absolute intensity of the write beam 29 by ad~usting the power supply of the write ~aser 30 and by ad~usting the potentiometer 236 in the stabllizing circuit 48 to a level wherein an aperture is formed beginning with a forty-~ive degree rotation of the angle of polarization 3~ in the write beam 29.
Referring again to the aperture forming process illustrated with reference to Figures 7 and 8, melting o~
a thin metal coating 26 occurs when the power in the light spot exceeds a threshold characteristic o~ the composition 3~

and ~hickness of the metal film and the properties o~ the substrate. The spot power is modulated by the light intensit~ modulacing assembly 44. The on-o~ transltions are kept short to make the loca~ion o~ the hole ends pre-cise in spite o~ variations in the melt-Lng threshold.
S~ch variations in the melting threshold can occur due to variations in the thickness of the metal coating and/or the use of a di~ferent 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 o~ the order of 200 milliwatts. Since the FM carrier frequency 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 o~ 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 reflectivity to an im-pinging reading beam.
When the forming of first and second indicia is being undertaken using a coating of photoresist, the inten-sity of the write beam 29' is adjus~ed to a level such that a forty-five degr~e rotation of the plane of polari-zation generates a light beam 29' of threshold intensity ~or exposing and/or interacting with the photoresist coating 2~ 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 com-prises a light intensity modulating member which operates from the ~orty-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 o~ operation.
~nen the intensity of the write light beam 29' increases above the initially adjusted level or predetermine start ~53~8 intensi~y, and increases towards the higher light trans-mitting state the i~lcident ~rite light beam 29' exposes the phovoresist illuminated thereby. This eY.posure con-tinues after the intensity of the write beam reaches the 5 maximum light transmitt~ng state and starts back down towards the initial predetermined intensity associated with a fort~-five degree rotation of the plane of polari-zation of the light issuing from the ~rite laser 30. As the rotation drops below the forty-five degree value, the 10 ~ntensi~y of the wrlte beam 29' exiting the Glan-prism 70 drops below the threshold intensity at which the focused write beam fails to expose the photoresist illuminated thereby. This failure to expose the photoresist illumin-ated thereby continues after the intensity of the write 15 beam reaches the minimum light transmittlng 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 s typically a high gain and high voltage amplifier having an output signal providing an output voltage swing o~ 100 volts.
This signal is ~ntended to match the driving re~uirements of the Pockels cell 68. TypicallyJ this means that the mid-voltage value o~ the output of the Pockels cell driver 72 provides a sufficient control voltage for d~i:ving the Pockels cell 68 through forty-~ive degrees so that about one half of the total available light from the laser 30 issues ~rom the linear polarizer 70. As the output signal from the driver 72 goes positive, mid voltage value, more light from the laser is passed. As the output signal from the driver 72 goes negat~ve, 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 1OJ when the output from the driver 72 is zero and the operating point o~ the Pockels cell is forty-five degrees. Accordingly, as the output ~.~5~B

from the driver 72 goes posit~a, melting continues. Also, when the outp~t from ~he driver 72 goes negative, melting stops.
In 2 second embodiment us~ng the photoresist coating 25S the output ~rom the laser 30 is adjusted so as ~o produce an intensity which both ~lluminates and exposes the photoresist coating 26 when the output from the driver 72 is generating its mid-voltage value. Accordingly, as the output ~rom the driver 72 goes positive, the illumina-tion and exposure of the photoresist illumLnated by thewrite beam continues. Also, when the output from the driver 72 goes negative, the illumination continues but the energy in the write beam is insuf~icient to expose the illuminated region. The term expose is herein being used for its technical meaning which describes ~hat physical phenomenon which accompanies exposed photoresist. Exposed photoresist is capable of being developed and the developed photoresist is removed by standard procedures. Photo-resist which is illuminated by light, insufficient ~n intensity to expose the photoresist, cannot be developed and removed.
In both the first and second embodiments just described, the absolute power level 80 lllustrated by the line 80 in Figure 6 is ad~usted upward and downward to achieve this effect by adjusting the power supply of the write laser 30. In combination with th~s adJustment of the absolute power level o~ the write laser 30, the potentiome~er 236 is also used to cause indicia ~o be formed in the coating 26 when the beam 29 is rotated above 3 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 sllgh~ attenua-tion in the reflected path thus requ~ g a slight increase in the intensity of the read laser 152 to insure the same intensity at the detector 158 when compared to a read only system which does not use a ~ilter 180.
The converging lens 182 is optional. In a properly arranged raad system the re~lected read beam 150' ~53~

has essentiall~T the same diameter as the working area of the photodetector 158. If this is nct the case, a con-verging lens 182 -is employed for col~centrating the re-flected read beam 150~ upon the smaller worklng area of the photodetector 158 selected.
Prior to giving the detailed mode o~ operation of an improved version of a mastering machine, it would do well to establish a number of terms which have a special meaning in the description contained hereinaf~er. The laser intensity generated by the writing laser source as it impinges upon the master video disc is employed to interact with the information bearing portion of the video disc to form indicia representing the carrier frequency and the frequency variations in time from the carrier 15 frequency~
The threshold power level required of the laser beam at the point of impact with the information bearing layer of the video disc differs depending upon the mater-ial from which the information bearing layer is made.
20 In the two examples given hereinabove describing a metal such as bismuth and a photosensitive material such as photoresist, the threshold power level required to form indicia differs significantly and represents a good ex-ample for illustrating the term threshold power. Obvious-25 ly, the threshold power of other materials would alsodiffer from each of the examples explained.
The indicia formed in a bismuth coated video disc master are alternate regions of light reflectivity.and light non-reflectivity. The areas of llght non-reflecti-30 vlty are caused by the melting of the bismuth followed bythe retracting of the bismuth before cooling to expose an underlying portion of the glass substrate. Light imping-ing upon the metal layer is highly rsflectiveg while light impir.ging upon the exposed portion of the glass substrate 35 is absorbed and hence light non-reflectivity is achleved.
The threshold power is that power from the laser beam required to achieve melting and retracting o~ the metal layer in the presence of a laser beam of lncreasing light intensity. The threshold power level is also \

~s~

represented as tha~ intens~ty of a decreasing light inten-sity signal when ~he metal layer ceases to melt and retract from the region having incident light impinging thereupon. More speciLically, when the power in the impinging light beam exceeds the threshold power require-ments of the recordi.ng material; a hole is rormed in the recording material. When the light power intensity in the impinging light beam is below the threshold power level of the recording material~ no hole is formed in the recording medium. The forming of a hole and the non-forming of a hole by the impinging light beam is the principal manner in which the light beam impinging upon a bismuth coated master interacts with the bismuth layer to form indicia on the recording surface. The indicia represents a carrier frequency having frequency changes in time varying about the carrier ~requency.
A video disc master having a thin layer o~ photo-resist formed thereover has its own threshold power level.
The mechanism whereby a light beam exposes a photoresist layer is pursuant to a photon theory requiring a suffi-cient number of photons in the impinging light beam toexpose a portion of the photoresist. When the positive going modulated light beam contains sufficient photons above this threshold power level, the photoresist in that area is exposed so that subsequent development removes the exposed photoresist. ~hen the photon level in a decreas-ing light intensity modulated light beam falls below the normal threshold power level of the photoresist, the phot~
resist ceases to be exposed to the extent that subsequent development does not remove the photoresist illuminated by an impinging light beam having photons below the threshold power level.
The impinging light beam from the modulated laser source interacts with the information bearing layer to fully expose or underexpose the photoresist layer illumin-ated by the impinging light beam. This is an interaction of the photons in the impinging light beam with the infor-mation bearing member to form indicia o~ the carrier fre-quency having frequency changes in time varying about the \

~53~
_3C_ carrier frequency. The indicia storln~ the carrier fre-quency and frequency change in time are more fully appre-ciated after the developmellt step ~?hereb~ those portions of fully exposed photoresist material are e~fectivel~J
removed leaving the underexposed portions on the video disc member.
Referring to Figure 23~ there is a block diagram of the Pockels cell bias servo system e~ployed in the pre-ferred embodiment of the present invention for maintaining the operating bias on the Pockels cell ~$ at the half power point. The DC bias of the Pockels cell is first adjusted to its steady state condition such that the half power point of the Pockels cell-Glan prism combination coincides with the forty-five degree rotation point of the Pockels cell 58. This DC bias point is identlfied as the fixed bias point. In a system wherein the input video signal to the FM modulator 36 does not contain any second harmonic distortionS the DC bias position selected in the procedure just identified operates satisfactorily. ~ow-everJ when the video information inp~.t signal to the FMmodulator contains second harmonic distortion products~
these distortion products show up in the modulated light beam at 29l. The output from the FM modulator is applied to a Pockels cell driver 72 for developing the voltage required to drive the Pockels cell through its zero to ninety degree rotational shift. The unmodulated light beam from the laser 29 is applied to the Pockels cell 68 as previously described.
The purpose of the Pockels cell bias servo i5 to bias the Pockels cell 68 so that the output light signal detected at a photo diode 26a is as free of second harmonic content as possible.
The second harmonic distortion is introduced into the modulated light beam at 29' from a plurality of sources.
A first of such sources is the non-linear transfer func-tions of both the Pockels cell 68 and the Glan prism 70.
I~hen the input video signal on the line 18 itself contains , second harmonic distortion products this further increases the total second harmonic distortion products in the light beam at 29 .
The Pockels cell bias servo functions to adjust the DC bias applied to the Pockels cell 58, which DC
bias biases the Pockels cell to its half power point9 so as to minimize the second harmonic content o~ the output light beam.
The change in DC bias level from the half power point is achieved in the ~ollowing sequence of steps. The modulated light beam 28' from the Pockels cell 68 is applied to a photo diode 260. The photo diode 260 oper-ates in its standard mode o~ operation and generates a signal having the ~orm of a carrier frequency with fre-quency variations about the carrier frequency. This fre-quency modulated wave~orm is a su~iciently linear repre-sentation of the light impinging upon the photo diode 261to accurately re~lect the signal content of the light modulated beam 29' imp~nging upon the disc surface. More speci~ically, the output signal from the photo diode 260 contains the di~tortion products present in the modulated 20 light beam 29 ~ . The output from the photo diode 260 is applied to a second harmonic detector 261 over a line 262 which forms a part o~ the bias control circuit 264. The output ~rom the second harmonic detector is to a high voltage amplifier 266 which generates the DC bias signal over a line 268. The line 268 is connected to a summation circuit 270 which has as its second input signal the output from the Pockels cell driver 72. The DC bias signal on the line 268 is summed with the output ~rom the Pockels cell driver 72 and is applied to the Pockels cell 68 for chang-ing the DC bias of the Pockels cell 68.
~ eferring back to the operation of the secondharmonic detector 261g this device generates a voltage which is approximately linear in the ratio of the second harmonic to the fundamental of the output light beam.
Furthermore~ the output signal re~lects the phase charac-teristics of the second harmonic and if the second harmonic is in phase with the fundamentalg the output of the second harmonic detector is in a first voltage level~ ie. g a positive level. I~ the second harmonic is opposite in

-4 1 -phase with the ~ur~amental~ t~ner, the cutp~t Or the secGnd harmonic detector is at a second voltage level, ie., a negative voltaOe level. The output fr~m the second harmor~
de'ector is ampl-,fied through 2 high voltage amplifier 265 which provides a range of zero to three hundred volts of DC bias. This DC bias is summed with the signal from the FM modulator 20 ampli~ied in the Pockels cell driver 72 and applied to the Pockels cell 68.
The second harmonlc detector includes a limiter 272 shown with reference to Figure 24 and a differential amplifier 27~ shown with reference to Figure 24. The out-put signal from the photo diode 260 is AC coupled ~o the limiter 272 over lines 276 and 278. The limiter 272 has a first output signal for appllcation to the differential amplifier over a first output leg 280. The second output from the limiter 272 is applied to the second input of the dlfferential amplifier over a second output leg 282.
The output signals from the lim~;ter 272 are logical comple-ments of each other. ~ore specificallyl when one output is at a relatively high voltage level, the other output is at a relatively low voltage level. The two output signals on the legs 280 and 282 are fed into the differen-tial amplifier 2~4. The output of this differential amplifier reflects the content of the second harmonic available on the inpu~ signal lines 276 and 278.
In a standard mode of operation3 when the input signal from the photo diode 260 is substantially free of second harmonic distortion; then the output signal from the differential amplifier 274 at terminal 284 is a square-3 wave with exactly a 50~ duty cycle and with voltage levelsextending between two predetermined voltage levels above and below a constant reference level. The duty cycle of 50% refers to a high voltage half cycle being e~ual in width to the following low voltage half cycle. In this condition, the effectlve DC level of these two half cycles offset one another. Accordinglyg the output of the diffe~
ential amplifier 274 is, on the average9 zero.
When ?, degree of second harmonic d~stortion is present in the output from the photo diode 260, the value

5~3~8 of harmGnic distGr'io~ shirts the mean value crossing from a symmetri^al case to a non-symmetrical case. In this si~uationS the oulput from the ~iL ferential ampli41er is other than a squarewave ~ith a fif~y-fifty duty cycle.
The dif~erential ampllfier therefor~Q detects the effec~
tive DC level shif'G of the incoming signal and generates an output which is above or below ~ero~ on the average9 depending upon ~he asymmetrical nature of ~he lnput signal.
The output of the differential ampli~ier is there~ore applied to the high voltage amplifier which DC smooths the output ~rom the dif~erential amplifier 274 and amplifies the negative or posi-cive resulting DC level. The resulting product is the required shift in bias signal for applica-tion to the Pockels cell to return the operating point o~
the Pockels cell to the half power point at which zero harmonic distortion occurs.
A summary of the standard operating mode of Pockels cell bias servo includes the generation o~ a light signal representing the distortion products present on the modulated light beam. Means are provided for dete~t-ing the value of second harmonic distortion present in thls light beam and generating a signal representation of this distortion. The signal generated to represent the amount of second harmonic distortion also includes whether the second harmonic distortion is in phase with the fundamen-tal frequency or the second harmonic distortion is out of phase wlth the fundamental fre~uency. The output signal representing the amount of second harmonic distortion and the phase o~ the second harmonic distortion with reference to the fundamental frequency is applied to a means for generating a bias signal necessary for application to the Pockels cell to bring it to an operating point at which second harmonic distortion ceases to exist. A summation circuit is provided L or summing the change in bias signal with the input ~requency modulated video signal. Th~s summed voltage is appl-ied as an input to the Pockels cell 58.
Figure 14 shows a series of waveforms illustraing an improved form of light modulation o~ a wrlting light 31 ~S39~6~3 beam 29. Line ~ o~ ~igure 14 shows an ldealized or simpl~
fied video wavefor~i that is typically supplie~ as a video signal ~rom a video tape recsrder or television camera.
This waveform is essentiall-~ the same as ~hat shown in 5 Figure 4 and represents a video signal ~hat is applied to the FM modulator circuit 20. Tl!o output signals are shown on lines B and Cs and each is an FM modulated output signal and each carries the same frequency information.
The waveform on line ~ is a ~epeat of the waveform in Figure 5 and is repeated here for convenience. This wave-form on line ~ shows the output normally generated by a mlulti-vibrator type FM modulator 20. The waveform shown on line C shows the output generated by an FM modulator 20 having a triangular shaped output waveform. ~oth wave-forms contain the same frequency information. The triangu-lar shaped waveform gives enhanced results when used in driving a Pockels cell 68 for light modulation o~ a con~
stant intensity light beam applied through the Pockels cell.
The frequencies contained in each waveform B and C are at all times identical and each represents the voltage level o~ the video waveform shcwn in line A. ~y inspectiong it can be seen that the lower amplitude region of the video waveform generally indicated by the numeral 75 corresponds to the low carrier frequencies andhigher amplitude regions of the video waveform as gener-ally indicated at 77 corresponds to the higher frequency shown ln lines E and C. It is the custom and practice of the televiaion industry to utilize a one volt peak to peak voltage signal having voltage variations in time as the video signal generated by a television camera. These signal characteristics are the same required to drive a television monitor 166. The advantage of using a triangu-lar shaped waveform for driving a Pockels cell 68 is to match the Pockels cell's transfer characteristic with a selected waveform of the modulating signal to achieve a sinusoidal modulation of the light beam passing through the Pockels cell and to the Glan prism 78. The triangular waveform shown in line ~ is a linear voltage change with 3~8 ~~4-time. The linear voltage change versus time of the trl-angular driving waverorm when multiplled by a sinusoldal voltage change versus light transfer functiorl of the Pockels cell 68 gives a sinusoidally varylng light inten~
5 sity output from the Glan prism.
The waveform shown on line D illustrates the sinusoidal waveform which corresponds to the light inten-sity output from the Glan prism when the Pockels cell is driven by the triangular waveform shown on line C.
Referring specifically to the bottommost point at 285 and the topmost point at 285 of the waveform shown on line D, the point exactly equally distant from each is identified as the half power point. An understanding of the utilization of this half power point f~eature is re-15 quired for high quality mastering operations.
The peak to peak voltage of the triangular wave~
form is represented by a first maximum voltage level V2 shown on lina 287 of line C and by a second minimum vol-tage level Vl on line 288. The voltage differential 20 between points 287 and 288 is the driving voltage for the Pockels cell 68. This voltage differential is adjusted to equal that voltage required by the Pockels cell 68 to give a ninety degree rotation of the output polarization of the light passing through the Pockels cell 68, The bias 2~ on the Pockels cell is maintained such that voltage levels Vl and V2 always correspond to the zero degree rotation and the ninety degree rotation respectively of the light beam passing through the Pockels cell 68. The forty-five degree rotation of a light beam is half way between 30 the two extremes of a triangle waveform. That half-way voltage is always the same for the Pockels cell 68. But the half-way voltage with respect to zero volts may dr~ft due to thermal instabilities causing the half power volt-age point to drift also. The correct biaslng of the half-35 way voltage is completely described hereinafter with refe~ence to Figures 18, 19 and 20.
While the waveform shown on line C of Figure 14 shows the triangular wave shape generated by the FM modu-lator 20, it also represents the wave shape of the signal ~5~

generated by ~he Pockels cell driver 7~. The ou~put from the F~ modulator is typically in a smaller voltage range, typically under 10 volts wh~le the output from the Pockels cell driver 72 typically swings 100 volts in order to provide suitable driving voltage to the Pockels cell 68 to drive ~t from i~s zero rotational state to its ninety degree rotational state. In discussing the voltage levels V1 and V2 and the lines 288 and 287~ respectlvely~ repre-senting such voltage points, reference is made to line C
of Figure ll~s because the output from the Pockels cell driver 68 has the identical shape while differing in the amplitude of the wave~orm. This was done for convenience and the elimination of a substantially identical waveform different only in amplitude.
Referring to Figure 15, there is shown a cross sectional, schematic view of a video disc formed according to the mastering process of the invention described herein.
A substrate member is shown at 300 having a planar sllaped upper surface indicated at 302. An information bearing layer 30~ is formed to top the upper surface 302 of the substrate 300. The information bearing layer 304 is of uniform thickness over the entire surface 302 of the sub-strate 300. The information layer 304 itself has a planar shaped upper surface 306.
Figure 6 is shown positioned beneath line C of Figure 14 showing the intensity of the light beam passing ~rom the Pockels cell - Glan prism combination in the improved embodiment which utilizes a voltage control oscillator in the FM modulator 20 generating a triangular shaped output waveform as the driving waveform shape to the Pockels cell 68. As previously described, the thres-hold power level of the information bearing layer is deflned as that power required to form indicia in the information bearing layer in response to the impinging light beam. For a metal surface, the th2rmal threshold point is that power required to melt khe metal layer and have the metal layer retract from the heated region o~
impingement. For a photoresist layer, the threshold power is that power level required to supply sufficient photons 3~

to completel~T expose the photoresist information bearing layer. In the case of the metal layer, the heated metal retracts from the impinging area to exp~se the substrate 300 position thereunder. In the case of the photoresist material~ the photon power is sufficient to fully expose the total thickness of the photoresist layer 324 completely down to the upper surface 322 of the substrate 320 as shown in Figure 7.
It has been previously discussed how the half power point of the Pockels cell - Glan prism combination is located at a point hallway between a first operating point at which maximum transmission from a fixed intensity beam passes through the Glan prism and a second operating point at which minimum transmission from a fixed intensity beam passes through the Glan prism 70. The half power point is the point at which the light passing through the Po^kels cell has been rotated forty-five degrees from the point of zero power transmission, In operation9 the output power from the laser is adjusted such that the half power.point of the Pockels cell-Glan prism combination provides suf~cient energy to equal the threshold power level of the lnformation bearing member employed, such as the member 304. The matching of the half power point of the Pockels cell-Glan prism com-2~ bination ensures hlghest recording fidelity of the videofrequency signal to be recorded and ensures minimum inter-modulation distortion of the signal played back from the video disc recording member.
~his matching of the power levels is illustrated with reference to line D of Figure 14 and Figure 15 and by the construction lines drawn vertically between the hal~ power point represented by the line 290 shown on line D of Figure 14 and the apertures shown ~enerally at 310 in Figure 1~. The length of an aperture 310 is coextensive with the time that the transmitted intensity of the modu~
lated light beam exceeds the half power point line 290 shown with reference to line D of Figure 14.
In this embodiment the half power point line 290 also represents the zero crossing of the triangular wave ~s~

shape shown on line C of Figure 14. The zero crossing points are represel1ted by lines 291 and 292 shown in Figures 14~ and 15Cg and the importance of regulating the half power point is explained in grea~er detail with refer-ence ~o Figures 20 and 21.
Figure 16 shows an in~ormation storage memberincluding a substrate 320 having a planar upper surface 322. A thin layer of photoresist 324 o~ uniform thi~kness is ~ormed over the planar upper surface 322 of the sub-strate 320. The thin photoresist layer 324 is also rormedwith a planar upper surface 326. The photoresist layer 32~ is a light responsive layer just as the metal bismuth layer, 304 is a light responsive layer. Both the thin opaque metallized coating 304 and the photoresist layer 15 324 function to retain indicia representative of the video input signal. In the case of the metal layer 304, aper tures 310 are formed in the metallized layer to form successive light reflective and light non-re~lective regions in the information storage member.
Re~erring to Figure 17 showing the photoresist coated informatlon storage memberg regions 330 are formed in substantially the same manner as regions 310 were ~ormed with reference to the structure shown in Figure 15.
Rather than apertures 310 be~ formed as shown with refe~
ence to Figure 6, exposed regions 330 are ~ormed corres-ponding to the apertures 310. The exposed photoresist material is represented in Figure 16 by cross hatching of the regions in the photoresist information storage layer 324. Subsequent development of the exposed photoresist material removes such exposed photoresist material leaving apertures comparable to the apertures 310 shown with refer-ence to Figure 15.
In operation, when using the photoresist coated substrate video disc memberg the output power of the writ-ing laser is adjusted such that the power o~ the modulatedlaser beam passing through the Pockels cell-Glan prism combination at the half power point of the Glan prism equals the photon threshold power required to completely expose the photoresist illuminated by the impinging light ~ ~S;~6~
4~
beam. .Tust as with the bismuth ~oated master video disc s~stem; this ensures hlghest f~elity recording and minimum intermodulation distortion during the playback o~ the re-corded ~ideo s~gnal.
In referring to both Fi~ures 15 and 16~ that por-tion o~ tne light bea~ passing through the Glan prism above the half power point as represented by that portion of the waveform shown on line D of Figure 14 which is above the llne 290J causes an irreversible change in the character-istics of the light sensitive surface 304 in the case of the bismuth coated video disc shown in Figure 15 and the photoresist coating 324 shown with re~erence to the photo-resist coated video disc shown in Figure 16. In the case of the bismuth coated video disc member 300, the irrever-sible changes take the ~orm of successively formed aper-tures 310 in the opaque metallized coating 304. In the case of the pbotoresist coated substrate 320, The lrrever-sible alteration of the characteristic of the photoresist layer 324 occurs in the form of successive fully exposed regions 332.
While bismuth is listed as the preferred metal layer, other m~tals can be used such as tellurium/ inconel and nickel.
Re~erring to Figure 18, there is shown the trans-fer characteristic of the Poc~els-Glan prism combination as a result of the sinusoidal rotatinn in degrees of the light pas~ing through the Pockels cell 68 versus linear voltage change of input drive to the Pockels cell 68.
The ninety degree rotation point is shown at point 340 and equals the maximum light transmission through the Glan prism 70. The zero degree rotation point is shown at points 342 and equals the zero or minimum llght trans-mission througll the Glan prism 70. The zero light tEan mission point 342 corresponds to the voltage level Vl represented by the line 288 in line C o~ Figure 14. The ninety degree rotation point corresponds with the voltage level`V2 represented by the line 287 on line C of Figure 14. The point hal~ way between these two voltages repre-sented by the line 292 is equal to V2 minus Vl over Z

~53~68 and corresponds to a ~orty-f~ive degree rotation o~ the light beam passing through ~he Pockels cell.
As ~s well kn~n~ the power through the Pockels cell is substantially unchanged. The only characteristics 5 being changed in Jhe Pockels cell is the degree of rota-tion of the light passing therethrough. In normal practice, a Pockels cell 68 and Glan prism 70 are used together to achieve ligh~ modulation. In order to do this~ the prin~i-pa'l axes o~ the Pockels cell 68 and the Glan prism 70 are put into alignment such that a light beam polarized at ninety degrees rotation passes substantially undiminished through the Glan prism. When the same highly polærlzed light is rotated by the Pockels cell 68 for ninety degrees rotation back to the zero degree rotation, the light beam does not pass through the Glan prism.70. In actual prac-tice, the ~ull transmission state and zero transmission state is not reached at high ~requencies o~ operations.
The waveform shown in Figure 18 shows the transfer charac-teristics o~ the Pockels cell 68 rotated to correspond with two cycles o~ frequency modulated video information.
This demonstrates that the transfer function continuously operates over the zero to ninety degree portion of the trans~er function curve.
Referring to Figure 195 there is shown the trans-~er characteristic of a Glan prism 70. At point 3~0,maximum transm~ssion through the Glan prism 70 is achieved with a n~nety degree rotation of the incoming light beam.
At point 352, minimum or zero light transmission through the Glan prism 70 is achieved at zero rotation of the 3 incoming light beam. Hal~ o~ the intenslty o~ the imping-ing light beam is passed through the Glan prism 70 as i~dicated at points 354 which corresponds to forty-~ive degrees rotation of the light entering'the Glan prism 70.
Obviously, the absolute power of the li~ht passing through the Glan prism 70 at the ~orty-~lve degree rota-tion can be adJusted by ad~usting the light output inten-sity of the light source. In th~s embodimentg the light source is the ~riting laser 30.
In the pre~erred embodiment, the power output -5c-from the writin, laser 30 is adjusted such that the inten-si'~v of the 1~ ~h~ passin~ ti~r~ugh ~he Glan prism at the half ool~er pC' !l~ ccil~cides wite the threshold ~ower level of Jhe recording medium. Since more power is required to melt a bismuth layer thar, is required to fully expose a photoresist layer~ the absolute intensity o~ a writlng beam used in writing on a bismuth master disc is greater than the intensity o~ a writing laser used to interact with a photoresist covered master video disc.
Referring collectively to Figures 20 and 21, there is shown a series of waveforms useful in explaining the relationship between length of a hole cut in a master video disc by the writing laser 30 and the length of uncut land area between successively formed holes. ~his rela-tionship can be referred to as a relationship formed by the value of the peak cutting power~ the average cutting power and focus of the spot on the metal layer. Collec-tively, these terms have evolved into a single term known as duty cycle which term represents all three such charac-20 teristics.
As previously described, the energy required tointeract with the information bearing layer on video disc substrate is that energy necessary to cause irreversible changes in the material selected for placement on the master video disc member. In the case of a bismuth coated master~ the energy required is that needed to selectively remove the portion of the bismuth coated layer in those locations when the energy is above the threshold energy level of the bismuth layer. If this energy contained in the spot of light is not focused properly upon the bismuth layer, then the energy cannot be used for its intended function and it will be dissipated without effecting lts intended function. If some cutting occurs due solely to an out of focus spot distortions are introduced into the 3~ mastering process.
If the peak cutting power greatly exceeds the threshold power level of the recording medium, destructive removal of material occurs and provides a surface con~
taining distortion products caused by this destructive ~.~.53~
-5l-removal. The a~erage cutting power is that power at a point midway be~ een a first higher cutting power and a second lol~er cutting power. As just describedg the average cutting power is preferably ~xed to equal the threshold power level of the recording medium. In this sense, the intensity of the light beam above the average cutting power interacts with the informatiGn bearing layer to form indicia of the signal to be recorded. The intensity of the light beam below the average cutting power fails to heat a bismuth coated master to a point needed in the hold forming process or fails to fully expose a portion of a photoresist coated master.
Referring briefly to lines B and C of Figure 14, the ad~ustment of the average cutting power to coincide with the line 291 shown in line ~ and with the line 292 shown with reference to the line C of Figure 14, results in a duty cycle where the length of a hole equals the length o~ the "land" area position and successively thereafter.
This is known as a 50% or fifty-fifty duty cycle. A
fifty-fifty duty cycle is the preferable duty cycle in a recordlng procedure but commercially acceptable playback signals can be achieved in the range from sixty-forty to forty-sixty. This means that elther the hole or the inte~
vening land member becomes larger while the other member becomes smaller.
Referring to Figure 20, there is shown a waveform r`epresented by a line 360 corresponding to two cycles of the llght intensity transmitted through the Poc~els cell-Glan prism combination and represented more specifically on line D of Flgure 14. The threshold power level of the recording medium is represented by a line 362. The threshold power level of the reading medium is caused to be equal to the half power point of the light intensity transmitted by the Pockels cell-Glan prism combinatlon by ad~usting the absolute intensity of the writing laser 30.
I~en the threshold level is properly ad~usted at the half power point, an indicia is formed on the information surface layer of the master video disc begin-ning ~t point 364 and continuing ~or the time until the ~3!.539~i8 intensity falls to a point 366. Dash lines shown at 364 ' and 356 ~ are drawn to line A of Figure 12 showing an indici~ represented by the eclipse 368 ~lhich has been ~ormed for the period of time when the light intensity continues to rise past the poin~ 36~ to a maximum at 370 and then falls to a point 366. The light intensity below point 366 falls to a minimum at 372 and continues to rise towards a new maximum at 374. At a certain point between the lower intensity level 372 and the upper intensity level 374, the light intensity equals the threshold power level of the recording medium at 376. Beginning at point 376J the energy in the light beam begins to form an indicia represented by the eclipse 378 shown on line A of Figure 20. A dotted line 376 ~ shows the start of forma-tion of indicla 378 at the point when the light intensityexceeds the threshold level 362. The indicia 378 con-tinues to be formed while the light intensity reaches a maximum at 37~ and begins to fall to a new minimum at 375.
Howeverf at the intersection of line 360 with the thres-hold power level at 362 the light intensity falls belowthe threshold power level and the indicia is no longer formed. In the preferred embodiment, the length of the indicia represented by a line 384 ~quals the length of the land region shown generally at 386 as represented by the length of the line 388. Accordingly~ the matching of the half power point light intensity output from the Pockels cell-Glan prism combination with the threshold power level of the recording surface results in a fifty-fifty duty cycle wherein the length of the indicia 368 equals the 3 length of the next succeeding land reglon 386. Points 364, 366, 376 and 382 shown on the line 360 represent the zero crosslng of the original frequency modulated video si~nal. Hence, it can be seen how the indicia 368 and 386 represent the frequency modulated video signal. This 35 representation in the preferred embodiment represents a fifty-fifty duty cycle and is achieved by ad~usting the half power level of the beam exlting from the Pockels cell-Glan prism combination to equal the threshold power level of the recording medium.

~Q5;~

The waveform shown with reference to Figure 20, including the variable light in~ensity represented by the line 3~0, represents a preferred mode of operatlon to achieve 50/50 duty cycle independent of the recording medium employed on the master video disc member. The absolute intensities at the various points change accord-ing to the absolute intensities requlred ~or the modulated llgh~ beam to interact with the recording surf~e, but the relative wave shapes and their relative locations remain the same. More specifically~ the absolute intenslty of the threshold power level for bismuth is different than the absolute intensi~y of the threshold power level for photoresist~ but the relationship with the intensity line 360 is the same.
Referring to the combination of Figure 20 and line ~ of Figure 21~ there will be described the results of faillng to match the half power point output of the Pockels cell-Glan Prism combination with the threshold power level of the recording medium. Referring to Figure 20, a second dash line 380 represents the relationship between the actual threshold power level of the recording medlum being used with the light intensity output from the Pockels cell 68-Glan Prism 70 combination. The thres-hold power level line 380 intersects the intensity line 360 at a plurality of locations 390, 392, 394 and 396. A
line 390' represents the intersection of the light inten-sity line 360 with the threshold power level 380 and signals the start of the formation of an indicia 398 shown on line E of Flgure 21. The indicia 398 is formed during the time that the light intensity is above the threshold power level. The length of the indicia 398 is represented by the time required for the light intenslty to move to its maximum at 370 and fall to the threshold point 392 as is shown by a line 399. The length of a land area indi-35 cated generally at 400 has a length represented by a line402. The length of a line 402 is determined by the time requlred for the light intensity to move from threshold point 392 to the next threshold point 394. ~uring this time, the intensity of the light beam is sufficiently low

6~ -`
-5~-as to cause no interaction ~)ith the recording medium. A
second lndicla is shown at 406 and its leng~ corresponds with the point at ~hich the intenslty of the waveform represented by the line 360 exceeds the threshold power level at point 39~. The length of the lndicia 406 is shown b a line 408 and is determined by the time required for the llght intensity to rise to a maximum at 374 and ~all to the threshold level at point 396.
Various lines are shown indicating the beginning and ending of the indicia and intravening land areas by employing primed numbers to identify the corresponding intersections of the light intensity line 3~0 with the threshold power level lines 362 and 380.
The successively posit~oned indicia 398 and land region 400 represent a single cycle of the recorded fre-quency modulated video signal. The indicia 398 repre-sents approximately 65 percent of the sum of the length of the line 399 and the line 402. This represents a duty cycle of 65/35 percent. Sixty-five percent of the 20 available space is an indicia ~hile thirty-five percent of the available space is land area. Typically, the indicia in the final format is a light scattering member such as a bump or hole, and the land area is a planar surface covered with a highly reflective material.
The frequenc~ modulated video information repre-sented by the sequentially positioned light non-reflective member 368 and light reflective member 3~6 shown in line A
of Figure 21 represents the preferred duty cycle of 50/50.
When the photoresist mastering procedure is employed, the 30 reflectivity of the upper surface of the photoresist layer is not significantly altered by the impingement of the writlng beam such as to be able to detect a dif~erence between rerlected light beams from the developed and not developed portions of the photoresist member. It is 35 because of this effect that a read-after-write procedure3 using a photoresist coated master video disc is not possible.
Referring to line C of Figure 213 there is shown a re~resentation of the recovered video signal represented 3~68 by the sequence of~in~icla 368 and land area 386 shown on line A. The ~Yave~orm shown ln line C is an undlstorted sine wave 410 and contains the same undistorted frequency modulated information as represented b~J the lig~t intensity waveform represented b~J the line 360 shown in Figure 11.
The slne wave shoT~n in line C of Figure 21 has a center line represented by a llne 412 which intersects the sine wave 410 in the same polnts of intersection 8S the line 362 intersects the intensity line 360 shown in Figure 20.
Referring to line D of ~igure 21, there is sho-~n a recovered frequency modulated video signal having bad second harmonic distortion. The fundamental frequency of the wave~orm represented by a line 414 shown in line D
is the same as that contained in the waveform shown on line C. However, the information shown in Line D contains bad second harmonic distortion. I~hen used in a system in which bad second harmonic distortion is not a disabling problem, the attention to a 50/50 duty cycle situation explained hereinabove need not be strictly followed.
However, when it is necessary to have a substantially undistorted output signal recovered from the video disc surface, it is necessary to follow the procedure described hereinabove.
Referring to Figure 13, there is shown the rela-tionship between the intensity of the reading spot in thereading beam as it impinges upon successively posltioned light reflective and light non-reflective regions formed during a preferred form of ~he mastering process. In a preferred embodiment, a metal is used for this purpose and 3 the preferred metal as disclosed is bismuth.
Line A of Figure 13 shows a plurality of indicia formed in the surface of a video disc master. In the preferred embodiment the holes formed in a bismuth layer 420 are shown at 422, 424 and 426 ~ The intervening por-tions of the la~rer 420 which are unaffected by the forma-tion of the holes 422j 424 and 426 are sometimes called "land areas and are indicated generally at 428 and 430.
The land areas are highly reflective. The formation of the holes 422, 424 and 426 expose the underlying glass substrab2 .

~3~6Z5 which is essentiall~ light absorbing and hence the glass substrate is a ligh~ no~-reflective region. The waveform shown at 432 represents the light intensity wave~orm of the spot in the read beam as t'~e spot passes over a light non-reflective region. This indicates the spacial rela-tlonship between the spot as it moves over a light non-reflective region.
Referring to line B of Figure 13, there is shown a waveform represented b~ a line 434 lndicating the inten-sity waveform of the reflected light as a spot having thein~ensity relationship shown in Figure A passes over a successivel~J positioned light reflective and light non-reflective region. A solid line portlon 436 of the line 434 shows the intensity waveform of the reflected light as the spot passes over the light non-reflective region 424.
The intensity of the reflected light shows a minimum at point 438 which corresponds with the center o~ the non-reflective region 424. The center of the non-reflective portion ~24 is shown on a llne 440 at a point 442. The 20 intensity waveform of the reflected light is a maximum~
as shown at 444, when corresponding to a center point 446 of the land area 428 positioned between successive non-reflective regions 422 and 424 respectivel~. The center point 445 is shown on a line 448 representing the center 25 line o~ the information track. The dotted portion of the line 434 represents the past history of the intensit~
waveform of the re~lected light when the light passed over the non-re~lective region 422. A dotted portion 452 o~ the waveform 434 shows the expected intensit~ of the reflected light beam when the reading spot passes over ~he non-reflective region ~26.
Re~erring to line C o~ Figure 13, there is shown the recovered electrical representation of the light intensity signal shown on line B. The electrical repre-sentation is shown as a line 454 and is generated ln thephotodetector 70 sllown in Figure 1.
A schematic diagram of a suita~le hlgh voltage amplifier is shown in Figure 16.
A special advantage o~ the read while write capability of the mastering procedure herein described includes the use of t.~e instantaneous monitoring of tne in~ormation ~ust written as a means for controlling the duty cycle of the reflective and non-reflective regions.
By displaying the recovered frequency modulated video signal on a television monitor during the writing proce-dure, the duty cycle can be monitored. Any indication of the distortion visible on the mo~-t~or indicates that a change in duty cycle has occurred. Means are provided for ad~usting the duty cycle of the written information to eliminate the distortion ~y ad~usting the duty c~Jcl~
to its 5Q/50 pre~erred operating point. A change in duty cycle is typically corrected by adjusting the absolute intensity of the light beam generated in the laser 30 in a system having either an average intens~ty biasing servo or a second harmonic biasing servo and in conjunction with circuitry for adjusting the half power point output of the Pockels Cell-Glan Prism combination to equal the threshold power level of the recording medium. The term half power point and average intensity are interchanged in the portions of the specilications and claims which concern the use of the triangular shaped wave form gener-ated by the FM modulator. The modulated light beam 40 exiting from the Glan Prism 38 is of sinusoidal shape. In ~5 this sltuation the half power point equals the average intensity, and tllis would be the case for any symmetrical wave form. A frequency modulated output from an FM modu-lator has been found to act as such a symmetrical wave form.
While the invention hss been particularly shown and described with reference to a preferred embodiment and alterations theretoj it would be understood by those skllled in the art that vario~us changes in form and detail may be made therein without departing from the spirit and scoQe of the invention.

Claims (22)

1. An apparatus for recording an electrical information signal upon an information storage member, comprising:
first means for providing an information signal to be recorded and said signal having its informational content in the form of a variable amplitude, cyclical signal alternating between a first higher amplitude and a second lower amplitude; an information storage member having a substrate and a thin, light-sensitive surface layer overlying said substrate; means for moving said storage member in a prescribed fashion; a laser light source for providing a write beam of light having sufficient intensity to interact with said light-sensitive layer and produce indicia representative of the information signal; said thin surface layer having a substantially uniform threshold power level; said threshold power level being of the type wherein said layer reacts to said laser write beam having an intensity greater than said threshold power level and said layer does not react to said laser write beam having an intensity less than said threshold power level; an optical modulator positioned in said laser write beam intermediate said thin surface layer and responsive to the electrical signal for modulating the intensity of the laser write beam to include a predetermined first intensity at which said beam forms an indicia in said thin surface layer and a predetermined second intensity at which said beam does not form an indicia in said thin surface layer; and feedback apparatus for stabilizing the operating level of said optical modulator to issue said write beam at a predetermined average power level equal to the threshold power level of said layer.

2. An apparatus as claimed in Claim 1, wherein said feedback apparatus further includes: light-sensing means for sensing at least a portion of said laser write beam issuing from said optical modulator and responsive thereto for producing and applying to said optical modulator a corresponding bias signal to stabilize the operating level of said optical modulator.

3. An apparatus as claimed in Claim 1, and further including: said intensity of said laser light beam being adjustable relative to said threshold power level of said light-sensitive surface.

4. The apparatus as claimed in Claim 1, wherein the surface layer is a metallic layer.

5. The apparatus as claimed in Claim 1, wherein the surface layer is a photoresist layer.

6. The apparatus as claimed in Claim 1, wherein said feedback apparatus includes: first bias means responsive to the average intensity level of said intensity of said modulated light transmitted through said optical modulator for adjusting said average intensity of said modulated light beam at a fixed first level; and second bias means responsive to a change in average intensity level established in said modulated light transmitted through said optical modulator for restoring the changed intensity level to said fixed first level.

7. The apparatus as claimed in Claim 1, wherein the-optical modulator further includes: means responsive to the electrical signal for rotating the plane of polariza-tion of the laser write beam between zero and ninety degrees; and said feedback apparatus includes means for adjusting and stabilizing said optical modulator for fixing the rotation of the plane of polarization of said laser write beam at forty-five degrees when said modulated beam has an intensity equal to the threshold power level.

8. The apparatus as claimed in Claim 7, wherein one-half of the total available light of said laser beam passes through said optical modulation at forty-five degrees of rotation.

9. Apparatus for recording an electrical signal upon an information storage member, comprising: first means for providing an information signal to be recorded and said signal having its informational content in the form of a variable amplitude, cyclical signal alternating be-tween a first higher amplitude and a second lower amplitude;

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, and said light beam being of sufficient intensity for interacting with said coating while said coating is in motion and said coating is positioned upon said moving information storage member, and said light beam being of sufficient intensity for altering said coating to retain indicia representative of said information signal; optical means for defining an optical path between said light source and said record carrier including said coating, and said optical means being further employed for imaging said light beam to a spot upon said coating;
light intensity modulating means positioned in said optical path between said light source and said record carrier, and said light intensity modulating means operating over a range between a maximum light transmitting state and a minimum light transmitting state for intensity modulating said light beam with said information to be stored; said light intensity modulating means being responsive to said information signal and changing between its maximum light transmitting state and its minimum light transmitting state during each cycle of said information signal for modulating said light beam with the information signal to be stored; said maximum light transmitting state corresponding to said first voltage level of said information signal, and said minimum light transmitting state corresponding with said second voltage level of said information signal;
said light beam passing through said light intensity modulating means and imaged upon said light responsive coating by said optical means having sufficient intensity to form a first permanent and substantially uniform physical change in said surface for a first portion of each cycle of said information signal during the time that said information signal is at said first voltage level; and stabilizing means responsive to the average intensity of said modulated light transmitted through said light intensity modulating means for generating a bias control signal for application to said light intensity modulating means, and said bias control signal being employed for maintaining said average intensity of said modulated light beam at a fixed first level intermediate said maximum and said minimum; and said light beam passing through said light intensity modulating means and imaged upon said light responsive coating by said optical means falling below said intensity required to form a physical change in said surface for the remaining portion of each cycle of said information signal during the time that said information signal is at said second voltage level.

10. Apparatus for recording an electrical signal upon an information storage member, comprising: first means for providing an information signal to be recorded and said signal having its information content in the form of a variable amplitude, cyclical signal alternating between a first higher amplitude and a second lower amplitude; 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 laser light source for providing a collinated writing laser beam of polarized monochromatic light, and said light beam being of sufficient intensity for interacting with said coating while said coating is in motion and said coating is positioned upon said moving information storage member, and said light beam being of sufficient intensity for altering said coating to retain indicia representative of said information signal; optical means for defining an optical path between said light source and said record carrier including said coating, and said optical means being further employed for imaging said light beam to a spot upon said coating; said optical means includes an optical lens and an objective lens; said objective lens having an entrance aperture; said optical lens being positioned a fixed distance from said objective lens to diverge the substantially parallel beam generated by said light source for filling the entrance aperture of said objective lens; carriage means for mounting said lenses; means for translating said carriage parallel to said optical path for moving said light beam in a radial direction with respect to said moving storage member while keeping said light beam imaged upon said coating; light intensity modulating means positioned in said optical path between said light source and said record carrier, and said light intensity modulating means operating over a range between a maximum light transmitting state and a minimum light transmitting state for intensity modulating said light beam with said information to be stored; said light intensity modulating means being responsive to said information signal and changing between its maximum light transmitting state and its minimum light transmitting state during each cycle of said information signal for modulating said light beam with the information signal to be stored; said maximum light transmitting state corresponding to said first amplitude level of said information signal, and said minimum light transmitting state corresponding with said second amplitude level of said information signal; said light beam passing through said light intensity modulating means and imaged upon said light responsive coating by said optical means having sufficient intensity to form a first permanent and substantially uniform physical change in said surface for a first portion of each cycle of said information signal during the time that said information signal is at said first voltage level; and stabilizing means responsive to the average intensity of said modulated light transmitted through said light intensity modulating means for generating a bias control signal for application to said light intensity modulating means, and said bias control signal being employed for maintaining said average intensity of said modulated light beam at a fixed first level intermediate said maximum and said minimum; and said light beam passing through said light intensity modulating means and imaged upon said light responsive coating by said optical means falling below said intensity required to form a physical change in said surface for the remaining portion of each cycle of said information signal during the time that said information signal is at said second voltage level.

11. Apparatus for storing an electrical signal upon an information storage member, comprising: first means for providing an information signal to be recorded; said signal having its informational content in the form of a variable amplitude, cyclical signal alternating between a first higher amplitude and a second lower amplitude; 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;
said coating having a threshold power level above which said indicia are formed; means for imparting uniform motion to said storage member; a laser light source for providing a write light beam, and said write beam being of sufficient intensity for interacting with said coating while said coating is in motion and said coating is positioned upon said moving information storage member, and said light beam being of sufficient intensity for altering said coating to retain indicia representative of said information signal; said intensity of said light beam having a predetermined value relative to said threshold power level of said coating; optical means for defining an optical path between said light source and said storage member including said coating, and said optical means being further employed for imaging said light beam to a spot upon said coating;
light intensity modulating means positioned in said optical path between said light source and said coating, and said light intensity modulating means operating over a range between a maximum light transmitting state and a minimum light transmitting state for intensity modulating said light beam with said information to be stored; said light intensity modulating means being responsive to said information signal and changing between its maximum light transmitting state and its minimum light transmitting state during each cycle of said information signal for modulating said light beam with the information signal to be stored, said light intensity modulating means has an intermediate light transmitting state at which the instantaneous power in said modulated light beam equals one half that of said modulated light beam transmitted at said maximum state and said intermediate light transmitting state also equals the threshold power level of said coating; and said light passing through said light intensity modulating means and imaged upon said coating by said optical means begins to form indicia in said coating representative of said information signal to be stored when said intermediate light transmitting state is exceeded.

12. The apparatus as claimed in Claim 11, and further including: stabilizing means responsive to the average intensity of said modulated light transmitted through said light intensity modulating means for generating a bias control signal for application to said light intensity modulating means, said bias control signal being employed for maintaining said average intensity of said modulated light beam at a prescribed level.

13. The apparatus as claimed in Claim 12, wherein said stabilizing means further includes: first bias means responsive to the average intensity level of said intensity of said modulated light transmitted through said light intensity modulating means for adjusting said average intensity of said modulated light beam at a fixed first level; said average intensity being set equal to said intensity present at said intermediate light transmitting state; and second bias means responsive to a change in average intensity level established in said modulated light transmitted through said light intensity modulating means for restoring said changed intensity level to said fixed first level.

14. The apparatus as claimed in Claim 11 and further comprising: said light beam issuing from said light intensity modulating means during the period corresponding to the period said light beam exceeds said half power level being of sufficient intensity for interacting with said light responsive coating while said surface is in motion to impart a permanent and substantially uniform physical change during said period, representative of said information signal; said light beam issuing from said light intensity modulating means during the period corresponding to the time said beam is at a power level less than said half power level having an intensity less than the intensity required for altering said light responsive coating; said maximum light transmitting state corresponding to said first voltage level of said information signal, and said minimum light transmitting state corresponding with said second voltage level of said information signal; said light beam passing through said light intensity modulating means and focused upon said light responsive coating by said optical means having sufficient intensity to form a first permanent and substantially uniform physical change in said surface for a first portion of each cycle of said information signal during the time that said information signal is at said first voltage level;
and said light beam passing through said light intensity modulating means and imaged upon said light responsive coating by said optical means falling below said intensity required to form a physical change in said surface for the remaining portion of each cycle of said information signal during the time that said information signal is at said second voltage level.

15. The apparatus as claimed in Claim 11, and further comprising: said intensity of said light beam being adjustable relative to said threshold power level of said coating.

16. The apparatus as claimed in Claim 11, and further comprising: said indicia having the form of a linear series of alternately positioned first and second indicia positioned in a track-like fashion upon said coating; each first indicia having a variable length representing the instantaneous frequency of said frequency modulated signal, and each second indicia being substantially equal in length to its associated first indicia for representing a 50-50 duty cycle in the formation of the pair of indicia.

17. A method for recording an information signal on a rotatable recording disc, comprising the steps of:
providing an electrical signal to be recorded, and said signal having its informational content in the form of a variable amplitude, cyclical signal alternating between a first higher amplitude and a second lower amplitude; providing a rotatable recording disc having a substrate and a thin, light-sensitive coating overlying the substrate; rotating the recording disc in a prescribed fashion; providing a beam of light having sufficient intensity to interact with said light-sensitive coating and produce indicia representative of the information signal; directing the beam of light along an optical path to impinge on said light-sensitive coating;
controllably moving the point of impingement of the beam of light on said recording disc radially in a prescribed fashion, such that the beam of light impinges on the rotating disc in a succession of substantially circular and concentric recording tracks; modulating the intensity of the beam of light in accordance with the information signal to be recorded, the intensity varying between a prescribed maximum intensity and a prescribed minimum intensity during each cycle of the signal, the prescribed maximum intensity corresponds to the first higher amplitude portion of the information signal and is greater than a predetermined recording threshold of said light-sensitive coating, and the prescribed minimum intensity corresponds to the second lower amplitide portion of the informative signal and is less than the predetermined recording threshold; and stabilizing the average intensity of the intensity-modulated beam of light at a prescribed level corresponding both to one-half the prescribed maximum intensity and to the predetermined recording threshold of said light-sensitive coating, the indicia being arranged in a succession of substantially circular and concentric recording tracks.

18. A method as defined in claim 17, wherein said step of stabilizing includes the steps of: monitoring the intensity-modulated beam of light and producing an average intensity signal representative of the average intensity of the beam; comparing the average intensity signal with a prescribed selectable level, to produce a difference signal representative of the difference therebetween; and using the difference signal in said step of modulating, to controllably adjust the average intensity of the intensity-modulated beam of light accordingly.

19. A method as defined in claim 18, wherein: the beam of light provided in said second step of providing is linearly polarized; said step of modulating includes a step of rotating the polarization plane of the beam of light through an angle of 90 degrees during each cycle of the information signal to be recorded; and said step of stabilizing stabilizes said light intensity modulating means such that the midpoint in the 90 degree range of rotation of the polarization plane of the beam of light results in an intensity corresponding to the pre-determined recording threshold of said light-sensitive coating.

20. A method as defined in claim 17, wherein each of the successive indicia formed in said light-sensitive coating has a length substantially equal to that of the adjacent space between successive indicia.

21. A method for recording information on an information storage member using a laser beam, comprising the steps of: providing an electrical signal to be recorded, and said signal having its informational content in the form of a variable amplitude, cyclical signal alternating between a first higher amplitude and a second lower amplitude; using said electrical signal as a control signal for controlling the application of a light beam upon a light sensitive surface of an information storage member; adjusting the average intensity of said light beam to equal the threshold power level of said light sensitive surface; moving the information storage member at a constant rate while focusing said stationary light beam to a spot upon the light sensitive surface of said information storage member; using said focused light spot to irreversibly alter the characteristics of said light sensitive surface of said information storage member as said member moves at a constant rate and under the control of said higher amplitude portion of said information signal; and blocking the transmission of said focused light beam to said light sensitive surface of said information storage member as said member moves at a constant rate and under the control of said lower amplitude portion of said information signal.

22. The method as claimed in Claim 21, including the steps of: generating a fixed intensity beam of light;
using said information signal as a control signal for varying the amount of said fixed intensity light beam impinging upon a light sensitive surface of an information storage member; selecting said fixed intensity of said light beam in said adjusting step to provide an average intensity of the modulated light beam to equal the threshold power level of said light sensitive surface;

and stabilizing the average intensity of said beam of light to equal the threshold power level of said light sensitive surface.