CA1147057A - 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 video disc member and for recover-ing the video information from the video record is de-scribed. The video disc member formed by the writing apparatus is also described. The writing apparatus in-cludes 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 video disc member. The video disc member comprises a substrate member carrying a light responsive coating on at least one surface. The write optical system focuses the beam to a small spot of light approximately one micron in diameter upon the light re-sponsive layer. The intensity of the focused spot is changeable under the control of a light intensity modu-lating 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 system is employed to retrieve the video signal stored on the video disc member. This read system includes a read laser for generating a read beam, and a motion control assembly for rotating and translating the video disc member relative to the read light beam. A read optical system directs the read beam to impinge upon the successively positioned specular light reflective and non-specular light reflec-tive regions carried by the video disc member. The read optical system also collects the light reflected from these regions. 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 video disc member.

Description

~ L'7~5~

MASTERING MACHINE

TECHNICAL FIELD
The present invention relates to the writlng o~ a ~requency modulated electrical signal upon an information bearing surface of a vldeo disc member in the form o~ a lineal series of ~lrst and second indlcia positioned ln track-like fashion upon such surface.
~ACKGROUND OF THE PRIOR ART
The apparatus ~or writing a frequency modulated si~nal upon a. video disc member includes a movable writing beam and a vldeo disc member mou~t;ed on a kurntable. Ihe turntable is drlven by a motion control assembly which rotates the disc precisely in a clrcle at a constant rate 15 o~ rotation and a ~ranslational drive assembly for trans-latlng the writing beam at a very constant, and very low veloclty along a radius of the rotating disc. The rota-tional drive of the disc ls synchronized with the trans-lætional drlve of the writing beam to create a spiral track o~ predetermined pitch. In a pre~erred embodiment, the spacing between ad~acent tracks of the splral is two microns, center to center. The indicia i formed having a width of one micron. This leaves an lntertrack or guard area o~ one micron between indicia in ad~acent tracks.
I~ desired, the indicla can be formed as concentric circles by translating ~n incremental steps rather than by trans-lating a.t a constant velocity 8S just described.
In the preferred embodiment, a microscope objec-tive lens is posi~loned at a constant height above the .~ ``, ~
. . ~ ;, .

video disc member on an air bearing. This ob~ective lens ls employed for focusing the write beam upon the light sensitive surface of the video dlsc member. The constant height is necessary because of the shallow focal depth of the objective lens. A 0.65 NA dry microscope ob~ective lens is employed to focus the write laser beam to a spot one micron in diameter upon the light sensitive coating.
Because the coating is rotating at a relatively high rate, the length of the indicia formed in the light sensitive coatlng depends upon the length of time the spot intensity exceeds that needed to form such an indicia.
A linearly polarized ion laser ls used as the source o~ the writing beam. A Pockels cell is used to rotate the plane of polarlzation of the wrlting beam with respect to its fixed plane of linear polarization. A
llnear polarizer attenuates the roiated writing beam in an amount p~oportional to the difference in polarization between the light ln the writing beam and the axis Or the linear polariz~r. The combination of a Pockels cell and linear polarizer modulates the writin~ beam with the video ~ ormation to be stored. This modulation follows the pattern pr~vided by control slgnals furnlshed by a Pockels cell driver.
The video signal to be recorded is applied to a ~requency mDdulator clrcult. The output from the modu-lator clrcuit ls a rectangular wave whose frequency is proportional to the video signal. The duration of each cycle of the rectangular wave~orm is variable as is characteristlc of a frequency modulated signal. As is characterlstic of a rectangular wave~ it has an upper voltage level and a lower voltage level. The upper and lower voltage levels of the rectangular wave are ampli-fied by a Pockels cell driver and used to control tha Pockels cell. The Pockels cell changes the angle of pola~
lzatlon of the light passing therethrough in response to the instantaneous voltage level Or 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 ~7~5~7 applied to a Pockels cell driver, the light beam passes unhlndered through the Pockels cell linear polarizer comblnation at a first intensity sufficient to form a first indicla in a light responsive coating. When the control signal changes to represent its second voltage level, the Pockels cell rotates the polarization of the light which forms the writing beam to a new angle 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 lssuing from the Pockels cell and the preferred angle of polarization of the linear polarizer. In this situation, the linear polarizer acts as an attenuator and less light passes through the linear polarizer. This reduces the light intensity of the wrlting beam below the intensity required to form such first indicla in the light responsive coating.
A portion of the writing beam is sensed by a Pockels cell stabilizing clrcuit for maintaining the average power of the modulated wr:lting beam at a pre-determined level in spite of changes in the Pockels celltransfer characteristic produced by small temperature variations. The stabilizing circu~t includes a level adJusting circuit ~or selectively ad~usting the power level to form indicia in dlfferent light sensitive coat-ings as identified hereina~ter.
Circuitry ls described for achieving a predeter-mined duty cycle m~dulation ~ the indlc~a formed during the writ~ng process. The preferred duty cycle lies wlthin the range o~ 60/40 to 40/60, with a preferred value of 50/50. The output o~ the linear polarizer is adjusted such that the halr power point from the polarizer equals the threshold power level of the material forming the informatlon storage Iayer. This is achieved in part by matching the 45 rotatlon of the Pockels Cell with the 3~ half power output level of the linear polarlzer.
Different types of video disc members can be used with this writing proce~s and apparatus. Each such member has a different configuration. In a flrst configuration, the video disc member includes a glass substrate havlng ~7-~5 an ~pper sur~ace carrying a thin metal coating as a light responsive coatlng. In this configuration, the write beam forms variable length apertures in a krack-like fashion in the metal coating.
The intenslty o~ the write beam is ad~usted such that an aperture is formed, ~or example, during each positive hal~ cycle of the ~requency modulated slgnal to be stored, and no aperture is formed during the negative half cycle. Accordingly, the first and second indicia representative of the stored information is a lineal series o~ apertures separated by an intervening portion of the surface coating.
In this first con~iguration, a portion of the glass substrate is exposed in each aperture. The exposed portion o~ the glass substrate appears as a region of non-specular light re~lectivity to an impinging beam. The intervening portion of the metal coating remaining between specular reflectivity means a significant portion of the reflected light returns along the path of the light beam, le., a 180 reversal in paths between the incident and reflected beam paths. Non-specular re~lectivity means that no significant portion o~ the incident beam is re-~lected along the path of the incident beam.
In a second configuration, the video disc member includes a glass substrate having an upper surface carry-ing a thin layer o~ photoresist as the light responsive coating. In this con~lguration, the write beam forms variable length regions o~ exposed and unexposed photo-resist material in a track-l~ke fashion in the photo-resist coating. The lntensity o~ the write beam isad~usted such that a region of exposed photoresist ma-terial is ~ormed, ~or example, during positive half cycles of the frequency modulated signal to be stored and a region o~
unexposed photoresist material is left during the nega-3~ tive half cycles. Accordingly, the first and secondindicia representative-of the stored information is a lineal series o~ exposed and unexposed portions of the surface coating, respectively.
A preferred embodiment of a reading apparatus is s~

described employing a read laser for producing a polar-ized collimated beam of light having a preferred angle of polarizat~on. A read optical system directs and images the laser beam to implnge upon the lndicia carried upon the surface o~ the video disc member. The video disc me~mber 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 reflectlve 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 serles of regions.
The intensit~ of the read beam is ad~usted such that a sufficiently strong re~lected read beam signal ls gathered 15 by the read optical system.
A motion control assembly rotates the video disc member at a unlform rate of speed sufficient to reconstruct the frequency of the originally stored fre-quency modulated signal. A typical frequency modulated signal stored ln this matter varies in frequency between two megacycles and ten megacycles. The rotational rate of 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 translatlng the reading beam at a very con-stantJ and very low velocity along the radius of the rotating disc so as to impinge upon the lineal series of light reflective and light scatterlng regions contained thereon, The reflected read beam gathered by the read optical system is directed to a light sensing circuit for changlng the intensity modulated reflected light beam to a frequency modulated electrical signal corresponding to the lntensity 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 7 ~ 5'~

splitting element the real light beam is linearly polar-ized in the preferred plane. A quarterwave plate is posit~oned il~termediate vhe output of the polarlzation selectlve beam splitting element and the video disc - 5 member. The quarterwave plate changes the light in the read beam from linear po~arization to circular polariza-tlon. The reflected light retains its circular polariza-tion until it passes through the quarterw~ve plate a second tlme. Durlng this second pass through the quarterwave plate the reflected light is changed by circular polariza-tion back into linear polarized light rotated ninety degrees from the preferred plane established by the polar-ization selective beam split~ing element as described hereinabove.
The polarization selective beam splitting element is responsive to this ninety degrèe shift in the reflected light beam for divertlng 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 o~ the ob~ective lens.
In a second embodiment of the read optical

2~ system, an opticai filter is placed in the reflected read beam path for ~iltering out all wavelen~ths of light other than the wavelength o~ light generated by the read laser source.
In a recording apparatus, tl~e write function 30 alone is employed for writing the frequency modulated informatlon onto a video disc member~ In a video disc player~ the read function alone is employed for recover-ing the frequency modulated information stored on the surface of the video disc member. In a thlrd mode of

3~ operation, the read and write functions are combined in a single machine. In thls comblned apparatus, the read apparatus is employed for chec~ing the accuracy of the information being written by the write apparatus.
To implement ths monitoring function~ the read beam from the Hellum-Neon (He-Ne) read laser is added into the writing beam path. The read optics are ad~usted to direct the read beam through the microscope objective lens at a light angle with respect to the writing beam.
The angle is chosen so that the reading beam illuminates an area on the same track being written by the write beamJ
but at a point that is approximately four to six microns downstream from the writing spot. More specifically, the read beam is imaged upon the information track that was ~ust formed by the write beam. Sufficient time has been - allowed for the informatlon indicia to be formed on the vldeo disc member. In this manner, the read beam is im-pipged upon alternate regions of different reflectivity.
In one form o~ the read apparatus, the read beam impinges 1~ upon the portions of the metal not heated by the write beam and also impinges upon the glass substrate exposed in the apertures just formed by the writing spot. The regions of different reflectivity function to change an impinging read beam of constant intensity into an intensity modu-lated reflected read beam.
In this monitoring mode of operation, the readlaser beam is selected to operate at a wavelength di~fer-ent from that of the write laser beam. A wavelength selective optical filter is placed in the reflected llght beam path having a band pass which includes the reading laser beam. Any write laser beam er.ergy which follows the read reflected path is excluded by the filter and therefore cannot lnterfere with the reading process. The monitoring mode of operation is employed at the time of writing the video information onto the video disc member as an aid ~n 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 haIf cycle of a frequency modulated signal, is the same as the next succeeding region of ~7057 non-specular refiectlvltyJ which represents the next consec~tive half cycle of a frequency modulated signal.
The read after write or monitoring mode of oper-ation ls also utilized in an error checking mode, especl-ally i~ digital type in~ormation is being written. Theinput video ~nformation is delayed ~or an interval equal to the accumulative val~ s of the time delay beginning with ~he ~requency modulation of the lnput video informa-tion signal during the write process and contlnuing through 10 the rrequency demDdulation of the recovered reflected signal from the senslng circuit, and including the delay o~ travel time of the point on the storage member movlng ~rom the point of storing the input video information signal to the point of impingement of the read light 15 beam. The recovered information ls then compared with the delayed inp~t ln~ormatlon for accuracy. The existence of too many dlssimllarities would be a basls for either rechecking and realigning the apparatus or re~ecting the ~sc .
The read apparatus is suitable ~or use with a standard home television recelver by addlng an RF modu-lator ~or adding the video signal t~ a suitable carrier frequency matched to one o~ the channels of a standard home televislon receiver. me standard television re-celver then handles this signal in the sa~e manner as are recelved ~rom a standard transmittlng statlon.
In accordance with one aspect of the invention there isprovided an in~ormation carrying record for ~toring a rrequency modulated ~ignal having sn lnformational content ln the ~orm o~ a carrler ~requency having ~requency changes wlth tlme varying from sald carrier frequency, said record comprising: an information st,orage member ha~ing an upper surrace, and said surface carrylng a llneal serles of alter-nately posltloned flrst and second lndlcia in ~rack-llke rashlon upon sald sur~ace; sald indicia representing a ~requency modulated slgnal ha~ing lts ln~ormational content 7~
-8a-ln the rorm Or a carrier frequency havin~ ~requency changes wlth time ~arylng from said carrier ~requency; each rlrst indlcla having a variable length representlng the in~tantan-eous ~requency of sald frequency modulated slgnal; and each second lndlcla belng substantlally equal ln length to its associated ~irst lndlcla ror representlng a 50 - 50 duty cycle ln the Yormatlon o~ the palr of indlcla.
In accordance wit~ another aspect of the invention there is provided an information carrying record for storing a 10 rrequency modulated ~ignal havlng an lnrormatlonal content ln the rorm of a carrier rrequency havlng ~requency changes wlth tlme varying rrom 8 ld carrler Prequency, and the ~requency modulat~d 8~ gnal 1~ ~tored in the ro~n of a non-image, spatially varlable linear ~eries Or dlscrete regions forming a physlcal pattern~ said inrormation carrylng record comprising: sn ln~ormation storage member in the rorm of a rotary disc havlng an upper surface upon whlch in~ormation 13 recorded, sald storage member havlng a non-lmage, ~patlal-ly variable physlcal pattern derlned by a llneal series of discrete regions, posltloned in track-llke rashion upon sald sur~ace, the width o~ the tracX being substantially 1 micron or less; said reglons being alternately specular light re.lectl~e and non-specular llght reflectlve, the lengths : and spacing between such alternate dlscrete regions being contlnuously varlable and representing a ~requency modulated signal havlng its lnforma~ional conten~ in the rorm of a carrler frequency ha~lng ~requency changes wlth tlme varylng rrom sald carrier ~requency; and the lengths and spacing between said reglons having been determined by a writlng laser beam lssutng ~rom laser lntenslty modulating means havlng a hlgh laser transmlttlng state o~ ~u~riclent in-tensity to produce sald non~specular llght re~lectlve regions and a lower laser transmltting state o~ lnsurriclent ln-tenslty to produce sald non-speculaT- regl~ns, the varying intenslty o~ said wrl~lng laser beam issuing from ~ald laser lntensity modulatlng means during the recording process ~7~
-8b-being malntained at an average value lntermedlate the laser intensltles assoclated with said higher laser transmittlng state and said lower laser transmittlng state.
r BRIEF DESCRIPTIOI~ OF T~IE DRAWINGS
~ . . . .
FIGURE 1 is a block dlagram of the wrlte appara-tus;
FIGURE 2 is a cross-sectional vlew o~ a video dlsc member prlor to writing thereon using the write apparatus shown in Fi~ure l;
FIGURE 3 is a parttal top view of a video disc member a~ter writing has taken place using the write apparatus shown in Figure l;

~7~35~
g FI W RE 4 is a wave~orr.. of a video slgnature e~ployed in the write apparatus shown in Figure 1, FIGURE 5 is a wave~or.m o~ a frequency dulated signal used in the write apparatus shown in Figure l;
FIGURE 6 ls a graph showlng the intensity of the wr~te laser used in the write apparatus shcwn ln Figure l;
FIG~RE 7 is a graph shcwlng the modulatèd write beam as changed by the write apparatus shown in Figure l;
FIGURE 8 is a radial cross-sectlonal view t~cen lGng the line ~-8 of` the disc shcwn ln F~gure 3~
FIGURE 9 is a detailed block diagram of a suit-able motion control assemblyj ` FIGURE 10 is a block diagram shcwing a read ap-paratusj FIGU~E 11 is a block diagram shcwing the ccmbl-natlon of a rea~ and write apparatus;
FIGUFE 12 ls a schematlc representatlon showing the read and wrlte beams passln~ through a single ob~ec-tive lens as u~ed in the block diagram of` Figure l;
FIGUFE 13 is a schematic diagram of` a suitable stabllizing circuit f`or use in the write apparatus sha~n in Figure 1.
FIGUFE 14 shows various wave~orms used in lllus-tr~ting the operatlon of a mastering machlnej FIGU~E 15 shows a cross-sectio~al schematic view of one ~orm Or a video disc~
FIUU~ 16 shcws a photoresis~ coded storage mem~
ber;
FIGURE 17 shows certain portions removed ~ro~
the photoresist coded storage me~ber o~ Figure 16g FIGURE 18 shows the transfer characterlstlc o~ a Pockels cell used herein;
FIGURE 19 shcws the transfer characteristlc of a Glan prism used herein;
FIGURE 20 shcws a light intenslty wave~orm~
-.

S7' FIGURE 21 shows ln cor~unction wlth Figure 20 a series of wavefo~.ms useful in explainin~ the du~y cycle o~
recording~
FIGURE 22 shows an additional waveforn used in illustrating the operatlon o~ a mastering machine FIGUR~ 23 is a block diagram o~ a Pockels cell bias servo system3 PIGURE 24 is a dlagram of a second h~monlc detec-tor used ir. Figure 23; and 10FIGUR~ 25 is a diagram of a hlgh voltage ampllfier used in Flgure 23.

The same r~D~eral is used to identify the same ele-nent in the several views. The terms recor~n~ and stor-lng are used interchangeably for the te:nm writlng. The term retrievlng ls used lnterchangeably for the term read ~ng.
The apparatus ror storing video lnformation in the form of a frequency modulated signal upon an in~ormation storage nenber 10 is shown ~lith reference to Figure 1. An ln~ormation signal source circuit 12 ls employed for provid-ing an information signal to be recorded. Ihis informa-tlon signal p~3sent on a line 14 is a frequency mDdulated signal having its lnformatlon 1 content in the ~orm of a carrier frequency having frequency changes ln time repre-senting said inrormation to be recorded. Figure 5 shows ~o a typical example of a frequency modulated signal. ~he lnformatlon siænal source circuit l? employs a video slg-nal circ1lit 16 for providing ~n informat~on signal on a line 18 havlng its informational content in the rorm of a voltage varying wlth time ~onmat. Figure 4 shows a typical example of a voltage varying with time signal. A

~7~;7 frequency modulator circult 20 s responsive to the video signal circuit 16 for converting the voltage varying with time signal to the frequency modulated signal on the line 14 as shown in Figure 5.
The information storage member 10 is mounted upon a turntable 21. The member 10 is shown in Figure 2 with no indicia formed thereon and includes a substrate 22 having a first surface 24 and a light responsive coating 25 covering the first surface 24. A motion control 10 assembly 28 imparts uniform motion to the storage member 10 relative to a write beam 29 ' generated by a light source 30.
The motio~ 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 00tion to thc informa-tion storage member 10 and translational drive circuit 34 synchronized with the rotational drive circuit 32 for moving the ~ocused light be2m 29' radially across the coating 26. The motion col:itrol assembly 28 further in-cludes an electrical synchronizing assembly 36 for main-taining a constant re~lationship between the rDtational motion imparted to the member 10- by the rotational drive circuit 32 and the translational motion imparted to the light beam 29 b~T the translational drive circuit 34.
The light source 30 provides a beam of light 29 which is of sufficient intensity for interacting wlth or altering the coating 26 while the coating 26 is in motion and positioned upon the moving information storage member 10~ Additionally,the intensit~ of the light beam 29 ' is sufficient for pr~ducing permanent indlcia in the coating 26 representatlve of the lnformation to be recorded. A
suitable light source 30 comprises a writing laser for pro~ucing a collimated writing beam of polarized mono-chromat~c light.
3~ Referring again to Figure 2, there is shown a cross-sectional view of a Eirst conf'iguration 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 light responsive coati!lg 26 is formed upon the ~7~S'7 ~l2-sur~ace 24.
In one ol the disclosed embodiments3 the coating 26 ls a thin, opaque metallized layer having sultable physical properties to permit localized heating responslve to the lmpingement o~ the write light beam 29 ~rom the writlng laser 30. In operation, the heating causes local-ized melting of the coating 26 accompanied by withdrawal of the molten material towards the perimeter ~ the melted area. Upon freezing, this leaves a permanent aperture such as at 37, shown in Figures 3 and 8, in the thin metal coa~
lng 26. The aperture 37 is one type of indicia employed for representing information. In this embodiment, succes-sively positioned apertures 37 are separated by a portion 38 of the undisturbed coating 25. 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 optlcal assembly 40 and a beam steering optical assembly 41 collectively de~ine an optical path ~or the light beam 29 issuing from the light source 30.
The optical assemblies image the read beam 29 into a spot 42 upon the coating 26 carried by the storage member 10.
The optical path is also represented by the line identifiedby the numerals 29 and 29'.
A light intensity modulating assembly 44 is posi-tioned in the opt~cal path 29 between the light source 30 and the coating 26. In its broadest mode ~ operation, the light intensity modulating assembly intensity modu-lates the light beam 29 with the information to be stored.
~he light intensity modulating assembly 44 operates under the control of an amplified form of the frequency modu-lated signal shown in Figure 5. This ~requency modulated signal causes the assembly 44 to change between its higher light transmitting state and its lower light transmitting state during each cycle of the frequency modulated signal.
This rapid change between transmitting states modulates the light beam 29 with the ~requency modulated signal to ~7~57 be stored.
The light beam 29 is modulated as it passes through the light lntensity modulating assembly 44. Thereafter, the modulated light beam, now represented by the numeral 29', is imaged upon the coating 26 by the optical assem-blles 40 and 41. As the modulated llght beam 29' impinges upon the coating 26, indicia is formed in said coating 26 representative of the frequency 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 25 carried by the lnformation sotrage member 10. Addi-tionally~ the electrically controllable subassembly 46 is responsive to the frequency modulator 20 for varying the intensity o~ the light beam below a predetermined intensity at which the focused beam 29' fails to alter the coating 26. The alterations formed in the coating 26 are repre-sentative of the frequency modulated signal to be stored.
When a photoresist layer forms the coating 26 carried by the information storage member 1OJ the alterations are in the form of exposed and unexposed photoresist members analogous to the size as previously described 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 of the writing beam 29' above a first predetermlned intensity at which the focused beam 29' melts the metal coating without vaporizing it and ~urther varies the intensity of the writing beam below the predetermined intensity at which the focused beam 29' fails to melt the metal surface.
The light intensity modulating assembly 44 in-cludes a stabili~ing circuit 48 for providing a feedback signal employed for temperature stabilizing the operating level of the electrical controllable subassembly 46 to operate between a predetermined higher light intensity and ~47~35~

predetermined lower light lntensity level. The llght intensity modulating assembly 44 includes a light sensing circuit for sensing at least a portion o~ the light beam, indicated at 2~ !~ issuing ~rom the electrically controllable subassembly 46 to produce an electrical feedback signal representative of the average intensity of the beam 29l.
The feedback signal is connected to the electrically con-trollable subassembly 46 over the llnes 50a and 50b to stabillze its operating level.
The light sensing means produces an electrical feedback signal whlch is representative of the average intensity of the modulated light beam 29'. In this manner3 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 includeslevel adjustment means ~or 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 26 or a photoresist coating 26, or any other materlal used as the coatlng 26.
The movable optlcal assembly 40 includes an ob~ectlve lens 52 and a hydrodynamic alr bearlng 54 for supportlng the lens 52 above the coating 26. The laser beam 29' generated by the laser source 30 ls ~ormed of substantlally parallel light rays. In the absence o~ the lens 66, these substantially parallel light rays have substantially no natural tendency to diverge. Then the ob~ective lens 52 has an entrance aperture 56 larger in dlameter than the dlameter of the light beam 29'. A
planar convex diverging lens 66 posltioned in the light beam 29' is ~mplo~Jed for spreading the substantlally parallel light beam 29l 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 6C is shown as a movable mirror and is emplo~Jed for making strictl~ circular tracks rather than the preferred spiral tracks. Sprial tracks require only a fixed mirror.

-~5-A~ previously descrcribed, the light source 30 produces a polarized laser beam 29. The electrically con-trollable subassembly 46 rotates the plane of polarization of this laser beam 29 under the control of the frequency modulated ~ignal. A sultable electrically controllable subassembly includes a Pockels cell 58, a linear polarizer 70 and a Pockels cell driver 72. The Pockels cell driver 72 is essentiall~J a linear amplifier and is responsive to the frequency modulated signal on the llne 14. The output from the ~ockels cell driver 72 provldes driving signals to the Pockels cell 68 for rotating the plane of polarization of the laser beam 29. The linear polarizer 70 is orlenta-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 the angle of polarization of the light issuing from th.e source 30. ~ecause of this arrangement, minimum light exits the polarizer 70 with zero degree rotation added to the write beam 29 by the Pockels cell 68. Maximum light exits the polarizer 70 with ninety degree rotation added to the write beam 29 by the Pockels cell 68. This positioning of the linear polarizer as described is a matt;er of choice. By aligning the maximu~ light transmitting ~is of the pola~zer 70 wlth the angle of polarlzation 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 appaxatus would essentially operate the same. The linear polarizer 70 functions to attenuate the intensity of the beam 29 which is rotated away from its natural polarization angle. It is this attenuating action by the linear polarizer 70 which forms a modulated laser beam 29' cor-responding to the frequency 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 DC coupled to the Pockels cell 68.
Referring ~ollectively to Figures 4 through 7, there are shown selectlve waveforms of electrical and optical signals which are present in the embodiment shown with reference to Figure 1. A video signal generated by the video signal source circuit 16 is shown in Figure 4.
A typical device for generating Ruch a video signal is a television camera or a video tape recorder pla~ing back a previously recorded signal generated by a television camera. A ~lying spot scanner is a still further source of such a video signal. The information signal shown in Figure 4 is typically a one volt peak-to-peak slgnal having its informational content in the ~orm of a voltage varying witn time format is represented by a llne 73. The maximum instantaneous rate of change of a typical video sig~al is limited b~T the 4.5 megacycles banclwidth. This video signal is of the t~Jpe which is directly displayable on a television monitor.
The video signal ~hown in Figure 4 is applied to the frequency modulator 20 as shown in Figure 1. The modulator 20 generates the frequency modulated waveform 74 shown in Figure 5. The informational content of the wave~orm shown in Figure 5 is the same as the lnformation-al content o~ the waveform shown ln Figure 4, but the form is different. The informational signal shown in Figure 5 is a frequency modulated signal having its informational content in the form of a carrier signal having ~requency changes in time about a center frequency.
By inspec~ion, it can be seen that the lower amplitude region, generally indicated b~ a numeral 75, of the video wave~orm 73 shown in Figure 4, corresponds to the lower frequency portion of the frequency modulated signal 74 shown in Figure 5. One such c~cle of the lower frequency portion of the frequency modulated signal 74 is indicated generally by a bracket 7~. A higher amplitude region, indicated generally b~J the numeral 77 of the video waveform 73J corresponds to the higher frequency portions of the frequency modulated signal 74. One complete cycle of the higher frequency portion of the ~requency modulated signal '74 is represented by a bracket 78. An intermediate amplitude region, generally indicated with a numeral 79 of the video waveform 73, corresponds to the intermediate frequency portions of the frequency modulated signal 74.
A single cycle of the higher frequency portion of the frequency modulated slgnal representing the intermediate amplitude region 79 is indicated by a bracket 79a.
By an inspection of Figures 4 and 5, lt can be seen that the frequency modulator 20, shown in Figure 1 converts the voltage varying with time signal shown in Figure 4, to a frequency modulated signal as shown ln Figure 5.
Figure 6 illustrates the intensity of the writing beam 29 generated by the write laser 30. The intensity of the write beam 29 is shown to be at a constant level represented by the line 80. After initial setup procedures, this intensity remains unchanged.
Figure 7 illustrates the intensity of the writing beam 29' after its passage through the light intensity modulating assembly 44. The intensity modulated writing beam i9 shown having a pluralit~ of upper peaks 92 repre-sentlng the higher light transmitting state of the llght intensity~modulating assembly 44, and having a plurality of valleys 94 representing the low light transmltting 3tate o~ the light intensity moclulating assembly 44. The line 80 representing the maximum intensity of the laser 30 is superimposed with the wave~orm 29' to show that some loss in light intensity occurs ~n the assembly 44.
This loss ls indicated by a line 96 showing the difference in the intensity of the light beam 29' generated by the laser 30 and the maximum intensity 92 o~ the light beam 29' modulated by the assembly 44.
This intensity modulation of the writing beam 29 to ~orm an intensity modulated writing beam 29' is best illustrated by an inspection of Figures 5 and 7. Figure 6 shows t'ne unmodulated beam 29 having a constant intensi~
represented by the line 80. Figure 7 shows the modulated beam 29' having maximum levels of intenslty indicated at 92 and minimum levels o~ intensity indicated at 94.

~7/~57 The lntensity modulation of the writing beam 29 ls compared to the rotational effect of the Pockels cell 68 by reference to lines 98, 100 and 102. The intersec-tion of the line 98 with the line 29 ' shows tne intensity 5 of the beam 29 ' :Lssuing 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~
section of ,the line 100 with the line 29l shows the inten-sity of the beam 29 ' issuing from 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 ' lssuing from the linear polarizer 70 when the Pockels cell 68 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 Figures 3 and 8, by the inten~ity modulated beam 29', shown in Figure 7 can best be understood by a comparison 20 between the two Fi~;ures 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 by the assembly 4LI when the Pockels cell 68 rotates the angle of polarization of the write beam 29 passing therethrough through an angle o~ forty-five degrees. Additionally, the line 100 repre-sents the threshold intensity of the modulated beam 29 ~
30 required to form an lndicla in the light responsive coat-ing 26. This threshold is reached upon rotatio n of the angle of polarization of the write b eam 29 through an angle o~ forty-five degrees.
By a comparison between Figures 7 and 8J it can 35 be seen that an aperture 37 is formed while the Pockels cell 68 is rotating the angle sf polarization of the write beam 29 passing therethrough between the angle of` forty-five degrees and niilety degrees and back to forty-five degrees. No aperture is formed while the Pockels cell 68 ~7~57 is rotating the angle of polarization of the write beam 29 passing therethrough between the angle of forty-flve degrees and ninety degrees and back tG forty-five degrees.
No aperture is formed while the Pockels cell 68 is rotating the angle of polarization of the write beam 29 passing therethrough between the angle of forty-five degrees and zero degrees and back to forty-five degrees.
Referring again to Flgure 3, there is shown a top view of the video disc member shown in radial cross-sectional view in Figure 8. An inspection of this Figure3 is helpful in understanding the manner in whlch the lineal serles of light reflecting and light scatter-regions 38 and 37 are ~ormed 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 1, ~orms the apertures 37 in circular track-like fashion. A numeral 104 is employed to identify a sectlon of an inner track, and a numeral 105 is employed to identi~y a section of an outer track. A
dashed line 106 represents the center line of the track 105 and a dashed line 107 represents the center line of the track 104. The length of a line 108 represents the distance between the center lines 106 and 107 of ad~acent tracks 105 and 104. Two microns is a typical dista~ce between center lines o~ ad~acent tracks. The width o~ an aper~ure 37 is lndicated by the len~h of a line 109. A
typical width o~ an aperture ls one micron. The distance between ad~acent apertures is represented by the length of a line 110. This distance between ad~acent tracks is known as the intertrack region and typically is one micron in length. The length o~ an aperture ls represented b~J 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 ~requency range generated by the fre-quency modulator 20, the slze of the spot 42 formed by the write optical systems 41 and 42 and the rotational .~

7C~

speed selected f'or the disc lO.
~ e~erring to Figure 9~ there can be seen a more detailed block diagram of the motion control assembly ~8 shown with reference to Figure l. The rotational drive clrcuit 32 includes a spindle servo circuit 130 and a spi~dle shaft 132. The spindle shaft 132 is integrally ~oined to the turntable 21. The splndle shaft 132 is driven by a printed circuit type motor 134. The rota-tional motion provided by the printed circuit motor 134 ls controlled by the spindle servo circuit 130 which phase locks the rotational speed of the turntable 21 to a signal generated by a color subcarrier crystal oscillator 136 which forms a portion of the synchronization assembly 36. The synchronization assembly 36 further includes a first dlvider circuit 138 and a second divider circuit 140. The flrst divider circuit 138 reduces khe color subcarrier frequency generated ln the oscillator circuit 136 down to a rotational reference frequency. The spindle ~haft 132 contalns a tachometer 143 for generating a ~re~uency lgnal indicatlnæ the exact rotational speed of the shaft 132 and turntable 21 combination. The tach-ometer signal is avallable over a line 142 and the rota-tional reference signal from the first divider circuit 138 is available on a line 144. The tachometer signal on line 142 is applied to the spindle servo circuit 130 and the rotational reference signal on the line 144 is also applied to the spindle servo circuit 130. The spindle servo circult 130 phase compares these two input signals.
~hen the phase of the tachometer signal leads the phase of the rotational reference signal, the rate o~ rotation ls 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 re~erence signal. When the phase of the tachometer signal lags the phase of the rotational reference signal as compared in the spindle servo circuit 130~ the rate of rotation is too slow and a signal is generated in the spindle servo circuit 130 ~or application to the motor 134 over a line ~7~1S~

148 to increase the rotational speed and bring the phase of the tachometer signal into agreement with the phase of the rotational reference signal.
The second divider circuit 140 reduces the color subcarrier frequency generated by the oscillator~136 down to a translational reference frequency for advancing the translational drive circuit 34 a fixed distance for each complete revolution of the member 10. In the pre-ferred 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 d~vider circuits 138 and 140 functions as an electrical synchronizing circuit for maintaining a con-stant relationship between the rotational motion of the disc as provided by the rotational drive assembly 32 and the translational motion between the write beam 29 and the coating 26 is provided by the translation drive assembly 34.
The movable optlcal assemblies illustrated in Figures ls 10 and 11 are mounted on a platform indica~ed at 142. This movable platform is driven radially by the translational drive 34 which advances the platform 142 2.0 microns per revolukion of the spindle shaft 132. This translational movement is radial with respect to therotating disc 10. This radial advancement per revolution of the spindle shaft 132 is identified as the pitch of khe recording. Since the pitch uniformity of the finished recording 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 34J pre-load a translation drlve nut 144 which engages the lead screw lL~3 and make the connection between the nut 144 and the platform 142 as stiff as possible as represented by a bar 146.
Referring to Figure 10~ there is shown a read apparatus which is employed for retrieving the frequency modulated signal stored on the information storage member 10 as a lineal series of indicia 37 amd 38 previously ~7~357 -~2-described. A reading beam 150 ls generated by a read laser 152 wllich produces a polarized, collimated beam 150 of light. A support member~ such as the turntable 21, is employed for holding the information storage member 10 in a substantially predetermined position.
A stationary read optical assembly 154 and a mova-ble optical assembly 156 define a read optical path over which the read light beam 15Q 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 ligh~ scatter-ing regiDns 37 carried in successive positions upon the information storage member 10. The movable optical assem-bly 156 ls employed for collecting the reflections fromthe light reflective regions 38 and the light scattering regions 37. The motion control assembly 28 provides relative motion between the read beam 150 and the 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'. ~his re~lected 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 1~0' are used. A light senslng element 1~8 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 ~requency modulated electrical signal generated by the light sensing element 158 is present on a line 160 and has its in~ormational content in the form of a carrier frequency having frequency changes in time corresponding to the stored lnformation. The output of the light sensing circuit 158 is applied to a discriminator circuit 162 by an amplifier 154. The discriminator circuit 162 is responsive to the output o~ the light sensing circuit ~7~S7 158 and ls employed ~or changing the frequency modulated electrical signal into a time dependent voltage signal representing the stored information. The tlme dependent voltage signal is also identified as a video signal and it is present on a line 165. This time dependent voltage signal has its informational content in the form of a voltage varying with time format and is sultable for display over a standard television monitor 166 and/or an oscilloscope 168.
The read optical assemblles 154 and 156 further include a polarization selective beam splitting member 170 which ~unctions as a beam polarizer to the incident beam 150 and which functions as a selective beam splitter to the reflected beam 150 ~ . The read optical assemblies further include a quarterwave plate 172. The beam polar-lzer 170 filters DUt from the read beam 150 any spurious light waves whlch are not aligned with the axis of polar-ization of the beam polarlzer 170. With the axis of polarizatlon of the read beam 150 fixed in a particular orientation by the member 170, the quarterwave plate 172 changes the plane o~ polarizatloll 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~
bet~een the source 15? of the read beam 150 and the quarter-25 wave plate 172. The quarterwave plate 172 is also locatedin the reflècted read beam path 150 t. Therefore, not only does the quarterwave plate 172 change the read beam polar-ization from linear to circular during lts travel from the read laser 1~2 to the information storage member 10, 30 but the quarterwave plate 172 further changes the cir-cularly polarized re~lected light back lnto linearly polarized light which is rotated ninety degrees with respect to the preferred direction fixed by the source 152 and the member 170. This rotated beam 150 ' is 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 ~7~5~7 -2~
compensated for by setting the initial intensit~ of the read beam 150 to a level sufflcient to of,set this re-duction. The quarterwave plate 172 gives a total rota-tion o~ ninet~ degrees to the reflected beam 15G' with respect to the lncident beam 150 during the change from linear polarization to circular polarlzatlon and back to linear polarization. As previously mentioned, the member 170 is also a beam splitting cube in the reflected read beam path 150'. As the plane of polarization of the re-10 flected read beam 150' is shifted ninety degrees due toits double pass~ge 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 ln the capacity of a llght sensing element 158 is a photodiode. Each such element 158 ls capable of changing the reflected fre-quency modulated light beam 150' into an electrical signal having lts information content ln the form of a carrier frequency having frequency va~iations in time varying from the carrier frequency. The optlcal assemblies 154 and 156 further comprise the ob~ective lens 52 supported by a hydrodynamic air bearing member 54 which supports the lens 52 above the coating 26 carried b~- the lnformation storage member 10.
~ As previously described, the read beam 150 ls formed with substantially parallel light rays. The objec-tive lens 52 has an entrance aperture 55 larger in diameter than the diameter of the read beam 150 as it ls generated by the laser source 152. A planar convex diverging lens 3 174 ls provided intermediate the laser source 152 and the entrance aperture 56 of the ob~ective lens 52 for spreading the substantially parallel light rays forming the reading beam 150 into a light beam 150 having a diameter sufficient to at least fill the entrance aperture 56 of the objec~ive lens 52. The optical assemblies 154 and 156 further include a number of stationary or ad~ust-able mirrors 175 and 178 for folding the read light beam 150 and the reflected light beam 150' along a path cal-culated to impinge upon the previously mentioned elements.

7~57 An optlonal op~ical filter 180 is positioned in the reflected beam path 150' and filters out all wave-lengths other than that of the incident beam. The use of this filter 180 improves picture quality as displayed over the televisisn monltor 166. This ~llter 180 is essential when the read system is used wlth the write s~stem as discussed in greater detail with reference to Figure 11. In this read after write environment, a por-tion o~ the write beam 29 travels al~ng the reflected 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 imaging the reflected beam onto the active area of the light sensing element 158. Thls c~nverging lens 182 reduces the diameter of the reflected beam 150' and concentrates the light lnten-lty of the reflected beam upon the active area of the light sensing element 158.
The amplifler 164 amplifies the output of the light sensing element 158 and raises the amplitude of the frequency modulat.ed electrlcal signal generated by the light sensing element 158 to match an input signal requlrement of the demodulator 162.
Referring again to the electrical and optical waveforms shown in Figures 4 through 7, these wave~orms are also generated by the read apparatus, shown in Figure 10 durlng the ret~ieval o~ the frequency modulated signal stored in the coating 26 carried by the dlsc 10. Figure 6 3 shows a laser source generating a write laser beam having a constant intensity represented by the line 80d. The read laser 152 generates a read beam 150 having a constant in-tensity but at a lower level.
Figure 7 shows an lntensity modulated write laser beam. The reflected read beam 150' is intensity modulated by the act o~ 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 sh~wn in Figure 7. Rather, the ~7 square edges are rounded by the ~inite size of the read spot.
Figure 5 shows a ~requency modulated electrical slgnal havlng its informational content in the ~orm of a carrier signal having ~requency changes in time varying about the center ~requency. The output o~ the llght sensing element 158 is the same type of signal. Figure shows a video signal having its in~ormational content in the ~orm of a voltage varying with time ~ormat. The output of the demodulator 162 is the same type of signal.
The motion control assembl~ 28 shown in Flgure 10 operates in the same manner as the motion control assembly 28 shown in Figure 1. In the read apparatus, the motion control assembly 28 produces a rotational motlon to the disc member under the control of a rotational drlve 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.
The assembly 28 ~urther lncludes a synchronizing circuit for malntaining a constant relationship between the rotational motion and the translational motion so that the read beam 150 impinges upon tl~e in~ormation tracks ~arried by the disc member 10. P~rtions o~ typical in-formation tracks are shown as 104 and 105 in Figure 3.
Re~erring to Figure 11, there i~ shown a bl~ck diagram illustrating the comblnation o~ the write apparatus shown in Figure 1, and the read apparatus shown in Figure 10. The elements shown in Figure 11 operate in an identlcal manner as previously descri~ed and this de-30 tailed operation ls not repeated here. Only a brie~
descr~ption i9 given to avoid repetltion and confusion.
The unmodulated write beam path is shown at 29 and the modulated beam path is shown at 29'. A ~irst optical assembly de~ines the modulated beam path 29' 35 between the output of the linear polarizer 70 and the coating 26. The 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 mirrGr 60 and the objective lens 52.

s~

The modulated write beam 29' is imaged to a wrlt.e spot 42 upon the light responsive coating and interacts with the coating to form indicia as prevlously described.
The read beam path is shown at 150. The read optlcal assemblies de~ine a second optical path for the read beam 150 between the read laser 152 and the informa-tion storage record carrler 10. The fixed, read optical assembly 154 includes the mirror 176. The movable, read optical assembly 156 includes the diverging lens 174~ the polarization shifting means 172, a second fixed mirror 202, the partially transmissive mirror 200, the movable mirror 60 and the lens 52. The read beam 150 is imaged to a read spot 157 at a point spaced downstream from the write spot 42, as is more completely described wlth refer-ence to Flgure 12.
The mirror 200 ls a dlchroic mirror which is transmisslve at the wavelength of the wrlte beam 29' and which ls reflective at the wavelength of the read beam 150`.
The intensity of the write beam 29' is higher than the intensity o~ the read beam 150. Whlle the write : beam 29' must alter the light responsive coating 26 to retain indicia representative of the video slgnal to be stored, the intensity of the read beam 150 should only be su~ficient to illuminate the lndlcia formed in the coatlng26 and provlde a reflected light beam 150' of sufficlent intenslty tD provlde a good signal a~ter collectlon by the read optical assembly and conversion from an intensity modulated reflected beam 150' to a frequency modulated 3o electrical s~gnal by the light sensing circuit 15B.
The fixed mirror 58 in the write optlcal path and the two flxed mirrors 176 and 202 ln the read optical path are employed for directing the write beam 29' toward the ob~ectlve lens 56 at a controlled angle with respect ko 35 the read beam 150. This angle between the two incident beam~ provides a spacing between the write spot 42 and read spot 157 as they are each respectively lmaged upon `the coating 26.
In operation, a sufficlent spacing has been found ~47~
-2&-to be ~our to six mlcrons. This distance corresponds to an angle ~oo small to show clearly in F~gure 12. Accord-ingly, this angle is exaggerated in Figure 12 for purpose of illustratlon only.
The read beam 150' is demodulated ln a discrim-inator circuit 162 and displayed on a standard television monltor 156 and an oscilloscope 168. The televislon monitor 166 shows the pictorial quality of the recordlng &nd the oscilloscope 168 shows the vldeo signal ln more detail. This read after write function allows the quallty of the video si~nal being stored during a write operation to be instantaneously monitored. In the event that the quality of the stored slgnal is poor~ lt is known immedi-ately and the write procedure can be corrected or the information storage member 10 storing the poor quality video information signal can be discarded.
In the read after write mode of operation, the write laser 30 and the read laser 1~2 are operating at the same time. A dichroic mlrror 200 ls employed for combininr, the read beam 150 lnto the write beam 29'. In this read a~ter wrlte mode of operation, the wavel~ngth o~ the write beam 29 is chDsen to be different from the wavelength of the read beam 150. ~n optical filter 180 is employed for blocking any port:Lon of a write beam which 25 has followed the reflected read beam path. Accordingly, the optical fi1ter 180 passes the reflected read beam 150' and filters out any part of the write laser beam 29~ follow-ing the reflected read beam path 150'.
In the comparis~on mode of opera~ion, the read 30 after write operatlon is practiced as described with refe~
ence to Figure 11. ~hen operating in this monitorin~ mode of operation, a comparator circuit 204 compæres the output of the demodulator 162 with the original video information signal provlded by the source 18 More specifically, the video output o~ the dis-criminator 162 is applied to a comparator 204 over a line 206 The other input of the comparator 204 is taken ~rom the video source 16 over the line 18, an additional line 208 and through a del~y line 210. The delay line 210 ~7~S~

imparts a tlme delay to the input video informatlon 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 senslngcircult 158. This delay also includes the delay of travel time from the polnt 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 imp~ngement of the read spot 157.
The correct amount of delay ls best generated by making the delay circult 210 a variable delay clrcuit which is ad~usted for optimum operation.
Ideally, the vldeo output signal of the discrim-inator 162 is identical in all respects to the vldeo inputsignal on the lines 18 and 208. Any differences noted represent errors which might be caused by imperfections in the disc's surface or malfunctlons of the writing clr-cuits. Thls application, while essential ir recordlng digital information, is less critical 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 erroræ present. on any disc. When the errors counted exceed the predetermined selected number, the writing operation is terminated. If necessary, a ~ew disc can be written. Any disc with excessive errors can then ~e reprocessed.
In Figure 11~ the comparator 204 compares the output signals available on the lines 208 and 206. An alternative and more direct connection of~the comparator 204 is to compare the output of the ~requency modulator 20 and the amplifier 164 shown with reference to Figure 10.
Turning next to Figure 12, there is shown in somewhat exaggerated ~orm, the slightly differing ~ptical 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 informatlon storage member 10 is moving in the direction indicated by an arrow 217.

~L713'~7 This shows an unexposed coating 2~ approaching the write beam 29 ~ and a lineal series of apertures 37 leaving the intersection of the write beam 29' and the coatlng 26.
The writing beam 29 ~ coincides with the optical axis of the 5 microscope objective lens 52. The central axis of the reading beam 150 shown as 212 makes an angle with the central axis of the write beam 29' shown a3 214. The angle is represented by a double headed arrow 216. Due to this slight difference in optical paths of the write beam 29' and read beam 150 through the lens 52, the write spot 42 falls a distance ahead of the read spot 157. The write spot 42 lead~ the read spots 157 by a distance equal to the length of a line 218. The length of the line 218 is equal to the angle times the focal length of the objective lens 52. The resulting delay between writing and reading allows the molten metal coating 26 to solidify so that the recording is read in its final solidified state. If it were read too soon while the metal was still molten, the reflection from the edge of the aperture would fail to provide a high quality signal for display on the monitor 166.
Referring to Figure 13, there is shown an ideal-ized diagram of a Pockels cell stabillzlng circuit 48 suitable for use in the apparatus of Figure 1. As is known, a Pockels cell 68 rotates the plane of polarlzation of the applied write light beam 29 as a function of an applied voltage as illustrated with reference to Figure 7.
Depending upon the individual Pockels cell 68, a voltage change of the order of 100 volts causes the cell to rotate the plane of polarization of the light pa~sing therethrough a full ninety degrees. The Pockels cell driver functions to amplify the output from the in-formation signal source 12 to a peak-to-peak output swlng of 100 volts. This provides a proper input driving signal 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-pealc voltage swing of 100 volts.
The Pockels cell should be operated at an average rotation of forty-five degrees in order to make the 7~ 7 modulated light beam intensity most faithfully reproduce the electrical drive signal. A bias voltage must be pro-videq to the Pockels cell for keeping the Pockels cell at thls average operating point. In practice, the electrical bias voltage corresponding to a forty-five degree rotation operating point varies continuously. This continuously changing bias voltage is generated through the use of a servo feedback loop. This feedback loop includes the comparlson of the average value of the transmitted light to an ad~ustable reference value and applying the differ-ence signal to the Pockels cell by means of a DC amplifier, This arrangement stabillzes the operating point. The reference value can be adjusted to correspond to the average transmission corresponding tc the forty-five degree operating point and the servo feedback loop provides 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 senslng means. The diode 225 senses a portion 29"
of the writing beam 29' issuing from the optical modulator 44 and passing throug~ the partia:lly reflective mirror 58 as shown in Figure 1. The sillcon diode 225 .functions in much the same fashion as a solar oell and is a source of electrical energ~ when illuminated bVI incident radiation.
One output lead of the s~licon diode 225 is connected to common,reference potential 226 by a llne 227. The other output lead of the diode 225 is connected to one input of a differentlal amplifier 228 by a line 230. The output leads of the silicon cell 225 are shunted b~ a load resis-tor 232 which enables a linear response mode.
The other input to the differential amplifier 228 is connected to an adjustable arm 234 of a potentiometer 236 by a llne-238. One end of the potentiometer 236 is connected to reference potential 225 by a line 240. A
source of power 242 is cDupled to the other end of the potentiometer 236 which enables the ad~ustment of the differential amplifier 228 to generate a feedback signal on the lines 244 and 246 for adjùstlng the average power s~

level of the modulated laser beam 29' to a predetermined value.
The output terminals o~ the differential ampli~ier 228 are/ respectively, connected through resistive elements 248 and 250 and output lines 244 and 246 to the input terminals of the Pockels cell 58 in Figure 1. The Pockels cell driver 72 ls AC coupled to the Pockels cell 68 by way of capacitive elements 252 and 254, respectively~
while the differential amplifier 228 is DC coupled to the 10 Pockels cell 68.
In operation, the system is energized. The por-tion 29" of the light from the wrltlng beam 29' impinglng on the silicon diode 225 generates a differential voltage at one input to the differential ampllfier 228. Initia 15 the potentiometer 236 is ad~usted so that the average transmission through the Pockels cell corresponds to fort~
five degree of rotation. Thereafter, if the average level of intensity impinging on the silicon cell 22~ either in-creases or decreases, a correcting voltage wlll be gener-20 ated by the differential ampllfier 228. The correcting voltage applled to the Pockels cell 58 ls of a polarity and magnitude adequate to restore the average level of . lntensity to the predetermined level selected by adJustment v of the input voltage to the other input of the differen-25 tial ampli~ier over the line 238, ~y movement of the movable arms 234 along the potentiometer 236.
The adjustable arms 234 o~ potentiometer 236 is the means for selecting the average level of intensity of the light generated by the write laser 30. Optimum re-30 sults are achieved when the length of an aperture 37 ex-actly equals the length of the next succeeding space 38 as previously described. The ad~ustment of potentiometer 236 is the means for achieving this equall~y of length.
When the length of an aperture equals the length of its 35 next adjacent space, a duty cycle of fifty-fifty is achieved. Such duty cycle is detectable by examining the display of the just written information on the TV monitor and/or oscillosoope 165 and 168, respectively, as pre-viously described. Commercially acceptable results occur 7~i7 when the length of an aperture 37 varies between forty and slxty perce~t of the combined length of an aperture and its neY~t successively positioned space. In other words, the length of an aperture and t~ next successively posi-tioned space is measured. The aperture can then be alength falling within the range of forty and sixty percent of the total length.
Re~erring to Figure 8, there is shown a radial cross-section of an information track shown with reference to Figure 3 in which a specular light reflectlve region 38 is positioned intermediate a pair of non-specular light reflective regions 37. In the radial cross-sectional view shown in Figure 8, the impinging read or write beam is moving relative to the member 10 in the direction represented by the arrow 217. This means that a reading beam impinges first upon the specular light reflective region 38a followed by its impingement upon the non-specular light reflective region 37a. In thls conflgur-ation, the positive half cycle of the si~nal to be re-corded is represented by a specular light reflectiveregion 38a and t`ne negative half cycle of the signal to be recorded is represented by the non-specular light reflective region 37a. The duty cycle of the signal shown with ~'è~erence to Figure 8 is a flfty percent duty cycle insofar as the length of the specular light re-~lected region 38a as represented by a bracket 250, is equal in length to the length o~ the non-specular l~ght reflective region 37a ~s represented by the bracket 252 This preferred duty cycle set up by the combination o~
ad~usting the absolute intensity of the write beam 29 by adjusting the power supply of the write laser 30 and by adjusting the potentiometer 236 in the stabilizing circuit 48 to a level wherein an aperture is ~ormed beginning with a ~orty-five degree rotation of the angle of polarlzation in the write beam 29.
Referring again to the aperture forming process illustrated with reference to Figures 7 and 8, melting of a thin metal coating 25 occurs when the power in the light spot exceeds a threshold characteristic of the composition ~4~57 -j4-and thickness of the metal film and the properties of the substrate. The spot power is modulated by the light intensity modulatillg assembly 44. The on-off transitions are kept short to make the location of the hole ends pre-cise in spi~e of variations in the melting threshold.Such variatlons in the melting threshold can occur due to variations in the thickness of the metal coatlng and/or the use of a different materlal as the ~nformatlon storing layer.
The average power in the spot required to form an aperture in a thln metal coating 26 having a thickness between 200 and 300 Angstroms is of the order of 200 milliwatts. Since the FM carrier frequency is about 8 MHz, 8 x 105 holes o~ 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 o~ the glass substrate appears as a region of non-specular li~ht reflectlvlty to an impinging reading beam. The portlon o~ the metal coatlng remaining between successively positioned apertures appears as a region of high light reflectivlty to an im-pinging reading beam.
~en the forming of first and second indicia is being undertaken using a coating of photoresist, the inte~
sity of the write beam 29' is adjusted to a level such that a forty-five degnee rotation of the plane of polari-zation generates a light beam 29' o~ threshold intensity for exposing and/or interacting with the photoresist coating 25 while the photoresist coating is ln motion and positioned upon the moving in~ormation storage member 10.
The Pockels cell 68 and Glan-prism 70 combination com-prises a light intensit~ modulating member which operates from the forty-flve degree setup condition to a lower light transmitting state associated with a near zero degree state of operation, to a higher light transmitting state, associated with a near ninety degree state of operation.
When the intensity of the write light beam 29~ increa~es above the initially adjusted level or predetermine start ~7`~357 intensit~, and increases towards the higher light trans-mitting state the incident write light beam 29' exposes the photoresist illuminated thereby. This exposure con-tinues after the intensity of the write beam reaches the maximum light transmitting state and starts back down towards the initial predetermined intensity associated with a forty-flve degree rotation of the plane of polari-zation of the light issuing from the write laser 30. As the rotation drops below the forty-five degree value, the 10 intensity of the write beam 29' exiting the ~lan-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 rotatlon 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 hig~l voltage amplifier havlng an output signal providin~ an output voltage swing of 100 volts.
This signal is intended to match the driving requirements of the Pockels cell 68. Typical].y, this means that the mid-voltage value of the output of the Pockels cell driver 72 provides a sufficient control voltage for d~lving the Pockels cell 68 throùgh forty-~ive degrees so that about one half of the total available light from the laser 30 issues from the linear polarlzer 70. As the output signal from the driver 72 goes positlve, mid-voltage value, more light from the laser is passed. As the output signal from the drlver 72 goes negative, less light from the laser is passed.
In the first embodiment using a metal coating 26, the output from the laser 30 is adjusted so as to produce an lntensit~ which begins to melt the metal layer coating 26, positioned on the disc 10, when the output from the driver 72 is zero and the operating polnt of the Pockels cell is forty-five degrees. Accordingly, as the output .

, ~7~5~7 from the driver 72 ~oes p~sit~a~ melting contlnues. Also, when the output from the driver 72 goes negative, melting stops.
In a second embodiment using the photoresist coating 26, the output from the laser 30 is adjusted so as to produce an intensity which both -llluminates and exposes the photoresist coating 25 when the output from the driver 72 ls generating its mid-voltage value. Accordingly, as the output from the driver 72 goes positive, the illumina-tion and exposure o~ the photoresist illuminated by thewrite beam continues. Also, when the output from the driver 72 goes negativej the illumination continues but the energy in the write beam is insuf~icient to expose the illuminated region. The term expose is herein bein~ used for its technical meaning which describes that physical phenomenon which accompanies exposed photoresist. Exposed photoresist is capable of being developed and the developed photoresist is removed by standard procedures. Photo-resist which is illumlnated by li~ht, insufficlent in ~ntensity to expose the photoresist, cannot be developed and removed.
In both the first and second embodiments ~ust described, the absolute power level 80 illustrated by the llne 80 ln Figure 6 is ad~usted upward and downward to achieve this effect by ad~usting the power 9upply of the write laser 30. In combination with this ad~ustment of the absolute power level of the write laser 30, the potentiometer 236 is also used to cause indicia to be formed in the coating 26 when the beam 29 is rotated above forty-five degrees as previously described.
In a read only system as shown in Figure 10, the optical ~ilter 180 is optional and usually is not required.
Its use in a read only system introduces a slight attenua-tion in the reflected path thus requi~lng a slight increase in t~e intensity o~ 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 filter 180.
The converging lens 182 is optional. In a properly arranged re~d system the reflected read beam 150' ~ 3~7~5~7 has essentially the same diameter as the working area o~
the photodetector 158. If this is not the case, a con-verging lens 182 is employed for concen~rating the re-flected read beam 150' upon the smaller worklng area of the photodetector 15~ selected.
Prior to giving the detailed mode of 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 hereinafter. The laser intensity generated by the writlng 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 ~requency and the frequency variations in time from the carrier 15 frequencY-The threshold power level required of tne laserbeam at the point of impact with the information bearing layer of the video disc di~fers depending upon the mater-lal from which the information bearing layer is made.
20 In the two examples given hereinabove describing a metal such as bismuth and a photosensitlve material such as photoresist, the threshold power level required to form indicla differs significantly and represents a good ex-ample for illustrating the term threshold power. Obvious-25 1~, the threshold power of other materials would alsodiffer from each of the examples explained.
The lndicia formed in a bismuth coated video disc master are alternate regions o~ light re~lectivity and light non-reflectivity. The areas of light non-reflecti 30 vity are caused by the melting of the bismuth followed by the retractlng o~ the bismuth before cooling to expose an underlying portion of the glass substrate. Light imping-ing upon the metal layer is highly reflective, while light impinging upon the exposed portion of the glass substrate 35 is absorbed and hence light non-reflectivity is achieved.
The threshold power is that power from the laser beam required to achieve melting and retracting of the metal layer in the presence of a laser beam of increasing light intensity. The threshold power level is also ~7~
-3~-represented as that intensity OL a decreasing light inten-sity signal when the metal layer ceases to melt and retract from the region ha~ing incident light impinging thereupon. ~ore specifically, when the power in the impinging light beam exceeds the threshold power require-ments of the recording material, a hole is formed ln the recording material. When the light power intensity in the impinging light beam is belo~ 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 wîth 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 frequency.
A video disc master having a thin layer of 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 llght beam to e~pose a portion of the photoresist. When the positive going modulated light beam contains sufficient photons above this threshold power level, the photoresist ln that area is exposed so that subsequent development removes the exposed photoresist. When the photon level in a decreas-ing light intensity modulated llght beam falls below the normal threshold power level of the photoresist, the phot~
resist caases to be exposed to the extent that subsequent development does not remove the photoresist illuminated by an implnging light beam havlng 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 of the carrier fre-quency having frequency changes in time varying about the ~ ~ ~7~57 -3~-carrier frequency. The indicia stor~ng the carrler rre~
quency and ~requency change in time are more fully appre-ciated after the development step whereb~ those portions of fully exposed photoresist material are ef~ectively removed leaving the underexposed portions on the video disc member.
Re~erring 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 polnt of the Pockels cell-Glan prism combination coincides with the forty-five degree rotation point of the Pockels cell 68. This DC bias point is identified as the flxed bias point. In a system wherein the input video signal to the FM modulator 36 does not contain any second harmonic distortion~ the DC bias position selected in the procedure just identified operates satisfactorily. How-ever, whe~ the video information lnput signal to the FMmodulator contains second harmonic distortion products, these distortion products show up in the modulated light beam at 29' 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 lts zero to ninety degree rotational shift. The unmodulated llght beam from the laser 29 is applied to the Pockels cell 68 as previously described.
The purpose of the Pockels cell b~as servo is to bias the Pockels cell 68 so that the output light signal detected at a photo diode 2~ is as free o~ second harmonic content as possible.
The second hzrmonic distortion is lntroduced into the modulated light beam at 29' from a plurality of sources.
3~ A first of such sources is the non-linear transfer func-tions of both the Pockels cell 68 and the Glan prism 70.
When 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 .

7~S'7 bea~ at 29l The Pockels cell bias servo functions to adjust the DC bias applied to the Pockels cell 68, which DC
bias biases the Pockels cell to -lts half power pointS so as to minimize the second harmonic content of the output light beam.
The change in DC bias level from the half power point is achieved in the following æequence 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 of operation and generates a signal having the form of a carrier frequency with fre-quency variations about the carrier frequency. This fre-quency modulated waveform ls a sufficiently linear repre-sentation of the light lmpinging upon the photo diode 261to accurately re~lect the signal content of the light modulated beam 29 ~ lmpinging upon the disc sur~ace. More speclfically~ the output signal from the photo diode 260 contains the distortion products present in the modulated ~0 light beam 29 ~ . The output from the photo diode 260 is applied to a second harmonic detector 261 over a line 262 whlch rorms a part of the blas control circuit 264. The output ~rom the second harmonlc detector is to a high voltage amplifier 266 which generates the DC bias signal 25 over a line 268. The line 268 is c onnected to a summation circuit 270 which has as its second input signal the output from the Pockels cell driver 72, The DC blas signal on the llne 268 is summed with the output from the Pockels cell driver 72 and is applied to the Pockels cell 68 for chang-ing the DC bias of the Pockels cell 68.
Referring back to the operation of the secondharmonic detector 261, 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 reflects the phase charac-teristics of the second harmonic and if the second harmonic is in phase with the fundamental, the output o~ the second harmonic detector is in a first voltage level, ie., a positive level. I~ the second harmonic is opposite in .

~7~5~

phase with the fundamental, t11en the output of the second harmonic detec~or is at a second voltage level, ie., a negative voltage level. The output from the second harmo~
detector is amplified through a high voltage amplifier 266 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 amplified in the Pockels cell drlver 72 and applied to the Pockels cell 6~.
The second harmonic detector includes a limiter 272 shown with reference to Figure 24 and a differential amplifier 274 shown with reference to Figure 24. The out-put signal from the photo dlode 260 is AC coupled to the limiter 272 over lines 276 and 278. The limiter 272 has a first output signal for application to the differential ampli~ier over a first output leg 280. The second output from the limiter 272 is applied to the second input of the differential amplifier over a second output leg 282.
The output signals from the limi;ter 272 are logical compl~
ments of each other. More speciflcally, when one output is at a relatively high voltage level, the other output ls at a relatively low voltage level. The two output slgnals on the legs 280 and 282 are fed into the differen-tial amplifier 274. The output o~ this difrerential amplifier reflects the content of the second harmonic available on the input signal lines 276 and 278.
In a standard mode of operation, when the input signal ~rom the photo diode 260 is substantially free of second harmonic distortion; then the output slgn~l from the differential amplifier 274 at terminal 284 is a square-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 effective DC level of these two h~lf cycles offset one another. Accordingly, the output of the diffe~
ential amplifier 274 is, on the average, zero.
When 2 degree of second harmonic distortion is present in the output from the photo diode 260, the value ~7~tj7

-4~-o~ harmonlc ~istortion shlfts the mean value crossing from a symmetrical case to a non~symmetrical case. In this situation, the output from the di~rerential ampli~ier is other than a squarewave with a ~i~ty~ ty duty cycle.
The dif~erential amplifier there~ore detects the effec-tive DC level shift of the incoming signal and generates an output which is above or below ~ero, on the average, depending upon the asymmetrical nature of the input signal.
The output of the differential amplifier is there~ore applied to the high voltage amplifier which DC sm~oths the outp`ut ~rom the di~ferential amplifler 274 and amplifies the negative or positive resulting DC level. The resulting product is the required shift in bias signal ~or applica-tion to the Pockels cell to return the operating point of the Pockels cell to the hal~ por~er point at which zero harmonic distortion occurs.
A summary o~ 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 ~or dete~t-ing the value of second harmonic distortion present in this light beam and generating a signal representation o~ this distortion. The signal generated to represent the amount o~ second harmonic distortion also includes whether the second harmonic distortion is in phase with the ~undamen-tal frequency or the second harmonic distortion is out of phase with the ~undamental ~requency. The output signal representing the amount of second harmonic distortion and the phase of the second harmonic distortion with reference to the ~undamental frequency is applied to a means ~or generating a bias signal necessary ~or application to the Pockels cell to bring it to an operating point at which second harmonic dlstortion ceases to exist. A summation circuit is provided for summing the change in bias signal wlth the input ~requency modulated video signal. This ; summed voltage is applied as an input to the Pockels cell 68.
Figure 14 shows a series of waveforms illustraing an improved form of light modulation o~ a writing light ~7~S7 beam 29, Line .~ o~ Figure 14 shows an idealized or simpl~
fied video waveform ~hat is typically supplied as a video signal from a video tape recorder or television camera.
This waveform is essentially the same as that shown in Figure 4 and represents a video signal that is applied to the FM modulator circuit 20. ~!0 output signals are shown on llnes B and C, and each is an FM modulated output signal and each carries the same frequency information.
The waveform on llne ~ is a ~epeat of the waveform in Figure 5 and is repeated here for convenience. This wave-form on line B 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. Both 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 of a con-stant intensity light beam applied through the Pockels cell.
The frequencies contained in each wave~orm B and C are at all times identical and each represents the voltage level of the video waveform shown in line A. By inspection, 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 and higher amplitude regions of the video waveform as gener-ally indicated at 77 corresponds ~o the higher frequency shown in lines R a;ld C. It is the custom and practice o~
the televi~ion industry to utilize a one volt peak to peak voltage signal having voltage variations in time as the vldeo signal generated by a television camera. These signal characteristics are the same requlred 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 C is a linear voltage change with ~7~Cj7 --time. The linear voltage change versus time of the tri-angular driving waveform when multiplied by a sinusoidal voltage change versus light transfer function of the Pockels cell 68 gives a sinusoidally varying light inten-sity output ~r~m the Glan prism.
The waveform shown on line D illustrates thesinusoidal 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 286 of the waveform shown on line D, the point exactly equally distant from each is ident~fied as the half power point. An understandlng o~
the utilization of this half power point feature 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 line 287 of line C and by a second minimum vol-tage level Vl on line 288. The voltage differentlal between points 287 and 288 is the driving voltage for the Pockels cell 68. This voltage differential is ad~usted to e~ual that voltage required by the Pockels cell 68 to give a nlnety degree rotation of the 0l1tpUt polarization of the light passing through the Poc~els cell 68. The bias 25 on the Pockels cell is maintained such that voltage levels Vl and ~2 always correspond tc the zero degree rotation and the ninety degree rotation respectively of the light beam passing through the Pockels cell 58. The forty-flve degree rotation of a light beam is half way between the two extremes of a trlangle 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 drift due to thermal instabilltles causing the half power volt-age point to drift also. The correct biasing of the half-way voltage is completely described hereinafter with refe~
ence ~o Figures 18, 19 and 20.
While the waveform shown on line C of Figure 1~
shows the triangular wave shape generated by the FM modu-lator 20, it also represents the wave shape of the signal 7~S~

generated by the Pockels cell driver 72. The output from the F~ modulator is typically in a smaller voltage range, typically under 13 volts while the output from the Pockels cell driver 72 typically swings 100 volts in order to provide suitable driving voltage to the Pockels cell 58 to drive it from its zero rotational state to its ninety degree rotational state. In discussing the voltage levels Vl and V2 and the lines 288 and 287, respectivelyJ repre- -senting such voltage points, reference is made to line C
of Flgure 145 because the output from the Pockels cell drlver 68 has the identical shape while differing in the amplitude o~ the waveform. This was done for convenience and the ellmination of a substantially identical wave~orm dlfferent only in amplltude.
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 shaped upper surface indicated at 302. An information bearing layer 30~ is ~ormed to top the upper surface 302 of the substrate 300. The information bearing layer 304 is of uniform thickness over the entire sur~ace 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 o~ the light beam passing from the Pockels cell - Glan prism comblnation in the improved embodiment which utili~es a voltage control oscillator ln the FM modulator 20 generating a triangular shaped output waveform as the driving waveform shape to the Pockels cell 6~. As previously described, the thres-hold power level o~ the information bearing layer is defined as that power required to form indicia in the information bearing layer in response to the impingi~g llght beam. For a metal surface, the th~rmal threshold point is that power required to melt the metal layer and have the metal layer retract from the heated region of implngement. For a photoresist layer, the threshold power is that power level required to supply sufficient photons ``.

i5~
-~6-to completely expose the photoreslst information bearlng layer. In the case of the metal layer, the heated metal retracts from the impinging area to exp~se the substrate 300 positlon thereunder. In the case of the photoresist material~ the photon power is suf~lcient to fully expose the total thickness of the photcresist layer 324 completely down to the upper surface 322 o~ 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 halfway 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 prlsm 70. The half power point is the point at which the light passing through the Pockels cell has been rotated forty-~ive degrees from the p~int of zero power transmission.
In operationJ the output power from the laser is ad~usted such that the half power point of the Pockels cell-Glan prism combination provides sufficient ener~y to _ equal the threshold power level o.~ the information bearing member employed, such as the member 304. The matching of the half power point of the Pocke:ls cell-Glan prism com-binatlon ensures highest recordinl3 ~idelity of the videofrequency signal to be recorded and ensures minimum inter-modulation distortion of the signal played back from the video disc recording member.
This matching of the power levels is illustrated 3 with reference to line D o~ Figure 14 and Figure 15 and by the construction lines drawn vertically between the half power point represented by the line 290 shown on line D of ~'igure 14 and the apertures shown generally at 310 in Figure 15. The length of an aperture 310 is coextensive wlth 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 ~7~5~

shape shown on line C of Figure 14. The zero crossing poi~ts are represented by lines 291 and ~92 shown in Figures 1~ and 15C, and the importance of regulating the hali powe- poin~ is e~plained in greater detail wlth refer-ence tc Figures 20 and 21.
Figure 16 shows an information storage memberincluding a substrate 320 having a planar upper surface 322. A thin layer of photoresist 324 of uniform thickness is formed over the planar upper surface 322 of the sub-strate 320. The thin photoresist layer 324 is also formedwith a planar upper surface 326. The photoresist layer 324 is a light responsive layer ~ust as the metal bismuth layer 304 is a light responsive layer. ~oth the thin opaque metallized coating 304 and the photoresist layer 15 324 function to retain indicia representative of the vldeo lnput 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-reflective regions in the information storage member.
Re~erring to Figure 17 showing the photoresist coated information storage member~, regions 330 are ~ormed in substantially the same manner as regions 310 were formed with reference to the structure shown in Figure 15.
Rather than apertures 310 b~g formed as shown with refe~
25 ence to Figure 6, exposed regions 330 are formed corres-pondlng to the apertures 310. The exposed photoresist material is represented in Figure 16 by cross hatching of the regions in the photoresist ~nformation storage layer 324. Subsequent develop~ent of the exposed photoresist mater~al removes such exposed photoresist material leaving apertures comparable to the apertures 310 shown with refer-ence to Flgure 15.
In operatlonJ when using the photoresist coated substrate video disc member, the output power of the wrlt-35 ing laser is ad~usted such that the power of 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 ;

~7~
-48-~
beam. J~s' as with the bismuth coated master v~deo disc system, this ensures hlghes~ f'delity recordlng and minimum in~ermodulation di.stortion during the playback o~ the re-corded vldeo signal.
In referring to both Figures 15 and 16, that por-tion of tne light beam passing through the Glan prism above the h~lf power point as represented by that portion o~ the wave~orm shown on line D of Figure 14 which is above the line 290J causes an irreversible change in the character-lstics 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 reference to the photo-reslst coated video disc shown in Figure 16. In the case Or the bismuth coated video disc member 300, the irrever-sible changes take the ~orm o~ successively formed aper-tures 310 in the opaque metalllzed coating 304. In the case of the p~otoresist coated substrate 320, The irrever-sible alteration of the characteristic of the photoresist layer 324 occurs in the form of successive ~ully exposed regions 332.
While bismuth is llsted as the preferred metal layerJ other metals can be used such as tellurlum, inconel and nlckel.
Re~erring to Figure 18, there is shown the trans-~er characteristlc o~ the Pockels-Glan prism combinatlon as a result of the sinusoidal rotatinn in degrees o~ the llght passing through the Pockels cell 68 versus linear voltage change o~ input drive to the Pockels cell 68.
The ninety degree rotation point is shown at point 340 and equals the maximum light transmisslon through the Glan prism 70. The zero degree rotation point is shown at points 342 and equals the zero or minimum llght trans-mlssion through the Glan prlsm 70. The zero light t~ans-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 t.he llne 292 is equal to V2 mlnus Vl over Z

~7~ ~

and corresponds to a forty-five degree rotation of the light beam passing through the Pockels cell.
As is well known, the power through the Pockels cell is substantially unch2nged. The only characteristics

5 being changed in the Pockels cell is the degree of rota-tion of the light passing therethrough. In normal practice, a Pockels cell 6~ and Glan prism 70 are used together to achieve light modulation. In order to do this, the prin~i-pal axes of the Pockels cell 68 and the Glan prism 70 are put into alignment such that a light beam polari2ed at ninety degrees rotation passes substantially undiminished through the Glan prism. When the same highly polarized 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-tlce, the full transmission state and zero transmission state is not reached at hlgh frequencies o~ operations.
The wave~orm shown in Figure 18 shows the trans~er charac-teristics of the Pockels cell 68 rotated to correspond with two cycles of frequency modulated video informatlon.
This demonstrates that the transfer function contlnuously operates over the zero to ninety degree portion of the transfer function curve.
Referring to Flgure 19, there is shown the trans-fer characteristic of a Glan prism 70. At point 350,maxlmum transmlssion through the Glan prism 70 is achieved wlth a ninety degree rotation of the incoming llght beam.
At point 352) minlmum or zero light transmission through the Glan prism 70 is achie~ed at zero rotation of the 3 lncomtng light beam. Hal~ o~ the intensity of the imping-ing light beam is passed through the Glan prism 70 as indicated at points 35~ which corresponds to ~orty-five degrees rotation of the light entering-the Glan prism 70.
Obviously, the absolute power of the light passing through the Glan prism 70 at the forty-five degree rota-tion can be adjusted by adjusting the light output inten-sity of the light source. In this embodiment, the light source is the writing laser 30.
In the preferred embodiment, the power output 7~5~

from the writln~ laser 30 is ad~usted such that the inten-sitj of the light passing through the Glan prism at the half power pCillt ccincides with the threshold power level of the recording medium. Since more power ls required to melt a bismuth layer than is require~ to fully expose a photoresist layer, the absolute lntenslty of a wrlting beam used ln writlng on a blsmuth master disc is greater than the intenslty of a writing laser used to interact with a photoresist covered master video disc.
Re~errlng collectively to Figures 20 and 21, there is shown a serles of waveforms useful in explaining the relationship between length of a hDle cut in a master video disc by the wrlting laser 30 and the length of uncut land area between successively formed holes. This rela-tionship can be referred to as a relationshlp 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 lnto a single term known as duty cycle wlllch term represents all three such charac-20 teristics.
As previously descrlbed, the energy required tointeract ~ith the lnformation bearing layer on video disc substrate is that ener~y necessary to cause lrreverslble changes in the material selected ~or placement on the master video disc member. In the case of a bismuth coated master, the energ~J required is that needed to selectively remove the portion of the bismuth coated layer in those locations when the energy ls a~ove the threshold energy level of the bismuth layer. If this energy contained in the spot of light is not ~ocused properly upon the bismuth layer~ then the energy cannot be used for its lntended function and it will be dissipated without effecting lts lntended function. If some cutting occurs due solely to an out of focus spot distortions are introduced into the 35 masterlng process.
I~ the peak cutting power greatly exceeds the threshold power level of the recording medium, destructive removal of material occurs and provides a surface con-talning dlstortion products caused by this destructive ~7~57 removal. The average cutting power ls that power at a polnt midwa~ between a first higher cutting power and a second lower cutting power. As ~ust described, the average cutting power is preferably fixed to equal the threshold power level of the recording medium. In this sense, the lntenslty of the light beam above the average cuttlng power lnteracts wlth the informatlon bearlng layer to form lndlcia Or the signal to be recorded. The lntensity of the llght beam below the average cutting power fails to heat a bismuth coated master to a polnt needed ln the hold forming process or fails to fully expose a portion of a photoresist coated master.
Referring brlefly to llnes ~ and C of Figure 14, the adJustment of the average cuttlng power to coincide wlth the line 291 shown in llne B 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 of the "land" area positlon and successlvely thereafter.
Thls is known as a 50% or flfty-flfty duty cycle. A
~lfty-flfty duty cycle ls the preferable duty cycle in a recordlng procedure but CommerclQlly acceptable pla~back slgnals can be achieved ln the range from sixty-forty to forty-sixty. This means that ell;her the hole or the lnte~
venlng land member becomes larger whlle the other member becomes smaller.
Referring to Figure 20, there is shown a waveform represented by a line 360 c-orresponding to two cycles of the light intensity transmltted through the Pockels cell-Glan prism combination and represented more æpecifically on llne D of Figure 14. The threshold power level of the recording medlum is represented by a llne 362. The threshold power level of the reading medium ls caused to be equal to the half power polnt of the llght lntenslty transmitted by the Pockels cell-Glan prlsm combination by ad~usting the absolute lntensity of the writing laser 30.
l~hen the threshold level is properly ad~usted at the half power polnt, an indicia is formed on the information surface layer of the master video disc begin-nlng at point 364 and continuing for the time until the ;7 intensity falls to a point 366. ~ash lines shown at 354' and 366 ~ are drah~n to line ~ of Figure 12 showing an indicia represented by the eclipse 358 which has been formed for the period of time when the light intensity continues to rise past the point 364 to a maximum at 370 and then falls to a point 365. The light intensity below point 366 falls to a minimum at 372 and continues to rise towards a new maximum at 374. At a certaln 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 376, the energy in the llght beam begins to form an indlcia represented by the eclipse 378 shown on line -A of Figure 20. A dotted line 376 ~ shows the start of ~orma-tion of indicia 378 at the point when the light intensityexceeds the threshold level 362. The indicla 378 con-tinues to be formed while the light intensity reaches a maxlmum at 374 and beglns to fall to a new minimum at 375.
However~ at the intersection o~ line 360 with the thres-hold power level at 362 the light intensity falls belowthe threshold power level and the indicia ~s no longer formed. In the preferred embodiment, the length of the lndicia represented by a llne 384 ~quals the length of the land re~ion shown generally at 3~6 as represented by the 25 length of the line 388. Accordingly, the matching of the half power point llght intensity output from the Pockels cell-Glan prism combination with the threshold power level of the recording sur~ace re~ults in a fifty-fifty duty cycle whereln the length of the indicia 368 equals the 30 length o~ the next succeeding land region 386. Points 364, 366, 376 and 382 shown on the line 360 represent the zero crossing o~ the original frequency modulated video signal. Hence, it can be seen how the lndicia 368 and 386 represent the frequency modulated video signal. This representation in the preferred embodiment represents a fifty-fifty duty cycle and is achieved by ad~usting the half power level of the beam exiting from the Pockels cell-Glan prism combination to equal the threshold power level o~ the recording medium.

~7~:3S~
-5~
The wave~orm shown wlth reference to Figure 20, including the variable light intensity represented by the line 360, represents a preferred mode of operation to achieve 50/50 duty cycle independent of the recording medium employed on the master video disc member. The absolute lntenslties at the various polnts change accord-ing to the absolute intenslties requ~red for the modulated light beam to interact with the recordlng sur~e, but the relative ~ave shapes and their relative locatlon~ remain the same. More specificallyJ the absolute intenslty of the threshold power level for bismuth is di~erent than the absolute intenslty of the threshold power level for photoresist, but the relatlonshlp wlth the intensity line 360 ls the same.
Referring to the combination of Figure 20 and llne ~ of Figure 215 there will be described the results o~ failing to match the half power point output o~ the Pockels cell-Glan Prism combination with the threshold power level of the recording medium. Referrlng to Figure 20, a second dash line 380 represents the relationshlp between the actual threshold power level o~ the recording medium being used with the llght intensity output ~rom the Pockels cell 68-Glan Prism 70 combination. The thres-hold power level line 3~0 intersec:ts the lntenslty line 25 360 at a plurality of locations 390, 392, 394 and 396. A
line 390' represents the intersectlon o~ the light inten-sity line 360 with the threshold power level 380 and signals the start o~ the formation of an lndicia 398 shown on line E of Figure 21. The ~ndici 398 is ~ormed during the time that the light lntensity is above the thres~old power level. The length of the indicia 398 is represented by the time required ~or the light intenslty to move to its maximum at 370 and fall to the threshold point 392 as is ~hown 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 o~ a line 402 is determined by the time required for the light intenslty to move ~rom threshold point 392 to the next threshold point 394. During this time, the intensity o~ the light beam is sufflciently 1DW

as to cause no interaction with the recording medium. A
second lndicia is shown at 406 and its length corresponds with the point at which the intensity of the waveform represented by the line 360 exceeds the threshold power level at point 394. The length of the lndlcia 406 i3 shown b a line 408 and is determined by the time requlred for the llght intensity to rise to a maxlmum at 374 and fall to the threshold level at point 396.
Various llnes are shown indicating the beginning and endlng of the lndlcia and intravening land areas by employing primed numbers to identify the correspondlng intersectlons of the light intenslty line 360 with the threshold power level llnes 362 and 380.
The successively positioned indicla 398 and land reglon 400 represent a single cycle of the recorded fre-quency modulated vldeo slgnal. The indicia 398 repre-sents approximately 65 percent of the sum of the length of the llne 399 and the line 402. This represents a duty cycle ,Df 65/35 percent. Sixty-five percent of the 20 avallable space ls an indicia w~llle thirty-five percent of the avallable space is land area. Typically, the lndlcla in the final format is a light scatterlng member such as a bump or hole, and the land area is a planar surface covered with a highly reflective material.
The frequency modulate~ video lnformation repre-sented by the sequentially positloned light non-reflective member 368 and light reflectlve member 386 shown in line A
of Fi~ure 21 represents the pre~erred 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 si~nificantly altered by the impingement of the writing beam such as to be able to detect a dlfference between reflected light beams from the developed and not developed portlons of the photoresist member. It is 35 because of this effect that a read-after-write procedure, using a photoresist coated master video disc is not posslble.
Referring to line C of Figure 21~ there is shown a representation of the recovered video signal represented ~:~4~{~5~

by the sequence o~`indicia 368 and land area 386 shown on llne A. The wave~orm shown in line C is an undistorted sine wave 410 and conkains the same undistorted ~requency modulated information as represented by the light lntensity wave~orm represented b~J the llne 360 shown in Figure 11.
The sine wave shown in llne C o~ Flgure 21 has a center line represented by a llne 412 which lntersects the sine wave 410 in the same polnts of lntersection a the line 362 lntersects the intenslty llne 360 shown ln Figure 20.
Re~erring to llne D o~ Figure 21, there is shown a recovered ~requenc~J modulated vldeo signal having bad second harmonlc dlstortlon. The fundamental ~requenc~J
of the wavefor~ represented by a llne 414 shown in line D
ls the same as that contained in the wave~orm shown on line C. However, the informatlon shown ln Llne D contains bad second harmonic distortion. When used in a system in which bad second harmonic dlstortlon is not a disabling problem, the attention to a 50/50 duty cycle situation explained hereinabove need not be strictly followed.
However, when it ls necessary to have a substantially undlstorted output signal recovered from the vldeo dlsc surface, ~t ls necessary to ~ollow the procedure described herelnabove.
Re~erring to ~igure 13, there is shown the rela-tionship between the intenslty of the reading spot ln the reading beam as lt lmplnges upon successlvely positioned llght reflec~ive and light non-re~lective regions ~ormed during a preferred form o~ the masterlng process. In a preferred embodlment, a metal ls used for this purpose and 3 the preferred metal as dlsclosed is bismuth.
Llne A of Figure 13 shows a plurallty of lndicia ~ormed in the surface of a video disc master. In the preferred embodimenk the holes ~ormed in a bismuth layer 420 are shown at 422, 424 and 426~ The lntervening por-tions of the la~Jer 420 which are unaf~ected by the forma-tion of the holes 422S 424 and 426 are sometimes called "land areas and are lndlcated generall~J at 428 and 430.
The land areas are highly reflective. The formatlon o~ the holes 422J 424 and 426 expose the underlylng glass substr~

~7~5~

which ls essentially light absorbing and hence the glass substrate ls a light non-reflective region. The waveform shown at 432 represents the light intensity waveform of the spot in the read beam as the spot passes over a light non-reflective region. This indicates the spacial rela-tionship between the spot as lt moves over a light non-reflective region.
~ eferring to line B of Figure 13, there is shown a waveform represented by a line 434 indicating the inten-sity waveform of the reflected light as a spot having theintensity relatlonship shown in Figure A passes over a successively positioned light reflective and light non-reflective region. A solid line portion 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 o~ 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 portlon 424 is shown on a line 440 at a point 442. The 20 intensity wave~orm of the reflected light is a maximum~
as shown at 444, when corresponding to a center polnt 446 of the land area 423 posltloned between successive non-reflective regions 422 and 424 respectively. The center point 446 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 intensity waveform of the reflected llght when the llght passed over the n~n-re~lective region 422. A dotted portlon 452 of the waveform 434 shows the expected lntensity of the reflected light beam when the reading spot passes over the non-reflective region 426.
Referring to line C of Figure 13, there is shown the recovered electrical representation of the light intensity signal shown on line B. The electrical repre-sentatlon is shown as a llne 454 and is generated in thephotodetector 70 shown in Figure 1.
` A schematic diagram o~ a suitable high voltage amplifier is shown in Figure 16.
A special advantage of the read while wrlte s~ ~

capability of the mastering procedure herein described includes the use of the instantaneous monitoring of the lnformation ~ust written as a means for controlling the duty cycle of tne reflective and non-reflective regions.
By dlsplaylng the recovered frequency modulated video signal on a television monitor during the writing proce-dure, the duty cycle can be monitored. Any indicatlon of the distortion visible on the mo~i~or indicates that a change in duty cycle has occurred. Means are provlded for adJusting the duty cycle of the wrltten lnformation to eliminate the distortion by ad~usting the duty cycle to its 50~50 preferred operating point. A change ln duty . cycle ls typically corrected by adjusting the absolute intenslty of the light beam generated in the laser 30 in a system having either an average intensity biasing servo or a second harmonic biasing servo and in conjunction with circuitry for ad~usting the half power polnt 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 intenslty are interchanged in the portions o~ the specifications and claims which concern the use of the trlangular shaped wave ~orm gener-ated by the FM modulator. The modulated light beam 40 exiting ~rom the Glan Prism 38 :ls o~ sinusoldal shape. In this sltuatlon the half power point equals the average intensity, and this would be the case for any symmetrical wave form. A frequency modulated output ~rom an FM modu-; lator has been found to act as such a symmetrical wave form.
While the inventlon has been particularly shown and descrlbed with reference to a pre~erred embodiment and alterations thereto~ it would be understood by those skllled in the art that various changes in ~orm and detail may be made thereln without departing from the splrit and scope of the invention.

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Claims (8)

1. An information carrying record for storing a frequency modulated signal having an informational content in the form of a carrier frequency having frequency changes with time varying from said carrier frequency, said record comprising: an information storage member having an upper surface, and said surface carrying a lineal series of alter-nately positioned first and second indicia in track-like fashion upon said surface; said indicia representing a frequency modulated signal having its informational content in the form of a carrier frequency having frequency changes with time varying from said carrier frequency; each first indicia having a variable length representing the instantan-eous 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.

2. The information carrying record as claimed in Claim 1, wherein said first indicia are fully exposed portions of a layer of a layer of photoresist; and said second indicia are unexposed portions of a layer of photresist.

3. The information carrying record as claimed in Claim 1, wherein: said first indicia are specular light reflective regions, and said second indicia are specular light non-reflective regions.

4. The information carrying record as claimed in Claim 1, wherein said first indicia are light reflective regions, and said second indicia are light non-reflective regions.

5. The information carrying record as claimed in Claim 1, wherein said duty cycle falls within the range of 60 - 40 to 40 - 60.

6. An information carrying record for storing a frequency modulated signal having an informational content in the form of a carrier frequency having frequency changes with time varying from said carrier frequency, and the frequency modulated signal is stored in the form of a non-image, spatially variable linear series of discrete regions forming a physical pattern, said information carrying record comprising: an information storage member in the form of a rotary disc having an upper surface upon which information is recorded, said storage member having a non-image, spatial-ly variable physical pattern defined by a lineal series of discrete regions, positioned in track-like fashion upon said surface, the width of the track being substantially 1 micron or less; said regions being alternately specular light reflective and non-specular light reflective, the lengths and spacing between such alternate discrete regions being continuously variable and representing a frequency modulated signal having its informational content in the form of a carrier frequency having frequency changes with time varying from said carrier frequency; and the lengths and spacing between said regions having been determined by a writing laser beam issuing from laser intensity modulating means having a high laser transmitting state of sufficient in-tensity to produce said non-specular light reflective regions and a lower laser transmitting state of insufficient in-tensity to produce said non-specular regions, the varying intensity of said writing laser beam issuing from said laser intensity modulating means during the recording process being maintained at an average value intermediate the laser intensities associated with said higher laser transmitting state and said lower laser transmitting state.

7. The information carrying record as set forth in Claim 6, wherein each specular light reflective region is substantially equal in length to its associated adjacent non-specular light reflective region for representing a substantially 50-50 duty cycle in the formation of said adjacent specular light reflective and non specular light reflective regions.

8 . The information carrying record as set forth in Claim 6 , wherein each specular light reflective region has a length relative to its associated non-specular light reflective region for representing a duty cycle in the range of 60-40 to 40-60 in the formation of said adjacent specular light reflective and non-specular light reflective regions.