CA1125434A - 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 recovering video information from the video record . The video disc member formed by the writing apparatus is also described.
The writing apparatus includes a laser as a source of a laser write beam and a write optical system for directing the stationary laser write beam to a moving video disc member. The video disc member comprises a substrate member carrying a light responsive coating on at least one sur-face. The write optical system focuses the beam to a small spot of light approximately one micron in diameter upon the light responsive layer. The intensity of the focused spot is changeable under the control of a light intensity modulating assembly. This light intensity modu-lator 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. Circuitry is described for using a triangular frequency modulator for driving the Pockels Cell driver and for obtaining a sinusoidal-shaped light modulation output signal from the linear polarizer. The output from the linear polarizer is adjusted such that the half power point from the polar-izer equals the threshold power level of the material forming the information storage layer. A second harmonic bias circuit is employed for removing the second harmonic intermodulation distortion of the modulated write beam.

Description

ii2S43~

MASTERING MACHINE

TECXNICA~ FIELD
The present lnvention relates to the writing of a ~requency modulated electrical signal upon an ln~ormatlon bearing surface of a video disc member ln the form of a lineal series of flrst and second indicia positioned in ; track-like fashion upon such surface.
BACKGROUND OF THE P~IOR ART
The apparatus ~or writing a ~requency modulated signal upon a video di8c member includes a movable writing beam and a video disc member mounted on a turntable. The turntable ~s driven by a motion control assembly which rotates the disc precisely ln a circle at a constant rate 15 of rotation and a translational drive assembly for trans-lating the wrlting beam at a very constant~ and very low velocity along a radius ~f the rotating disc. The rota-tional drive of the disc i8 synchronized wtth the trans-latlonal drlve oP the writing beam to create a splral traok of predetermined pitch. In a pre~erred embodiment, the 9paoing between ad~acent trac~s o~ the spiral 1B two microns, center to center. The indicia ls formed having a width of one micron. This leaves an intertrack or guard area of one micron between indicia in ad~acent tracks.
If des~red, the lndicla can be formed as concentric circles by translating in incremental steps rather than by trans-lating at a constant velocity as just described.
In the pre~erred embod~ment~ a microscope obJec-tive lens is positloned at a constant height above the , 1~L2~39~

video disc member on an air bearing. This objectlve lens is employed for focusing the write beam upon the light sensitive surface of the video disc member. The constant height is necessary because of the shallow focal depth of the objective lens. A o.6~ NA dry microscope objective lens is employed to focus the write laser beam to a spot one micron in diameter upon the light sensitive coating.
Because the coating is rotating at a relatively high rate, the length of the indicia formed in the light sensitive coating depends upon the length o~ tl~e the spot lntensity exceeds that needed to form such an indicia.
A linearly polarized ion laser is used as the source of the writlng beam. A Pockels cell i5 used to rotate the plane of polarization of the wrlting beam with respect to its fixed plane of linear po-arization. A
; linear polarlzer attenuates the rotated writing beam in an amount p~portional to the difference in polarization between the light in the writing beam and the axis of the linear polarizer. The combination of a Pockels cell and linear polarizer modulates the writing beam with the video i~ormation to be stored. This modulation follows the pattern provided by control signals furnished by a Pockels cell driver.
The video signal to be recorded is applied to a frequency modulator clrcuit. The output from the modu-lator circuit is a rectangular wave whose frequency is proportional to the video signal. The duration of each cycle of the rectangular waveform is variable as is characterlstic of a frequency modulated signal. As i5 characteristic o~ a rectangular wave, 1~ has an upper voltage level and a lower voltage level. The upper and lower voltage levels of the rectangular wave are empll-fled by a Pockels cell driver and used to control the ; Pockels cell. The Pockels cell changes the angle of pola~
3~ lzation of the light passing therethrough in response to the instantaneous vol~age level of the control signal supplied ~y the Pockels cell driver.
In a first mode of operation responsive to one voltage level of the rectangular-shaped control signal ` ' ~12~i434 applied to a Pockels cell driverg the light beam passes unhindered through the Pockels cell linear polarizer combination at a first intensity sufficienJ to form a first lndicia in a light responsive coating. When the control signal changes to represent its second voltage level, the Pockels cell rotates the polarization o~ the light which forms the writing beam to a new angle of pola~
ization. ~ue to this change in polarlzation of the llght ~orming the writing beam, a mlsmatch occurs between the angle of polarization of the light 13suing from the Pockels cell and the preferred angle o~ polarization of the linear polarizer. In this situation, the linear polarizer acts as an attenuator and less light passes through the linear polarizer. This reduces the light intensity of the writing beam below the intensity requlred to form such f~rst j indicia in the light responslve coating.
A portion of the writing beam is sensed by a Pocke-s cell s~abillzing circuit for maintaining the average power o~ the modulated writing beam at a predeter-mlned level in spite of changes in the Pockels cell trans-fer characteristic produced by small temperature variations.
~he stabilizing circuit includes a level ad~usting circuit for selectively ad~ustlng the power level to form indicia in dif~erent light sensitive coatings as identi~ied here-inafter.
Circuitry is described for using a triangular~req~ency modulator ~or driving the Pockels Cell driver and r obtaining a 3inusoidal-shaped light modulation output slgnal ~rom the llnear polarizer. The output ~rom the llnear polarizer is ad~usted such that the half power polnt from the polarizer equals the threshold power level of the material forming the in~ormation storage layer.
A second harmonic bias circuit is employed for removing the second harmonic intermodulation distortion o~ the modulated write beam.
Different types of video disc members can be used with this writing process and apparatus. Each such member haq a di~ferent configuratlon. In a first configuration~
the video disc member includes a glass substrate having ~ .

i l Z ~

an upper surface carrying a thin metal coating as a light responsive coating. In this configurationg the write beam forms variable length apertures in a track-like fashion in the metal coating.
The intensity of the write bea~ is adjusted such that an aperture is formed, for example, during each positlve half cycle of the frequency modulated signal to be stored, and no aperture is for~ed durlng the negative half cycle. Accordingly, the first and second indicia representative o~ the stored information is a lineal series o~ apertures separated by an intervening portion of the surface coating.
In this first configuration, a portion of the glass substrate is exposed in each aperture. The exposed portion of the glass substrate appears as a reglon of non-specular light reflectivity 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, ie., a 180 reversal in paths between the incident and reflected beam paths. Non-specular reflectivity means that no significant portion of the incident beam is re-flected along the path of the incident beam.
In a second configuration, the video disc member includes a glass substrate having an upper surface carry-ing a thin layer of photoresist as the light responsive coating. In this configuration~ the write beam forms variable length regions of exposed and unexposed photo-resist material in a track-like fashion in the photo-reslst coating. The lntenslty of the wrlte beam isad~usted such that a region of exposed photoresist ma~erial ls formed, ~or example, during positive half cycles of the frequency modulated signal to be stored and a region of unexposed photoresist material is left during the nega-tive half cycles. Accordingly, the first and secondindicla representative o~ the stored lnformation is a lineal series of exposed and unexposed portions of the surface coating, respectively.
A preferred embodiment of a reading apparatus is ~12~D~3~

described empl~ying a read laser for producing a polar-ized collimated beam of light having a preferred angle of polarization. A read optical system directs and images the laser beam to impinge upon the indicia carrled upon the surface of the video disc member. The video disc member is employed for storing a frequency modulated sig-nal on its surface in the form of a lineal series of regions. The regions are alternately specular light reflective and non-specular llght reflective. A read optical system focuses the read beam to a spot of light approximately one micron in diameter and dlrects the focused spot to impinge upon the lineal series of regions.
The intensity of the read beam is ad~usted such that a sufficiently strong reflected read beam signal is gathered 15 by the read optical system.
A motion control assembly rotates the video disc member at a uniform rate of speed su~ficient t~
reconstruct the frequency of the originally stored fre-quency modulated slgnal. A typical frequency modulated signal stored in this matter varles in frequency between two megacycles and ten megacycles. The rotational rate Or the video disc member is preferentially set at about 1800 rpm to chànge the spatially stored frequency modu-lated signal into a real time electrical signal. The motion control assembly includes a translational drive assembly for translating the reading beam at a very con-stant, and very low velocity along the radius of the rotating disc so as to impinge upon the lineal series of light reflective and light scatterlng regions contained thereon.
The reflected read beam gathered by the read optlcal system is dlrected to a light senslng circuit for changing the intensity modulated reflected light beam to a frequency modulated electrical signal corresponding to the intensity modulated reflected light beam.
A polarization selective beam splitting element is positioned in the read beam path intermediate the read laser source and the video disc member. A~ter the read beam passes through the polarization selective beam . :~ 3L2S~34 splitting element the real light beam is linearly polar-ized in the preferred plane. h quarterwave plate is positloned intermedia~e the ou~put of the polarization selective beam splitting element and the video disc member. The quarterwave plate changes the light in the read beam from linear po~arization to circular polariza-tion. The reflected light retains its circular polariza-tion until it passes through the quarterwave plate a second time. During this second pass through the quarterwave plate the reflected light is changed by circular polariza-tlon back into linear polarized light rotated ninety degrees from the preferred plane established by the polar-ization selective beam splitting element as described hereinabove.
The polarization selective beam splitting element is responsive to this ninety degree shift in the reflected light beam for diverting the reflected beam to the light sensing circuit and prevents the reflected ~ight beam from reentering the read laser source.
A diverging lens is employed in the read optlcal system for spreading the substantially parallel light beam from the read laser source to at least fill the entrance aperture of the ob~ective lens.
In a second embodiment of the read optical system~ an optical ~ilter is placed in the re~lected read beam path for filtering out all wavelengths of light other than the wavelength of light generated by the read laser source.
In a reaording apparatus, tlle write functlon 30 alone is employed for writlng the frequenoy modulated information onto a video dlsc member. In a video dlsc 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 35 operatiDnJ the read and write functions are combined in a single machine. In this comblned apparatus~ the read apparatus is employed for checking the accuracy of the information being written by the write apparatus.
To implement the monitoring function, the read 1~125~34 beam from the Helium-Neon (He-Ne) read laser is added into the writ~ng 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 ls chosen so that the readlng beam illuminates an area on the same t~ac~ being written by the write beam, but at a point that ls approxlmately four to six microns downstream from the writlng spot. More speciflcall~g the read beam ls imaged upon the in~ormation track that was ~ust formed by the write beam. Sufficient tlme has been -~ allowed for the ~nformatlon indicia to be formed on the video disc member. In this manner, the read beam is im-pin~ed upon alternate regions of different reflectivity.
In one form of the read apparatus, the read beam impinges upon the portions of the metal not heated by the write beam and also impinges upon the glass substrate exposed in the apertures just formed by the writing spot. The regions of different reflectivity function to change an impinging read beam of constant intensity into an intens~ty modu-lated reflected read beam.
In this monitoring mode of operation, the readlaser beam is selected to operate at a wavelength differ-ent from that of the wri~e laser beam. A wavelength selective optical filter is placed in the reflected light beam path having a band pass which includes the reading laser beam. Any write laser beam energy which follows the read reflected path is excluded by the filter and therefore cannot interfere with the reading process. The monitoring mode of operatlon i6 employed at the tlme o~
writlng the video in~ormation onto the video dlsa member as an aid in checking the quality of the signal belng recorded. The output signals from the read path are displayed on an oscilloscope and/or a television monitor.
The visual inspectlon of this dlsplayed signal indicates whether the lndicla are being formed wlth the preferred duty cycle. The pre~erred duty cycle is achleved when on the average the length of a specular reflective region~
whlch represents one half cycle of a frequency modulated slgnal, ls the same as the next succeeding region of , ' 1~25~34 non-specular rerlect~ity, which represents the next consecutive half c~rcle o~ a ~requenc-~ modulated signal.
The read after wr~te or mon~toring mode of oper-atlon ls als~ utilized ir. an error checking modeJ especi-ally lf digital type information is belng written. The lnput video inform~tion is delayed for an lnterval equal to the accumulative val~ s Df the time delay beglnning with the frequency modulation of the input video informa-tlon signal during the write process and contlnuing through 10 the rrequency demodulation of the recovered re~lected signal rrom the sensing circuit, and lncluding the delay of travel tlme of the point on the storage member mo~lng ~rom the point of storing the input video in~ormation signal to the point of impingement of the read light 15 ~eam. The recovered in~Drmation is then compared with the delayed input lnformatiDn f~r accuracy. The exlstence of too many dissimilarities would be a basis for either rechecklng and reallgning the apparatus or re~ecting the ~isc .
The read apparatus is suitable rOr use with a standard ho~e television receiver by adding an RF modu-lator for adding the video signal to a suitable carrier ~requency matched to one Or the channels Or a stanaard home television receiver. The standard televlsion re-ceiver then handles this signal in the same manner as are received ~rom a standard transmitting statlon~
More particularly, there is provided:
Apparatus ~or storing vldeo in~ormation ln the ~orm Or a frequency modulated sl~nal upon an in~ormation storage member, comprlslng: first means for providing an lnformatlon slgnal to be recorded, and said signal havlng lts in~ormatlonal content in the ~orm of a carrier frequency havlng rrequency changes in tlme representlng said inrormation to be recorded; an lnrormatlon storage member including a substrate havlng a ~lrst sur~ace and a llght respon~lve coatlng coverlng sald flrst surface for retainlng lndlcla representatlve Or said lnrormatlon sl6nal; means ror imparting unlrorm motlon to sald storage member; a light source ~or rovldlng a llght beamJ sald light beam belng Or surrlclent v . - ~
:

112~434 -8a-lntenslty ~or lnteracting wlth sald coating whlle sald coatlng is ln motlon and sald coating ls positloned upon said movlng lnformation storage member, said llght beam t~elng of suf~lclent lntenslty ~or altering sald coatlng to retaln lndicla representative of said lnformatlon slgnal;
optlcal means for de~lning an optlcal path between sald llght source and said record carrler lncluding sald coatlng, sald optlcal means belng ~urther employed for focuslng said llght beam to a spot upon said coating; llght lntenslty modulatlng mesns posltloned ln sald optical path between said llght source and sald record carrler, sald light ln-tenslty modulatlng means operatlng over a range between a maxlmum llght transmittln~ state and a minlmum llght tran-smlttlng state for lntensity modulating said light beam with said lnformatlon to be stored; sald light intenslty modulating means bein& responslve to sald frequency modulated slgnal and changlng between lts maxlmum llght transmittlng state and lts mlnlmum llght transmitting state durlng each cycle of said frequency modulated slgnal for modulating sald llght beam wlth the frequency modulated electrlcal slgnal to be stored; and stablllzlng means responsive to sald modulated light beam for ~eneratlng a bias control slgnal lndlcatlve o~ the second harmonlc dlstortlon present ln sald modulated light beam, sald blas control slgnal belng employed for blaslng sald modulatlng means at lts operating polnt at whlch mlnlmum second harmonlc dlstortton ls present ln sald modulated llght beam at its average power lntenslty; and whereln ~ald llght passlng through said llght lntensity modulatlng means and ~ocused upon sald coating by said optl¢al means forms lndlcla ln sald coating representative o~ sa~d frequency modulated slgnal to be stored.
There is further provided:
Apparatus for recording input vldeo infornation on an lnPormatlon storage member, comprislng: flrst means for provldlng a ~ldeo lnformatlon slgnal to be recorded and sald vldeo slgnal havlng lts informational content ln the form of a voltage varying wlth time signal suitable for dlsplay on a standard televislon monitor; a wrltlng laser for produclng a collimated writlng beam of polarized mono-chromatlc light; a smooth ~lat rigld dlsc havlng a planar ,.~

. .

-8b-surface covered wlth a llght responslve coatlng for retaln-lng lndlcla representative of said vldeo slgnal; sald coat-lng havlng a threshold power level above which sald lndlcla are formed; ~irst optical means for dlrecting said llght beam to lmpinge upon sald coatlng, and for focusing sald lmpinglng wrlting beam down to a spot at sald coating; sald coatlng havlng sultable physlcal properties to be altered ln response to the impingement of light from said writing beam for formlng a permanent alteration in sald coating; rotational drlve means for producing uni~orm rotatlonal motlon of sald disc; translational drive means synchronlzed wlth said rotatlonal drlve means for moving said ~ocused light spot radially across said planar surface of said disc, electrical synchronlzlng means for malntalnlng a constant relatlonshlp between sald rotational motion and sald translational motion, fre~uency modulatlon means ~or covering said lnput vldeo lnformation to a correspondlng frequency modulated slgnal, said frequency modulated signal havlng its informational content ln the form of a carrler frequency havlng frequency changes with tlme correspondlng to sald voltage variatlon with time slgnal; electrically controllable means responsive to sald frequency modulated means for varying the lntenslty of said wrlting beam between a flrst predetermlned intensity at uhlch the focused spot alters said coatlng and a second predetermlned lntensity at which the Pocused spot ~alls to alter sald coating, said alteration belng representative of sald frequency modulated signal, means for ad~ustlng the average lntensity o~ the output of sald electr~cally con-trollable means to equal said threshold power of sald llght responslve coatlng; and stablllzln~ means responsl~e to said modulated llght beam for generatlng a blas control signal lndlcatlve of the second harmonlc dlstortlon present ln said modulated llght beam, sald blas control slgnal belng employed for blaslng said electrically controllable means at its operating point at whlch mlnlmum second harmonlc dlstortlon is present ln sald modulated ll~ht beam at lts average power intenslty.

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1~2S43~
-8c-There is also pr~vided:
A method for recording ln~or3atlon on an lnformatlon storage member uslng a laser beam, comprlsing the steps of: providlng an electrlcal signal to be recorded, and sald signal h~ing its lnformational content ln the rorm of a voltage varying with time format; changing sald volta~e ~arylng with time signal to a frequency modulated electrical slgnal haYlng its in~ormational content ln the rorm of a carrier frequency having fre~uency changes wlth time cor-responding to sald voltage variations wlth tlme; shaplng sald frequency modulated electrical signal lnto a triangular shaped wa~e~orm; modulatin~ a flxed 1ntenslty llght beam wlth the triangular shaped, fre~uency modulated slgnal, senslng the amount of second harmonlc distortion present ln 3ald llght beam ~ust prlor to its applicatlon upon sald light sen~ltlve sur~ace for generating a blas control ci~-cuit; using sald ~la~ control slgnal for biaslng said modu-latlon to produce mlnlmum second harmonic dlstortlon; movlng the lnformatlon qtorage member at a constant rate while ~ocusing a ~ald li~ht beam down to a spot upon the llght sensitlve sur~ace of said lnformatlon storage member; ùslng said focused light spot to lrreverslbly alter the char-acterlstlcs of sa~d llght sensitlve surface of said lnfor-mation storage member as said member moves at a constant rate and under the control of one portlon of said ~requency varying signal; and blocklng the transmlssion o~ said foc~sed 11ght beam to said ll~ht ~ensitive sur~ace o~ sald ln~ormatlon storage member as sald member moves at a constant rate and under the control by a oecond portlon oP sald ~requency varylng ~i~nal.
RIEF DESCRIPTIOI~ QF THE DRAWINGS
FIGURE 1 ls a block diagram o~ the write appara-tus;
FIGURE 2 ls a cross-sectional view of a video dlsc member prior to wrltlng thereon uslng the wrlte apparatus shown in Figure l;
FIGURE 3 is a partial top view of a video dlsc member after writlng has taken place uslng the wrlte apparatus shown in Figure l;
.~

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~2~i434 FIGURE 4 is a waveform of a video signature e~ployed in the mrite apparatus shown in Figure 1, FIGU~E 5 is a ~avefor.m of a f.requency dulated signal used ln the write apparatus shown in Figure l;
FIGVFE 6 ls a graph showing the intensity of the write laser used in the write apparatus shcwn in Figure l;
FIGURE 7 is a graph shcwlng the modulat`ed write beam as changed by the write apparatus shcwn in Figure l;
FIGURE 8 is a radial cross sectional view taken along the llne 8-8 of the disc æhown in Figure 3;
FIGURE 9 is a detailed block diagram of a suit-able motlon control asse~blyS
FIGURE 10 ls a block diagram shGwing a read ap-paratusj FIGU~E 11 is a block diagr~m showlng the combi-nation of a read and wrlte apparatus~
FIGURE 12 ls a schematlc representatlon showing the read and wrlte beams passing through a single ob~ec-tive lens as ~lqed ln the block diagran of Flgure l;
FIGURE 13 is a schematic diagram of a suitable stabll~zing circuit for use in the write apparatus shown in Figure 1.
FIGURE 14 shows various waveforms used in illus-trating the operation of a ~astering machine~
FIGURE 15 shows a cross-sectional schematic view o~ one ~orm of a video disc~
PIGU~E 16 shows a photoreslst coded storage nenr ber, FIGURE 17 shows certaln portions rem~ved rram the photoresist coded storage nEs~er of Figure 16, FIGURE 18 shows the trans~er characterlst1c o~ a Pockel~ cell used herein, FIGURE 19 shows the transfer characteristlc of a Glan prism used herein;
FIGURE 20 shcws a light intenslty wave~orm~

~S~L34 FIGURE 21 shGws in conJunction ~ith Figure 20 a series of waveforms usefhl in explaining the duty cycle of recordin~
FIGUR 22 shows an addltional wave~orm used in illustrating the operation of a mastering machlne FIGURE 23 is a block diagram of a Pockels cell bias servo system;
FIGURE 24 is a diagram of a second harnonic detec-tor used in Figure 23; and 10FIGURE 25 is a diagr2m o~ a high voltase amplifier used ln Flgure 23.

15D~TAILED DESCRIPTION OF THE INVENIION
The same nu~Aeral is used to identl~y the same ele-ment in the several views. The terms recording and stor-ing are used lnterchangeably ~or the term wrlting. m e term retrieving ls used interchangeably for the term read-ing.
m e apparatus for storing video informatlon in theform of a frequency modulated signal upon an informa~ion storage mP~ber 10 is shown with reference to Figure 1. An information signal source circuit 12 ls employed for pr~vld-ing an informatlon signal to be recorded. Ihis in~orma-tion signal present on a line 14 is a ~re~uency modulated signal havin~ its informational oontent in the form of a carrier frequency havlng frequency changes in time repre-senting said in~ormatlon to be recor~ed. Figure 5 shows a typical example of a frequency modulated slgna~. m e information signal source circuit 12 employs a video slg-nal cir~uit 16 for providing an information signal on a line 18 havlng its informational content in the form of a voltage varying with t~me format. Figure 4 shows a typical example of a ~oltage varying with tin~e slgnal. A

~L2~3~

frequenc~ modulator circuit 20 is responsive to the video signal circuit 1~ lor converting the voltage varying with time signal to the frequency modulated signal on the line 14 as shown in Figure 5.
The lnformation 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 flrst surface 24 and a light responsive coating 26 covering the first surface 24. A motlon control assembly 28 imparts uniform motion to the storage member 10 relative to a write beam 29' generated by a light source 30.
The motion control assémbly 28 is shown and described in greater detail with re~erence to Figure 9. The motion control assembly 28 includes a rotational drive circuit 32 for providing uniform rotational motion to the informa-tion storage member 10 anq translational drive circuit 34 synchronized with the rotational drive circuit 32 for moving the focused light beam 29' radially across the coating 26. The motion co~trol assembly 28 further in-cludes an electrical synchronizing assembly 36 for main-taining a constant relationship between the rotational motion imparted to the member 10,by the rotational drive : circuit 32 and the translational motion imparted to the : light beam 29 by the translational drive circuit 34.
The light source 30 provides a beam of light 29 which is of sufficient intensity for interacting with or -: altering the coating 26 while the coating 26 is in motion and positioned upon the moving information storage member 10. Additionally~the intensit~ of the llght beam 29' i~
suf~iclent f'or producing permanent indlcia in the coatlng 26 representative of the information to be recorded. A
suitable light source 30 comprises a writing laser ~or produclng a collimated writing beam o~ polarized mono-chromatic light.
Referring again to Flgure 2~ there is shown a cross-sectional view of a first configuratlon of a suit-able video disc member 10. A suitable substrate 22 is made of glass and has a smooth5 flat, planar first surface 24. The light responsive coating 2~ is formed upon the ~2~ 4 sur~ace 24.
In one o~ the disclosed embodiments, the coating 26 is a thin, opaque metallized layer having suitable physical properties to permlt localized heating responsive to the impingement of the write light beam 29 from the writing 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 ~reezing, this leaves a permanent aperture such as at 3~, shown in Figures 3 and 8, in the thin metal coa~
ing 26. The aperture 37 is one type of indicia employed for represent~ng in~ormation. In this embodiment, succes-sively positioned apertures 37 are separated by a portion 38 of the undisturbed coating 26. The portion 38 is the second type of indicia employed for representing informa-tion A more detailed description concerning the process by which the indicia 37 and 38 represent the frequency modulated signal ls given with reference to Figures 5 through 8.
A movable optical assembly 40 and a beam steering optical assembly 41 collectively define an optical path for the light beam 29 issuing from the light source 30.
The optical assemblies image the read beam 29 into a spot 42 upon the coating 26 carried by the storage member lO.
The optical path is also represented by the line identified ` by the numerals 2g and 29'.
A light inten3ity modulating a~sembly 44 i9 posl-tioned in the optical path 29 between the light 3 ource 30 and the coating 2~. In its broadest mode ~ operatlon, the light intensity modulating assemoly intensity modu-lates the light beam 29 with the in~ormation to be stored.
The llght intensity modulating assembly 44 operates under the control of an amplified form of the frequency modu-lated signal shown in Figure 5. This frequency modulated signal causes the assembly 44 to change between its higher light transmitting state and its lower light transmitting state during each cycle of the frequency modulated signal.
This rapid change between transmitting states modulates the light beam 29 with the frequency modulated signal to li2S43~
-~3-be stored.
The light beam 29 is modulated as it passes through the light intensity modulating assembly 44. Thereafter, the modulated light beam~ now represented by the numeral 29'~ is imaged upon the coating 2~ by the optical assem-blies 40 and 41. As the modulated light beam 29' impinges upon the coating 26, indicia is ~ormed in said coating 26 representative of the frequency modulated slgnal 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 varylng the intensity of the light beam 29' above a predetermined intensity at which the focused beam 29' alters the coating 26 carried by the information sotrage member 10. Addi-tlonallyJ the electrically controllable subassembly 46 is responsive to the frequency modulator 20 for varylng the intensity o~ the light beam below a predetermlned intensity at which the ~ocused beam 29' fails to alter the coating 26. The alterations formed in the coating 25 are repre-sentatlve o~ the ~requency modulated signal to be stored.
When a photoresist layer forms the coating 26 carried by the information storage member 10, the alterations are in the form of exposed and unexposed photoresist members analogous to the size as previously descrlbed with respect to indicia 37 and 38, respectively.
~` When the coating 26 carried by the information storage member 10 is a metal coating, the eleatrically controllable subassembly 46 varies the lntensity o~ the 3 writing beam 29' above a ~irst predetermined intensity at which the ~oaused beam 29' melts the metal coating wit~out ~- vaporizlng it and further varies the intenslty of the wrlting beam below the predetermlned intensity at which the focused beam 29' fails to melt the metal sur~ace.
The light intensity modulating assembly 44 in-cludes a stabilizing circuit 48 for providing a feedback signal emplo~ed ~or temp~rature stabilizlng the operating level of the electrical controllable subassembly 46 to operate between a predetermined higher light intensity and , . , .

~Z~434 predetermined lower light intensity level. The light intensity modulating assembly ~ includes a light sensing circuit ~or sensing at least a portion of the light beam, indicated at 29 !1~ issuing from the electrically controllable subassembly 46 to produce an electrical feedback signal representat,ive of the average intensity of the beam 29l.
The feedback signal is connected to the electrically con-trollable subassembly ~6 over the lines 50a and 50b to stabillze its operating level.
The light sensing means produces an electrical feedback signal which is representative o~ the average intensity of the modulated light beam 29'. In this manner, the light intensity modulating assembly 44 is stabilized to issue the light beam at a substantially constant aver-age power level. The stabilizing circuit 48 also includeslevel ad~ustment means for selectively adjusting the average power level of the light beam 29' to a predeter-mined value to achieve the preferred duty cycle in either a metal coating 26 or a photoresist coating 26, or any other material used as the coating 26~
The movable optical assembly 40 includes an ob~ective lens 52 and a hydrodynamic air bearing 54- for supporting tha lens 52 above the coating 26. The laser beam 29' generated by the laser source 30 is formed of substantially parallel light rays. In the absence of the lens 66, these substantially parallel light rays have substantially no natural tendency to diverge. Then the ob~ective lens 52 has an entrance aperture 56 larger in diameter than the dlameter of the light beam 29'. A
planar convex diverging lens 65 positloned in the light beam 29' is ~mplo~ed for spreading the substantially parallel-light beam 29' to at least fill the entrance ~ aperture 55 of the obaective lens 52.
; The beam steering optical assembly 41 further in-cludes a number of mirror members 58~ 50, 62 and 54 for ~ folding the light beams 29' and 29" as desired. The mirror `~ 60 is shown as a movable mirror and ls employed for making strictly circular tracks rather than the preferred spiral tracks. Sprial tracks require only a fi,Yed mirror.

-` ~12~i;434 -~5-As previously des~.rc~ibed, 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 signal. A suitable electrically controllable subassembly includes a Pockels cell 68, a linear polarizer 70 and a Pockels cell driver 72. The Pockels cell driver 72 ls essentially a linear amplifier and is responsive to the ~requency modulated signal on the line 14. The output from the ~ockels cell driver 72 provides drivln~ signals to the Pockels cell 68 for rotating the plane of polarization o~ the laser beam 29. The linear polarizer 70 is orienta-ted ln 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 the source 30. ~ecause of this ; 20 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 ~- 25 polarizer as described is a matter o~ choice. By aligning the maximum iight transmittin~ axis o~ the pola~zer 70 with the angle o~ polarization of the light issuing from the laser source 30, the maximum and minimum states would be opposlte rrom that described when subjected to zero degrees and ninety de~ree rotation. However, the write apparat~s would essentially operate the same. The llnear polarizer 70 ~unctions to attenuate the intensity o~ the beam 29 which is rotated away from its natural polarization angle. It is this attenuating action b~J the linear polarizer ro wllich forms a modulated laser beam 29' cor-responding to the ~requency modulated signal. A Glan-prism is suitable ~or use as a llnear polarizer 70.
The Pockels cell driver 72 is AC coupled to the Pockels cell 68. The stabilizing ~eedback circuit 48 is ~ , ' .

~12~i434 l6-DC coupled to the Pockels cell 68.
Referring CollectiVely to Figures 4 through 7g there are shown selective waveforms of electrical and optical signals which are present in the embodiment shown with reference to Figure 1. A video signal generated by the video signal source circuit 16 is shown in Figure 4.
A typical device for generating such a video signal is a television camera or a video tape recorder playing back a previously recorded signal generated by a television camera. A flying spot scanner is a still further source of such a video signal. The information signal shown in Figure 4 is typically a one volt peak-to-peak signal having its informational content in the form of a voltage varylng witn time format is represented by a line 73. The maximum instantaneous rate of change of a typical video slgnal is limited by the 4.5 megacycles bandwidth. This video signal is o~ the type which is directly displayable on a televieion monitor.
The video slgnal shown in Figure 4 is applied to the frequency modulator 20 as shown in Figure 1. The modulator 20 generates the frequency modulated waveform 74 . ~
shown in Figure 5. The informational content of the waveform shown in Figure 5 is the same as the lnformation-al content of the waveform shown in Figure 4, but the form ' 25 i9 different. The informational signal shown in Figure 5 is a ~requency modulated signal having its informational content ln the form of a carrier signal having frequency changes in time about a center frequency.
B~ in~peation, it can be seen that the lower amplltude region, generally lndicated by a numeral 75, of the video waveform 73 shown ln ~igure 4, oorresponds to the lower ~requency portion of the frequency ~odulated signal 74 shown ~n Figure 5. One such cycle of the lower -frequency portion of the frequency modulated signal 74 is indicated generally by a bracket 75. A higher amplitude region, indicated generally by the numeral 77 of the video waveform 733 corresponds to the higher frequency.portions of the frequency modulated signal 74. One complete cycle of the higher frequency portion of the frequency modulated ` ~Z5~34 signal 74 is represented by a bracket 78. An intermediate amplitude region3 generally indicated with a numeral 79 o~ the video wave~orm 73, corresponds to the intermediate frequency portions o~ the frequency modulated signal 74.
A single cycle of the higher frequency portion o~ the frequency modulated slgnal representing the intermedlate amplitude reglon 79 is indicated by a bracket 79a.
By an inspection of Figures 4 and 5, it can be seen that the ~requency modulator 20, shown in Figure 1, converts the voltage varylng with time signal shown in Figure 4, to a frequency modulated signal as shown in Figure 5.
: Figure 6 illustrates the intensity o~ the writlng beam 29 generated by the write laser 30. The intensity o~
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' a~ter its passage through the light intensity modulating assembly 44. The intensity modulated writing ~ beam iq shown having a plurality of upper peaks 92 repre-; senting the higher light transmitting state of the light ~: intensity~modulatlng assembly 44, and having a plurality :~ of valleys 94 representing the low light transmitting state o~ the light intensity modulating 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 in the aqsembly 44.
Thi~ 108s i9 indicated by a line 96 showing the difference in the intensity of the light beam 29' generated by the laser 30 and the maximum intensity 92 of the light beam 29' modulated by the assembly 44.
This intensity modulation of the writing beam 29 to ~orm an intensity modulated writing beam 29' is best illustrated by an inspection of Figures 6 and 7. Figure 6 shows the unmodulatad beam 29 having a constant intensi~
represented b~ the line 80. Figure 7 shows the modulated beam 29' having maximum levels of intensIty indicated at 92 and minimum levels o~ lntensity lndicated at 94.

The intensity modulation of the writing beam 29 is compared to the rotational ef~ect o~ the Pockels cell 68 by reference to lines 98, lOO and 102. The intersec-tion of the line 98 with the line 29 l shows the intensity of the beam 29 ' issuing from the linear polarizer 70 when the Pockels cell 68 adds no rotation to the angle of polarization of the light passing therethrough. The inte~
section of,the line lOO with the line 29 ~ shows the lnten-sity of the beam 29 ' issuing from the linear polarizer 70 lO 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 ~rom 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, b~ the intensity modulated beam 29 l, shown in Figure 7 can best be understood by a comparison , 2~ between the two Figures 7 and 8.
- The line lOO 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 assembl~ 44. The line lOO

2~ represents the lntensity generated by the assembly 44 when the Pockels cell 68 rotates the angle of polarization of ~; the wrlte beam 29 passing therethrough through an angle ~; o~ forty-five degrees. Additionally, the line 100 repre-sents the threshold lntenslty o~ the modulated beam 29' required to ~orm an indicia ln the llght responsive coat-lng 26. This threshold ls reached upon rotatlon of the angle of polarizatlon of the write ~eam 29 through an angle of forty-five degrees.
By a comparison between Figures 7 and 8, it can

3~ be seen that an aperture 37 is formed while the Pockels cell 68 is rotating the angle of polarization o~ the write beam 29 passing therethrough between the angle of forty-five degrees and ninety degrees and back to forty-five degrees. No aperture is formed while the Pockels cell 68 i43~

is rotating the angle of polarization o~ the write beam 29 pass~ng theretl~rough between the angle o~ fort~-five degrees and ninety degrees and back tc forty-~ive degrees.
No aperture is formed while the Pockels cell 68 is rotating the angle of polarization o~ the write beam 29 passing therethrough between the angle of forty-five degrees and zero degrees and back to forty-five degrees.
Re,~erring agaln to Figure 3, there is shown a top vlew of the video disc member shown in radial cross-sectional view in Flgure 8. An inspection of this Figure3 is help~ul in understanding the manner in which the lineal series of light reflecting and light scatter-regions 38 and 37 are formed upon the video disc member 10. The disc member 10 is rotated at a preferred rota-tional rate of 1800 rpm and the indicia 37 and 38 areformed in the light responsive coating 2~ as shown with re~erence to Figure 8. ~he motion control assembly 28, shown with reference to Figure 1~ forms the apertures 37 in circular track-like fashion. A numeral 104 is employed to identify a section of an inner track, and a numeral 105 is employed to identi~y a section of an outer track. A
dashed line 106 represents the center line of the track 135 and a dashed line 107 represents the center line of the track 104. The length of a line 108 represents the distance between the center lines 10~ and 107 of adjacent tracks 105 and 104. Two mlcrons is a typical distance between center lines of ad~acent tracks. The width of an aperture 37 is indicated by the length of a llne 109. A
typlcal width o~ an aperture is one micron. ~he dlstance between ad~acent apertures is represented by the length of a line 110. This distance between ad~acent tracks ls known as the intertrack region and typically is one micron in length. The length of an aperture is represented by a line 112 and typically varies between 1.0 and 1.5 microns.
All o~ these dimensions depend upon many variables in the write apparatus. For example, these dimensions vary depending upon the frequency range generated by the fre-quency modulator 20, the size of the spot 42 formed by the write optical systems 41 and 42 and the rotatlonal llZSA34 speed selected for the disc 10.
Re~erring to Figure 99 there can be seen a more detailed block diagram of the motion control assembly ~8 shown with reference to Figure 1. The rotatlonal drive circuit 32 includes a spindle servo circuit 130 and a spindle shaft 132. The spindle shaft 132 is integrally ~oined to the turntable 21. The spindle shaft 132 is driven by a printed circuit type motor 134. The rota-tional motion provided by the printed circuit motor 134 0 i9 controlled by the spindle servo circuit 130 which phase locks the rotational speed o~ the turntable 21 to a signal generated by a color subcarrier crystal oscillator 136 whlch forms a portlon of the synchronlzation assembly ; 36. The synchronization assembly 36 further includes a ~irst divider circuit 138 and a second divider circuit 140. The first divider circuit 138 reduces the color subcarrier frequency generated in the oscillator circuit 136 down to a rotational re~erence frequency. The spindle shaft 132 contains a tachometer 143 for generating a frequency signal indicating the exact rotational speed of the shaft 132 and turntable 21 combination. The tach-ometer signal is available 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 ~, 25 line 142 is applied to the spindle servo clrcult 130 and the rotational reference signal on the line 144 is also applied to the spindle servo circuit 130. The spindle servo circuit 130 phase compares these two lnput signals.
When the phase of the tachometer signal leads the phase Or the rotational re~erence signal, the rate o~ rotation i8 too high and a sl~nal i8 generated ln the spindle servo circuit 130 ~or application to the motor 134 over a line 146 to slow the rotational speed and bring the tachometer signal into phase agreement with the rotational reference signal~ When the phase of the tachometer signal lags the phase of the rotatlonal reference signal as compared in the splndle servo circuit 130, the rate of rotation is too slow and a signal is generated in t'ne splndle servo clrcult 130 for application to the motor 134 over a line 112~;~34 148 to increase the rotat~onal speed and bring the phase of the tachome~er 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~135 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 member 10 is a distance of two microns.
The color subcarrier crystal oscillator 136 with , its two divider circuits 138 and 140 functions as an electrical synchronizing circuit for maintaining a 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 optical assemblies ill~strated in ~igures 1~ 10 and 11 are mounted on a platform indicated at 142. ~his movable platform is driven radially by the translational drive 34 which advances the platform 142 2.0 microns per revolution of the spindle shaft 132. This translational movement is radlal with respect to the rotating disc 10. This radial advancement per revolution of the spindle shaft 132 is identified as the pitch of the recording. Since the pitch uniformity of the finished recording depends on the steady advance of the optical 3 assemblies mounted on the platform 142, care ls taken to lap a lead screw 143 in the translation drlve 341 pre-load a translation drlve nut 144 which engages the lead screw 143 and make the connection between the nut 144 and the platform 142 as stlff as possible as represented by a bar 146.
Referring to Figure 10, there ~s shown a read apparatus which is employed for retrieving the ~requency modulated signal stored on the information storage member 10 as a lineal series of indicia 37 amd 38 previously i ~Z5434 described. A reading beam 150 is generated by a read laser 152 which produces a polarized$ collimated beam 150 of light. A support member9 such as the turntable 21, is employed for holding the information storage member 10 in a substantially predetermined position.
A stationary read optical assembly 15~ and a mova-ble optical assembly 156 define a read opt~cal path over which the read light beam 150 travels between the laser source 152 and the information storage me~ber 10. Addi-tionally~ either of the optical assemblies can be employedto ~ocus the light beam 150 upon the alternately posi-tioned light reflective regions 38 and the llght scatter-ing regions 37 carried in successive positions upon the information storage member 10. The movable optical assem-bly 156 is employed for collecting the reflections fromthe light reflective regions 38 and the light scattering regions 37. mhe motion control assembly 28 provides ; relative motion between the read beam 150 and the alter-nate regions of light reflectivity 38 and light scattering 37.
The optical assemblies 154 and 156 also define the optical path travelled by the beam reflected from the coating 26. The path of the reflected beam is identified by the numeral 150'. This reflected light path 150' includes a portioll of the initial read beam path 150. In those portions where the reflected beam 150' coincides with the read beam 150, both numerals 150 and 150' are used. A light sensing element 158 is positioned in the reflected light beam path 150' and ls employed for gener-3 ating a ~requency modulated electrical slgnal correspond-ing to the reflections impinging thereupon. The frequency modulated electrical signal generated by the light sensing element 158 is present on a line 160 and has lts informational content in the form of a carrier frequency having ~requency changes in time corresponding to the stored information. The output of the light sensing circuit 158 is applied to a discriminator circuit 162 by an amplifier 154. The discriminator clrcuit 162 is responsive to the output of the light sensing circuit ~25434 158 and is employed ~or changing the fre~uency modulated electrical signal into a time dependent voltage signal representing the stored information. The time dependent voltage signal is also identified as a video signal and it is present on a line 165. This time dependent voltage signal has its infor~ational content in the form of a voltage varying with time format and is suitable for display over a standard television monitor 166 and/or an oscilloscope 168.
The read optical assemblies 154 and 156 ~urther include a polarization selectlve beam splitting member 170 which functions as a beam polarlzer to the lncident beam 150 and which functions as a selective beam splitter to the reflected beam 150'. The read optical assemblies further lnclude a quarterwave plate 172. The beam polar-izer 170 filters out from the read beam 150 any spurious light waves which are not allgned with the axis of polar-lzation of the beam polarizer 170. With the axis of polarization of the read beam 150 fixed in a particular orientation by the member 170, the quarterwave plate 172 changes the plane of polarization from linear to circular.
The member 170 and the quarterwave plate 172 are disposed in the read light beam path 150. The member 170 is locat~
between the source 152 of the read beam 150 and the quarter-25 wave plate 172. The quarterwave plate 172 is also locatedin the reflected read beam path 150 t. Therefore, not only does the quarterwave plate 172 change the read beam polar-ization from linear to circular durin, its travel from the read laser 152 to the ln~ormation storage member 1OJ
but the quarterwave plate 172 further changes the clr-cularly polarized reflected light back into linearly polarized light which is rotated ninety degrees with respect to the preferred direction fixed by the source 152 and the member 170. This rotated beam 150' is selec-tively dlrected to the light 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 ~12~i434 compensated for by setting the initial intensity of the read beam 150 to a level sufficien' to offset this re-duction. The quarterwave plate 172 glves a total rota-tion of ninety degrees to the reflected beam 150 t with 5 respect to the incident beam 150 during the change from linear polarization to circular polarization and back to linear polarization. As previously mentioned, the member 170 is also a beam splitting cube in the reflected read beam path 150'. As the plane of polarization of the re-10 flected read beam 150' is shlfted ninety degree~ due toits double passage through the quarterwave plate, 172, the beam splitting cube portion of the member 170 dlrects the re~lected read beam 150' to the light sensing circuit 158 -' A suitable element ~or ~unctioning in the capacity of a light sensing element 158 is a photodiode. Each such element 158 is capable of changing the reflected fre-quency modulated light beam 150 ~ into an electrical signal having its information content in the form of a carrier frequency having frequency variations in time varying from the carrier frequency. The optical assemblies 154 and 156 further comprise the ob~ective lens 52 supported by a hydrodynamic air bearlng member 54 which supports the lens 52 above the coatlng 26 carried by the information storage member 10.
` As previously described, the read beam 150 is formed with substantially parallel light rays. The objec-tive lens 52 has an entrance aperture 56 larger in diameter than the dlameter of the read beam 150 as it is generated by the laser source 152. A planar convex diverging lens 174 is provided intermediate the laser source 152 and the entrance aperture 56 of the objective lens 52 ~or spreadlng the substantially parallel light rays forming the reading beam 150 into a light beam 150 having a diameter sufficient to at least fill the entnance aperture 56 of the obJective lens 52. The optical assemblies 154 and 156 further include a number of stationary or ad~ust-able mirrors 176 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.

:. .

hn optional optical fllter 180 is positioned in the re~lected 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 television monitor 166. This rllter 180 is essential when the read system is used wlth the write system as discussed in greater deta~l with re~erence to F ~ ure 11. In this read after write environment, a por-tion of the write beam 29 travels along the reflected read beam path 150'. The filter stops this portion o~ the write beam and passes the full intensity of the reflected beam 150'.
An optlonal converging lens 182 is positioned in the reflected beam path 150' for imaging the reflected beam onto the active area of the light sensing element 158. This converglng lens 182 reduces the diameter of the reflected beam 150' and concentrates the light lnten-slty of the reflected beam upon the actlve area of the llght senslng element 158.
The amplifier 164 amplifies the output o~ the light sensing element 158 and raises the amplitude of the frequency modulated electrical signal ~enerated by the light sensing element 158 to match an input slgnal requirement of the demodulator 162.
Referring again to the electrical and optical waveforms shown in Figures 4 through 7, these waveforms are also generated by the read apparatus, shown in Figure 10 during the retrleval o~ the frequenoy modulated signal stored in the coatlng 26 carried by the di~c 10. Flgure 6 3 shows a laser source generating a write laser beam having a constant intenslty represented by the llne 80~. The read laser 152 generates a read beam 150 having a constant in-tensity but at a lower level.
Flgure 7 shows an intens~ty modulated wrlte laser beam. The reflected read beam 150' is lntensity modulated by the act o~ implnglng upon the llght reflective and `llght scatterlng regions 38 and 37 carrled on the dlsc member 10. The reflected read beam 150' wlll not be a perfect squarewave as shown ln Figure 7. Rather, the .

ii2~A34 square edges are rounded by the fin~te size of the read spot.
Figure 5 shows a frequency modulated electrical signal having its informational content in the ~orm of a , 5 carrier signal having frequency changes in time varying about the center frequency. The output o~ the light sensing element 158 is the same type o~ signal. Figure 4 shows a video signal having its informational content in the f~rm of a voltage varying with time ~ormat. The output o~ the demodulator 162 is the same type of signal.
The motion control assembly 28 shown in Figure 10 operates in the same manner as the motion control assembly 28 shown in Figure 1. In the read apparatus J the motion control assembl~ 28 produces a rotational motion to the disc member under the control Or a rotational drlve assem-bly 32. The assembly 28 further produces a translational motion for moving the movable read optical assembly 156 radlally across the surface of the storage member.
~he assembly 28 further includes a synchronizing - 20 circuit for maintaining a constant relationship between the rotational motion and the translational motion so that the read beam 150 impinges upon the information tracks carried by the disc member 10. Portions of typical in-formation tracks are shown as 104 and 105 in Flgure 3.
Referring to Figure 11, there $s shown a block d$agram illustrating the comb$nation of the wrlte apparatus shown ln Figure 1J and the read apparatus shown in Figure 10. The elements shown in Figure 11 operate ln an ldentical manner as previously described and this de-30 talled operatlon is not repeated here. Only a brie~
descriptlon is given to avoid repetit$on and confusion.
The unmodulated write beam path i9 shcwn at 29 and the modulated beam path is shown at 29'. A first optical assembly de~$nes the modulated beam path 29' 35 between the output of the linear polarizer 70 and the coat$ng 26. The f$xed, wr$te opt~cal assembly 41 $ncludes the mlrror 58. me movable, write optical assembly 40 includes the diverging lens 66, a partially transmlssive mirror 200~ a movable mirror 60 and the ob~ective lens 52.

1~2~;434 The modulated write beam 29 ' is imaged to a write spot 42 upon the light responsive coating and interacts with the coating to for~ indicia as previously described.
The read beam path is shown at 150. The read optical assemblies define a second optical path for the read beam 150 between the read laser 152 and the 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 polarlzation shifting means 172, a second fixed mlrror 202, the partially transmisslve 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 ~rom the write spot 42, as is more completely described with refer-ence to Flgure 12.
The mirror 200 is a dichroic mirror whlch is transmissive at the wavelength of the write beam 29' and which is reflective at the wavelength of the read beam 150 '.
The intensity of the write beam 29 ' is higher than the intensity of the read beam 150. While the write beam 29 ' must alter the light responsive coating 26 to retain indicia representative of the video signal to be stored, the intenslty of the read beam 150 should only be sufficient to illuminate the indicia formed in the coating 26 and provide a reflected light beam 150' of sufficlent intensity to provide a E;ood ~ignal a~ter collectlon by the read optical assembly and conversion from an int~nsity modulated reflected beam 150 ' to a frequency modulated 3o electrical signal by the light sensing circuit 15~.
The fixed mirror 58 in the write optlcal path and the two fixed mirrors 176 and 202 in the read optical path are employed for directing the write beam 29' toward the ob~ective lens 56 at a controlled angle with respect to 35 the read beam 150. This angle between the two incident beams provides a spacing between the write spot 42 and read spot 157 as they are each respectively imaged upon the coating 26.
In operation, a sufficient spacing has been found ....

5~34 to be four to six microns. This distance corresponds to an angle too small to show clearly in Figure 12. Accord-ingly, this angle is exaggerated in Figure 12 for purpose Or illustration only.
The read beam 150' is demodulated in a discrim-inator clrcuit 162 and displayed on a standard television monitor 156 and an oscilloscope 16~. The television monitor 166 shows the picto.rial quality of the recording and the oscilloscope 168 shows the video signal in more detail. Thls read after write function allows the quality of the video signal belng stored during a write operation to be instantaneously monitored. In the event that the quality of the stored signal is poor~ it is known immedi-ately and the write procedure can be corrected or the information stora~e member lO storlng the poor quality video information signal can be discarded.
In the read after write mode of operation, the write laser 30 and the read laser 152 are operating at the same time. A dichroic mlrror 200 is employed for combinin~ the read beam 150 lnto the write beam 29'. In this read after write mode of operation, the wavel~ngth of the write beam 29 is chosen to be different from the wavelength of the read beam 150. ~n optical filter 180 is employed for bloc~ing any portion of a write beam which 25 has followed the re~lected read beam path. Accordingly, the optical ~ilter 180 passes the reflected read beam 150' and filters out any part o~ the write laser beam 29' ~ollow-lng the reflected read beam path 150'.
In the comparison mode of operationJ the read 30 after write operation i9 practlced as descrlbed wlth refe~
ence to Figure ll. When operating in thls monltoring mode of operatlon, a comparator circuit 204 comp~res the output o~ the demodulator 162 with the original video information signal provided by the source 18.
M~re speci~ically, the video output of 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 delay line 210. The delay line 210 .. ~. , ~125~34 imparts a time delay to the input video information signal equal to the accumulated values of the delay beginning with the ~requency modulation of the input video informa-tlon signal and extending through the frequency demodula-tion of the recovered electrical signal from the sensingcircuit 158. This delay also lncludes the delay of travel time from the point on said storage member 10 at which the input video information signal is stored upon the informa-tion s~orage member by the write spot 42 and continuing to the point of impingement of the read spot 157.
The correct amount of delay is best generated by making the delay circuit 210 a varlable delay clrcuit which ls ad~usted for optimum operation.
Ideally~ the video output signal of the discrim-inator 162 is identical in all respects to the video lnputslgnal on the lines 18 and 208. Any dif~erences noted represent errors which might be caused by imperfections in the disc's surface or malfunctions of the wrlting cir-cults. This application, while essential if recordlng dlgital information, is less critical when other informa-tion is recorded.
The output signal fro~ the comparator circuit 204 may be counted, in a counter (not shown) 3 ~or establishing the actual number of errors present on any disc. When the errors ccunted exceed the predetermined selected number, the writlng operation is terminated. If necessary, a new disc can be written. Any disc with excesslve errors can then be reprocessed.
In Figure 11, the comparator 204 compares the output signals available on the lines 208 and 206. An alternative and more direct connection o~ the comparator 204 i9 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 form, the slightly di~ering optical paths of the intensity modulated write beam 29' from the writing laser 30 and the unmodulated read beam 150 from the reading laser 152. The information storage member 10 is moving in the direction indicated by an arrow 217.

i~.Z~43 This shows an unexposed coating 26 approaching the wrlte beam 29' and a lineal series o~ apertures 37 leaving the intersection of the write beam 29~ and the coa~ing 26.
The writing beam 29' coincides with the optical axis of the microscope objective lens 52. The central axis of the reading beam 150 shown as 212 makes an angle with the central axis of the wrlte beam 29~ shown as 214. me angle is represented by a double headed arrow ~16. 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 1~7. The write spot 42 leads the read spots 157 by a distance equal to the length of a line 21B. 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 readlng allows the molten metal coating 26 to solidify so that the recording ls read in its final solidified state. If it were read too soon while the metal was still molten, the re~lection from the edge of the aperture would fail to provide a high quality signal for display on the monitor 166.
Referring to Flgure 13, there is shown an ideal-ized diagram of a Pockels cell s~abilizing circuit 48 suitable for use in the apparatus of Figure 1. As is 2~ known, a Pockels cell 68 rotates the plane of polari~ation of the applied write light beam 29 as a function of an applied voltage as illustrated with reference to Flgure 7.
gepending upon the individual Pockels cell 68, a voltage change of the order of 100 volts causes the 3 cell to rotate the plane of polarization o~ the light passing therethrough a ~ull ninety degrees. The Pockels cell driver functions to ampli~y the output from the in-formation signal source 12 to a peak-to-peak output swing o~ 100 volts. This provides a proper input drlving signal to the Pockels cell 68. The Pockels cell driver 72 gener-ates a waveform having the shape shown in Figure 5 and having a peak-to-peak voltage swing of 100 volts.
The Pockels cell should be operated at an average rotation of forty-five degrees in order to make the i modulated light heam intensity most faithfully reproduce the electrical dr~ve signal. A bias voltage must be pro-videq to the Pockels cell for keeping the Pockels cell at this average operating point. In practice, the electrical blas voltage corresponding to a forty-~ive degree rotation operating point varies continuously. This continuously changing bias voltage is generated through the use of a servo feedback loop. Thls feedback loop includes the comparison of the average value of the transmltted light to an ad~ustable re~erence value and applylng the differ-ence signal to the Pockels cell by means of a DC ampllfier.
Thls arrangement stabilizes the operatlng point. The reference value can he ad~usted to correspond to the average transmission corresponding to the forty-five degree operating point and the servo feedback loop provides cor-rective bias volta~es to maintain the Poc~els cell at this average rotation of forty-five degrees.
The stabilizing circuit 48 includes a light sens-ing means 225. A silicon diode operates as a suitable light sensing means. The diode 225 senses a portion 29'l of the writing beam 29' issuing from the optical modulator 44 and passing through the partially reflective mirror 58 as shown in Figure 1. The silicon diode 225 functions in much the same fashion as a solar cell and ls a source of electrical energy when llluminated b~ incident radiation.
One output lead of the silicon diode 225 is connected to common reference potential 226 by a line 227. The other output lead of the diode 225 is connected to one input of a dif~erential amplifler 228 by a line 230. The output leads of ~he silicon cell 225 are shunted by a load resis-tor 232 which enables a linear response mode.
The other input to the differential ampllfier 228 is connected to an adjustable arm 234 of a potentiometer 236 by a line 238. One end of the potentiometer 236 is connected to reference potential 226 by a llne 240. A
source of power 242 is cDupled to the other end of the potentiometer 236 ~hich enables the ad~ustment of the differential ampli~ier 228 to generate a feedback signal on the lines 244 and 246 for ad~usting the average power level o~ the modulated laser beam 29' to a predetermined value.
The output terminals of the differential amplifier 228 are) respectively, connected through resistive elements 248 and 250 and output lines 244 and 246 to the input terminals of the Pockels cell 68 in Figure 1. ~he Pockels cell driver 72 is AC coupled to the Pockels cell 58 by way of capacitive elements 2~2 and 254, respectively, while the differential amplifier 228 ls DC coupled to the Pockels cell 68.
In operatlon, the system is energlzed. The p~r-tion 29" of the light from the writing beam 29' impinging on the silicon diode 225 generates a differential voltage at one input to the differential amplifier 228. InitiallyJ
the potentiometer 236 is adjusted so that the average transmission through the Pockels cell corresponds to fort~
five degree of rotatior.. Thereafter, lf the average level of intensity impin~ing on the silicon cell 225 either in-creases or decreases, a correcting voltage will be gener-20 ated by the differential amplifier 228. The correctingvoltage ap~lied to the Pockels cell 68 is of a polarity and magnitude adequate to restore the average level of intensity to the predetermined level selected by ad~ustment of the input voltage to the other input of the differen-25 tial ampllfier over the line 238, by movement of themovable arms 234 along the potentiometer 236.
The adjustable arms 234 of potentiometer 236 is the means for selecting the average level of intensity of the light generated by the write laser 30. Optimum re-sults are achleved when the length Or an aperture 37 ex-actly equals the length of the next succeeding space 38 as previously described. The ad justment of potentiometer 236 is the means for achievlng this equality of length.
When the length of an aperture equals the length of its next ad~acent spaceg a duty cycle of fifty-flfty is achieved. Such duty cycle is detectable by examining the display o~ the just written information on the TV monitor and/or osclllosoope 166 and 168, respectively, as pre-viously descrlbed. Commercially acceptable results occur . ~

` 11;~5~L34 ~hen the length o~ an aperture 37 varies between ~orty and sixty percent of the combined length of an aperture and its next successively positioned space. In other words, the length of an aperture and the next successively posi-tioned space is measured. The aper'ure can then be alength ~alling within the range of ~orty and sixty percent of the total length.
Referring to Figure 8, there is shown a radial cross-section of an information track shown with reference to F~gure 3 1n 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 lO in the direction represented by the arrow 217. m is means that a reading beam impinges first upon the specular light reflective region 38a followed by its impingement upon the non-specular light reflective region 37a. In this configur-ation, the positive half c~Jcle of the signal to be re-corded is represented by a specular light reflectiveregion 38a and the negative half 'c~cle of the signal to be recorded is represented by the non-specular light reflectlve,region 37a. The duty cycle of the signal shown with ~.eference to Fi~ure 8 is a fifty percent duty cycle insofar as the length of the specular light re-flected region 38a as represented by a bracket 260, is equal in length to the length of the non-specular light reflective region 37a as represented by the bracket 262.
This preferred dut~ cyole set up by the comblnatlon o~
adJusting the absolute intensity of the wrlte beam 29 by ad~usting the power supply of the write laser 30 and by ad~usting the potentlometer 236 in the stabilizing circult 48 to a level wherein an aperture ls formed beginning with a forty-five degree rotation of the angle of polarization in the write beam 29.
Referring again to the aperture forming process illustrated with reference to Figures 7 and 8, meltlng of a thin metal coating 26 occurs when the power in the light spot exceeds a threshold characteristic of the composition 3.~ L34 and thickness o~ the metal film and the properties o~ the substrate. The spot power ls modula'ed by the light intensity modulating assembly 4~. The on-o~ transitions are kept short to make the location o~ ~he hole ends pre-cise in spite of variations in the melting threshold.Such variations in the melting threshold can occur due to variations in the thickness of the metal coating and/or the use of a dif~erent material as the information storing layer.
The average power in the spot required to form an aperture in a thin metal coating 26 having a thickness between 200 and 300 Angstroms is of the order of 200 milliwatts. Since the FM carrier frequency is about 8 MH7, 8 x 106 holes of variable length are cut per second and the energy per hole is 2.5 and 10-9 joul.
In this ~irst embodiment of a video disc member 10, a portion of the glass substrate is exposed in each aperture. The exposed portion of the glass substrate appears as a region o~ non-specular light re~lectivity to an impinging reading beam. The portion of the metal coating remaining between successively positioned apertures appears as a region o~ high light reflectivity to an im-pinging reading beam.
When the forming of first and second lndicia is being undertaken using a coating of photoresist~ the inten-sity of' the write beam 29' is adjusted to a level such that a forty-~iYe deg~ee rotation of the plane o~ polar-i-zation generates a light beam 29' of threshald intensity for exposing and/or interactl~g with the photoresist coatlng 26 while the photoresist coating ls in motion and posltioned upon the moving information storage member 10.
The Pockels cell 68 and Glan-prism 70 combination com-prises a light intensity modulating member which operates from the forty-five degree setup condition to a lower light transmitting state associated with a near zero degree state of operation, to a higher light transmitting state, associated with a near ninety degree state o~ operation.
When the intensity o~ the write light beam 29l increases above the initially adjusted level or predetermine start intenslty, and increases ~owards the higher llght trans-mitting state the incident write light beam 29' exposes the photoresist illuminated thereby. This exposure con-tinues after the intenslty of the write beam reaches the maximum light transmittlng state and starts back down towards the initial predetermined intensit~ associated with a forty-~ive degree rotation of the plane of polarl-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 Glan-prism 70 drops below the threshold intensity at which the focused write beam ~ails to expose the photoresist illuminated thereby. This failure to expose the photoresist illumln-ated thereby continues after the intensity of the write 1~ beam reaches the minimum light transmitting state and starts back up towards the initial predetermined intensity associated with a forty-five degree rotation of the plane of polarization of the light issuing from the write laser 30.
The Pockels cell driver circuit 72 ~s typically a high gain and high voltage amplifier having an output signal providing an output voltage swing of 100 volts.
~his signal is intended to match the driving requirements - of the Pockels cell 68. Typically, this means that the mid-voltage value of the output of the Pockels cell driver 72 provides a suf~icient control voltage for d~ving the Pockels cell 68 through ~orty-~ive degrees so that about one half of the total available light ~rom the laser 30 issues ~rom the linear polarizer 70. As the output signal fro~ the driver 72 goes positive, mid-voltage value, more light from the laser is passed~ As the output signal ~rom the driver 72 goes negativeJ less light ~rom the laser ls passed.
In the first embodiment us~ng a metal coating 26, the output from the laser 30 ls adjusted so as to produce an intensity which begins to melt the metal layer coating 26, positioned on the disc 10, when the output from the driver ~2 is zero and the operating polnt of the Pockels cell is ~orty-f~ve degrees. Acoordingly, as the output from the driver 72 goes P~sitl~a, melting continues. Also, when the output ~rom the dr~ver ~2 goes negative, melting stops.
In a second embodiment using the photoresist coating 255 the output ~rom the laser 30 is adjusted so as to produce an intensity which both illuminates and exposes the photoresist coating 26 when the output from the driver 72 is generating its mid-voltage value. Accordingly, as the output from the driver 72 goes positive, the illumina-tion and exposure of the photoresist illuminated by thewrite beam continues. A1SOJ when the output from the driver 72 goes negative, the illumlnation continues but the energy in the write beam is insuf~icient to expose the illuminated region. The term expose is herein being used for its technical meaning which describes that physical phenomenon which accompanies exposed photoresist. Exposed photoresist is capable o~ being developed and the developed photoresist ls removed by standard procedures. Photo-resist which is illuminated by light, insuf~icient ln intensity to expose the photoresistl cannot be developed and removed.
In both the first and second embodiments just described$ the absolute power level 80 illustrated by the line 80 in Figure 6 is ad~usted upward and downward to achieve this effect by ad~usting the power supply of the write laser 30. In combination with this ad~ustment of the absolute power level of the write laser 30, the potentlometer 236 is also used to cause indlcia to be formed in the coating 26 when the beam 29 is rotated above 3 forty-five degrees as prevlously descrlbed.
In a read only system as shown in ~igure 1OJ the optical rilter 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 requ~r~ng a slight increase in the intensity of the read laser 152 to insure the same intensity at the detector 158 when compared to a read only system which does not use a filter 180~
The converging lens 182 is optional. In a properly arranged re~d system the reflected read beam 150' S~4 has essen~ially the same diameter as the working area o.
the photodetector 15~. If this is not the case, a con-verging lens 182 is employed for concentrating the re-flected read beam 150' upon the smaller working area of the photodetector 158 selected.
Prior to giving the detailed mode of operation of an improved vers~on 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 writing laser source as it impinges upon the master video disc is employed to interact with ~he in~ormation bearing portion of the video dlsc to form indicia representing the carrier frequency and the f`requency variations in tlme from the carrier 15 frequencY-- The threshold power level required of the laser beam at the point of impact with the information bearing layer of the video disc differs depending upon the mater-ial from which the in~ormation bearing layer is made.
20 In the two examples given hereinabove describing a metal such as bismuth and a photosensitive material such as photoresist, the threshold power level required to form indicia differs significantly and represents a good ex-ample for illustrating the term threshold power. Obvious-25 l~ the threshold power of other materials would alsodif~er from each of the examples explained.
The lndicia formed in a bismuth coated video disc master are alternate regions o~ light reflectivity and light non-reflectlvity. The areas o~ light non-reflecti-30 vlty are caused by the melting of the bismuth ~ollowed bythe retractlng of the bismuth be~ore 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 e~posed portion of the glass substrate 35 ls absorbed and hence light non-reflectivity is achieved.
The threshold power is thàt power ~ro~ 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 ~L~Z~ 3~

represented as that intens~ty of a decreasing l~ght inten-sity signal when the metal layer ceases to melt and retract from the region having incident light impinging thereupon. More specifically, when the power in the impinging light beam exceeds the threshold power require-ments of the recording material, a hole is formed in the recording material. When the light power intensity in the impinging light beam is below the threshold power level of the recording material, no hole is formed in the recording medium. The forming of a hole and the non-form~ng of a hole by the impinging light beam is the principal manner ln which the light beam impinging upon a bismuth coated master interacts with the bismuth layer to form indicia on the recording surface. The indicia represents a carrier frequency havlng 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 requirlng a suffi-cient number of photons ln the impinging light beam to expose a portion of the photoresist. When the positive going modulated light beam contains sufficient photons above this threshold power level, the photoresist in that area is exposed so that subsequent development removes the ; exposed photoresist. When the photon level in a decreas-ing light intensity modulated light beam falls below the normal threshold power level of the photoresist, the phot~
resi~t caases to be exposed to the extent that subsequent development does not remove the photoresist illuminated by an impinging light beam having photons below the threshold power level.
The impinging light beam from the modulated laser source interacts with the information bearing layer to fully expose or underexpose the photoresist layer illumin-ated by the impinging light beam. This is an i~teraction of the photons in the impinging light beam with the infor-mation bearing member to form lndicia of the carrier fre-quency having frequency changes in time varying about the ' 5~3~

carrier ~requency. The indicia stor~ng the carrier ~re-quency and ~requency change in time are more fully appre-ciated a~ter the development step ~!hereby those por~ions of fully exposed photoresist material are effectively removed leaving the undere~posed 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 maintainlng the operating bias on the Pockels cell ~ at the hal~
power point. ~he DC bias of the Pockels cell is ~irst adjusted to its steady state condition such that the half power point of the Pockels cell-Glan prism combination coincides with the forty-five degree rotation point o~ the Pockels cell 68. Thls DC bias point is identified as the ~ixed blas 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, when the video information input 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 ~or developing the voltage required to drive the Pockels cell through its zero to ninety degree rotational shift. The unmodulated light beam from the laser 29 is applied to the Pockels cell 68 as previously,described.
The purpose of the Pockels cell bias servo is to bias the Pockels cell 68 so that the output llght signal detected at a photo diode 2~0 i9 as free of second harmonic content ~s posæible.
The second harmonic distortion is introduced into the modulated light beam at 29' ~rom a plurality of sources.
A first o~ such sources is the non-linear transfer func-tions of both the Pockels cell 68 and the Glan prism 70.
Ilhen 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 ,, :~lZ5~3~

beam at 29'.
The Pockels cell bias servo functioIls to adjust the DC bias applied to the Pockels cell 58, which DC
bias biases the Pockels cell to its half power point, so as to minimize the second harmonic content of the output light beam.
The change in DC bias level from the half power point ls achieved in the following sequence of steps. The modulated light beam 28' from the Pockels cell 68 is applied to a photo diode 260. The photo diode 260 oper-ates in its standard mode of operation and generates a signal having the form of a carrier frequency with fre-quenc~r variations about the carrier frequency. This fre-quency modulated waveform is a sufficiently linear repre-sentation of the light impinging upon the photo diode 261 to accurately reflect the signal content of the light modulated beam 29' impinging upon the disc surface. More specifically, the output signal from the photo diode 260 contains the distortion products present in the modulated 20 light beam 29'. The output from the photo diode 260 is applied to a second harmonic detector 261 over a line 262 which forms a part of the bias control circuit 264. The output from the second harmonic detector is to a high voltage amplifier 266 which generates the DC bias slgnal ; 25 over a line 268. The line 268 is connected to a summation circuit 270 which has as its second input slgnal the output from the Pockels cell driver 72, The DC bias signal on the line 268 is summed wlth the output ~rom the Pockels cell drlver 72 and is applied to the Pookels cell 68 ~or chang-ing the DC bias of the Pockels cell 68.
Re~erring back to the operation of the second harmonic detector 261, this device generates a voltage which is approximately linear in the ratio of the second harmonic to the fundamental of the output llght beam.
Furthermore, the output signal reflects the phase charac-teristics of the second harmonic and if 'he second harmonic is in phase with the fundamentalg the output o~ the second harmonic detector is in a first voltage level, ie.g a positive level. If the aecond harmonic is opposite in ~2S4;~

phase with the ~ul~damental; then the output ef the second harmonic detector is at a second voltage level~ ie., a negative vcltaOe level. The output ~rom the second harmo~
detector is amplified through a high voltage amplifier 266 which provides a range of 7ero to three hundred volts of DC bias. This DC bias is summed with the si~nal from the FM modulator 20 amplified in the Pockels cell driver 72 and applied to the Poc~els cell 68.
The second harmonic detector includes a limiter 272 shown with reference to Figure 24 and a dif~erential amplifier 274 shown with reference to Figure 24. The out-put signal from the photo diode 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 di~ferential amplifier 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 limiter 272 are logical comple-ments of each other. More specifically, when one output is at a relatively high voltage level, the other output is at a relatively low voltage level. The two output signals on the legs 280 and 2~2 are fed into the differen-tial amplifier 274. The output of this differential ampllfier 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 from the photo diode 260 is substantially free o~
æecond harmonic distortion; then the output sig~al from the dlfferential ampllfier 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 condltion, the ef~ective DC level of these two half cycles offset one another. Accordingly, the output of the diffe~
ential amplifier 274 is, on the average, zero.
When ~. degree of second harmonic distortion is pr~sent ln the output from the photo diode 260, the value ~12~34 of harmonic dlstortion shifts the mean value crossing ~ro~
a symmetrical case to a non-symmetrical case. In this situation5 the outpu~ ~rom the di~ferential amplifier is other than a squarewave with a fi~t~-fifty duty cycle.
~he differential amplifier therefore detects the effec-tive DC level shift of the incoming s~gnal and generates an output which is above or below sero, on the average, depending upon the asymmetrical nature of the input signal.
The output o~ the differential amplifier is therefore applled to the high voltage ampli~ier which DC smooths the output from the di~ferential amplifier 274 and amplifies the negative or positive resulting DC level. The resulting product is the required shift in bias signal for applica-tion to the Pockels cell to return the operating point of the Pockels cell to the half power point at which ~ero harmonic distortion occurs.
A summary of the standard operating mode of Pockels cell bias servo includes the generation of a light signal representing the distortion products present on the modulated light beam. Means are provided for dete~t-ing the value of second harmonic distortion present in this light beam and generating a signal representation of this distortion. The signal generated to represent the amount of second harmonic distortion also includes whether the second harmonic distortion is in phase with the fundamen-tal frequency or the second harmonic distortion is out of phase with the fundamental frequency. The output signal representing the amount of second harmonic dlstortion and the phase of the second harmonic distortion with reference to the fundamental frequency ls applied to a means for generating a bias signal necessary for appllcation to the Pockels cell to bring it to an operating point at which second harmonic distortion ceases to exist. A summation circuit is provided for summing the change in bias signal wlth the lnput frequency modulated video signal. This summed voltage is applied as an input to the Pockels cell 68.
Fi~ure 14 shows a series of waveforms illustraing an improved form of light modulation of a writing light 25~34 beam 29. Line ~ of Figure 14 shows an idealized or simpl~
ried video waveform that is ty~ically 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 circult 20. ~!0 output signals are shown on lines ~ and C~ and each is an FM modulated output signal and each carries the same frequency information.
The waveform on line B is a ~epeat of the waveform in Figure 5 and ls repeated here ~or convenience. ~his 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. Eo~h 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 waveform ~ and C are at all times identical and each represents the voltage level of the video waveform shown in line A. By inspection9 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 amplltude regions of the video waveform as gener-ally indicated at 77 corresponds to the higher frequency shown in lines ~ and C. It is the custom and practice of the televi~ion lndustry to utllize a one volt peak to peak voltage signal having voltage varlations in time as the video signal generated by a television camera~ These signal characteristics are the same required to drive a television monitor 166. The advantage of using a triangu-lar shaped waveform for driving a Pockels cell 68 is to 3~ 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 ~he Pockels cell and to the Glan prism 78. The triangular waveform shown in line C is a linear voltage change with 3~12~

tlme. The linear voltage change versus time o~ the tri-angular driving uaveform when multiplied b~ a sinusoidal voltage change versus light trans~er function of the Po~kels cell 68 gives a sinusoidally varying light inten-sity output from 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 ls driven by the triangular waveform shown on line C.
Re~erring specl~ically to the bottommost point at 285 and the topmost point at 286 o~ the waveform shown on line D, the point exactly equally distant from each is identified as the half power point. An understanding o~
the utilization of this hal~ power point ~eature is re-quired ~or high quality mastering operations.
The peak to peak voltage of the triangular wave-~orm is represented by a first maxlmum voltage level V2 shown on line 287 of line C and by a second minimum vol-tage level Vl on line 288. The voltage di~ferential between points 287 and 288 is the driving voltage for the Pockels cell 68. This voltage differential is ad~usted to equal that voltage required by- the P~ockels cell 68 to give a ninety degree rotation o~ the output polarization of the light passing through the Pockels cell 68. The bias on the Pockels cell is maintained such that voltage levels Vl and V2 always correspond to the zero degree rotation and the ninety degree rotation respectively of the light beam passlng through the Pockels cell 68. The forty-~ive degree rotation Or a light beam is half way between the two extremes of a trlangle waveform. That half-way voltage is alway~ 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 powçr volt-age polnt to dri~t also. The correct biasing of the half-way voltage is completely described hereinafter with re~e~ence to Figures 18J 19 and 20.
While the waveform shown on line C of Figure 14 shows the triangular wave shape generated by the FM modu-lator 20J it also represents the wave shape of the signal , ~ ~

. .

generated by the Pockels cell driver 7~. The output from the FM modulator is typiGally in a smaller voltage range, typically under 13 olts wh11e the output frcm the Pockels cell driver 72 typically swings 100 volts in order to provide suitable driving voltage to the Pockels cell 68 to drive it from lts zero rotational state to its ninety degree rotational state. In discussing the voltage levels Vl and V2 and the lines 288 and 2B7, respectively, repre-senting such vcltage points, reference is made to line C
of Flgure 145 because the output from the Pockels cell driver 68 has the identical shape while dlffering in the amplitude of the waveform~ This was done for convenience and the elim~nation of a substantially identical waveform dif~erent only in amplitude.
Referring to Flgure 153 there ls 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 304 is formed to top the upper surface 302 of the substrate 300. The in~ormation bearing layer 304 is of uniform thickness over the entire surface 302 of the sub-strate 300. The in~ormation layer 304 itself has a planar shaped upper surface 306.
Figure 5 is shown positioned beneath line C of Figure 14 showing the intensity of the llght beam passing from the Pockels cell - Glan prism combination in the improved embodiment which utilizes a voltage control oscillator in the FM modulator 20 generating a triangular 30 shaped output waveform as the driving waveform shape to the Pockels cell 68. As previously described, the thres-hold power level of the in~ormatlon bearlng layer is defined as that power requlred to form indicia in the information bearing layer in response to the lmplnging light beam. For a metal surface, the t~mal threshold point is that power required to melt the metal layer and have the 0etal layer retract from the heated region of impingement. For a photoresist layer, the threshold power is that power level required to supply sufficient photons , -~6-to completely expose the photoreslst information bearing layer ln the case o~ the metal layer, the heated metal retracts from the impinging area to exp~se the substrate 300 position thereunder. In the case of the photoresis~
material, the photon power is sufficient to fully expose the total thickness of the photoresist layer 324 completely down to the upper surface 322 of the substrate 320 as shewn in Figure 7.
It has been prevlously discussed how the half power polnt of the Pockels cell - Glan prism combination is located at a point halfway be~ween a first operating point at which maximum transmission from a fixed intensity beam passes through the ~lan prism and a second operatlng 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 passlng through the Pockels cell has been rotated forty-five degrees from the p~int of zero power transmission.
In operation, the output power from the laser is adjusted such that the half power point of the Pockels cell-Glan prism combination provides sufficient energy 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 Pockels cell-Glan prism com-bination ensures highest recording fidelity of the videofrequency signal to be recorded and ensures minimum inter-modulation distortion of the signal played back ~rom the video disc recording member.
This matching o~ the power levels i8 illustrated 3 with re~erence to line D of Figure 14 and Figure 15 and by the oonstructlon lines drawn vertically between the half power point represented by the line 290 shown on llne : D of Figure 14 and the apertures shown generally at 310 in Flgure 15. The length o~ an aperture 310 i-9 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 tr:angular wave ., ~, .

shape shown on line C of Figure 14. The zero crossing points are represented b~r lines 291 and 292 shown in Figures 14~ and 155~ and the importance of regulating the half power point is explained in greater detail with refer-ence to Figures 20 and 21.
Figure 16 shows an informat~on storage memberincludlng 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 l~ght responsive layer just as the metal bismuth layer 304 is a light responsive layer. Both the thin opaque metallized coating 304 and the photoresist layer 324 function to retain indicia representative of the video input signal. In the case of the metal layer 304, aper-tures 310 are formed in the metallized layer to form successive light reflective and light non-reflective regions in the information storage member.
Referring to Flgure 17 showing the photoresist coated information storage member, regions 330 are formed in substantially the same manner as regions 310 were formed with reference to the structure shown in Figure 15.
Rather than apertures 310 be~ formed as shown with refe~
ence to Figure 6, exposed regions 330 are formed corres-ponding to the apertures 310. The exposed photoreslst material is represented in Figure 16 by cross hatching of the regions in the photoresist information storage layer 324. Subsequent development of the exposed photoresist material removes SUCIl exposed photoresist material leaving apertures comparable to the apertures 310 shown with refer-ence to Figure 15.
In operation, when using the photoresist coated substrate video disc member, the output power of the writ-ing laser is adJusted such that the power of the modulatedlaser beam passing through the Pockels cell-Glan prlsm 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 .

beam. Just as with the bismuth coated master video disc system, this ensures highes~ f~delity recording and minimum intermodulation distortion during tlle playback of the re-corded video signal.
In re~erring to both Fi~ures 15 and 16, that por-tion of the light beam passing through the Glan prism above the half power point as represented by that portion o~ the wave~orm shown on line D of Figure 14 which is above the llne 2905 causes an irreversible change in the character-istics of the light sensltive surface 304 ln the case of the bismuth coated vldeo disc shown in Figure 15 and the photores~st coating 324 shown with reference to the photo-resist coated video disc shown ln Figure 16. In the case of the bismuth coated video disc member 300, the irrever-slble changes take the form o~ successively formed aper-tures 310 in the opaque metallized coating 304. In the case of the p~otoresist coated substrate 320, The irrever-sible alteration o~ the characteristic o~ the photoresist layer 324 occurs in the form of successive fully exposed regions 332.
While bismuth is listed as the preferred metal layer$ other metals can be used such as tellurium, inconel and nickel.
Referrlng to Figure 18, ~here is shown the trans-fer characteristic of the Pockels-Glan prism combination as a result of the sinusoidal rotatinn in degrees of the light passing through the Pockels cell 68 versus linear voltage change of input drive to the Pockels cell 68.
The ninety degree rotation point ls shown at point 340 and equals the maxlmum l~ght transmission through the Glan prism 70. The zero degree rotation point i9 shown at points 342 and equals the zero or minimum light trans-misslon through the Glan prism 70. The zero light t~ans-mission point 342 corresponds to the voltage level Vl represented by the line 288 in line C of Flgure 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 halr way between these two voltages repre-sented by the line 292 is equal to V2 minus Vl over Z

~1~2543g~

-4~-and corresponds to a forty-five degree rotation of the light beam passing through the Pockels cell.
As is well knowng the power through the Pockels cell is substantially unchanged. The only characteristics being changed in the Pockels cell is the degree of rota-tion of the light passing therethrough. In normal practice, a Pockels cell 58 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 polarized at ninety degrees rotatlon passes substantially undiminished through the Glan prism. When the same highly p~larized light is rotated by the Pockels cell 68 for ninety degrees rotation back to the zero degree rotation, the light beam does nDt pass through the Glan prism.70. In actual prac-tice, the full transmission state and zero transmission state is not reached at high frequencles o~ operations.
The waveform shown in Figure 18 shows the transfer charac-teristics of the Pockels cell 68 rotated to correspond witn two cycles of frequency modulated video information.
This demonstrates that the transfer function continuously operates over the zero to ninety degree portion of the transfer function curve.
Referring to Figure 199 there is shown the trans-fer characteristic of a Glan prism 70. At point 350,maximum transmisslon through the Glan prism 70 is achieved with a ninety degree rotation of the incoming light beam.
At point 352, minlmum or zero light transmission through the Glan prism 70 ls achieved at zero rotation of the 3 incoming light bea~. Hal~ o~ the intensity o~ the imping-ing light beam is passed through the Glan prism 70 as indicated at points 354 which corresponds to forty-five degrees rotation of the light entering the Glan prism 70.
Obviously, the absolute power o~ the light passing through the Glan prism 70 at the forty-flve 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 ~ ~`S~34 ~rom the writing laser 30 is adjusted such that the inten-sity of the light passing tnrough tlle Glan prism at the half power point coinc~des wit'l the threshold power level of the recording medium. Since more power is required to melt a bismuth la-~er than is required to fully expose a photoresist layerg the absolute intensity of a writing beam used in writing on a bismuth master disc is greater than the lntensity of a writing laser u~ed to lnteract with a photoresist covered master video disc.
Referring collectively to Figures 20 and 21, there is shown a series of waveforms useful in explaining the relationship between length o~ a hole cut in a master vldeo disc by the writlng laser 30 and the length of uncut land area between successively ~ormed holes. This rela-tionship can be referred to as a relationship formed by the value of the pea~ cutting power, ~he average cutting power and focus of the spot on the metal layer. Collec-tively, these terms have evolved into a single term known as duty cycle which term represents all three such charac-20 teristics.
As previously described, the energy required tointeract with the information bearing layer on video disc substrate is that energy necessary to cause irreversible changes in the material selected for placement on the master video disc memberO In the case of a bismuth coated masterS the energy required is that needed to selectively remove the portion Or the bismuth coated layer in those locatlons when the energy is above the threshold energy level of the bismuth layer. If this energy contained ln the spot of light is not focused properly upon the bismuth layer, then the energy cannot be used for lts lntended function and it will be dissipated without effecting its intended function. If some cutting occurs due solely to an out of focus spot, distortions are introduced into the 35 mastering process.
If the peak cutting power greatly exceeds the threshold power level of the recording medium, destructive removal o~ material occurs and provides a surface con-taining distortlon products caused by this destructive .

removal. ~he average cutting power is that power at a polnt midway between a ~irs~ 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 intensity of the light beam above the average cutting power lnteracts with the informatlon bearing layer to form indicia of the signal to be recorded. The intensity o~
the light beam below the average cutting power fails to heat a bismuth coated master to a polnt needed in the hold forming process or fails to ~ully expose a portion of a photoresist coated master.
Referring brlefly to lines B and C of Figure 14, the ad~ustment o~ the average cutting power to coincide with the line 291 shown in line 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 position and successively thereafter.
Thls ls known as a 50~ or fifty-fifty duty cycle. A
fifty-fifty duty cycle is the preferable duty cycle in a recordlng procedure but commercially acceptable playback slgnals can be achieved in the range from sixty-forty to forty-sixty. This means that either the hole or the inte~
;~ venlng land member becomes larger while the other member 2~ becomes smaller.
Referring to Figure 20~ there is shown a waveform ~` represented by a line 350 corresponding to two cycles of the light intenslty transmitted through the Pockels cell-Glan prlsm comblnatlon and represented more specifically on llne D of Flgure 14. The threshold power level of the recordlng medlum is represented by a line 362. The ~hreshold power level of the reading medium ls caused to be equal to the half power point of the light intensity transmitted by the Pockels cell-Glan prism combination by ad~usting the absolute intensity of the writlng laser 30.
l~en the threshold level is properly adjusted at the half power point, an indicia is ~ormed on the information surface layer of the master vldeo disc begin-nlng at point 364 and continuing for the time until the , ~Z5~34

-5~-intensity ~alls to a point 366. Dash lines shown at 364 ' and 356 ~ are drawn to line A of Figure 12 showing an indicia represented by the ecllpse 36& 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 366. The light intensity below point 366 falls to a minimum at 372 and continues to rise towards a new maximum at 374. At a certain point between the lower intensity level 372 and the upper intensity level 374, the llght intensity equals the threshold power level of the recording medium at 376. ~eginnlng at point 376, the energy in the light beam begins to form an indicla represented by the eclipse 378 shown on line A of Figure 20. A dotted line 376 l shows the start of forma-tion of indicia 378 at the point when the llght intensityexceeds the threshold level 362. The indicia 378 con-tinues to be formed while the light intensity reaches a maximum at 374 and begins to fall to a new minimum at 375.
However, at the intersection of line 360 with the thres-hold power level at 362 the light intensity falls belowthe threshold power level and the indicia is no longer formed. In the preferred embodiment, the length of the indicla represented by a line 384 ~uals the length of the land region shown generally at 386 as represented by the 2~ length of the line 388. Accordingly, the matching o~ the half power point light intensity output from the Pockels cell-Glan prism combination wlth the threshold power level of the recording sur~ace results in a fifty-flfty duty cycle wherein the length of the lndicia 368 equals the length of the next s~cceeding land region 386. Points 364, 366, 376 and 382 shown on the llne 360 represent the zero crosslng of the original frequency modulated video signal. Hence, it can be seen how the indicia 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 adJusting 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.

~Z~3~
-5~
The wave~orm shown with reference to Figure 20, including the variable light intensity represented by the line 350, represents a preferred mode of operation to achieve 50/50 duty cycle independent of the recording medium empl~yed on the master vldeo disc member. The absolute intensities at the various points change accord-ing to the absolute intensities requlred for the modulated llght beam to lnteract with the recording sur~e, but the relative wave shapes and ~helr relative locations remain the same. More speci~ically, the absolute intensity of the threshold power level for bismuth ls dlfferent than the absolute intensity of the threshold power level for photoresist, but the relatlonshlp with the intensity line 360 is the same.
Referring to the combination of Figure 20 and line B of Figure 21, there will be described the results of failing to match the hal~ power point output of the Pockels cell-Glan Prism combination with the threshold power level of the recording medium. Referring to Figure 20, a second dash llne 380 represents the relationship between the actual threshold power level of the recordlng medlum belng used with the light intensity output from the Pockels cell 68-Glan Prism 70 combinatlon. The thres-hold power level line 380 ~ntersects the intensity llne 360 at a plurality of locations 390, 392, 394 and 396. A
line 390' Yepresents the lntersection o~ the llght inten-slty line 360 with the threshold power level 380 and slgnals the start of the formatlon of an lndicla 398 shown on llne B of Figure 21. The indicla 398 is ~ormed during the time that the light lntensity ls above the threshold power level. The length of the indlcia 398 i9 rspresented by the tlme rçqulred for the light intenslty to move to its maximum at 370 and fall to the threshold polnt 392 as ls shown by a line 399. The length of a land area lndi-cated generally at 400 has a length represented by a line402. The length of a line 402 is determined by the time required for the light intensity to move ~rom threshold point 392 to the next threshold point 394. During thls time, the intensity of the llght beam is sufficiently low . ~

i~25434 as to cause no interaction wlth the recording medium. A
second ind~cia is sho~n at 405 and ~ts le~lgth corresponds with the point at which the intensity o~ the waveform represented by the line 360 exceeds the threshold power level at point 39~. ~he length of the indicia 406 is shown b a line 408 and is determined by the time required ~or the light intensity to rise to a maximum at 374 and fall to the threshold level at point 39~.
Various lines are shown lndicating the beginning and ending of the indicia and intravening land ar0as by employlng primed numbers to identify the corresponding intersections of the light intensity line 360 with the threshold power level llnes 362 and 380.
The successively positioned indicia 398 and land region 400 represent a slngle cycle of the recorded fre-quency modulated video signal. The indicia 398 repre-sents approximately 65 percent of the sum of the length of the line 399 and the line 402. This represents a duty cycle of 65/35 percent. Sixty-~ive percent of the 20 available space is an indlcia whlle thirty-five percent of the available space is land area. T~pically, the indicia in the final format is a light scattering member such as a bump or hole, and the land area is a planar sur~ace covered with a highly reflective material.
The frequency modulated video information repre-sented by the sequentially positioned light non-reflective member 368 and llght reflective member 386 shown in line A
of Figure 21 represents the preferred duty cycle of 50/50.
When the photoresist mastering procedure ls employed, the 30 re~leotivity of the upper sur~ace o~ the photoresist layer i8 not ~ignificantly altered by the implngement of the writing beam such as to be able to detect a difference between reflected light beams from the developed and not developed portions of the photoresist member. It ls 35 because of this ef~ect that a read-after-write procedure, uslng a photoresist coated master video disc is not possible.
Referrlng to line C of Figure 21~ there ls shown a representatlon o~ the recovered vldeo signal represented 4~4 by the sequence o~indlcia 368 and land area 386 shown on line A. The waveform shown in l~ne C is an undistorted sine wave 410 and contains the same undistorted frequency modulated information as represented by the light intensity waveform represented b~J the line 350 shown in Figure 11.
The sine wave shown in line C of Figure 21 has a center llne represented by a llne 412 which intersects the sine wave 410 in the same points of lntersectlon as the line 362 intersects the intensity line 360 shown in Figure 20.
~eferrlng to line D of Figure 21, there is shown a recovered frequency modulated vldeo signal having bad second harmonlc distortion. The fundamental frequency of the waveform represented by a line 414 shown in line D
is the same as that contained in the waveform shown on line C. However~ the information shown in Line D contains bad second harmonic distortion. When used in a system in which bad second harmonic distortion is not a disabling problem, the attention to a 50/50 duty cycle situation explained hereinabove need not be strictly followed.
However, when it is necessary to have a substantially undistorted output signal recovered from the video disc sur~ace, it is necesisary to follow the procedure described hereinabove.
Referring to Figure 13, there is shown the rela-tionship between the intensity of the reading spot in thereadlng beam as it impinges upon successively positioned light reflective and light non-reflective regions formed dur~ng a preferred form of the mastering processi. In a preferred embodiment, a metal is used for thls purpose and 3 the pre~rred metal as d~sclosed ls bismuth, Line A of Figure 13 shows a plurallty of indlcla formed in the surface of a vldeo disc master. In the preferred embodiment the holes ~ormed ln a bismuth layer 420 are shown at 422; 424 and 426~ The intervening por-tlons of the la~Jer 420 which are unaffected by the forma-tlon of the holes 422j 424 and 426 are sometimes called "landi areas and are indicated generallJ at 428 and 430.
The land areas are highly reflective. The formation of the holes 422, 424 and 426 expose the underlying glass substr~e which is essentlally light absorbing and hence the glass subs~rate is a light non-reflective region. The waveform shown at 432 represents the light intens~ty wave~orm of the spot ln the read beam as the spot passes over a light non-reflective region. This indicates the spacial rela-tionship between the spot as it moves over a light non-reflective region.
Referring to line B of Figure 13, there is shown a waveform represented b~ a line 434 indicating the inten-sity wave~orm of the reflected light as a spot having theintensity relationship shown in Figure A passes over a successively positioned light reflective and light non-reflective region. A solid line portion 43~ of the line 434 shows the intensity waveform of the reflected light as the spot passes over the light non-reflective region 424.
The inten~it~y of the reflected light shows a minimum at point 438 which corresponds with the center of the non-re~lective region 424. The center of the non-re~lective portion 424 is shown on a line 440 at a point 442. The 20 intensity waveform of the reflected light is a maximum, as shown at 444, when corresponding to a center point 446 of the land area 428 positioned between successive non-reflective regions 422 and 424 respectively. The center point 446 is shown on a line 448 representing the center 25 line of the information track. The dotted portion of the llne 434 represents the past history of the lntensity waveform of the reflected light when the light passed over the non-reflective region 422. A dotted portion 452 of the waveform 434 shows the expected intensity of the re~lected light beam when the reading spot passes over the non-reflective region 426.
Referrlng 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- -sentation is shown as a line 454 and is ~enerated in the photodetector 7Q shown in-Figure 1.
A schematic diagram of a suitable hiæh voltage amplifier is shown in Figure 16~
A special advantage of the read while write capability of the mastering procedure herein described includes the use of the instantaneous monitorlng o~ the information ~ust written as a means for controlling the duty cycle of tne reflectlve and non-reflective regions.
By displayin~ the recovered frequency modulated video slgnal on a television monitor during the writing proce-dure, the duty cycle can be monitored. Any indication of the distortion visible on the mo~or indicates that a change in duty cycle has occurred. Means are provided for ad~usting the duty cycle of the written information to ellminate the distortion by ad~usting the duty cycle to its 50/50 preferred operating point. A change in duty cycle is typically corrected by adjusting the absolute intensity of the light beam generated in the laser 30 in a system having either an average intensity biasing servo or a second harmonic biasing servo and in conjunction with circuitry for adjusting the half power point output of the Pockels Cell-Glan Prism combination to equal the threshold power level of the recording medium. The term . 20 half power point and average intensity are interchanged in the portlons of the speciLicatlon~ and clalms which concern the use of the triangular shaped wave form gener-ated by the FM modulator. The modulated light beam 40 exitlng from the Glan Prism 38 is of slnusoidal'shape. In this situation the hal~ power point equals the average lntensity, and this would be the case for any s~Jmmetrlcal wave form. A frequency modulated output from an FM modu-lator has been found to act as such a symmetrical wave form.
Whlle the lnvention has been particularly shown and descrlbed with re~erence to a preferred embodiment and alterations thereto, it would be understood by those skilled ln the art that various changes ln form and detall may be made therein without departing ~rom the spirit and scope of the invention.

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for storing video information in the form of a frequency modulated signal upon an information storage member, comprising: first means for providing an information signal to be recorded, and said signal having its informational content in the form of a carrier frequency having frequency changes in time representing said information to be recorded; an information storage member including a substrate having a first surface and a light responsive coating covering said first surface for retaining indicia representative of said information signal; means for imparting uniform motion to said storage member; a light source for providing a light beam, said light beam being of sufficient intensity for interacting with said coating while said coating is in motion and said coating is positioned upon said moving information storage member, said light beam being of sufficient intensity for altering said coating to retain indicia representative of said information signal;
optical means for defining an optical path between said light source and said record carrier including said coating, said optical means being further employed for focusing said light beam to a spot upon said coating; light intensity modulating means positioned in said optical path between said light source and said record carrier, said light in-tensity modulating means operating over a range between a maximum light transmitting state and a minimum light tran-smitting state for intensity modulating said light beam with said information to be stored; sald light intensity modulating means being responsive to said frequency modulated signal and changing between its maximum light transmitting state and its minimum light transmitting state during each cycle of said frequency modulated signal for modulating said light beam with the frequency modulated electrical signal to be stored; and stabilizing means responsive to said modulated light beam for generating a bias control signal indicative of the second harmonic distortion present in said modulated light beam, said bias control signal being employed for biasing said modulating means at its operating point at which minimum second harmonic distortion is present in said modulated light beam at its average power intensity; and wherein said light passing through said light intensity modulating means and focused upon said coating by said optical means forms indicia in said coating representative of said frequency modulated signal to be stored.

2. The apparatus as claimed in Claim 1, wherein said first means comprises: signal source means for provid-ing an initial information signal having its informational content in the form of a voltage varying with time format;
and frequency modulator means responsive to said signal source means for converting said voltage varying with time signal to a corresponding frequency modulated signal, and said frequency modulated signal having its informational content in the form of a carrier frequency having frequency changes with time corresponding to said voltage variations with time.

3. The apparatus as claimed in Claim 1, wherein said first means includes wave shaping means for issuing said information signal as a continuous triangular waveform.

4. The apparatus as claimed in Claim 3, including means for adjusting said modulating means to generate average power intensity at an operating point midway between said maximum state and said minimum state.

5. The apparatus as claimed in Claim 4, wherein said output waveform of said light intensity modulating means is sinusoidally shaped in response to said triangularly shaped output signal from said wave shaping means.

6. Apparatus for recording input video information on an information storage member, comprising: first means for providing a video information signal to be recorded and said video signal having its informational content in the form of a voltage varying with time signal suitable for display on a standard television monitor; a writing laser for producing a collimated writing beam of polarized mono-chromatic light; a smooth flat rigid disc having a planar surface covered with a light responsive coating for retain-ing indicia representative of said video signal; said coat-ing having a threshold power level above which said indicia are formed; first optical means for directing said light beam to impinge upon said coating, and for focusing said impinging writing beam down to a spot at said coating; said coating having suitable physical properties to be altered in response to the impingement of light from said writing beam for forming a permanent alteration in said coating; rotational drive means for producing uniform rotational motion of said disc; translational drive means synchronized with said rotational drive means for moving said focused light spot radially across said planar surface of said disc; electrical synchronizing means for maintaining a constant relationship between said rotational motion and said translational motion, frequency modulation means for covering said input video information to a corresponding frequency modulated signal said frequency modulated signal having its informational content in the form of a carrier frequency having frequency changes with time corresponding to said voltage variation with time signal; electrically controllable means responsive to said frequency modulated means for varying the intensity of said writing beam between a first predetermined intensity at which the focused spot alters said coating and a second predetermined intensity at which the focused spot fails to after said coating, said alteration being representative of said frequency modulated signal; means for adjusting the average intensity of the output of said electrically con-trollable means to equal said threshold power of said light responsive coating; and stabilizing means responsive to said modulated light beam for generating a bias control signal indicative of the second harmonic distortion present in said modulated light beam, said bias control signal being employed for biasing said electrically controllable means at its operating point at which minimum second harmonic distortion is present in said modulated light beam at its average power intensity.

7. A method for recording information on an information storage member using a laser beam, comprising the steps of: providing an electrical signal to be recorded, and said signal having its informational content in the form of a voltage varying with time format; changing said voltage varying with time signal to a frequency modulated electrical signal having its informational content in the form of a carrier frequency having frequency changes with time cor-responding to said voltage variations with time; shaping said frequency modulated electrical signal into a triangular shaped waveform; modulating a fixed intensity light beam with the triangular shaped, frequency modulated signal;
sensing the amount of second harmonic distortion present in said light beam just prior to its application upon said light sensitive surface for generating a bias control cir-cuit; using said bias control signal for biasing said modu-lation to produce minimum second harmonic distortion; moving the information storage member at a constant rate while focusing a said light beam down to a spot upon the light sensitive surface of said information storage member; using said focused light spot to irreversibly alter the char-acteristics of said light sensitive surface of said infor-mation storage member as said member moves at a constant rate and under the control of one portion of said frequency varying signal; and blocking the transmission of said focused light beam to said light sensitive surface of said information storage member as said member moves at a constant rate and under the control by a second portion of said frequency varying signal.

8. A method for recording information on an information storage member using a laser beam, comprising the steps of: providing a frequency modulated electrical signal to be recorded; and said frequency modulated elec-tical signal having a carrier frequency with frequency changes over time corresponding to said information to be stored; using said frequency modulated signal as a control signal for varying the intensity of a fixed intensity light beam impinging upon a light sensitive surface of an infor-mation storage member; providing a sinusoidally shaped modulated light beam having a first maximum intensity level, and a second minimum intensity level; adjusting the midpoint between said first intensity level and said second intensity level to equal the average intensity of said modulated light beam; adjusting the average intensity to equal the threshold power level of the light sensitive surface by adjusting the output intensity of said light beam source; moving the information storage member at a constant rate while focusing said light beam down to a spot upon said light sensitive surface of said information storage member; using said transmitted light beam for irreversibly altering said light sensitive surface of said information storage member as said member moves at a constant rate and under the control of one portion of said frequency modulated signal; and blocking the transmission of said light beam to said light sensitive surface of said information storage member as said member moves at a constant rate and under the control of a second portion of said frequency varying signal.