CA1055157A - Apparatus and method for storing information on and retrieving information from a videodisc - Google Patents
Apparatus and method for storing information on and retrieving information from a videodiscInfo
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- CA1055157A CA1055157A CA313,005A CA313005A CA1055157A CA 1055157 A CA1055157 A CA 1055157A CA 313005 A CA313005 A CA 313005A CA 1055157 A CA1055157 A CA 1055157A
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Abstract
ABSTRACT OF THE DISCLOSURE
The present invention relates to reading while writing on an information storage videodisc. Prior art techniques of recording on videodiscs have involved processing procedures producing recorded intelligence information on a disc directly from a corresponding intelligence signal in amplitude varying with time format. Such procedures have definite limitations as to the quality of the reproduced signal. Additionally, it has previously not been possible to instantaneously read and check the recorded signal. In accordance with the present invention, a writing system includes an apparatus for recording information on a videodisc having a light responsive coating covering the videodisc sub-strate for retaining indicia representing a frequency modulated information signal. A light beam of sufficient intensity is provided to interact with the responsive coating of the videodisc forming the indicia representing the information signal. A reading system is provided and includes a reading beam directed along an optical path between a reading light source and the videodisc.
The reading beam is of sufficient intensity to generate reflections from the indicia on the videodisc. A
light-sensor is responsive to the reflections for gener-ating a frequency modulated signal having its information content in the form of a carrier frequency which has frequency changes with time. The reading while writing apparatus and method provides a comparative system for analyzing the initial frequency modulated signals used in writing on the videodisc to the actual frequency m odulated signal produced in reading the videodisc.
The present invention relates to reading while writing on an information storage videodisc. Prior art techniques of recording on videodiscs have involved processing procedures producing recorded intelligence information on a disc directly from a corresponding intelligence signal in amplitude varying with time format. Such procedures have definite limitations as to the quality of the reproduced signal. Additionally, it has previously not been possible to instantaneously read and check the recorded signal. In accordance with the present invention, a writing system includes an apparatus for recording information on a videodisc having a light responsive coating covering the videodisc sub-strate for retaining indicia representing a frequency modulated information signal. A light beam of sufficient intensity is provided to interact with the responsive coating of the videodisc forming the indicia representing the information signal. A reading system is provided and includes a reading beam directed along an optical path between a reading light source and the videodisc.
The reading beam is of sufficient intensity to generate reflections from the indicia on the videodisc. A
light-sensor is responsive to the reflections for gener-ating a frequency modulated signal having its information content in the form of a carrier frequency which has frequency changes with time. The reading while writing apparatus and method provides a comparative system for analyzing the initial frequency modulated signals used in writing on the videodisc to the actual frequency m odulated signal produced in reading the videodisc.
Description
PPARATUS FOR SIMULTAMEOUSLY RETRIEVING INFORMATION
FROM AND STORING INFORMATION ON A VIDEODISC
TECHNICAL FIELD
The present invention relates to storage and retrieval o~ information upon an in~ormatlon storage member, and more particularly to simultaneously writing in~ormation on and retrieving information from a video-disc. ~ACKGROUND OF THE PRIOR ART
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Systems have heretofore been developed for re-cording and reproducing signals at video frequencies upondiscs, tapes, or other media. Such systems have utilized, àmong other thing~, optical recording upon photosensitive media, electron beam recording on thermo-plastic sur-~aces, and still other systems provide an instantaneously 1~ reproducible record of video information.
The prior art can generally be divided into systems utilizing photographic surfaces, systems util-izing electron beam sensitive surfaces, magnetic record- -lng systems, and as in the present invention, system~
20 ln which a radiant energy beam causes an irreversible -change to a surface, thereby "writing" in~ormation thereon to be read out by a compatible reading apparatus.
Photographic systems have been described in the patents to P. C. Goldmark et al, No. 3,234,326, - 25 which teaches recordlng on a continuou~ wdb such as a tape or ~ilm, or the patent to W. R~ John30n~ No.
3,361,873j which teache~ the photographic recordatlon o~
vldeo inrormation on a rotating disc in a spiral path.
The patent to W. C. Hughes et al, No. 3,283,310, .
, ~ -2- 1055157 i~ illustratlve o~ recordation o~ inrormation on a thermo-plastic film surface, which utilizes an electronic beam writin~ apparatus such as was disclosed ln U. S.
Patent No. 3,120,991.
Yet other systems have employed an eleetron beam to record lnformation on a special storage medium.
One such system has been disclosed in the patent to D. P.
Gregg, No. 3,350,503. An alternative scheme utilizing an electron beam on a ph~tosensitive medium such as .10 photographic rilm has been taught in the patent to R. F.
Dubbe, et al, No. 3,444,317.
In recent years, alternative methods have been disclosed ror high densit~ recording which are based upon either removal of material or vaporization of mater-ial by laser beam bombardment. These methods have been discussed briefly in the magazine "Electronics" of March 3, 1969J at page 110. Further, a "laser thermal micro image recorder" was described in some detail by C. O. Carlson and H. D. Ives in a paper given at the 1968 WESCON meet~ng (~Jestern Electronic Show and Conven-tion~, which paper was published in Volume 12 of tESCON
Technical Papers for 1968, at page 1 of Section 16/1.
The authors have referred to articles in the December 23, 1966, issue of "5clence:, Volume 54, No. 3756, at pages 1550 and 1551, the Proceedings ~ the Fall Joint Computer Conference of 1966, pages 711-716, and an article in the "Bell Systems Technical Journal" o~ March 1968, pages 385, 405.
These publications disclose a recording techni~ue which utllizes a thin metallic film coating upon a sub-8trate. The thin metal ~ilm, under applied heat, melts rapidly and forms small globules within a recorded spot.
A highly concentrated spot of laser illumination can apply su~icient heat in a short enough time so that a 35 . suitably modulated laser beam impinging upon a moving surrace can produce a pattern o~ holes in.the metallic ~urrace whlch, when "read back"J can reproduce the in-rormation.recorded.
A~ pointed out ln the Carlson and Ives paper, .. . . . . . . . . . . .
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3 1055~57 supra, the sizc ~ the recorded spot or hole can be much smaller than the diameter of the imaged laser beam By an approprlate choice Or metal film materlal, ~llm thlck-nes~J laser divergence and spot power, an appropriate system can be designed to record video frequencles wlth reasonably high resolution. However, the quallty o~ the video signal reproduced from such recordings has not been good due to a low signal-to-noise ratio resulting ~rom the direct recording o~ the video signal onto the 10 recording medium. Additionally, the use of a modulated laser beam to selectively produce a pattern o~ holes on the metallic surface o~ a vldeodisc is not conducive to ~aithfully represent an amplitude varying aniog video signal. Likewlse, readlng back the recorded in~ormation in the ~orm o~ a track of holes or spots on the surrace o~ the disc is ine~fective to faith~ully recreate an analog vldeo slgnal from the recorded track on the disc.
The lack of fidelity in the reproduced video signal can be attributed prlmarily to the inability o~ the laser beam modula~or to "write" precise analog signals in the coating of the disc. ConsequentlyJ the reproduced signal deve~oped by the reader suffers in the same manner.
BRIEF SUMMARY OF THE INVENTION
The present invention provides an improved apparatus and method for storin~ in~ormation on and re-trievin~ information from a v-ideodisc, the stored in-~ormation having the form of a lineal series of regions representing a frequency ~odulated information signal.
In accordance with the invention, a videodlsc 18 provided which has a light responsive coating covering the disc substrate for retaining indicia representing-the rrequency modulated information signal. A writing light source provides a light beam o~ su~icient lntensity t~ interact with the responsive coating of the video-dlsc as the disc uniformly rotates. An optical systemde~lnes an optical path between the light source and the responsive coating on the disc ~or ~ocusing the light ~eam on the coating. A light intensity modulator is po~ltloned in the optical path and 13 respon~iv~ to the .
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fre~uency modulated lnformation slgnal to intensity modulate the llght beam. The light intensity modulator i8 ef~ective to transmit light over a predetermined lntensity range, the intenslty of the modulated light beam changing between a relatively high light level and a relatively lower light level during each cycle of the frequency modulated signal. The light beam passing through the intensity modulator and optically focused on the responsive coating ~orms the indic~a representing the frequency modulated information signal. The resultlng indicia is in the form of a llneal series of regions of alternately high and l~w directional re~lectivity normal to the surface of the disc. That is, when viewed from a point above the surface of the disc, an impinging light beam directed normal to the surface would reflect alter-nately with high and low levels pf reflected light in a direction opposite that of the impinging light beam.
In a preferred embodimentJ an initial input signal has its information content in the ~orm of a voltage varying with time. Such a signal is suitable for display on a standard television monitor. In such an embodiment, a frequency modulator, responsive to the voltage varying with time signal converts the signal to a ~requency modulated signal for driving the light in-tensity modulator.
The relatively higher and lower light transmit-ting states ~ the light intensity modulator result in the varying of the intensity of the resultant modulated llght beam above a predetermined intensity at which the ~ocused beam alters the responsive coating, and helow the predetermined intensity at which the focused beam rails to alter the responsive coating.
In order to establish an optimum average in-tensity level for the modulated light beam, the light intensity modulator may further include feedback cir-cuitry ~or stabilizing the operating level Or the electrical control of the light modulator to produce optlmum ~irst and second predetermined inten~itie~.
The ~eedback clrcuitry may include a level ad~ustment to .
_5_ ~055~57 selectively ad~ust the average power level Or the light beam to a predetermined value. Toward this end, a light-sensor senses at least a portion of the light beam Prom the optical modulator to produce an electrical feedback signal representative oP the average intensity oP the light beam such that when the average level of modulated light beam increases, the Peedback signal tends to lower the operating level oP the optical modulator in order to keep the average intensity of the modulated light beam stabilized at a constant operating intensity level.
At the heart of the optical modulator is a Poc-kels cell driven by a Pockels cell driver. The Pockels cell driver has as its input the ~requency modulated information signal, and the Pockels cell driver responds 1~ to the frequency modulated signal to provide correspond-lng driving signals to the Pockels cell device. The Pockels cell driver is A.C. coupled to the Pockels cell device, and the aPorementioned feedback circuitrv is D.C. coupled to the Pockels cell device.
The light source for producing the writing light beam is a writing laser, preferably an Argon ion laser~
which produces a collimated writing beam of polarized mo~ochromatic light. The writing laser produces local-ized heating upon ~mpingement of the llght responsive coating oP the videodisc.
The videodisc has a rigid disc-shaped substrate wlth a surface of opaque metallized coating. The coating has suitable physical properties to permit localized heating by the writing laser to, in turn, cause local-ized melting accompanied withdrawal Or the molten mater-lal toward the perimeter O~ the ~olten area. Upon rreezing, a permanent aperture is lePt in the metallized coating. Under proper operating conditions, the optical -modulator varies the intensity Or the wrlting laser beam rrom an lntensity above the aPorementloned predetermlned intensity, at whlch the ~ocused beam melts the metallized ~oatlng without vaporizlng it to an lntensity below the predetermined intensity, at which the Pocused beam Pail~
to melt the metallized coatin~.
-- 1055157 ~6--The disc-shaped videodisc ls uni~ormly rotated, and radlal translational motion is glven to the video-disc relative to the writing beam ~or writing the in~or-mation in a spiral path on the disc. Both rotational and translational motion are under control of an elec-trical synchronizer whlch maintain~ a constant relation-ship between the rotational motion o~ the disc and the relative translational motion between the writing laser beam and the disc`.
A mlcroscope ob~ective lens for focusing the writing beam on the vldeodisc surrace may be maintained ~n a spaced r~lationship to the'videodisc by a hydro-d ynamic air bearing created as the disc rotates relative to the objective lens support. The objective lens has an aperture larger in diameter than the diameter of the reading light beam, and the optical sys~em makes use o~
mirrors and lenses for diverging the writing light beam ~rom the laser source to at least fill the entrance aperture Or the objective l~ns.
me rotational dri~er includes a spindle whlch rotates the disc precisely ln a circle, and the radial translating ariver includes a lead screw mechanism for e~recting relative translational motion between the disc and the writing beam at a very constant velocity along a radius Or the rotating disc.
Synchronization of the disc drive with the translating drive creates a spiral track of predetermined pitch. Ir desirPd, concentric circles can be created by alternately translating and writing. In a preferred emb'o*iment employing a spiral trackJ the spacing between adJacent turns Or the sp.'.ral is 2pm , center to center.
Assuming a spot diameter on the order of l~m , this would produce a guard area Or l~lm between ~pots in ad~acent track~. ' The microscope ob~ective lens "~lie~" at a constant height above the disc on an air bearing. The ¢onstant height is desirable because o~ the shallow ~ocal depth Or the ob~ective lens. A 40X dry micro~cope obJective len~ ha~ been ~ound to be satls~actory in terms , , ' ,' '~ :
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.. ~,.,, . , . ,, : , 7 ~.055157 Or concentrating the energy Or the laser beam at the disc surrace to enable the writing of the 1 ~m spot.
The reading portion Or the invention include~ a polarized collimated reading beam of light, and an op-tical system between the light source and the videodiscfor focuslng the reading beam upon the videodisc. As the disc rotates, relative motion between the reading light beam and the alternate regions of indicia produces corresponding alternate reflections from the high re-~lective regions on the videodisc. A light sensor isresponsive to the reflections for generating a frequency modulated electrical signal. The frequency modulated electrical signal so developed has its inrormational content in the form o~ a carrier rrequency which has a frequency varying with time.
As mentioned previouslyj the v~deodisc has a lineal serles o~ regions positioned in track-like fashion upon its upper surface. The sequence of alter-nate regions of high and low reflectivity normal to the disc surface represents a ~requency modulated signal.
The reading system pre~erably includes a low power laser for producing a reading light beam~ a polarizing beam splitting cube, and a quarter wave plate. Li~ht passes from the laser source through the polarizing beam ~plitter and to the quarter wave plate. The quarter ~ave plate is used to rotate the incident reading light beam forty~five degrees on passage through to the video-dlsc.
The reading beam is focused upon the disc surface through the same ob~ective lens as used with the wrlting beam. The ob~ective lens has an aperture larger ln diameter than the diameter of the reading beam. The readlng optical system ma~es use of mirrors and lenses for diverging the readlng beam from the laser source to at least flll the entrance aperture of the ob~ectlve lens.
The reflected light from the videodisc returns by means of mirror~ and lenses through the quarter wave plate where it is once a~ain rotated forty-flve de~rees to impart a tot~l Or nlnety degrees rotation relatlve to ~, , ~., , lOS5157 ,....... . o ~, the beam from the source. The emerglng reading beam passes on to the polari%lng beam splitting cube which i3 adapted to direct the re~lected reading beam to a sensor and away ~rom the reading laser source.
The rerlected light from the videodisc directed toward the sensor represents a ~requency modulated signal having its informational content in the form o~ a carrier rrequency having a frequency varying with tlme. The ~requency modulated output signal o~ the sensor may be changed into a time dependent voitage signal ~or display on a standard television monitor.
The optical system further includes a beam director ~or directing the ~ritlng beam toward the ob~ective lens at an angle with respect to the readlng beam, thereby spacing the beams as they emerge ~rom the ob~ective lens to impinge the videodisc, the reading beam impinging the disc sur~ace downstream of the writ-ing beam.
The writing beam is preferably an Argcn ion laser beam, while the reading beam is preferably a Helium-Neon laser beam. The re~lected reading beam passes through a filter, opaque to an Argon ion beam, prior to being sent to the sensor, ~hereby writing beam energy is substantially prevented from reaching the sensor.
An F.M. demodulator responsive to the output of the sensor produces a voltage varying with time output video information signal. A comparator may be provided ror comparing the video output information signal developed from reading the videodisc with a video input slgnal used to write information on the videodisc. The comparator output can be used to verlfy the quallty of the ln~ormation being stored on the vldeodisc, and it can output a sl~nal indicating that proper comparison or reglstration has not been achieved.
A delay clrcuit receives a portion of the video lnput slgnal before routing to the comparator. The delay clrcuit imparts a delay in the input video ~lgnal e~ual to the accumulated valu~s o~ the delay fronl rrequency g modulation of the input vldeo signal through rrequency demodulation Or the signal from the sensor. Included in the delay time is the time necessary for the videodisc to move from the point of stora~e of the lnformation signal by the writlng beam to the point o~ implngement by the reading light beam.
BRIEF DESCRIPTION OF THE DR~WINGS
. . . _ FIG. 1 is a generalized diagram of the appar-- atus of the present invention;
~IG. 2 is a view ~ the optical path through the ob~ective lens of FIG. l;
~ IG. 3 is a representational indication of the relative spacing between a point of impingement of the writing beam and of the reading beam; and F~G. 4 is a diagram of a novel Pockels cell -stabilizing circuit.
D~TAILED DESCRIPTION OF THE INVENTION
Turning first to FIG. 1, the writing apparatus 10 includes a writing head 12 which is, in the preferred embodiment, a dry microscope ob~ective-lens 14 mounted upon an air bearing suDport member 16. A 40X lens has been found t.o be satisfactory. A disc 18 is specially prepared and may be constructed according to the teach-ings o~ the prior art, in which a substrate has coated 25 thereon a very thin film of a metal with a reasonably low melting point and a high surface tension.
A crystal oscillator 20 controls the drive elements. The disc 18 is rotated by a first, rotational drlve element 22 which is coupled to a spindle 24. A
30 8econd, translational drive element 26 controls the - position of the writing head 12.
A translatlng carriage 28, whlch ls driven by the translational drive element 26 through a lead screw and travellin~ nut, moves the writing head 12 in the 35 radial directlon relative to the rotating disc 18. The .
` 1055157 radial dlrection relative to the rotating di~c 18. The carriage 28 is provlded with approprlate mlrrors and lenses so that the remainder of the optics and elec-tronics necessary to the writing devlce may be perman-ently mounted.
In the prererred embodiment of the`presentinvention, the beam of a polarized cutting laser 30, which ls an argon ion laser, is passed through a Pockels cell 32 which is driven by the Pockels cell driver 34.
An FM modulator 36 receives the video signal that is to be recorded and applies the appropri~te control signals to the Pockels cell driver 34. `
As described hereinarter, the video in signal is o~ the t~rpe displayable on a TV monitor. Accordingly, it is a voltage varying with time slgnal. The FM modu-lator 36 is o~ standard design and converts the voltage varying with time signal to a frequency modulated signal having its informational content in the form of a carrier frequency havinæ frequency changes with time correspondin~ to said voltage variations with time As is known, the Pockels cell 32 responds to applied sign~l voltages by rotating the plane of polar-ization of the light beam. Since a linear polarizer transmits light only in a predetermlned polarization plane, a polarizer, such as a Glan prism 38 in the pre-rerred embodiment, is included in the writing beam path to provlde a modulated writing beam 40. The modulated writlng beam erfectively follows the output o~ the FM
modulator 36.
The modulated writing beam 40 emerging from the Pockel~ cell-Glan prism combination 32, 38 is applied to a first mirror 42 which directs the writing beam 40 to the translatlng carrlage 28. The rirst mirror 42 tranæmlts a portion o~ the writlng beam 40 to a Poc~els cell stabilizing circuit 44 which responds to the aver-age intensity o~ the writlng beam to maintaln the energy level Or the beam.
- ~ lens 46 1~ inserted in the path o~ the writ-lng beam 40 to dlverge the æubstantlally parallel beam '' ~ ' ' ' ' ~
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~055~57 so that it wlll spread to fill the entrance aperture of the ob~ective lens 14 for optimum resolution. A di-chroic mlrror 48 is included in the path oriented to substantlally transmit all of the writing beam 40 to a second, articulated mirror 50. A mirror, such as has been shown in applicant's Canadlan Patent No. 1,012,643, issued June 21, 1977, may be employed in the present in-ventlon. The articulated mirror 50 then directs the beam through the lens 14 and is capable Or shiftlng the point of implngement ôf the beam 40 on the surface of the disc 18.
A series of holes ls formed in the metal coat-ing by the writing beam. One hole is formed for each cycle of the FM modulated signal represented by the m~dulated writing beam 40. Since the modulated writing beam tracks the output of the FM modulator 36, the holes formed in the coating also track the output of the FM
modulator. Obviously, since the informational content in the output signal of the FM modulator 36 is in the form of a frequency changes in time about a carrier fre-quency, and since the "hole", "no hole'i sequence repre-sents the stored information, and since the disc 18 is rotating at a unlform speed, the "hole" 'lno hole"
sequence changes to represent the stored video informa-tion by the holes being formed closer or farther apart 25 and the size of the hole becomes larger or smaller as -the writing beam 40 changes under the control of the FM
modulated output signal from the FM modulator 36.
m e ob~ectlve lens 14 and the assoclated air bearing 16 effectively fly on a cushion of air a~ a 3 substantially fixed distance from the surface of the dlsc 18. That dlstance is determined by the geometry of the bearing 16, the linear velocity of the dlsc 18, and the force used to load the head against the disc 18.
m e flxed spacing is required because the focal toler-ance Or a lens capable of resolving a 1 ~m spot ls alsoof the order of 1 ~m.
A second, relatively low-power laser 52 pro-vldes a monltoring beam 54. In the preferred embodlment, the reading laser 52 ls a helium-neon device which , , -12~
enables the reading beam 54 to be distlnguish~d ~ro~
the ~Jriting beam 40 by wavelen~th. A polarizing, beam splitter cube 56 transmits the reading beam 54 to a mirror 58 that directs the beam 54 through a second diver~i~g lens 60 that spreads the reading beam 54 to ~ill the entrance aperture o~ the objective lens 14.
A quarterwave plate 62 is placed in the opti~
cal path and~ in con~unction with the plane polarizing beam splitter 56, prevents light reflected from the disc 18 ~rom re-entering the laser 52 and upsetting its mode Or oscillation. The quarterwave plate 62 rotates the plane of polarization o~ the beam by 45 degrees on each pass so that the reflected beam is rotated 90 degrees wit`n respect to the polarizing beam spl~tter 56 and is there~ore not passed by it.
A second mlrror 64 in the reading beam 54 path directs the beam into the dichroic mirror 43 and is capable o~ llmited adjustment so that the paths o~ the writing and reading beams are substantially identical, except that the reading beam "spot" impinges on the dlsc 18 downstream from the writing beam spot as explained in-greater detail below.
A filter 66 that is opaque to the argon ion beam is interposed in the path of light reflected from ~5 the beam splitter 56. The He.Ne reading beam 54 that i8 returned from the disc suxface ls able to pass through the filter 66 and through a lens 68 onto a photodetec-tor 70.
The re~lected light o~ the reading beam im-plnges upon the photodetector 70. The photodetector 70operates in its standard manner and generates an elec-trlcal current representative of the light impinglng thereupon. In this case, the photodetector generates the ~ignal represented by the "hole", "no hole" con-~lguration formed in the coating. The "hole", "nohole" con~i~uration is representative o~ the output o~ the FM modulator 36. The output Q~ the FM modulator 36 i8 a carrier ~requency having frequency changes with tlme reprcsenting the vldeo signal to be record~d. The , , "hole", "no hole' configuration 1~ representatlve of a carrler frequency havlng ~requency c`anges with time representing the stored vldeo slgnal. The output of the photodetector 70 is an electricai slgnal representin~
the stored carrier frequency having frequency changes wlth time representing the stored vldeo signal.
The output of the photodetector 70 ls applied to a preamplifier 72 which provides a si~nal of suffi-clent amplitude and signal strength for subsequent 10 utilization. A video discriminator 74 then provides a vldeo output signal which can be utllized in several ways, two of which are shownJ as examples only.
The discriminator 74 is of standard design and function. It takes the frequency modulated signal from i5 the photodetector 70 and changes it to a time dependent voltage signal having its informational content in the form of a voltage varying with time format suitable for dlsplay in the TV monitor 76.
In a first application, the video output is applied to a TV monitor 76 and an oscilloscope 78. As ls well known, the TV monitor is responsive to a vol-tage varying~with time signal. The information to be displayed on the TV monitor is represented by a voltage change with time.
The TV monitor 76 shows the picture fidelity Or the recording, and the oscilloscope 78.indicates the slgnal-to-noise ratio of the record and the quality of the cutting, whether it is light or heavy. Not shown, an appropriate ~eedback loop could be provided through 3o the Pockels cell stabilizing circuit 44 to assure an adequate discrimination on the disc between a "hole" or "black" area and "no hole" or "white" area.
As an alternative utilization, the ~ideo out-put of the discriminator 74 is applied to a comparator 35 80. The other lnput of the comparator 80 ls taken from th~ video input signal which ls directed through a delay llne 81. A delay that is equal the accumulated delays o~ the writlnG system and the time elapsed between the lnstant of writln~ o~ the information and the tl~e required for that lncremental area of the disc to reach the reading point must be lmparted to the lnput vldeo signal.
Ideally, the video output signal of the dis-crimina~or 74 should be identical in all respects to thevldeo input signalJ after the proper delay.
As previously mentloned, the output from the dlscrimlnator 74 is a voltage varying with time signal.
The video ln signal is also a voltage varying with time signal. Any dlf~erences noted represent errors which might be caused by imperfections in the disc's surface or malfunctions of the writing circuits. Thls applica-tlon, while essential if recording digital information, is less critical when other inrormation is recorded.
The output of the comparator circuit 80 can be quantized and counted, so tha~ an acceptable number o~ errors can be established for any disc. I^lhen the errors counted exceed the standard, the writing opera-tion can be terminated. If necessary, a new disc can be written. Any disc with excessive errors can then be reprocessed to serve as a "new" disc ror a subsequent recording.
Well-known techniques are available to trans-late the writing head assembly 12 in the radial direc-tion with respect to the rotating disc 18. I~lhile in FIG. 1 the rotational and translational drives 20, 22 are indicated as independent, the drives are synchronized to enable the writing assembly 12 to translate a pre-determined increment for each revolution of the disc 18, 3 by means of the common crystal oscillator 20.
Turning next to FIG. 2 there ls shcwn, in somewhat exaggerated form, the slightly differing opti-¢al paths of beam 40 from the writing laser 30 and the beam 54 rrom reading laser 52. The writing beam 40 coincldes with the optical axis Or the mlcroscope ob~ec-tlve ~ens 14. me readlng beam 54, in contrast, makes an angle ~ with the axis so that it ralls some distance X, equal to ~ tlmes the focal length o~ the obJective, "downstream' ~rom where the writing bcam 40 is "cuttlng".
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The resulting delay between reading and writing allows the molten metal to ~olldlfy so that the recording is read in it~ final state. I~ lt were read too 800n while the metal was still molten, lt would not provide per-tlnent informatlon ror ad~usting the recording parameters.
Thls ls best indlcated in FIG. 3 where twopolnts in the same informatlon channel are sho~1n as displaced. The point A, which is the polnt of impinge-ment of the writing beam 40, is shown as being on the 10 optical axis of the ob~ective lens 14. Separated from point A, in the direction of medium motion, as indicated by the arrow, is the reading point B, which is at an .angle ~ from the axis of the microscope objective lens 14. A distance between points A and ~ of two~ has provided a satis~actory monitoring of the writing oper-ation.
Turning finally to FIG. 4, there is shown an idealized diagram of a Pockels cell stabilizing circuit 44, suitable for use in the apparatus of FIG. 1. As is known, a Poc~els cell rotates the plane of polarization of the applied light as a function of an applied vol-tage. Therefore, the Pockels cell is used to rotate plane polarized light, and the rotated light is passed through a plane polarizer, such as a Glan prism. The llght issuing from the polarizer will be amplitude modulated in accordance with the applied voltage, Stated another way, the standard operating mode of a Pockels cell 32 and Glan prism 38 is for use as a light intensity modulating means. Each cycle from the FM modulator drives the Pockels cell through its full operating range ~ nlnety degrees. Within this operatlng range of ninety degrees, one operating point passes all light applied thereto and identified as a ~ull light transmitting state. A second operating point passes no light and is identlfied as a full light blocking sta'ce. The Pockels cell itself only rotates the plane of polarlzation. The Glan prism passes light ln one plane of polarization and no llght in the plane dl~placed ninety degrees rrom that plane ln which all ..
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-- 1055~57 the llght passes.
Dependlng upon the lndividual Pockels cell, a - voltage change Or approximately 100 volts wlll cause the cell to rotate the plane of polarlzation through 360 degrees. However, the transfer characteristic o~ an indlvidual cell may drift spontaneously, corresponding to a voltage change of + 50 volts, and accordingly, a ~eedback loop is desirable to maintain the cell within a use~ul, reasonably linear, operating ra~ge.
The stabilizing circuit 44 includes a photo-sensitive silicon diode 82, which is positioned to `
receive a portion of the writlng beam 40 reflect~d ~rom the mirror 42 o~ FIG. 1. The silicon diode 82 ~unctions ` in much the same fashion as a solar cell and is a source of electrical energy when illuminated by incident radiation. One terminal o~ the silicon diode 82 is connected to common re~erence potential 84, indicated by the conventional ground symbol, and the other terminal is connected to one input Or a differential ampli~ier 86; The silicon cell 82 is shunted by a load 88 which enables a linear response mode.
me other input to the dirferential amplifier 86 is connected through an appropriate potentiometer 90 to the common reference 84. A source o~ power 92
FROM AND STORING INFORMATION ON A VIDEODISC
TECHNICAL FIELD
The present invention relates to storage and retrieval o~ information upon an in~ormatlon storage member, and more particularly to simultaneously writing in~ormation on and retrieving information from a video-disc. ~ACKGROUND OF THE PRIOR ART
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Systems have heretofore been developed for re-cording and reproducing signals at video frequencies upondiscs, tapes, or other media. Such systems have utilized, àmong other thing~, optical recording upon photosensitive media, electron beam recording on thermo-plastic sur-~aces, and still other systems provide an instantaneously 1~ reproducible record of video information.
The prior art can generally be divided into systems utilizing photographic surfaces, systems util-izing electron beam sensitive surfaces, magnetic record- -lng systems, and as in the present invention, system~
20 ln which a radiant energy beam causes an irreversible -change to a surface, thereby "writing" in~ormation thereon to be read out by a compatible reading apparatus.
Photographic systems have been described in the patents to P. C. Goldmark et al, No. 3,234,326, - 25 which teaches recordlng on a continuou~ wdb such as a tape or ~ilm, or the patent to W. R~ John30n~ No.
3,361,873j which teache~ the photographic recordatlon o~
vldeo inrormation on a rotating disc in a spiral path.
The patent to W. C. Hughes et al, No. 3,283,310, .
, ~ -2- 1055157 i~ illustratlve o~ recordation o~ inrormation on a thermo-plastic film surface, which utilizes an electronic beam writin~ apparatus such as was disclosed ln U. S.
Patent No. 3,120,991.
Yet other systems have employed an eleetron beam to record lnformation on a special storage medium.
One such system has been disclosed in the patent to D. P.
Gregg, No. 3,350,503. An alternative scheme utilizing an electron beam on a ph~tosensitive medium such as .10 photographic rilm has been taught in the patent to R. F.
Dubbe, et al, No. 3,444,317.
In recent years, alternative methods have been disclosed ror high densit~ recording which are based upon either removal of material or vaporization of mater-ial by laser beam bombardment. These methods have been discussed briefly in the magazine "Electronics" of March 3, 1969J at page 110. Further, a "laser thermal micro image recorder" was described in some detail by C. O. Carlson and H. D. Ives in a paper given at the 1968 WESCON meet~ng (~Jestern Electronic Show and Conven-tion~, which paper was published in Volume 12 of tESCON
Technical Papers for 1968, at page 1 of Section 16/1.
The authors have referred to articles in the December 23, 1966, issue of "5clence:, Volume 54, No. 3756, at pages 1550 and 1551, the Proceedings ~ the Fall Joint Computer Conference of 1966, pages 711-716, and an article in the "Bell Systems Technical Journal" o~ March 1968, pages 385, 405.
These publications disclose a recording techni~ue which utllizes a thin metallic film coating upon a sub-8trate. The thin metal ~ilm, under applied heat, melts rapidly and forms small globules within a recorded spot.
A highly concentrated spot of laser illumination can apply su~icient heat in a short enough time so that a 35 . suitably modulated laser beam impinging upon a moving surrace can produce a pattern o~ holes in.the metallic ~urrace whlch, when "read back"J can reproduce the in-rormation.recorded.
A~ pointed out ln the Carlson and Ives paper, .. . . . . . . . . . . .
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. . .
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3 1055~57 supra, the sizc ~ the recorded spot or hole can be much smaller than the diameter of the imaged laser beam By an approprlate choice Or metal film materlal, ~llm thlck-nes~J laser divergence and spot power, an appropriate system can be designed to record video frequencles wlth reasonably high resolution. However, the quallty o~ the video signal reproduced from such recordings has not been good due to a low signal-to-noise ratio resulting ~rom the direct recording o~ the video signal onto the 10 recording medium. Additionally, the use of a modulated laser beam to selectively produce a pattern o~ holes on the metallic surface o~ a vldeodisc is not conducive to ~aithfully represent an amplitude varying aniog video signal. Likewlse, readlng back the recorded in~ormation in the ~orm o~ a track of holes or spots on the surrace o~ the disc is ine~fective to faith~ully recreate an analog vldeo slgnal from the recorded track on the disc.
The lack of fidelity in the reproduced video signal can be attributed prlmarily to the inability o~ the laser beam modula~or to "write" precise analog signals in the coating of the disc. ConsequentlyJ the reproduced signal deve~oped by the reader suffers in the same manner.
BRIEF SUMMARY OF THE INVENTION
The present invention provides an improved apparatus and method for storin~ in~ormation on and re-trievin~ information from a v-ideodisc, the stored in-~ormation having the form of a lineal series of regions representing a frequency ~odulated information signal.
In accordance with the invention, a videodlsc 18 provided which has a light responsive coating covering the disc substrate for retaining indicia representing-the rrequency modulated information signal. A writing light source provides a light beam o~ su~icient lntensity t~ interact with the responsive coating of the video-dlsc as the disc uniformly rotates. An optical systemde~lnes an optical path between the light source and the responsive coating on the disc ~or ~ocusing the light ~eam on the coating. A light intensity modulator is po~ltloned in the optical path and 13 respon~iv~ to the .
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fre~uency modulated lnformation slgnal to intensity modulate the llght beam. The light intensity modulator i8 ef~ective to transmit light over a predetermined lntensity range, the intenslty of the modulated light beam changing between a relatively high light level and a relatively lower light level during each cycle of the frequency modulated signal. The light beam passing through the intensity modulator and optically focused on the responsive coating ~orms the indic~a representing the frequency modulated information signal. The resultlng indicia is in the form of a llneal series of regions of alternately high and l~w directional re~lectivity normal to the surface of the disc. That is, when viewed from a point above the surface of the disc, an impinging light beam directed normal to the surface would reflect alter-nately with high and low levels pf reflected light in a direction opposite that of the impinging light beam.
In a preferred embodimentJ an initial input signal has its information content in the ~orm of a voltage varying with time. Such a signal is suitable for display on a standard television monitor. In such an embodiment, a frequency modulator, responsive to the voltage varying with time signal converts the signal to a ~requency modulated signal for driving the light in-tensity modulator.
The relatively higher and lower light transmit-ting states ~ the light intensity modulator result in the varying of the intensity of the resultant modulated llght beam above a predetermined intensity at which the ~ocused beam alters the responsive coating, and helow the predetermined intensity at which the focused beam rails to alter the responsive coating.
In order to establish an optimum average in-tensity level for the modulated light beam, the light intensity modulator may further include feedback cir-cuitry ~or stabilizing the operating level Or the electrical control of the light modulator to produce optlmum ~irst and second predetermined inten~itie~.
The ~eedback clrcuitry may include a level ad~ustment to .
_5_ ~055~57 selectively ad~ust the average power level Or the light beam to a predetermined value. Toward this end, a light-sensor senses at least a portion of the light beam Prom the optical modulator to produce an electrical feedback signal representative oP the average intensity oP the light beam such that when the average level of modulated light beam increases, the Peedback signal tends to lower the operating level oP the optical modulator in order to keep the average intensity of the modulated light beam stabilized at a constant operating intensity level.
At the heart of the optical modulator is a Poc-kels cell driven by a Pockels cell driver. The Pockels cell driver has as its input the ~requency modulated information signal, and the Pockels cell driver responds 1~ to the frequency modulated signal to provide correspond-lng driving signals to the Pockels cell device. The Pockels cell driver is A.C. coupled to the Pockels cell device, and the aPorementioned feedback circuitrv is D.C. coupled to the Pockels cell device.
The light source for producing the writing light beam is a writing laser, preferably an Argon ion laser~
which produces a collimated writing beam of polarized mo~ochromatic light. The writing laser produces local-ized heating upon ~mpingement of the llght responsive coating oP the videodisc.
The videodisc has a rigid disc-shaped substrate wlth a surface of opaque metallized coating. The coating has suitable physical properties to permit localized heating by the writing laser to, in turn, cause local-ized melting accompanied withdrawal Or the molten mater-lal toward the perimeter O~ the ~olten area. Upon rreezing, a permanent aperture is lePt in the metallized coating. Under proper operating conditions, the optical -modulator varies the intensity Or the wrlting laser beam rrom an lntensity above the aPorementloned predetermlned intensity, at whlch the ~ocused beam melts the metallized ~oatlng without vaporizlng it to an lntensity below the predetermined intensity, at which the Pocused beam Pail~
to melt the metallized coatin~.
-- 1055157 ~6--The disc-shaped videodisc ls uni~ormly rotated, and radlal translational motion is glven to the video-disc relative to the writing beam ~or writing the in~or-mation in a spiral path on the disc. Both rotational and translational motion are under control of an elec-trical synchronizer whlch maintain~ a constant relation-ship between the rotational motion o~ the disc and the relative translational motion between the writing laser beam and the disc`.
A mlcroscope ob~ective lens for focusing the writing beam on the vldeodisc surrace may be maintained ~n a spaced r~lationship to the'videodisc by a hydro-d ynamic air bearing created as the disc rotates relative to the objective lens support. The objective lens has an aperture larger in diameter than the diameter of the reading light beam, and the optical sys~em makes use o~
mirrors and lenses for diverging the writing light beam ~rom the laser source to at least fill the entrance aperture Or the objective l~ns.
me rotational dri~er includes a spindle whlch rotates the disc precisely ln a circle, and the radial translating ariver includes a lead screw mechanism for e~recting relative translational motion between the disc and the writing beam at a very constant velocity along a radius Or the rotating disc.
Synchronization of the disc drive with the translating drive creates a spiral track of predetermined pitch. Ir desirPd, concentric circles can be created by alternately translating and writing. In a preferred emb'o*iment employing a spiral trackJ the spacing between adJacent turns Or the sp.'.ral is 2pm , center to center.
Assuming a spot diameter on the order of l~m , this would produce a guard area Or l~lm between ~pots in ad~acent track~. ' The microscope ob~ective lens "~lie~" at a constant height above the disc on an air bearing. The ¢onstant height is desirable because o~ the shallow ~ocal depth Or the ob~ective lens. A 40X dry micro~cope obJective len~ ha~ been ~ound to be satls~actory in terms , , ' ,' '~ :
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.. ~,.,, . , . ,, : , 7 ~.055157 Or concentrating the energy Or the laser beam at the disc surrace to enable the writing of the 1 ~m spot.
The reading portion Or the invention include~ a polarized collimated reading beam of light, and an op-tical system between the light source and the videodiscfor focuslng the reading beam upon the videodisc. As the disc rotates, relative motion between the reading light beam and the alternate regions of indicia produces corresponding alternate reflections from the high re-~lective regions on the videodisc. A light sensor isresponsive to the reflections for generating a frequency modulated electrical signal. The frequency modulated electrical signal so developed has its inrormational content in the form o~ a carrier rrequency which has a frequency varying with time.
As mentioned previouslyj the v~deodisc has a lineal serles o~ regions positioned in track-like fashion upon its upper surface. The sequence of alter-nate regions of high and low reflectivity normal to the disc surface represents a ~requency modulated signal.
The reading system pre~erably includes a low power laser for producing a reading light beam~ a polarizing beam splitting cube, and a quarter wave plate. Li~ht passes from the laser source through the polarizing beam ~plitter and to the quarter wave plate. The quarter ~ave plate is used to rotate the incident reading light beam forty~five degrees on passage through to the video-dlsc.
The reading beam is focused upon the disc surface through the same ob~ective lens as used with the wrlting beam. The ob~ective lens has an aperture larger ln diameter than the diameter of the reading beam. The readlng optical system ma~es use of mirrors and lenses for diverging the readlng beam from the laser source to at least flll the entrance aperture of the ob~ectlve lens.
The reflected light from the videodisc returns by means of mirror~ and lenses through the quarter wave plate where it is once a~ain rotated forty-flve de~rees to impart a tot~l Or nlnety degrees rotation relatlve to ~, , ~., , lOS5157 ,....... . o ~, the beam from the source. The emerglng reading beam passes on to the polari%lng beam splitting cube which i3 adapted to direct the re~lected reading beam to a sensor and away ~rom the reading laser source.
The rerlected light from the videodisc directed toward the sensor represents a ~requency modulated signal having its informational content in the form o~ a carrier rrequency having a frequency varying with tlme. The ~requency modulated output signal o~ the sensor may be changed into a time dependent voitage signal ~or display on a standard television monitor.
The optical system further includes a beam director ~or directing the ~ritlng beam toward the ob~ective lens at an angle with respect to the readlng beam, thereby spacing the beams as they emerge ~rom the ob~ective lens to impinge the videodisc, the reading beam impinging the disc sur~ace downstream of the writ-ing beam.
The writing beam is preferably an Argcn ion laser beam, while the reading beam is preferably a Helium-Neon laser beam. The re~lected reading beam passes through a filter, opaque to an Argon ion beam, prior to being sent to the sensor, ~hereby writing beam energy is substantially prevented from reaching the sensor.
An F.M. demodulator responsive to the output of the sensor produces a voltage varying with time output video information signal. A comparator may be provided ror comparing the video output information signal developed from reading the videodisc with a video input slgnal used to write information on the videodisc. The comparator output can be used to verlfy the quallty of the ln~ormation being stored on the vldeodisc, and it can output a sl~nal indicating that proper comparison or reglstration has not been achieved.
A delay clrcuit receives a portion of the video lnput slgnal before routing to the comparator. The delay clrcuit imparts a delay in the input video ~lgnal e~ual to the accumulated valu~s o~ the delay fronl rrequency g modulation of the input vldeo signal through rrequency demodulation Or the signal from the sensor. Included in the delay time is the time necessary for the videodisc to move from the point of stora~e of the lnformation signal by the writlng beam to the point o~ implngement by the reading light beam.
BRIEF DESCRIPTION OF THE DR~WINGS
. . . _ FIG. 1 is a generalized diagram of the appar-- atus of the present invention;
~IG. 2 is a view ~ the optical path through the ob~ective lens of FIG. l;
~ IG. 3 is a representational indication of the relative spacing between a point of impingement of the writing beam and of the reading beam; and F~G. 4 is a diagram of a novel Pockels cell -stabilizing circuit.
D~TAILED DESCRIPTION OF THE INVENTION
Turning first to FIG. 1, the writing apparatus 10 includes a writing head 12 which is, in the preferred embodiment, a dry microscope ob~ective-lens 14 mounted upon an air bearing suDport member 16. A 40X lens has been found t.o be satisfactory. A disc 18 is specially prepared and may be constructed according to the teach-ings o~ the prior art, in which a substrate has coated 25 thereon a very thin film of a metal with a reasonably low melting point and a high surface tension.
A crystal oscillator 20 controls the drive elements. The disc 18 is rotated by a first, rotational drlve element 22 which is coupled to a spindle 24. A
30 8econd, translational drive element 26 controls the - position of the writing head 12.
A translatlng carriage 28, whlch ls driven by the translational drive element 26 through a lead screw and travellin~ nut, moves the writing head 12 in the 35 radial directlon relative to the rotating disc 18. The .
` 1055157 radial dlrection relative to the rotating di~c 18. The carriage 28 is provlded with approprlate mlrrors and lenses so that the remainder of the optics and elec-tronics necessary to the writing devlce may be perman-ently mounted.
In the prererred embodiment of the`presentinvention, the beam of a polarized cutting laser 30, which ls an argon ion laser, is passed through a Pockels cell 32 which is driven by the Pockels cell driver 34.
An FM modulator 36 receives the video signal that is to be recorded and applies the appropri~te control signals to the Pockels cell driver 34. `
As described hereinarter, the video in signal is o~ the t~rpe displayable on a TV monitor. Accordingly, it is a voltage varying with time slgnal. The FM modu-lator 36 is o~ standard design and converts the voltage varying with time signal to a frequency modulated signal having its informational content in the form of a carrier frequency havinæ frequency changes with time correspondin~ to said voltage variations with time As is known, the Pockels cell 32 responds to applied sign~l voltages by rotating the plane of polar-ization of the light beam. Since a linear polarizer transmits light only in a predetermlned polarization plane, a polarizer, such as a Glan prism 38 in the pre-rerred embodiment, is included in the writing beam path to provlde a modulated writing beam 40. The modulated writlng beam erfectively follows the output o~ the FM
modulator 36.
The modulated writing beam 40 emerging from the Pockel~ cell-Glan prism combination 32, 38 is applied to a first mirror 42 which directs the writing beam 40 to the translatlng carrlage 28. The rirst mirror 42 tranæmlts a portion o~ the writlng beam 40 to a Poc~els cell stabilizing circuit 44 which responds to the aver-age intensity o~ the writlng beam to maintaln the energy level Or the beam.
- ~ lens 46 1~ inserted in the path o~ the writ-lng beam 40 to dlverge the æubstantlally parallel beam '' ~ ' ' ' ' ~
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~055~57 so that it wlll spread to fill the entrance aperture of the ob~ective lens 14 for optimum resolution. A di-chroic mlrror 48 is included in the path oriented to substantlally transmit all of the writing beam 40 to a second, articulated mirror 50. A mirror, such as has been shown in applicant's Canadlan Patent No. 1,012,643, issued June 21, 1977, may be employed in the present in-ventlon. The articulated mirror 50 then directs the beam through the lens 14 and is capable Or shiftlng the point of implngement ôf the beam 40 on the surface of the disc 18.
A series of holes ls formed in the metal coat-ing by the writing beam. One hole is formed for each cycle of the FM modulated signal represented by the m~dulated writing beam 40. Since the modulated writing beam tracks the output of the FM modulator 36, the holes formed in the coating also track the output of the FM
modulator. Obviously, since the informational content in the output signal of the FM modulator 36 is in the form of a frequency changes in time about a carrier fre-quency, and since the "hole", "no hole'i sequence repre-sents the stored information, and since the disc 18 is rotating at a unlform speed, the "hole" 'lno hole"
sequence changes to represent the stored video informa-tion by the holes being formed closer or farther apart 25 and the size of the hole becomes larger or smaller as -the writing beam 40 changes under the control of the FM
modulated output signal from the FM modulator 36.
m e ob~ectlve lens 14 and the assoclated air bearing 16 effectively fly on a cushion of air a~ a 3 substantially fixed distance from the surface of the dlsc 18. That dlstance is determined by the geometry of the bearing 16, the linear velocity of the dlsc 18, and the force used to load the head against the disc 18.
m e flxed spacing is required because the focal toler-ance Or a lens capable of resolving a 1 ~m spot ls alsoof the order of 1 ~m.
A second, relatively low-power laser 52 pro-vldes a monltoring beam 54. In the preferred embodlment, the reading laser 52 ls a helium-neon device which , , -12~
enables the reading beam 54 to be distlnguish~d ~ro~
the ~Jriting beam 40 by wavelen~th. A polarizing, beam splitter cube 56 transmits the reading beam 54 to a mirror 58 that directs the beam 54 through a second diver~i~g lens 60 that spreads the reading beam 54 to ~ill the entrance aperture o~ the objective lens 14.
A quarterwave plate 62 is placed in the opti~
cal path and~ in con~unction with the plane polarizing beam splitter 56, prevents light reflected from the disc 18 ~rom re-entering the laser 52 and upsetting its mode Or oscillation. The quarterwave plate 62 rotates the plane of polarization o~ the beam by 45 degrees on each pass so that the reflected beam is rotated 90 degrees wit`n respect to the polarizing beam spl~tter 56 and is there~ore not passed by it.
A second mlrror 64 in the reading beam 54 path directs the beam into the dichroic mirror 43 and is capable o~ llmited adjustment so that the paths o~ the writing and reading beams are substantially identical, except that the reading beam "spot" impinges on the dlsc 18 downstream from the writing beam spot as explained in-greater detail below.
A filter 66 that is opaque to the argon ion beam is interposed in the path of light reflected from ~5 the beam splitter 56. The He.Ne reading beam 54 that i8 returned from the disc suxface ls able to pass through the filter 66 and through a lens 68 onto a photodetec-tor 70.
The re~lected light o~ the reading beam im-plnges upon the photodetector 70. The photodetector 70operates in its standard manner and generates an elec-trlcal current representative of the light impinglng thereupon. In this case, the photodetector generates the ~ignal represented by the "hole", "no hole" con-~lguration formed in the coating. The "hole", "nohole" con~i~uration is representative o~ the output o~ the FM modulator 36. The output Q~ the FM modulator 36 i8 a carrier ~requency having frequency changes with tlme reprcsenting the vldeo signal to be record~d. The , , "hole", "no hole' configuration 1~ representatlve of a carrler frequency havlng ~requency c`anges with time representing the stored vldeo slgnal. The output of the photodetector 70 is an electricai slgnal representin~
the stored carrier frequency having frequency changes wlth time representing the stored vldeo signal.
The output of the photodetector 70 ls applied to a preamplifier 72 which provides a si~nal of suffi-clent amplitude and signal strength for subsequent 10 utilization. A video discriminator 74 then provides a vldeo output signal which can be utllized in several ways, two of which are shownJ as examples only.
The discriminator 74 is of standard design and function. It takes the frequency modulated signal from i5 the photodetector 70 and changes it to a time dependent voltage signal having its informational content in the form of a voltage varying with time format suitable for dlsplay in the TV monitor 76.
In a first application, the video output is applied to a TV monitor 76 and an oscilloscope 78. As ls well known, the TV monitor is responsive to a vol-tage varying~with time signal. The information to be displayed on the TV monitor is represented by a voltage change with time.
The TV monitor 76 shows the picture fidelity Or the recording, and the oscilloscope 78.indicates the slgnal-to-noise ratio of the record and the quality of the cutting, whether it is light or heavy. Not shown, an appropriate ~eedback loop could be provided through 3o the Pockels cell stabilizing circuit 44 to assure an adequate discrimination on the disc between a "hole" or "black" area and "no hole" or "white" area.
As an alternative utilization, the ~ideo out-put of the discriminator 74 is applied to a comparator 35 80. The other lnput of the comparator 80 ls taken from th~ video input signal which ls directed through a delay llne 81. A delay that is equal the accumulated delays o~ the writlnG system and the time elapsed between the lnstant of writln~ o~ the information and the tl~e required for that lncremental area of the disc to reach the reading point must be lmparted to the lnput vldeo signal.
Ideally, the video output signal of the dis-crimina~or 74 should be identical in all respects to thevldeo input signalJ after the proper delay.
As previously mentloned, the output from the dlscrimlnator 74 is a voltage varying with time signal.
The video ln signal is also a voltage varying with time signal. Any dlf~erences noted represent errors which might be caused by imperfections in the disc's surface or malfunctions of the writing circuits. Thls applica-tlon, while essential if recording digital information, is less critical when other inrormation is recorded.
The output of the comparator circuit 80 can be quantized and counted, so tha~ an acceptable number o~ errors can be established for any disc. I^lhen the errors counted exceed the standard, the writing opera-tion can be terminated. If necessary, a new disc can be written. Any disc with excessive errors can then be reprocessed to serve as a "new" disc ror a subsequent recording.
Well-known techniques are available to trans-late the writing head assembly 12 in the radial direc-tion with respect to the rotating disc 18. I~lhile in FIG. 1 the rotational and translational drives 20, 22 are indicated as independent, the drives are synchronized to enable the writing assembly 12 to translate a pre-determined increment for each revolution of the disc 18, 3 by means of the common crystal oscillator 20.
Turning next to FIG. 2 there ls shcwn, in somewhat exaggerated form, the slightly differing opti-¢al paths of beam 40 from the writing laser 30 and the beam 54 rrom reading laser 52. The writing beam 40 coincldes with the optical axis Or the mlcroscope ob~ec-tlve ~ens 14. me readlng beam 54, in contrast, makes an angle ~ with the axis so that it ralls some distance X, equal to ~ tlmes the focal length o~ the obJective, "downstream' ~rom where the writing bcam 40 is "cuttlng".
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The resulting delay between reading and writing allows the molten metal to ~olldlfy so that the recording is read in it~ final state. I~ lt were read too 800n while the metal was still molten, lt would not provide per-tlnent informatlon ror ad~usting the recording parameters.
Thls ls best indlcated in FIG. 3 where twopolnts in the same informatlon channel are sho~1n as displaced. The point A, which is the polnt of impinge-ment of the writing beam 40, is shown as being on the 10 optical axis of the ob~ective lens 14. Separated from point A, in the direction of medium motion, as indicated by the arrow, is the reading point B, which is at an .angle ~ from the axis of the microscope objective lens 14. A distance between points A and ~ of two~ has provided a satis~actory monitoring of the writing oper-ation.
Turning finally to FIG. 4, there is shown an idealized diagram of a Pockels cell stabilizing circuit 44, suitable for use in the apparatus of FIG. 1. As is known, a Poc~els cell rotates the plane of polarization of the applied light as a function of an applied vol-tage. Therefore, the Pockels cell is used to rotate plane polarized light, and the rotated light is passed through a plane polarizer, such as a Glan prism. The llght issuing from the polarizer will be amplitude modulated in accordance with the applied voltage, Stated another way, the standard operating mode of a Pockels cell 32 and Glan prism 38 is for use as a light intensity modulating means. Each cycle from the FM modulator drives the Pockels cell through its full operating range ~ nlnety degrees. Within this operatlng range of ninety degrees, one operating point passes all light applied thereto and identified as a ~ull light transmitting state. A second operating point passes no light and is identlfied as a full light blocking sta'ce. The Pockels cell itself only rotates the plane of polarlzation. The Glan prism passes light ln one plane of polarization and no llght in the plane dl~placed ninety degrees rrom that plane ln which all ..
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-- 1055~57 the llght passes.
Dependlng upon the lndividual Pockels cell, a - voltage change Or approximately 100 volts wlll cause the cell to rotate the plane of polarlzation through 360 degrees. However, the transfer characteristic o~ an indlvidual cell may drift spontaneously, corresponding to a voltage change of + 50 volts, and accordingly, a ~eedback loop is desirable to maintain the cell within a use~ul, reasonably linear, operating ra~ge.
The stabilizing circuit 44 includes a photo-sensitive silicon diode 82, which is positioned to `
receive a portion of the writlng beam 40 reflect~d ~rom the mirror 42 o~ FIG. 1. The silicon diode 82 ~unctions ` in much the same fashion as a solar cell and is a source of electrical energy when illuminated by incident radiation. One terminal o~ the silicon diode 82 is connected to common re~erence potential 84, indicated by the conventional ground symbol, and the other terminal is connected to one input Or a differential ampli~ier 86; The silicon cell 82 is shunted by a load 88 which enables a linear response mode.
me other input to the dirferential amplifier 86 is connected through an appropriate potentiometer 90 to the common reference 84. A source o~ power 92
2~ is coupled to the potentiometer 90, which enables the setting of the differential a~plifier 86 to establish the average light level transmitted by the Pockels cell 32.
Accordln~ly, a palr of output termlnals Or the
Accordln~ly, a palr of output termlnals Or the
3 dirrerentlal amplifier 86 are respectively connected through resistive elements 94, 96 to the input terminals o~ the Pockels cell 32 of FIG. 1. It is noted that the Pockels cell driver 34 is a.c. coupled to the Pockels cell 32, while the dir~erential amplifier 86 is d.c.
coupled to tlle Pockels cell 32.
In operation, the system is energized. The l~ght rrom the writing beam impinging on the silicon dlode ~2 generates a dl~rerentlal volta~e at the input to the dirrerential amplirier ~6. Initially, the .
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. ' ' ' ' ~ ', "'~
, . , , potentlometer 90 is adjusted to produce llght at a predetermlned averaF,e level of lntensity. Therearter, lr the average level o~ intensity i.mplnging on the silicon cell 82 either increases or decreases, a correct-ing voltage will be generated in the difrerential am-plirier ~6. The correcting voltage applied to the Pockels cell 32 is of a polarity and magnitud2 adequate to restore the average level of intensity to the pre-determined level.
Thus there has been sho~n an improved video disc recording assembly. A microscope ob~ective lens mounted on an air bearing "flies" at a predetermined distance ~rom the sur~ace of a metallized disc. The metallized coating is such that a laser beam can, under suitable modulation, deliver su~icient energy to melt localized areas o~ the surface. Under surface tension, the molten metal retracts leaving a clear area of appro~
im~tely one micron in diameter.
A second, low-energy laser utilizing substan tially the same optical path is directed through the same microscope objectlve lens, but is brought to the surface ~ the disc at a slight distance "do~lnstream"
rrom the point of writing. The reading beam is returned through an appropriate optical system that excludes the re~lected energy o~ the writing beam and enables an analysis of the lnformation that has been written on the disc.
The playback information can, among other things, control the intensity o~ the writing beam to assure adequate "recording levels", determine whether an unacceptable number of errors have been made in the . recording process.
coupled to tlle Pockels cell 32.
In operation, the system is energized. The l~ght rrom the writing beam impinging on the silicon dlode ~2 generates a dl~rerentlal volta~e at the input to the dirrerential amplirier ~6. Initially, the .
.
.
. . .
. ' ' ' ' ~ ', "'~
, . , , potentlometer 90 is adjusted to produce llght at a predetermlned averaF,e level of lntensity. Therearter, lr the average level o~ intensity i.mplnging on the silicon cell 82 either increases or decreases, a correct-ing voltage will be generated in the difrerential am-plirier ~6. The correcting voltage applied to the Pockels cell 32 is of a polarity and magnitud2 adequate to restore the average level of intensity to the pre-determined level.
Thus there has been sho~n an improved video disc recording assembly. A microscope ob~ective lens mounted on an air bearing "flies" at a predetermined distance ~rom the sur~ace of a metallized disc. The metallized coating is such that a laser beam can, under suitable modulation, deliver su~icient energy to melt localized areas o~ the surface. Under surface tension, the molten metal retracts leaving a clear area of appro~
im~tely one micron in diameter.
A second, low-energy laser utilizing substan tially the same optical path is directed through the same microscope objectlve lens, but is brought to the surface ~ the disc at a slight distance "do~lnstream"
rrom the point of writing. The reading beam is returned through an appropriate optical system that excludes the re~lected energy o~ the writing beam and enables an analysis of the lnformation that has been written on the disc.
The playback information can, among other things, control the intensity o~ the writing beam to assure adequate "recording levels", determine whether an unacceptable number of errors have been made in the . recording process.
Claims (39)
1. Information handling apparatus, comprising:
first means for providing a video information signal to be recorded, said signal having its informational con-tent in the form of a carrier frequency having frequency changes in time representing said information to be recorded; a record carrier including a substrate having a first surface and a light responsive coating covering said first surface for retaining indicia representative of said video signals; a first light source for provid-ing a light beam, said first light beam being of suffi-cient intensity for interacting with said coating, and for altering said coating to retain indicia representa-tive of said video information; means for imparting relative motion to said record carrier with respect to said light beam; first optical means for defining a first optical path between said first light source and said record carrier including said coating, and for focusing said first light beam upon said coating; light intensity modulating means positioned in said optical path between said light source and said coating on said record carrier, said light intensity modulating means operating over a range between a higher light trans-mitting state and a lower light transmitting state for intensity modulating said light beam with said informa-tion to be stored; said light intensity modulating means being responsive to said frequency modulated signal and changing between its higher light transmitting state and its lower light transmitting state during each cycle of said frequency modulated signal for intensity modulating said light beam with the frequency modulated electrical signal to be stored; said light passing through said light intensity modulating means and focused upon said coating by said optical means for altering said coating to retain indicia representative of said video information; a second light source for providing a second light beam; a second optical means for defining an optical path between said second light source and said record carrier, including a portion of said first optical path, for focusing said second light beam upon said coating; said second light beam being of sufficient intensity for illuminating selected por-tions of said coating on said carrier, said second light beam being highly reflected from certain ones of said illuminated portions; said second optical means being further employed for collecting said reflections from said certain ones of said illuminated portions; and sensing means responsive to said reflected light for generating a frequency modulated electrical signal cor-responding to said reflections, and said last mentioned frequency modulated signal having its informational con-tent in the form of a carrier frequency having frequency changes in time corresponding to the stored video in-formation.
first means for providing a video information signal to be recorded, said signal having its informational con-tent in the form of a carrier frequency having frequency changes in time representing said information to be recorded; a record carrier including a substrate having a first surface and a light responsive coating covering said first surface for retaining indicia representative of said video signals; a first light source for provid-ing a light beam, said first light beam being of suffi-cient intensity for interacting with said coating, and for altering said coating to retain indicia representa-tive of said video information; means for imparting relative motion to said record carrier with respect to said light beam; first optical means for defining a first optical path between said first light source and said record carrier including said coating, and for focusing said first light beam upon said coating; light intensity modulating means positioned in said optical path between said light source and said coating on said record carrier, said light intensity modulating means operating over a range between a higher light trans-mitting state and a lower light transmitting state for intensity modulating said light beam with said informa-tion to be stored; said light intensity modulating means being responsive to said frequency modulated signal and changing between its higher light transmitting state and its lower light transmitting state during each cycle of said frequency modulated signal for intensity modulating said light beam with the frequency modulated electrical signal to be stored; said light passing through said light intensity modulating means and focused upon said coating by said optical means for altering said coating to retain indicia representative of said video information; a second light source for providing a second light beam; a second optical means for defining an optical path between said second light source and said record carrier, including a portion of said first optical path, for focusing said second light beam upon said coating; said second light beam being of sufficient intensity for illuminating selected por-tions of said coating on said carrier, said second light beam being highly reflected from certain ones of said illuminated portions; said second optical means being further employed for collecting said reflections from said certain ones of said illuminated portions; and sensing means responsive to said reflected light for generating a frequency modulated electrical signal cor-responding to said reflections, and said last mentioned frequency modulated signal having its informational con-tent in the form of a carrier frequency having frequency changes in time corresponding to the stored video in-formation.
2. The apparatus as claimed in Claim 1, where-in said first means comprises: means for providing an initial information signal having its informational content in the form of a voltage varying with time format; and frequency modulator means, responsive to said means providing an initial information signal for converting said voltage varying with time signal to said frequency modulated signal.
3. The apparatus as claimed in Claim 2, in-cluding demodulator means responsive to the output of said sensing means for changing said frequency modulated electrical signal into a time dependent voltage signal representing said stored video information, said time dependent voltage signal having its informational content in the form of a voltage varying with time format and being suitable for display by a standard television monitor.
4. The apparatus as claimed in Claim 3, in-cluding means for comparing said voltage varying with time video information signal from said demodulator means with said initial information signal.
5. The apparatus as claimed in Claim 4, wherein said second light beam is focused upon said record carrier at a point on said carrier downstream of the point of impingement of said first light beam, and wherein said apparatus further comprises delay means in the signal path of said initial information signal, said delay means imparting a time delay of said initial information signal equal to the accumulated values of the delay from frequency modulation of said initial information signal through frequency demodulation of said signal from said sending means and including the delay of travel time of the point on said carrier moving from the point of impingement of said first beam to the point of impingement of said second beam.
6. The apparatus as claimed in Claim 2, wherein said light intensity modulating means includes electrically controllable means responsive to said fre-quency modulator means for varying the intensity of said first light beam above a predetermined intensity at which the focused first beam alters said coating and below said predetermined intensity at which the focused first beam fails to alter said coating, said alteration being representative of said frequency modulated signal.
7. The apparatus as claimed in Claim 6, where-in: said initial information signal has its informational content in the form of a voltage varying with time sig-nal suitable for display on a standard television monitor; said first light source comprises a writing laser for producing a collimated writing beam of polar-ized monochromatic light, said substrate defines a smooth flat rigid disc, said first surface being a planar surface; said coating is a thin opaque metallized coating having suitable physical properties to permit localized heating responsive to the impingement of light from said writing laser, said heating causing localized melting accompanied b withdrawal of the molten material toward the perimeter of the melted area, leav-ing upon freezing a permanent aperture in the thin metallized coating; and said electrically controll-able means is responsive to said frequency modulator means for varying the intensity of said writing beam above said predetermined intensity at which the focused beam melts said metallized coating without vaporizing it and below said predetermined intensity at which the focused beam fails to melt said metallized surface.
8. The apparatus as claimed in Claim 1, where-in: said record carrier is disc-shaped; said means for imparting relative motion comprises rotational drive means for producing uniform rotational motion of said disc; and wherein said apparatus further comprises:
translational drive means synchronized with said rota-tional drive means for relatively moving said first and second focused light beams radially across said first surface of said disc-shaped record carrier; and elec-trical synchronizing means for maintaining a constant relationship between said rotational motion and said translational motion.
translational drive means synchronized with said rota-tional drive means for relatively moving said first and second focused light beams radially across said first surface of said disc-shaped record carrier; and elec-trical synchronizing means for maintaining a constant relationship between said rotational motion and said translational motion.
9. The apparatus as claimed in Claim 6, where-in said light intensity modulating means further includes feedback apparatus for stabilizing the operating level of said electrically controllable means to produce the intensity levels of said first light beam above and below said predetermined intensity, said light intensity modulating means including light-sensing means for sens-ing at least a portion of the modulated first light beam issuing from said electrically controllable means to produce an electrical feedback signal representative of the intensity of the modulated first beam and apply-ing the feedback signal to said electrically controll-able means to stabilize its operating level.
10. The apparatus as claimed in Claim 9, where said light-sensing means produces an electrical feed-back signal which is representative of the average intensity of the first light beam issuing from said electrically controllable means the operating level of said light intensity modulating means being stabi-lized to issue the modulated first light beam at a substantially constant average power level.
11. The apparatus as claimed in Claim 7, wherein said first and second optical means include:
an objective lens; and hydrodynamic air bearing means for supporting said lens above said first surface of said record carrier.
an objective lens; and hydrodynamic air bearing means for supporting said lens above said first surface of said record carrier.
12. The apparatus as claimed in Claim 11, wherein said collimated first beam of light has sub-stantially parallel light rays; said objective lens has an entrance aperture larger in diameter than the diam-eter of said first light beam as provided by said first light source; and said first optical means further includes mirror means for folding said first light beam path provided by said first light source, and a diverg-ing lens for spreading the substantially parallel light beam from said first light source to at least fill said entrance aperture of said objective lens.
13. The apparatus as claimed in Claim 6, wherein said first light source produces a polarized laser beam, and said electrically controllable means includes: means for rotating the plane of polarization of said first laser beam from said first source under control of said initial frequency modulated signal;
and a linear polarizer, the output of which is a modu-lated laser beam corresponding to said initial fre-quency modulated signal.
and a linear polarizer, the output of which is a modu-lated laser beam corresponding to said initial fre-quency modulated signal.
14. The apparatus as claimed in Claim 10, wherein said feedback apparatus includes level adjust-ment means for selectively adjusting the average power level of said modulated first light beam to a predeter-mined value.
15. The apparatus as claimed in Claim 10, wherein: said electrically controllable means comprises a Pockels cell driver and a Pockels cell device, said Pockels cell driver responding to said initial fre-quency modulated signal to provide corresponding driving signals to said Pockels cell device; and wherein said Pockels cell driver is A.C. coupled to said Pockels cell, and said stabilizing feedback apparatus is D.C.
coupled to said Pockels cell.
coupled to said Pockels cell.
16. The apparatus as claimed in Claim 1, further comprising: a beam polarizer, and a polarization shifting means for shifting the plane of polariza-tion of said second beam, said polarizer and shifting means disposed in the path of said second light beam with the beam polarizer located between said second light beam source and said shifting means, said shifting means rotating said second beam during accumulative incident and reflected passages through the shifting means to and from the record carrier, said polarizer substantially reducing the intensity of the reflected second beam passed through said shifting means toward said second light beam producing means.
17. The apparatus as claimed in Claim 16, wherein said shifting means is a quarter wave plate for rotating said second light beam ninety degrees by accumulative incident and reflected passages through said plate.
18. The apparatus as claimed in Claim 16, wherein said beam polarizer is a polarizing beam split-ting cube adapted to direct said reflected second beam passed through said shifting means toward said light sensing means.
19. The apparatus as claimed in Claim 12, wherein said second beam of light is collimated and has substantially parallel light rays; the entrance aperture of said objective lens being larger in diameter than the diameter of said second light beam as provided by said second light source, and said second optical means further includes mirror means for folding said second light beam path provided by said second light source, and a second diverging lens for spreading the substantially parallel light beam from said second light source to at least fill said entrance aperture of said objective lens.
20. The apparatus as claimed in Claim 19, wherein said first and second optical means include beam directing means for directing said first beam toward said objective lens at an angle with respect to said second beam, thereby spacing the beams as they emerge from said objective lens and impinge upon said record carrier, said second beam impinging said record carrier downstream of said first beam.
21. The apparatus as claimed in Claim 1, wherein: said first light source produces an argon ion laser beam; said second light source produces a Helium-Neon laser beam; and wherein said second optical means includes a filter, opaque to an argon ion beam, in the path of said second beam reflected from said record carrier toward said sensing means.
22. A method for storing information on and retrieving information from an information storage member using a pair of laser beams, comprising the steps of: providing a frequency modulated electrical signal to be recorded, said frequency modulated elec-trical signal having a carrier frequency with frequency changes over time corresponding to said information to be stored; controlling the intensity of the transmission of a first light beam upon a light responsive surface of an information storage member, using said frequency modulated signal as a control signal; moving the information storage member at a constant rate relative to said first light beam while focusing said first light beam upon said light responsive surface of said information storage member; said controlling step in-cluding using said transmitted first light beam for irreversibly altering said light responsive surface of said information storage member under the control of one portion of said frequency modulated signal, as said member moves at a constant rate, and lowering the intensity of the transmitted first light beam to said light responsive surface of said information storage member for leaving said light responsive surface un-altered under the control of a second portion of said frequency varying modulated signal, as said member moves at a constant rate, said controlling step thereby producing in the light responsive surface of the storage member a lineal series of regions stored in track-like fashion upon said surface, said regions being of alternately high and low directional reflectivity normal to said surface, the sequence of alternate regions representing said frequency modulated signal; imaging a second light beam of polarized colli-mated light upon said series of regions, said moving step providing relative motion between said second imaged beam of light and said alternate regions for generating reflections from said light reflective regions representing said stored frequency modulated signal; and sensing said reflections and generating a frequency modulated electrical signal corresponding to said reflections, said frequency modulated electrical signal having its informational content in the form of a carrier frequency having frequency changes in time from said carrier frequency.
23. The method as claimed in Claim 22, where-in said step of providing a frequency modulated elec-trical signal includes: providing an initial electrical signal having its informational content in the form of a voltage varying with time format; and changing said voltage varying with time signal to said frequency modulated electrical signal having its informational content in the form of a carrier frequency having fre-quency changes with time corresponding to said voltage variations with time.
24. The method as claimed in Claim 23, where-in said first and second light beams are held stationary and said storage member is moved at said constant rate relative to the stationary first and second beams.
25. The method as claimed in Claim 23, where-in said controlling step includes using said frequency modulated signal for varying the intensity of said first light beam above a predetermined intensity at which the focused beam alters said light responsive surface and below a predetermined intensity at which the focused beam fails to alter said light responsive sur-face, said alteration being representative of said frequency modulated signal.
26. The method as claimed in Claim 25, where-in: said initial information signal has its informational content in the form of a voltage varying with time signal suitable for display on a standard television monitor; said controlling step includes producing a modulated collimated writing laser beam of polarized monochromatic light for impinging upon said light responsive surface of the storage member, said surface being a thin planar opaque metallized coating having suitable physical properties to permit localized heating responsive to the impingement of light from said writ-ing laser, said heating causing localized melting accompanied by withdrawal of the molten material toward the perimeter of the melted area, leaving upon freezing a permanent aperture in the thin metallized coating;
and said controlling step includes using said frequency modulated signal for varying the intensity of said writing beam above said predetermined intensity at which the focused beam melts said metallized coating without vaporizing it and below said predetermined intensity at which the focused beam fails to melt said metallized surface.
and said controlling step includes using said frequency modulated signal for varying the intensity of said writing beam above said predetermined intensity at which the focused beam melts said metallized coating without vaporizing it and below said predetermined intensity at which the focused beam fails to melt said metallized surface.
27. The method as claimed in Claim 24, where-in the storage member is disc-shaped and said storage member moving step includes: producing uniform rotational motion of said disc; and synchronizing said rotational motion with movement of said storage member for effect-ing relative movement of said first and second light beams radially across said surface of said disc-shaped storage member to maintain a constant relationship between said rotational motion and said translational motion.
28. The method as claimed in Claim 26, wherein said controlling step further includes stabilizing the level of modulation of said first light beam to produce the intensity levels of said first light beam above and below said predetermined intensity; sensing at least a portion of the laser writing beam after modula-tion of the writing beam to produce an electrical feedback signal representative of the intensity of the writing beam; and utilizing the feedback signal in said controlling step to effect stabilization of the level of modulation of said writing beam.
29. The method as claimed in Claim 28, where-in said step of sensing at least a portion of said writing beam produces an electrical feedback signal which is representative of the average intensity of the modulated writing beam, the operating level of light beam modulation being stabilized to issue the modulated writing beam at a substantially constant average power level.
30. The method as claimed in Claim 26, where-in said controlling step includes: rotating the plane of polarization of said writing laser beam under control of said frequency modulated signal; and linearly polar-izing the rotating beam to produce a modulated laser writing beam corresponding to said frequency modulated signal.
31. The method as claimed in Claim 28, where-in said controlling step includes selectively adjusting the average power level of said modulated writing beam to a predetermined value.
32. The method as claimed in Claim 28, where-in said controlling step comprises: amplifying said frequency modulated signal to provide corresponding driving signals to a Pockels cell device; A.C. coupling the amplified frequency modulated signal to the Pockels cell; and D.C. coupling said feedback signal to said Pockels cell.
33. The method as claimed in Claim 23, in-cluding the step of demodulating said frequency modu-lated electrical signal produced in said sensing step to produce a time dependent voltage signal representing said stored information, said time dependent voltage signal having its informational content in the form of a voltage varying with time format and being suitable for display by a standard television monitor.
34. The method as claimed in Claim 22, further comprising the step of shifting the plane of polari-zation of said polarized second beam by rotating said second beam during incident and reflected passages to and from the storage member, thereby substantially reducing the intensity of the reflected second beam passed to a light beam source utilized in producing the second light beam in said imaging step.
35. The method as claimed in Claim 24, where-in said shifting step includes rotating said second light beam ninety degrees by accumulative incident and re-flected passages, of said second light beam to said storage member surface.
36. The method as claimed in Claim 33, in-cluding the step of comparing the time dependent vol-tage signal produced in said demodulating step with said initial information signal.
37. The method as claimed in Claim 36, where-in said second light beam is focused upon said storage member at a point on said storage member downstream of the point of impingement of said first light beam, and wherein said method further comprises delaying said initial information signal, said delaying step impart-ing a time delay of said initial information signal equal to the accumulated values of the delay from frequency modulation of said initial information signal through frequency demodulation of said signal produced in said sensing step and including the delay of travel time of the point on said storage member moving from the point of impingement of said first beam to the point of impingement of said second beam.
38. The method as claimed in Claim 22, in-cluding providing an objective lens adjacent said storage member for receiving both of said first and second beams and for directing the beams toward said storage member, said method including the step of direct-ing said first beam toward the objective lens at an angle with respect to said second beam, thereby spacing the beams as they emerge from said objective lens and impinge upon said storage member.
39. The method as claimed in Claim 22, where-in: said first light beam is an argon ion laser beam;
said second light beam is a Helium-Neon laser beam;
and wherein said method includes optically blocking any portion of the argon ion beam in the path of re-flection from said storage member, prior to sensing said reflected second beam in said sensing step.
said second light beam is a Helium-Neon laser beam;
and wherein said method includes optically blocking any portion of the argon ion beam in the path of re-flection from said storage member, prior to sensing said reflected second beam in said sensing step.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA289,623A CA1069214A (en) | 1973-02-20 | 1977-10-27 | Videodisc mastering system |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1055157A true CA1055157A (en) | 1979-05-22 |
Family
ID=4109872
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA313,005A Expired CA1055157A (en) | 1977-10-27 | 1978-10-10 | Apparatus and method for storing information on and retrieving information from a videodisc |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1055157A (en) |
-
1978
- 1978-10-10 CA CA313,005A patent/CA1055157A/en not_active Expired
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