CA1057398A - Apparatus and method for stabilizing light beam modulation of a videodisc - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention relates to light modu-lator stabilization. The prior art has involved record-ing processes in open loop fashion, thereby being subject to random inaccuracies in the level of light intensity modulation. The present invention overcomes this problem by providing an apparatus and method for stabilizing a light modulator. In accordance with the invention, stabilization of the light modulator is effected by control of the average intensity of the modulated laser write beam impinging a metallized video-disc surface. Optical modulation is accomplished by an optical modulator positioned in the path of the laser write beam between a laser source and the metallized videodisc surface. A portion of the laser write beam, after optical modulation, is used to produce an elect-trical feedback signal. The feedback signal control affects stabilizations of the level of modulation of the laser write beam. The optical modulator varies the intensity of the laser write beam from above a pre-determined intensity at which an aperture is formed in the metallized videodisc surface to below the pre-determined intensity at which the beam forms no aperture in the metallized videodisc surface. The modulated laser beam is stabilized at a predetermined average intensity level between the levels above and below the predetermined intensity.

Description

~Os7398 LIGHT MODULATOR STABILIZATION
TECHNICAL FIELD
The present invention relates to light modu-lator stabilization, and more particularly to servo-control of a light modulator.
BACKGROUND OF THE PRIOR ART
Systems have heretofore been developed forrecording signals at video frequencies upon discs, tape~, or other media. Such systém~ have utilized, 10 among othér thlngs, optical recording upon photosensi- i tivè media, eleckron beam recording on thermo-plastic surfaces, and ~till other systems provide an instan-taneously reproducible record of video information.
The prior art can generally be divided into sy~tems utilizing photographic surfaces, systems util-izing electron beam sensitive surfaces, magnetic record-ing systems, and as in the present invention, systems in which a radiant energy beam cause~ an irreversible c-hange to a surface, thereby "writing" information thereon.
Photographic systems have bee~ described in the patents to P. C. Goldmark, et al, No. 3,234,326, which teaches recording on a continuous web such as a tape or film, or the patent to W. R. Johnson, No.
3,361,873, which teaches the photographic recordation of video information on a rotating disc in a spiral path.
The patent to W. C. Hughes, et al, No.
3,283,310, is lllustrative of recordation of information ~.

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on a thermo-plastic film surface~ which utilizes an electronic beam writing apparatus such as was disclosed ~n U. S. Patent No. 3J120,991.
Yet other systems have employed an electron beam to record information 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 utlllzing an electron beam on a photosensitive medium such as photographic film has been taught in the patent to R. F. Dubbe, et al) No. 3,444,317.
In recent years, alternative methods have been disclosed for high density recording which are based upon either removal of material or vaporization of material by laser beam bombardment. These methods have been discussed briefly in the magazine "Electronics' of March 3, 1969, 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 meetlng (Western Electronic Show and Co~vention), whlch paper was published ln Volume 12 of WESCON ~echnlcal Paper~ for 1968, at page 1 of Section 16/1. The authors have referred to articles in the December 23, 1966; issue of "S¢ience !1~ Volume 54, No.
3756, at pages 1550 and 1551, the Proceedings of the Fall Joint Computer Conference of 1966, pages 711-716, and an article in the "~ell Systems Technical Journal"
of March 1968, pages 385, 405.
These publications disclose a recordlng technique which utilizes a thin metallic film coating 30 upon a substrate. The thin metal film, under applied heat, melts rapidly and forms small globules within a recorded spot. A highly concentrated spot of laser illumination can apply sufficient heat in a short enough time so that a suitably modulated laser beam 35 impinging upon a moving surface can produce a pattern of holes in the metallic surface which, when '1read back", can reproduce the information recorded.
As pointed out in the Carlson and Ives paper, supra, the size of the recorded spot or hole can be much .

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, 105'7~3 smaller than the diameter of the imaged laser beam.
By an appropriate choice of metal film material, film thickness, laser divergence and spot power, an appro-priate system can be designed to record video frequen-5 Gies wlth reasonably high resolution. However, the quality of the video signal reproduced from such record-ings has not been good due to a low signal-to-noise ratio resulting from the direct recording of the video signal onto the recording medium. Additionally, the 10 use of a modulated laser beam to selectively rpdocue a pattern of holes on the metallic surface of a vldeodisc is not conducive to faithfully represent an amplitude varying analog video signal. Likewise, reading back t the recorded information in the form of a track of 15 holes or spots on the surface of the disc is ineffec-tive to faithfully recreate an analog video signal from the recorded track on the disc. The lack of fidelity in the reproduced video signal can be attributed pri~
marily to the inability of the laser beam modulator to 20 "write" precise analog signals in the coating of the disc. The law quality of wrlting was also due in part to the lack o~ control of the optical modulator.
BRIEF SUMMARY 0~ THE INVENTION
The present invention provides an apparatus 25 and method for stabilizing a light modulator.
In accordance with the invention, stabiliza-tion of the light modulator is effected by control of the intensity of a modulated laser write beam imping-ing a metallized videodisc surface.
Optical modulation is accomplished by an opti-cal modulator positioned in the path of the laser write beam between a laser source and the metallized video-disc surface.
A portion of the laser write beam, after opti-cal modulation, is sensed to produce an electrical feedback signal. The feedback signal control affects stabilization of the level of modulation of the laser write beam.
The optical modulator varies the intensity o~

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the laser write beam above a predetermined intensity, at which an aperture is formed ln the metallized vldeo-disc surfaceg and below the predetermined intensity, at which the beam forms no aperture in the metallized videodisc surface. The modulated laser beam is stabi-lized at a predetermined average intensity level between the levels above and below the predetermined intensity.
In the preferred embodiment, an initial video signal is provided having its informational content in the form of a voltage varying with time signal. A
frequency modulator responsive to the voltage varying with time signal converts the signal into a frequency modulated signal. The frequency modulated signal optically modulates the laser write beam with corres-ponding frequency modulated information.
The laser wrike beam source produces a colli-mated writing beam of polarized monochromatic light.
The writing laser produces localized heating upon im-pingement of the vldeodisc. The videodisc is a flat rigid disc having an upper surface with a thin opaque metallized coating. The coating has suitable physical properties to permit localized heat~.ng by the writing laser causing localized rnelting accompanied by with-drawsl of the molten material toward the perimeter of the molten area. Upon ~reezing~ a permanent aperture is ].eft ln the metal coating.
Electrical control circuitry responsive to the frequency modulator varies the intensity of the writing beam to above the predetermined intensity at which the focused beam melts the metallized coating w~thout vaporizing it, and below the predetermined intensity, at which the focused beam fails to melt the metallized coating.
The feedback apparatus includes a level ad-justment to selectively ad~ust the average power levelof the light beam to a predetermined value. The elec-trical con~rol circuitry includes a Pockels cell driver controlling a Pockels cell device which~ together with a linear po]arizer effects intensity modulation of the ,' '' ~ , -:: , . - , ~: . ,,, : . .
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writing laser beam. The Pockels cell driver responds to the initial frequency modulated signal to provide corresponding driving signals to the Pockels cell de-vice. The Pockels cell driver is A.C. coupled to the 5 Pockels cell device and the feedback apparatus is D.C.
¢oupled to the Pockels cell device.
The novel ~eatures which are believed to be t characteristic of the inventiong both as to organiza-tion and method of operation, together with further 10 ob~ects and advantages thereof, will be better under-stood from the ~ollowing description considered in connection with the accompanying drawings in which several preferred embodiments of the invention are illus-trated by way of example. It is to be expressly under-15 stood, however, that the drawings are for the purpose of illu~tration and description only and are not in-tended as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
~IG. l is a ge~eralized diagram of the appar-20 atus of the present inventlon;
FIG. 2 is a view ~ the optical path through the ob~ective lens of FIG. l;
FIG. 3 is a representational indication of the relative spacing between a point o~ impingement of 25 the writing beam and of the reading beam; and FIG. 4 is a diagram of a novel Pockels cell stabilizing circui~.
DETAILED DESCRIPTION OF INVENTION
Turning first to FIG. l, the writing appara-30 tus ~ includes a writing head 12 which is, in the preferred embodiment, a dry microscope ob~ective lens 14 mounted upon an air bearing support member 16. A
40X lens has been found to be satisfactory. A disc 18 is specially prepared and may be constructed accord-35 ing to the teachings ~ the prior art, in which a sub-strate has coated 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 . . .:

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~0~'~3~3 , elements. The disc 18 is rotated by a first, rotational drive element 22 which is coupled to a spindle 24. A
aecond, translational drive element 26 controls the position of t:~e wrlting head 12.
A translating carriage 28, which is driven by the translational drive element 26 through a lead screw and travelling nut, moves the writing head 12 in the radial direction relative to the rotating disc 18. The carriage 28 is provided with appropriate mirrors and lenses so that the remainder of the optics and electronics necessary to the writing device may be permanently mounted.
In the preferred embodiment of the present invention, the beam of a polarized cutting laser 30, which is 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 applles the appropriate control signals to the Pockels cell driver 34.
~0 A~ described hereinafter, the video in signal i~ of the t~pe displayable on a TV monitor. Accordingly, it is a voltage varying with tlme signal. The FM modu-lator 36 ls of standard design and converts the voltage varying with time signal to a frequency modulated signal having its informatlon~l content in the form of a carrier frequency having frequency changes with time corresponding to said voltage variations with time.
As is known, the Pockels cell 32 responds to applied signal voltages by rotating the plane of polar-ization of the light beam. Since a linear polarizertransmits light only in a predetermined polarization plane, a polarizer, such as a Glan prism 38 in the preferred embodiment, is included in the writing beam path to provide a modulated writlng beam 40. The modu-lated writing beam effectively follows the output ofthe FM modulator 36.
The modulated writing beam 40 emerging from the Pockels cell-Glan prism combination 32, 38 is ap-plied to a first mirror 42 which directs the writing , ~ .

~ 1057398 7 t beam 40 to the translating carriage 28. The first mirror 42 transmits a portion of the writing beam 40 to a Pockels cell stabilizing circuit 44 which respo.lds to the average intensity of the writing beam to maintain the energy level of the beam.
~ lens 46 is inserted in the path of the writing beam 40 to diverge the substantially parallel beam so that it will spread to fill the entrance aperture of the ob~ective lens 14 for optimum resolution. A dichroic mirror 48 is included ln the path oriented to substantially transmit all of the writing beam 40 to a second, articulated mirror 50. The articulated mirror 50 then directs the beam through the lens 14 and is capable of shifting the point of impingement of the beam 40 on the surface of the disc 18.
A series of holes is formed in the metal coating by the wri~ing beam. One hole is formed for each cycle of the FM modulated signal represented by the modulated 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 frequency changes in time about a carrier frequency, and since the "hole", "no hole" sequence represents the stored informa-tion, and since the disc 18 is rotating at a uniform speed, the "hole", "no hole" sequence changes to represent the stored video information by the holes being formed mb/ ~

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105'7398 closer or farther apart and the size of the hole becomes larger or smaller as the writing beam 40 changes under the oo~rol o the FM modulator output signal from the FM modulator 36.
The ob~ective lens 14 and the associated air bearing 16 effectively fly on a cushion of air at a substantially fixed distance from the surface of the disc 18. That distance is determined by the geometry of the bearing 16, the linear velocity of the disc 18, and the force used to load the head against the disc 18, The fixed spacing is required because the focal tolerance . of a lens capable of resolving a l~m spot is also of the order of l~m.
A second, relatively low-power laser 52 provides a monitoring beam 54. In the preferred embodiment, the reading laser 52 is a helium-neon device which enables the reading beam 54 to be distinguished from the writing beam 40 by wavelength. A polarizing, beam splitter cube 56 transmits the reading beam 54 to a mirror 58 that directs the beam 54 through a second diverging lens 60 that spreads the reading beam 54 to fill the entrance aperture of the ob~ective lens 14.
A quarterwave plate 62 is placed in the optical path and, in con~unction with the plane polarizing beam splitter 56, prevents light reflected from the disc 18 from re-entering the laser 52 and upsetting its mode of 08cillation. The quarterwave plate 62 rotstes the plane of polarization of the beam by 45 degrees on each pass so mb/~

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105739~
-8a-that the reflected beam is rotated 90 degrees with respect to the polari~ing beam splitter 56 and is therefore not passed by it.
A second mirror 64 in the reading beam 54 path directs the beam into the dichroic mirror 48 and is .
capable of limited adjustment so that the paths of the writing and reading beams are substantially identical, except that the reading beam "spot" impinges on the disc 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 the beam splitter 56. The He.Ne reading beam 54 that is returned from the disc surface is able to pass through the filter 66 and through a lens 68 onto a photodetector 70.
The `reflected light of the reading beam impinges upon the photodetector 70. The photodetector 70 operates in its standard manner and generates an mb/ ~

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electrical current represen~ative o~ the light imping-ing thereupon. In this case, the photodetector gener-ates the signal represented by the "hole", no hole"
configuratlon formed in the coating. The "hole 13 ~ino hole" configuration is representative of the output of the FM modulator 3~. The output of the FM modulator 36 is a carrier frequency having frequency changes with time representing the video signal to be recorded. The "hole", "no hole' configuration is representative of a carrier frequency having frequency changes with time representing the stored video signal.
The output of the photodetector 70 is an electrical signal representing the stored carrier frequency having frequency changes with time repre-senting the stored video signal.
The output of the photodetector 70 is appliedto a preamplifier 72 which provides a signal of suffi-cienk amplitude and signal strength for subsequent utilization. A video discriminator 74 then provides a video output signal which can be utilized in several ways, two of which are shown, as examples only.
The discrimlnator 74 is of standard design and function. It takes the frequency modulated signal from 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 display 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 of the recording, and the oscilloscope 78 indicates the signal-to-noise ratio of the record and the quality of the cutting, whether it is light or heavy. Not shown, an appropriate feedback loop could be provided through the Pockels cell stabilizing circuit 44 to assure 1~5~73~

an adequate discrlmination on the disc between a "hole"
or "black" area and "no hole or "white" area.
As an alternative utilization, the video out-put of the discriminator 74 is applied to a comparator 80. The other input of the comparator 80 is taken from the video input signal which is directed through a delay line 81. A delay that is equal the accumulated delays of the writing system and the time elapsed between the instant of writing of the information and lO the time required for that incremental area of the disc to reach the reading point must be imparted to the input video signal.
Ideally~ the video output signal of the dis-criminator 74 should be identical in all respects to the video input signal, after the proper delay.
As previously mentioned, the output from the discriminator 74 is a voltage varying with time signal.
The video in signal is also a voltage varying with time signal. Any differences noted represent errors which might be caused b~ imperfections in the disc's surface or malfunctions of the writing circuits. This applica-tion, while essential if recording digital information, is less c.ritical when other information is recorded.
The output of the comparator circuit 80 can 25 be quantized and counted, so that an acceptable number of errors can be established for any disc. When 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 for a subsequent recording.
Well-known techniques are available to trans-late the write head assembly 12 in the radial direction with respect to the rotating disc 18. While in FIG. l the rotational and translational drives 20, 22 are in-dicated as independent, the drives are synchronized to enable the writing assembly 12 to translate a predeter-mined increment for each revolution of the disc 18, by means of the common crystal oscillator 20.

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Turnlng next to FIG. 2 there is sho~1n, in somewhat exaggerated form, the slightly dif~ering op~i-cal paths of beam 40 from the writing laser 30 and the beam 54 from reading laser 52. The writing beam 40 co incides with the optical axis of the microscope ob~ective lens 14. The reading beam 54, in contrast9 makes an angle with the axis so that it falls some distance X, equal to ~ times the focal length of the ob~ective, "downstream" from where the writing beam 40 is "cutting". The resulting delay between reading and writing allows the molten metal to solidify so that the recording is read in its final state. If it were read ~D
soon while the metal was still molten, it would not pro-vide pertinent in~ormatlon for ad~usting the recording parameters.
This is best indicated in FIG. 3 where two points in the same information channel are shown as dis-placed. The point A, which is the point of impingement of the writing beam 40, is shown as being on the optical axis ~ the ob~ective lens 14. Separated from point A, in the direction of medium motion, as indicated by the arrow, is the reading point Bg which is at an angle ~ from the axis ~ the microscope objective lens 14.
A di~tance between points A and B of two ~ has provided a satis~actory monitoring of the writing operation.
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 Pockels cell rotates the plane of polarlzation of the applied light as a function of an applied voltage.
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 light 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. ~ach cycle from the FM modulat,or drives the Pockels cell through its ~ull . :

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operating range of ninety degrees. Within this opera-ting range of ninety degrees, one operating point passes all light applied thereto and identified as a full light transm~tting state. A second operating point passes no light and is identified as a full light blockln~ state. The Pockels cell itself only rotates the plane of polarization. The Glan prlsm passes light ln one plane of polarization and no light in the plane displaced ninety degrees from that plane in which all 10 the light passes. ~;
Depending upon the individual Poc~els cell, a voltage change ~ approximately 100 volts will cause the cell to rotate the plane of polarization through 360 degrees. However, the transfer characteristic of an individual cell may drift spontaneously, correspond-ing to a voltage change of + 50 volts, and accordingly, a feedback loop is desirable to maintain the cell within a useful, reasonably linear, operating range.
The stabilizing circuit 44 includes a photo-gen~itive sillcon diode 82~ which i~ positioned toreceive a portion of the writing beam 40 reflected from the mirror L~2 of FIG. 1. The silicon diode 82 functions in much the same fashion as a solar cell and iB a source of electrical energy when illuminated by incident radia-tion. One terminal of the silicon diode 82 is connectedto common reference potential 84, indicated by the conventional ground symbol, and the other termlnal is connected to one input of a differential amplifier 86.
The silicon cell 82 is shunted by a load 88 which 3 enables a linear response mode.
The other input to the differential amplifier 86 is connected through an appropriate potentiometer 90 to the common reference 84. A source of power 92 is coupled to the potentiometer 90, which enables the setting of the differential amplifier 86 to establish the average light level transmitted by the Pockels cell 32.
Accordingly, a pair of output terminals of the differential amplifier 86 are respectively connected . ,. :. , : . - . ' , . . . . .
- . . -, . . .
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through resistive elements 94, 96 to the input terminals of 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 d-lfferential amplifier 86 is d.c.
coupled to the Pockels cell 32.
In operation~ the system is energized. The light from the writing beam impinging on the silicon diode 82 generates a differential voltage at the input to the differential amplifier 86. Initlally, the potentiometer 90 is adjusted to produce light at a pre-determined average level of intensity. Thereafter, i~
the average level of intensity impinging on the silicon cell 82 either increases or decreases, a correcting voltage will be generated in the differential amplifier 86. The correcting voltage applied to the Pockels cell 32 is of a polarity and magnitude adequate to restore the average level of intensity to the predetermined level.
Thus there has been shown an improved video disc recording as~embly. A micro~cope ob~ectlve lens mounted on an air be~ring "flles" at a predetermined distance from the surface of a metallized disc. The metallized coating is such that a laser beam can, under suitable modulation, deliver sufficlent energy to melt localized areas of the surface. Under surface tension, the molten metal retracts leaving 2, clear area of approximately one micron in diameter.
A second, low-energy laser utilizing substan-tially the same optical path is directed through the same microscope objective lens, but is brought to the surface of the disc at a slight distance "downstream"
from the point of writing. The .reading beam is re-turned through an appropriate optical system that ex-cludes the reflected energy of the writing beam and enables an analysis of the information that has been written on the disc.
The playback information can, among other thlngs, control the intensity of the writing beam to - assure adequate "recording levels", determine whether an . :

0 5~i'3 ~ 8 unacceptable number o~ errGrs have been made in the ~
recording process. :

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Claims (14)

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

2. The apparatus as claimed in Claim 1, wherein said light-sensing means produces an electrical feedback signal which is representative of the average intensity of the write beam, the operating level of said modulator being stabilized to issue the write beam at a substantially constant average power level.

3. The apparatus as claimed in Claim 1, in-cluding: means for providing an initial video informa-tion signal having its informational content in the form of a voltage varying with time format; and frequency modulator means responsive to said means providing an initial information signal for converting said voltage varying with time signal to a frequency modulated signal, said optical modulator responsive to said frequency modulated signal to optically modulate said laser write beam with corresponding frequency modulated information.

4. The apparatus as claimed in Claim 1, wherein: said laser write beam source produces a colli-mated writing beam of polarized monochromatic light;
said recording surface is defined by a smooth flat rigid disc having a planar upper surface; said surface layer is a thin opaque metallized coating having suit-able physical properties to permit localized heating responsive to the impingement of light from said write laser, said heating causing localized melting accom-panied 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 means is responsive to said modulated signal for varying the intensity of said laser write beam above said predetermined intensity at which the write beam melts said metallized coating without vapor-izing it and below said predetermined intensity at which the laser write beam fails to melt said metallized surface.

5. The apparatus as claimed in Claim 1, where-in said laser light source produces a polarized laser beam, and said optical modulator comprises: means for rotating the plane of polarization of said laser beam from said source under control of said modulated signal;
and a linear polarizer the output of which is a modu-lated laser beam corresponding to said modulated signal.

6. The apparatus as claimed in Claim 5, where-in said feedback apparatus includes level adjustment means for selectively adjusting the average power level of said modulated laser beam to a predetermined value.

7. The apparatus as claimed in Claim 1, where-in: said controlling means comprises a Pockels cell driver and a Pockels cell device, said Pockels cell driver responding to said modulated signal to provide corresponding driving signals to said Pockels cell device; and wherein said Pockels cell driver is A.C.
coupled to said Pockels cell, and said stabilizing feed-back apparatus is D.C. coupled to said Pockels cell.

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

9. The method as claimed in Claim 8, wherein said sensing step produces an electrical feedback signal which is representative of the average intensity of the modulated write beam, the operating level of optical modulation being stabilized to issue the write beam at a substantially constant average power level.

10. The method as claimed in Claim 9, includ-ing the step of developing said modulated electrical signal by: providing an initial electrical signal having its informational content in the form of a voltage varying with time format; and changing said voltage varying with time signal to a frequency modu-lated 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.

11. The method as claimed in Claim 8, where-in: said controlling step includes producing a modulated collimated laser write beam of polarized monochromatic light for impinging upon said recording surface, said surface being a thin planar opaque metallized coating on a smooth flat substrate, said coating having suit-able physical properties to permit localized heating responsive to the impingement of light from said laser write beam, said heating causing localized melting accompanied by withdrawal of the molten material toward the perimeter of the melted area, leaving upon freezing a permanent aperture in the thin metallized coating;
and said controlling step includes using said modulated signal for varying the intensity of said laser write beam above said predetermined intensity at which the laser write beam melts said metallized coating without vaporizing it and below said predetermined intensity at which the laser write beam fails to melt said metallized surface.

12. The method as claimed in Claim 8, wherein said controlling step includes: rotating the plane of polarization of said laser write beam under control of said modulated signal; and linearly polarizing the rotating beam to produce a modulated laser beam cor-responding to said modulated signal.

13. The method as claimed in Claim 12, wherein said controlling step includes selectively adjusting the average power level of said modulated laser write beam to a predetermined value.

14. The method as claimed in Claim 9, where-in said controlling step comprises: amplifying said modulated signal to provide corresponding driving signals to a Pockels cell device; A.C. coupling the amplified modulated signal to the Pockels cell; and D.C. coupling said feedback signal to said Pockels cell.