Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
. 106641~
APPARAT115 AND METHOD FOR STORING
INFOR~ATION ON A VIDEODISC
- TECHNICAL FIELD
The present inventlon relates to the storage
Or lnrormation upon an information storage member, and
more particularly to the writing of vldeo lnformation
on a vldeodlsc.
BACKG ~lND OF ~XE PRIOR ART
Systems have heretofore been developed ~or
10 recordlng signals at video frequencles upon dl~cs,
tapes, or other.media. Such system3 have utlllzed,
.... among other things, optical recording upon photosensl-
- tlve media, electron beam recordlng on thermo-plastlc
~urraces, and stlll other system~ prov~de an lnstan-
taneously repro~.'ucible r~cord Or vid~o inrormatlon.
The prlor art can generally be dlvided into
.. systems utlllzlng photographic sur~aces, systems utll-
lzlng electron beam sensitive surraces, magnetic
. .recording systems, and æs ln the present inventlon,
20 .systems in whlch a radlant energy beam ¢auses an lrre-
.versible.çharge to a surrace, thereby "wrltlng" inrorma-
... . .
tion thereon.
Photographic systems have been described ln
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, whlch teaches the photogr~.phic recordatlon
Or video in~ormatlon on a rotatlng dlsc in a splral
path.
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The patent to W. C. Hughes, et al, ~o. 3,283,310
is lllustrative Or recordation of lnformation on a
thermo-plastic fllm surface, which utilizes an electron-
lc beam writing apparatus such as was dlsclosed ln U. S.
Patent No. 3,120,991.
Yet other systems have emplbyed an electron
beam to record information on a special storage medlum.
One such system has been disclosed in the patent to D. P.
Gregg, No. 3,350,503. An alternative scheme utllizlng
an electron beam on a photosensitive medium such as
photographic film has been taught ln the patent to
R. F. Dubbe, et al, No. 3,444,317.
In recent years, alternative methods have been
disclosed for hlgh density recording whlch are based
upon either removal of material or vaporization of
materlal by laser beam bombardment. These methods have
been dlscussed brlefly in the magazine "Electronics",
of March 3, 1969, at page 110. Further, a "laser thermal
mlcro image recorder" was descrlbed ln some detall by
C. O. Carlson and H. D. Ives in a paper glven at the
1968 WESCON meetlng (Western Electronlc Show and Con-
ventlon), whlch paper was published ln Volume 12 of
WESCON Technical Papers for 1968, at page 1, of Sectlon
16/1. The authors have referred to artlcles in the
December 23, 1966, issue o~ "Science", 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 ln the "Bell Systems Technlcal Journal",
o~ March 1968, pages 385, 405.
These publlcatlons disclose a recording
technique which utilizes a thln metalllc film coating
upon a substrate. The thin metal film, under applied
heat, melts rapidly and forms small globules within a
recorded spot. A hlghly concentrated spot of laser
35 lllumlnation can apply sufficient heat in a short
enough time so that a sultably modulated laser beam
impinging upon a moving surface can produce a pattern
of holes ln the metalllc surface which, when "read back ,
can reproduce the lnformatlon recorded.
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10664~4
As pointed out in the Carlson and Ives paper,
supra, the size of the recorded spot or hole can be much
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 appropriate system can
be designed to record video frequencies with reasonably
high resolution. However, the quality of the video signal
reproduced from such recordings 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 use of a modulated laser beam to selectively produce a
pattern of holes on the metallic surface of a videodisc
is not conducive to faithfully represent an amplitude varying
analog video signal. Likewise, reading back the recorded
information in the form of a track of holes or spots on the
surface of the disc is ineffective 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
prlmarily to the inability of the laser beam modulator to
0 "write" precise analog signals in the coating of the disc.
BRIEF SUMMARY OF THE INVENTION
The present invention provides an improved apparatus
and method for storing information on a videodisc, the stored
information having the form of a lineal series of regions
representing a frequency modulated informational signal~
Specifically, the invention relates to apparatus
for storing information in the form of a frequency modulated
signal compising: means for providing an initial information having its
informational content in the form of voltage varying with time format;
frequency modulator means, responsive to the means providing
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an initial information signal, for converting the voltage
varying with time signal to a frequency modulated signal
having its informational content in the form of a carrier
frequency having frequency changes in time representing
the information to be recorded; a disc-shaped information
storage member including a substrate having a first surface
and a light responsive coating covering the first surface
for retaining indicia representative of the information
signal; means for imparting uniform rotational motion to
the storage member~ a light source for providing a light
beam, the light beam being of sufficient intensity for
interacting with the coating while the coating is in motion
and positioned upon the moving information storage member,
and for altering the coating to retain indicia respresentative
of the information; optical means for defining an optical
- path between the light source and the coating on the storage
member, and for focusing the light beam upon the coating; and
light intensity modulating means positioned in the optical
path between the light source and the coating on the storage
member, the light intensity modulating means operating over
a range between a higher light transmitting state and a lower
light transmitting state for intensity modulating the light
beam with the information to be stored; wherein the light
intensity modulating means is responsive to frequency modulated
signal and changes between its higher light transmitting state
and its lower light transmitting state during each cycle of
the frequency modulated signal for modulating the light beam
with the frequency modulated signal to be stored; the light
intensity modulating means including electrically controllable
means responsive to the frequency modulator means for varying
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the intensity of the light beam above a predetermined intensity
at which the focused beam physically alters the coating and
below the predetermined intensity at which the focused beam
fails to physically alter the coating, to form alternating
discrete regions of altered and unaltered coating being
representative of the frequency modulated signal.
In its method aspect, the invention relates to a
method for recording information an information storage
member using a laser beam, comprising the steps of: providing
an intial electrical signal having its informational content
in the form of a voltage varying with time format; changing
the voltage varying with time signal to a frequency modulated
electrical signal having its informational content in the
form of a carrier frequency having frequency changes with time
corresponding to the voltage variations with time; controlling
the intensity of the transmission of a laser light beam upon
a light responsive surface of a disc-shaped information storage
member, using the frequency modulated signal as a control
signal; and rotating the information storage member at a
constant rate relative to the light beam while focusing the
light beam upon the light responsive surface of the information
storage member; the controlling step including using the
frequency modulated signal for varying the intensity of the
light beam above a predetermined intensity at which the focused
beam physically alters the light responsive surface and below
the predetermined intensity at which the focused beam fails to
physically alter the light responsive surface, the alteration
being representative of the frequency modulated signal; the
controlling step further including using the transmitted light
beam for irreversibly altering the light responsive surface of
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the information storage member under the control of one
portion of the frequency modulated signal, as the member
moves at a constant rate, and lowering the intensity of tbe
transmitted light beam to the light responsive surface of
the information storage member for leaving the light responsive
surface unaltered under the control of a second portion of the
frequency varying signal, as the member moves at a constant
rate.
In accordance with the invention, a videodisc is
provided which has a light responsive coating covering the
disc substrate for retaining indicia representing the frequency
modulated information signal. A light source provides a light
beam of sufficient intensity to interact with the responsive
coating of the videodisc as the disc uniformly rotates. An
optical system defines an optical path between the light source
and the responsive coating on the disc for focusing the light
beam on the coating. A light intensity modulator is positioned
in the optical path and is responsive to
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the frequency modulated information signal to intensity
modulate the light beam. The light intensity modulator
i~ effective to transmit light over a predetermined
i~tensity range, the lntensity of the modulated light
5 ~eam changlng between a relatively high light level and
a relatlvely lower light level durlng each cycle of the
frequency modulated signal. The llght beam passing
through the intensity modulator and optically focused
on the responsive coatlng forms the indicia represent-
lO ing the frequency modulated information signal.
In a preferred embodiment, an initial input
slgnal has its information content ln the form of a
voltage varylng with tlme. Such a slgnal ls sultable
ror dlsplay on a standard televlslon monitor. In such
15 an embodlment, a frequency modulator, responsive to
the voltage varying with time signal converts the slgnal
to a frequency modulated signal for drlving the light
lntensity modulator.
The relatively higher and lower llght trans-
20 mittlng states of the light lntenslty modulator result
in the varylng of the lntenslty of the resultant
modulated llght beam above a predetermined lntensity
at whlch the focused beam alters the responslve coatlng,
and below the predetermlned lntenslty at whlch the
25 ~ocused beam fails to alter the responslve coatlng.
In order to establish an optlmum average in-
tenslty level for the modulated light beam, the light
intensity modulator may further include feedback
circuitry for stabllizing the operating level Or the
3 electrlcal control of the light modulator to produce
optlmum first and second prede~ermined intensities.
The ~eedback clrcultry may include a level ad~u~tment
to selectlvely ad~ust the average power level of the
light beam to a predetermined value. Toward thls end,
35 a light-sensor senses at least a portion of the light
beam from the optical modulator to produce an elec-
trical feedback signal representatlve of the average
intenslty of the light beam such that when the average
~r level Or modulated light beam increases, the feedback
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~ignal tends to lower ~he operat~ng level of the optical
modulator in order to keep the average intensity of the
modulated l~ght beam stabilized at a c~nstant operatlng
lntenslty level.
At the heart of the optical modulator i5 a
Pockels cell driven by a Pockels cell driver. The
Pockels cell driver has as lts input the frequency
modulated information slgnal, and the Pockels cell
drlver responds to the frequency modulated signal to
provide corresponding driving signals to the Pockels
cell devlce. The Pockels cell drlver is A.C. coupled
to the Pockels cell devlce, and the a~orementioned
~eedback circultry ls D.C. coupled to the Pockels cell
device.
The light source for produclng the wrltlng
llght beam ls a writing laser, preferably an Argon ion
laser, whlch produces a colllmated wrlting beam of
polarlzed monochromatic light. The wrltlng laser pro-
duces locallzed heatlng upon lmplngement of the llght
20 responslve coatlng of the videodisc.
The videodisc has a rigid disc-shaped sub-
strate wlth a surface of opaque metalllzed coatlng.
The coatlng has sultable physical properties to permit
looallzed heating by the writlng laser to, ln turn,
cauae localized meltlng accompanled by wlthdrawal of
the molten materlal toward the perlmeter of the molten
area. Upon freezlng, a permanent aperture ls left ln
the metallized coating. Under proper operating condl-
tions, the optlcal modulator varles the intel~lty of the
wrlting laser beam from an lntensity above the afore-
mentloned predetermlned intenslty, at whlch the focused
peam melts the metalllzed coating without vaporlzing lt
to an lntenslty below the predetermined lntenslty, at
whlch the focused beam falls to melt the ~,etallized
coating.
The dlsc-shaped vldeodlsc ls uniformly rota-
ted, and radial tranælational motion ls glven to the
vldeodlsc relatlve to the writing beam for writing the
lnformation in a spiral path on the disc. Both rotatlon-
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--6--al and translational notion are under control of an
electrlcal synchronizer which maintains a con~tant re-
latlonship between the rotational mot-lon of the disc
and the relatlve translational motion between the writ-
lng laser beam and tl~e disc.
A microscope objective lens for focusing thewrltlng beam on the videodisc surface may be maintained
ln a spaced relationship to the videodisc by a hydro-
dynamic air bearing created as the disc rotates rela-
tive to the ob~ective lens support. The ob~ective lenshas an aperture larger in diameter than the diameter
Or the reading light beam, and the optlcal system makes
use of mirrors and lenses for diverglng the writing
light beam from the laser source to at least fill the
entrance aperture of the ob~ective lens.
The rotational driver includes a spindle
which rotates the disc precisely in a circle, and the
radial translating driver includes a lead screw mechan-
lsm for effecting relative translational motion between
the disc and t,le writing beam at a very constant
veloclty along a radlus of the rotating dlsc.
Synchronlzatlon of the dlsc drlve wlth the
translatlng drive creates a spiral track of predeter-
mined pltch. If deslred, concentrlc clrcles can be
created by alternately translating and wrlting. In a
preferred embodiment employlng a splral track, the
~paclng between adJacent turns of the splral is 2 ~m,
center to center. Assuming a spot diameter on the
order of 1 ~m, thls would produce a guard area of 1 ~m
between spots in adJacent tracks.
The mlcroscope ob~ective lens "flies" at a
conBtant height above the disc on an alr bearing. The
constant height ls desirable because of the shallow
focal depth of the ob~ective lens A 40X dry mlcro-
scope ob~ective lens has been found to be satisfactoryln terms of concentrating the energy of the laser beam
at the disc surface to enable the writlng of the l~m
spot.
The novel features which are belleved to be
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characteristic of the invention~ both as to organiza-
tion and method of operation, together with further
ob~ects and adYantages thereof, will be better under-
stood from the following description considered in
connection with the accomp~nying drawings in which
several preferred embodiments of the invention are
illustrated by way of example. It is to be expressly
understood, however, that the drawings are for the
purpoæe of illustration and description only and are not
lntended as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a generalized diagram of the appar-
atus of the present invention;
FIG. 2 ~s a view ~ the optical path through
the ob~ective lens of FIG. l;
FIG. 3 is a representational lndlcation o~
the relative spacing between a point of impingement of
the writing beam and of the reading beam; and
FIG. 4 is a diagram of a novel Pockels cell
stabillzlng circuit.
DETAILED DESCRIPTION OF THE INVENTION
Turnillg first to FIG. 1, the writing apparatus
~ lncludes a writing head 12 wh~ch is, in the preferred
l; embodlmentJ a dry microscope ob~ective lens 14 mounted
upon an alr bearing support member 16. A 40X lens has
been ~ound to be satlsfactory. A disc 18 is speclally
prepared and may be constructed according to the teach-
lngs of the prior art, in whlch a substrate has coated
thereon a very thin film of a metal with a reasonably
low meltlng polnt and a high surface tension.
A crystal oscillator 20 controls the drlve
elements. me disc 18 is rotated by a first, rotational
drlve element 22 which is coupled to a spindle 24. A
second, translatlonal drive element 26 controls the
posltlon of the writing head 12.
A translating carrlage 28, whlch is driven by
the translational drive element 26 through a lead screw
and travelling nut, moves the writing head 12 in the
radlal direction relative to the rotating disc 18. The
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carriage 28 is provided with appropriate mirrors and
lenses so that t~le remainder of the optics and elec-
tronlcs neces~ary to the writing device may be perman-
ently mounted.
In the preferred embodiment of the present
lnvention, 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 applies the appropriate control signals
to the Pockels cell driver 34.
As described hereinafter, the video in signal
i~ of the type displayable on a TV monitor. Accordingly,
lt is a voltage varying with time signal. The FM modu-
lator 36 is of 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 having frequency changes with time correspond-
ing to said voltage variations with time.
As is known, the Pockels cell 32 responds to
applled signal voltages by rotating the plane of polar-
lzation of the light beam. Since a linear polarizer
transmits light only ln a predetermined polarlzation
plane, a polarizer, such as a Glan prism 38 in the pre-
ferred embodiment, is lncluded in the writing beam path
to provide a modulated writing beam 40. The modulated
wrltlng beam effectively follows the output of the FM
modulator 36.
The modulated writing beam 40 emerging from
the Pockels cell-Glan prlsm combination 32, 38 is ap-
plied to a first mirror 42 which directs the writlng
beam 40 to the translating carriage 28. The first
mirror 42 transmlts a portion of the wrlting beam 40 to
a Pockels cell stabllizing circuit 44 which responds to
the average intensity of the writing beam to maintain
the energy level of the beam.
A ~ens 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
1066414
of the ob~ective lens 14 for optimum resolution. A
dichroic mirror 48 is incluted in the path oriented
to substantially transmit all of the writing beam 40 to
a second, articulated mirror 50. me articulated mirror
50 then directs the beam through the lens 14 ant is capable
of shifting the point of impingement of the beam 40 on
the surface of the tisc 18.
A series of holes is formet in the metal coat-
ing by the writing beam. One hole is fonmed 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. Obviousl~, since the informational content
in the output signal of the FM modulator 36 is in the
form of frequencg changes in time about a carrier fre-
quenc~, and since the "hole", 'bo hole" sequence repre-
sents the stored information, and since the disc 18 is
rotating at a uniform speed, the "hole'~ ~bo hole"
sequence changes to represent the stored ~ideo informa-
tion by the holes being foroed closer or farther apart
nd the Ize of the hole beco~es l-rger or ~aller as
the ~riting beao 40 changes under the control of the PM
modulated output signal fro- the FM modulator 36.
m e ob~ecti~e lens 14 and the associated air
bearing 16 effectively fly on a cushlon of air at a
Jubstantially flxed distance fro~ the surface of the
disc 18. Th t distance is determined b~ the geometry
of the bearing 16~ the linear ~elocity of the disc lô,
and the force used to load the head agalnst the disc 18.
me fixed spacing is required because the focal toler-
ance of a lens capable of resol~ing a 1 ~m spot is al~o
of the order of 1 ~m .
A second~ relati~ely low-power laser 52 pro-
~ides a nitoring beam 54. In the preferred embodi-
ment, the reàding laser 52 i9 a helium-neon de~ice which
enables the reading beam 54 to be dlstinguished from
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the wrlting beam 40 by wavelength. A polarizing, beam
splltter cube 55 transmits the reading beam 54 to a
mirror 58 t~Aat directs the beam 54 through a second
diverging lens 60 that spreads the reading beam 54 to
Pill the entrance aperture of the ob~ective lens 14.
A quarterwave plate 62 i8 placed in the opti-
cal path and, in conjunction with the plane polarizing
beam splitter 56, prevents light reflected from the disc
18 ~rom re-entering the laser 52 and upsetting its mode
of oscillation. me quarterwave plate 62 rotates the
plane of polarization of the beam by 45 degrees on each
pass so that the reflected beam is rotated 90 degrees
wlth respect to the polarlzing beam splitter 56 and ls
there~ore 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 ad~ustment so that the paths of the
writing and reading beams are substantlally ldentical,
except that the reading beam "spot" implnges on the disc
18 downstream from the writing beam spot as explalned in
greater detail below.
A filter 66 that is opaque to the argon ion
beam ls interposed in the path of light reflected from
the beam splltter 56. me He.Ne reading beam 54 that i8
returned ~rom the disc sur~ace is able to pass through
the ~llter 66 and through a lens 68 onto a photodetector
7o-
me reflected llght of the reading beam lm-
plnges upon the photodetector 70. me photodetector 70
operates ln its standard manner and generates an elec-
trical cur~ent representative of the light impinging
thereupon. In this case, the photodetector generates
the slgnal represented by the "hole", "no hole" con-
~iguratlon formed ln the coatlng. me 'hole", no hole"
configuration is representative of the output of the
FM modulator 36. me output of the FM modulator 36 ls
a carrier frequency having frequency changes with tlme
representing the video signal to be recorded. The
"hole`', "no hole" configuration is representative of a
~ 1 066~4
carrier frequency having frequency changes with time
repre~enting the stored video signal The output of the
photodetector 70 is an electrical signal representing
the stored carrier frequency having frequency changes
wlth tlme representing the stored video signal.
~ ne output of the photodetector 70 is applied
to a preamplifier 72 which provides a signal of suffi-
clent amplitude and signal strength for subsequent
utllization. A video discriminator 74 then provldes a
video output signal which can be utillzed in several
ways, two of which are shown, as examples only.
me discriminator 74 is of standard design and
~unctlon. It takes the frequency modulated signal from
the photodetector 70 and ohanges it to a time dependent
voltage signal having its informatlonal content in the
~orm of a voltage varying with time format suitable for
display in the TV monitoF 76.
In a first application, the video output i8
applied to a TV monitor 76 and an oscilioscope 78. As
i~ well known, the TV monitor is responsive to a voltage
varylng with time signal. The information to be dis-
played on the TV monitor ls repreS~ented by a voltage
. change with time.
The TV monitor 76 shows the plcture fldellty
o~ the recordlng, and the oscilloscope 78 indicates the
81gna1-to-noise ratlo of the record ~nd the quallty of
the cuttingJ whether it is light or heavy. Not shown,
an appropriate feedback loop could be provided through
the Pockels cell stabilizing circuit 44 to assure an
30 adequate dlscrimination 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. me other input of the comparator 80 is taken from
35 the video input signal which is directed through a delay
line 81. A delay that is equal the accumulated delays
o~ the writing system and the time elapsed between the
instant of writing of the information and the time re-
quired for that incremental ar~a of the disc to reach
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the reading polnt must be imparted to the input videG
signal.
Ide~lly, the vldeo output signal of the dis-
crimlnator 74 should be identical in all respects to
the vldeo input signal, after the proper delay.
As previously mentioned, the output from the
discrlminator 74 is a voltage varying with time signal.
me video in signal is also a voltage varying with time
slgnal. Any differences noted represent errors which
might be caused by imperfections in the disc~s surface
or malfunctions of the writing circuits. This applica-
tion, while essential if recording digital information,
i8 le~s critical when other information ls recorded.
The output of the comparator circuit 80 can
be quantized and counted, so that an acceptable number
of errors can be established for any disc. ~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't disc for a subse-
quent recording.
Well-known techniques are avallable to trans-
late the writing head assembiy 12 in the radial direc-
tion with respe¢t to the rotating dlsc 18. While in
FIG. 1 the rotatlonal and translatlonal drives 20, 22
are lndlcated as independent, the drives are synchron-
ized to enable the wrltlng assembly 12 to translate a
predetermined increment for each revo}ution of the disc
18j by means of the common crystal osclllator 20.
Turnlng next to FIG. 2 there ls shown, ln
somewhat exaggerated form, the sllghtly dlffering opti-
¢al paths of beam 40 from the wrltln~ laser 30 and the
beam 54 from readlng lase~ 52. The wrltlng beam 40
¢oincldes with the optical axls of the mlcroscope ob~ec-
tlve lens 14. The reading beam 54, in contrast, 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 writlng beam 40 is "cutting".
The resulting delay between reading and writing allows
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the molten metal to solidity so that the recording is
read in its final ~tate. If lt were read too soon while
the metal was still molten, it would not provide per-
tinent lnformation for ad~usting the recording parameters.
This i~ best indicated in FIG. 3 where two
points in the ~ame information channel are shown as
displaced. The point A, which is the polnt of implnge-
ment o~ the wrltlng beam 40, ls shown as belng on the
optical axls of the ob~ective lens 14. Separated from
point A, in the dlrectlon of medium motion, as lndicated
by the arrow~ is the reading point B, whlch is at an
angle ~ from the axis of the mlcroscope ob~ectlve lens
14. A dlstance between polnts A and B of two ~m has pr~
vlded a satisfactory monltorlng of the wrltlng operation.
Turning finally to FIG. 4, there is shown an
idealized diagram o~ a Pockels cell stabillzlng clrcult
44, sultable for use in-the apparatus of FIG. 1. As
18 known, a Pockels cell rotates the plane of polariza-
tlon of the applied light as a function of an applied
voltage. There~ore, the Pockels cell i8 used to rotate
plane polarlzed llght, and the rotated llght ls passed
through a plane polarlzer, such as a Glan prism. me
- llght issuing from the polarizer will be amplitude
modulated in accordance with the applied voltage.
Stated another wsy~ the standard operatlng
mode Or a Pockels cell 32 and Glan prism 38 is for use
as a llght lntenslty modulating means. Each cycle ~rom
the FM modulator drlves the Pockels cell through lts
~ull operating range of nlnety degrees. Wlthln thls
operatlng range of nlnety degrees, one operating polnt
passes al~ llght applled thereto and ldentlfled as a
full light transmlttlng state. A second operatlng polnt
passes no llght and 18 ldentlfled as a full llght block-
lng state. The Pockels cell itself only rotates the
plane of polarlzation. me Glan prism passes light in
one plane of polarization and no light in the plane dls-
placed nlnety degrees from that plane ln whlch all the
llght passes.
Dependlng upon the indivldual Pockels cell, a
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voltage change of approximately lOO volts wlll cause
the cell to rotate the plane of polarization through
360 degrees. However, the transfer characteristic of
an indivldual cell may drift spontaneously. correspond-
ing to a voltage change of ~ 50 volts, and accordingly,a ~eedback loop i5 deslrable to maintain the cell within
a use~ul, reasonably linear, operating range.
The stabilizing circuit 44 includes a photo-
Bensltlve silicon diode 82, which iB positioned to
recelve a portion of the writlng beam 40 reflected from
the mlrror 42 of FIG. 1. The silicon diode 82 functions
ln much the same fashion as a solar cell and is a source
Or electrlcal energy when illuminated by incident radla-
tlon. One terminal Or the silicon diode 82 is connected
to common re~erence potential 84, indicated by the con-
ventlonal ground symbol, and the oth~r terminal is con-
nected to one input of a dlfferential ampllfier 86.
me sllicon cell 82 is shunted by a load 88 which
enables a linear response mode.
me other input to the differential ampllfier
86 19 connected through an approprlate potentiomater 90
to the common reference 84. A source of power 92 is
coupled to the potentlometer 90, whlch enables the
oettlng o~ the dl~erential ampiifler 86 to establlsh
the average light level transmltted by the Pockels cell
32.
Accordingly, a pair of output termlnals of the
di~ferentlal ampllfier 86 are respectively connected
through resistlve elements 94, 96 to the input terminals
3 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, whlle the dlfferentlal ampllfler 86
18 d.c. coupled t~ the Pockels cell 32.
In operation, the system is energized. The
light from the writing beam implnging on the silicon
diode 82 generates a dlfferential voltage at the lnput
to the differentlal ampllfier 86. Inltiallyj the
potentlometer 90 is ad~usted to produce llght at a pre-
determlned average level of lnte-nslty. Thereafter, lf
r-
i
~` ~0664~4
-15-
the average level of intensity impinging on the silicon
cell 82 either increases or decreases, a correctlng
voltage will be generated ln the differential amplifier
86. m e correcting voltage applied to the Pockels cell
32 is of a polarity and magnitude adequate to restore
the averaKe level Or intensity to the predetermined
level.
m us there has been shown an improved vldeo
dlsc recording assembly. A microscope ob~ective lens
mounted on an air bearing "flies" at a predetermined
distance from the surface of a metallized dlsc. The
metallized coating is such that a laser beam can, under
suitable modulation, dellver sufflcient energy to melt
localized areas of the surface. Under surface tension,
the molten metal retracts leaving a clear area of approx-
lmately one micron in diameter.
A second, low-energy laser utilizing substan-
tially the same optlcal path is directed through the
samd microscope objective lens, but is brought to the
surface of the disc at a slight dlstance "downstream"
from the point of writing. me reading beam 1~ returned
through an approprlate optical system that excludes the
reflected energy of the writing beam and enables an
analysis of the information that has been written on
the disc.
me playback information can, among other
things, control the intensity of the wrlting beam to
a~sure adequate "recordlng levels", determlne whether
an unacceptable number of errors have been made in the
recordlng process.
