Note: Descriptions are shown in the official language in which they were submitted.
ii2S43~
MASTERING MACHINE
TECXNICA~ FIELD
The present lnvention relates to the writing of a
~requency modulated electrical signal upon an ln~ormatlon
bearing surface of a video disc member ln the form of a
lineal series of flrst and second indicia positioned in
; track-like fashion upon such surface.
BACKGROUND OF THE P~IOR ART
The apparatus ~or writing a ~requency modulated
signal upon a video di8c member includes a movable writing
beam and a video disc member mounted on a turntable. The
turntable ~s driven by a motion control assembly which
rotates the disc precisely ln a circle at a constant rate
15 of rotation and a translational drive assembly for trans-
lating the wrlting beam at a very constant~ and very low
velocity along a radius ~f the rotating disc. The rota-
tional drive of the disc i8 synchronized wtth the trans-
latlonal drlve oP the writing beam to create a splral
traok of predetermined pitch. In a pre~erred embodiment,
the 9paoing between ad~acent trac~s o~ the spiral 1B two
microns, center to center. The indicia ls formed having
a width of one micron. This leaves an intertrack or guard
area of one micron between indicia in ad~acent tracks.
If des~red, the lndicla can be formed as concentric circles
by translating in incremental steps rather than by trans-
lating at a constant velocity as just described.
In the pre~erred embod~ment~ a microscope obJec-
tive lens is positloned at a constant height above the
,
1~L2~39~
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video disc member on an air bearing. This objectlve lens
is employed for focusing the write beam upon the light
sensitive surface of the video disc member. The constant
height is necessary because of the shallow focal depth of
the objective lens. A o.6~ NA dry microscope objective
lens is employed to focus the write laser beam to a spot
one micron in diameter upon the light sensitive coating.
Because the coating is rotating at a relatively high rate,
the length of the indicia formed in the light sensitive
coating depends upon the length o~ tl~e the spot lntensity
exceeds that needed to form such an indicia.
A linearly polarized ion laser is used as the
source of the writlng beam. A Pockels cell i5 used to
rotate the plane of polarization of the wrlting beam with
respect to its fixed plane of linear po-arization. A
; linear polarlzer attenuates the rotated writing beam in
an amount p~portional to the difference in polarization
between the light in the writing beam and the axis of the
linear polarizer. The combination of a Pockels cell and
linear polarizer modulates the writing beam with the video
i~ormation to be stored. This modulation follows the
pattern provided by control signals furnished by a Pockels
cell driver.
The video signal to be recorded is applied to a
frequency modulator clrcuit. The output from the modu-
lator circuit is a rectangular wave whose frequency is
proportional to the video signal. The duration of each
cycle of the rectangular waveform is variable as is
characterlstic of a frequency modulated signal. As i5
characteristic o~ a rectangular wave, 1~ has an upper
voltage level and a lower voltage level. The upper and
lower voltage levels of the rectangular wave are empll-
fled by a Pockels cell driver and used to control the
; Pockels cell. The Pockels cell changes the angle of pola~
3~ lzation of the light passing therethrough in response to
the instantaneous vol~age level of the control signal
supplied ~y the Pockels cell driver.
In a first mode of operation responsive to one
voltage level of the rectangular-shaped control signal
` ' ~12~i434
--3--
applied to a Pockels cell driverg the light beam passes
unhindered through the Pockels cell linear polarizer
combination at a first intensity sufficienJ to form a
first lndicia in a light responsive coating. When the
control signal changes to represent its second voltage
level, the Pockels cell rotates the polarization o~ the
light which forms the writing beam to a new angle of pola~
ization. ~ue to this change in polarlzation of the llght
~orming the writing beam, a mlsmatch occurs between the
angle of polarization of the light 13suing from the Pockels
cell and the preferred angle o~ polarization of the linear
polarizer. In this situation, the linear polarizer acts
as an attenuator and less light passes through the linear
polarizer. This reduces the light intensity of the writing
beam below the intensity requlred to form such f~rst
j indicia in the light responslve coating.
A portion of the writing beam is sensed by a
Pocke-s cell s~abillzing circuit for maintaining the
average power o~ the modulated writing beam at a predeter-
mlned level in spite of changes in the Pockels cell trans-
fer characteristic produced by small temperature variations.
~he stabilizing circuit includes a level ad~usting circuit
for selectively ad~ustlng the power level to form indicia
in dif~erent light sensitive coatings as identi~ied here-
inafter.
Circuitry is described for using a triangular~req~ency modulator ~or driving the Pockels Cell driver and
r obtaining a 3inusoidal-shaped light modulation output
slgnal ~rom the llnear polarizer. The output ~rom the
llnear polarizer is ad~usted such that the half power
polnt from the polarizer equals the threshold power level
of the material forming the in~ormation storage layer.
A second harmonic bias circuit is employed for removing
the second harmonic intermodulation distortion o~ the
modulated write beam.
Different types of video disc members can be used
with this writing process and apparatus. Each such member
haq a di~ferent configuratlon. In a first configuration~
the video disc member includes a glass substrate having
~ .
i l Z ~
--4--
an upper surface carrying a thin metal coating as a light
responsive coating. In this configurationg the write beam
forms variable length apertures in a track-like fashion
in the metal coating.
The intensity of the write bea~ is adjusted such
that an aperture is formed, for example, during each
positlve half cycle of the frequency modulated signal to
be stored, and no aperture is for~ed durlng the negative
half cycle. Accordingly, the first and second indicia
representative o~ the stored information is a lineal
series o~ apertures separated by an intervening portion
of the surface coating.
In this first configuration, a portion of the
glass substrate is exposed in each aperture. The exposed
portion of the glass substrate appears as a reglon of
non-specular light reflectivity to an impinging beam. The
intervening portion of the metal coating remaining between
specular reflectivity means a significant portion of the
reflected light returns along the path of the light beam,
ie., a 180 reversal in paths between the incident and
reflected beam paths. Non-specular reflectivity means
that no significant portion of the incident beam is re-
flected along the path of the incident beam.
In a second configuration, the video disc member
includes a glass substrate having an upper surface carry-
ing a thin layer of photoresist as the light responsive
coating. In this configuration~ the write beam forms
variable length regions of exposed and unexposed photo-
resist material in a track-like fashion in the photo-
reslst coating. The lntenslty of the wrlte beam isad~usted such that a region of exposed photoresist ma~erial
ls formed, ~or example, during positive half cycles of the
frequency modulated signal to be stored and a region of
unexposed photoresist material is left during the nega-
tive half cycles. Accordingly, the first and secondindicla representative o~ the stored lnformation is a
lineal series of exposed and unexposed portions of the
surface coating, respectively.
A preferred embodiment of a reading apparatus is
~12~D~3~
--5--
described empl~ying a read laser for producing a polar-
ized collimated beam of light having a preferred angle of
polarization. A read optical system directs and images
the laser beam to impinge upon the indicia carrled upon
the surface of the video disc member. The video disc
member is employed for storing a frequency modulated sig-
nal on its surface in the form of a lineal series of
regions. The regions are alternately specular light
reflective and non-specular llght reflective. A read
optical system focuses the read beam to a spot of light
approximately one micron in diameter and dlrects the
focused spot to impinge upon the lineal series of regions.
The intensity of the read beam is ad~usted such that a
sufficiently strong reflected read beam signal is gathered
15 by the read optical system.
A motion control assembly rotates the video
disc member at a uniform rate of speed su~ficient t~
reconstruct the frequency of the originally stored fre-
quency modulated slgnal. A typical frequency modulated
signal stored in this matter varles in frequency between
two megacycles and ten megacycles. The rotational rate
Or the video disc member is preferentially set at about
1800 rpm to chànge the spatially stored frequency modu-
lated signal into a real time electrical signal. The
motion control assembly includes a translational drive
assembly for translating the reading beam at a very con-
stant, and very low velocity along the radius of the
rotating disc so as to impinge upon the lineal series of
light reflective and light scatterlng regions contained
thereon.
The reflected read beam gathered by the read
optlcal system is dlrected to a light senslng circuit for
changing the intensity modulated reflected light beam to
a frequency modulated electrical signal corresponding to
the intensity modulated reflected light beam.
A polarization selective beam splitting element
is positioned in the read beam path intermediate the read
laser source and the video disc member. A~ter the read
beam passes through the polarization selective beam
. :~ 3L2S~34
-6 -
splitting element the real light beam is linearly polar-
ized in the preferred plane. h quarterwave plate is
positloned intermedia~e the ou~put of the polarization
selective beam splitting element and the video disc
member. The quarterwave plate changes the light in the
read beam from linear po~arization to circular polariza-
tion. The reflected light retains its circular polariza-
tion until it passes through the quarterwave plate a second
time. During this second pass through the quarterwave
plate the reflected light is changed by circular polariza-
tlon back into linear polarized light rotated ninety
degrees from the preferred plane established by the polar-
ization selective beam splitting element as described
hereinabove.
The polarization selective beam splitting element
is responsive to this ninety degree shift in the reflected
light beam for diverting the reflected beam to the light
sensing circuit and prevents the reflected ~ight beam from
reentering the read laser source.
A diverging lens is employed in the read optlcal
system for spreading the substantially parallel light
beam from the read laser source to at least fill the
entrance aperture of the ob~ective lens.
In a second embodiment of the read optical
system~ an optical ~ilter is placed in the re~lected read
beam path for filtering out all wavelengths of light
other than the wavelength of light generated by the read
laser source.
In a reaording apparatus, tlle write functlon
30 alone is employed for writlng the frequenoy modulated
information onto a video dlsc member. In a video dlsc
player, the read function alone is employed for recover-
ing the frequency modulated information stored on the
'surface of the video disc member. In a thlrd mode of
35 operatiDnJ the read and write functions are combined in a
single machine. In this comblned apparatus~ the read
apparatus is employed for checking the accuracy of the
information being written by the write apparatus.
To implement the monitoring function, the read
1~125~34
--7--
beam from the Helium-Neon (He-Ne) read laser is added
into the writ~ng beam path. The read optics are ad~usted
to direct the read beam through the microscope objective
lens at a light angle with respect to the writing beam.
The angle ls chosen so that the readlng beam illuminates
an area on the same t~ac~ being written by the write beam,
but at a point that ls approxlmately four to six microns
downstream from the writlng spot. More speciflcall~g the
read beam ls imaged upon the in~ormation track that was
~ust formed by the write beam. Sufficient tlme has been
-~ allowed for the ~nformatlon indicia to be formed on the
video disc member. In this manner, the read beam is im-
pin~ed upon alternate regions of different reflectivity.
In one form of the read apparatus, the read beam impinges
upon the portions of the metal not heated by the write
beam and also impinges upon the glass substrate exposed in
the apertures just formed by the writing spot. The regions
of different reflectivity function to change an impinging
read beam of constant intensity into an intens~ty modu-
lated reflected read beam.
In this monitoring mode of operation, the readlaser beam is selected to operate at a wavelength differ-
ent from that of the wri~e laser beam. A wavelength
selective optical filter is placed in the reflected light
beam path having a band pass which includes the reading
laser beam. Any write laser beam energy which follows
the read reflected path is excluded by the filter and
therefore cannot interfere with the reading process. The
monitoring mode of operatlon i6 employed at the tlme o~
writlng the video in~ormation onto the video dlsa member
as an aid in checking the quality of the signal belng
recorded. The output signals from the read path are
displayed on an oscilloscope and/or a television monitor.
The visual inspectlon of this dlsplayed signal indicates
whether the lndicla are being formed wlth the preferred
duty cycle. The pre~erred duty cycle is achleved when on
the average the length of a specular reflective region~
whlch represents one half cycle of a frequency modulated
slgnal, ls the same as the next succeeding region of
,
' 1~25~34
--8--
non-specular rerlect~ity, which represents the next
consecutive half c~rcle o~ a ~requenc-~ modulated signal.
The read after wr~te or mon~toring mode of oper-
atlon ls als~ utilized ir. an error checking modeJ especi-
ally lf digital type information is belng written. The
lnput video inform~tion is delayed for an lnterval equal
to the accumulative val~ s Df the time delay beglnning
with the frequency modulation of the input video informa-
tlon signal during the write process and contlnuing through
10 the rrequency demodulation of the recovered re~lected
signal rrom the sensing circuit, and lncluding the delay
of travel tlme of the point on the storage member mo~lng
~rom the point of storing the input video in~ormation
signal to the point of impingement of the read light
15 ~eam. The recovered in~Drmation is then compared with
the delayed input lnformatiDn f~r accuracy. The exlstence
of too many dissimilarities would be a basis for either
rechecklng and reallgning the apparatus or re~ecting the
~isc .
The read apparatus is suitable rOr use with a
standard ho~e television receiver by adding an RF modu-
lator for adding the video signal to a suitable carrier
~requency matched to one Or the channels Or a stanaard
home television receiver. The standard televlsion re-
ceiver then handles this signal in the same manner as are
received ~rom a standard transmitting statlon~
More particularly, there is provided:
Apparatus ~or storing vldeo in~ormation ln the
~orm Or a frequency modulated sl~nal upon an in~ormation
storage member, comprlslng: first means for providing an
lnformatlon slgnal to be recorded, and said signal havlng
lts in~ormatlonal content in the ~orm of a carrier frequency
havlng rrequency changes in tlme representlng said inrormation
to be recorded; an lnrormatlon storage member including a
substrate havlng a ~lrst sur~ace and a llght respon~lve
coatlng coverlng sald flrst surface for retainlng lndlcla
representatlve Or said lnrormatlon sl6nal; means ror imparting
unlrorm motlon to sald storage member; a light source ~or
rovldlng a llght beamJ sald light beam belng Or surrlclent
v . - ~
:
112~434
-8a-
lntenslty ~or lnteracting wlth sald coating whlle sald
coatlng is ln motlon and sald coating ls positloned upon
said movlng lnformation storage member, said llght beam
t~elng of suf~lclent lntenslty ~or altering sald coatlng to
retaln lndicla representative of said lnformatlon slgnal;
optlcal means for de~lning an optlcal path between sald
llght source and said record carrler lncluding sald coatlng,
sald optlcal means belng ~urther employed for focuslng said
llght beam to a spot upon said coating; llght lntenslty
modulatlng mesns posltloned ln sald optical path between
said llght source and sald record carrler, sald light ln-
tenslty modulatlng means operatlng over a range between a
maxlmum llght transmittln~ state and a minlmum llght tran-
smlttlng state for lntensity modulating said light beam with
said lnformatlon to be stored; sald light intenslty modulating
means bein& responslve to sald frequency modulated slgnal
and changlng between lts maxlmum llght transmittlng state
and lts mlnlmum llght transmitting state durlng each cycle
of said frequency modulated slgnal for modulating sald llght
beam wlth the frequency modulated electrlcal slgnal to be
stored; and stablllzlng means responsive to sald modulated
light beam for ~eneratlng a bias control slgnal lndlcatlve
o~ the second harmonlc dlstortlon present ln sald modulated
light beam, sald blas control slgnal belng employed for
blaslng sald modulatlng means at lts operating polnt at
whlch mlnlmum second harmonlc dlstortton ls present ln sald
modulated llght beam at its average power lntenslty; and
whereln ~ald llght passlng through said llght lntensity
modulatlng means and ~ocused upon sald coating by said
optl¢al means forms lndlcla ln sald coating representative
o~ sa~d frequency modulated slgnal to be stored.
There is further provided:
Apparatus for recording input vldeo infornation
on an lnPormatlon storage member, comprislng: flrst means
for provldlng a ~ldeo lnformatlon slgnal to be recorded and
sald vldeo slgnal havlng lts informational content ln the
form of a voltage varying wlth time signal suitable for
dlsplay on a standard televislon monitor; a wrltlng laser
for produclng a collimated writlng beam of polarized mono-
chromatlc light; a smooth ~lat rigld dlsc havlng a planar
,.~
. .
434
-8b-
surface covered wlth a llght responslve coatlng for retaln-
lng lndlcla representative of said vldeo slgnal; sald coat-
lng havlng a threshold power level above which sald lndlcla
are formed; ~irst optical means for dlrecting said llght
beam to lmpinge upon sald coatlng, and for focusing sald
lmpinglng wrlting beam down to a spot at sald coating; sald
coatlng havlng sultable physlcal properties to be altered ln
response to the impingement of light from said writing beam
for formlng a permanent alteration in sald coating; rotational
drlve means for producing uni~orm rotatlonal motlon of sald
disc; translational drive means synchronlzed wlth said
rotatlonal drlve means for moving said ~ocused light spot
radially across said planar surface of said disc, electrical
synchronlzlng means for malntalnlng a constant relatlonshlp
between sald rotational motion and sald translational motion,
fre~uency modulatlon means ~or covering said lnput vldeo
lnformation to a correspondlng frequency modulated slgnal,
said frequency modulated signal havlng its informational
content ln the form of a carrler frequency havlng frequency
changes with tlme correspondlng to sald voltage variatlon
with time slgnal; electrically controllable means responsive
to sald frequency modulated means for varying the lntenslty
of said wrlting beam between a flrst predetermlned intensity
at uhlch the focused spot alters said coatlng and a second
predetermlned lntensity at which the Pocused spot ~alls to
alter sald coating, said alteration belng representative of
sald frequency modulated signal, means for ad~ustlng the
average lntensity o~ the output of sald electr~cally con-
trollable means to equal said threshold power of sald llght
responslve coatlng; and stablllzln~ means responsl~e to said
modulated llght beam for generatlng a blas control signal
lndlcatlve of the second harmonlc dlstortlon present ln said
modulated llght beam, sald blas control slgnal belng employed
for blaslng said electrically controllable means at its
operating point at whlch mlnlmum second harmonlc dlstortlon
is present ln sald modulated ll~ht beam at lts average power
intenslty.
~4
~ ~ .
:
1~2S43~
-8c-
There is also pr~vided:
A method for recording ln~or3atlon on an
lnformatlon storage member uslng a laser beam, comprlsing
the steps of: providlng an electrlcal signal to be recorded,
and sald signal h~ing its lnformational content ln the rorm
of a voltage varying with time format; changing sald volta~e
~arylng with time signal to a frequency modulated electrical
slgnal haYlng its in~ormational content ln the rorm of a
carrier frequency having fre~uency changes wlth time cor-
responding to sald voltage variations wlth tlme; shaplng
sald frequency modulated electrical signal lnto a triangular
shaped wa~e~orm; modulatin~ a flxed 1ntenslty llght beam
wlth the triangular shaped, fre~uency modulated slgnal,
senslng the amount of second harmonlc distortion present ln
3ald llght beam ~ust prlor to its applicatlon upon sald
light sen~ltlve sur~ace for generating a blas control ci~-
cuit; using sald ~la~ control slgnal for biaslng said modu-
latlon to produce mlnlmum second harmonic dlstortlon; movlng
the lnformatlon qtorage member at a constant rate while
~ocusing a ~ald li~ht beam down to a spot upon the llght
sensitlve sur~ace of said lnformatlon storage member; ùslng
said focused light spot to lrreverslbly alter the char-
acterlstlcs of sa~d llght sensitlve surface of said lnfor-
mation storage member as said member moves at a constant
rate and under the control of one portlon of said ~requency
varying signal; and blocklng the transmlssion o~ said foc~sed
11ght beam to said ll~ht ~ensitive sur~ace o~ sald ln~ormatlon
storage member as sald member moves at a constant rate and
under the control by a oecond portlon oP sald ~requency
varylng ~i~nal.
RIEF DESCRIPTIOI~ QF THE DRAWINGS
FIGURE 1 ls a block diagram o~ the write appara-
tus;
FIGURE 2 ls a cross-sectional view of a video
dlsc member prior to wrltlng thereon uslng the wrlte
apparatus shown in Figure l;
FIGURE 3 is a partial top view of a video dlsc
member after writlng has taken place uslng the wrlte
apparatus shown in Figure l;
.~
~ ,~
L
, `~:
~2~i434
FIGURE 4 is a waveform of a video signature e~ployed
in the mrite apparatus shown in Figure 1,
FIGU~E 5 is a ~avefor.m of a f.requency dulated
signal used ln the write apparatus shown in Figure l;
FIGVFE 6 ls a graph showing the intensity of the
write laser used in the write apparatus shcwn in Figure l;
FIGURE 7 is a graph shcwlng the modulat`ed write
beam as changed by the write apparatus shcwn in Figure l;
FIGURE 8 is a radial cross sectional view taken
along the llne 8-8 of the disc æhown in Figure 3;
FIGURE 9 is a detailed block diagram of a suit-
able motlon control asse~blyS
FIGURE 10 ls a block diagram shGwing a read ap-
paratusj
FIGU~E 11 is a block diagr~m showlng the combi-
nation of a read and wrlte apparatus~
FIGURE 12 ls a schematlc representatlon showing
the read and wrlte beams passing through a single ob~ec-
tive lens as ~lqed ln the block diagran of Flgure l;
FIGURE 13 is a schematic diagram of a suitable
stabll~zing circuit for use in the write apparatus shown
in Figure 1.
FIGURE 14 shows various waveforms used in illus-
trating the operation of a ~astering machine~
FIGURE 15 shows a cross-sectional schematic view
o~ one ~orm of a video disc~
PIGU~E 16 shows a photoreslst coded storage nenr
ber,
FIGURE 17 shows certaln portions rem~ved rram
the photoresist coded storage nEs~er of Figure 16,
FIGURE 18 shows the trans~er characterlst1c o~ a
Pockel~ cell used herein,
FIGURE 19 shows the transfer characteristlc of
a Glan prism used herein;
FIGURE 20 shcws a light intenslty wave~orm~
~S~L34
--10--
FIGURE 21 shGws in conJunction ~ith Figure 20 a
series of waveforms usefhl in explaining the duty cycle of
recordin~
FIGUR 22 shows an addltional wave~orm used in
illustrating the operation of a mastering machlne
FIGURE 23 is a block diagram of a Pockels cell
bias servo system;
FIGURE 24 is a diagram of a second harnonic detec-
tor used in Figure 23; and
10FIGURE 25 is a diagr2m o~ a high voltase amplifier
used ln Flgure 23.
15D~TAILED DESCRIPTION OF THE INVENIION
The same nu~Aeral is used to identl~y the same ele-
ment in the several views. The terms recording and stor-
ing are used lnterchangeably ~or the term wrlting. m e
term retrieving ls used interchangeably for the term read-
ing.
m e apparatus for storing video informatlon in theform of a frequency modulated signal upon an informa~ion
storage mP~ber 10 is shown with reference to Figure 1. An
information signal source circuit 12 ls employed for pr~vld-
ing an informatlon signal to be recorded. Ihis in~orma-
tion signal present on a line 14 is a ~re~uency modulated
signal havin~ its informational oontent in the form of a
carrier frequency havlng frequency changes in time repre-
senting said in~ormatlon to be recor~ed. Figure 5 shows
a typical example of a frequency modulated slgna~. m e
information signal source circuit 12 employs a video slg-
nal cir~uit 16 for providing an information signal on a
line 18 havlng its informational content in the form of
a voltage varying with t~me format. Figure 4 shows a
typical example of a ~oltage varying with tin~e slgnal. A
~L2~3~
-I 1-
frequenc~ modulator circuit 20 is responsive to the video
signal circuit 1~ lor converting the voltage varying with
time signal to the frequency modulated signal on the line
14 as shown in Figure 5.
The lnformation storage member 10 is mounted upon
a turntable 21. The member 10 is shown in Figure 2 with
no indicia formed thereon and includes a substrate 22
having a flrst surface 24 and a light responsive coating
26 covering the first surface 24. A motlon control
assembly 28 imparts uniform motion to the storage member 10
relative to a write beam 29' generated by a light source 30.
The motion control assémbly 28 is shown and described in
greater detail with re~erence to Figure 9. The motion
control assembly 28 includes a rotational drive circuit
32 for providing uniform rotational motion to the informa-
tion storage member 10 anq translational drive circuit 34
synchronized with the rotational drive circuit 32 for
moving the focused light beam 29' radially across the
coating 26. The motion co~trol assembly 28 further in-
cludes an electrical synchronizing assembly 36 for main-
taining a constant relationship between the rotational
motion imparted to the member 10,by the rotational drive
: circuit 32 and the translational motion imparted to the
: light beam 29 by the translational drive circuit 34.
The light source 30 provides a beam of light 29
which is of sufficient intensity for interacting with or
-: altering the coating 26 while the coating 26 is in motion
and positioned upon the moving information storage member
10. Additionally~the intensit~ of the llght beam 29' i~
suf~iclent f'or producing permanent indlcia in the coatlng
26 representative of the information to be recorded. A
suitable light source 30 comprises a writing laser ~or
produclng a collimated writing beam o~ polarized mono-
chromatic light.
Referring again to Flgure 2~ there is shown a
cross-sectional view of a first configuratlon of a suit-
able video disc member 10. A suitable substrate 22 is
made of glass and has a smooth5 flat, planar first surface
24. The light responsive coating 2~ is formed upon the
~2~ 4
-12-
sur~ace 24.
In one o~ the disclosed embodiments, the coating
26 is a thin, opaque metallized layer having suitable
physical properties to permlt localized heating responsive
to the impingement of the write light beam 29 from the
writing laser 30. In operation, the heating causes local-
ized melting of the coating 26 accompanied by withdrawal
of the molten material towards the perimeter ~ the melted
area. Upon ~reezing, this leaves a permanent aperture such
as at 3~, shown in Figures 3 and 8, in the thin metal coa~
ing 26. The aperture 37 is one type of indicia employed
for represent~ng in~ormation. In this embodiment, succes-
sively positioned apertures 37 are separated by a portion
38 of the undisturbed coating 26. The portion 38 is the
second type of indicia employed for representing informa-
tion A more detailed description concerning the process
by which the indicia 37 and 38 represent the frequency
modulated signal ls given with reference to Figures 5
through 8.
A movable optical assembly 40 and a beam steering
optical assembly 41 collectively define an optical path
for the light beam 29 issuing from the light source 30.
The optical assemblies image the read beam 29 into a spot
42 upon the coating 26 carried by the storage member lO.
The optical path is also represented by the line identified
` by the numerals 2g and 29'.
A light inten3ity modulating a~sembly 44 i9 posl-
tioned in the optical path 29 between the light 3 ource 30
and the coating 2~. In its broadest mode ~ operatlon,
the light intensity modulating assemoly intensity modu-
lates the light beam 29 with the in~ormation to be stored.
The llght intensity modulating assembly 44 operates under
the control of an amplified form of the frequency modu-
lated signal shown in Figure 5. This frequency modulated
signal causes the assembly 44 to change between its higher
light transmitting state and its lower light transmitting
state during each cycle of the frequency modulated signal.
This rapid change between transmitting states modulates
the light beam 29 with the frequency modulated signal to
li2S43~
-~3-
be stored.
The light beam 29 is modulated as it passes through
the light intensity modulating assembly 44. Thereafter,
the modulated light beam~ now represented by the numeral
29'~ is imaged upon the coating 2~ by the optical assem-
blies 40 and 41. As the modulated light beam 29' impinges
upon the coating 26, indicia is ~ormed in said coating 26
representative of the frequency modulated slgnal to be
stored.
The light intensity modulating assembly 44 in-
cludes an electrically controllable subassembly 46 which
is responsive to the frequency modulator 20 for varylng the
intensity of the light beam 29' above a predetermined
intensity at which the focused beam 29' alters the coating
26 carried by the information sotrage member 10. Addi-
tlonallyJ the electrically controllable subassembly 46 is
responsive to the frequency modulator 20 for varylng the
intensity o~ the light beam below a predetermlned intensity
at which the ~ocused beam 29' fails to alter the coating
26. The alterations formed in the coating 25 are repre-
sentatlve o~ the ~requency modulated signal to be stored.
When a photoresist layer forms the coating 26 carried by
the information storage member 10, the alterations are in
the form of exposed and unexposed photoresist members
analogous to the size as previously descrlbed with respect
to indicia 37 and 38, respectively.
~` When the coating 26 carried by the information
storage member 10 is a metal coating, the eleatrically
controllable subassembly 46 varies the lntensity o~ the
3 writing beam 29' above a ~irst predetermined intensity at
which the ~oaused beam 29' melts the metal coating wit~out
~- vaporizlng it and further varies the intenslty of the
wrlting beam below the predetermlned intensity at which
the focused beam 29' fails to melt the metal sur~ace.
The light intensity modulating assembly 44 in-
cludes a stabilizing circuit 48 for providing a feedback
signal emplo~ed ~or temp~rature stabilizlng the operating
level of the electrical controllable subassembly 46 to
operate between a predetermined higher light intensity and
,
. , .
~Z~434
--14--
predetermined lower light intensity level. The light
intensity modulating assembly ~ includes a light sensing
circuit ~or sensing at least a portion of the light beam,
indicated at 29 !1~ issuing from the electrically controllable
subassembly 46 to produce an electrical feedback signal
representat,ive of the average intensity of the beam 29l.
The feedback signal is connected to the electrically con-
trollable subassembly ~6 over the lines 50a and 50b to
stabillze its operating level.
The light sensing means produces an electrical
feedback signal which is representative o~ the average
intensity of the modulated light beam 29'. In this manner,
the light intensity modulating assembly 44 is stabilized
to issue the light beam at a substantially constant aver-
age power level. The stabilizing circuit 48 also includeslevel ad~ustment means for selectively adjusting the
average power level of the light beam 29' to a predeter-
mined value to achieve the preferred duty cycle in either
a metal coating 26 or a photoresist coating 26, or any
other material used as the coating 26~
The movable optical assembly 40 includes an
ob~ective lens 52 and a hydrodynamic air bearing 54- for
supporting tha lens 52 above the coating 26. The laser
beam 29' generated by the laser source 30 is formed of
substantially parallel light rays. In the absence of the
lens 66, these substantially parallel light rays have
substantially no natural tendency to diverge. Then the
ob~ective lens 52 has an entrance aperture 56 larger in
diameter than the dlameter of the light beam 29'. A
planar convex diverging lens 65 positloned in the light
beam 29' is ~mplo~ed for spreading the substantially
parallel-light beam 29' to at least fill the entrance
~ aperture 55 of the obaective lens 52.
; The beam steering optical assembly 41 further in-
cludes a number of mirror members 58~ 50, 62 and 54 for
~ folding the light beams 29' and 29" as desired. The mirror
`~ 60 is shown as a movable mirror and ls employed for making
strictly circular tracks rather than the preferred spiral
tracks. Sprial tracks require only a fi,Yed mirror.
-
` ~12~i;434
-~5-
As previously des~.rc~ibed, the light source 30
produces a polarized laser beam 29. The electrically con-
trollable subassembly 46 rotates the plane of polarization
of this laser beam 29 under the control of the frequency
modulated signal. A suitable electrically controllable
subassembly includes a Pockels cell 68, a linear polarizer
70 and a Pockels cell driver 72. The Pockels cell driver
72 ls essentially a linear amplifier and is responsive to
the ~requency modulated signal on the line 14. The output
from the ~ockels cell driver 72 provides drivln~ signals to
the Pockels cell 68 for rotating the plane of polarization
o~ the laser beam 29. The linear polarizer 70 is orienta-
ted ln a predetermine relationship with respect to the
original plane of polarization of the laser beam 29 issuing
from the laser source 30.
As seen with reference to Figure 7, the maximum
~ light transmitting axis of the linear polarizer 70 is
- positioned at right angle with the angle of polarization
of the light issuing from the source 30. ~ecause of this
; 20 arrangement, minimum light exits the polarizer 70 with
zero degree rotation added to the write beam 29 by the
Pockels cell 68. Maximum light exits the polarizer 70
~ with ninety degree rotation added to the write beam 29 by
;~ the Pockels cell 68. This positioning of the linear
~- 25 polarizer as described is a matter o~ choice. By aligning
the maximum iight transmittin~ axis o~ the pola~zer 70
with the angle o~ polarization of the light issuing from
the laser source 30, the maximum and minimum states would
be opposlte rrom that described when subjected to zero
degrees and ninety de~ree rotation. However, the write
apparat~s would essentially operate the same. The llnear
polarizer 70 ~unctions to attenuate the intensity o~ the
beam 29 which is rotated away from its natural polarization
angle. It is this attenuating action b~J the linear
polarizer ro wllich forms a modulated laser beam 29' cor-
responding to the ~requency modulated signal. A Glan-
prism is suitable ~or use as a llnear polarizer 70.
The Pockels cell driver 72 is AC coupled to the
Pockels cell 68. The stabilizing ~eedback circuit 48 is
~ , ' .
~12~i434
l6-
DC coupled to the Pockels cell 68.
Referring CollectiVely to Figures 4 through 7g
there are shown selective waveforms of electrical and
optical signals which are present in the embodiment shown
with reference to Figure 1. A video signal generated by
the video signal source circuit 16 is shown in Figure 4.
A typical device for generating such a video signal is a
television camera or a video tape recorder playing back a
previously recorded signal generated by a television
camera. A flying spot scanner is a still further source
of such a video signal. The information signal shown in
Figure 4 is typically a one volt peak-to-peak signal
having its informational content in the form of a voltage
varylng witn time format is represented by a line 73. The
maximum instantaneous rate of change of a typical video
slgnal is limited by the 4.5 megacycles bandwidth. This
video signal is o~ the type which is directly displayable
on a televieion monitor.
The video slgnal shown in Figure 4 is applied to
the frequency modulator 20 as shown in Figure 1. The
modulator 20 generates the frequency modulated waveform 74
. ~
shown in Figure 5. The informational content of the
waveform shown in Figure 5 is the same as the lnformation-
al content of the waveform shown in Figure 4, but the form
' 25 i9 different. The informational signal shown in Figure 5
is a ~requency modulated signal having its informational
content ln the form of a carrier signal having frequency
changes in time about a center frequency.
B~ in~peation, it can be seen that the lower
amplltude region, generally lndicated by a numeral 75, of
the video waveform 73 shown ln ~igure 4, oorresponds to
the lower ~requency portion of the frequency ~odulated
signal 74 shown ~n Figure 5. One such cycle of the lower
-frequency portion of the frequency modulated signal 74 is
indicated generally by a bracket 75. A higher amplitude
region, indicated generally by the numeral 77 of the video
waveform 733 corresponds to the higher frequency.portions
of the frequency modulated signal 74. One complete cycle
of the higher frequency portion of the frequency modulated
` ~Z5~34
--17--
signal 74 is represented by a bracket 78. An intermediate
amplitude region3 generally indicated with a numeral 79
o~ the video wave~orm 73, corresponds to the intermediate
frequency portions o~ the frequency modulated signal 74.
A single cycle of the higher frequency portion o~ the
frequency modulated slgnal representing the intermedlate
amplitude reglon 79 is indicated by a bracket 79a.
By an inspection of Figures 4 and 5, it can be
seen that the ~requency modulator 20, shown in Figure 1,
converts the voltage varylng with time signal shown in
Figure 4, to a frequency modulated signal as shown in
Figure 5.
: Figure 6 illustrates the intensity o~ the writlng
beam 29 generated by the write laser 30. The intensity o~
the write beam 29 is shown to be at a constant level
represented by the line 80. After initial setup procedures,
this intensity remains unchanged.
Figure 7 illustrates the intensity of the writing
beam 29' a~ter its passage through the light intensity
modulating assembly 44. The intensity modulated writing
~ beam iq shown having a plurality of upper peaks 92 repre-
; senting the higher light transmitting state of the light
~: intensity~modulatlng assembly 44, and having a plurality
:~ of valleys 94 representing the low light transmitting
state o~ the light intensity modulating assembly 44. The
line 80 representing the maximum intensity of the laser
30 is superimposed with the wave~orm 29' to show that
some loss in light intensity occurs in the aqsembly 44.
Thi~ 108s i9 indicated by a line 96 showing the difference
in the intensity of the light beam 29' generated by the
laser 30 and the maximum intensity 92 of the light beam
29' modulated by the assembly 44.
This intensity modulation of the writing beam 29
to ~orm an intensity modulated writing beam 29' is best
illustrated by an inspection of Figures 6 and 7. Figure
6 shows the unmodulatad beam 29 having a constant intensi~
represented b~ the line 80. Figure 7 shows the modulated
beam 29' having maximum levels of intensIty indicated at
92 and minimum levels o~ lntensity lndicated at 94.
25434
The intensity modulation of the writing beam 29
is compared to the rotational ef~ect o~ the Pockels cell
68 by reference to lines 98, lOO and 102. The intersec-
tion of the line 98 with the line 29 l shows the intensity
of the beam 29 ' issuing from the linear polarizer 70 when
the Pockels cell 68 adds no rotation to the angle of
polarization of the light passing therethrough. The inte~
section of,the line lOO with the line 29 ~ shows the lnten-
sity of the beam 29 ' issuing from the linear polarizer 70
lO when the Pockels cell 68 adds a forty-five degree rotation
to the angle of polarization of the light passing there-
through. The intersection of the line 102 with the line
29' shows the intensity of the read beam 29 ~ lssuing ~rom
the linear polarizer 70 when the Pockels cell 68 adds a
15 ninety degree rotation to the angle of polarization of the
~ light passing therethrough.
`~ The formation of an aperture, such as 37 shown in
Figures 3 and 8, b~ the intensity modulated beam 29 l,
shown in Figure 7 can best be understood by a comparison
, 2~ between the two Figures 7 and 8.
- The line lOO is drawn midpoint between the inten-
, sity 92 representing the higher light transmitting state
~' of the assembly 44 and the intensity 94 representing the
' lower transmitting state of the assembl~ 44. The line lOO
2~ represents the lntensity generated by the assembly 44 when
the Pockels cell 68 rotates the angle of polarization of
~; the wrlte beam 29 passing therethrough through an angle
~; o~ forty-five degrees. Additionally, the line 100 repre-
sents the threshold lntenslty o~ the modulated beam 29'
required to ~orm an indicia ln the llght responsive coat-
lng 26. This threshold ls reached upon rotatlon of the
angle of polarizatlon of the write ~eam 29 through an
angle of forty-five degrees.
By a comparison between Figures 7 and 8, it can
3~ be seen that an aperture 37 is formed while the Pockels
cell 68 is rotating the angle of polarization o~ the write
beam 29 passing therethrough between the angle of forty-
five degrees and ninety degrees and back to forty-five
degrees. No aperture is formed while the Pockels cell 68
i43~
--19--
is rotating the angle of polarization o~ the write beam 29
pass~ng theretl~rough between the angle o~ fort~-five
degrees and ninety degrees and back tc forty-~ive degrees.
No aperture is formed while the Pockels cell 68 is rotating
the angle of polarization o~ the write beam 29 passing
therethrough between the angle of forty-five degrees and
zero degrees and back to forty-five degrees.
Re,~erring agaln to Figure 3, there is shown a
top vlew of the video disc member shown in radial cross-
sectional view in Flgure 8. An inspection of this Figure3 is help~ul in understanding the manner in which the
lineal series of light reflecting and light scatter-
regions 38 and 37 are formed upon the video disc member
10. The disc member 10 is rotated at a preferred rota-
tional rate of 1800 rpm and the indicia 37 and 38 areformed in the light responsive coating 2~ as shown with
re~erence to Figure 8. ~he motion control assembly 28,
shown with reference to Figure 1~ forms the apertures 37
in circular track-like fashion. A numeral 104 is employed
to identify a section of an inner track, and a numeral 105
is employed to identi~y a section of an outer track. A
dashed line 106 represents the center line of the track
135 and a dashed line 107 represents the center line of
the track 104. The length of a line 108 represents the
distance between the center lines 10~ and 107 of adjacent
tracks 105 and 104. Two mlcrons is a typical distance
between center lines of ad~acent tracks. The width of an
aperture 37 is indicated by the length of a llne 109. A
typlcal width o~ an aperture is one micron. ~he dlstance
between ad~acent apertures is represented by the length of
a line 110. This distance between ad~acent tracks ls known
as the intertrack region and typically is one micron in
length. The length of an aperture is represented by a
line 112 and typically varies between 1.0 and 1.5 microns.
All o~ these dimensions depend upon many variables in the
write apparatus. For example, these dimensions vary
depending upon the frequency range generated by the fre-
quency modulator 20, the size of the spot 42 formed by
the write optical systems 41 and 42 and the rotatlonal
llZSA34
-20-
speed selected for the disc 10.
Re~erring to Figure 99 there can be seen a more
detailed block diagram of the motion control assembly ~8
shown with reference to Figure 1. The rotatlonal drive
circuit 32 includes a spindle servo circuit 130 and a
spindle shaft 132. The spindle shaft 132 is integrally
~oined to the turntable 21. The spindle shaft 132 is
driven by a printed circuit type motor 134. The rota-
tional motion provided by the printed circuit motor 134
0 i9 controlled by the spindle servo circuit 130 which
phase locks the rotational speed o~ the turntable 21 to
a signal generated by a color subcarrier crystal oscillator
136 whlch forms a portlon of the synchronlzation assembly
; 36. The synchronization assembly 36 further includes a
~irst divider circuit 138 and a second divider circuit
140. The first divider circuit 138 reduces the color
subcarrier frequency generated in the oscillator circuit
136 down to a rotational re~erence frequency. The spindle
shaft 132 contains a tachometer 143 for generating a
frequency signal indicating the exact rotational speed
of the shaft 132 and turntable 21 combination. The tach-
ometer signal is available over a line 142 and the rota-
- tional reference signal from the first divider circuit
138 is available on a line 144. The tachometer signal on
~, 25 line 142 is applied to the spindle servo clrcult 130 and
the rotational reference signal on the line 144 is also
applied to the spindle servo circuit 130. The spindle
servo circuit 130 phase compares these two lnput signals.
When the phase of the tachometer signal leads the phase
Or the rotational re~erence signal, the rate o~ rotation
i8 too high and a sl~nal i8 generated ln the spindle servo
circuit 130 ~or application to the motor 134 over a line
146 to slow the rotational speed and bring the tachometer
signal into phase agreement with the rotational reference
signal~ When the phase of the tachometer signal lags the
phase of the rotatlonal reference signal as compared in
the splndle servo circuit 130, the rate of rotation is
too slow and a signal is generated in t'ne splndle servo
clrcult 130 for application to the motor 134 over a line
112~;~34
-21-
148 to increase the rotat~onal speed and bring the phase
of the tachome~er signal into agreement with the phase of
the rotational reference signal.
The second divider circuit 140 reduces the color
subcarrier frequency generated by the oscillator~135
down to a translational reference frequency for advancing
the translational drive circuit 34 a fixed distance for
each complete revolution of the member 10. In the pre-
ferred embodiment, the distance advanced by the trans-
lational drive circuit 34 for each revolution of the member
10 is a distance of two microns.
The color subcarrier crystal oscillator 136 with
, its two divider circuits 138 and 140 functions as an
electrical synchronizing circuit for maintaining a con-
stant relationship between the rotational motion of the
disc as provided by the rotational drive assembly 32 and
the translational motion between the write beam 29 and the
coating 26 is provided by the translation drive assembly
34.
The movable optical assemblies ill~strated in
~igures 1~ 10 and 11 are mounted on a platform indicated
at 142. ~his movable platform is driven radially by the
translational drive 34 which advances the platform 142
2.0 microns per revolution of the spindle shaft 132. This
translational movement is radlal with respect to the
rotating disc 10. This radial advancement per revolution
of the spindle shaft 132 is identified as the pitch of the
recording. Since the pitch uniformity of the finished
recording depends on the steady advance of the optical
3 assemblies mounted on the platform 142, care ls taken to
lap a lead screw 143 in the translation drlve 341 pre-
load a translation drlve nut 144 which engages the lead
screw 143 and make the connection between the nut 144 and
the platform 142 as stlff as possible as represented by a
bar 146.
Referring to Figure 10, there ~s shown a read
apparatus which is employed for retrieving the ~requency
modulated signal stored on the information storage member
10 as a lineal series of indicia 37 amd 38 previously
i
~Z5434
-22-
described. A reading beam 150 is generated by a read
laser 152 which produces a polarized$ collimated beam 150
of light. A support member9 such as the turntable 21, is
employed for holding the information storage member 10 in
a substantially predetermined position.
A stationary read optical assembly 15~ and a mova-
ble optical assembly 156 define a read opt~cal path over
which the read light beam 150 travels between the laser
source 152 and the information storage me~ber 10. Addi-
tionally~ either of the optical assemblies can be employedto ~ocus the light beam 150 upon the alternately posi-
tioned light reflective regions 38 and the llght scatter-
ing regions 37 carried in successive positions upon the
information storage member 10. The movable optical assem-
bly 156 is employed for collecting the reflections fromthe light reflective regions 38 and the light scattering
regions 37. mhe motion control assembly 28 provides
; relative motion between the read beam 150 and the alter-
nate regions of light reflectivity 38 and light scattering
37.
The optical assemblies 154 and 156 also define
the optical path travelled by the beam reflected from the
coating 26. The path of the reflected beam is identified
by the numeral 150'. This reflected light path 150'
includes a portioll of the initial read beam path 150. In
those portions where the reflected beam 150' coincides
with the read beam 150, both numerals 150 and 150' are
used. A light sensing element 158 is positioned in the
reflected light beam path 150' and ls employed for gener-
3 ating a ~requency modulated electrical slgnal correspond-
ing to the reflections impinging thereupon. The frequency
modulated electrical signal generated by the light
sensing element 158 is present on a line 160 and has lts
informational content in the form of a carrier frequency
having ~requency changes in time corresponding to the
stored information. The output of the light sensing
circuit 158 is applied to a discriminator circuit 162
by an amplifier 154. The discriminator clrcuit 162 is
responsive to the output of the light sensing circuit
~25434
158 and is employed ~or changing the fre~uency modulated
electrical signal into a time dependent voltage signal
representing the stored information. The time dependent
voltage signal is also identified as a video signal and
it is present on a line 165. This time dependent voltage
signal has its infor~ational content in the form of a
voltage varying with time format and is suitable for
display over a standard television monitor 166 and/or an
oscilloscope 168.
The read optical assemblies 154 and 156 ~urther
include a polarization selectlve beam splitting member
170 which functions as a beam polarlzer to the lncident
beam 150 and which functions as a selective beam splitter
to the reflected beam 150'. The read optical assemblies
further lnclude a quarterwave plate 172. The beam polar-
izer 170 filters out from the read beam 150 any spurious
light waves which are not allgned with the axis of polar-
lzation of the beam polarizer 170. With the axis of
polarization of the read beam 150 fixed in a particular
orientation by the member 170, the quarterwave plate 172
changes the plane of polarization from linear to circular.
The member 170 and the quarterwave plate 172 are disposed
in the read light beam path 150. The member 170 is locat~
between the source 152 of the read beam 150 and the quarter-
25 wave plate 172. The quarterwave plate 172 is also locatedin the reflected read beam path 150 t. Therefore, not only
does the quarterwave plate 172 change the read beam polar-
ization from linear to circular durin, its travel from
the read laser 152 to the ln~ormation storage member 1OJ
but the quarterwave plate 172 further changes the clr-
cularly polarized reflected light back into linearly
polarized light which is rotated ninety degrees with
respect to the preferred direction fixed by the source
152 and the member 170. This rotated beam 150' is selec-
tively dlrected to the light sensing element 158 whichchanges the reflected light beam 150' into a correspond-
ing electrical signal. It is to be noted that the member
170 reduces the intensity of the incident light beam 150
as it passes therethrough. This drop in intensity is
~12~i434
-24-
compensated for by setting the initial intensity of the
read beam 150 to a level sufficien' to offset this re-
duction. The quarterwave plate 172 glves a total rota-
tion of ninety degrees to the reflected beam 150 t with
5 respect to the incident beam 150 during the change from
linear polarization to circular polarization and back to
linear polarization. As previously mentioned, the member
170 is also a beam splitting cube in the reflected read
beam path 150'. As the plane of polarization of the re-
10 flected read beam 150' is shlfted ninety degree~ due toits double passage through the quarterwave plate, 172, the
beam splitting cube portion of the member 170 dlrects the
re~lected read beam 150' to the light sensing circuit 158
-' A suitable element ~or ~unctioning in the capacity of a
light sensing element 158 is a photodiode. Each such
element 158 is capable of changing the reflected fre-
quency modulated light beam 150 ~ into an electrical signal
having its information content in the form of a carrier
frequency having frequency variations in time varying from
the carrier frequency. The optical assemblies 154 and
156 further comprise the ob~ective lens 52 supported by a
hydrodynamic air bearlng member 54 which supports the
lens 52 above the coatlng 26 carried by the information
storage member 10.
` As previously described, the read beam 150 is
formed with substantially parallel light rays. The objec-
tive lens 52 has an entrance aperture 56 larger in diameter
than the dlameter of the read beam 150 as it is generated
by the laser source 152. A planar convex diverging lens
174 is provided intermediate the laser source 152 and
the entrance aperture 56 of the objective lens 52 ~or
spreadlng the substantially parallel light rays forming
the reading beam 150 into a light beam 150 having a
diameter sufficient to at least fill the entnance aperture
56 of the obJective lens 52. The optical assemblies 154
and 156 further include a number of stationary or ad~ust-
able mirrors 176 and 178 for folding the read light beam
150 and the reflected light beam 150 ' along a path cal-
culated to impinge upon the previously mentioned elements.
:. .
34
-25-
hn optional optical fllter 180 is positioned in
the re~lected beam path 150' and filters out all wave-
lengths other than that of the incident beam. The use of
this filter 180 improves picture quality as displayed
over the television monitor 166. This rllter 180 is
essential when the read system is used wlth the write
system as discussed in greater deta~l with re~erence to
F ~ ure 11. In this read after write environment, a por-
tion of the write beam 29 travels along the reflected read
beam path 150'. The filter stops this portion o~ the
write beam and passes the full intensity of the reflected
beam 150'.
An optlonal converging lens 182 is positioned in
the reflected beam path 150' for imaging the reflected
beam onto the active area of the light sensing element
158. This converglng lens 182 reduces the diameter of
the reflected beam 150' and concentrates the light lnten-
slty of the reflected beam upon the actlve area of the
llght senslng element 158.
The amplifier 164 amplifies the output o~ the
light sensing element 158 and raises the amplitude of
the frequency modulated electrical signal ~enerated by
the light sensing element 158 to match an input slgnal
requirement of the demodulator 162.
Referring again to the electrical and optical
waveforms shown in Figures 4 through 7, these waveforms
are also generated by the read apparatus, shown in Figure
10 during the retrleval o~ the frequenoy modulated signal
stored in the coatlng 26 carried by the di~c 10. Flgure 6
3 shows a laser source generating a write laser beam having
a constant intenslty represented by the llne 80~. The read
laser 152 generates a read beam 150 having a constant in-
tensity but at a lower level.
Flgure 7 shows an intens~ty modulated wrlte laser
beam. The reflected read beam 150' is lntensity modulated
by the act o~ implnglng upon the llght reflective and
`llght scatterlng regions 38 and 37 carrled on the dlsc
member 10. The reflected read beam 150' wlll not be a
perfect squarewave as shown ln Figure 7. Rather, the
.
ii2~A34
-26-
square edges are rounded by the fin~te size of the read
spot.
Figure 5 shows a frequency modulated electrical
signal having its informational content in the ~orm of a
, 5 carrier signal having frequency changes in time varying
about the center frequency. The output o~ the light
sensing element 158 is the same type o~ signal. Figure 4
shows a video signal having its informational content in
the f~rm of a voltage varying with time ~ormat. The output
o~ the demodulator 162 is the same type of signal.
The motion control assembly 28 shown in Figure 10
operates in the same manner as the motion control assembly
28 shown in Figure 1. In the read apparatus J the motion
control assembl~ 28 produces a rotational motion to the
disc member under the control Or a rotational drlve assem-
bly 32. The assembly 28 further produces a translational
motion for moving the movable read optical assembly 156
radlally across the surface of the storage member.
~he assembly 28 further includes a synchronizing
- 20 circuit for maintaining a constant relationship between
the rotational motion and the translational motion so that
the read beam 150 impinges upon the information tracks
carried by the disc member 10. Portions of typical in-
formation tracks are shown as 104 and 105 in Flgure 3.
Referring to Figure 11, there $s shown a block
d$agram illustrating the comb$nation of the wrlte apparatus
shown ln Figure 1J and the read apparatus shown in Figure
10. The elements shown in Figure 11 operate ln an
ldentical manner as previously described and this de-
30 talled operatlon is not repeated here. Only a brie~
descriptlon is given to avoid repetit$on and confusion.
The unmodulated write beam path i9 shcwn at 29
and the modulated beam path is shown at 29'. A first
optical assembly de~$nes the modulated beam path 29'
35 between the output of the linear polarizer 70 and the
coat$ng 26. The f$xed, wr$te opt~cal assembly 41 $ncludes
the mlrror 58. me movable, write optical assembly 40
includes the diverging lens 66, a partially transmlssive
mirror 200~ a movable mirror 60 and the ob~ective lens 52.
1~2~;434
--27--
The modulated write beam 29 ' is imaged to a write spot 42
upon the light responsive coating and interacts with the
coating to for~ indicia as previously described.
The read beam path is shown at 150. The read
optical assemblies define a second optical path for the
read beam 150 between the read laser 152 and the informa-
tion storage record carrler 10. The fixed, read optical
assembly 154 includes the mirror 176. The movable, read
optical assembly 156 includes the diverging lens 174, the
polarlzation shifting means 172, a second fixed mlrror
202, the partially transmisslve mirror 200, the movable
mirror 60 and the lens 52. The read beam 150 is imaged
to a read spot 157 at a point spaced downstream ~rom the
write spot 42, as is more completely described with refer-
ence to Flgure 12.
The mirror 200 is a dichroic mirror whlch is
transmissive at the wavelength of the write beam 29' and
which is reflective at the wavelength of the read beam
150 '.
The intensity of the write beam 29 ' is higher
than the intensity of the read beam 150. While the write
beam 29 ' must alter the light responsive coating 26 to
retain indicia representative of the video signal to be
stored, the intenslty of the read beam 150 should only be
sufficient to illuminate the indicia formed in the coating
26 and provide a reflected light beam 150' of sufficlent
intensity to provide a E;ood ~ignal a~ter collectlon by the
read optical assembly and conversion from an int~nsity
modulated reflected beam 150 ' to a frequency modulated
3o electrical signal by the light sensing circuit 15~.
The fixed mirror 58 in the write optlcal path and
the two fixed mirrors 176 and 202 in the read optical path
are employed for directing the write beam 29' toward the
ob~ective lens 56 at a controlled angle with respect to
35 the read beam 150. This angle between the two incident
beams provides a spacing between the write spot 42 and read
spot 157 as they are each respectively imaged upon the
coating 26.
In operation, a sufficient spacing has been found
....
5~34
-28-
to be four to six microns. This distance corresponds to
an angle too small to show clearly in Figure 12. Accord-
ingly, this angle is exaggerated in Figure 12 for purpose
Or illustration only.
The read beam 150' is demodulated in a discrim-
inator clrcuit 162 and displayed on a standard television
monitor 156 and an oscilloscope 16~. The television
monitor 166 shows the picto.rial quality of the recording
and the oscilloscope 168 shows the video signal in more
detail. Thls read after write function allows the quality
of the video signal belng stored during a write operation
to be instantaneously monitored. In the event that the
quality of the stored signal is poor~ it is known immedi-
ately and the write procedure can be corrected or the
information stora~e member lO storlng the poor quality
video information signal can be discarded.
In the read after write mode of operation, the
write laser 30 and the read laser 152 are operating at
the same time. A dichroic mlrror 200 is employed for
combinin~ the read beam 150 lnto the write beam 29'. In
this read after write mode of operation, the wavel~ngth
of the write beam 29 is chosen to be different from the
wavelength of the read beam 150. ~n optical filter 180
is employed for bloc~ing any portion of a write beam which
25 has followed the re~lected read beam path. Accordingly,
the optical ~ilter 180 passes the reflected read beam 150'
and filters out any part o~ the write laser beam 29' ~ollow-
lng the reflected read beam path 150'.
In the comparison mode of operationJ the read
30 after write operation i9 practlced as descrlbed wlth refe~
ence to Figure ll. When operating in thls monltoring mode
of operatlon, a comparator circuit 204 comp~res the output
o~ the demodulator 162 with the original video information
signal provided by the source 18.
M~re speci~ically, the video output of the dis-
criminator 162 is applied to a comparator 204 over a line
206. The other input of the comparator 204 is taken ~rom
the video source 16 over the line 18, an additional line
208 and through a delay line 210. The delay line 210
.. ~. ,
~125~34
-29-
imparts a time delay to the input video information signal
equal to the accumulated values of the delay beginning
with the ~requency modulation of the input video informa-
tlon signal and extending through the frequency demodula-
tion of the recovered electrical signal from the sensingcircuit 158. This delay also lncludes the delay of travel
time from the point on said storage member 10 at which the
input video information signal is stored upon the informa-
tion s~orage member by the write spot 42 and continuing to
the point of impingement of the read spot 157.
The correct amount of delay is best generated by
making the delay circuit 210 a varlable delay clrcuit which
ls ad~usted for optimum operation.
Ideally~ the video output signal of the discrim-
inator 162 is identical in all respects to the video lnputslgnal on the lines 18 and 208. Any dif~erences noted
represent errors which might be caused by imperfections
in the disc's surface or malfunctions of the wrlting cir-
cults. This application, while essential if recordlng
dlgital information, is less critical when other informa-
tion is recorded.
The output signal fro~ the comparator circuit 204
may be counted, in a counter (not shown) 3 ~or establishing
the actual number of errors present on any disc. When the
errors ccunted exceed the predetermined selected number,
the writlng operation is terminated. If necessary, a new
disc can be written. Any disc with excesslve errors can
then be reprocessed.
In Figure 11, the comparator 204 compares the
output signals available on the lines 208 and 206. An
alternative and more direct connection o~ the comparator
204 i9 to compare the output of the ~requency modulator 20
and the amplifier 164 shown with reference to Figure 10.
Turning next to Figure 12, there is shown in
somewhat exaggerated form, the slightly di~ering optical
paths of the intensity modulated write beam 29' from the
writing laser 30 and the unmodulated read beam 150 from
the reading laser 152. The information storage member 10
is moving in the direction indicated by an arrow 217.
i~.Z~43
-30-
This shows an unexposed coating 26 approaching the wrlte
beam 29' and a lineal series o~ apertures 37 leaving the
intersection of the write beam 29~ and the coa~ing 26.
The writing beam 29' coincides with the optical axis of the
microscope objective lens 52. The central axis of the
reading beam 150 shown as 212 makes an angle with the
central axis of the wrlte beam 29~ shown as 214. me angle
is represented by a double headed arrow ~16. Due to this
slight difference in optical paths of the write beam 29'
and read beam 150 through the lens 52, the write spot 42
falls a distance ahead of the read spot 1~7. The write
spot 42 leads the read spots 157 by a distance equal to
the length of a line 21B. The length of the line 218 is
equal to the angle times the focal length of the objective
lens 52. The resulting delay between writing and readlng
allows the molten metal coating 26 to solidify so that the
recording ls read in its final solidified state. If it
were read too soon while the metal was still molten, the
re~lection from the edge of the aperture would fail to
provide a high quality signal for display on the monitor
166.
Referring to Flgure 13, there is shown an ideal-
ized diagram of a Pockels cell s~abilizing circuit 48
suitable for use in the apparatus of Figure 1. As is
2~ known, a Pockels cell 68 rotates the plane of polari~ation
of the applied write light beam 29 as a function of an
applied voltage as illustrated with reference to Flgure 7.
gepending upon the individual Pockels cell 68,
a voltage change of the order of 100 volts causes the
3 cell to rotate the plane of polarization o~ the light
passing therethrough a ~ull ninety degrees. The Pockels
cell driver functions to ampli~y the output from the in-
formation signal source 12 to a peak-to-peak output swing
o~ 100 volts. This provides a proper input drlving signal
to the Pockels cell 68. The Pockels cell driver 72 gener-
ates a waveform having the shape shown in Figure 5 and
having a peak-to-peak voltage swing of 100 volts.
The Pockels cell should be operated at an average
rotation of forty-five degrees in order to make the
434
-31-
i modulated light heam intensity most faithfully reproduce
the electrical dr~ve signal. A bias voltage must be pro-
videq to the Pockels cell for keeping the Pockels cell at
this average operating point. In practice, the electrical
blas voltage corresponding to a forty-~ive degree rotation
operating point varies continuously. This continuously
changing bias voltage is generated through the use of a
servo feedback loop. Thls feedback loop includes the
comparison of the average value of the transmltted light
to an ad~ustable re~erence value and applylng the differ-
ence signal to the Pockels cell by means of a DC ampllfier.
Thls arrangement stabilizes the operatlng point. The
reference value can he ad~usted to correspond to the
average transmission corresponding to the forty-five degree
operating point and the servo feedback loop provides cor-
rective bias volta~es to maintain the Poc~els cell at this
average rotation of forty-five degrees.
The stabilizing circuit 48 includes a light sens-
ing means 225. A silicon diode operates as a suitable
light sensing means. The diode 225 senses a portion 29'l
of the writing beam 29' issuing from the optical modulator
44 and passing through the partially reflective mirror 58
as shown in Figure 1. The silicon diode 225 functions in
much the same fashion as a solar cell and ls a source of
electrical energy when llluminated b~ incident radiation.
One output lead of the silicon diode 225 is connected to
common reference potential 226 by a line 227. The other
output lead of the diode 225 is connected to one input of
a dif~erential amplifler 228 by a line 230. The output
leads of ~he silicon cell 225 are shunted by a load resis-
tor 232 which enables a linear response mode.
The other input to the differential ampllfier 228
is connected to an adjustable arm 234 of a potentiometer
236 by a line 238. One end of the potentiometer 236 is
connected to reference potential 226 by a llne 240. A
source of power 242 is cDupled to the other end of the
potentiometer 236 ~hich enables the ad~ustment of the
differential ampli~ier 228 to generate a feedback signal
on the lines 244 and 246 for ad~usting the average power
-32-
level o~ the modulated laser beam 29' to a predetermined
value.
The output terminals of the differential amplifier
228 are) respectively, connected through resistive elements
248 and 250 and output lines 244 and 246 to the input
terminals of the Pockels cell 68 in Figure 1. ~he Pockels
cell driver 72 is AC coupled to the Pockels cell 58 by
way of capacitive elements 2~2 and 254, respectively,
while the differential amplifier 228 ls DC coupled to the
Pockels cell 68.
In operatlon, the system is energlzed. The p~r-
tion 29" of the light from the writing beam 29' impinging
on the silicon diode 225 generates a differential voltage
at one input to the differential amplifier 228. InitiallyJ
the potentiometer 236 is adjusted so that the average
transmission through the Pockels cell corresponds to fort~
five degree of rotatior.. Thereafter, lf the average level
of intensity impin~ing on the silicon cell 225 either in-
creases or decreases, a correcting voltage will be gener-
20 ated by the differential amplifier 228. The correctingvoltage ap~lied to the Pockels cell 68 is of a polarity
and magnitude adequate to restore the average level of
intensity to the predetermined level selected by ad~ustment
of the input voltage to the other input of the differen-
25 tial ampllfier over the line 238, by movement of themovable arms 234 along the potentiometer 236.
The adjustable arms 234 of potentiometer 236 is
the means for selecting the average level of intensity of
the light generated by the write laser 30. Optimum re-
sults are achleved when the length Or an aperture 37 ex-
actly equals the length of the next succeeding space 38
as previously described. The ad justment of potentiometer
236 is the means for achievlng this equality of length.
When the length of an aperture equals the length of its
next ad~acent spaceg a duty cycle of fifty-flfty is
achieved. Such duty cycle is detectable by examining the
display o~ the just written information on the TV monitor
and/or osclllosoope 166 and 168, respectively, as pre-
viously descrlbed. Commercially acceptable results occur
. ~
` 11;~5~L34
~hen the length o~ an aperture 37 varies between ~orty and
sixty percent of the combined length of an aperture and
its next successively positioned space. In other words,
the length of an aperture and the next successively posi-
tioned space is measured. The aper'ure can then be alength ~alling within the range of ~orty and sixty percent
of the total length.
Referring to Figure 8, there is shown a radial
cross-section of an information track shown with reference
to F~gure 3 1n which a specular light reflectlve region
38 is positioned intermediate a pair of non-specular light
reflective regions 37. In the radial cross-sectional
view shown in Figure 8, the impinging read or write beam
is moving relative to the member lO in the direction
represented by the arrow 217. m is means that a reading
beam impinges first upon the specular light reflective
region 38a followed by its impingement upon the non-
specular light reflective region 37a. In this configur-
ation, the positive half c~Jcle of the signal to be re-
corded is represented by a specular light reflectiveregion 38a and the negative half 'c~cle of the signal to
be recorded is represented by the non-specular light
reflectlve,region 37a. The duty cycle of the signal
shown with ~.eference to Fi~ure 8 is a fifty percent duty
cycle insofar as the length of the specular light re-
flected region 38a as represented by a bracket 260, is
equal in length to the length of the non-specular light
reflective region 37a as represented by the bracket 262.
This preferred dut~ cyole set up by the comblnatlon o~
adJusting the absolute intensity of the wrlte beam 29 by
ad~usting the power supply of the write laser 30 and by
ad~usting the potentlometer 236 in the stabilizing circult
48 to a level wherein an aperture ls formed beginning with
a forty-five degree rotation of the angle of polarization
in the write beam 29.
Referring again to the aperture forming process
illustrated with reference to Figures 7 and 8, meltlng of
a thin metal coating 26 occurs when the power in the light
spot exceeds a threshold characteristic of the composition
3.~ L34
-34-
and thickness o~ the metal film and the properties o~ the
substrate. The spot power ls modula'ed by the light
intensity modulating assembly 4~. The on-o~ transitions
are kept short to make the location o~ ~he hole ends pre-
cise in spite of variations in the melting threshold.Such variations in the melting threshold can occur due to
variations in the thickness of the metal coating and/or
the use of a dif~erent material as the information storing
layer.
The average power in the spot required to form an
aperture in a thin metal coating 26 having a thickness
between 200 and 300 Angstroms is of the order of 200
milliwatts. Since the FM carrier frequency is about 8
MH7, 8 x 106 holes of variable length are cut per second
and the energy per hole is 2.5 and 10-9 joul.
In this ~irst embodiment of a video disc member
10, a portion of the glass substrate is exposed in each
aperture. The exposed portion of the glass substrate
appears as a region o~ non-specular light re~lectivity to
an impinging reading beam. The portion of the metal
coating remaining between successively positioned apertures
appears as a region o~ high light reflectivity to an im-
pinging reading beam.
When the forming of first and second lndicia is
being undertaken using a coating of photoresist~ the inten-
sity of' the write beam 29' is adjusted to a level such
that a forty-~iYe deg~ee rotation of the plane o~ polar-i-
zation generates a light beam 29' of threshald intensity
for exposing and/or interactl~g with the photoresist
coatlng 26 while the photoresist coating ls in motion and
posltioned upon the moving information storage member 10.
The Pockels cell 68 and Glan-prism 70 combination com-
prises a light intensity modulating member which operates
from the forty-five degree setup condition to a lower light
transmitting state associated with a near zero degree
state of operation, to a higher light transmitting state,
associated with a near ninety degree state o~ operation.
When the intensity o~ the write light beam 29l increases
above the initially adjusted level or predetermine start
-35-
intenslty, and increases ~owards the higher llght trans-
mitting state the incident write light beam 29' exposes
the photoresist illuminated thereby. This exposure con-
tinues after the intenslty of the write beam reaches the
maximum light transmittlng state and starts back down
towards the initial predetermined intensit~ associated
with a forty-~ive degree rotation of the plane of polarl-
zation of the light issuing from the write laser 30. As
the rotation drops below the forty-five degree value, the
10 intensity of the write beam 29~ exiting the Glan-prism 70
drops below the threshold intensity at which the focused
write beam ~ails to expose the photoresist illuminated
thereby. This failure to expose the photoresist illumln-
ated thereby continues after the intensity of the write
1~ beam reaches the minimum light transmitting state and
starts back up towards the initial predetermined intensity
associated with a forty-five degree rotation of the plane
of polarization of the light issuing from the write laser
30.
The Pockels cell driver circuit 72 ~s typically
a high gain and high voltage amplifier having an output
signal providing an output voltage swing of 100 volts.
~his signal is intended to match the driving requirements
- of the Pockels cell 68. Typically, this means that the
mid-voltage value of the output of the Pockels cell
driver 72 provides a suf~icient control voltage for d~ving
the Pockels cell 68 through ~orty-~ive degrees so that
about one half of the total available light ~rom the laser
30 issues ~rom the linear polarizer 70. As the output
signal fro~ the driver 72 goes positive, mid-voltage
value, more light from the laser is passed~ As the output
signal ~rom the driver 72 goes negativeJ less light ~rom
the laser ls passed.
In the first embodiment us~ng a metal coating 26,
the output from the laser 30 ls adjusted so as to produce
an intensity which begins to melt the metal layer coating
26, positioned on the disc 10, when the output from the
driver ~2 is zero and the operating polnt of the Pockels
cell is ~orty-f~ve degrees. Acoordingly, as the output
34
-36-
from the driver 72 goes P~sitl~a, melting continues. Also,
when the output ~rom the dr~ver ~2 goes negative, melting
stops.
In a second embodiment using the photoresist
coating 255 the output ~rom the laser 30 is adjusted so as
to produce an intensity which both illuminates and exposes
the photoresist coating 26 when the output from the driver
72 is generating its mid-voltage value. Accordingly, as
the output from the driver 72 goes positive, the illumina-
tion and exposure of the photoresist illuminated by thewrite beam continues. A1SOJ when the output from the driver
72 goes negative, the illumlnation continues but the
energy in the write beam is insuf~icient to expose the
illuminated region. The term expose is herein being used
for its technical meaning which describes that physical
phenomenon which accompanies exposed photoresist. Exposed
photoresist is capable o~ being developed and the developed
photoresist ls removed by standard procedures. Photo-
resist which is illuminated by light, insuf~icient ln
intensity to expose the photoresistl cannot be developed
and removed.
In both the first and second embodiments just
described$ the absolute power level 80 illustrated by the
line 80 in Figure 6 is ad~usted upward and downward to
achieve this effect by ad~usting the power supply of the
write laser 30. In combination with this ad~ustment of
the absolute power level of the write laser 30, the
potentlometer 236 is also used to cause indlcia to be
formed in the coating 26 when the beam 29 is rotated above
3 forty-five degrees as prevlously descrlbed.
In a read only system as shown in ~igure 1OJ the
optical rilter 180 is optional and usually is not required~
Its use in a read only system introduces a slight attenua-
tion in the reflected path thus requ~r~ng a slight increase
in the intensity of the read laser 152 to insure the same
intensity at the detector 158 when compared to a read only
system which does not use a filter 180~
The converging lens 182 is optional. In a
properly arranged re~d system the reflected read beam 150'
S~4
-37-
has essen~ially the same diameter as the working area o.
the photodetector 15~. If this is not the case, a con-
verging lens 182 is employed for concentrating the re-
flected read beam 150' upon the smaller working area of
the photodetector 158 selected.
Prior to giving the detailed mode of operation
of an improved vers~on of a mastering machine, it would
do well to establish a number of terms which have a special
meaning in the description contained hereinafter. The
laser intensity generated by the writing laser source as
it impinges upon the master video disc is employed to
interact with ~he in~ormation bearing portion of the video
dlsc to form indicia representing the carrier frequency
and the f`requency variations in tlme from the carrier
15 frequencY-
- The threshold power level required of the laser
beam at the point of impact with the information bearing
layer of the video disc differs depending upon the mater-
ial from which the in~ormation bearing layer is made.
20 In the two examples given hereinabove describing a metal
such as bismuth and a photosensitive material such as
photoresist, the threshold power level required to form
indicia differs significantly and represents a good ex-
ample for illustrating the term threshold power. Obvious-
25 l~ the threshold power of other materials would alsodif~er from each of the examples explained.
The lndicia formed in a bismuth coated video disc
master are alternate regions o~ light reflectivity and
light non-reflectlvity. The areas o~ light non-reflecti-
30 vlty are caused by the melting of the bismuth ~ollowed bythe retractlng of the bismuth be~ore cooling to expose an
underlying portion of the glass substrate. Light imping-
ing upon the metal layer is highly reflective, while light
impinging upon the e~posed portion of the glass substrate
35 ls absorbed and hence light non-reflectivity is achieved.
The threshold power is thàt power ~ro~ the laser
beam required to achieve melting and retracting of the
metal layer in the presence of a laser beam of increasing
light intensity. The threshold power level is also
~L~Z~ 3~
-38-
represented as that intens~ty of a decreasing l~ght inten-
sity signal when the metal layer ceases to melt and
retract from the region having incident light impinging
thereupon. More specifically, when the power in the
impinging light beam exceeds the threshold power require-
ments of the recording material, a hole is formed in the
recording material. When the light power intensity in the
impinging light beam is below the threshold power level of
the recording material, no hole is formed in the recording
medium. The forming of a hole and the non-form~ng of a
hole by the impinging light beam is the principal manner
ln which the light beam impinging upon a bismuth coated
master interacts with the bismuth layer to form indicia on
the recording surface. The indicia represents a carrier
frequency havlng frequency changes in time varying about
the carrier frequency.
A video disc master having a thin layer of photo-
resist formed thereover has its own threshold power level.
The mechanism whereby a light beam exposes a photoresist
layer is pursuant to a photon theory requirlng a suffi-
cient number of photons ln the impinging light beam to
expose a portion of the photoresist. When the positive
going modulated light beam contains sufficient photons
above this threshold power level, the photoresist in that
area is exposed so that subsequent development removes the
; exposed photoresist. When the photon level in a decreas-
ing light intensity modulated light beam falls below the
normal threshold power level of the photoresist, the phot~
resi~t caases to be exposed to the extent that subsequent
development does not remove the photoresist illuminated by
an impinging light beam having photons below the threshold
power level.
The impinging light beam from the modulated laser
source interacts with the information bearing layer to
fully expose or underexpose the photoresist layer illumin-
ated by the impinging light beam. This is an i~teraction
of the photons in the impinging light beam with the infor-
mation bearing member to form lndicia of the carrier fre-
quency having frequency changes in time varying about the
'
5~3~
-39-
carrier ~requency. The indicia stor~ng the carrier ~re-
quency and ~requency change in time are more fully appre-
ciated a~ter the development step ~!hereby those por~ions
of fully exposed photoresist material are effectively
removed leaving the undere~posed portions on the video disc
member.
Re~erring to Figure 23, there is a block diagram
of the Pockels cell bias servo system e,~ployed in the pre-
ferred embodiment of the present invention for maintainlng
the operating bias on the Pockels cell ~ at the hal~
power point. ~he DC bias of the Pockels cell is ~irst
adjusted to its steady state condition such that the half
power point of the Pockels cell-Glan prism combination
coincides with the forty-five degree rotation point o~ the
Pockels cell 68. Thls DC bias point is identified as the
~ixed blas point. In a system wherein the input video
signal to the FM modulator 36 does not contain any second
harmonic distortion~ the DC bias position selected in the
procedure JUSt identified operates satisfactorily. How-
ever, when the video information input signal to the FMmodulator contains second harmonic distortion products,
these distortion products show up in the modulated light
beam at 29~. The output from the FM modulator is applied
to a Pockels cell driver 72 ~or developing the voltage
required to drive the Pockels cell through its zero to
ninety degree rotational shift. The unmodulated light beam
from the laser 29 is applied to the Pockels cell 68 as
previously,described.
The purpose of the Pockels cell bias servo is to
bias the Pockels cell 68 so that the output llght signal
detected at a photo diode 2~0 i9 as free of second harmonic
content ~s posæible.
The second harmonic distortion is introduced into
the modulated light beam at 29' ~rom a plurality of sources.
A first o~ such sources is the non-linear transfer func-
tions of both the Pockels cell 68 and the Glan prism 70.
Ilhen the input video signal on the line 18 itself contains
second harmonic distortion products this further increases
the total second harmonic distortion products in the light
,,
:~lZ5~3~
-40-
beam at 29'.
The Pockels cell bias servo functioIls to adjust
the DC bias applied to the Pockels cell 58, which DC
bias biases the Pockels cell to its half power point, so
as to minimize the second harmonic content of the output
light beam.
The change in DC bias level from the half power
point ls achieved in the following sequence of steps. The
modulated light beam 28' from the Pockels cell 68 is
applied to a photo diode 260. The photo diode 260 oper-
ates in its standard mode of operation and generates a
signal having the form of a carrier frequency with fre-
quenc~r variations about the carrier frequency. This fre-
quency modulated waveform is a sufficiently linear repre-
sentation of the light impinging upon the photo diode 261
to accurately reflect the signal content of the light
modulated beam 29' impinging upon the disc surface. More
specifically, the output signal from the photo diode 260
contains the distortion products present in the modulated
20 light beam 29'. The output from the photo diode 260 is
applied to a second harmonic detector 261 over a line 262
which forms a part of the bias control circuit 264. The
output from the second harmonic detector is to a high
voltage amplifier 266 which generates the DC bias slgnal
; 25 over a line 268. The line 268 is connected to a summation
circuit 270 which has as its second input slgnal the output
from the Pockels cell driver 72, The DC bias signal on the
line 268 is summed wlth the output ~rom the Pockels cell
drlver 72 and is applied to the Pookels cell 68 ~or chang-
ing the DC bias of the Pockels cell 68.
Re~erring back to the operation of the second
harmonic detector 261, this device generates a voltage
which is approximately linear in the ratio of the second
harmonic to the fundamental of the output llght beam.
Furthermore, the output signal reflects the phase charac-
teristics of the second harmonic and if 'he second harmonic
is in phase with the fundamentalg the output o~ the second
harmonic detector is in a first voltage level, ie.g a
positive level. If the aecond harmonic is opposite in
~2S4;~
-41-
phase with the ~ul~damental; then the output ef the second
harmonic detector is at a second voltage level~ ie., a
negative vcltaOe level. The output ~rom the second harmo~
detector is amplified through a high voltage amplifier 266
which provides a range of 7ero to three hundred volts of
DC bias. This DC bias is summed with the si~nal from the
FM modulator 20 amplified in the Pockels cell driver 72
and applied to the Poc~els cell 68.
The second harmonic detector includes a limiter
272 shown with reference to Figure 24 and a dif~erential
amplifier 274 shown with reference to Figure 24. The out-
put signal from the photo diode 260 is AC coupled to the
limiter 272 over lines 276 and 278. The limiter 272 has
a first output signal for application to the di~ferential
amplifier over a first output leg 280. The second output
from the limiter 272 is applied to the second input of
the differential amplifier over a second output leg 282.
The output signals from the limiter 272 are logical comple-
ments of each other. More specifically, when one output
is at a relatively high voltage level, the other output
is at a relatively low voltage level. The two output
signals on the legs 280 and 2~2 are fed into the differen-
tial amplifier 274. The output of this differential
ampllfier reflects the content of the second harmonic
available on the input signal lines 276 and 278.
In a standard mode of operation, when the input
signal from the photo diode 260 is substantially free o~
æecond harmonic distortion; then the output sig~al from
the dlfferential ampllfier 274 at terminal 284 is a square-
wave with exactly a 50% duty cycle and with voltage levelsextending between two predetermined voltage levels above
and below a constant reference level. The duty cycle of
50~ refers to a high voltage half cycle being e~ual in
width to the following low voltage half cycle. In this
condltion, the ef~ective DC level of these two half cycles
offset one another. Accordingly, the output of the diffe~
ential amplifier 274 is, on the average, zero.
When ~. degree of second harmonic distortion is
pr~sent ln the output from the photo diode 260, the value
~12~34
-42-
of harmonic dlstortion shifts the mean value crossing ~ro~
a symmetrical case to a non-symmetrical case. In this
situation5 the outpu~ ~rom the di~ferential amplifier is
other than a squarewave with a fi~t~-fifty duty cycle.
~he differential amplifier therefore detects the effec-
tive DC level shift of the incoming s~gnal and generates
an output which is above or below sero, on the average,
depending upon the asymmetrical nature of the input signal.
The output o~ the differential amplifier is therefore
applled to the high voltage ampli~ier which DC smooths the
output from the di~ferential amplifier 274 and amplifies
the negative or positive resulting DC level. The resulting
product is the required shift in bias signal for applica-
tion to the Pockels cell to return the operating point of
the Pockels cell to the half power point at which ~ero
harmonic distortion occurs.
A summary of the standard operating mode of
Pockels cell bias servo includes the generation of a
light signal representing the distortion products present
on the modulated light beam. Means are provided for dete~t-
ing the value of second harmonic distortion present in this
light beam and generating a signal representation of this
distortion. The signal generated to represent the amount
of second harmonic distortion also includes whether the
second harmonic distortion is in phase with the fundamen-
tal frequency or the second harmonic distortion is out of
phase with the fundamental frequency. The output signal
representing the amount of second harmonic dlstortion and
the phase of the second harmonic distortion with reference
to the fundamental frequency ls applied to a means for
generating a bias signal necessary for appllcation to the
Pockels cell to bring it to an operating point at which
second harmonic distortion ceases to exist. A summation
circuit is provided for summing the change in bias signal
wlth the lnput frequency modulated video signal. This
summed voltage is applied as an input to the Pockels cell
68.
Fi~ure 14 shows a series of waveforms illustraing
an improved form of light modulation of a writing light
25~34
-43-
beam 29. Line ~ of Figure 14 shows an idealized or simpl~
ried video waveform that is ty~ically supplied as a video
signal from a video tape recorder or television camera.
This waveform is essentially the same as that shown in
Figure 4 and represents a video signal that is applied to
the FM modulator circult 20. ~!0 output signals are shown
on lines ~ and C~ and each is an FM modulated output
signal and each carries the same frequency information.
The waveform on line B is a ~epeat of the waveform in
Figure 5 and ls repeated here ~or convenience. ~his wave-
form on line B shows the output normally generated by a
mlulti-vibrator type FM modulator 20. The waveform shown
on line C shows the output generated by an FM modulator 20
having a triangular shaped output waveform. Eo~h wave-
forms contain the same frequency information. The triangu-
lar shaped waveform gives enhanced results when used in
driving a Pockels cell 68 for light modulation of a con-
stant intensity light beam applied through the Pockels
cell.
The frequencies contained in each waveform ~ and
C are at all times identical and each represents the
voltage level of the video waveform shown in line A. By
inspection9 it can be seen that the lower amplitude
region of the video waveform generally indicated by the
numeral 75 corresponds to the low carrier frequencies and
higher amplltude regions of the video waveform as gener-
ally indicated at 77 corresponds to the higher frequency
shown in lines ~ and C. It is the custom and practice of
the televi~ion lndustry to utllize a one volt peak to peak
voltage signal having voltage varlations in time as the
video signal generated by a television camera~ These
signal characteristics are the same required to drive a
television monitor 166. The advantage of using a triangu-
lar shaped waveform for driving a Pockels cell 68 is to
3~ match the Pockels cell's transfer characteristic with a
selected waveform of the modulating signal to achieve a
sinusoidal modulation of the light beam passing through
~he Pockels cell and to the Glan prism 78. The triangular
waveform shown in line C is a linear voltage change with
3~12~
-44-
tlme. The linear voltage change versus time o~ the tri-
angular driving uaveform when multiplied b~ a sinusoidal
voltage change versus light trans~er function of the
Po~kels cell 68 gives a sinusoidally varying light inten-
sity output from the Glan prism.
The waveform shown on line D illustrates thesinusoidal waveform which corresponds to the light inten-
sity output from the Glan prism when the Pockels cell ls
driven by the triangular waveform shown on line C.
Re~erring specl~ically to the bottommost point at
285 and the topmost point at 286 o~ the waveform shown on
line D, the point exactly equally distant from each is
identified as the half power point. An understanding o~
the utilization of this hal~ power point ~eature is re-
quired ~or high quality mastering operations.
The peak to peak voltage of the triangular wave-
~orm is represented by a first maxlmum voltage level V2
shown on line 287 of line C and by a second minimum vol-
tage level Vl on line 288. The voltage di~ferential
between points 287 and 288 is the driving voltage for the
Pockels cell 68. This voltage differential is ad~usted to
equal that voltage required by- the P~ockels cell 68 to give
a ninety degree rotation o~ the output polarization of
the light passing through the Pockels cell 68. The bias
on the Pockels cell is maintained such that voltage levels
Vl and V2 always correspond to the zero degree rotation
and the ninety degree rotation respectively of the light
beam passlng through the Pockels cell 68. The forty-
~ive degree rotation Or a light beam is half way between
the two extremes of a trlangle waveform. That half-way
voltage is alway~ the same for the Pockels cell 68. But
the half-way voltage with respect to zero volts may drift
due to thermal instabilltles causing the half powçr volt-
age polnt to dri~t also. The correct biasing of the half-
way voltage is completely described hereinafter with re~e~ence to Figures 18J 19 and 20.
While the waveform shown on line C of Figure 14
shows the triangular wave shape generated by the FM modu-
lator 20J it also represents the wave shape of the signal
, ~ ~
. .
-45-
generated by the Pockels cell driver 7~. The output from
the FM modulator is typiGally in a smaller voltage range,
typically under 13 olts wh11e the output frcm the Pockels
cell driver 72 typically swings 100 volts in order to
provide suitable driving voltage to the Pockels cell 68 to
drive it from lts zero rotational state to its ninety
degree rotational state. In discussing the voltage levels
Vl and V2 and the lines 288 and 2B7, respectively, repre-
senting such vcltage points, reference is made to line C
of Flgure 145 because the output from the Pockels cell
driver 68 has the identical shape while dlffering in the
amplitude of the waveform~ This was done for convenience
and the elim~nation of a substantially identical waveform
dif~erent only in amplitude.
Referring to Flgure 153 there ls shown a cross
sectional, schematic view of a video disc formed according
to the mastering process of the invention described herein.
A substrate member is shown at 300 having a planar shaped
upper surface indicated at 302. An information bearing
layer 304 is formed to top the upper surface 302 of the
substrate 300. The in~ormation bearing layer 304 is of
uniform thickness over the entire surface 302 of the sub-
strate 300. The in~ormation layer 304 itself has a planar
shaped upper surface 306.
Figure 5 is shown positioned beneath line C of
Figure 14 showing the intensity of the llght beam passing
from the Pockels cell - Glan prism combination in the
improved embodiment which utilizes a voltage control
oscillator in the FM modulator 20 generating a triangular
30 shaped output waveform as the driving waveform shape to
the Pockels cell 68. As previously described, the thres-
hold power level of the in~ormatlon bearlng layer is
defined as that power requlred to form indicia in the
information bearing layer in response to the lmplnging
light beam. For a metal surface, the t~mal threshold
point is that power required to melt the metal layer and
have the 0etal layer retract from the heated region of
impingement. For a photoresist layer, the threshold power
is that power level required to supply sufficient photons
,
1125A34
-~6-
to completely expose the photoreslst information bearing
layer ln the case o~ the metal layer, the heated metal
retracts from the impinging area to exp~se the substrate
300 position thereunder. In the case of the photoresis~
material, the photon power is sufficient to fully expose
the total thickness of the photoresist layer 324 completely
down to the upper surface 322 of the substrate 320 as
shewn in Figure 7.
It has been prevlously discussed how the half
power polnt of the Pockels cell - Glan prism combination
is located at a point halfway be~ween a first operating
point at which maximum transmission from a fixed intensity
beam passes through the ~lan prism and a second operatlng
point at which minimum transmission from a fixed intensity
beam passes through the Glan prlsm 70. The half power
point is the point at which the light passlng through the
Pockels cell has been rotated forty-five degrees from the
p~int of zero power transmission.
In operation, the output power from the laser is
adjusted such that the half power point of the Pockels
cell-Glan prism combination provides sufficient energy to
equal the threshold power level o~ the information bearing
member employed~ such as the member 304. The matching of
the half power point of the Pockels cell-Glan prism com-
bination ensures highest recording fidelity of the videofrequency signal to be recorded and ensures minimum inter-
modulation distortion of the signal played back ~rom the
video disc recording member.
This matching o~ the power levels i8 illustrated
3 with re~erence to line D of Figure 14 and Figure 15 and
by the oonstructlon lines drawn vertically between the
half power point represented by the line 290 shown on llne
: D of Figure 14 and the apertures shown generally at 310 in
Flgure 15. The length o~ an aperture 310 i-9 coextensive
wlth the time that the transmitted intensity of the modu-
lated light beam exceeds the half power point line 290
- shown with reference to line D of Figure 14.
In this embodiment the half power point line 290
also represents the zero crossing of the tr:angular wave
.,
~,
.
434
-47-
shape shown on line C of Figure 14. The zero crossing
points are represented b~r lines 291 and 292 shown in
Figures 14~ and 155~ and the importance of regulating the
half power point is explained in greater detail with refer-
ence to Figures 20 and 21.
Figure 16 shows an informat~on storage memberincludlng a substrate 320 having a planar upper surface
322. A thin layer of photoresist 324 of uniform thickness
is formed over the planar upper surface 322 of the sub-
strate 320. The thin photoresist layer 324 is also formedwith a planar upper surface 326 The photoresist layer
324 is a l~ght responsive layer just as the metal bismuth
layer 304 is a light responsive layer. Both the thin
opaque metallized coating 304 and the photoresist layer
324 function to retain indicia representative of the video
input signal. In the case of the metal layer 304, aper-
tures 310 are formed in the metallized layer to form
successive light reflective and light non-reflective
regions in the information storage member.
Referring to Flgure 17 showing the photoresist
coated information storage member, regions 330 are formed
in substantially the same manner as regions 310 were formed
with reference to the structure shown in Figure 15.
Rather than apertures 310 be~ formed as shown with refe~
ence to Figure 6, exposed regions 330 are formed corres-
ponding to the apertures 310. The exposed photoreslst
material is represented in Figure 16 by cross hatching of
the regions in the photoresist information storage layer
324. Subsequent development of the exposed photoresist
material removes SUCIl exposed photoresist material leaving
apertures comparable to the apertures 310 shown with refer-
ence to Figure 15.
In operation, when using the photoresist coated
substrate video disc member, the output power of the writ-
ing laser is adJusted such that the power of the modulatedlaser beam passing through the Pockels cell-Glan prlsm
combination at the half power point of the Glan prism
equals the photon threshold power required to completely
expose the photoresist illuminated by the impinging light
.
34
-48-
beam. Just as with the bismuth coated master video disc
system, this ensures highes~ f~delity recording and minimum
intermodulation distortion during tlle playback of the re-
corded video signal.
In re~erring to both Fi~ures 15 and 16, that por-
tion of the light beam passing through the Glan prism above
the half power point as represented by that portion o~ the
wave~orm shown on line D of Figure 14 which is above the
llne 2905 causes an irreversible change in the character-
istics of the light sensltive surface 304 ln the case of
the bismuth coated vldeo disc shown in Figure 15 and the
photores~st coating 324 shown with reference to the photo-
resist coated video disc shown ln Figure 16. In the case
of the bismuth coated video disc member 300, the irrever-
slble changes take the form o~ successively formed aper-
tures 310 in the opaque metallized coating 304. In the
case of the p~otoresist coated substrate 320, The irrever-
sible alteration o~ the characteristic o~ the photoresist
layer 324 occurs in the form of successive fully exposed
regions 332.
While bismuth is listed as the preferred metal
layer$ other metals can be used such as tellurium, inconel
and nickel.
Referrlng to Figure 18, ~here is shown the trans-
fer characteristic of the Pockels-Glan prism combination
as a result of the sinusoidal rotatinn in degrees of the
light passing through the Pockels cell 68 versus linear
voltage change of input drive to the Pockels cell 68.
The ninety degree rotation point ls shown at point 340 and
equals the maxlmum l~ght transmission through the Glan
prism 70. The zero degree rotation point i9 shown at
points 342 and equals the zero or minimum light trans-
misslon through the Glan prism 70. The zero light t~ans-
mission point 342 corresponds to the voltage level Vl
represented by the line 288 in line C of Flgure 14. The
ninety degree rotation point corresponds with the voltage
level V2 represented by the line 287 on line C of Figure
14. The point halr way between these two voltages repre-
sented by the line 292 is equal to V2 minus Vl over Z
~1~2543g~
-4~-
and corresponds to a forty-five degree rotation of the
light beam passing through the Pockels cell.
As is well knowng the power through the Pockels
cell is substantially unchanged. The only characteristics
being changed in the Pockels cell is the degree of rota-
tion of the light passing therethrough. In normal practice,
a Pockels cell 58 and Glan prism 70 are used together to
achieve light modulation. In order to do this, the prin~i-
pal axes of the Pockels cell 68 and the Glan prism 70 are
put into alignment such that a light beam polarized at
ninety degrees rotatlon passes substantially undiminished
through the Glan prism. When the same highly p~larized
light is rotated by the Pockels cell 68 for ninety degrees
rotation back to the zero degree rotation, the light beam
does nDt pass through the Glan prism.70. In actual prac-
tice, the full transmission state and zero transmission
state is not reached at high frequencles o~ operations.
The waveform shown in Figure 18 shows the transfer charac-
teristics of the Pockels cell 68 rotated to correspond
witn two cycles of frequency modulated video information.
This demonstrates that the transfer function continuously
operates over the zero to ninety degree portion of the
transfer function curve.
Referring to Figure 199 there is shown the trans-
fer characteristic of a Glan prism 70. At point 350,maximum transmisslon through the Glan prism 70 is achieved
with a ninety degree rotation of the incoming light beam.
At point 352, minlmum or zero light transmission through
the Glan prism 70 ls achieved at zero rotation of the
3 incoming light bea~. Hal~ o~ the intensity o~ the imping-
ing light beam is passed through the Glan prism 70 as
indicated at points 354 which corresponds to forty-five
degrees rotation of the light entering the Glan prism 70.
Obviously, the absolute power o~ the light passing
through the Glan prism 70 at the forty-flve degree rota-
tion can be adjusted by adjusting the light output inten-
sity of the light source. In this embodiment, the light
source is the writing laser 30.
In the preferred embodiment~ the power output
~ ~`S~34
-50-
~rom the writing laser 30 is adjusted such that the inten-
sity of the light passing tnrough tlle Glan prism at the
half power point coinc~des wit'l the threshold power level
of the recording medium. Since more power is required to
melt a bismuth la-~er than is required to fully expose a
photoresist layerg the absolute intensity of a writing
beam used in writing on a bismuth master disc is greater
than the lntensity of a writing laser u~ed to lnteract
with a photoresist covered master video disc.
Referring collectively to Figures 20 and 21,
there is shown a series of waveforms useful in explaining
the relationship between length o~ a hole cut in a master
vldeo disc by the writlng laser 30 and the length of uncut
land area between successively ~ormed holes. This rela-
tionship can be referred to as a relationship formed by
the value of the pea~ cutting power, ~he average cutting
power and focus of the spot on the metal layer. Collec-
tively, these terms have evolved into a single term known
as duty cycle which term represents all three such charac-
20 teristics.
As previously described, the energy required tointeract with the information bearing layer on video disc
substrate is that energy necessary to cause irreversible
changes in the material selected for placement on the
master video disc memberO In the case of a bismuth coated
masterS the energy required is that needed to selectively
remove the portion Or the bismuth coated layer in those
locatlons when the energy is above the threshold energy
level of the bismuth layer. If this energy contained ln
the spot of light is not focused properly upon the bismuth
layer, then the energy cannot be used for lts lntended
function and it will be dissipated without effecting its
intended function. If some cutting occurs due solely to
an out of focus spot, distortions are introduced into the
35 mastering process.
If the peak cutting power greatly exceeds the
threshold power level of the recording medium, destructive
removal o~ material occurs and provides a surface con-
taining distortlon products caused by this destructive
.
112SA34
-51-
removal. ~he average cutting power is that power at a
polnt midway between a ~irs~ higher cutting power and a
second lower cutting power. As ~ust described, the average
cutting power is preferably fixed to equal the threshold
power level of the recording medium. In this sense, the
intensity of the light beam above the average cutting
power lnteracts with the informatlon bearing layer to form
indicia of the signal to be recorded. The intensity o~
the light beam below the average cutting power fails to
heat a bismuth coated master to a polnt needed in the hold
forming process or fails to ~ully expose a portion of a
photoresist coated master.
Referring brlefly to lines B and C of Figure 14,
the ad~ustment o~ the average cutting power to coincide
with the line 291 shown in line B and with the line 292
shown with reference to the line C of Figure 14, results
in a duty cycle where the length of a hole equals the length
of the "land" area position and successively thereafter.
Thls ls known as a 50~ or fifty-fifty duty cycle. A
fifty-fifty duty cycle is the preferable duty cycle in a
recordlng procedure but commercially acceptable playback
slgnals can be achieved in the range from sixty-forty to
forty-sixty. This means that either the hole or the inte~
;~ venlng land member becomes larger while the other member
2~ becomes smaller.
Referring to Figure 20~ there is shown a waveform
~` represented by a line 350 corresponding to two cycles of
the light intenslty transmitted through the Pockels cell-
Glan prlsm comblnatlon and represented more specifically
on llne D of Flgure 14. The threshold power level of the
recordlng medlum is represented by a line 362. The
~hreshold power level of the reading medium ls caused to
be equal to the half power point of the light intensity
transmitted by the Pockels cell-Glan prism combination by
ad~usting the absolute intensity of the writlng laser 30.
l~en the threshold level is properly adjusted
at the half power point, an indicia is ~ormed on the
information surface layer of the master vldeo disc begin-
nlng at point 364 and continuing for the time until the
,
~Z5~34
-5~-
intensity ~alls to a point 366. Dash lines shown at 364 '
and 356 ~ are drawn to line A of Figure 12 showing an
indicia represented by the ecllpse 36& which has been
formed for the period of time when the light intensity
continues to rise past the point 364 to a maximum at 370
and then falls to a point 366. The light intensity below
point 366 falls to a minimum at 372 and continues to rise
towards a new maximum at 374. At a certain point between
the lower intensity level 372 and the upper intensity
level 374, the llght intensity equals the threshold power
level of the recording medium at 376. ~eginnlng at point
376, the energy in the light beam begins to form an
indicla represented by the eclipse 378 shown on line A of
Figure 20. A dotted line 376 l shows the start of forma-
tion of indicia 378 at the point when the llght intensityexceeds the threshold level 362. The indicia 378 con-
tinues to be formed while the light intensity reaches a
maximum at 374 and begins to fall to a new minimum at 375.
However, at the intersection of line 360 with the thres-
hold power level at 362 the light intensity falls belowthe threshold power level and the indicia is no longer
formed. In the preferred embodiment, the length of the
indicla represented by a line 384 ~uals the length of the
land region shown generally at 386 as represented by the
2~ length of the line 388. Accordingly, the matching o~ the
half power point light intensity output from the Pockels
cell-Glan prism combination wlth the threshold power level
of the recording sur~ace results in a fifty-flfty duty
cycle wherein the length of the lndicia 368 equals the
length of the next s~cceeding land region 386. Points
364, 366, 376 and 382 shown on the llne 360 represent the
zero crosslng of the original frequency modulated video
signal. Hence, it can be seen how the indicia 368 and
386 represent the frequency modulated video signal. This
representation in the preferred embodiment represents a
fifty-fifty duty cycle and is achieved by adJusting the
half power level of the beam exiting from the Pockels
cell-Glan prism combination to equal the threshold power
level o~ the recording medium.
~Z~3~
-5~
The wave~orm shown with reference to Figure 20,
including the variable light intensity represented by the
line 350, represents a preferred mode of operation to
achieve 50/50 duty cycle independent of the recording
medium empl~yed on the master vldeo disc member. The
absolute intensities at the various points change accord-
ing to the absolute intensities requlred for the modulated
llght beam to lnteract with the recording sur~e, but the
relative wave shapes and ~helr relative locations remain
the same. More speci~ically, the absolute intensity of
the threshold power level for bismuth ls dlfferent than
the absolute intensity of the threshold power level for
photoresist, but the relatlonshlp with the intensity line
360 is the same.
Referring to the combination of Figure 20 and
line B of Figure 21, there will be described the results
of failing to match the hal~ power point output of the
Pockels cell-Glan Prism combination with the threshold
power level of the recording medium. Referring to Figure
20, a second dash llne 380 represents the relationship
between the actual threshold power level of the recordlng
medlum belng used with the light intensity output from
the Pockels cell 68-Glan Prism 70 combinatlon. The thres-
hold power level line 380 ~ntersects the intensity llne
360 at a plurality of locations 390, 392, 394 and 396. A
line 390' Yepresents the lntersection o~ the llght inten-
slty line 360 with the threshold power level 380 and
slgnals the start of the formatlon of an lndicla 398 shown
on llne B of Figure 21. The indicla 398 is ~ormed during
the time that the light lntensity ls above the threshold
power level. The length of the indlcia 398 i9 rspresented
by the tlme rçqulred for the light intenslty to move to
its maximum at 370 and fall to the threshold polnt 392 as
ls shown by a line 399. The length of a land area lndi-
cated generally at 400 has a length represented by a line402. The length of a line 402 is determined by the time
required for the light intensity to move ~rom threshold
point 392 to the next threshold point 394. During thls
time, the intensity of the llght beam is sufficiently low
. ~
i~25434
-54-
as to cause no interaction wlth the recording medium. A
second ind~cia is sho~n at 405 and ~ts le~lgth corresponds
with the point at which the intensity o~ the waveform
represented by the line 360 exceeds the threshold power
level at point 39~. ~he length of the indicia 406 is
shown b a line 408 and is determined by the time required
~or the light intensity to rise to a maximum at 374 and
fall to the threshold level at point 39~.
Various lines are shown lndicating the beginning
and ending of the indicia and intravening land ar0as by
employlng primed numbers to identify the corresponding
intersections of the light intensity line 360 with the
threshold power level llnes 362 and 380.
The successively positioned indicia 398 and land
region 400 represent a slngle cycle of the recorded fre-
quency modulated video signal. The indicia 398 repre-
sents approximately 65 percent of the sum of the length
of the line 399 and the line 402. This represents a duty
cycle of 65/35 percent. Sixty-~ive percent of the
20 available space is an indlcia whlle thirty-five percent
of the available space is land area. T~pically, the
indicia in the final format is a light scattering member
such as a bump or hole, and the land area is a planar
sur~ace covered with a highly reflective material.
The frequency modulated video information repre-
sented by the sequentially positioned light non-reflective
member 368 and llght reflective member 386 shown in line A
of Figure 21 represents the preferred duty cycle of 50/50.
When the photoresist mastering procedure ls employed, the
30 re~leotivity of the upper sur~ace o~ the photoresist layer
i8 not ~ignificantly altered by the implngement of the
writing beam such as to be able to detect a difference
between reflected light beams from the developed and not
developed portions of the photoresist member. It ls
35 because of this ef~ect that a read-after-write procedure,
uslng a photoresist coated master video disc is not
possible.
Referrlng to line C of Figure 21~ there ls shown
a representatlon o~ the recovered vldeo signal represented
4~4
-55-
by the sequence o~indlcia 368 and land area 386 shown on
line A. The waveform shown in l~ne C is an undistorted
sine wave 410 and contains the same undistorted frequency
modulated information as represented by the light intensity
waveform represented b~J the line 350 shown in Figure 11.
The sine wave shown in line C of Figure 21 has a center
llne represented by a llne 412 which intersects the sine
wave 410 in the same points of lntersectlon as the line
362 intersects the intensity line 360 shown in Figure 20.
~eferrlng to line D of Figure 21, there is shown
a recovered frequency modulated vldeo signal having bad
second harmonlc distortion. The fundamental frequency
of the waveform represented by a line 414 shown in line D
is the same as that contained in the waveform shown on
line C. However~ the information shown in Line D contains
bad second harmonic distortion. When used in a system in
which bad second harmonic distortion is not a disabling
problem, the attention to a 50/50 duty cycle situation
explained hereinabove need not be strictly followed.
However, when it is necessary to have a substantially
undistorted output signal recovered from the video disc
sur~ace, it is necesisary to follow the procedure described
hereinabove.
Referring to Figure 13, there is shown the rela-
tionship between the intensity of the reading spot in thereadlng beam as it impinges upon successively positioned
light reflective and light non-reflective regions formed
dur~ng a preferred form of the mastering processi. In a
preferred embodiment, a metal is used for thls purpose and
3 the pre~rred metal as d~sclosed ls bismuth,
Line A of Figure 13 shows a plurallty of indlcla
formed in the surface of a vldeo disc master. In the
preferred embodiment the holes ~ormed ln a bismuth layer
420 are shown at 422; 424 and 426~ The intervening por-
tlons of the la~Jer 420 which are unaffected by the forma-
tlon of the holes 422j 424 and 426 are sometimes called
"landi areas and are indicated generallJ at 428 and 430.
The land areas are highly reflective. The formation of the
holes 422, 424 and 426 expose the underlying glass substr~e
56-
which is essentlally light absorbing and hence the glass
subs~rate is a light non-reflective region. The waveform
shown at 432 represents the light intens~ty wave~orm of
the spot ln the read beam as the spot passes over a light
non-reflective region. This indicates the spacial rela-
tionship between the spot as it moves over a light non-
reflective region.
Referring to line B of Figure 13, there is shown
a waveform represented b~ a line 434 indicating the inten-
sity wave~orm of the reflected light as a spot having theintensity relationship shown in Figure A passes over a
successively positioned light reflective and light non-
reflective region. A solid line portion 43~ of the line
434 shows the intensity waveform of the reflected light as
the spot passes over the light non-reflective region 424.
The inten~it~y of the reflected light shows a minimum at
point 438 which corresponds with the center of the non-
re~lective region 424. The center of the non-re~lective
portion 424 is shown on a line 440 at a point 442. The
20 intensity waveform of the reflected light is a maximum,
as shown at 444, when corresponding to a center point 446
of the land area 428 positioned between successive non-
reflective regions 422 and 424 respectively. The center
point 446 is shown on a line 448 representing the center
25 line of the information track. The dotted portion of the
llne 434 represents the past history of the lntensity
waveform of the reflected light when the light passed
over the non-reflective region 422. A dotted portion 452
of the waveform 434 shows the expected intensity of the
re~lected light beam when the reading spot passes over
the non-reflective region 426.
Referrlng to line C of Figure 13, there is shown
the recovered electrical representation of the light
intensity signal shown on line B. The electrical repre- -
sentation is shown as a line 454 and is ~enerated in the
photodetector 7Q shown in-Figure 1.
A schematic diagram of a suitable hiæh voltage
amplifier is shown in Figure 16~
A special advantage of the read while write
5434
-57-
capability of the mastering procedure herein described
includes the use of the instantaneous monitorlng o~ the
information ~ust written as a means for controlling the
duty cycle of tne reflectlve and non-reflective regions.
By displayin~ the recovered frequency modulated video
slgnal on a television monitor during the writing proce-
dure, the duty cycle can be monitored. Any indication of
the distortion visible on the mo~or indicates that a
change in duty cycle has occurred. Means are provided
for ad~usting the duty cycle of the written information
to ellminate the distortion by ad~usting the duty cycle
to its 50/50 preferred operating point. A change in duty
cycle is typically corrected by adjusting the absolute
intensity of the light beam generated in the laser 30 in
a system having either an average intensity biasing servo
or a second harmonic biasing servo and in conjunction with
circuitry for adjusting the half power point output of
the Pockels Cell-Glan Prism combination to equal the
threshold power level of the recording medium. The term
. 20 half power point and average intensity are interchanged
in the portlons of the speciLicatlon~ and clalms which
concern the use of the triangular shaped wave form gener-
ated by the FM modulator. The modulated light beam 40
exitlng from the Glan Prism 38 is of slnusoidal'shape. In
this situation the hal~ power point equals the average
lntensity, and this would be the case for any s~Jmmetrlcal
wave form. A frequency modulated output from an FM modu-
lator has been found to act as such a symmetrical wave
form.
Whlle the lnvention has been particularly shown
and descrlbed with re~erence to a preferred embodiment and
alterations thereto, it would be understood by those
skilled ln the art that various changes ln form and detall
may be made therein without departing ~rom the spirit and
scope of the invention.
