CA1051550A - System for reproducing pulse time modulated waveforms stored along a diffractive track - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:
A record is provided for storing a pulse time modulated waveform along a recorded track having an axis. The record com-prises a substrate having at least one engraved face carrying an embossed print of the recorded track. The engraved face is a phase object constituted by a succession of diffractive elements distributed along the axis in accordance with a sequence of pul-ses of the waveform. These diffractive elements have non-uniform length and spacing and the width of the embossed print is subs-tantially constant along the axis. The non-uniform length and spacing of the diffractive elements respectively measured along said axis are at least equal to the width of the embossed print.
The diffractive elements are engraved in the form of depressions or relief depressions in the engraved face.

Description

1~5~
The present applicati~n is a divisional of parent ap-plication No. 179,5~4, filed on ~UCJUSt 24, 1973.
The present invention relates to a record or data~
carrier useful in storing pulse time modulated waveform along a track.
The invention particularly relates to a record or data-carrier appropriate to methods of recording by virtue of depres-sions or projections, and in cooperation with reproducing devices to effect optical read-out of the signals stored in relief form :
at the surface of the carrier.
In known methods of an apparatus for reproducing si-gnals which are stored in the form of superficial deformations of the surface of a record, for read-out purposes, use is made of a ;~ stylus which displaces in a groove imparting to the stylus lateral or depth displacements. The movements imparted by the modulation in the groove or track, to the tip of the stylus, are communica-ted to an electromechanical transducer which supplies a voltage ~ .
proportional to the instantaneous velocity or amplitude of the ` tip of the read-out stylus. Those methods have numerous draw-backs, the major of which are the wear in the stylus, in the carrier or substrate, the play back distortion and the difficulty . .~ . .
of making the stylus assembly of the transducer follow the high fre~uency components which succeed one another at very close intervals.
Also known are methods of optically reading out a car~
rier carrying a photographically recorded track and exhibiting variations in optical density or in width, corresponding to the time variation of a signalO A difficulty with these systems, resides in the fact that the copies of the carrier are obtained by photochemical methods, more expensive than the die-stamping methods employed with ordinary sound record discs. Another difficulty of these optical methods which are based upon the , ~ .

- . :

projection of enla~ged images, resides in the small depth of field of the objective lenses used to ~orm the enlarged image of the track, and in the limitation imposed upon the resolving power, due to the phenomenon of diffraction. In order to overcome these difficulties, it has been proposed a me-thod of holographic record-ing by which the carrier may be copied by a stamping operation, but this technique involves the use of relatively powerful co-herent light sources since the diffraction efficiency of the recorded holograms is relatively poor. Hitherto, the major dif-ficulty with systems for mechanically or optically scanning the carriers on which the signal i5 recorded, has consisted in main-taining a proportional relationship between the amplitude of the ;
deformation recorded on the track, and the electrical signal fur-nished by the read-out equipment. This amplitude proportionality can be roughly satisfied if the stored signals are pulse time modulated waveforms taking the form of successive square wave envelopes containing transitions between two levels. The precise location of such transitions in time, serves to faithfully trans-late the inormation. This is particularly the case in telecom-

2 ~unications systems utilising frequency or phase modulation.
There is nothing to prevent this principle from being applied to the field of optical recordin~ with a view to simplifying the process of scanning and reading out a record wherein, the embos-sed pattern engraved corresponds to a purely time variation on ~-the part of an alternating data-carrying signal.
The above-mentioned parent application is particularly concerned with a reproducing device for the reading-out of a pulse time modulated waveform stored onto a record -~
The present invention more specifically resides in a record for storing a pulse time modulated waveform along a re-corded track having an axis, said record comprising a substrate having at least one engraved face carrying an embossed print of .

~5~
said recorde~ track; said engraved face beiny a phase object constituted by a succession o~ diffractive elements distributed along said axis in accordance with a sequence of pulses of said waveform; said diffractive elements having non-uniform length and spacing; the width of said embossed print being substantially constant along said axis, and the non-uniform length and spacing of said diffractive elements respectively measured along said axis being at least equal to the width of said embossed print.
Preferably, the present invention is directed to a material data-carrier or record designed to be read out optically by transmission or by reflection, at least one of the engraved faces of which comprises, arranged in accordance with at least one recorded track, a series of irregulari-ties in the form of depressions or projections, corresponding to the time variation .
of an alternating signal. One level of the alternating signal is translated by a portion of the track not containing an irre-gularity, another level of the alternating signal is translated ;- by a portion of the track which does con-tain an irregularity, the width of which, measured perpendicularly to the track axis, does not exceed two microns, and the length of which, along the track, is equal to or greater than said width. Microscopic examination of the surface of the record, would exhibit the pre-sence of a chain of diffractive elements of more or less elonga-ted form and it is purely by virtue of their diffractive power, that the depressions or projections at the surface of the car-rier serve to reconstitute the recorded alternating signal.
Their shape is in no way critical and they can be readily obtained for example by chemically etching a smooth plate through a suita-bly exposed photosensitive resin mask. The optical accuracy and fidelity of the transcription of the signal depend upon the loca-tion, upon the track, of the diffractive elements which constitute it, and not upon particularly careful cutting of its surface or ~ . .: - , . - . ~

~L~)5~5~
upon the change in its optical properties, in the ~ay which is the case in the known systems. The small transverse dimensions of the track, due to the systematic exploitation of the phenome-non of diffraction, mean that an extremely high data-storage capacity is achieved. If the record takes the form of a disc and if the track is of spiral shape, then the neigh~ouring turns ;~ of the spiral can be located very close together indeed. As a ~ result, read-out is performed using a concentrated light spot ;~ having a diameter comparable with the width of the trac~; the centring of this light spot to within a fraction of a micron, is achieved by positional control of the light source illuminat-ing the track, during the read-out phase.
Prefarred embodiments of the present invention will be hereinafter described with reference to the accompanying drawings, wherein:
- Fig. 1 is an isometric view illustrating a record, in accordance with the invention, and an optical read-out device;
- - Fig. 2 is a partial isometric view illustrating a fragment of the record shown in figure 1, at substantial magni-fication;
- Fig. 3 is a sectional view illustrating the elements of a read-out device centred in relation to the track being read out;
- Fig. 4 is a sectional view illustrating the device shown in Fig. 3 in an eccentric posi'cion;
- Fig. 5 is a sectional view of a first variant embodi-ment of the read-out device shown in Fig. 3; and - Fig. 6 is a sectional view of a second variant embodi-men~ of the read-out device shown in Fig. 3.
Fig. 1 illustrates a record 1 in the form of a circular disc which can rotate in its own plane, at an axis 4, thanks to the provision of a drive pin 2 mechanicallyconnected to a motor 3.
The bottom face of the disc 1, parallel to the plane x o y, is `~ assumed to be smooth, and the top face 16, parallel to the lat-_ 4 _ ~, . .
: - . . . .

~55~
ter is also smooth but contains a succession of difEractive ele-ments 14 in the form of depressions or projectior~s, arranyed in the form of the turns 15 of a spiral track. Each of the elements 14 has a contour, in the plane of the face 1~, of more or less elongated shape, the width Q of which is substantially constant an and does not exceed two microns. The element 14 can take the form of a shallow trough hollowed out of the surface of the face 16, or of a bead. These various relief irregularities, are produced for example by chemical etching circumscribing the contours of 10 these superficial irregularities; the areas subjected to this et- -ching, are delimited by a masking technique which involves selec-tive exposure of a photosensitive resin. It is also possible to in~pri~t use an ~}~t pressed by a die, in the manner employed in the manufacture of micro-circuits. The important thing to bear in mind as far as the manufacture of the disc is concerned, is that the exposure of the resin used for the masking function, is ef-; fected by a light spot the intensity of which is modulated by a pulse time modulated squarewave electrical signal. The incorpo-ration of the data into the modulating signal is carried out, . or 20 as required, by frequency 4~ phase-modulation or by any other co-ding method eapable of producing a pulse-coded message.
To eonclude the description of the make-up of the record 1, in Fig. 1, it has been assumed that the disc is trans-parent so that it can be read out by transmission. It has also been assumed that it is rigid although it is equally possible to envisage utilisation of a flexible disc which slides between sta-biliser shoes which have not been shown. Self-evidently, the shape of the record 1 is in no way limitative; a record of ta~e form, containing one or more rectilinear traeks, is eonceivable, and in the case of a cireular disc, instead of the spiral track a set of eoncentric circular tracks could be substituted, giving step-by-step access to the reeorded data.

In addition to the record 1 and its drive system, Fig. 1 also shows the optical read-out device employed in relation to the track 15. This read-out dev:ice is constituted by a light source S and an objective lens 7. The source S, parallel to the ~ axis oz, produces substantially parallel light beams 6 and the : microscope objective lens 7 causes the beam 6 to converge at the point 0 on the track lS. The light rays 9 which converge towards the point 0, intersect and diverge beyond said point; after having passed through the disc 1, a fragment of which has been removed in order to simplify the drawing, they illuminate an area 10 which ~: overlaps to a greater or lesser extent the receiving surfaces of two side-by-side photodetector elements 12 and 13. The space se-~: parating the receiving surfaces of the photodetector elements 12 : and 13, is located plumb in line with the direction oz and orien-.. tated along the axis of the track, tangentially to ox at the read-out point 0.
:-~
The photodetector elements 12 and 13 furnish electrical :`~ signals which are applied respectively to the inputs of a first -:; differential amplifier 17. The output of the amplifier 17 is ;
connected to a low-pass filter 21. This filter 21 supplies an error voltage which, through the medium of an electromechanical transducer 8, controls the radial displacement in the o y direction, .
of the objective lens 7. The electrical signals furnished by the photodetectors 12 and 13 are also applied to resistors 19, which, with the resistor 20 and-the operational amplifier 18, constitute an electrical transmission circuit furnishing a signal S(t) propor- .
tional to the sum of the signals produced by the two photodetec-tors 12 and 13.
When the point of convergence 0 of the beam 9 encoun-;~ 30 ters the surface 16 between two diffractive elements 1~ succeeding one another on the track 15, no diffraction occurs and the light energy received hy the photodetector elements 12 and 13 is confi-.

~.35~3~
ned to the interior of the area 10.
sy contrast, as soon as the point of convercJency 0 ofthe beam encounters a diffractive element 14 on the record 1, the light experiences substantial diffraction, this tending to distribute the light energy over a cross-hatched area 11 which substantially exceeds the area 10. The result is a variation in the sum S (t) of the signals furnished by the two photodetector elements 12 and 13O Rt the time of passage of the elements 1~, there is picked up at the output of the amplifier 18 a signal S(t) of squarewave form, which faithfully translates the time varia-tions in the signal engraved in the track 15. As far as the si-gnal S(t) is concerned, it is not mandatory to provide two photo-detector elements 12 and 13. The sensitive faces of 'chese two transducers can be combined into one, and an optical mask provided to cover the area 10. Under these conditions, overlapping of the area 10 which is masked, will indicate the passage of a diffractive element 14, in the form of an appreciable variation in the voltage produced by the single photodetector. Instead of providing a mask, it is simpler to separate the photodetector into two parts, as shown in Fig. 1. This doubling-up has the additional advantage o~ making it possible to detect deviations on the part of the point of convergence 0 in relation to the axis of the track 15.
Fig. 2, at a very large magnification, illustrates a -tiny fragment of the record 1, located at the position of the read-out device. To illustrate the shape of the depressions and projec~
tions which the engraved profile can exhibit, on a turn 15, surface irregularitics 14 in the form of more or less elongatod troughs have becn shown; on the neighbouring turn 150, the i~regularities 141, 142 and 143 are constituted by more or less elongaced beads.
The read out beam ~ has also been illustrated by showing its contour. The objective lens 7, because of its imperfections and because of the diffraction of the light, forms at the surface 16 35~5~
of the record 1 a spot 22 which substantially occupies the width ~ of the track 15. This spot has a minimum diameter of the same order of magnitude as the wavelength ~ separatiny two successive wave surfaces ~ of the read-out radiation 9. The envelope of the illuminating beam has its minimum section at the foc~s 0 of the objective lens 7 and it flares only gently around this position so that the record system is able to make vertical displacements of several microns without any substantial change in the conditions of illumination of the track.
To illustrate the foregoing, in Fig. 2 there has been produced in relation to the projecting elements 141, 142 and 143 of the turn 150, a diagram which represents the time variation of the signal S recorded in this section of the turn. The signal S is a pulse ti~e modulated squarewave signal, the top levels ~B, C and DE of which correspond to the scanning of the beads 141, 142 and 143 by the beam 9. During the scanning of the turn 150, the carrier 1 moves at the speed V = ~R, where ~) is the angular velo-`~ city of the disc and R the radius of the turn. The bottom levels of the signals S, correspond to the transitions away from the parts of the turn. The pitch p of the turns is chosen so that the light spot 22 can only read one turn at a time. The pitch p will be for example two to three times greater than the width of the turns.
So far as the distribution of the light energy is con-cerned, from the enlarged view of Flg. 2 it will be seen that the light rays 9 emerging from the objective lens 7 cross one another ~; in a pseudo-conical volume which has its minimum section 22 at the level of the ocus 0. In the absence of diffraction at the surace 16, the light energy is confined to this volume. By contrast, if the beam 9 is intercepted by a diffracting element `14, dispersion of the light energy within the solid angle having its apex at the point 0 takes place. A photodetector located in .

105~5~
the volume delimite~ by the dotted lines flaring beneath the light spot 22 will experience a variation in the luminous inten-sity received in the presence of diffraction. The same photode-tector could e~ually well be arranged outside this volume. In addition, if the light volume forming at the exit from the lens 7 is split into two parts and if these parts are located to the left and to the right of the plane xoz containing the axis of the track and the axis of the beam, then symmetrical distribution of ; the light energy will be obtained when the light spot 22 is cente-red on a diEfractive element 14 and the track 15.
If the light spot 22 is eccentric by the quantity ~ y in relation to the axis of the track 15, the ligh-t energy fractions will be distributed asymmetrically to either side of the plane xoz; it is thus possible to detect the offset ~ y by utilising two photodetector elements.
The sectional view o~ Fig. 3 corresponds to the plane of the section yoz of Fig. 1, and illustrates the path taken by -- the light rays when the light beam 9 concentrated by the objective ~;~
lens 7 is centred on a diffractive element 14. In the absence of any diffraction, the light energy is confine~ between the rays 26 illustrated in dotted line, and a small portion of this light ener~
;~ gy reaches the receiving surfaces of the photodetector elements 12 and 13 which are separated by the distance s. In this case, equal currents Sl and S2 are delivered by the photodetector elements 12 and 13 and if we consider the circuits of Fig. 1, it will be seen that the amplifier 17 produces a zero voltage, this being trans-lated into terms of an error signal which produces no displace men~ of the obJective lens 7 under the influence of the electro-- mechanical transducer 8.
When a diffractive element enters the path of the light the marginal rays 25 appear and define, with the rays 26, the illumination zones 27 and 28 which cause the currents Sl and .. .. ~ , .

5~$~
S2 to vary by ~ile same quantity. The sum of these variations is available at the output of the ampli~ier 18, in the ~orm of a signal S(t) which represents the scanning of the eleMent 14 by a change in level. By contrast, because of the quality of these variations, the voltage produced by the dif~erential amplifier 17 remains unmodified.
In Fig. 4, the path of the light rays has been illus-trated in the case of an offset ~ y on the part of the axis of the objective lens 7, in relation to the track element 14 being read out. It will be seen that the zone 27 contains a different energy fraction to the zone 28, because of the offset; the result is variations in the currents Sl and S2 and in the error voltage produced by the filter 21, this voltage acquiring a value which, under the action of the transducer, tends to return the objective lens 7 to the centre of the track being read. The positional con-trol or feed-back thus achieved through the agency of the trans-ducer 8, makes it possible to maintain the read-out beam in a -perfectly centred relationship in the case of radial fluctuations of the track being read, of as much as several tens of microns;
the low-pass filter serves to elimlnate the high-frequency compo-nents which could disturb the operation of the control.
The control of the objective lens 7 makes it possible to correctly follow a small-pitch spiral trace, despite eccentrici-ty errors in the disco In the event that it loses the track, the system tends to lock onto an adjacent track, but this could take place without being detected in the case of recording a television video signal, because it is possible to arrange for each revolu-tion of the track to contain the video signal of a television frame. In this case, the track jump will be unobserved becaùse the synchronism of operation is undistributed.
Fig. 5 illustrates a practical example of -the system in accordance with the invention. This example is in no way limi-., .

.5~i~
tative of tile scope of the inven~lon and relates to the case of atransparent disc engraved on one face. The beam 6 is produced by a helium-neon laser operatiny at a power of 1 milliwatt. The wavelength of emission is equal to 0.6328 microns makiny it pos-sible to choose a diffractive track having a width Q equal to 1 micron. The diffractive elements 1~ take the form of more or ` less elongated troughs with a depth of 0.5 microns. The two pho-todetector elements 12 and 13 are silicon cells whose sensitive surfaces have areas of 2 mm and are separated from one another`by a distance s equal to 0.8 mm. The sensitive surfaces are located at around 6 mm from the surface 16 of the disc 1 and are fully il-luminated by the light beam coming from a microscope objective ~ns 7 having a ma~nification of 80. The electromechanical trans-ducer ~ which is used to radially displace the objective lens 7, ~: ``"
`~ is constituted by a ceramic electrostrictive bimorph device; it is excited by a low-pass filter 21 whose cut-off frequency is some few hundredsof cycles per secondO
; In the variant embodiment described hereinbefore, the read-out of the record is effected by transmission so that only one face of the disc can be engraved. In order to double the sto- -`
rage capacity of a disc, it can be engraved on both faces provided that the track is read out by reflection.
- In Fig. 6, a record 1 has been illustrated the faces 16 and 160 of which can be engraved in a similar fashion to that ` illustrated in Figs. 1 and 2. The record 1 can be made or a non-transparent material and its engraved faces can, if required, be given a thin reflective coating. The read-out head comprises the same elements as the one hereinbefore described, but the beam 6 is hereby transmitted to the objective lens 7 through a semi-reflective plate 100; this serves to reElect the light in a re-verse direction which it takes through the objective lens 7, to-wards the photodetector elements 12 and 13 arranged laterally and : . . . . :.~ .

~(3~ O
above the surface 16. In the absence of any diffractive element 14 in the illumlnated zone of the surface 16, a re~lected be~m si~lilar to that 6 is received by the pho~odetector elements 12 and 13. By contrast, in the presence of a dif~ractive element, the reflected light is diffracted in the directions 25 outside the pupil of the objective lens 7; the result is a reduction in the luminous intensity incident upon the photodetector elements 12 and ; 13. Apart from these modifications, the mode of operation is es-sentially the same as that of the read-out devices operating by transmission.
Utilisation of the system described above makes i-t pos-sible to record a band of frequencies reaching the order of video-frequency signals, upon a disc-type record to which the technique of reproduction by hot-pressing a thermoplastic material, can be applied. A disc oE this kind can be enclosed in a dustproof cas-sette and equipped with a radial window to enable it to be optical-ly read out on a record player of light-scanner type, without it being necessary to utilise a coherent light source. Self-evidently, read-out is not limited to the visible spectrum and it is merely necessary to use a read-out radiation which corresponds to the sources and photodetectors employed.

- .

Claims (9)

The embodiments of the invention for which an exclusive property or privilege is claimed, are defined as follows:

1. A record for storing a pulse time modulated wave form along a recorded track having an axis, said record compri-sing a substrate having at least one engraved face carrying an embossed print of said recorded track; said engraved face being a phase object constituted by a succession of diffractive ele-ments distributed along said axis in accordance with a sequence of pulses of said waveform; said diffractive elements having non-uniform length and spacing; the width of said embossed print being substantially constant along said axis, and the non-uniform length and spacing of said diffractive elements respec-tively measured along said axis being at least equal to the width of said embossed print.

2. A record as claimed in claim 1, wherein said dif-fractive elements are engraved in the form of depressions in said engraved face.

3. A record as claimed in claim 1, wherein said dif-fractive elements are engraved in the form of relief projections on said engraved face.

4. A record as claimed in claim 1, wherein said substrate is made of a transparent material.

5. A record as claimed in claim 1, wherein said engraved face carries a reflective coating.

6. A record as claimed in claim 1, wherein said substrate has the form of a disc.

7. A record as claimed in claim 1, wherein said axis has the form of a spiral; the pitch of said spiral being higher than twice said width.

8. A record as claimed in claim 1, wherein said engraved face comprises a plurality of evenly spaced recorded tracks; the pitch of said recorded tracks being higher than twice said width.

9. A record as claimed in claim 1, wherein said substrate is made of a thermoplastic material; said thermo-plastic material being hot-pressed.