US3463572A - Optical phase modulation apparatus - Google Patents

Description

oni-mun NUUIW K. PRESTON, JR

OPTICAL PHASE MODULATION APPARATUS Aug. 26, 1969 Filed Oct. 2l. 1966 66 INVENTOR. Kerala/i Pres'02z,fJ/ BY 615 63 ,W D, Zh-61 m United States Patent() 3,463,572 OPTICAL PHASE MODULATION APPARATUS Kendall Preston. Jr.. New Haven, Conn.. assignor to The Perkin-Elmer Corporation, Norwalk, Conn., a corporation of New York Continuation-impart of application Ser. No. 418,719, Dec. 16, 1964. This application Oct. 21,1966, Ser. No. 588,384

f Int. Cl. G02f I/34 U.S. Cl. 350-161 1l Claims ABSTRACT OF THE DISCLOSURE Apparatus for use in phase modulating a beam of light. A reflective and conductive film is stretched across a constant impedance layer of material having downwardly extending cavities. The layer is formed on nonconductive support. A variable impedance layer of material is located below the constant impedance layer and is in communication with the cavities. Potential differences are established'between the film and the top of the constant impedance layer and between the top of the constant impedance layer and the variable impedance layer. Exciting the variable impedance layer causes changes in the potential difference between the two layers which in turn causes the portions of the film extending over the cavities to deflect downward. In use, a beam of light striking the film is reflected and at the same time phase modulated.

This application is a continuation-impart of my copending application Ser. No. 418,719, filed Dec. 16, 1964.

The present invention relates to an optical phase modulating apparatus of the typev wherein a reflecting surface is selectively altered so as to change the phase of a beam of light incident upon the surface.

Optical phase modulators find particular application in display systems. In one well known display system. the modulator used comprises an electron beam gun and an oil surface or film. A modulating signal is supplied to the electron beam gun which scans the oil surface. As `the electron beam passes over the oil film the thickness of the film is altered by an amount dependent upon the magnitude of the electron beam which, in turn. is controlled by the modulating signal. As the film thickness varies the optical path length through the film varies. thus altering the phase of a light beam incident upon the film and imparting the modulation information to the reflected light beam. This system, known as the Eidophor system is more fully described in the book Televisiom the Electronics of Image Transmission in Color and Monochrome? second edition, V. K. Zworkin and G. A. Morton, copyright 1940 and 1954, Library of Congress Catalog Card Number 54-11162, John Wiley and Sons, Inc., New York, pp. 284-287.

Because thisl system uses an oil film, electron beam gun, oil film type modulator it has certain inherent disadvantages and undesirable characteristics. The oil is apt to become contaminated after a period of time and when this happens the oil obviously cannot perform its intended function. This contamination also affects other portions of the system such as the electron beam gun to which the oil particles eventually migrate.

A number of techniques have been suggested to overcome the problems associated with electron beam gun,

voil film type modulator. One involves the use of a modulator comprising a reflective and conductive membrane of parabolic shape supported at its periphery by a ring. A plurality of conductor elements either of pie shape or concentric rings configuration is located below the membrane. Each conductor element is intended to be associl 3,463,572 Patented Aug. 26, 1969 ICC ated with a particular sector of the membrane. As energizing potentials are applied between the membrane and selected conductor elements` the sectors of the membrane associated with the selected elements are deflectedA lt has been found, however, that other sectors of the membrane are also deflected to some extent because the various sectors of the membrane are not isolated from each other. This cross-talk effect is obviously undesirable since it results in improper deflection which, in turn. causes improper transfer of the modulation information to a light beam incident upon the membrane.

Another technique which overcomes the problems associated with both the elect-ron beam gun oil film type modulator and the parabolic membrane reflector type modulator uses a plurality of individual reflecting elements. Each reflecting element has associated with it a tranducer to which the modulation information is supplied. The transducer movement controls the position of the reflecting element. It is apparent that this arrangement provides localized operation in that the various transducers and reflecting elements are isolated. The resolution of such a system is limited only by the size of the individual reflecting elements and transducers. While the limitations and disadvantages of the oil film and parabolic membrane reflector modulators are overcome by using a plurality of reflecting elements, it has been found that a complete and operative system based upon this technique is extremely expensive to fabricate since the individual reflector elements and transducers are very costly.

It is therefore, an object of the present invention to provide a new and improved optical phase modulation apparatus.

It is another object of the present invention to provide an optical phase modulation apparatus which is simple in construction and inexpensive to fabricate.

It is a further object of this invention to provide an optical phase modulation apparatus wherein the surface contour ofcertainl portions of a unitary reflective member are deformed while other portions remain unchanged.

It is another object of the present invention to provide an optical phase modulation apparatus which is rcsponsive to modulation information in the form of a light pattern.

It is still another object of this invention to provide an optical phase modulation apparatus which is responsive to modulation information in the form of a thermal image. l

It is yet still another object of this invention to provide an optical phase modulation apparatus in which the modulation information is in the form of an acoustical wave pattern.

It is a further object of this invention to provide an optical phase modulation apparatus that is reliable and not subject to an unusually short lifetime.

An optical phase modulation apparatus constructed in accordance with this invention includes a layer of constant impedance material having apertured or cavity portions and a coating of conductivematerial deposited on the top surface of said layer. The apparatus further includes a reflective and conductive member stretched across and carried by the layer of constant impedance material. The apparatus further includes a layer of variable impedance material having conductive elements in communication with the bottoms of the apertures of the constant impedance material. The apparatus further includes a nonconductive substrate for supporting the abovementioned layers and a voltage source connected to the reflective and conductive member, the conductive coating on the top surface of the constant impedance layer, and the variable impedance layer.

In one version the variable impedance layer is a material adapted to change in response to a light pattern input; in another version the variable impedance layer is a material adapted to change in response to an acoustical input and in still another version the variable impedance layer is a material adapted to change in response to a thermal image. These changes result in deflections of those portions of the membrane located above the apertures.

For a better understanding of the present invention, together with other and further objects thereof, reference 'is had to the following detailed description, taken in connection with the accompanying drawings, and its scope will be pointed out by the appended claims.

Referring to the drawings;

'FIGURE l shows one embodiment of an apparatus for use in an optical phase modulator constructed in accordance with the present invention;

FIGURE 2 is a section view of the apparatus shown in FIGURE 1 taken along lines 2-2 in FIGURE 1;

FIGURE 3 is an equivalent electrical diagram of the apparatus in FIGURE 1; and

FIGURE 4 is a section view of another of the invention.

Referring now to FIGURES 1 and 2, wherein similar elements have been given the same reference numerals, there is shown an apparatus for use in an optical phase modulator constructed according to this invention and adapted to respond to modulation information in the form of a light pattern.

The apparatus includes a unitary reective and conductive member 11. Member 11 is comprised of a thin membrane 12 of optically at collodian. One surface of the membrane 12 is covered with a reliective and conductive coating 13. The coating 13 may be, for example, aluminum.

The apparatus further includes a layer 14 of constant impedance material having apertured or cavity portions. Layer 14 may be a material of constant high resistivity, such as for example, titanium rich titanium oxide or silicon rich silicon oxide and may be shaped in the form of a plurality of interconnected ribs or ridges 15, 16, 17 with the spaces between the ribs defining apertures or cavities 19, 21, 22. In the embodiment shown, the ribs are running in two directions as opposed to one direction so that the device will respond to modulation information in two directions as opposed to one direction. The top surface of the ribs is provided with a coating of electrically conductive material 23, such as for example aluminum, so that the entire top surface will be at an equipotential. A plurality of electrodes 24, 25, 26 which may also be fabricated from aluminum or other suitable conductive material are located at the bottom of the respective cavity portions 19, 21, 22

The reflective and conductive member 11 is stretched across and carried by the tops of the ribs of the layer 14 with its uncoated lower surface in contact with the conductive coating 23.

Layer 14 is supported on the top surface of a substrate 27v of non conductive material, such as for example, glass.

A layer 28 of material of variable impedance, such as for example photoconductive material, is deposited on the bottom surface of the substrate 27. The photoconductive layer 28, may be for example, cadmium sulphide or zinc oxide. A layer 29 of transparent conductive material is deposited on the layer of photoconductive material 27 so that the entire bottom surface of the photoconductive material will be at an equipotential. The transparent conductive material, may be for example, stannous oxide or a thin layer of gold or similar metal. A plurality of electrodes 31, 32, 33 which are in contact with the photoconductive layer 28 are connected to electrodes 24, 25, 26 respectively by means of wires 34, 35, 36. Wires 34, 35, 36 are imbedded in the body of insulating material 27.

A D.C. voltage source 37, such as for example a battery, is connected at one end to the transparent conductor 29, and at the other end the coated surface 23 of the embodiment ribs. The coated surface 23 of the ribs is in turn electrically connected to the reective and conductive member 11. Thus, a potential difference is established between the transparent conductor 29 and the combination of the coated surface 23 and the refiective and conductive member 11. While the potential difference between the coated surface 23 and the reflective and conductive member 11 remains constant, the potential difference between the electrodes in the cavities and the reective and conductive member 11 will change in proportion to the change in impedance of the variable impedance layer 27.

The apparatus operates in the following manner. When modulation information which in this instance is in the form of a light pattern is imaged in the plane of the photoconductive element 27, the current flow through the wires 34, 35, 36 is increased in proportion to the intensity of light mpinging on the respective electrodes 31, 32, 33. This in turn causes a proportional voltage drop at corresponding electrodes 24, 25, 26. Thus, the 4potential difference between each electrode 24, 25, 26 and the portion of the reflective and conductive member 11 located above the respective electrode is increased in proportion to the intensity of the signal at the individual electrodes 31, 32, 33. This electrostratic potential difference causes a deflection of those portions of the reective and conductive member 11 located above the electrodes 24, 25, 26. Accordingly, a beam of light incident on the reflective and conductor member 11 will on reflection be phase modulated wherever it strikes deflected areas. The distance between successive electrodes should be large in relation to the thickness of the current carrying layers, or, in other words, the distance A from electrode 24 to electrode 25 should be much greater than thickness B of the support member 14 and the thickness C of the photoconductive layer 28.

FIGURE 3 shows the equivalent electrical diagram for the apparatus shown in FlGURES l and 2. In this diagram V corresponds to the voltage source 37; wire 41 corresponds to the transparent conductor; wire 42 corresponds to the conductive coating 23 on the tops of the ribs; variable resistors 43, 44 and 45 correspond to electrodes 31, 32, 33; points 46, 47, 48 correspond to electrodes 24, 25, 26; and, resistors 49, 51, 52 correspond to the resistance of the side walls of the ribs 15, 16, 17.

In another version of the invention, a layer of piezo.

resistive material, such as for example silicon or germanium, is substituted in place of the photoconductive layer 28. In this version, the apparatus is adapted to respond to an acoustical image input pattern.

Similarly, in still another version, a layer of pyroresistive material such as for example silicon or germanium may be substituted in place of the photoconductive layer 28 in which case the apparatus is adapted to respond to a thermal image input signal.

In FIGURE 4 is shown still another modification of the invention in which the wires connecting the companion pairs of electrodes are eliminated. In this arrangement, the reflective member 61, constant impedance layer 62 having a conductive coating 63, substrate 64, the variable impedance layer 65' and transparent conductor layer 66 correspond to elements 11, 14, 27, 28 and 29 respectively. However, as can be seen, the variable impedance layer 65 and conductive layer 66 are now deposited on the top surface of the substrate 64 in between the constant impedance layer 62 and reflective member 61. Electrodes 67, 68, 69 correspond to electrodes 31, 32 and 33 respectively. Accordingly, electrodes 24, 25, 26 and their associated wires 34, 35 and 36 are eliminated.

Other arrangements are also considered within the scope of this invention. For example, the voltage source 37 may be an oscillator or other A.C. equivalent rather than a D.C. type as described above. In these arrangements the constant impedance layer l4 is in the form of a material having a constant dielectric constant; that is, a I

material having a constant impedance to A.C. current at a given frequency. Silicon dioxide is one example of such a material. The variable impedance layer 28 is accordingly a layer of material whose dielectric constant changes in response to the modulating input signal. When the input signal is in the form of a light pattern or acoustical pattern, a material such as barium titanate may be used. When the input signal is a thermal image tryglycine sulphate may be used. It should be noted that although the A.C. current is cyclical, the refiective element will always be deflected in one direction, that is, toward the electrodes in the cavities. Thus, very high frequency A.C. currents may be utilized.

While there have been described what are at present considered to be preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is therefore aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. Apparatus for use in modulating the phase of a beam of light comprising:

(a) a support member having a top surface;

(b) a layer of constant impedance material formed on the top surface of said support member, said layer of constant impedance material having a plurality of cavities extending downward from the top;

(c) a reflective and conductive member stretched across the top of the layer of constant impedance material and extending over the cavities;

(d) individual electrodes located in the cavities spaced downward from the top surface of the support member;

(e) means for establishing a potential difference between the reflective and conductive member and the electrodes in the cavities;

(f) a layer of variable impedance material located below the layer of constant impedance material and connected on one side at a different location to a diierent one of said electrodes for receiving a pattern of radiant energy and producing changes in the potential difference between the reflective and conductive member and selected electrodes so as to cause the portions of the reflective and conductive member extending over said selected electrodes to deflect downward into the cavities; and

(g) a conductive coating on the other side of said layer of variable impedance material for maintaining said other side of said layer of variable impedance material at equipotential.

2. Apparatus for use in modulating the phase of a beam of light comprising:

(a) a layer of constant impedance material having a plurality of cavities extending therethrough;

(b)a first layer of conductive material formed on top of said layer of constant impedance material;

(c) a reflective and conductive membrane member stretched across the top of said first layer of conductive material and extending over the cavities;

(d) a layer of variable impedance material for converting a pattern of radiant energy into a pattern of electrical currents located below said layer of constant impedance material;

(e) a separate electrode disposed in each cavity spaced downward from the top of the layer ofA constant impedance material and in electrical contact with a different portion of one side of said layer of variable impedance material;

(f) a second layer of conductive material formed on the other side of said layer of variable impedance material;

(g) a voltage source connected at one end to said second layer of conductive material and at the other end to both the retiective and conductive membrane member and the first layer of conductive material; and

(h) a substrate for supporting said layers; whereby, a pattern of radiant energy striking the layer of variable impedance material will produce changes in the potential difference between the reective and conductive membrane member and the electrodes in the cavities, resulting in a downward deection of the portions of the reflective and conductive member extending over the cavities so as to cause a beam of light striking the reflective and conductive member to be phase modulated on reection.

3. The invention according to claim 2 and wherein the layer of constant impedance material and the layer of variable impedance material are formed on the top side of the substrate.

4. The invention according to claim 2 and wherein the layer of constant impedance material is formed on the topV side of the substrate and the layer of variable impedance material is formed on the bottom side of the substrate.

5. The invention according to claim 2 and wherein the layer of variable impedance material is photoconductive, whereby the apparatus is responsive to a pattern of light.

6. The invention according to claim 2 and wherein the layer of variable impedance material is pyroresistive, whereby the apparatus is responsive to a thermal image.

7. The invention according to claim 2 and wherein the laver of variable impedance material is piezoresistive, whereby the apparatus is responsive to an acoustical image.

8. The invention according -to claim 2 and wherein the voltage source is D.C.

9. The invention according to claim 2 and wherein the voltage source is A.C.

10. The invention according to claim 2 and wherein the cavities are arranged in a series of rows, each row containing at least one cavity.

11. The invention according to claim 2 and wherein said reective and conductive membrane comprises a thin sheet of collodian having retiective and conductive coatings on its top side.

References Citedv UNITED STATES PATENTS 2,281,637 5 1942 Sukumlyn. 2,289,205 7/1942 Nagy et al 350-161 X 3,001,447 9/1961 Ploke 350-161 3,137,762 6/ 1964 Baumgartner et al. 350--161 3,306,160 2/1967 Dinhobclet al. 350-161 X 3,322,485 5/ 1967 Williams 350--160 FOREIGN PATENTS 778,376 7/ 1957 Great Britain.

JEWELL H. PEDERSEN, Primary Examiner T. R. MOHR, Assistant Examiner U.S. Cl. X.R.

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