Microchannel Spatial Light Modulator

The present invention relates to microchannel spatial light modulators for real-time phase or intensity modulation of light.

By way of background, attention is called to two theses supervised by one of the present inventors at the Massachusetts Institute of Technology and based on ideas proposed jointly by the present inventors: "Speed and Resolution of a Microchannel-Addressed Optical Modulator", (Cocco) and "Resolution and Imaging Characteristics of a Proximity-Addressed Electro-Optic Plate", (Burmawi); the latter was deposited in the M.I.T. Library System on Nov. 12, 1976. Attention is also called to a paper entitled "Low-Visibility Optical Communication: Devices and Techniques for Improved Performance," given by one of the present inventors at a National Science Foundation usergrantee meeting in New York, New York, June 1977 (called "Warde paper" herein) and to a Varian bulletin MCP-2819A. Other parallel, spatial light modulators to perform real-time phase or intensity modulation use photoconductive electro-optic crystals or elastomers or liquid crystals. Examples are PROM of Itek Corporation; Phototitus as described in two journal articles, "A Pockels-Effect Light Valve: Phototitus. Application to Optical Image Processing" (Donjon et al.), IEEE Transactions on Electron Devices, Vol. ED-20, No. 11, November. 1973 and "Information Input in an Optical Pattern Recognition System Using a Relay Tube Based on the Pockels Effect," (Groh et al), Optics Communications, Vol. 2, No. 3, August, 1970; Ruticon of Xerox Corporation; and liquid crystal devices of Hughes Corporation.

The background need for modulators of the type herein disclosed and the context within which such modulators may be found are found in the Warde paper as well as in a journal article entitled "Spatial Light Modulators" (Casasent), Proceedings of the IEEE, Vol. 65, No. 1, 1977, pp. 143-157. The present system is called a "distributed electron beam type" in the Warde paper. Such a system can be used in coherent optical signal processing. Because optical systems have the ability to perform parallel signal processing at ultra-high speeds, more and more optical systems are being used in computer apparatus. High speed, re-usable spatial light modulators are badly needed to take full advantage of parallel optical signal processing at high speeds.

Accordingly, it is an object of the present invention to provide a novel spatial light modulator.

Another object is to provide a microchannel light modulator capable of high resolution (.about.50 lines/mm), high speed (.about.1000 frames/second) and high sensitivity (less than 10.sup.-2 ergs/cm.sup.2) and which can perform both phase and intensity modulation as well as beam steering and beam deflection.

In addition to coherent optical signal processing, the modulator herein disclosed can also be used as a light valve for large screen projection television, for atmospheric phase compensation (see the Warde paper), and as an optical memory and for laser beam steering and deflection. It is still another object to provide a modulator adapted to such other applications.

These and still further objects are addressed hereinafter.

The invention is hereinafter described with reference to the accompanying drawing in which:

FIG. 1 is a diagrammatic representation of a microchannel spatial light modulator, including a schematically represented electrical biasing means and an elevation view of the device wherein modulation is effected, the modulator shown being adapted to effect phase modulation;

FIG. 2 is a diagrammatic representation, like FIG. 1, showing, among other things, a grid within said device for phase modulation;

FIG. 3 is a diagrammatic representation, like FIG. 1, of a system wherein intensity modulation is effected;

FIG. 4 is a diagrammatic representation, like FIG. 2, of a system wherein intensity modulation is effected.

FIG. 5 is a diagrammatic representation, like FIG. 1, wherein the device is used to effect laser beam steering or deflection;

FIG. 6 is a diagrammatic representation, like FIG. 2, wherein the device is used to effect laser beam steering or deflection;

FIG. 7 is an isometric view of said device of FIG. 1; and

FIG. 8 is a diagrammatic representation showing three microchannel spatial light modulators in the context of a large-screen television projection arrangement for color television.

In the explanation that now follows, there is first an overall discussion to place the later, more detailed discussion, in perspective. Referring now to FIG. 1, there is shown at 101A a system that includes a microchannel spatial light modulator or modulator system 102A. The system 101A, as later discussed, further includes a source of incident or write light 30 (which may be a distant star, for example), a source of read light 31 (which may be coherent light such as laser light) a pulsed source of blue or near UV erase light 33 and a processed light interpreter 32 (which may be an array of light detectors or a photographic film). Light modulation is actually accomplished in a modulation device 1A which is electrically biased by electrical circuitry that includes dc source 20, 21 and 22, an ac or pulsed source 23 and an electric current limiting resistor 24; the numerals 20-24 are used in later figures to designate these same circuit elements.

The modulation device 1A, when appropriately biased, is a microchannel spatial light modulator as now explained. As shown in FIG. 1, the device 1A includes a photocathode 2, a microchannel plate 3 and an electro-optic plate 4 within a vacuum housing 5. The elements 2, 3 and 4, as shown in FIG. 7, have planar major surfaces. The photocathode 2 is spaced from the microchannel plate by a vacuum gap 6 and the microchannel plate 3 is spaced from the electro-optic plate 4 by a further vacuum gap 7. Further structures are associated with the elements 2, 3 and 4 to form composite structures, as explained in the next few paragraphs.

The photocathode 2 is a thin film of a photosensitive material (see RCA Electro-optics Handbook, pp. 152-153) on the upper surface of which there is deposited a thin metal layer (e.g., chromium) that serves as a photocathode electrode 2A. The photocathode and its electrode are usually supported by a transparent substrate 2B as shown in FIGS. 1 and 7, although for some applications the photocathode material can be deposited directly onto the first electrode 3A (discussed below) of the microchannel plate, while in other applications involving extreme ultraviolet radiation and soft x-ray radiation, no photocathode is necessary. The photocathode 2, as later explained in some detail, receives a photon image at its upper planar surface and acts as a source of photoelectrons whose spatial distribution is proportional to the spatial distribution intensity of light incident upon the photocathode; that is, the photocathode provides an electron image of the image in the incident light.

The microchannel plate 3 is a thin wafer of a porous semiconducting glass (see the Varian bulletin for details) having at each of its major surfaces a conductive coating to serve, respectively, as a first planar electrode 3A to receive electrons from the photocathode and a second planar electrode 3B to interact with the first electrode 3A and create an electric field in the porous semiconducting glass 3 to draw electrons through the porous semiconducting glass from the first electrode 3A to the second electrode 3B, there being amplification in the number of electrons between the first electrode and the second electrode so that there is an amplified electron image at the second electrode.

The electro-optic plate is a thin, plane parallel wafer of electro-optic material (later specified). The electro-optic plate 4 has at its upper planar surface a planar dielectric mirror 4A in the form of an insulating layer made by depositing multiple coatings, for example, of silicon dioxide and titanium dioxide on the electro-optic plate 4 and an insulating light-blocking layer 4C deposited on top of the dielectric mirror 4A. The electro-optic plate 4 is sandwiched between the dielectric mirror 4A and a transparent planar electrode 4B. The light-blocking layer 4C faces the second electrode 3B and is separated therefrom by the vacuum gap 7; it receives the amplified electron image from the microchannel plate 3. Whether the microchannel light modulator 102A is used to modulate the phase of an incoming light beam or the intensity thereof, the electro-optic plate 4 must have a low half-wave surface charge density and either high or controlled resistivity. By a material with controlled resistivity is meant a material whose resistivity can be controlled by some convenient process, e.g., a material that is both electro-optic and photo-conductive such as bismuth silicon oxide. Alternatively, the electro-optic plate may have low half-wave surface charge density and the controlled resistivity may be built into the dielectric mirror and the light-blocking layer. For phase modulation, lithium niobate (LiNbO.sub.3) used in a longitudinal electro-optic mode may be employed, for example; and, for intensity modulation, deuterated cesium dihydrogen arsenate (CD*A) may be employed, for example. Other examples of electro-optic materials are listed below in Table 1.

Electrical biasing of the elements 2, 3 and 4 is now discussed. The dc source 20 applies a voltage V.sub.c between the photocathode electrode 2A and the first electrode 3A of the microchannel plate to draw the photoelectrons across the vacuum gap 6. The dc source 21 (having a terminal voltage V.sub.m) applies a voltage between the first electrode 3A and the second electrode 3B through the resistor 24 (having a resistance R.sub.p) to create the necessary electric field within the microchannel plate 3. The pulsed source 23 (having a pulsed output voltage V.sub.e) and the dc source 22 (having a terminal voltage V.sub.b) apply biasing potentials between the second electrode 3B and the transparent electrode 4B to draw the amplified electron image across the gap 7. Typically, the gaps 6 and 7 are 1/10 millimeter the electro-optic plate 4 is 1/2 millimeter thick, V.sub.c is 1-10 volts, V.sub.m is 1000 volts, R.sub.p is 1 M.OMEGA., V.sub.b is 2,000-10,000 volts and V.sub.e is a source of 1-10 kV repetitively pulsed at the framing rate of the device during real-time operation. The voltage V.sub.e serves a reset function, as later explained.

The system 101A is set up to effect phase modulation. For operation of the system 101A in the cycled mode, the pulse generator 23 is off during the write period and it behaves like a short-circuit. A control signal from the source of write light 30 (coherent or incoherent light) illuminates the photocathode 2 and produces, at the back of the microchannel plate 3, an amplified electron image of the optical intensity. This amplified electron image is proximity focused onto the light-blocking layer 4C; that is, photoelectrons from the photocathode, amplified by the microchannel plate, are distributed on a planar upper surface of the image plane 4C such that the spatial density of the electrons is proportional to the spatial intensity of the incident light on the photocathode. The constant-voltage supply 22 maintains the light-blocking layer 4C at a potential above that of the bottom electrode 3B of the microchannel plate 3 to draw the electrons across the gap 7 to the flat upper surface of the light-blocking layer 4C. The charge distribution of the thusly deposited electrons induces a corresponding spatially-varying refractive index in the electro-optic material which is typically an electro-optic crystal of symmetry 3 m (e.g., lithium niobate) cut perpendicular to the c-axis, but any of the materials in Group I of Table 1 will suffice. This spatially varying index of refraction is utilized by means of longitudinal electro-optic effect to phase modulate the readout laser beam from the source 31, as explained below.

The source 31 is typically a laser beam and is the light to be processed in the device 1A. The output beam from the source 31 is reflected off a beam splitter 50 through a bottom window 47 of the housing 5, through the transparent electrode 4B, into the electro-optic plate 4; it is reflected by the dielectric mirror 4A back downward in FIG. 1 through the beam splitter 50 to the interpreter 32 (for example, a TV camera, a photographic plate or a light detector array). The read light from the source 31 is, in this way, processed in the device 1A by the electro-optic plate 4 whose index of refraction, as above explained, is spatially modulated by the electron image on the dielectric mirror 4A. Erasure of the electron image on the light-blocking layer 4C can be assisted by a source 33 of erase light which is a source of pulsed blue or ultraviolet light, it being possible with some materials to achieve erasure by the processes of photoconduction and photoemission. The source 33 of pulsed erase light 30 must be synchronized with an erase or reset pulse V.sub.e from the pulsed source 23.

The systems labeled 101B, 101C, 101D, 101E and 101F in FIGS. 2, 3, 4, 5 and 6, respectively, have elements that are common with the system 101A and are given identical labels. The modulation devices in FIGS. 2-6 are labeled 1B, 1C, 1D, 1A and 1B, respectively. The functions of the common elements in the later figures are not explained in any further detail in what follows. The later systems also contain elements not included in FIG. 1, which are now taken up.

The system 101B in FIG. 2, like the system 101A, is set up to effect phase modulation. It differs from the system 101A in that a grid 8 is inserted in the gap 7 between the microchannel plate 3 and the light-blocking layer 4C. The grid 8 allows an external electrical modulation signal to be inserted into the processing capability of the device 1B to control electron flow in the gap 7, and, in addition, it provides an alternative means of stripping electrons from the light-blocking layer 4C during erasure. The grid can also be used to achieve operation of the device in a continuous mode in which (unlike the framed mode described for the system 101A) the writing and erasing of the electron image is a continuous rather than a discrete process.

The system 101C in FIG. 3 is set up to effect either phase or amplitude modulation and each of the materials shown in the first two groups of Table 1, among others, can be used. It differs from the system 101A in that two polarizers, 51 and 52 are placed in the read light and the processed light. To effect phase modulation the axis of polarizer 51 in the read beam is generally oriented parallel to one of the induced axes of the electro-optic material and the processed beam polarizer 52 is either omitted or has its axis aligned along that of the read beam polarizer 51. To effect amplitude modulation, the read beam polarizer 51 is usually aligned with its axis at 45.degree. to the induced axes of the electro-optic material and the processed beam polarizer 52 is crossed with respect to the read beam polarizer 51.

The system 101D in FIG. 4 is set up to effect either phase or intensity (i.e., amplitude) modulation. It differs from the system 101A in that a grid (again marked 8) is inserted in the gap 7 for the purposes described with regard to the system 101B and that polarizers 51 and 52 are inserted in the read beam and the processed beam as in the system 101C and employed in exactly the same fashion as in the system 101C.

The system 101E in FIG. 5 is set up to effect beam steering or beam deflection. It is essentially the same as the system 101A except that a beam splitter is not needed. The principle employed to steer the light is that of diffraction. By using a write light (control signal) whose intensity is periodic in space, a periodic index of refraction profile (i.e., a phase grating) is set up in the electro-optic material by the process described earlier. The periodic index of refraction then diffracts the input light. By varying the periodicity and orientation of the control write light, two-dimensional beam steering is accomplished.

The system 101F in FIG. 6 is also set up to effect beam steering. The principle and operation of the system 101F is similar to that in the system 101E except that a control grid (again labeled 8) is inserted for the same purposes as described with reference to the system 101B.

In the isometric view of FIG. 7, the modulation device is again labeled 1A and the further numerical designations applied are also the same as in FIG. 1, except that further elements are identified in the structure 1A of FIG. 7, as now discussed. The mechanical structural elements in FIG. 7 that serve to maintain the elements 2, 3 and 4 in spaced relationship to one another, include a photocathode flange 40, microchannel plate flanges 41 and 42 and an electro-optic plate flange 43. The flanges are attached to the vacuum enclosure 5 by insulated support studs (e.g., the stud labeled 44) and held in spaced relationship by ceramic spacers (e.g., the spacers labeled 45A-45D) which also serve to electrically isolate the flanges. Electrical feedthroughs (such as 49) serve to pass electrical connections through the housing 5 to specific components of the device 1A. Write light enters the device 1A in FIG. 7 through a quartz or other window 46. Read light enters the device 1A through another quartz or other window 47. The vacuum enclosure 5 is a shielded glass housing, that is, the housing is glass with metal shielding except for the windows 46 and 47. The various facing surfaces of the interacting elements, as above indicated, are all planar and the elements themselves are flat and plane parallel to minimize distortion.

A few matters of a general nature are taken up in this paragraph. The write source 30, as above indicated, may be light from a distant star approximately focused on the photocathode using telescopic apparatus, it may be a small hole like a camera aperture, it may be a camera or some other imaging mechanism. However, the present system takes that optical image and uses it to process read light which is modulated or processed by the write light in the devices 1A-1D and the processed light contains a message that is later interpreted. The devices 1A-1D will operate in a storage mode as optical memories if the electrons are not actively erased from the dielectric mirror 3A. The material PLZT which is found in the third group of Table 1 can also be used in the device as an electro-optic plate to effect phase and intensity modulation as well as storage in the systems 101A and 101B. In the microchannel spatial light modulator, for extremely low level write light applications, the single microchannel plate can be replaced by two (or more) microchannel plates cascaded in series in the conventional chevron configuration and appropriately biased, as discussed in the Varian bulletin.

The system labeled 103 in FIG. 8 is set up to effect the projection of color television images on a large screen 70 (.about.31/2 feet.times.5 feet). The system 103 consists of three microchannel spatial light modulators 102, 102', 102" that may be like the modulator 102A in FIG. 1 (or may have grids as in FIG. 2, for example). The sources of write light marked 30A, 30B and 30C are red, green and blue CRTs whose signal can be either taken from the commercial transmitted TV video signals (e.g., by a receiver 59) or from video recording and playback machines or the like or from special circuitry. The source of read light is a high intensity Xenon arc lamp 67 which is reflected from the parabolic reflector 68 and filtered to remove its ultraviolet components by a filter 66 and its infrared components by a filter 65. After passing through and reflecting from a polarizing beam splitter 50B it is further separated into its red, green and blue components by filters 63 and 64 and a green-blue selective dichroic mirror 50A. In fact, the final red, green and blue beams that enter the modulators 102, 102' and 102", respectively, constitute light that provides the read function. The light beams reflected back by the modulators 102, 102' and 102" are mixed in the polarizing beam splitter 50B and reflected and/or transmitted as a beam 71 that contains the three colors. The beam splitter 50B acts as an input polarizer for light from the source 67 and as an output polarizer for light from the modulators 102, 102' and 102". The combined light beam 71 is focused by a projection lens 69 onto the large screen 70. The projection scheme discussed in this paragraph, except for the microchannel spatial light modulators, is similar to that disclosed in a paper entitled "A New Color-TV Projector," Jacobson et al, SID International Symposium, April 1977.

Further modifications of the invention herein disclosed will occur to persons skilled in the art, and all such modifications are deemed to be within the scope of the invention as defined by the appended claims.