Mechanical interface for proximity coupled electro-optic devices
BACKGROUND OF THE INVENTION 
     This invention relates to electro-optic devices and, more particularly, to proximity coupled light valves for electro-optic line printers and the like. 
     It has been shown that an electro-optic element having a plurality of individually addressable electrodes can be used as a multi-gate light valve for line printing. See, for example, a copending and commonly assigned U.S. patent application of R. A. Sprague et al., which was filed June 21, 1979 under Ser. No. 040,607 on a &#34;TIR Electro-Optic Modulator with Individually Addressed Electrodes.&#34; Also see &#34;Light Gates Give Data Recorder Improved Hardcopy Resolution.&#34; Electronic Design, July 19, 1979, pp. 31-32; &#34;Polarizing Filters Plot Analog Waveforms,&#34; Machine Design, Vol. 51, No. 17, July 26, 1979, p. 62; and &#34;Data Recorder Eliminates Problem of Linearity,&#34; Design News, Feb. 4, 1980, pp. 56-57. 
     As is known, almost any optically transparent electro-optical material can be used as the electro-optic element of such a light valve. The most promising materials now appear to be LiNbO 3  and LiTaO 3 , but there are other materials which qualify for consideration, including BSN, KDP, KD x  P, Ba 2  NaNb 5  O 15  and PLZT. In any event, the electrodes of such a light valve are intimately coupled to the electro-optic element and are distributed in non-overlapping relationship widthwise of the electro-optic element (i.e., orthogonally relative to its optical axis), typically on equidistantly separated centers so that there is a generally uniform interelectrode gap spacing. A copending and commonly assigned U.S. patent application of W. D. Turner, which was filed Sept. 17, 1980 under Ser. No. 187,936 on &#34;Proximity Coupled Electro-Optic Devices&#34;, shows that the electrodes of an electro-optic device, such as a multi-gate light valve, may be fabricated on a suitable substrate and pressed against or held very close to the electro-optic element to provide what is referred to a &#34;proximity coupling&#34;. 
     To perform line printing with a multi-gate light valve of the foregoing type, a photosensitive recording medium, such as a xerographic photoreceptor, is exposed in an image configuration as it advances in a cross line direction (i.e. a line pitch direction) relative to the light valve. More particularly, to carry out the exposure process, a sheet-like collimated light beam is transmitted through the electro-optic element of the light valve, either along its optical axis for straight through transmission or at a slight angle relative to that axis for total internal reflection. Furthermore, successive sets of digital bits or analog signal samples (hereinafter collectively referred to as &#34;data samples&#34;), which represent respective collections of picture elements or pixels for successive lines of the image, are sequentially applied to the electrodes. As a result, localized bulk or fringe fields are created within the electro-optic element in the immediate vicinity of any electrodes to which non-reference level data samples are applied. These fields, in turn, cause localized variations in the refractive index of the electro-optic element within an interaction region (i.e., a light beam illuminated region of the electro-element which is subject to being penetrated by the electric fields). Thus, the phase front or polarization of the light beam is modulated (hereinafter generically referred to as &#34;p-modulation&#34; of the light beam) in accordance with the data samples applied to the electrodes as the light beam passes through the interaction region. Schlieren readout optics may be usedto convert a phase front modulated light beam into a light beam having a correspondingly modulated intensity profile. For example, the phase front modulated light beam may be imaged onto the recording medium by central dark field or central bright field imaging optics. Alternatively, if the input light beam is polarized, a polarization modulation to intensity modulation conversion process may be performed by passing the polarization modulated output beam through a polarization analyizer. In more generic terms, the p-modulation of the light beam is converted into a correspondingly modulated intensity profile by using &#34;p-sensitive readout optics&#34; to image or project (hereinafter collectively referred to as imaging) the light beam onto the recording medium 
     SUMMARY OF THE INVENTION 
     It has been found that proximity coupled electro-optic devices are sensitive to pressure differentials which are likely to exist if the electrodes bear directly against the electro-optic element. If the electro-optic element is a crystal, differential contact pressures tend to cause undesireable coupling between the electro-optic and piezoelectric characteristics of the crystal, thereby detracting from the spatial uniformity of its electro-optic output. It has also been found that such devices are sensitive to variations in the physical displacement of different points on the electrodes from the electro-optic element, such as may be caused by the ordinary roughness of the mating surfaces of the electrodes and the electro-optic element, by defects in those surfaces, and by dust or other particles which may be entrapped therebetween. 
     Therefore, in accordance with this invention, the electrodes of a proximity coupled electro-optic device, such as a multi-gate light valve for an electro-optic line printer, are mechanically gapped from the electro-optic element to provide a relatively uniform gap distance between the electrodes and the electro-optic element, despite any ordinary surface roughnesses, entrapped dust particles, and even conventional surface defects. To create the gap, there suitably are spaced apart rails disposed between the electro-optic element and the electrode bearing substrate to maintain the electrodes at a predetermined small nominal gap distance from the electro-optic element. Preferably, the rails are located on opposite sides of the interaction region of the electro-optic element so that no differential contact pressures are applied to the interaction region. The rails may be formed on the electrode support substrate to bear against the electro-optic element or vice-versa. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Still other features and advantages of this invention will become apparent when the following detailed description is read in conjunction with the attached drawings, in which: 
     FIG. 1 is a schematic side view of an electro-optic line printer including a proximity coupled TIR multi-gate light valve which embodies the present invention; 
     FIG. 2 is a schematic bottom plan view of the electro-optic line printer shown in FIG. 1; 
     FIG. 3 is an enlarged side view of a TIR light valve for the electro-optic line printer of FIGS. 1 and 2; 
     FIG. 4 is an enlarged cutaway bottom view of the TIR light valve of FIG. 3 showing a pattern of individually addressable electrodes and the rails which are provided to mechanically gap the electrodes from the electro-optic element in accordance with this invention; 
     FIG. 5 is a simplified block diagram of a system for applying differentially encoded serial input data to the individually addressable electrodes of the electrode pattern shown in FIG. 4; 
     FIG. 6 is an enlarged and fragmentary sectional view of the TIR light valve shown in FIG. 3 to better illustrate the proximity coupling of the electrodes to the electro-optic element and the interaction which occurs between the light beam and the electric fringe fields within the interaction region of the electro-optic element; and 
     FIG. 7 is an enlarged and fragmentary schematic plan view of the electrode pattern of FIG. 4 as embodied on a silicon integrated circuit in accordance with this invention; 
     FIG. 8 is a enlarged cutaway bottom view of a TIR light valve having an alternative electrode pattern in combination with the mechanical gapping rails provided in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     While the invention is described in some detail hereinbelow with reference to certain illustrated embodiments, it is to be understood that there is no intent to limit it to those embodiments. On the contrary, the aim is to cover all modifications, alternatives and equivalents falling within the spirit and scope of the invention as defined by the appended claims. 
     Turning now to the drawings, and at this point especially to FIGS. 1 and 2, there is an electro-optic line printer 11 comprising a multi-gate light valve 12 for exposing a photosensitive recording medium 13 in an image configuration. The recording medium 13 is depicted as being a photoconductively coated xerographic drum 14 which is rotated (by means not shown) in the direction of the arrow. It nevertheless will be evident that there are other xerographic and non-xerographic recording media that could be used, including photoconductively coated xerographic belts and plates, as well as photosensitive film and coated paper in web or cut sheet stock form. The recording medium 13 should, therefore, be visualized in the generalized case as being a photosensitive medium which is exposed in an image configuration while advancing in a cross line or line pitch direction relative to the light valve 12. 
     As shown in FIGS. 3 and 4, the light valve 12 includes an electro-optic element 17 and a plurality of individually addressable electrodes 18a-18i. For a total internal reflection (TIR) mode of operation as illustrated, the electro-optic element 17 typically is a y cut crystal of, say, LiNbO 3  having an optically polished reflecting surface 21 which is integral with and disposed between optically polished input and output faces 22 and 23, respectively. The electrodes 18a-18i are intimately coupled to the electro-optic element 17 adjacent the reflecting surface 21 and are distributed across essentially the full width thereof. Typically, the electrodes 18a-18i are 1-30 microns wide and are on centers which are more or less equidistantly separated to provide a generally uniform interelectrode gap spacing of 1-30 microns. In this particular embodiment the electrodes 18a-18i extend generally parallel to the optical axis of the electro-optic element 17 and have projections of substantial length along that axis. Alternatively, the electrodes 18a-18could be aligned at the so-called Bragg angle relative to the optical axis of the electro-optic element 17. As will be appreciated, if the electrodes 18a-18i are aligned parallel to the optical axis of the electro-optic element 17, the light valve 12 will produce a diffraction pattern which is symmetrical about the zero order diffraction component. If, on the other hand, the electrodes 18a-18i are at the Bragg angle relative to the optical axis of the electro-optic element 17, the light valve 12 will produce an asymmetrical diffraction pattern. 
     Briefly reviewing the operation of the line printer depicted in FIGS. 1-4, a sheet-like collimated beam of light 24 from a suitable source, such as a laser (not shown), is transmitted through the input face 22 of the electro-optic element 17 at a grazing angle of incidence relative to the reflecting surface 21. The light beam 24 is brought to a wedge shaped focus (by means not shown) at approximately the centerline of the reflecting surface 21 and is totally internally reflected therefrom for subsequent transmission through the output face 23. As will be seen, the light beam 24 illuminates substantially the full width of the electro-optic element 17 and is phase front modulated while passing therethrough in accordance with the differentially encoded data samples applied to the electrodes 18a-18i. 
     More particularly, as shown in FIG. 5, serial input data samples, which represent picture elements for successive lines of an image, are applied to a differential encoder 25 at a predetermined data rate. The encoder 25 differentially encodes the input samples on a line-by-line basis in response to control signals from a controller 26, and a multiplexer 27 ripples the encoded data samples onto the electrodes 18a-18i at a ripple rate which is matched to the data rate in response to further control signals from the controller 26. The input data may, of course, be buffered (by means not shown) to match the input data rate to any desired ripple rate. Additionally, the input data may be processed (by means also not shown) upstream of the encoder 25 for text editing, formatting or other purposes, provided that the data samples for the ultimate image are applied to the encoder 25 in adjacent picture element sequence. See, for example, a commonly assigned U.S. patent application of William Gunning et al., which was filed Apr. 5, 1978 under Ser. No. 893,658. 
     Differential encoding is described in substantial detail in a copending and commonly assigned U.S. patent application of W. D. Turner et al., which was filed Sept. 17, 1980 under Ser. No. 187,916 on &#34;Differential Encoding for Fringe Field Responsive Electro-Optic Line Printers.&#34; Thus it will suffice to note that each differentially encoded data sample, other than the first sample for each line of the image, has a magnitude whose difference from the previous differentially encoded data sample corresponds to the magnitude of a respective input data sample. The first sample for each line of the image is referenced to a common reference potential, such as ground. Thus, all picture elements are faithfully represented by the electrode to electrode voltage drops that are produced in response to the differentially encoded data. 
     Referring to FIG. 6, the electrode to electrode voltage drops create localized fringe fields 28 within an interaction region 29 of the electro-optic element 17, and the fringe fields 28 cause localized variations in the refractive index of the electro-optic element 17 widthwise of the interaction region 29. The voltage drop between any adjacent pair of electrodes, such as 18b and 18c or 18c and 18d, determines the refractive index for the portion of the interaction region 29 which bridges between those two electrodes. Hence, the refractive index variations within the interaction region 29 faithfully represent the input data samples appearing on the electrodes 18a-18i in differentially encoded form at any given point in time. It therefore follows that the phase front of the light beam 24 (FIG. 3) is sequentially spatially modulated in accordance with the data samples for successive lines of the image as the light beam 24 passes through the interaction region 27 of the electro-optic element 17. 
     Returning for a moment to FIGS. 1 and 2, to expose the recording medium 13 in an image configuration, there suitably are Schlieren central dark field imaging optics 31 which are optically aligned between the electro-optic element 17 and the recording medium 13 for imaging the light beam 24 onto the recording medium 13. The imaging optics 31 convert the spatial phase front modulation of the light beam 24 into a correspondingly modulated intensity profile and provide any magnification that is required to obtain an image of a desired width. To accomplish that, the illustrated imaging optics 13 include a field lens 34 for focusing the zero order diffraction components 32 of the phase front modulated light beam 24 onto a central stop 35 and an imaging lens 36 for imaging the higher order diffraction components onto the recording medium 13, i.e., the image plane for the light valve 12. The field lens 34 is optically aligned between the electro-optic element 17 and the stop 35 so that substantially all of the zero order components 32 of the light beam 24 are blocked by the stop 35. The higher order diffraction components of the light beam 24 scatter around the stop 35 and are collected by the imaging lens 36 which, in turn, causes them to fall onto the light valve image plane defined by the recording medium 13. Of course, other p-sensitive readout optics could be used to convert the phase front or polarization modulated light beam provided by the electro-optic element 17 into a light beam having a correspondingly modulated intensity profile. 
     To summarize, as indicated in FIG. 2 by the broken lines 39, each neighboring pair of electrodes, such as 18b and 18c (FIG. 6), cooperates with the electro-optic element 17 and with the p-sensitive readout optics 31 to effectively define a local modulator for creating a picture element at a unique, spatially predetermined position along each line of the image. Accordingly, the number of electrodes 18a-18i determines the number of picture elements that can be printed per line of the image. Moreover, by sequentially applying successive sets of differentially encoded data samples to the electrodes 18a-18i while the recording medium 13 is advancing in a cross line direction relative to the light valve 12, successive lines of the image are printed. 
     As best shown in FIG. 7, the electrodes 18a-18i are preferably defined by a suitably patterned, electrically conductive layer, generally indicated by 30, which is deposited on and is a part of an electrical integrated circuit 31, such as a LSI (large scale integrated) silicon integrated circuit, to make electrical contact to the integrated drive electronics 32b-32g. For example, as illustrated, the multiplexer 27 is embodied in the integrated circuit 31, and the electrodes 18a-18i are an extension of the metalization or polysilicon layer 30 which is used to make electrical connections to the output transfer gates or pass transistors 32b-32g and other individual components (not shown) of the multiplexer 27. The pass transistors 32b-32g and the other components of the multiplexer 27 are formed on the integrated circuit 31 by using more or less standard LSI component fabrication techniques, and the metalization or polysilicon layer 30 is thereafter deposited on the outer surface 33 of the integrated circuit 31. An etching process or the like is then used to pattern the electrically conductive layer 30 as required to provide electrically independent connections to the electrically independent components of the multiplexer 27 and to form the electrically independent electrodes 18a-18i(only the electrodes 18b-18g can be seen in FIG. 7). Thus, the data transfer lines 34b-34g for the pass transistors 32b-32g are defined in the metalization or polysilicon layer 30 by the same etching process which is used to define the electrodes 18a-18i. 
     Referring again to FIG. 6, the electrodes 18a-18i are proximity coupled to the electro-optic element 17. To that end, a clamp, which is schematically represented by the arrows 42 and 43, is engaged with the electro-optic element 17 and with the silicon integrated circuit 31 to urge the electrodes 18a-18i into close contact with the reflecting surface 23 of the electro-optic element 17. Alternatively, the integrated circuit 31 could be bonded to the electro-optic element 17 by an adhesive or by suction. 
     In accordance with the present invention, as shown in FIGS. 4 and 6, rails 44 and 45 or similar spacers are provided to mechanically maintain the electrodes 18a-18i at a small predetermined nominal gap distance from the electro-optic element 17. The rails 44 and 45 are preferably located on opposite sides of the interaction region 27 of the electro-optic element 17 so that the interaction region 27 is free of pressure differentials which might cause undesireable piezoelectric effects. For example, as shown the rails 44 and 45 are formed on the electrode bearing integrated circuit 31 near its lead edge and trail edges, repectively, to engage the electro-optic element 17 forward and aft, respectively, of the interaction region 27. Alternatively, the rails 44 and 45 could be formed on the integrated circuit 31 near its front and back edges, repectively, to engage the electro-optic element 17 on opposite lateral sides of the interaction region 27. Still another alternative is to form the rails 44 and 45 on the electro-optic element 17 to engage the electrode substrate 31 either forward and aft or on opposite lateral sides of the electrodes 18a-18i. The interelectrode gap spacing of the electrodes 18a-18i must be sufficiently large compared to the nominal gap spacing provided by the rails 44 and 45 to ensure that the fringe fields 28 span the gap 46 and penetrate the electro-optic element 17 as previously described. For example, the rails 44 and 45 are each selected to have sufficient to provide a nominal gap 46 on the order of 0.10 microns if the interelectrode gap spacing is on the order of 5 microns. Of course, increased or decreased interelectrode gap spacings may require or permit a corresponding change in the heights of the rails 44 and 45. 
     A hard dielectric film 47 may be overcoated on the electrodes 18a-18i to physically protect the electrodes 18a-18i and to isolate any point defects in the silicon integrated circuit 31 from the gap 46. The film 47 preferably has a dielectric constant to thickness ratio which is higher than the corresponding ratio for an air gap of the same thickness, so that it does not appreciably increase the effective electrical thickness of the gap 46 as measured by the voltage drop occurring between any one of the electrodes 18a-18i and the electro-optic element 17. 
     As shown in FIG. 8, the present invention may also be used to mechanically gap an alternating pattern of individually addressable electrodes 18a 1  -18i 1  and ground plane electrodes 19a 1  -19i 1  from the electro-optic element 17. As is known, such an electrode pattern may be used if the input data samples are not differentially encoded. 
     CONCLUSION 
     In view of the foregoing, it will be understood the mechanical gapping provided by this invention eliminates several possible sources of unwanted noise in the output of a proximity coupled electro-optic device, such as multi-gate light valve for an electro-optic line printer.