LIGHT-MODULATING DEVICE COMPRISING A MULTIGATE LIGHT VALVE

Abstract: To obtain a more uniform optical output from an electro-­ optic light valve (17), the lengths of the electrodes (18) for the light valve are selected in accordance with a function which is inversely related to the predicted and/or measured data-inde­ pendent variations in the uncompensated or incompletely compensated optical output of the light valve, thereby com­ pensating for such variations.

Description: Non-uniformity compensation for multigate light valves This invention relates to fringe field responsive multigate light valves and, more particularly, to methods and means for increasing the uniformity of the optical output of such light valves. As is known, an electro-optic element having a plurality of individually addressable electrodes may be employed as a multigate light valve in, say, an electro-optic line printer. "Light Gates Give Data Recorder Improved Hardcopy Resolution," Electronic Design, July 19, 1979, pp. 31-32; "Polarizing Filters Plot Analog Waveforms," Machine Design, Vol. 51, No. 17, July 26, 1979, p. 62; and "Data Recorder Eliminates Problems of Linearity," Design News, February 4, 1980, pp. 56-57. Substantial progress has been made in developing such light valves and in applying them to electro-optic line printing. However, one of the issues which has not yet been addressed is the problem of obtaining a substantially uniform optical output across the full width of a multigate light valve. Undesirable vairations may occur in the output of such a light valve because of system irregularities, such as non-uniform illumination of the light valve and vignetting in the imaging optics, and elemental irregularities, such as variations in the electrode spacing and in the electrode drive electronics. Furthermore, some of the optical variations are predictable, while others can be ascertained only by actual measurement. In accordance with the present invention, to obtain a more uniform optical output from an electro-optic light valve, the lengths of the electrodes are selected to compensate for the system and/or elemental irregularities that tend to cause a non-uniform optical output To the extent that such irregularities are predictable, the compensation may be provided by initially fabricating the electrodes to have lengths complementing the predicted optical output irregularities. Otherwise, the output irregularities are measured so that the electrodes may be trimmed as necessary to compensate for the non uniformity. In any event, in keeping with this invention, the lengths of the electrodes vary widthwise of the electro-optic element in accordance with a function which is inversely related to the predicted or measured variations in its uncompcnsated or incompletely compcnsated optical outpuL 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: : Figure 1 is a schematic side view of an electro-optic line printer having a TIR multigate light valve embodying the present invention; Figure 2 is an enlarged bottom plan view of a portion of the printer shown in Figure 1; Figure 3 is an enlarged side view of the TIR light valve, Figure 4 is an enlarged, cut-away, bottom view of the light valve shown in Figure 3 to illustrate the varying length electrodes that are provided in accordance with this invention to compensate for non-uniformities in the optical output of the light valve, Figure 5 is a simplified block diagram of a system for applying differentially encoded input data samples to the electrodes of the light valve shown in Figure 5 Turning now to the drawings, and at this point especially to Figures 1 and 2, there is an electro-optic line printer 11 having a fringe field responsive multigate light valve 12 for printing an image on a photosensitive recording medium 13. As shown, the recording medium 13 is a photoconductively coated drum 14 which is rotated (by means not shown) in the direction of the arrow. Nevertheless, it will be evident that there are other xerographic and non-xerographic recording media that could be used, including photoconductively coated belts and plates, as well as photosensitive films and coated papers Thus, in the gencralized case, the recording medium 13 should be visualized as being a photosensitive medium which is exposed while advancing in a cross line or line pitch direction relative to the light valve 12. As best illustrated in Figure 3 and 4, the light valve 12 comprises an optically transparent electro-optic element 17 and a plurality of individually addressable electrodes 18a- 18a-18r. The most promising electro-optic materials for such a device presently appear to be UNbO3 and Lira03, but there are others which warrant consideration, including BSN, KDP, KDXP, Ba2NaNb5015 and PLZT. In this particular embodiment, the light valve 12 is operated in a TIR mode. Consequently, the electro-optic element 17 is suitably a ycut crystal of, say, LiNbO3 having an optically polished reflecting surface 21 extending between opposed, optically polished input and output faces 22 and 23, respectively. Typically, each of the electrodes 1a- 1Si is 1- 30 microns wide, and the interelectrode gap spacing is 1-30 microns. In operation, as shown in Figs. 1-4, a sheet-like collimated light beam 24 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 (i.e., an angle no greater than the critical angle of incidence for total internal reflectance from the reflecting surface 21). The input beam 24 is supplied by a suitable source, such as a laser, and is laterally expanded (by means not shown) so that it more or less uniformly illuminates substantially the full width of the electro-optic element 17. Furthermore, the input beam 24 is brought to a wedge shaped focus (by means also not shown) on the reflecting surface 21 approximately midway through the electro-optic element 17 and is totally internally reflected therefrom to provide an output beam 25 which exits from the electro-optic element 17 through its output face 23. The phase front or the polarization of the output beam 25 is spatially modulated in accordance with the data applied to the electrodes 18a- 18i. More particularly, if the data creates a voltage drop between any adjacent pair of electrodes, such as the electrodes 18b and 18= a corresponding fringe field is coupled into the electro-optic element 17, thereby producing a localized variation in its refactive index. To efficiently couple such fringe fields into the electro-optic element 17, the electrodes 18a-18i arc supported on or vcry near the reflecting surface 21 thereof. lndecd, as shown, the electrodes 18--18i preferably are deposited in laycrs on a suitable substrate, such as a VLSI silicon circuit 28, which is pressed or otherwise firmly held against the electro-optic clement 17, as indicated by the arrows 29 and 30, to maintain the elcctrodes iSa-iSi in contact with or closely adjacent to the reflecting surface 21. The advantage of this construction is that the VLSI circuit 28 may be used to make the necessary electrical connections to the electrodes 18a--18i. Alternatively, the electrodes 18a-18i could be deposited on the reflecting surface 21 of the electro-optic element 17. For illustrative purposes, it has been assumed that the phase front of the output beam 25 is modulated in accordance with the data applied to the electrodes iSa- iSi. Accordingly, Schlieren central dark field or bright field imaging optics are used to convert the spatial phase front rnodulation of the output beam 25 into a correspondingly modulated intensity profile and to supply any magnification that may be needed to obtain an image of the desired size on the recording medium 13. More particularly, as illustrated, there are central dark field imaging optics 31 comprising a field lens 34, a central stop 35, and an imaging lens 36. The field lens 34 is optically aligned between the output face 23 of the electro-optic element 17 and the stop 35 to focus substantially all of the zero order diffraction components of the output beam 25 onto the stop 35. However, the higher order diffraction components of the output beam 25 scatter around the stop 35 and are collected by the imaging lens 36 which, in turn, focuses them on the recording medium 13 to provide an intensity modulated image of the light valve 12. Of course, if the input beam 24 is polarized (by means not shown), the polarization of the output beam 25 will be spatially modulated by the light valve 12 in accordance with the data applied to the electrodes iSa- 18a1Si. In that event, a polarization analyzer (also not shown) may be used to convert the spatial polarization modulation of the output beam 25 into a correspondingly modulated intensity profile. Accordingly, to generically cover the alternatives, the phase front or polarization modulation of the output beam 25 will be referred to as "p-modulation" and the read out optics 31 will be described as "psensitive imaging optics." Unfortunately, there are system and elemental irregularities which tend to cause unwanted, data independent non-uniformities or irregularities in the output image of the light valve 12 For example, the laser beam 24 typically has a Gaussian energy profile, such that illumination of the light valve 12 tends to peak at or near its center and to more or less symmetrically and progressively drop off from that point outwardly toward its opposite ends. Any vignetting in the imaging optics 31 may also detract from the uniformity of such image. In addition, there are likely to be elemental variations in, say, the drive electronics for the electrodes 18a-18i or in the spacing of the electrodes 18an381 to further detract from the uniformity of the optical output of the light valve 12. It has been found that the exact relationship of the spatial intensity of the output beam 25 to the lengths of the electrodes 18a-18i is a complicated function of the angle of incidence of the input beam 24 on the reflecting surface 21, the cone angle with which the input beam 24 is brought to focus on the reflecting surface 21, the wavelength of the input beam 24, the spacing of the electrodes 18a-18i, and the drive voltages applied to the electrodes 18a--18i. However, once that relationship is empirically determined for any given set of conditions, its inverse may be used to adjust the lengths of the electrodes so as to obtain a more or less spatially uniform optical output from the light valve 12 Therefore, in accordance with the present invention, as shown in Fig. 4, to obtain a more uniform optical output from the light valve 12, the lengths of the electrodes 18¯a-18h are selected to vary in accordance with a function which is inversely proportional to the predicted or measured, data independent variations in the optical output of the light valve 12. To the extent that such variations are predictable, the lengths of the electrodes may be appropriately controlled during the fabrication of the light valve 12, such as by photolithographically defining such electrodes 18a-18i on the metallization layer of the VLSI silicon circuit 28 to have lengths matching a predetermined compensating function Otherwise, the data independent optical output variations of the light valve 12 are measured so that the electrodes 18a181 may be trimmed, such as by re-etching or laser ablation, to match the required compensating function. Turning to Figure 5, it will be seen that the electrodes 18a- 1 & are individually addressable. Therefore, to print an image, differentially encoded data samples for successive lines of the image are sequentially applied to the electrodes iSa- 18a-18i. To supply the differentially encoded data samples, serial input data samples representing adjacent picture elements for successive lines of an image are applied to a differential encoder 61 at a predetermined data rate. The encoder 61 differentially encodes these input samples on a line-by-line basis, and a multiplexer 62 ripples the encoded data samples onto the electrodes 18a-18i at a ripple rate which is matched to the data rate. A controller 53 synchronizes the encoder 61 and the multiplexer 62. Of course, the input data may be buffered (by means not shown) to match the input data rate to any desired ripple rate. As a matter of definition, each diffcrentially encoded data sample. other than the first sample for each line of the image, has a magnitude which diffcrs from the magnitude of the previous differentially encoded sample by an amount corresponding to the magnitude of a particular input data sample. The first differentially encoded sample for each line of the image is referenced to a predetermined potential, such as ground Thus, when the differentially encoded data samples for any given line of an image are applied to the electrodes 18a-i & , all picture elements for that line are faithfully represented by the magnitude of the voltage drops appearing between respective pairs of neighboring electrodes. Preferably, the differentially encoded data samples are binary digital data, but analog data may also be differentially encoded for application to the electrodes8a-1i. It will be evident that there are many variations to the present invention. For example, the individually addressable electrodes lSa-iSl may be interleaved with and referenced to ground plane electrodes (not shown), thereby avoiding the need for any differential encoding. As another example, the electrodes 18a-18i could be tilted at the Bragg angle relative to the optical axis of the light valve 12 to provide for Bragg diffraction of the output beam 25. Still another possibility is that the electrodes 18a-i & could be convergent on the entrance pupiI of the imaging lens 36, thereby allowing for the use of non-telecentric imaging optics In view of the foregoing, it will now be understood that the present invention provides a technique for improving the spatial uniformity of the optical output of a multi-gate light valve. As will be appreciated, the electrode lengths may be selected prior to fabrication of a light valve to compensate for predictable spatial variations in the optical output of the light valve and/or may be trimmed following assembly of the light valve to compensate for spatial variations which can be determined only by empirical measurements.

Claims: Claims: 1. A multi-gate light valve (12) for spatially modulating a light beam (24), said light valve including an electro-optic element (17) positioned to be illuminated by said light beam, said electro-optic element having a predetermined optical axis a plurality of electrodes (18) intimately associated with said electrooptic element, said electrodes extending lengthwise of said optical axis, and means (62) for applying data samples to at lest every other one of said electrodes, whereby localized electric fields are created within said electrooptic element to spatially modulate said light beam in accordance with said data samples; characterised in that the electrodes are of different effective lengths selected to compensate at least partially for data-independent, spatial non-uniformities in said modulated light beam. 2. A valve as claimed in claim 1, wherein the lengths of said electrodes are preselected to compensate for predictable, data-independent, spatial energy variations in said modulated light beam. 3. A valve as claimed in claim 1 or 2, wherein said electrodes are supported on a separate substrate (28) and are held closely adjacent said electro-optic element, whereby electric fringe fields are proximity coupled into said electro-optic element to modulate said light beam. 4. A valve as claimed in claim 3, wherein said electro-optic element has a reflecting surface (21), said light beam is applied to said electro-optic element at a grazing angle of incidence relative to said surface and is brought to a wedge-shaped focus thereon for total internal reflection therefrom, said substrate is an integrated circuit having a metallised layer patterned to define said electrodes, and said integrated circuit is immediately adjacent said reflecting surface to proximity couple said fringe fields into said electro-optic element. 5. A valve as claimed in any preceding claim, each of said electrodes is individually addressable; said data samples are differentially encoded for application to said electrodes, and said electro-optic element is an electro-optic crystal. 6. A valve as claimed in any preceding claim, the lengths of the electrodes are trimmed after fabrication in order to compensate for measured variations in said modualted light beam.

Inventors: SPRAGUE, ROBERT A.
Applicants: XEROX CORPORATION
Application year: 1982
Main classification: G02F1
IPC codes: G02F1, B41J2, G06K15