OPTICAL IMAGE BARS

Abstract: An optical image bar (12) utilizing polarized light and a coherent spatial light modulator (16) includes birefrigent spatial or angular shearing means (51) for uniformly dividing the spatially-modulated output radiation of a coherent image bar into a pair of laterally-offset, redundantly-modulated, orthogonally-polarized optical field distributions. Imaging optics (17), which include any spatial filtering and/or polarization filtering elements needed for converting those field distributions into correspondingly-modulated spatial intensity distributions, bring the filtered field distributions to focus on an output image plane (13), thereby producing redundantly-modulated, laterally-offset, intensity profiles on the output image plane which spatially sum with each other on an intensity basis. The magnitude of the shear is selected so that the image plane offset of those intensity profiles is approximately equal to half the nominal image plane pixel pitch of the image bar, whereby each of the intensity profiles redundantly overwrites the interpixel intensity nulls of the other. Spatial shearing is preferred for image bars having telecentric imaging optics, but angular shearing is a practical alternative for some embodiments.

Description: This invention relates to polarized optical image bars and, more particularly, to methods and means for suppressing unwanted interpixel intensity nulls in their output images. As a matter of definition, an "optical image bar" comprises an array of optical picture element ("pixel") generators for converting a spatial pattern, which usually is represented by the information content of electrical input signals, into a corresponding optical intensity profile. If the spatial pattern is superimposed on polarized optical radiation, the image bar is "polarized." Although there is a variety of applications for these image bars in a number of different fields, a significant portion of the effort and expense that have been devoted to their development has been directed toward their application to electrophotographic printing, where they may prove to be a relatively low cost and reliable alternative to the flying spot raster scanners which have dominated that field since its inception. Optical displays may also benefit from the use of such image bars, although their application to that field is a secondary consideration at present. Some of the more interesting image bar proposals relate to EO TIR (electrooptic total internal reflection) spatial light modulators. As disclosed in US-A-4 396 252 an EO TIR spatial light modulator characteristically comprises a plurality of laterally-separated, individually-addressable, electrodes which are maintained on or closely adjacent a reflective surface of an optically-transparent electrooptic element, such as a lithium niobate (LiNbO₃) crystal. In operation, substantially the full width of the electrooptic element of such a modulator is illuminated by a linearly-polarized, transversely-collimated, light beam. Thus, when voltages representing the pixels of a linear pixel pattern (e.g., the pixels for a given line of an image) are applied to its individually-addressable electrodes, the modulator spatially phase modulates the wavefront of the light beam in accordance with the applied pixel pattern. As a general rule, of course, the spatial wavefront modulation varies as a function of time in accordance with the pixel patterns for successive lines of a two-dimensional image, thereby providing a line-by-line representation of the image. For image bar applications of EO TIR spatial light modulators, prior proposals typically have incorporated Schlieren imaging optics for imaging the modulator on to its output image plane. The frequency plane filtering of a Schlieren imaging system effectively transforms the spatially-modulated output radiation of the modulator into a series of correspondingly-modulated intensity profiles, but there are embodiments in which a polarization analyzer may be used alone or in combination with a Schlieren stop to read out the spatial modulation produced by the modulator. Thus, as used herein, the phrase "electrooptic image bar" applies to all image bars which embody electrooptic spatial light modulators, regardless of whether the modulators are read out by spatial filtering and/or by polarization filtering. There have been several significant developments which have reduced the cost and increased the reliability of EO TIR spatial light modulators. Among the improvements that are of particular relevance to image bar applications of these modulators are a "differential encoding" technique that is described in US-A-4,450,459 and an electrical interconnect strategy that is described in US-A-4,367,925. Briefly, it has been shown that the number of electrodes which such a modulator requires, when used in an image bar having a predetermined resolution, may be reduced by a factor of almost two if the input data samples are differentially encoded on a line-by-line basis prior to being applied to the modulator. Furthermore, it has been demonstrated that more or less conventional VLSI circuit technology can be employed to integrate the modulator electrodes with their addressing and drive electronics, thereby facilitating the orderly and reliable distribution of data samples to the relatively large number of electrodes which customarily are required for reasonably high resolution printing. Electrooptic image bars intrinsically are spatially coherent devices. Axially illuminated EO TIR spatial light modulators (i. e., those wherein the incident radiation propagates in a direction that is essentially parallel to the optical axis of the modulator) are especially well suited for use in higher resolution image bars, but they inherently tend to produce interpixel intensity nulls because they spatially modulate the incident radiation by diffractively scattering optical energy into positive and negative diffraction orders which are more or less angularly symmetrical about a zero-order or unmodulated component. These positive and negative diffraction orders (collectively referred to herein as "higher order diffraction components") define the upper and lower spatial frequency sidebands, respectively, of the modulated radiation, so they coherently contribute to the effective spatial modulation bandwidth of the modulator, provided that their relative phase is preserved. Unfortunately, however, whenever such spatially-coherent radiation is brought to focus to form an image, adjacent pixels of opposite phase destructively interfere with each other, thereby producing undesirable interpixel intensity nulls. For example, differential encoding produces adjacent pixels of opposite phase. Others who have attempted to develop essentially null-free image bars embodying axially-illuminated EO TIR spatial light modulators have recognized that the unwanted interpixel intensity nulls are caused by destructive interference, so their work is most interesting. As described in US-A-4,437,106, one of the prior null-suppression proposals suggests scattering light into the null regions in accordance with a pattern having an angular orientation and/or a spatial frequency which tends to inhibit the ability of the unaided eye to resolve the nulls, even when the imaging is performed at normal exposure levels. This approach preserves the internal spatial coherency of the output radiation of the image bar (i. e., the relative phase of its positive and negative diffraction orders), while reducing the observable effects of the nulls. US-A-4,483,596 describes an alternative approach, pursuant to which a polarization retardation plate or the like is provided for orthogonally polarizing the positive and negative diffraction orders of the modulated output radiation of the image bar, thereby preventing them from destructively interfering with each other. That effectively suppresses the interpixel intensity nulls, but it also destroys the relative phase information between the positive and negative diffraction orders, thereby reducing the effective spatial bandwidth of the image bar by a factor of two. Moreover, it may be relatively difficult and expensive to take full advantage of this orthogonal polarization concept in practice because of the wide range of incident angles at which light from different points along an image bar of appreciable width would fall on the polarization retardation plate. The wavelength spread that is required is a significant limitation because many image bars, including those which embody laser-illuminated EO TIR spatial light modulators, are monochromatic. However, the basic concept of decomposing the spatially-modulated output radiation of a coherent image bar into mutually-orthogonal components not only preserves the internal spatial coherency of the output radiation of the image bar, but also is instrumental in producing laterally-offset and redundantly-modulated intensity profiles which spatially sum with each other on an intensity basis to produce an essentially null-free image having relatively uniformly shaped pixels. The challenge, therefore, is to provide a suitable mechanism for applying such a null-suppression technique to monochromatic image bars. In accordance with the present invention, an optical image bar utilizing polarized light and a coherent spatial light modulator, includes birefringent spatial or angular shears for uniformly dividing the spatially-modulated output radiation of a coherent image bar into a pair of laterally-offset, redundantly-modulated, orthogonally-polarized, pixel distributions. Imaging optics, which include any spatial filtering and/or polarization filtering elements needed for converting those pixel distributions into correspondingly-modulated spatial intensity distributions, bring the filtered pixel distributions to focus on an output image plane, thereby producing redundantly-modulated, laterally-offset intensity profiles on the output image plane which spatially sum with each other to produce an image essentially free of inter-pixel intensity nulls. The magnitude of the shear is selected so that the image plane offset of those intensity profiles is approximately equal to one half of the nominal image plane pixel pitch of the image bar, whereby each of the intensity profiles redundantly overwrites the inter-pixel intensity nulls of the other. Spatial shearing is preferred for image bars having telecentric imaging optics, but angular shearing is a practical alternative for some embodiments. The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic sagittal plane view of a line printer embodying a spatial shearing mechanism for carrying out the present invention; Figure 2 is a schematic tangential plane view of the printer shown in Figure 1; Figure 3 is a partially cut-away, enlarged bottom view of the spatial light modulator for the printer shown in Figures 1 and 2; Figure 4 is a simplified block diagram of a system for applying differentially encoded input data samples to the electrodes of the modulator shown in Figure 3; Figure 5 is a spatial intensity diagram illustrating the image plane intensity summation of orthogonally-polarized sheared intensity profiles, whereby interpixel intensity nulls are suppressed in accordance with this invention; Figure 6 is an enlarged tangential plane view of a plane parallel birefringent shear plate embodiment of the spatial shearing mechanism for the printer shown in Figures 1 and 2; Figure 7 is an enlarged sagittal plane view of the embodiment shown in Figure 6; Figure 8 is a wave-vector diagram illustrating the spatial shear produced by the embodiment shown in Figures 6 and 7: Figure 9 is an enlarged tangential plane view of another plane parallel birefringent shear plate embodiment of the spatial shearing mechanism for the printer shown in Figures 1 and 2; Figure 10 is an enlarged sagittal plane view of the embodiment shown in Figure 9; Figure 11 is a wave-vector diagram illustrating the spatial shear produced by the embodiment shown in Figs. 9 and 10: Figure 12 is an enlarged tangental plane view of an optically-active birefringent crystal implementation of spatial shearing for the printer shown in Figures 1 and 2, and Figure 13 is a partial tangential plane view of a line printer having an angular shearing mechanism for carrying out an alternative implementation of this invention. Turning now to the drawings, and at this point especially to Figs. 1 and 2, there is an electrophotographic line printer 11 having an electrooptic image bar 12 for printing an image on a photosensitive record medium 13. As shown, the record medium 13 is a photoconductively coated drum 14 which is rotated (by means not shown) in the direction of the arrow 15. However other xerographic and non-xerographic recording media could be used, including photoconductively coated belts and plates, as well as photosensitive films and coated papers. The record medium 13, therefore, can be more generally described as being a photosensitive medium which is exposed while advancing across an image plane in a cross-line direction (i.e., the "sagittal direction") relative to the image bar 12. In keeping with accepted practices, the image bar 12 comprises an EO TIR spatial light modulator 16 and a Schlieren imaging system 17. The modulator 16 includes an optically-transparent electroöptic element 18, such as an optically-polished, y-cut crystal of LiNbO₃, and a plurality of individually-addressable electrodes 19 a -19 i (Figs. 3 and 4). As will be seen, the electrodes 19 a -19 i are located on, or closely adjacent, a longitudinal reflective surface 20 of the electroöptic element 18. For example, as in other modulators of this type, the electrodes 19 a -19 i may be integrated with their addressing and drive electronics on a VLSI silicon circuit 21, and the modulator 16 may then be assembled so that the electrodes 19 a -19 i are pressed firmly against the reflective surface 20. Typically, the electrodes 19 a -19 i extend lengthwise of the electrooptic element 18 (i. e., parallel to its optical axis) in mutually parallel alignment and are spaced apart laterally (i.e., in the "tangential direction") on essentially equidistant centers. An illuminator 24, such as a monochromatic laser, supplies a polarized input light beam 25 in (it typically is linearly polarized along the tangential direction) which is transversely collimated and, if necessary, expanded (by means not shown) to illuminate substantially the full width or tangential dimension of the electrooptic element 18. Means (also not shown) are provided for bringing the input beam 25 in to a wedge-shaped focus on the reflective surface 20 of the electrooptic element 18, thereby enabling the imaging system 17 to establish a relatively straightforward object/image relationship between the modulator 16 and the record medium 13 (i. e., the output image plane). As will be seen, the input beam 25 in is incident on the electrooptic element 18 at a near-grazing angle of incidence with respect to its reflective surface 20, so the input beam 25 in is totally internally reflected from the surface 20 to provide an output beam 25 out . To print a two-dimensional image on a line-by-line basis, successive sets of input voltage data samples are sequentially applied to the electrodes 19 a -19 i , thereby causing the modulator 16 to phase modulate the wavefront of the output beam 25 out spatially as a function of time in accordance with the pixel patterns for successive lines of the image. Preferably, the input data samples are applied to the electrodes 19 a -19 i in differentially-encoded form. As is known, differential encoding causes the voltage levels of the raw input data samples for any given line of an image to determine the sample-to-sample voltage transitions of the differentially-encoded data samples for that line. It, therefore, simplifies the construction of the modulator 16 by eliminating the need for ground or reference plane electrodes. For that reason, as shown in Fig. 4, there is an encoder 31 for differentially encoding the raw data samples on a line-by-line basis, a multiplexer 32 for rippling the encoded data samples on to the electrodes 19 a -19 i , and a controller 33 for synchronizing the encoder 31 and the multiplexer 32. The incoming or raw data samples usually are received as a serial stream at a predetermined synchronous data rate, so the encoder 31 and the multiplexer 32 generally are synchronized by the controller 33 to operate at that rate. Nevertheless, it will be evident that a suitable buffer (not shown) could be provided for temporarily storing either the raw data samples or the encoded data samples should it be found, for example, that a rate change is required or that asynchronous operation is desired. Returning to Figs. 1 and 2, the illustrated imaging system 17 comprises a field lens 36 and an imaging lens 37 for bringing the zero-order and the higher-order diffraction components of the spatially-modulated light beam 25 out to focus on an opaque stop 38 and on the record medium 13, respectively. The stop 38 is centrally located in the rear focal plane of the lens 36 (i. e., the exit pupil of the imaging optics 17), so it blocks the transversely-collimated, zero-order, components of the light beam 25 out , as indicated by the solid line rays in Fig. 2. However, the higher-order diffraction components (collectively represented by dashed line rays in Fig. 2) scatter around the stop 38 and then are collected by the imaging lens 37 to form an image of the modulator 16 on the record medium 13. This is an example of a central dark field system. It will be apparent however, that a central bright field Schlieren imaging system could be used to read out the spatial modulation produced by the modulator 16, although the phase-to-intensity conversion characteristics of the imaging system would be logically inverted. A polarization analyzer (not shown) could also be used, either alone or in combination with a Schlieren system, for imaging the modulator 16 when both the phase and the polarization of the output beam 25 out are spatially modulated. In accordance with the present invention, as illustrated in Fig. 5 by reference to the net effect, the spatially-modulated output radiation 25 out of the modulator 16 is spatially or angularly sheared into two orthogonally-polarized, redundantly-modulated, components 25 out-1 (l₁) and 25 out-2 (l₂) which are laterally offset from each other on the image plane 13 by a distance equal to approximately one half the nominal image plane pixel pitch of the image bar 12 (i. e., the pitch of its electrodes 19 a -19 i times the magnification occurring between the image bar 12 and the image plane 13). In the absence of the shearing, the coherent interference between the positive and negative diffraction components of the output beam 25 out would cause the printed pixels to have an image plane diameter, d, equal to approximately one half of the nominal image plane pixel pitch of the image bar 12 (see Fig. 5), thereby producing significant interpixel intensity nulls. In accordance with this invention, however, such interpixel intensity nulls are substantially eliminated, even if the image bar 12 is monochromatic, because the orthogonal polarization of the redundantly-modulated sheared components 25 out-1 and 25 out-2 causes them to sum with each other spatially on an intensity basis (l t ) when they are read out on the output image plane 13. As illustrated in Figures 1 and 2, the imaging optics 17 are telecentric, so there advantageously is a shearing mechanism 51 in the telecentric region between the modulator 16 and the imaging optics 17 for spatially shearing the modulated output beam 25 out . The telecentricity which exists in that region causes the spatial shearing mechanism 51 to have substantially the same effect on all points of the image, with the result that little, if any, distortion is introduced. As will be seen, the spatial shearing mechanism 51 may take a variety of forms. For example, as shown in Figs. 6 and 7, a tilted, plane parallel birefringent shear plate 51 a may be employed to provide a single-component mechanism for performing the spatial shearing. The shear plate 51 a is an uniaxial crystal, such a LiNbO₃ crystal, which is cut so that its optical axis (i. e., its z-axis) and it x-axis are coplanar with its face plates and are oriented at approximately + 45° and -45°, respectively, with respect to the tangential direction (i. e., the axis along which the spatially-modulated light beam 25 out is linearly polarized). Moreover, the shear plate 51 a is tilted about a tangentially-directed axis, so that a normal to the plate is deviated sagitally from the principal rays (i. e., the unmodulated or "zero-order diffraction" components) of the light beam 25 out by a predetermined tilt angle. As will be appreciated, the shear plate 51 a is essentially equidistant from the modulator 16 at all points across the wavefront of the modulated light beam 25 out , so the depth (sagittal dimension) of the space needed to accommodate the shear plate 51 a is substantially independent of the tangential dimension of the modulator 16. This is an important consideration, especially for relatively compact image bars and for image bars employing tangentially elongated modulators, such as full image width modulators. In operation, the crystal shear plate 51 a (i) decomposes the light beam 25 out into two more or less equally-intense, redundantly-modulated, field distributions 25 out-1 and 25 out-2 , which have ordinary and extraordinary polarizations within the shear plate 51 a , and (ii) produces a lateral (i. e., tangential direction) offset beween those field distributions which is determined by the birefringence and thickness of the shear plate 51 a and by the incident angle of the light beam 25 out . Minor adjustments to the angular orientation of the x- and z-axes of the birefringent shear plate crystal may improve the intensity balance of the two optical field distributions 25 out-1 and 25 out-2 . For example, In one case it was found that substantial balance was achieved by orienting the z-axis of a LiNbO₃ crystal shear plate at almost 46° with respect to the tangential direction. Additionally, as shown in Fig. 8, it was found the double refraction "beam walk-off", caused by the tilted incidence of the light beam 25 out on the optical axis of such a shear plate, compensates over a substantial range of tilt angles (at least from about 10° to about 17° of tilt) for the relative sagittal displacement the sheared field distributions 25 out-1 and 25 out-2 would otherwise experience with a birefringent shear plate tilted about a tangential direction axis. Thus, the tilt angle may be adjusted to optimize the shear displacement. Similar self-compensating shear plates can be formed from uniaxial birefringent crystals with different birefringence values. To that end, it is noted that the compensation which is achieved is attributable to the superposition of the relative displacements caused (1) by the different refractions of the ordinary and extraordinary wavenormals because of the birefringence of the crystal, and (2) by the "walkoff" of the extraordinary polarization within the crystal which causes its principal ray to deviate from its wave normal. Those superposed effects vectorally sum with each other, so the direction of the net relative displacement may be controlled by careful selection of the magnitude and direction of each of the effects. The thickness of the shear plate 51 a , on the other hand, maybe increased and decreased to increase and decrease, respectively, the relative lateral offset of the field distributions 25 out-1 and 25 out-2 as required to accommodate modulators having different electrode pitches inasmuch as their displacement is proportional to the plate thickness. Alternatively, as shown in Figs. 9 and 10, a shearing mechanism 51 b comprising a half-wave plate 61 and a plane parallel birefringent shear plate 62 may be located in the telecentric region between the modulator 16 and the imaging optics 17 for spatially shearing the modulated output beam 25 out . The half-wave plate 61 is disposed between the modulator 16 and the shear plate 62 and has its optic axis angularly oriented and about 22.5° with respect to its tangential axis, so it rotates the polarization plane of the output beam 25 out to an angle of approximately 45° with respect to the tangential direction. The shear plate 62, on the other hand, is cut from a LiNbO₃ crystal or the like, so its optic (z) and x-axes lie in its face planes and are oriented along the tangential and sagittal directions, respectively. The shear plate 62 spatially shears the output beam 25 out into more or less equally-intense, orthogonally-polarized, redundantly-modulated field distributions 25 out-1 and 25 out2 . To perform that function, the shear plate 62 is tilted about a sagittally-directed axis which is normal to a plane containing its optical axis and the tangential axis of the modulator 16 (i.e., the axis of symmetry of the modulated light beam 25 out ). As a result, the relative displacements caused by the birefringence of the shear plate 62 and by the "beam walkoff" of the extraordinary polarization are both tangentially directed (Fig. 11). If desired, a shear plate not having the "walkoff" effect could be produced by interchanging the x- and z-axes of the shear plate 62. Again, minor adjustments may be made to the angular orientation of optic axis of the half-wave plate 61 to balance the intensities of the optical field distributions 25 out-1 and 25 out-2 more precisely, and the tilt angle of the shear plate 62 may be adjusted to optimize the shear separation. Also, the thickness of the shear plate 62 may be increased or decreased to increase or decrease, respectively, the lateral offset between the sheared field distributions 25 out-1 and 25 out-2 because the magnitude of the shear essentially varies linearly therewith. Still another spatial shearing mechanism is shown in Fig. 12 where there is an optically active uniaxial crystal 51 c , such as a quartz crystal, in the telecentric region between the modulator 16 and the imaging optics 17. The crystal 17 has its optic or z-axis tangentially aligned with the optical axis of the image bar 12, but sagittally tilted at a predetermined angle with respect thereto. Thus, the crystal 51 c decomposes the spatially-modulated light beam 25 out into two laterally-offset, redundantly-modulated optical field distributions 25 out-1 and 25 out-2 which have orthogonal elliptical polarizations. In this case, the relative lateral offset of the field distributions 25 out-1 and 25 out-2 is determined by the circular birefringence and the tilt angle of the crystal 51 c , and is proportional to its thickness. While spatial shearing of the modulated light beam 25 out in the telecentric region of the image bar 12, as above-described, has the advantage of producing essentially the same amount of relative lateral offset between the decomposed field distributions 25 out-1 and 25 out-2 at all points across the full width or tangential dimension of the image bar 12, angular shearing is another practical alternative for achieving the broader goals of this invention. Thus, as shown in Fig. 13, there is a birefringent prism 71 which is located in or near the aperture plane of the imaging optics 17(i.e., the plane of the stop 38) for angularly shearing the spatially-modulated light beam 25 out to produce two more or less equal-intensity, orthogonally-polarized, redundantly-modulated components 25 out-1 and 25 out-2 which are sagitally aligned but angularly offset from each other tangentially by a predetermined offset angle. Thus, the relative lateral offset of the decomposed components 25 out-1 and 25 out-2 is not uniform at all points across the image plane 13, but the non-linear distortion that is introduced because of that non-uniformity may be minimized by selecting the prism 71 so that it produces the desired amount of relative offset at approximately the tangential midpoint of the output image, The present invention provides methods and means for suppressing interpixel intensity nulls in the output images of coherent image bars, including monochromatic image bars, without reducing their effective spatial modulation bandwidth or degrading their resolution. In other words, achromatic null suppression is achieved while employing the full aperture of the imaging optics, so there is no significant loss of single pixel contrast or of pixel edge sharpness.

Claims: Optical apparatus (11) including a coherent optical image bar (12) for converting a spatial input pattern into a corresponding output image, the image bar including: a modulator (16) for spatially modulating a polarized coherent light beam in accordance with a spatial input pattern to provide a spatially-modulated output beam; imaging optics (17) for reading out the spatial modulation of the output beam on to an output image plane as an array of individual pixels, the image bar having a predetermined nominal image plane pixel pitch; and a shearing device (51, 51a, 51b, 51c, 71) interposed between the modulator and the image plane for shearing the output beam into a pair of laterally-offset, redundantly-modulated and orthogonally-polarized pixel distributions of substantially equal intensities, whereby the two distributions sum with each other on an intensity basis on the output image plane to form the image; characterised in that the shearing device is a birefringent shearing device adapted to offset said pixel distributions from each other on the output image plane, by approximately half the nominal image plane pixel pitch of the image bar, the pixels having image plane diameters equal to approximately half the pixel pitch, whereby the image is essentially free of interpixel intensity nulls. The apparatus of Claim 1, wherein the modulator comprises an electrooptic element (18) and a plurality of individually-addressable electrodes (19a, 19b, 19c, 19i) located on a flat surface (20) of the electrooptic element, the electrodes extending lengthwise of the electrooptic element in mutually parallel alignment, and being spaced apart laterally in a tangential direction, the modulator being illuminated by the polarized light beam substantially perpendicularly to the tangential direction. The apparatus of Claim 1 or 2, wherein the imaging optics make up a telecentric lens system, and in which the birefringent device is a spatial shearing mechanism disposed between the modulator and the imaging optics. The apparatus of Claim 3, wherein the birefringent device comprises a uniaxial birefringent crystal (62). The apparatus of Claim 4, wherein the crystal has orthogonal axes, of which one is the optical axis, and parallel input and output faces which contain the optical axis and the other axis. The apparatus of claim 2 or claim dependent therefrom, wherein the input beam is linearly polarized along the tangential direction. The apparatus of Claim 6, wherein the optical axis and the other axis of the crystal are oriented at approximately ±45°, respectively, with respect to the tangential direction in which the incident beam is polarised, whereby the crystal decomposes the incident beam into two substantially equally intense, redundant-modulated, diffracted components having ordinary and extraordinary polarizations within the crystal, and wherein the crystal is tilted a predetermined angle about a tangentially-directed axis with respect to the incident beam, whereby the incident beam strikes the optic axis of the crystal at a predetermined tilt angle selected to align the diffracted components sagittally with each other. The apparatus of Claim 7, wherein the crystal is substantially parallel to the modulator in the tangential direction. The apparatus as claimed in claim 5 wherein: the optical axis and the other axis of the crystal are aligned in parallel direction and a normal direction, respectively, with respect to the plane defined by the output beam and the tangential direction; the light beam is linearly polarized along a predetermined one of the directions; the birefringent device further includes a half-wave plate (61) disposed between the modulator and the crystal, the half-wave plate having an optical axis oriented in a plane perpendicular to the output beam at approximately 22.5° with respect to the normal direction, whereby the half-wave plate rotates the polarization of the output beam by approximately 45°, and the crystal has its input and output faces tilted a predetermined angle about an axis along the normal direction. The apparatus of Claim 8 or 9, wherein the modulator has an axially-extending reflective surface, and the input beam strikes the reflective surface at a near-gazing angle of incidence, whereby the light beam is totally internally reflected from the reflective surface to cause the modulator to operate in a TIR mode.

Inventors: FLORES, L. PRASADAM, HECHT, DAVID L., SPRAGUE, ROBERT A.
Applicants: XEROX CORPORATION
Application year: 1986
Main classification: G02F1
IPC codes: G02F1, G06K15