Electro-optic modulator and imaging device

An imaging device comprising an electro-optic modulator for modulating incident laser beams; an array of adjacent electrodes comprising a group of imaging electrodes addressed in accordance with image information; means to illuminate an area slightly larger than the width of said imaging electrodes; and one or more additional electrodes located on each side of said group of imaging electrodes to permanently direct extraneous marginal beams to light-stop means so that only the light emerging from the imaging electrodes is allowed to reach a light sensitive medium.

BACKGROUND OF INVENTION
 1. Field of the Invention
 This invention generally relates to the exposure of light on a
 light-sensitive medium involving a spatial modulator to produce successive
 columns of individually controlled light spots and particularly relates to
 electro-optic modulators used in an imaging device for modulating incident
 light beams, which light is then allowed to reach the light-sensitive
 medium.
 2. Description of the Prior Art
 The electro-optic effect, in general, permits extremely rapid and direct
 modulation of a light phase front with an electronic drive signal.
 Various types of electro-optic modulators have been proposed, such as
 described, for example, in U.S. Pat. Nos. 4,281,904, 4,316,196, 4,804,251,
 and 4,746,942.
 According to U.S. Pat. No. 4,281,904, a TIR (total internal reflection)
 type of electro-optic device which has each electrode individually
 addressed is utilized. The operation of a TIR modulator depends on the
 effect of applying a voltage to a symmetrical electrode pattern to induce
 a change of the refractive index in an electro-optic element in the region
 of the surface of the element where the light is totally internally
 reflected. The electrode pattern is deposited on the surface of the
 element as an array with the electrodes being arranged parallel to the
 incident light beam. A voltage is applied to the electrode pattern and
 induces an electric field adjacent to the surface which alters the
 refractive index of the element. Thus, incident phase fronts are modulated
 by the TIR modulator to produce modulated light phase fronts. The
 electrodes within the electrode pattern are selectively activated in
 accordance with the desired image pattern.
 TIR modulators are also used in U.S. Pat. No. 4,639,073 issued to Yip et
 al. and U.S. Pat. No. 4,554,561 issued to Daniele et al.
 Another electro-optic modulator is the PLZT modulator, which is shown, e.g.
 in U.S. Pat. No. 4,746,942 to Moulin and U.S. Pat. No. 4,316,196 to
 Jacobs. The PLZT modulator has a plurality of interleaved electrodes,
 which, together with a crossed polarizer, forms an array of very small
 light gates. If a voltage is applied to the electrodes of the PLZT
 modulator, an electric field is created thus shifting the relative phases
 of light polarized parallel and perpendicular to the applied field. The
 plane of polarization of light transmitted to the zones between the
 electrodes is rotated upon the application of proper voltages to the
 electrodes.
 Hence, electro-optic modulators are used to produce successive columns of
 individually controlled light spots. Images are produced on the
 light-sensitive medium by a succession of adjoining bands of spots to
 produce text and graphics on a film, a printing plate or other medium on
 which images are to be produced.
 In order to avoid any noticeable discontinuity between adjacent bands, it
 is not only necessary that the relative displacement of the bands and the
 light-sensitive medium exactly correspond to the size of a column of
 spots, but also that all the spots be substantially identical in form and
 intensity. In addition, it is preferred that only the light emerging from
 the independently selected spot-producing elements of the electro-optic
 modulator reach the light-sensitive medium at the imaging plane.
 To achieve desired uniformity between selected spots, all the selectable
 elements or gates of the modulator must be uniformly illuminated. This can
 better be achieved by illuminating an area larger than the zone occupied
 by the selectable modulator elements in order to compensate for the
 decrease in intensity of the incident light at the edges of the light
 phase front. It is then desirable to prevent the extraneous radiation
 overlapping said zone because of misalignment or for other reasons from
 reaching the light-sensitive medium.
 In general, multi-electrodes modulating systems associated with a light
 sensitive medium for imaging do not allow light (or other radiation) to
 reach the medium in the absence of energizing selected electrodes. In
 these systems, the light intensity of the spots reaching the medium is
 obtained by rays that have incurred a loss of energy caused by the
 modulating system as they pass through the modulator material. They can
 generally produce good image contrast, but at the expense of efficiency.
 Such systems may include deformable mirrors, crossed polarizers,
 deflection by diffraction. For imaging supports requiring higher radiant
 energy such as heat-sensitive polymer printing plates, it is desirable to
 lose as little energy as possible through the modulator, even at the
 expense of a loss of contrast. This can be achieved by letting light
 beams, unimpeded by the modulator, reach the sensitive medium. In this
 approach, the modulator electrodes are normally inactivated to allow all
 the energy from the incident light beams to reach the medium. Any
 activated electrode will block the beam it controls. In other words, all
 the electrodes are activated when no light should reach the medium. In
 this alternative, the illuminated area reaching the modulator should be
 exactly confined to the imaging electrode area of the modulator to avoid
 the influence of leakage of marginal rays that would expose the
 light-sensitive medium. The uniformity in illumination or exposure of the
 light-sensitive medium would be negatively affected by these marginal rays
 reaching the active zone of the medium at its edges.
 The insertion of a mask to limit the illumination to the active zone,
 although simple in appearance, presents difficulties of implementation and
 the marginal rays adjacent to the ends of the imaging zone are affected by
 diffraction by the mask edges.
 SUMMARY OF THE INVENTION
 One object of the invention is to provide an imaging device and method for
 eliminating edge effects in spatial modulators.
 Another object of the invention is to provide an electro-optic modulator
 which is highly efficient, results in uniform images on the
 light-sensitive medium and yet is relatively inexpensive.
 It is another object of the invention to provide an improved electro-optic
 modulator and method for modulating light.
 It is also an object of the invention to improve the performance and
 utility of electro-optical modulators.
 The present invention seeks to overcome the foregoing drawbacks by
 providing an electro-optic modulator comprising electronic masking means
 to prevent stray light rays from reaching the light sensitive media. The
 provision of the masking means at or inside the modulator minimizes the
 distance of the masking means to the plane of modulation. Having the
 masking means in the same or close to the same plane as the electrodes
 eliminates any diffraction effect that may be caused by having mechanical
 masking means upstream from the modulator.
 In an embodiment according to the present invention, said masking means is
 a permanent mask introduced into the modulator. This permanent mask can be
 a sheet or film or the like for masking extraneous light rays.
 According to a preferred embodiment, the masking means comprises one or
 more additional electrodes located on one or both sides of the imaging
 electrodes of the modulator. The additional electrodes permanently direct
 extraneous marginal beams to light-stop means so that only the light
 emerging from the imaging electrodes is allowed to reach the
 light-sensitive medium. This arrangement has the further advantage that
 the masking means in the form of the additional electrodes is in the same
 plane as the imaging electrodes. Therefore, any diffraction effects are
 avoided. The manufacturing of this arrangement is easy and inexpensive
 since the imaging electrodes and the additional electrodes can be
 manufactured in one step in one layer of the modulator.
 It is also preferred that the modulator be an electro-optic modulator such
 as a TIR modulator or a PLZT modulator.
 From another aspect thereof, the subject invention resides in an imaging
 device comprising an electro-optical modulator for modulating incident
 light beams; said device comprising an array of adjacent electrodes
 comprising a group of imaging electrodes addressed in accordance with
 image information; means to illuminate an area slightly larger than the
 width of said imaging electrodes; and one or more additional electrodes
 located on the modulator on one or both sides of said group of imaging
 electrodes to permanently direct extraneous marginal beams to light-stop
 means so that only the light emerging from the imaging electrodes group is
 allowed to reach the light-sensitive medium.
 In another aspect of the present invention, the imaging method of recording
 information on a light-sensitive medium comprises the steps of forming a
 light beam, projecting the light beam into a modulator comprising a group
 of imaging electrodes and masking means, wherein the light beam is
 projected into the modulator such that the illuminated area is larger than
 the area occupied by the imaging electrodes, selectively energizing the
 imaging electrodes in accordance with the desired image, and directing the
 modulated light beams emerging from the modulator to the light sensitive
 medium.
 In the present specification, "light" is typically, but without limitation
 UV, visible or IR radiation. Other objects, features and advantages of the
 invention will be apparent to those skilled in the art upon review of the
 following detailed description and drawings which show by way of
 illustration, and not limitation, preferred embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In the following, the term "height" describes the length of a column of
 dots produced by the assembly of imaging electrodes and the term "width"
 describes the thickness of a slice of dots generally of the order of 1
 micron on the medium.
 FIGS. 1 and 2 of the drawings shows a schematic representation of an
 assembly according to a first preferred embodiment of the present
 invention. According to this embodiment, a PLZT modulator is utilized. The
 collimated rays 1 of light incident on the modulator 5 are first blocked
 at both sides of the illumination zone by means of a mask 2. The remaining
 collimated rays pass the lens 3 and are thus directed to the surface of
 the modulator 5. Other suitable means to direct the remaining light to the
 surface of the modulator, such as mirrors or the like will be apparent to
 those skilled in the art. The modulator 5 blocks selected ones of the
 light rays, as it will be explained in greater detail below. The beams of
 light 10 emerging from the modulator pass a lens 9 or other similar means
 as necessary and a polarizer 12 and finally reach an objective 11. On the
 other side of the objective, the image 14 can be obtained.
 The modulator 5 comprises a first group 6 of adjacent electrodes which are
 the imaging electrodes. On each side of the modulating imaging group of
 electrodes 6, additional electrodes 8 are located causing extraneous light
 rays 15 to be blocked downstream. The electrodes 6 and 8 are preferably
 arranged parallel to each other and, adjacent to these electrodes, a
 common electrode 7 is provided. The additional electrodes 8 are connected
 to a voltage source 26 in order to energize the electrodes with a voltage
 high enough to block the extraneous light rays 15. The group of imaging
 electrodes 6 is supplied with control voltages via a driver circuit 13.
 Thus, each of the imaging electrodes can be controlled or selected
 independently from the other imaging electrodes, thus allowing control of
 the modulator in accordance with the desired image.
 FIG. 2 schematically shows how the extraneous light rays 15 are blocked.
 The thin sheet-like bundle of rays 1 produced by a laser and associated
 optics is first limited by a mask 2 to a width l.sub.m but still covers
 more than the total width of the imaging electrodes. Thus, not only the
 useful imaging zone defined by the imaging electrodes 6 is illuminated but
 also a certain area on both sides of the imaging electrodes where the
 additional electrodes are provided. It illuminates an area extending
 beyond the width l.sub.u of imaging electrodes group 6 by overlapping rays
 covering section e on each side of the imaging electrodes. The thickness
 of the bundle of rays falls within the thickness of the modulator elements
 but its width extends beyond the width of their assembly represented by
 l.sub.u +2e. On each side of the modulating imaging group of electrodes 6
 are located additional electrodes 8 to cause extra marginal light rays 15
 to be blocked downstream. Due to this structure, an area larger than the
 zone occupied by the imaging electrodes is illuminated resulting in a
 uniform illumination of the imaging electrodes of the modulator. The
 portion of the light phase front reaching the light-sensitive medium has a
 substantially uniform intensity whereas the edges of the light phase front
 illuminate the additional electrodes. This resulting extraneous radiation
 is however prevented from reaching the light-sensitive medium by the
 provision of the additional electrodes.
 In the case where a PLZT modulator is used, the added electrodes 8 are
 connected to a common voltage control 26 (FIG. 5). The field induced by
 this voltage interact with the inputted radiation to block its passage
 beyond the useful imaging zone. This is represented in FIG. 2 where it is
 shown that the rays 15, emerging from the electrodes 8 are blocked by
 polarizer 12 at location 12' independently of the operation of imaging
 electrodes 6.
 FIG. 3 illustrates a second embodiment of the present invention utilizing a
 TIR modulator. These modulators are well-known in the art, they operate to
 selectively deflect or bend high intensity beams from the laser. They
 depend on the effect of applying a voltage to an electrode pattern to
 induce change of the refractive index in an electro-optic element in the
 region of the surface of the element where the light is totally internally
 reflected. The emerging beams are diffracted into a series of orders. In
 general in images based on this system, rays of zero or low orders are
 prevented from reaching the light sensitive medium in the recording plane
 by a stop. The higher orders are focussed to form an image of the selected
 spots. In the preferred embodiment of the invention, only the zero order
 beams are allowed to reach the recording plane with a minimal loss of
 energy. To obtain blank areas at the imaging plane, the electrodes
 corresponding to these areas are activated, thus causing practically all
 the light energy emerging from the modulator to be concentrated in the
 higher order of the diffracted beams which are prevented from reaching the
 recording plane by a mask.
 Referring to FIG. 3, the collimated light beams emerging from a laser and
 associated optics to form a sheet-like bundle are shown at 1. Their width
 limited by baffles 2, is large enough to fill the full width of the
 modulator 16. The electrodes of the modulator are divided into a first
 group of imaging electrodes individually subjected to voltage variations
 for the projection of individually selected light spots located in zone 22
 (FIG. 5), and a second group of electrodes located in zones 21 and 21' on
 each side of the first group, permanently energized through common circuit
 26 in order to prevent extraneous "noise" rays extending beyond the
 imaging electrodes to reach the imaging plane. These rays shown at 23,
 after emerging from field lens 28, are blocked by mask 24 located at the
 focus of the field lens. This results in a uniform illumination of the
 imaging electrodes without allowing extraneous light rays to reach the
 light-sensitive medium.
 FIG. 4 illustrates in more detail how the extraneous light rays are blocked
 according to the present invention. A bundle of rays 32 is obtained by
 blocking the sheet-like bundle of light rays 30 by means of mechanical
 blocking means, such as a mask 2. The additional electrodes provided at
 the modulator 34 further reduce the illumination zone, so that only the
 light rays indicated by reference numeral 36 reach the imaging electrodes.
 According to the invention, the imaging and/or masking electrodes may be
 shaped and located as shown in FIG. 5. Other acceptable configurations
 will be apparent to those skilled in the art. One arrangement of the
 electrodes are shown, e.g. in U.S. Pat. No. 4,746,942, incorporated herein
 by reference. The electrodes are joined into two conducting blocks each
 comprising a plurality of electrode fingers or arms (6,6', 8,8'). The arms
 (6,8')of one block of electrodes are interleaved between adjacent arms
 (6',8') of the other block. The arms of the conducting blocks are divided
 into the imaging group 22 thus comprising two sets of adjacent electrodes,
 and the two sets of masking electrodes 21, 21' provided on both sides of
 the imaging group. The electrode arms 8 of the masking group are directly
 connected to a common voltage control 26, whereas control means, such as
 switches 27, are provided at the electrodes of the first set of electrodes
 of the imaging group. Thus, each electrode of the first set can be
 separately supplied with the control voltage.
 It is to be understood that the exemplary embodiments in no way limit the
 scope of the invention. Other modifications of the invention will be
 apparent to those skilled in the art in view of the foregoing
 descriptions. Accordingly, the invention is not limited to the described
 embodiments and all alternative modifications and variations of the
 present invention which fall within the spirit and scope of the appended
 claims are covered.