Laminar electrooptic assembly for modulator and printer

An electrooptic assembly for a light modulator, suitable for use in a printer, is constructed as a sequence of film-type layers of electrooptic material, particularly PLZT, supported upon a transparent substrate and having comb-electrode structures disposed as electrode layers at all of the interfaces between the layers of electrooptic material. The electrooptic layers and the electrode layers are readily fabricated by photolithography. A polarizer and an analyzer with crossed polarization axes are disposed on opposite sides of the electrooptic assembly along an optical path to provide, with the electrooptic assembly, an electrooptic modulator. Light passes via the optical path through the modulator upon energization of electrodes with an electric field transverse to a common direction of the electrodes. For use of the light modulator in a printer, a print-medium transport is provided to impart relative motion between the print medium and the optical path, the motion being in a direction perpendicular to a longitudinal dimension of comb electrode structures in the various electrode layers.

BACKGROUND OF THE INVENTION 
This invention relates to electrooptic modulation of a light beam to 
produce a line array of data suitable for imprinting pixels of an image on 
a print medium. More particularly, the invention provides a laminar 
assembly of comb electrode layers interleaved among layers of electrooptic 
film material wherein reduced interelectrode spacing lowers required 
voltage, and multiple arrays of electrodes produce an adequately long 
optical interaction region along an optical path. 
Electrooptic modulators have been employed for modulating beams of light. 
One form of modulator of considerable interest employs a line array of 
electrodes in the form of a comb electrode structure wherein sequential 
ones of the electrodes are provided with voltages from a suitable source 
of signal voltages to induce corresponding electric fields within the 
electrooptic material. A beam of light, incident upon the electrooptic 
material passes first through a polarizer which orients the electric 
vector of the incident light in a direction inclined at 45 degrees 
relative to the direction of the comb electrodes. In the construction of 
the modulator, the electrodes are spaced apart sufficiently to provide for 
a fringing field which extends in a plane transverse to the electrodes, 
the plane containing an optical path along which the light propagates. The 
term "light", as used herein, includes not only the visible portion of the 
spectrum, but includes also infrared and ultraviolet portions of the 
spectrum in those situations wherein the polarizing material and the 
electrooptic material is responsive to these portions of the 
electromagnetic spectrum. 
Light propagating through a region of the electrooptic material activated 
by the electric field experiences a change of polarization which develops 
in the following manner. The initially polarized wave can be regarded as 
having a component of electric field oriented perpendicularly to the 
electrodes, and a second component of electric field which is oriented 
parallel to the electrodes. In the absence of the applied electric field, 
both components of the incident optical signal propagate with the same 
speed of propagation along the optical path through the electrooptic 
material. However, in the presence of the applied electric field in the 
plane transverse to the electrodes, the component of the optical signal 
having its electric field in the transverse direction experiences a 
reduction in propagation speed relative to the portion of the optical 
signal having its electric field parallel to the electrodes. As the two 
components of the optical signal propagate through the interaction region, 
the differences in speeds of propagation introduce a continually 
increasing phase shift between the two optical components. 
The amount of phase shift experienced depends on the length of the 
interaction region, the wavelength of the light in the material, and upon 
the strength of the applied electric field up to a maximum value wherein 
the electrooptic effect saturates. The length of the interaction region 
and the magnitude of the applied electric field may be selected to produce 
a relative phase shift of 180 degrees between the two components of the 
optical signal in propagation through the interaction region. The result 
is a polarization of resultant optical signal which is perpendicular to 
that of the incident polarization. A second polarizing unit, or analyzer, 
is positioned at the output side of the electrooptic material, and is 
oriented at 90 degrees relative to that of the input polarization. 
Thereby, the outputted signal propagates through the analyzer in the 
presence of the applied transverse electric field to the electrodes. In 
the absence of the applied electric field, the exiting optical signal has 
a polarization perpendicular to that of the analyzer, and is blocked by 
the analyzer. Alternatively, it will be well understood that the analyzer 
may be parallel to the polarizer in which case the modulator will be light 
transmissive "open" when unexcited and light blocking "closed" when 
excited. 
It has been found useful to construct light modulation devices with 
electrooptic modulators of the foregoing form. By activating successive 
pairs or groups of electrodes to establish a presence or absence of 
electric field independently for each of the pairs or groups of 
electrodes, it is possible to establish a line array of pixels wherein 
individual ones of the pixels are characterized by the presence or absence 
of light exiting the modulator. 
A problem arises in the use of the foregoing arrangement of electrodes for 
directing the exiting light of the modulator upon a print medium, so that 
print markings appear on the print medium in response to the presence and 
absence of light of the various pixels which are to be printed. The 
effective length of the interaction region is related directly to the 
spacing between the electrodes. This is apparent from the geometry of the 
fringing electric field. In the case of a comb electric structure disposed 
directly upon the surface of a block of electrooptic material, the useful 
portion of the fringing field extends into the electrooptic material a 
distance approximately equal to the interelectrode spacing. In order to 
ensure an adequate strength of electric field throughout the interaction 
region, the electrodes must have a correspondingly large signal voltage of 
hundreds of volts. Such large voltage is inconvenient to generate by 
signal generators, requires specialized circuitry, and may cause 
electrical breakdown between electrodes. A further disadvantage is the 
relatively large size of the electrode array, particularly with respect to 
the size of pixels which are to be developed on a print medium. 
SUMMARY OF THE INVENTION 
The aforementioned problems are overcome and other advantages are provided 
by an electrooptic assembly for a light modulator suitable for use in a 
printer wherein, in accordance with the invention, the electrooptic 
assembly is constructed of a set of film-type layers of electrooptic 
material disposed on a transparent substrate, and wherein multiple 
electrode layers are included in the assembly with one electrode layer 
being disposed at each interface between adjacent electrooptic layers. In 
this way, an optical interaction region in the electrooptic material is 
provided with a desired length by a sequence of electrode structures 
disposed along a light propagation path. Thus, each electrode array need 
provide only a relatively small fraction of the applied electric fringing 
field for the interaction region. With closely spaced electrode layers, 
the electric field lines are substantially parallel, similar to the field 
of a parallel plate capacitor. Accordingly, the spacing between sequential 
electrodes in each electrode layer can be made much smaller than has been 
possible heretofore. Also, there can be a corresponding reduction in 
applied voltage between sequential electrodes in each electrode layer. In 
each layer, the electrode structure is a comb structure wherein, in a 
preferred embodiment of the invention, alternate electrodes are grounded 
and the intermediate electrodes operate at signal voltage. Thereby, both 
physical size of each comb electrode structure and the applied voltage are 
reduced to provide important benefits, both in terms of electrical drive 
circuitry and in terms of providing high-resolution small-pixel imprinting 
on a print medium. 
In each of the comb electrode structures, the electrodes are parallel to 
each other. Also, the electrodes of one layer are parallel to the 
electrodes of the comb structures of other ones of the layers. Both the 
film-type layers of electrooptic material and the electrode structures are 
readily fabricated by photolithography. This mode of construction removes 
restrictions on the thickness of the various electrooptic layers because, 
even if a layer need be made thin for dimensional stability, one need only 
add additional layers of electrodes to obtain the desired amount of 
electrooptic material for a sufficiently long interaction region. This 
provides convenience of assembly and accuracy in the locating of the 
electrodes. A suitable electrooptic material is hot-pressed lanthanum lead 
zirconate-titanate (PLZT). The substrate may be fabricated of sapphire, 
fused silica or Pyrex glass. Construction of the light modulator further 
comprises a polarizer and an analyzer with the electrooptic assembly 
disposed therebetween. A suitable signal generator is applied to all of 
the electrodes of all of the electrode layers for energizing the 
electrodes with image data which may be obtained in real time from a 
scanner of an image or from data which has been stored previously in a 
memory. For use of the modulator in a printer, apparatus is provided for 
producing relative motion between a print medium and the light-propagation 
path, the motion being, for example, in a direction parallel to the 
direction of the electrodes and perpendicular to the long dimension of 
each comb electrode structure. 
Upon application of light from a suitable source of light which may include 
a collimating lens for providing parallel rays of the light, the polarizer 
polarizes the light beam to provide the transverse and parallel components 
of the optical signal which undergo different speeds of propagation 
through the electrooptic material upon application of the applied electric 
field transverse to the electrodes. The resulting light exiting the 
electrooptic material is polarized in a direction perpendicular to that of 
the polarizer for those pixels having an applied electric field. Thus, 
light exiting the electrooptic material is passed or blocked by the 
analyzer depending on the presence or absence of the applied electric 
field. Photosensitive material of the print medium reacts to the presence 
of light to produce markings on the print medium corresponding to the 
activation of the various pixels. The invention provides an additional 
manufacturing benefit in that there is no longer a requirement for 
polishing and lapping surfaces of bulk electrooptic material to provide 
optically smooth surfaces to the bulk material as had been required by 
electrooptic light modulators of the prior art.

DETAILED DESCRIPTION 
The invention relates to a construction of an electrooptic assembly of 
multiple comb-electrode layers interleaved among electrooptic film layers. 
Inclusion of a polarizer and an analyzer disposed on opposite sides of the 
electrooptic assembly provides a light modulator suitable for printing a 
line of pixels of an image simultaneously upon a print medium. In the 
ensuing description, details in the construction of the electrooptic 
assembly are described with reference to FIGS. 1-4. Description of the 
completed light modulator in conjunction with printing apparatus is 
provided with reference to FIG. 5. 
With reference to FIGS. 1 and 2, a laminar electrooptic assembly 10 
comprises multiple electrooptic layers 12 interleaved among multiple 
electrode layers 14 in the form of a stack 16 supported by a transparent 
substrate 18. Each of the electrode layers 14 is positioned along an 
interface between two electrooptic layers 12. Each of the electrode layers 
14 comprises a comb electrode structure 20 including a first comb array 22 
of electrodes 24 and a second comb array 26 of electrodes 28. In the first 
comb array 22, the electrodes 24 are joined together electrically by a 
spine 30 which places all of the electrodes 24 at the same electric 
potential. The spine 30 also serves as a suitable pad for making 
electrical contact with an external electrical circuit, to be described 
hereinafter. In the second comb array 26, each of the electrodes 28 is 
provided with its own individual pad 32 by which electrical contact is 
made with the external electrical circuit. The comb electrode structure 20 
may be formed of electrically conductive material suitable for deposition 
by photolithography such as copper, aluminum, tin, gold, indium-tin oxide, 
or polysilicon. 
In accordance with the invention, the electrooptic material of each of the 
electrooptic layers 12 comprises lanthanum lead zirconate-titanate (PLZT) 
having a chemical composition given by the following formula: 
EQU Pb.sub.1-x La.sub.x (Zr.sub.y Ti.sub.z).sub.1-x/4 O.sub.3 
The composition of the PLZT material commonly used for electrooptic 
modulators has a value of y=0.65 and z=0.35. In the lanthanum 
concentration, x varies from 8.5 to 9.5 atomic percent. This composition 
range affords a reasonably slim hysteresis loop, and a reasonably strong 
quadratic transverse electrooptic effect. The difference in speeds of 
propagation of electromagnetic waves in the electrooptic material, for 
waves having electric fields parallel to and perpendicular to an applied 
external electric field, is proportional to the square of the magnitude of 
the applied electric field; hence, the use of the foregoing descriptive 
term "quadratic". 
The PLZT material is a ceramic which, on a microscopic scale, has 
crystalline properties but, on a macroscopic scale, is more like a 
mixture. The electrooptic material is initially isotropic, but becomes 
anisotropic upon excitation with an external electric field. Upon 
impressing the electric field through the electrooptic material, an 
electromagnetic wave propagating through the material normal to a plane 
containing the impressed electric field experiences a difference in speed 
of propagation such that a component of the wave polarized parallel to the 
external field is retarded in its propagation relative to a component of 
the wave which is polarized perpendicular to the external electric field. 
The individual electrooptic layers 12, as well as the individual electrode 
layers 14 are built up successively upon the substrate 18 by processes of 
deposition and photolithography. 
FIGS. 3 and 4 show sectional views of two different arrangements of the 
layers in the stack 16 of FIG. 1. In FIG. 3, the top layer is an 
electrooptic layer 12, and the bottom layer which is contiguous to the 
substrate 18 is an electrooptic layer 12. In FIG. 4, the top layer is an 
electrode layer 14, and the bottom layer which is contiguous to the 
substrate 18 is an electrode layer 14. The electrode layers 14 in each of 
FIGS. 3 and 4 are indicated diagrammatically by three electrodes of which 
the center electrode is an electrode 28 of the second comb array 26 of 
FIG. 1, and the two outer electrodes are electrodes 24 of the first comb 
array 22 of FIG. 1. Upon establishment of a difference of potential 
between the electrodes 24 and 28, electric fields are developed, the 
electric fields being indicated vectorially by means of field lines 34 
extending between the electrodes 28 and 24 in directions perpendicular to 
the electrodes 28 and 24. The field lines 34 spread apart with increasing 
distance from the electrodes 28 and 24 in the manner of a fringing field 
so as to occupy all of an interaction region 36 surrounding an optical 
path 38 within the stack 26 of the electrooptic layers 12. As shown in 
FIGS. 3 and 4, the configuration of the fringing fields along the optical 
path 38 about an electrode layer 14 produces substantial parallelism among 
the electric field lines 34 and reduction in length of the field lines 34 
upon a reduction in the spacing between successive electrode layers 14, 
particularly for spacings significantly less than the interelectrode 
spacing in any one electrode layer 14. Thereby, the voltage applied 
between electrodes, such as the electrodes 24 and 28 is reduced greatly 
from that employed in the prior art to achieve the same half-wave 
transformation of the transmitted light. 
FIG. 2 shows electrode dimensions employed in a preferred embodiment of the 
invention. Each of the electrodes 24 and 28 has a width, indicated as 
dimension A, of 10 microns. The interelectrode spacing between an 
electrode 24 and the next electrode 28, indicated as dimension B, is 40 
microns. The comb electrode structure 20 has a periodic form. With the 
foregoing dimensions, the repetition period of the electrode structure 20 
is 100 microns. Thus, each pixel extends 100 microns along the 
longitudinal dimension of the electrode structure 20. The thickness of an 
electrode 24 or 28 is in the range of approximately 1000-1500 angstroms. A 
cross-section 40 of a beam of light propagating along the optical path 38 
is indicated by a dashed line. The cross section 40 has a longitudinal 
shape and transects central portions of the electrodes 24 and 28 in each 
of the electrode layers 14. Within the cross section 40, the fringing 
electric fields, portrayed by the field lines 34 of FIG. 3, are at their 
maximum value. Thus, by configuring a beam of light to the shape and 
location of the cross section 40, maximum advantage is taken of the 
interaction region 36 for imparting maximum differential propagation speed 
as a function of applied signal voltage between a pair of electrodes 24 
and 28. 
In the preparation of each of the electrooptic layers 12, one technique for 
preparing a layer is to mix the component portions thereof in a solution, 
drip the solution onto the substrate 18, or other layers such as an 
electrode layer 14, and then spin the substrate 18 to attain a a spin 
layer, similar to the process of depositing a photoresist. Thereupon, the 
deposited material is heated to provide a gel, the gel then being baked at 
approximately 550 degrees centigrade to obtain a desired solid form of 
material of an electrooptic layer 12. The electrooptic material may be 
deposited also by chemical-vapor deposition (CVD). A typical thickness of 
an electrooptic layer 12 is approximately 10 microns, by way of example, a 
thickness which can be obtained by repeating the foregoing spin-deposition 
process. With this thickness, the fringing field at its maximum excursion 
is only five microns from the electrode layer 14, a distance only 
one-eighth of the interelectrode spacing of forty microns. This provides 
for the aforementioned parallelism of the electric field lines 34. In the 
construction of the electrooptic layers 12 of FIG. 3, the top layer and 
the bottom layer may be provided with only one-half the thickness (5 
microns) of the other layers to accommodate the fringing electric field of 
only one electrode layer 14, rather than the fringing fields of two 
electrode layers 14 as is the case with the centrally located electrooptic 
layers 12. 
FIG. 5 shows a simplified view of a printer 42 employing an electrooptic 
light modulator 44 for imprinting markings 46 on a print medium 48. The 
light modulator 44 includes the electrooptic assembly 10 (FIG. 1) plus a 
polarizer 50 and an analyzer 52 disposed respectively along a front side 
and a back side of the assembly 10. A light source 54 includes a 
collimator 56, which may be a barrel-shaped lens, and a suitable aperture 
(not shown) for forming a beam 58 of light having the cross-section 40 
(FIG. 2). The beam 58 propagates along the optical path 38 towards the 
print medium 48. A signal generator 60 is connected via a set of 
electrical leads 62 to the pads 32 of the second comb array 26 (FIG. 1) 
and the spine 30 of the first comb array 22 in each of the electrode 
layers 14 for applying signal voltage between various ones of the 
electrodes 24 and 28 in each electrode layer 14. The electrode structures 
20 of the respective electrode layers 14 are connected electrically in 
parallel to the signal generator 60. The parallel connection is 
facilitated by the introduction of electrically-conductive straps 64 which 
are emplaced along the outer surface of the stack 16. Thus, straps 64 
interconnect corresponding ones of the pads 32, and also interconnect the 
spines 30. In lieu of the straps 64, if desired, connecting rods (not 
shown) may be constructed through the electrooptic layers 12 to make 
connection between the corresponding pads 32 and the spines 30. 
As shown in each pixel region 66 of FIG. 2, there is one pad 32 with its 
electrode 28 associated with each pixel region 66. Thus, to print a 
marking 46 for a specific pixel, the signal generator 60 need apply a 
voltage to the pad 32 for the designated pixel region 66 of an electrode 
layer 14. In view of the parallel connections of the pads 32 and the 
spines 30, the application of the voltage to one pad 32 provides for the 
voltage to the corresponding pads 32 of all of the electrode layers 14. 
The voltage is applied between the designated pad 32 and the spine 30 
which is maintained at ground potential. Thereby, the signal generator 60 
can activate a plurality of designated pixels for printing a line of 
pixels simultaneously upon the print medium 48. 
In the operation of the electrooptic light modulator 44, the collimated 
beam 58 of light is polarized by the polarizer 50 such that the electric 
field of the light beam is angled 45 degrees to one side of the direction 
of the electrodes of the layers 14. The analyzer 52 has its direction of 
polarization oriented 45 degrees to the opposite side of the direction of 
the electrodes of the layers 14. Light propagating from the polarizer 50 
to the electrooptic assembly 10 has an electric field component 
perpendicular to the electrodes of the layers 14 and a second component 
parallel to these electrodes. The transverse and the parallel components 
of the polarization are shown by vectors in FIG. 5. In those pixels which 
have been activated electrically by the signal generator 60, by 
application of signal voltages to the electrodes 28 of the selected 
pixels, a differential propagation speed results between the perpendicular 
and the parallel components of the polarized light beam. This results, at 
the back side of the electrooptic assembly 10, in a shift in polarization 
of 90 degrees to line up with the polarization of the analyzer 52. 
Thereby, in response to electrical activation of a pixel region 66 in the 
electrooptic assembly 10 by the signal generator 60, the corresponding 
pixel region is printed in the print medium 48 in the form of a marking 
46. Other pixels along a print line of the print medium 48 wherein the 
corresponding pixel regions 66 of the assembly 10 have not been 
electrically activated by the signal generator 60 remain blank. 
In order to print a succession of lines of an image upon the print medium 
48, means such as a roller 68 are provided for introducing a relative 
motion between the print medium 48 and the optical path 38. Alternatively, 
means (not shown) may be provided for moving a scanning head comprising 
the modulator 44 and the light source 54 relative to the print medium 48 
which, in such case, would remain stationary. In the embodiment shown in 
FIG. 5, the print medium 48 is drawn by the roller 68 during rotation of 
the roller 68 by a motorized drive unit 70. Specific pixels in each print 
line which are to be activated by the signal generator 60 are stored as 
image data in a memory 72, and are read out of the memory 72 to the 
generator 60 in response to timing signals of a synchronizer 74. The 
synchronizer 74 also applies timing signals to the drive unit 70 for 
inducing rotary motion of the roller 68 in synchronism with transfer of 
data from the memory 72 to the generator 60. Thereby, the position of the 
print medium 48 is advanced a desired amount between each imprinting of a 
line of image data on the medium 48 by operation of the modulator 44. 
It will be understood that the polarizer 50 is required only when the light 
source is unpolarized; however, the source illumination may be a polarized 
laser in which case the polarizer 50 can be eliminated. 
It is to be understood that the above described embodiment of the invention 
is illustrative only, and that other modifications thereof may occur to 
those skilled in the art. Accordingly, this invention is not to be 
regarded as limited to the embodiment disclosed herein, but is to be 
limited only as defined by the appended claims.