Optically addressed liquid crystal light valve and optical logic device using the same

An optically addressed liquid crystal light valve includes a first substrate on which a plurality of liquid crystal driving circuits are formed, a second substrate provided with a transparent electrode, and a ferroelectric liquid crystal sandwiched between the two substrates. Each liquid crystal driving circuit includes a plurality of photodetectors and a logic circuit. An optical logic device includes an optically addressed liquid crystal light valve explained above and a plurality of masks for selectively transmit a plurality of lights. The masks may be replaced by a wavelength selecting filter including a plurality of regions each transmit a light having a specific wavelength to a corresponding photodetector of the light valve.

FIELD OF THE INVENTION 
This invention relates to an optically addressed liquid crystal light valve 
and an optical logic device using the same, and more particularly to, an 
optically addressed liquid crystal light valve used in optical information 
processing, optical computing, etc. and an optical logic device using the 
same. 
BACKGROUND OF THE INVENTION 
An optically addressed liquid crystal light valve is used widely in the 
field of optical information processing, optical computing, etc. A first 
conventional optically addressed liquid crystal light valve which is well 
known is one manufactured by Hughes Aircraft Company of the U.S. In the 
optically addressed liquid crystal light valve, nematic liquid crystal is 
sandwiched between a glass substrate and an opposite substrate. The glass 
substrate is provided with a plurality of photoconductors of CdS and 
dielectric mirrors, and the opposite substrate is provided with a 
transparent electrode. 
On the other hand, an optical arithmetic process carried out by an optical 
logic device using such an optically addressed liquid crystal light valve 
has been disclosed in an article titled "DIGITAL OPTICAL COMPUTING" on 
pages 758 to 779 of PROCEEDING OF THE IEEE, Vol. 72, No. 7July 1984. In 
the optical arithmetic process, AND and OR arithmetic processes, and so 
on, are carried out between two input images by using optical threshold 
characteristic of a reading light relative to a writing light in the 
optically addressed liquid crystal light valve. 
A second conventional optically addressed liquid crystal light valve is 
described in the previous text 30a-ZD-8 of 50th science lecture of APPLIED 
PHYSICS SOCIETY. In the optically addressed liquid crystal light valve, 
amorphous silicon and ferroelectric liquid crystal are used instead of CdS 
and nematic liquid crystal, and optical arithmetic processes using the 
light valve are carried out. The light valve has an advantage of high 
speed operation, because ferroelectric liquid crystal used therein has 
higher speed of response by two or three figures compared with nematic 
liquid crystal. 
According to the conventional optically addressed liquid crystal light 
valve, however, there are disadvantages as explained below. In the first 
conventional optically addressed liquid crystal light valve, the speed of 
response which is determined by the speed of response of CdS and nematic 
liquid crystal is as low as up to 30 ms. Such a low speed of response of 
the light valve may become obstacles to higher speed operation of optical 
arithmetic processes. Further, the optical contrast thereof is not high 
because the change of voltage applied across the liquid crystal is low 
between binary states. The voltage applied across the liquid crystal is 
determined by a capacitance dividing ratio of the photoconductor and the 
dielectric mirror composing the light valve. Capacitance of the dielectric 
mirror will not change even if capacitance of the photoconductor changes, 
so that the voltage applied to the liquid crystal will not change largely. 
Such a low contrast thereof may not badly affect optical arithmetic 
processes if the light valve is used discretely in the processes, because 
outputs of the optical arithmetic processes are supplied as binary data of 
states of bright and dark. However, it is difficult to supply outputs 
correctly from the previous stage of the light valve to inputs of the next 
stage thereof in case that a plurality of light valves are connected 
serially to form a multi-stage structure if the contrast is not 
sufficiently high. Additionally, it is difficult to carry out different 
arithmetic processes such as AND, OR or exclusive OR processes 
simultaneously in one light valve, because the arithmetic processes are 
carried out by using optical threshold characteristic of a reading light 
relative to a writing light in the light valve. The whole surface of the 
light valve is uniform and only one of either arithmetic process can be 
carried out, so that it is difficult to carry out two or more kinds of 
arithmetic processes simultaneously. 
In the second conventional optically addressed liquid crystral light valve, 
the speed of response is improved as compared with that of the first 
conventional optically addressed liquid crystal light valve, however, the 
same disadvantages of the low contrast and single-arithmetic-process 
ability as in the first conventional optically addressed liquid crystal 
remain. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide an optically 
addressed liquid crystal light valve having high speed of response and an 
optical logic device using the same. 
It is another object of the invention to provide an optically addressed 
liquid crystal light valve having high optical contrast and an optical 
logic device using the same. 
It is a further object of the invention to provide an optically addressed 
liquid crystal light valve in which two or more kinds of optical 
arithmetic processes can be carried out simultaneously and an optical 
logic device using the same. 
According to a first feature of the invention, an optically addressed 
liquid crystal light valve comprises: 
a first substrate on which a plurality of liquid crystal driving circuits 
are formed; 
a second substrate provided wtih a transparent electrode; and 
a ferroelectric liquid crystal sandwiched between the first and second 
substrates; 
wherein each the liquid crystal driving circuit comprises; 
a plurality (n) of photodetectors for detecting lights to be converted to 
electric signals; 
a logic circuit controlled by the electric signals supplied from the 
photodetectors; and 
pixel electrodes applied with a predetermined voltage as a result of a 
logic calculation in the logic circuit, whereby a predetermined electric 
field is applied across the transparent electrode and each the pixel 
electrode. 
According to a second feature of the invention, an optical logic device 
comprises: 
an optically addressed liquid crystal light valve comprising a first 
substrate on which a plurality of liquid crystal driving circuits are 
formed, a second substrate provided with a transparent electrode and a 
ferroelectric liquid crystal sandwiched between the first and second 
substrates, wherein each the liquid crystal driving circuit comprises a 
plurality (n) of photodetectors for detecting lights to be converted to 
electric signals, a logic circuit controlled by the electric signals 
supplied from the photodetectors, and pixel electrodes applied with a 
predetermined voltage as a result of a logic calculation in the logic 
circuit, whereby a predetermined electric field is applied across the 
transparent electrode and each the pixel electrode; 
a plurality (n) of masks for selectively transmit a plurality (n) of lights 
to corresponding photodetectors of the optically addressed liquid crystal 
light valve; 
an optical combining system for combining the plurality (n) of lights; and 
an optical focusing system for focusing the combined light.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an optically addressed liquid crystal light valve in a 
preferred embodiment according to the invention. The optically addressed 
liquid crystal light valve includes a circuit substrate 100 on which a 
plurality of liquid crystal driving circuits 101 are formed in line or in 
plane (though only one liquid crystal driving circuit 101 is shown in FIG. 
1), an opposite substrate 102 which is connected to ground, and 
ferroelectric liquid crystal 103 which is sandwiched with the circuit 
substrate 100 and the opposite substrate 102. Each liquid crystal driving 
circuit 101 includes a plurality of photodetectors 107 and a logic circuit 
106 which is supplied with outputs of the photodetectors 107. Each 
photodetector 107 is a voltage divider consisting of a photoconductor 104 
and a resistor 105 having no optical sensibility. The logic circuit 106 is 
either logic circuit of AND, OR, and exclusive OR circuits. 
FIG. 2 shows the optically addressed liquid crystal light valve of FIG. 1 
including a liquid crystal driving circuit 101 which has two 
photodetectors in the preferred embodiment in case that it includes an AND 
circuit as the logic circuit. The liquid crystal driving circuit 101 
includes two photoconductors 203 and 204, two resistors 208 and 209, and 
three thin film transistors 205 to 207. The photoconductor 203 and the 
resistor 208 composing the first photodetector are connected serially, 
while the photoconductor 204 and the resistor 209 composing the second 
photodetector are connected serially. Each one terminal of the 
photoconductors 203 and 204 is connected to a high voltage +V and each of 
the other terminals is connected to nodal points A and B, respectively. 
Each one terminal of the resistors 208 and 209 are connected to a low 
voltage -V and each of the other terminals is connected to the nodal 
points A and B, respectively. The thin film transistor 205 is connected at 
a gate to the nodal point A, at a source to a drain of the thin film 
transistor 206, and at a drain to an output point X which is connected to 
the ferroelectric liquid crystal 103. The thin film transistor 206 is 
connected at a gate to the nodal point B and at a source to the low 
voltage -V. The thin film transistor 207 is connected at a gate and a 
source to the high voltage +V and at a drain to the output point X. 
In operation, the thin film transistor 207 is constantly at ON state, 
because it is supplied with a high voltage constantly at the gate. On the 
other hand, the thin film transistors 205 and 206 become at ON state when 
lights 201 and 202 are irradiated to the photoconductors 203 and 204 to be 
detected, respectively. Therefore, the output point X becomes level of low 
voltage -V only if both the lights 201 and 202 are irradiated to the 
photoconductors 203 and 204 to make both the thin film transistors 205 and 
206 at ON state, otherwise the output point X remains level of high 
voltage +V. Consequently, the output voltage of the output point X, as a 
result of an optical logic AND process, is applied to the ferroelectric 
liquid crystal 103. 
FIG. 3 shows the optically addressed liquid crystal light valve of FIG. 1 
including a liquid crystal driving circuit 101 which has two 
photodetectors in the preferred embodiment in case that it includes an OR 
circuit as a logic circuit. The basic structure is the same as that in 
FIG. 2, except that the thin film transistors 205 and 206 are connected in 
parallel each other. In more detail, the thin film transistors 205 and 206 
are connected at sources to a low voltage -V in common and at drains to an 
output point X in common. 
In operation, the thin film transistor 207 is constantly at ON state, and 
the thin film transistors 205 and 206 become at ON state when lights 201 
and 202 are irradiated to the photoconductors 203 and 204, respectively, 
as like in FIG. 2. However, the output point X becomes level of low 
voltage -V if both or either of the lights 201 and 202 are irradiated to 
the photoconductors 203 and 204 to make both or either of the thin film 
transistors 205 and 206 at ON state. 
The output point X remains level of high voltage +V only if neither of the 
photoconductors 203 and 204 detect light. Consequently, the output voltage 
of the output point X, as a result of an optical logic OR process, is 
applied to the ferroelectric liquid crystal 103. 
FIG. 4 shows the optically addressed liquid crystal light valve of FIG. 1 
including a liquid crystal driving circuit 101 which has two 
photodetectors in the preferred embodiment in case that it includes an 
exclusive OR circuit as a logic circuit. The basic structure is the same 
as that in FIG. 2, except that connections of the thin film transistors 
205 and 206 are different. In more detail, the thin film transistor 205 is 
connected at a gate to a nodal point B, at a source to a nodal point A, 
and at a drain to an output point X, while the thin film transistor 206 is 
connected at a gate to the nodal point A, at a source to the nodal point 
B, and at a drain to the output point X. 
In operation, the thin film transistor 207 is constantly at ON state, and 
the thin film transistors 205 and 206 become at ON state when lights 201 
and 202 are irradiated to the photoconductors 203 and 204, respectively, 
as like in FIG. 2. When both the lights 201 and 202 are irradiated to the 
photoconductors 203 and 204, both the thin film transistors 205 and 206 
become at ON state and both the nodal points A and B become high level, so 
that the output point X becomes high level. When neither of the lights 201 
and 202 is irradiated thereto, both the thin film transistors 205 and 206 
become at OFF state, so that the output point X becomes high level. When 
only the light 201 is irradiated thereto, the thin film transistor 205 
becomes at ON state and the thin film transistor 206 becomes at OFF state, 
while the nodal point B remains low level, so that the output point X 
becomes low level. In the same manner, the output point X becomes low 
level when only the light 202 is irradiated thereto. Consequently, the 
output voltage of the output point X, as a result of an optical logic 
exclusive OR process, is applied to the ferroelectric liquid crystal 103. 
In the optically addressed liquid crystal light valve, an output voltage of 
the logic circuit 106 is applied to the ferroelectric liquid crystal 103. 
The output voltage thereof changes between positive and negative 
potentials relative to the voltage of the opposite substrate 102 in 
accordance with the results of the arithmetic processes, so that inversion 
of an electric field applied to the ferroelectric liquid crystal 103 
occurs to carry out switching the liquid crystal. Such mechanism 
contributes high speed of response. Further, the range of voltage changes 
applied to the liquid crystal is wide, so that high contrast can be 
obtained therein. 
Additionally, the optical arithmetic processes of the light signals are 
carried out by the logic circuit 106 which is an electric logic circuit. 
Such circuits can include any of AND, OR, and exclusive OR circuits in one 
light valve and can be positioned on any place thereof, so that different 
kinds of arithmetic processes of AND, OR, and exclusive OR can ben carried 
out simultaneously. 
FIG. 5 is shows the practical structure of the optically addressed liquid 
crystal light valve in the preferred embodiment in case that it includes 
an AND circuit as a logic circuit, and FIGS. 6A and 6B are cross-sectional 
views taken on lines A--A' and B--B' of FIG. 5, respectively. 
The fabrication process thereof will be explained. Here, photoconductors, 
resistors and active layers of thin film transistors consist of an 
amorphous silicon layer. First, a chromium layer is formed on a 
transparent insulating substrate 601 by sputtering. Then, the chromium 
layer is etched to have a predetermined pattern to form a light screening 
layer 505 and a gate electrode 503. Next, a silicon nitride layer is 
formed by mixing and decomposing silane (SiH.sub.4) and ammonia (NH.sub.3) 
by glow discharge. Then, an amorphous silicon layer is formed by 
discomposing the silane. Then, an n.sup.+ -amorphous silicon layer 604 
including a large amount of phosphorus for ohmic contact with electrodes 
is formed by mixing and decomposing the silane and phosphine (PH.sub.3). 
Then, the n.sup.+ -amorphous silicon layer 604 is patterned to form 
photoconductors 203 and 204, a resistor 105, and an active layer 504 of a 
thin film transistor. 
Next, contact holes 506 are formed through the gate electrode 503 and a 
high voltage electrode 501 to be formed thereafter and through the gate 
electrode 503 and a joint electrode 507 to be formed thereafter. Then, a 
chromium layer is formed by sputtering and patterned to form a high 
voltage electrode 501, a low voltage electrode 502, a joint electrode 507 
and an output electrode 508. Then, the n.sup.+ -amorphous silicon layer 
604 is removed by etching. Then, an interlayer insulating layer 606 of 
silicon nitride is formed on the fabricated surface. Then, the contact 
holes 506 are formed again through the output electrode 508 and a pixel 
electrode 607 to be formed thereafter. Then, a chromium layer is formed by 
sputtering and patterned to form a pixel electrode 607. Next, an 
orientating layer of polyimide is formed on the transparent insulating 
layer 601 by spin coating and then the surface thereof is rubbed. 
Next, an opposite substrate 602 provided with a transparent electrode 603 
which is rubbed is assembled with the transparent insulating substrate 601 
thus fabricated so that the rubbing directions of the two substrates are 
parallel but opposite (anti-parallel), and ferroelectric liquid crystal 
103 is poured therebetween. Thus, the optically addressed liquid crystal 
light valve is fabricated. Silicon oxide or organic insulating layers such 
as polyimide may be used as insulating layers. An LB layer may be used as 
the orientating layer. 
Next, an optical logic device using the optically addressed liquid crystal 
light valve will be explained. FIG. 7 shows an optical logic device in a 
first preferred embodiment. The optical logic device 706 includes an 
optically addressed liquid crystal light valve 701, two masks 702 and 703, 
a half mirror 704 and a lens 705. The masks 702 and 703 have openings 702a 
and 703a by which lights passing therethrough are separated to be 
transmitted to corresponding photoconductors of the optically addressed 
liquid crystal light valve 701, as shown in FIGS. 8A and 8B, respectively. 
In operation, lights 201 and 202 which are two-dimensional input images 
such as a still pattern or a CRT picture are irradiated to the masks 702 
and 703, respectively, where space-separation of the two lights is carried 
out. Then, the lights 201 and 202 are combined by the half mirror 704, and 
the combined light is focused by the lens 705 to become an input light for 
the light valve 701. Then, the optical arithmetic processes are carried 
out in the light valve 701 as explained before. That is, the masks 702 and 
703 are illustrated to have 3.times.3 picture cell regions each consisting 
of the opening 702a or 703a, respectively. In the optical arithmetic 
processes, logic calculations such as AND, OR, exclusive OR, etc. are 
carried out between respective two input lights transmitted through the 
openings 702a and 703a of each corresponding picture cell regions. 
FIG. 9 shows an optical logic device in a second preferred embodiment. The 
optical logic device 706 includes an optically addressed liquid crystal 
light valve 701, a half mirror 704, a wavelength selecting filter 801 and 
a lens 705. The wavelength selecting filter 801 includes red filters 802 
and blue filters 803 which respectively transmit red and blue lights and 
are arranged in position so that light passing therethrough are separated 
to be transmitted to corresponding photoconductors of the light valve 701, 
as shown in FIG. 10. 
In operation, lights 201 and 202 which are red and blue lights, 
respectively, are irradiated to the half mirror 704 where the two lights 
201 and 202 are combined. The combined light passes through the wavelength 
selecting filter 801, where the space-separation of the two lights is 
carried out, and then the light is focused by the lens 705 to become an 
input light for the light valve 701. Then, the optical arithmetic 
processes are carried out between the red and blue lights in the light 
valve 701 as explained before. 
Although the invention has been described with respect to specific 
embodiment for complete and clear disclosure, the appended claims are not 
to thus limited and alternative constructions that may occur to one 
skilled in the art which fairly fall within the basic teaching herein set 
forth.