Differentiating spatial light modulator

A differentiating spatial light modulator device in which a photoreceptor and an electro-optic crystal are isolated by a dielectric mirror. The electro-optic crystal is configured to have low or zero longitudinal response, yet is sensitive to transverse electric fields. The fringe field generated by the photoreceptor (photodiode) modultes the crystal birefringence. Readout via a polarizing beamsplitter gives an output light related to the spatial gradient of the input light. In a liquid crystal embodiment of the invention, reversal of the applied voltage gives a driven off state which speeds the erasure. Storage is possible in the smectic liquid crystal phase.

BACKGROUND OF THE INVENTION 
The present invention relates generally to the field of optical signal 
processing apparatus, and more specifically to a differentiating spatial 
light modulator of simplified construction and improved performance. 
Two-dimensional spatial light modulators are devices which allow control of 
an optical wavefront for processing or imaging operations. These devices, 
often referred to as light valves in the literature, have potential for 
application in large screen display systems as well as in optical data 
processing systems, including missile guidance and robotic vision systems. 
Listed below are several articles which describe their construction and 
operation. 
1. "A Fast Silicon Photoconductor-Based Liquid Crystal Light Value", P. O. 
Braatz, K. Chow, U. Efron, J. Grinberg and M. J. Little, IEEE 
International Electron Devices Meeting, pp. 540-543, 1979. 
2. "Oblique-cut LiN.sub.b O.sub.3 Microchannel Spatial Light Modulator", C. 
Warde and J. I. Thakara, Optics Letters, Vol. 7, No. 7, July 1982. 
3. "A First-Order Model of a Photo-Activated Liquid Crystal Light Valve", 
J. D. Michaelson, SPIE Vol. 218, Devices and Systems For Optical Signal 
Processing, 1980. 
4. "LiNbO.sub.3 and LiTaO.sub.3 Microchannel Spatial Light Modulators", C. 
Warde, A. M. Weiss and A. D. Fisher, SPIE Vol 218, Devices and Systems for 
Optical Signal Processing, 1980. 
5. "Silicon Liquid Crystal Light Valves: Status and Issues", U. Efron, P. 
O. Braatz, M. J. Little, R. N. Schwartz and J. Grinberg, Proc. SPIE Vol. 
388, Jan. 1983. 
6. "Applications of Priz Light Modulator", D. Casasent, F. Caimi, M. Petron 
and A. Khomenko, Applied Optics, Vol. 21., No. 21, November 1982, pp. 
3846-3854. 
7. PLZT Color Displays, G. Haertling, SID 84 Digest, pp. 137-140. 
Spatial light modulators often comprise a photosensitive semiconductor 
substrate (photodiode), a light blocking layer, a dielectric mirror and an 
electro-optic crystal (which may be a liquid crystal), arranged in a 
sandwich-like composite structure, and having a voltage applied 
thereacross. A control (write) illumination impinges on the face of the 
photosensitive semiconductor while an output (read) illumination makes a 
double pass through the electro-optic crystal. 
The photosensitive semiconductor responds to intensity variations in the 
control illumination impinging thereon. In the dark, most of the voltage 
applied across the composite structure appears across the reverse-biased 
photodiode. The write beam, however, excites carriers in the silicon, 
which are driven by the internal field to the Si/electro-optic crystal 
interface. The voltage across the silicon decreases, while the voltage 
across the electro-optic crystal increases. The read illumination passes 
through the electro-optic crystal, is reflected off of the dielectric 
mirror, and again passes through the electro-optic crystal before emerging 
from the device. Since the diffraction efficiency of the electro-optic 
crystal is a function of the voltage applied thereacross, (which is a 
function of the intensity of the write illumination), optical control of 
the output (read) illumination is achieved. 
It is known that in the recognition of images, about ninety percent of the 
information content resides at the edges of the image, where abrupt 
changes in light intensity occur. A form of spatial light modulator called 
the Priz modulator has been designed which modulates light by the 
transverse (rather than the longitudinal) electro-optic effect. Thus, only 
the edge contour of an object appears and edge enhancement is achieved. 
The operation of the Priz spatial light modulator involves the use of a BSO 
(bismuth silicon oxide, Bi.sub.12 Si O.sub.20) crystal which is cut such 
that the device modulates light by the transverse rather than the 
longitudinal electro-optic effect. The spatially varying light 
distribution is still incident on the crystal's large faces collinear with 
the applied electric field direction, and the spatially varying charge 
layer parallel to the crystal's large faces is still induced. However, the 
transverse component of this field is what is used to provide the spatial 
modulation of the incident light. 
In the Priz spatial light modulator, the photoconductor and electro-optic 
crystal are one and the same BSO element. This can have the disadvantage 
of erasure of the image by the readout light. Alternatively, cyclic 
operation requires an optical erasure operation which removes the 
previously recorded image. Moreover, the BSO crystal has a somewhat 
limited wavelength range of operation. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is therefore the general object of the present invention to provide an 
improved differentiating spatial light modulator. 
It is a further object of the present invention to provide a novel and cost 
effective means for eliminating readout erasure and increasing wavelength 
range in differentiating spatial light modulators. 
It is an additional object of the present invention to provide a 
differentiating spatial light modulator utilizing a liquid crystal as the 
electro-optic crystal in one embodiment thereof, and in which the cycle 
time of the device is substantially reduced. 
Briefly, in the spatial light modulator device of the present invention, a 
photoreceptor and an electro-optic crystal are isolated by a dielectric 
mirror. The electro-optic crystal is configured to have low or zero 
longitudinal response, yet is sensitive to transverse electric fields. The 
fringe field generated by the photoreceptor (photodiode) modulates the 
crystal birefringence. Readout via a polarizing beamsplitter gives an 
output light related to the spatial gradient of the input light. In a 
liquid crystal embodiment of the invention, reversal of the applied 
voltage gives a driven off state which speeds the erasure. Storage is 
possible in the smectic liquid crystal phase.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a diagram depicting the side view of a preferred embodiment of 
the spatial light modulator of the present invention which utilizes a 
solid electro-optic crystal. The various components thereof have not been 
drawn to scale in view of the large differences in their thicknesses. 
Representative thickness values are provided herein however, to facilitate 
an understanding of the invention. 
The modulator has a substrate 10 formed of silicon material having a 
thickness in the order of 100 microns. A p-type silicon material 12 of 
approximately 0.1 microns is diffused in the exposed surface thereof to 
form a silicon photodiode. A charge confinement grid 32 is formed on the 
inner surface of silicon substrate 10 to prevent lateral charge transfer 
in the semiconductor device. The grid 32 may be a boron-implanted p-grid, 
as taught by P. 0. Braatz et al in the article entitled "A Fast Silicon 
Photoconductor-Based Liquid Crystal Light Valve", or may be a microgroove 
lattice structure, as taught by the present inventor in his patent 
application Ser. No. 689,699 filed Jan. 8, 1985 and entitled "Charge 
Isolation In A Spatial Light Modulator". 
The inner surface of silicon substrate 10 is adjacent a layer of light 
blocking material 14, having a thickness in the order of one micron. In 
certain applications, this light blocking layer 14 may be unnecessary. 
Light blocking layer 14 is affixed to a dielectric mirror 16 and it, in 
turn is affixed to an electro-optic crystal 18 having a thickness in the 
order of 100 microns. 
As mentioned above, the light blocking layer may be omitted. The dielectric 
mirror transmits about 0.1% of the read illumination to the silicon 
substrate 10. In some applications this is not critical. If required, 
further isolation is provided by an absorbing or light blocking layer 14 
having a thickness less than 10 microns to avoid degrading the resolution 
of the device. The conductivity of the layer must be low to avoid charge 
confinement leakage. 
Electro-optic crystal 18 is affixed to a transparent electrode 20 having a 
thickness of approximately 0.25 microns and formed on one surface of a 
transparent supporting substrate 24 having a thickness in the order of 15 
millimeters. Transparent electrode 20 provides a uniform initial electric 
field across electro-optic crystal 18. A first electrical terminal 26 is 
connected to the transparent electrode 20 and a second electrical terminal 
28 is connected to the p-type material 12 diffused in silicon substrate 
10. A voltage V is applied across the terminals 26 and 28. 
Optical quality and voltage requirements, as well as the required 
sensitivity to a transverse electric field, suggest that electro-optic 
crystal 18 be formed of z-cut lithium tantalate (LT), or Lead 
Zirconate-Lead Titanate (PLZT). The transparent supporting substrate 24 
may be formed of glass or calcuim fluoride. 
The operation of the spatial light modulator shown in FIG. 1 is similar to 
that disclosed earlier. The control illumination, which could in some 
applications be the image of an object or scene of interest excites 
electrons and holes in the silicon photodiode formed in silicon substrate 
10. Electrons are driven by the internal electric field in the diode to 
the silicon substrate 10/electro-optic crystal 18 interface. (Since the 
light blocking layer 14 and dielectric mirror 16 are thin and of high 
resistivity, they can be ignored in this analysis.) This surface charge 
pattern in the silicon substrate 10 gives rise to a transversely varying 
voltage pattern containing the same information as the charge pattern. The 
transverse fringe field associated with the voltage pattern modulates the 
birefringence of the electro-optic crystal 18. The birefringence 
variations are sensed with the read beam via a polarizing beam splitter 
40, to give an output which is the spatial gradient of the input light. 
The dielectric mirror 16 is necessary for the read operation and prevents 
the read beam from destroying the charge pattern at the surface of silicon 
substrate 10 by creating more charge carriers in the silicon. The 
light-blocking layer 14, if used, attenuates any read beam leakage through 
the dielectric mirror 16. The transparent electrode 20 in conjunction with 
the applied voltage V, provides a uniform initial field across the 
electro-optic crystal 18. When the read operation has been completed, the 
voltage V may be removed from across the terminals 26 and 28, and the 
transversely varied charge pattern will discharge to equilibrium. 
As previously noted, about ninety percent of the information content useful 
for image recognition is contained in the edges of the image, where sudden 
changes of intensity level occur. This information is amplified and 
extracted directly by the differentiating spatial light modulator of the 
present invention, which differs from prior art devices by employing an 
electro-optic crystal having a low or zero response to a longitudinal 
electric field (direction of light propagation), yet a pronounced 
sensitivity to a transverse electric field. A sudden spatial change of 
intensity in the input plane of the differentiating SLM device produces a 
large local transverse electric field component in the electro-optic 
crystal. The electro-optic crystal will have an induced birefringence 
response to the electric field. This changes the polarization state of the 
readout light such as to increase the optical transmission through a 
polarizing beam splitter. The induced birefringence will depend on the 
field direction and crystal symmetry. For any solid crystal 18 there will 
be a direction of transverse field which has zero birefringence effect, 
e.g. for LT, the y direction. This means that the edge brightness is 
determined by edge orientation. This can be used to advantage in image 
recognition schemes. 
In the liquid crystal embodiment of the present invention, a slightly 
different structure is required as shown in FIG. 2. The dielectric mirror 
16 is adjacent a glass substrate 44 and spaced approximately 10 microns 
therefrom by sealing members 42. The space therebetween is filled with 
liquid crystal 19. The dielectric material 44 (glass) approximately 10 
microns thick between the electrode 20 and liquid crystal 19 eliminates 
the least effective region of the liquid crystal 19. This is advantageous 
since the liquid is turbid. The surfaces in contact with the liquid 
crystal are treated with a perpendicular aligning agent, e.g. 
octadecyltrichlorosilane. 
When the electro-optic crystal is a perpendicularly aligned liquid crystal 
19, there is always a birefringent effect as the optic axis is tilted away 
from perpendicular by the transverse electric field. Linearly polarized 
readout, as before, will relate line brightness to line orientation. If 
this is undesireable it can be avoided by using circularly polarized read 
light by insertion of a 1/4 wave retarder 46 as shown. The line brightness 
is now independent of orientation. 
Existing nematic liquid crystal devices are electrically driven in one 
direction and rely on elastic restoring forces to return to the initial 
state. This restricts the cycle time of the device. Two frequency 
addressing has had limited success in achieving driven on and off states. 
In the device described in FIG. 2, the off state is an undistorted 
perpendicular aligned nematic. This can be driven by reversing the 
polarity of the drive voltage which forward biases the photodiode, 
producing a uniform voltage across the nematic liquid crystal. The 
direction of the field is such as to produce the required off state 
alignment. 
If the liquid crystal is smectic-A phase, then memory is possible. The 
write and erase processes that have been described are applicable. If the 
voltage source is removed after the write pulse, then the written image 
will be stored for an indefinite time until erased. 
A differentiating SLM as described herein will also function in off-axis 
holography, since this is associated with carrier spatial frequency. The 
carrier spatial frequency provides a transverse electric field component. 
(In off-axis holography the image wavefront is added to a plane reference 
wave. Therefore, even when the image is a uniform plane wave, a sinusoidal 
interference pattern is generated at the SLM output.) 
Prior art devices which are based on photorefractive material, such as 
bismuth silicon oxide and where the photoconductor and electro-optic 
crystal are one and the same material can have the disadvantage of erasure 
by the readout light. Alternatively, cyclic operation requires an optical 
erasure operation which removes the previously recorded image. These 
limitations are eliminated in the present invention. Also a wider choice 
of materials is possible with the present invention. 
Although the invention has been described with reference to a particular 
embodiment, it will be understood to those skilled in the art that the 
invention is capable of a variety of alternative embodiments within the 
spirit and scope of the appended claims.