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Claims  |
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We claim:
1. An optical correlator comprising:
a laser for producing a beam of coherent polarized light;
a reference spatial light modulator (SLM) for modulating and reflecting an
incident beam of light, said reference SLM having:
(a) means for storing a reference image in the form of a two-dimensional
array of pixels; and
(b) means for selectively modulating said incident beam by rotating the
polarization of said beam for selected pixels corresponding to said
reference image;
a first polarizing beamsplitter for directing said laser beam onto said
reference SLM and receiving said beam reflected from said reference SLM,
said first beamsplitter being polarized to transmit modulated light
reflected from said reference SLM and to block unmodulated light reflected
from said reference SLM;
Fourier transform lenses for receiving said modulated light beam
transmitted by said first polarizing beamsplitter and producing a Fourier
transform of said beam;
a filter SLM for modulating and reflecting an incident beam of light, said
filter SLM having:
(a) means for storing a filter image in the form of a two-dimensional array
of pixels, said filter image being the complex conjugate of the Fourier
transform of a desired image; and
(b) means for selectively modulating said incident beam by rotating the
polarization of said beam for selected pixels corresponding to said filter
image;
a second polarizing beamsplitter for directing said Fourier transform beam
onto said filter SLM and receiving said beam reflected from said filter
SLM, said second polarizing beamsplitter being polarized to transmit
modulated light reflected from said filter SLM and to block unmodulated
light reflected from said filter SLM;
inverse Fourier transform lenses for receiving said modulated light beam
transmitted by said second polarizing beamsplitter and producing an
inverse Fourier transform of said beam; and
a camera for detecting any correlation peak produced by said inverse
Fourier transform lenses.
2. The optical correlator of claim 1, wherein said reference SLM comprises
an ferroelectric liquid crystal SLM.
3. The optical correlator of claim 1, wherein said SLM pixels are
electrically addressable.
4. The optical correlator of claim 1, further comprising a polarizing
filter between said inverse Fourier transform lenses and said camera.
5. The optical correlator of claim 1, further comprising a half-wave plate
between said first polarizing beamsplitter and said reference SLM for
adjustably aligning the polarization of light entering said reference SLM.
6. The optical correlator of claim 1, further comprising a half-wave plate
between said second polarizing beamsplitter and said filter SLM for
adjustably aligning the polarization of light entering said filter SLM.
7. The optical correlator of claim 1, further comprising a computer
processor connected to said reference SLM and to said image SLM for
downloading images to said SLM's.
8. The optical correlator of claim 1, wherein said Fourier transform lenses
have two focal planes and said reference SLM and said filter SLM are at
said focal planes of said Fourier transform lenses, and wherein said
inverse Fourier transform lenses have two focal planes and said filter SLM
and said camera are at said focal planes of said inverse transform lenses.
9. An optical correlator comprising:
a laser for producing a beam of coherent polarized light;
a reference spatial light modulator (SLM) for modulating and reflecting an
incident beam of light, said reference SLM having:
(a) means for storing a reference image in the form of a two-dimensional
array of pixels;
(b) a ferroelectric liquid crystal (FLC) layer for selectively modulating
said incident beam by rotating the polarization of said beam for selected
pixels corresponding to said reference image; and
(c) a reflective backplane beneath said FLC layer for reflecting said
incident beam;
a first polarizing beamsplitter for directing said laser beam onto said
reference SLM and receiving said beam reflected from said reference SLM,
said first beamsplitter being polarized to transmit modulated light
reflected from said reference SLM and to block unmodulated light reflected
from said reference SLM;
Fourier transform lenses for receiving said modulated light beam
transmitted by said first polarizing beamsplitter and producing a Fourier
transform of said beam;
a filter SLM for modulating and reflecting an incident beam of light, said
filter SLM having:
(a) means for storing a filter image in the form of a two-dimensional array
of pixels, said filter image being the complex conjugate of the Fourier
transform of a desired image;
(b) a ferroelectric liquid crystal (FLC) layer for selectively modulating
said incident beam by rotating the polarization of said beam for selected
pixels corresponding to said filter image; and
(c) a reflective backplane beneath said FLC layer for reflecting said
incident beam;
a second polarizing beamsplitter for directing said Fourier transform beam
onto said filter SLM and receiving said beam reflected from said filter
SLM, said second polarizing beamsplitter being polarized to transmit
modulated light reflected from said filter SLM and to block unmodulated
light reflected from said filter SLM;
inverse Fourier transform lenses for receiving said modulated light beam
transmitted by said second polarizing beamsplitter and producing an
inverse Fourier transform of said beam;
a CCD camera for detecting any correlation peak produced by said inverse
Fourier transform lenses; and
a computer processor connected to said reference SLM and to said filter SLM
for downloading images to said SLM's, and for analysis of correlation
peaks detected by said CCD camera.
10. The optical correlator of claim 9 wherein said SLM pixels are
electrically addressable by said computer processor.
11. The optical correlator of claim 9, further comprising a polarizing
filter between said inverse Fourier transform lenses and said CCD camera.
12. The optical correlator of claim 9, further comprising a half-wave plate
between said first polarized beamsplitter and said reference SLM for
adjustably aligning the polarization of light entering said reference SLM.
13. The optical correlator of claim 9, further comprising a half-wave plate
between said second polarized beamsplitter and said filter SLM for
adjustably aligning the polarization of light entering said filter SLM.
14. The optical correlator of claim 9, wherein said Fourier transform
lenses have two focal planes and said reference SLM and said filter SLM
are at said focal planes of said Fourier transform lenses, and wherein
said inverse Fourier transform lenses have two focal planes and said
filter SLM and said camera are at said focal planes of said inverse
transform lenses.
15. An optical correlator comprising:
a laser for producing a beam of coherent polarized light;
a reference spatial light modulator (SLM) for modulating and reflecting an
incident beam of light, said reference SLM having:
(a) an electrically addressable memory for storing a reference image in an
array of pixels forming a reflective backplane; and
(b) a ferroelectric liquid crystal layer (FLC) for selectively modulating
said incident beam by rotating the polarization of said beam for selected
pixels corresponding to said reference image;
a first polarizing beamsplitter for directing said laser beam onto said
reference SLM and receiving said beam reflected from said reference SLM,
said first beamsplitter being polarized to transmit modulated light
reflected from said reference SLM and to block unmodulated light reflected
from said reference SLM;
a first half-wave plate between said first polarized beamsplitter and said
reference SLM for adjustably aligning the polarization of said incident
beam entering said reference SLM;
Fourier transform lenses for receiving said modulated light beam
transmitted by said first polarizing beamsplitter and producing a Fourier
transform of said beam;
a filter SLM for modulating and reflecting an incident beam of light, said
filter SLM having:
(a) an electrically addressable memory for storing a filter image in a
two-dimensional array of pixels forming a reflective backplane, said
filter image being the complex conjugate of the Fourier transform of a
desired image; and
(b) a ferroelectric liquid crystal (FLC) layer for selectively modulating
said incident beam by rotating the polarization of said beam for selected
pixels corresponding to said filter image;
a second polarizing beamsplitter for directing said Fourier transform beam
onto said filter SLM and receiving said beam reflected from said filter
SLM, said second polarizing beamsplitter being polarized to transmit
modulated light reflected from said filter SLM and to block unmodulated
light reflected from said filter SLM;
a second half-wave plate between said second polarized beamsplitter and
said filter SLM for adjustably aligning the polarization of said incident
beam entering said filter SLM;
inverse Fourier transform lenses for receiving said modulated light beam
transmitted by said second polarizing beamsplitter and producing an
inverse Fourier transform of said beam;
a CCD camera for detecting any correlation peak produced by said inverse
Fourier transform lenses; and
a computer processor connected to said reference SLM and to said filter SLM
for downloading images to said SLM's, and for analysis of correlation
peaks detected by said CCD camera.
16. The optical correlator of claim 15, further comprising a polarizing
filter between said inverse Fourier transform lenses and said CCD camera.
17. The optical correlator of claim 15, wherein said Fourier transform
lenses have two focal planes and said reference SLM and said image SLM are
at said focal planes of said Fourier transform lenses, and wherein said
inverse Fourier transform lenses have two focal planes and said filter SLM
and said CCD camera are at said focal planes of said inverse transform
lenses. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of optical
correlators. More specifically, the present invention discloses an optical
correlator using ferroelectric liquid crystal spatial light modulators.
2. Statement of the Problem
Optical correlators were first suggested shortly after the advent of the
laser in the early 1960's by A. Vander Lugt et al. at the University of
Michigan. In early optical correlators, the input scene was introduced
into the correlator by means of a photographic film transparency. The
spatial filter was provided by means of a holographic film created by
generating a hologram of the filter image's Fourier transform. Special
care had to be taken to: (1) generate an acceptable spatial filter because
of the generally large dynamic range of the Fourier transform; and (2)
align the input scene's spectrum and filter encoded on the hologram. This
special care translated into many hours of filter preparation and tedious
mechanical alignment. However, once correlation was achieved, the
classical Vander Lugt correlator did a good job of recognizing and
locating patterns.
One major shortcoming of such simple matched spatial filtering is that the
filter is extremely sensitive to differences between the object in the
input scene and the object from which the filter is generated. If the
difference is more than a few degrees of in-plane rotation or a few
percent in scale, the focused points of light in the correlator quickly
defocus and the intensity of the signal diminishes. Thus, a robust pattern
recognition system for a dynamic application required a large number of
filters for each input to cover the different potential orientations and
scales of the target. This problem was magnified prior to the mid-1980's
by the fact that the process of changing the filter meant replacing the
piece of holographic film to within a few wavelengths of light. Thus,
until recently, optical correlation was viewed as good physics, but was
not practical for most pattern recognition applications in the field.
A key enabling technological development in recent years is the spatial
light modulator, or SLM. SLM's can be thought of as programmable
transparencies or pieces of film. The use of SLM's, instead of film, in an
optical correlator allows the system to rapidly change the input scene and
the spatial filter without mechanically moving or replacing pads, thus
accommodating the multiple-filter requirement necessary for practical
pattern recognition system. SLM's have different programming speeds and
resolutions depending on the type of material and the technique used to
encode the scene information on the SLM. The two types of SLM's that
appear best suited for use in two-dimensional pattern recognition systems
are the magneto-optic SLM (MOSLM) and the family of liquid crystal SLM's.
MOSLM devices are commercially available in pixel densities of up to
256.times.256 and have been demonstrated to operate at over 2000 Hz in
short bursts, with more practical operating frame rates of 500 Hz for a
128.times.128 device and 100 Hz for a 256.times.256 device. The modulating
principle of the MOSLM is Faraday rotation of the polarization vector of
the incident light as the light transmits through the MOSLM. The pixels
are independently and electronically addressed and are capable of binary
amplitude, binary phase, or ternary phase-amplitude (combination of binary
amplitude and binary phase) modulation.
Liquid crystal technology has long been used for incoherent imaging in such
applications as digital clocks, watches, and television displays. The
majority of these devices use a nematic liquid crystal material that
provides analog modulation, but is limited in switching speed. The most
prominent nematic liquid crystal SLM for coherent imaging is the liquid
crystal light valve, or LCLV. Unlike an MOSLM, an LCLV is optically
addressed, rather than electrically addressed. This requires the LCLV to
be programmed by another light source such as the illumination from a
mini-CRT display. An LCLV uses the birefringence property of the
crystalline structure and a controlled design thickness to achieve its
modulation capability. The device has a maximum resolution of
approximately 30 line pairs per millimeter, which equates to pixel
densities on the order of 750.times.750. It has a maximum operating speed
of approximately 25 to 30 Hz with the ability for modulating approximately
10 to 15 linear gray levels.
A second type of liquid crystal SLM that has recently made significant
advances in performance capabilities is the ferroelectric liquid crystal
(FLC) used in the present invention. The basic performance differences
between FLC SLM's and LCLV SLM's are their addressing capabilities and
frame rates. FLC's can be either optically or electrically addressed,
whereas LCLV's are only optically addressed. In addition, LCLV's operate
at only approximately 30 Hz. Optically addressed FLC's have been operated
at over 4500 Hz, and electrically addressed FLC's have been operated at
approximately 10,000 Hz. The modulation principle of the FLC SLM is
similar to the MOSLM in its operation (i.e., rotation of the modulator's
optic axis), but the rotation is due to a reorientation of the liquid
crystal molecules under an applied electric field instead of a Faraday
effect. The FLC SLM is optically more efficient than the MOSLM since its
axis rotation is .+-.22 degrees versus a few degrees for the MOSLM.
A number of optical correlators and SLM's have been patented in the past,
including the following:
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Inventor U.S. Pat. No.
Issue Date
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Yu 4,695,973 Sept. 22, 1987
Brooks 4,815,035 Mar. 21, 1989
Javidi 4,832,447 May 23, 1989
Moddel et al. 4,941,735 July 17, 1990
Juday 5,029,220 July 2, 1991
Marsh et al. 5,050,220 Sept. 17, 1991
Johnson et al.
5,073,010 Dec. 17, 1991
Capps 5,086,483 Feb. 4, 1992
Liu et al. 5,150,228 Sept. 22, 1992
Takesue et al.
5,150,229 Sept. 22, 1992
Moddel 5,177,628 Jan. 5, 1993
Moddel et al. 5,178,445 Jan. 12, 1993
Stappaerts et al.
5,221,989 June 22, 1993
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Yu discloses a real-time programmable optical correlator that incorporates
a magneto-optic spatial light modulator (MOSLM) and a liquid crystal light
valve (LCLV).
Javidi discloses an optical correlator that employs a spatial modulator
operating in a binary mode at the Fourier plane. The reference and input
images are illuminated by a coherent light at the object plane of a
Fourier transform lens system. An image detection device, such as a charge
couple device (CCD) is placed at the Fourier plane of this Fourier
transform lens system to detect the intensity of images. A thresholding
network generates a binary output for each pixel of the Fourier transform
interference intensity indicating whether the image intensity for that
pixel is greater than the median intensity.
Juday discloses an optical correlator for real-time tracking of the
position of the retina during laser eye surgery.
Capps discloses a hybrid optical/electronic processor in the general
configuration of a Vander Lugt optical correlator with an input SLM 12, a
first Fourier transform lens 16, a target SLM 14, a second Fourier
transform lens 18, and an electronic processing array 20. The processing
array 20 consists of a two-dimensional array of cells 40, each of which is
connected to its nearest neighbors to facilitate peak detection.
Takesue et al. disclose an optical correlator that generates pictorial
patterns of a sum of two patterns of pictorial information to be compared
and of a difference between the two patterns by a phase conjugate wave
form. The system then transforms the pictorial patterns into first Fourier
transform images, generates a pictorial pattern of a difference between an
intensity distribution of the first Fourier transform images by the phase
conjugate wave form, and transforms the pictorial pattern of a difference
between an intensity distribution of the first Fourier transform images
into second Fourier transform images. The optical correlator detects a
cross-correlation peak of the two patterns of pictorial information for
comparison at a high signal-to-noise ratio.
Liu et al. disclose another example of an optical correlator using liquid
crystal TV's (LCTV1 and LCTV2) to change the input and reference images in
real time.
Marsh et al. disclose an optical correlator for fingerprint identification.
Two spatial light modulators 28 and 32 are employed to input the unknown
fingerprint and a sequence of reference fingerprints for comparison.
The patents to Johnson et al., Moddel, and Moddel et al. disclose several
types of optically addressable spatial light modulators incorporating
ferroelectric liquid crystals.
Brooks discloses an example of a scrolling spatial light modulator using an
array of ferroelectric liquid crystal cells.
Stappaerts et al. disclose an example of a spatial light modulator using
non-ferroelectric PLZT ceramic.
3. Solution to the Problem
None of the prior art references uncovered in the search show an optical
correlator using electrically addressable FLC spatial light modulators in
the present optical configuration. The optical design is compact and
facilitates easier system alignment. The system offers real-time pattern
recognition at high image rates and at high resolution.
SUMMARY OF THE INVENTION
This invention provides an optical correlator using ferroelectric liquid
crystal spatial light modulators (FLC-SLM's) in both the reference and
filter planes. The SLM's include an electrically addressable memory to
store images in the form of a two-dimensional array of reflective pixels
beneath the FLC layer. The SLM's selectively rotate the polarization of
the light reflected by each pixel in accordance with the stored image. In
particular, a laser produces a polarized beam that is directed through a
first polarizing beamsplitter and onto the reference SLM. This
beamsplitter blocks unmodulated light reflected by the reference SLM and
transmits modulated light through a set of Fourier tranform lenses. The
resulting beam is directed through a second polarizing beam splitter and
onto a filter SLM that has been programmed with the complex conjugate of
the Fourier transform of a desired target image. Unmodulated light
reflected from the filter SLM is blocked by the second polarizing
beamsplitter and modulated light is reflected by the second polarizing
beamsplitter through a set of inverse Fourier transform lenses. A CCD
camera detects any correlation peak produced by the inverse Fourier
transform lenses. A computer system downloads images to the SLM's and
analyzes the correlation peaks detected by the camera. A half-wave plate
can be included between both sets of SLM's and polarizing beamsplitters to
allow manual adjustment of the polarization of the incident beam entering
the SLM.
A primary object of the present invention is to provide an optical
correlator capable of performing automated pattern recognition at high
speeds.
Another object of the present invention is to provide an optical correlator
that is compact and portable.
Yet another object of the present invention is to provide an optical
correlator that is easy to initially align and maintains its alignment.
These and other advantages, features, and objects of the present invention
will be more readily understood in view of the following detailed
description and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention can be more readily understood in conjunction with
the accompanying drawings, in which:
FIG. 1 is a simplified block diagram of the optical correlator.
FIG. 2 is a cross-sectional view of an electronically addressable
ferroelectric liquid crystal spatial light modulator.
FIG. 3 is simplified block diagram of the computer system used to interface
with and control the optical correlator.
DETAILED DESCRIPTION OF THE INVENTION
Turning to FIG. 1, a block diagram is provided of the overall optical
configuration of the present optical correlator. In short, the present
invention is a fully programmable Vander Lugt correlator. Two electrically
addressable ferroelectric liquid crystal (FLC) spatial light modulators
(SLM's) 16 and 26 are located at the focal planes of the Fourier transform
lenses 20. The reference SLM 16 is used to input image data. The filter
SLM 26 is used to spatially filter the input data for a desired target
image. The image data from the reference SLM 16 is transformed by the
Fourier transform lenses 20 into its spatial frequencies. The filter SLM
26 multiplies the spectral components of the image data with a pattern
that extracts the desired image from the input data. The filter is
designed such that its frequency response is the complex conjugate of the
desired target image (i.e., the filter is the complex conjugate of the
Fourier transform of the desired image). The filter is generated and
implemented as a binary phase-only filter (BPOF). The modulated light from
the filter SLM 26 is inverse transformed by an inverse Fourier transform
lens 28, resulting in the convolution of the image data with the filter
pattern. An image pattern that matches the filter will produce a
collimated wavefront that is focused to a bright spot in the correlation
plane where the camera 32 is located. The positions of any bright spots
will coincide with the locations of the matching patterns in the input
image. Phase distortion (i.e., spreading of the correlation peak) is
minimized since both the filter SLM 26 and the correlation plane are at
the focal planes of the inverse transform lens 28. Hence, all of the
correlator's processing elements are at the focal planes of the two
transform lenses 20 and 28. This general configuration is sometimes
referred to as a "4f correlator."
Spatial Light Modulator
FIG. 2 provides a cross-sectional view of the ferroelectric liquid crystal
SLM 16, 26. This device uses a dynamic memory on a very large scale
integration (VLSI) backplane to activate a liquid crystal modulator. A
transparent and conductive indium oxide (ITO) layer 51 is deposited on the
undersurface of the cover glass 50. This surface is then coated with an
alignment layer 52. A preferred alignment material is polybutylene
teraphthalate (PBT). Other alignment materials, such as polyvinyl alcohol,
silicon monoxide (SiO), silicon dioxide (SiO.sub.2), and Langmuir-Blodgett
films are also suitable. A metallic electrode 54 with an electrode wire 53
is mechanically bonded to the cover glass 50. This metallic electrode 54
is also electrically connected to the ITO layer 51 to produce a
transparent top electrode. A smectic C.sup.* ferroelectric liquid crystal
(FLC) layer 56 is placed within a SiO spacer 55 between the top electrode
and the VLSI chip 58. The VLSI chip is bonded to a PGA socket that
provides external electrical connections and mechanical stability.
The VLSI backplane consists of a two-dimensional array of conductive pads,
each of which form one pixel. These conductive pads act as electrodes to
apply voltage across the FLC layer. The conductive pads also serve as
mirrors that output the SLM's signal by reflection, since the VLSI
backplane is non-transmissive. Each pad is electrically connected to an
independent dynamic memory cell within the VLSI chip. Each memory cell
stores a binary bit of data (i.e., 1 or 0). Binary data is sequentially
loaded by rows into the dynamic memory cells by means of an external
computer and interface board, as shown in FIG. 3. A load cycle consists of
writing data to each row comprising one image frame. A load cycle either
writes new binary image data to the SLM or refreshes the old image data.
SLM's incorporating this design have been produced by Boulder Nonlinear
Systems, Inc., of Boulder, Colo., providing either 128.times.128 pads or
256.times.256 pads.
The 1's and 0's stored in each memory cell actually represent a voltage
(i.e., +5 V or 0 V, respectively). These voltages are applied to the
conductive pixel pad to produce an electric field between the pad and the
transparent top electrode. By applying 2.5 volts to the top electrode, the
electric field vectors at each pixel have equal magnitude, but the
electric field vectors change direction depending on whether the data bit
is a 1 or 0. The direction of the electric field vectors switches the FLC
into one of two states by interacting with the polarized FLC molecule to
produce either right or left handed torque on the molecule. A FLC molecule
is free to rotate through small angles and will pivot about the smectic
layer normal orientation (.alpha..sub.0) until the torque, viscous, and
elastic forces are equalized. This molecular rotation results in a bulk
reorientation (or tilt) of the liquid crystal's optical axis. A nonlinear
FLC material acts as a half-wave retarder. A half-wave retarder rotates
the light's polarization by 2.phi., where .phi. is the angle between the
light's polarization and the waveplate's optic axis. Here, the
polarization of the incident beam 40 is rotated by twice the tilt angle
(.PSI.) of the optic axis. For example, if the FLC's optic axis tilts
.+-.22 degrees about the smectic layer normal orientation, the net change
of 44 degrees in the optic axis will rotate the light's polarization by 88
degrees.
Rotation of the FLC material's optic axis (which is controlled by the
direction of the electric field) produces a change in the light's
polarization. This change in polarization can be converted to amplitude or
phase modulation depending on the orientation of the FLC layer with
respect to the polarization of the incident beam 40. Binary amplitude
modulation occurs when the input light enters the SLM polarized along the
optic axis orientation of one of the FLC's switched states and is
reflected back through an output analyzer that is cross polarized to the
input light. When the incident beam's polarization is aligned with the
optic axis director, the reflected beam's polarization remains unchanged
since the angle between the light's polarization and the optic axis
director is zero. The reflected beam 41 remains cross polarized to the
output analyzer and is blocked (Off state). When the input beam's
polarization is not aligned with the optic axis director, the light's
polarization is rotated by 2.phi., where .phi.=2.PSI.. A portion of this
light is transmitted by the output analyzer, producing an On state.
Binary phase or bipolar modulation occurs when the input light enters the
device polarized along the smectic layer normal orientation. Again the
reflected light 41 is analyzed by a crossed polarizer. In the bipolar
case, the switched states rotate the light's polarization by .+-.2.phi.
about the out analyzer's axis, where .phi.=.PSI.. Light transmitted by the
output analyzer has equal amplitude but varies in phase by 180 degrees.
Both phase and amplitude modulation have certain characteristics that are
useful in optical correlators. Amplitude modulation is very useful for
verifying SLM operation and troubleshooting the correlator system since
the image is visible. Phase modulation, on the other hand, provides better
performance because it reduces various noise sources without decreasing
signal power. The optical design for the correlator described herein
allows use of either phase or amplitude modulation with only minor
adjustments.
Optical Configuration
Returning to FIG. 1, the present invention uses polarization to modulate
and direct light through the correlator. A laser diode source 10 produces
a beam of coherent light that is polarized in a predetermined plane (i.e.,
S polarized). A first polarizing beamsplitter 12 intercepts this beam and
reflects it through a half-wave plate 14 onto the reference SLM 16. The
half-wave plate 14 rotates the polarization of the beam, so that the input
light's polarization is aligned with the FLC's smectic layer normal for
the reference SLM 16 | | |
