An optical theta-modulation based analog-to-digital converter having an acousto-optic deflector and an analog-to-digital conversion mask, for fast and flexible analog-to-digital conversion. The acousto-optic deflector deflects an optical beam at an angle corresponding to an input voltage signal having an analog value. The analog-to-digital conversion mask converts the deflected optical beam into N masked optical signals which each corresponds to a respective bit of a digital value representing the analog value of the input signal. Each optical signal is detected by a photodetector and converted into an output voltage signal by a comparator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to high precision numerical data processing, and 
more particularly to high speed optical analog-to-digital conversion. 
2. Background of the Invention 
For high precision numerical data processing, digital (rather than analog) 
computation is preferred. To increase the data processing rate, both fast 
signal processing and fast analog-to-digital (A/D) conversion are 
required. Optical A/D conversion has been investigated as an alternative 
to semiconductor or superconductor based A/D conversion. Wide-band, fast 
optical A/D conversion based on electro-optic (E-O) waveguide 
interferometric devices has been proposed. Recently, optical A/D 
conversion based on an optical logic gate array and an optical table 
look-up processor has been proposed. 
Optical A/D conversion based on an optical theta-modulation and table 
look-up (TMTL) has also been proposed. The concept of using TMTL for 
signal processing has been known for decades, and the TMTL concept has 
been implemented by deflecting an electron-beam between deflection plates 
in an oscilloscope. When used in a video display, the resultant device 
could reach over 1000 resolvable levels. 
The TMTL concept has also been implemented by an optical theta-modulation 
based on a variable grating mode (VGM) liquid-crystal (LC) device for 
optical signal processing. Such a VGM LC has also been proposed for the 
optical implementation of optical logic and signal processors. However, 
such a device has serious problems, such as a limited dynamic range and a 
limited switching time, which have inhibited high speed applications. 
SUMMARY OF THE INVENTION 
An object of the present invention is to overcome the problems and 
disadvantages of the prior art by an optical theta-modulation and table 
look-up (TMTL) based analog-to-digital (A/D) converter having an 
acousto-optical (A-O) deflector. 
Another object of the present invention is to provide an optical 
analog-to-digital conversion system wherein an input voltage can be 
substantially identically mapped into a beam deflection angle. 
A further object of the present invention is to provide an optical 
analog-to-digital conversion system having a wide bandwidth and a fast 
processing rate. 
Still another object of the present invention is to provide an optical 
analog-to-digital conversion system that performs real-time optical 
theta-modulation. 
Another object of the present invention is a reliable optical 
analog-to-digital conversion system. 
To achieve these and other objects and in accordance with the purpose of 
the invention, as embodied and broadly described herein, the optical 
analog-to-digital conversion system of the present invention comprises 
input signal source means for providing an input voltage signal having an 
analog value; light source means for generating an optical beam; 
acousto-optic (A/D) deflector means, coupled to the input signal source 
means and the light source means, for deflecting the optical beam at an 
angle corresponding to the analog value of the input voltage signal; an 
analog-to-digital conversion mask means, coupled to the acousto-optic 
deflector, for converting the deflected optical beam into N masked optical 
signals where N is an integer greater than unity and represents the number 
of bits of a digital value corresponding to the analog value; and 
detection means, coupled to the analog-to-digital conversion mask means, 
for detecting the N masked optical signals and converting each detected N 
masked optical signal into an output voltage signal corresponding to a 
respective bit of the digital value. 
The accompanying drawings, which are incorporated in and constitute a part 
of this specification, illustrate an embodiment of the present invention 
and together with the description, serve to explain the principles of the 
invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference will now be made in detail to the present preferred embodiment of 
the invention, an example of which is illustrated in the accompany 
drawings. Wherever possible, the same reference numbers will be used 
throughout the drawings to refer to the same or like parts. 
The optical analog-to-digital conversion system, for example a TMTL based 
A/D conversion system, of the present invention comprises input signal 
source means for providing an input voltage signal having an analog value. 
Referring to FIG. 1, the input signal source means, as embodied herein, 
comprises a voltage signal source 12 that provides an analog input signal 
having a voltage V.sub.m to be digitally converted. A light source means 
is provided for generating an optical beam. As embodied herein, the light 
source means comprises a one-dimensional input laser 13 (or optical), 
which has been spatially filtered. 
The optical beam is focused onto acousto-optic (A/D) deflector means, 
coupled to the input signal source means and the light source means, for 
deflecting the optical beam at an angle corresponding to the analog value 
of the input voltage signal. Acousto-optic (A/D) deflector means may 
comprise an A-O deflector RF driver 14, which is connected to voltage 
signal source 12, and generates an RF signal of a FM type, which 
represents a linear conversion of the input voltage signal from voltage 
signal source 12 into a frequency function. The RF driver 14, as embodied 
herein, may include a voltage controlled frequency oscillator and an RF 
amplifier. The RF signal generated by RF driver 14 modulates the 
deflection of the input laser beam or optical beam from A-O deflector 10 
in the Bragg's regime. The voltage (V.sub.m) controlled deflected angle 
(.theta.) is 
##EQU1## 
where C, in an ideal situation, is a constant, and where .lambda., 
.nu..sub.A and .DELTA.F are the optical wavelength, the acoustic wave 
velocity and the bandwidth of the cell of A-O deflector 10, respectively. 
The optical TMTL based conversion system, according to the present 
invention, further comprises an analog-to-digital conversion mask means, 
coupled to the acousto-optic deflector means, for converting the deflected 
optical beam into N masked optical signals where N is an integer greater 
than 1 and represents a number of bits of a digital value corresponding to 
the analog value. As embodied herein, the analog-to-digital conversion 
mask means comprises an anamorphic optical system 16 and a binary A/D 
conversion mask 18. 
The deflected optical beam passes through the anamorphic optical system 16, 
which horizontally (or vertically) images (or focuses) the optical beam 
onto binary A/D conversion mask 18. The anamorphic optical system 16 may 
include cylindrical lenses 16.sub.1, and 16.sub.3, and a lens 16.sub.2. 
For a rectangular beam aperture L.sub.s x L.sub.y at the cell of A-O 
deflector 10 coupled with anamorphic optical system 16, the deflected 
optical signal U.sub.M (x,y) at A/D mask 18 is: 
##EQU2## 
where F[] denotes a spatial Fourier transform, and where .alpha. and f are 
a magnification constant and the focal length of the Fourier transform 
lens, respectively. 
The A/D conversion mask 18, as embodied herein, has a two-dimensional (2D) 
rectangular array of N.times.2.sup.N pixels (or N columns and 2.sup.N 
rows), i.e.: 
EQU M.sub.nm (x,y)=a.sub.mn .delta.(x-nd.sub.x, 
y-md.sub.y)*rect[x/d.sub.x,y/d.sub.y ] (3) 
where * denotes a convolution; where n=0 . . . N-1; m=0 . . . 2.sup.N -1; 
and where d.sub.x and d.sub.y are the pixel aperture sizes, and a.sub.mn 
is the mn.sup.th binary coefficient of the code to be converted into. 
Since A/D conversion mask 18, as embodied herein, contains N columns, an 
N-bit A/D conversion precision is obtained. For example, FIG. 2 shows 
rectangular arrays of 4 (N=4) columns and 16 (2.sup.N) rows for binary and 
gray code outputs. 
Along each column of the array of the A/D conversion mask 18, the masked 
optical signal is spatially integrated and the integrated signal is 
provided to detection means, coupled to the analog-to-digital conversion 
mask means, for detecting the N masked optical signals and converting each 
detected N masked optical signal into an output voltage signal 
corresponding to a respective bit of the digital value. As embodied 
herein, the detection means comprises an N-channel photodetector array 20. 
The integrated signal is provided to a respective one of N photodetectors, 
of the N-channel photodetector array 20. 
At the n.sup.th (n.ltoreq.N) photodetector, the detected optical signal 
I.sub.n is: 
##EQU3## 
Each detected signal from photodetector array 20 passes through a 
respective one of a plurality of comparators of a comparator array 22 and 
is converted to an output voltage signal. Therefore, comparator array 22 
generates N-bit A/D converted signals. FIG. 1 shows the connection of an 
oscilloscope 25 to the output of the comparator array to display the 
output of the comparator 22. 
If the light passes through the A/D conversion mask 18 (6-bit binary code 
pattern) while being scanned upward, the photodetectors 20 produce 
respective output signals as shown in FIG. 3. As seen in the top part of 
FIG. 3, the difference between the on and off levels becomes small in the 
case of the lowest bit. The well-separated signals having 1 and 0 levels 
as shown in FIG. 4 can be obtained by comparing the output signals of the 
photodetector array 20 with the reference value. 
To ensure a low error rate detection, RF driver 14, as embodied herein, may 
optionally include an electronic sample-and-hold circuit therein, or, in 
the alternative, a pulsed laser can be used as a sampler to the cell of 
A-O deflector 10. To prevent the deflected beam from A-O deflector 10 from 
scanning multiple rows of the A/D conversion mask 18, the pixel width 
along the deflection direction should be greater than the main lobe width 
of the focused band-limited optical beam. 
Since the detecting rate of the photodetector 20 is inversely proportional 
to the size of its area, A/D conversion mask 18, as embodied herein, may 
optionally include a cylindrical lens 26 to spatially integrate the masked 
optical signal along the column direction of A/D conversion mask 18. 
One key advantage of the optical analog-to-digital conversion system of the 
present invention is conversion flexibility. For example, unlike the E-O 
interference-based A/D converter which requires analog input signals to be 
converted into binary gray codes, the optical TMTL A/D converter of the 
present invention can convert into any digital code. This advantage is 
significant because for various computation applications, various digital 
codes including weighted codes (such as binary, binary-coded decimal, 
2421, 6423, etc.) and non-weighted codes (such as cyclic, gray, excess-3, 
etc.) are often required. In the TMTL-based optical analog-to-digital 
conversion system of the present invention, to make a particular code 
change, it is sufficient only to change a precomputed mask array M.sub.mn 
(x, y) (i.e., equation (3) of A/D conversion mask 18) without modifying 
the remaining optical elements. For example, FIG. 2 shows an exemplary 
mask array having four columns, b0, b1, b2 and b3, and sixteen rows, 0-15, 
for binary, four columns, g0, g1, g2 and g3, and sixteen rows, 0-15, for 
gray code output. 
Yet another advantage of the TMTL-based analog-to-digital conversion system 
of the present invention is the use of the A-O deflector 10. Since the A-O 
deflector 10 is proven technology, a reliable converter can be constructed 
therewith. As compared to E-O waveguide devices, A-O deflectors provide a 
larger dynamic range, a larger time-bandwidth product, and are easier to 
operate and less expensive to implement. The E-O waveguide based A/D 
converter has further limitations. For example, to process each additional 
bit with an E-O waveguide based converter a pair of electrodes having 
lengths that are double the length of, the present, the longest electrode 
must be added. The use of long electrodes may cause problems such as 
nonuniform modulation of electrodes, limiting the precision of the 
conversion. 
The conversion precision of the A-O deflector-based TMTL A/D converter 
primarily depends on the time-bandwidth product (TBP) of the A-O 
deflector's. For example, to generate an error-free A/D conversion mask, a 
diffraction-related restriction on the minimum pixel size d.sub.y needs to 
be set, i.e.: 
##EQU4## 
where f, .lambda. and L.sub.y are the lens focal length, the optical 
wavelength, and the vertical mask length, respectively. Thus, the maximum 
number of useful pixels is M, where: 
##EQU5## 
where .DELTA.F and .tau. are the A-O bandwidth and the acoustic wave 
aperture time, respectively. Thus, the useful pixels for the A-O 
TMTL-based analog-to-digital conversion system of the present invention is 
half of the maximum number of A-O cell's resolvable points determined by 
its TBP. 
Since the number of converted bits is N=log.sub.2 M, the A-O deflector 
TMTL-based system of the present invention yields higher conversion 
precision than can be expected from conventional E-O and A/D converters. 
It has been shown that when a TeO.sub.2 A-O cell is used in a slow shear 
wave mode, a TBP of 2000 (40 MHz bandwidth and 50 .mu.s aperture time) is 
obtainable, which in turn provides about a 10-bit A/D conversion 
precision. For most A-O materials with a TBP of about 1000, the 
implementation of a 100 MHz 8-bit A-O deflector TMTL-based system is 
feasible. 
Yet another advantage of the TMTL A/D converter of the present invention is 
that it allows for calibration to compensate for device nonlinearity. The 
source of such a nonlinearity may come from Bragg's cell, or its driver, 
or both. As a result of fluctuations of the coefficient C in Eq.(1) as a 
function of V.sub.m, the deflection angle .theta.(V.sub.m) becomes a 
nonlinear function of input voltage. This parametric nonlinearity can be 
calibrated by using nonuniform pixel size along the mask's vertical (or 
column) direction. In this case, the same low A/D conversion error rate is 
achievable at the expense of wasting the device TBP. It can be shown that 
at the presence of nonlinearity the maximum number of convertible bits is: 
##EQU6## 
where .vertline.d.theta./ dV.sub.m .vertline..sub.min &gt;0 and V.sub.o is 
the voltage corresponding to the maximum deflection angle. 
The A-O deflector 10 of the TMTL-based analog-to-digital conversion system, 
as embodied herein, may include a GaP A-O deflector (Brimrose DF-30) which 
has a 200 MHz bandwidth centered at 650 MHz. The GaP A-O deflector is 
coated for a maximum transmittance at .lambda.=633 nm. The diffraction 
efficiency and Bragg's angle are 40% at 1 W and 1.7.degree., respectively. 
Although a much higher A/D sample rate, which is equal to the A-O cell's 
repetition rate (e.g., 10 MHz), is achievable, the GaP A-O deflector is 
operated at a rate of 5 KHz for the ease of detection with a 30 mW CW HeNe 
laser. 
A/D conversion mask 18, as embodied herein, may include a 6-bit laser 
printer printed mask that has been linearly demagnified 15 times to an 
approximate size of 9.times.10 mm.sup.2 and photoprinted onto a chromium 
plate. Each mask pixel of the mask is about 1500.times.250 .mu.m.sup.2. To 
avoid device nonlinearity, only part of the deflection range is used. To 
generate this angular scan, a scan of modulated voltages V.sub.m from 0.4 
to 28 V can be used. 
Referring to FIG. 3, the detected and converted signals from all six (or 
N=6) A/D converter channels are displayed from the lowest bit to the 
highest bit. This A-O deflector TMTL-based analog-to-digital conversion 
system can provide each detected signal simultaneously in parallel in 
response to a fixed analog voltage signal. 
FIGS. 4(a) through 4(d) show binary results 001010, 101010, 110101, and 
111111 of A/D conversion of input analog signals of 0.8 V, 20 V, 2.4 V, 
and 2.8 V, respectively. Error spikes appearing in the figures are due to 
cross-talk which can be minimized by using high quality optics and masks. 
The TMTL-based analog-to-digital conversion system of the present invention 
is simple in construction and the free space beam steering capability of 
the A-O deflector is fully utilized. Application of the system of the 
present invention can be further extended in conjunction with 
electro-optic and all-optic beam deflectors. An added advantage is the 
system's unique error correction capability. The system of the present 
invention can find applications where analog signals need to be processed 
digitally, e.g., optical sensing, digital signal and data processing, and 
optical computing. 
Other embodiments of the invention will be apparent to those skilled in the 
art from consideration of the specification and practice of the invention 
disclosed herein. It is intended that the specification and examples be 
considered as exemplary only with a true scope and spirit of the invention 
being indicated by the following claims.