Electro-optical analog-to-digital converter and method for digitizing an analog signal

An electro-optical analog-to-digital converter having an enhanced effective sample rate. Several embodiments of the electro-optical analog-to-digital converter (10, 100, 150, and 200) are described, each involving either space-division demultiplexing, time-division demultiplexing, or wavelength-division demultiplexing to increase the effective sample rate at which an analog signal provided by a source (36) can be converted to a digital signal using electrical analog-to-digital converters (66). A plurality of light pulses having a constant amplitude are modulated in response to the analog signal and demultiplexed using one of the three different techniques, so that the analog signal is sampled at successive points in time, varying the intensity of light pulses passing through each modulator channel (32, 104). In the space-division demultiplexing approach, light pulses produced by a mode-locked diode laser (12) are split into a plurality of channels and delayed by different time intervals so that the replicated pulses are spaced apart in time prior to being modulated by passage through a modulator array (32). The demultiplexed and modulated light pulses are input to a plurality of photodetectors (56) amplified by a plurality of amplifiers (60 ), and input to a plurality of electrical analog-to-digital converters (64). In the time-division multiplexing approach, either discrete optical switches (108) or an optical switch array (158) is used to transmit successive modulated light pulses into selected photodetectors for conversion to an electrical signal that is input to the analog-to-digital converters. The wavelength-division multiplexing approach uses a plurality of mode-locked diode lasers (210), each having a different characteristic wavelength. Pulses output from the mode-locked diode lasers are generated at different times and provided to a wavelength-division multiplexer (214) where they are combined and input to the modulator. A wavelength-division demultiplexer (220) separates the different wavelength pulses for input to selected photodetectors. For all the embodiments, the effective sample rate corresponds to the number of channels multiplied by the rate at which pulses are produced by the mode-locked laser diodes.

FIELD OF TECHNOLOGY 
This invention generally pertains to a high speed, analog-to-digital signal 
converter and a method for digitizing analog signals, and more 
specifically, to an A-D conversion apparatus and method that use a pulsed 
laser to sample the analog signal. 
BACKGROUND OF THE INVENTION 
Transformation of wideband data signals from the analog-to-digital domain 
may require sample rates that are generally not available in pure 
electronic A-D converters. Using currently available technology, 
electronic A-D converters are limited to about 2 gigasamples/second and 
about 4-bits/sample of resolution. Certain applications require A-D 
conversion of signals using a sample rate of from 5-10 gigasamples/second, 
with 6-8 bits of resolution. Ideally, the sample rate of a suitable A-D 
converter should be from 2.5 to 4 times the maximum bandwidth of the 
analog signal digitized. 
The fastest commercially available A-D converters are flash converters, 
which comprise a sample and hold circuit and a digitizer circuit. By using 
demultiplexing or deinterleaving techniques, the sample rate of such 
electronic A-D systems may be extended to about two gigasamples per second 
at about 6-bits of resolution. 
Recognizing the limitations on bandwidth, sampling rate, and resolution of 
electronic A-D converters, the prior art has turned to optical devices to 
substantially improve upon these parameters. For example, U.S. Statutory 
Invention Registration H353 discloses an optical converter with expanded 
dynamic range, which is achieved by dividing the input signal into an 
optically modulated light pulse signal comprising least significant bits 
(LSB) and most significant bits (MSB) representations. The LSB and MSB 
representations are then interleaved to form a final binary representation 
of the input analog signal. In the apparatus used to accomplish this task, 
a mode-locked laser provides a source of light pulses that are conveyed to 
a two channel linear, electro-optic, interferometric modulator, which is 
fabricated on a single crystal substrate. A phase bias is introduced in 
one of the modulated signals. Optical detectors convert the two LSB and 
MSB modulated signals emerging from the modulator and an unmodulated 
signal into analog electrical signals. A processor determines the digital 
values associated with the LSB and MSB representations based on values 
stored in a lookup table. While this method appears to improve the 
resolution of the conversion process, it is apparently limited to about 
one to two gigasamples/second. 
In U.S. Pat. No. 4,502,037, an A-D converter that includes an optical 
modulator is disclosed, which includes one interferometer channel for each 
bit of a digital output word. The output word corresponds to the magnitude 
of an analog input signal. The modulator applies a phase shift to each 
channel used to modulate light from a laser source, and the modulated 
light is demodulated by an array of photodetectors and comparators to 
produce a corresponding digital signal. The patent teaches that the 
response time of the A-D converter is around one nanosecond per output 
word, no matter what the number of stages, i.e., about one 
gigasamples/second. 
An alternative arrangement for an optical modulator A-D converter system is 
disclosed in U.S. Pat. No. 4,694,276. To achieve greater resolution, light 
from multiple wavelength lasers is combined and transmitted through an 
interferometric modulator. Light modulated therein in response to an 
analog input signal is split into two beams of different frequency using 
an optical grating or prism, and the beams are directed to photodetectors. 
The output of the photodetectors provides two bits of resolution. The 
patent teaches that additional lasers of different characteristic 
wavelengths or additional modulators and gratings can be provided to 
produce greater resolution. Since each bit-pair requires either two lasers 
or two modulator channels and a prism/grating, this technique appears to 
be unduly complex and inappropriate for use in some applications. 
Accordingly, it is an object of the present invention to provide an A-D 
converter having a sample rate of up to ten gigasamples/second and with at 
least 6-8 bits of resolution. A further object is to minimize the cost of 
the A-D converter by keeping its fabrication cost and parts count 
relatively low. These and other objects and advantages of the present 
invention will be apparent from the attached drawings and the Description 
of the Preferred Embodiment that follows. 
SUMMARY OF THE INVENTION 
A high speed, electro-optical analog-to-digital convertor is claimed. It 
includes clock means for producing synchronization signals, and 
synchronized light source means, connected to receive the synchronization 
signals and operative to produce pulses of light in response. Modulator 
means are provided for modulating the intensity of the light pulses, so 
that the intensity of successive light pulses varies in time in response 
to the amplitude of the analog signal. Demultiplexing means are operative 
to demultiplex the light pulses produced by the synchronized light source 
means. Light pulses that have been demultiplexed are received by detector 
means, which produce electrical signals having a magnitude that is 
proportional to the intensity of the demultiplexed light pulses. A 
plurality of electrical, analog-to-digital converters are each connected 
to receive a different one of the electrical signals and a synchronization 
signal, and are operative to produce digital data that vary as a function 
of the magnitude of the electrical signals. These digital data correspond 
to the amplitude of the analog signal at the times the light pulses were 
modulated. 
Preferably, the synchronized light source means comprise a mode-locked 
laser diode, and the demultiplexing means comprise splitter means for 
splitting each pulse of light produced by the mode-locked laser diode into 
a plurality of light pulses that are simultaneously output from the 
splitter means and conveyed to the modulator means. The demultiplexing 
means can further comprise delay means, for introducing a different delay 
interval in the time required for the light pulses simultaneously output 
from the splitter means to enter the modulator means. The modulator means 
include a plurality of modulation channels connected in parallel to the 
analog signal. Thus, each of the plurality of light pulses that are 
delayed enter a different modulation channel at different times, and the 
intensity of the light pulses passing through each modulation channel is 
modulated by the amplitude of the analog signal at one of the different 
times. Compensatory delay means can be disposed between the modulator 
means and the detector means, for delaying each of the demultiplexed and 
modulated light pulses by different intervals of time appropriate to 
compensate for the delay intervals introduced in the light pulses input to 
the modulator means, so that each of the demultiplexed and modulated light 
pulses arrive at the detector means at substantially the same time. 
One of the light pulses from the splitter means comprises a reference light 
pulse that is not modulated as a function of the amplitude of the analog 
signal. The reference light pulse is separately supplied to the detector 
means to provide a reference intensity with respect to other light pulses 
output from the splitter means, in order to compensate for variations in 
the intensity of the light pulses produced by the mode-locked laser over 
time. 
In one preferred form, the detector means comprise a plurality of separate 
photodetector channels. The demultiplexing means then can comprise switch 
means disposed between the modulator means and the detector means, for 
connecting successive modulated light pulses output from the modulator 
means to different photodetector channels, as a function of the 
synchronization signals produced by the clock means. The switch means can 
comprise an array of electronically controlled optical switches connected 
in series and spaced apart by a distance corresponding to the interval of 
time between successive modulated light pulses. Each optical switch has a 
plurality of outputs. At least one output of each optical switch is 
connected to one of the photodetector channels. Another output is 
connected to an input of another optical switch in the array, if any 
remain, and if not, to another of the photodetector channels. The switch 
means further comprise pulse divider means that are connected to receive 
the synchronization signals. For M optical switches in the array, the 
pulse divider means divide the synchronization signals by M, producing a 
gate control signal. In response to the gate control signal, the optical 
switches divert each of the modulated light pulses propagating through the 
array to a different photodetector. 
Alternatively, the switch means can comprise a plurality of optical 
switches, each of which has an input and an output. The optical switches 
are connected in a fan-out array in which input and outputs of at least 
some of the optical switches are connected together in a parallel and 
series relationship. Terminal outputs of the fan-out array are each 
connected to a different photodetector. In this case, the demultiplexing 
means comprise switch control means that are connected to receive the 
synchronization signals and are operative to produce a plurality of 
different control signals, as a function of the synchronization signals. 
The control signals close specific optical switches to select a different 
modulated light pulse in a stream of successive modulated light pulses 
passing through the fan-out array, for input to each of the 
photodetectors. Delay means can be disposed between the array of optical 
switches and the detector means to delay the modulated light pulses input 
into each photodetector by a different interval of time. The intervals of 
time are selected so that the modulated light pulses arrive at the 
photodetectors at substantially the same time. 
In another preferred form, the synchronized light source means comprise 
means for producing light pulses having substantially different 
wavelengths. Each light pulse is produced during a different portion of a 
period of the synchronization signal produced by the clock means. The 
analog-to-digital converter further comprises multiplexing means that are 
connected to receive the different wavelengths of light and are operative 
to produce a wavelength-division multiplexed signal that is input to the 
modulator means. Accordingly, the demultiplexing means comprise a 
wavelength-division demultiplexer disposed between the modulator means and 
the detector means, which produces a plurality of demultiplexed and 
modulated light pulses of the different wavelengths. 
A method for electro-optically digitizing an analog signal at a high speed 
comprises a further aspect of this invention. The method includes steps 
that generally correspond to the functions performed by the 
analog-to-digital converter described above.

DISCLOSURE OF THE PREFERRED EMBODIMENTS 
With reference to FIG. 1, a first embodiment of the electro-optical A-D 
system is shown, generally indicated at reference numeral 10. This 
embodiment uses a space-division demultiplexing architecture. 
Electro-optical A-D system 10 includes a mode-locked diode laser 12, which 
generates short optical pulses. As is common with mode-locked lasers, an 
appropriate length fiber-optic pigtail 13 (with an appropriate reflective 
tip - not shown) is connected at one end of the laser to establish the 
desired operational mode of the laser. The light pulses produced by 
mode-locked diode laser 12 are synchronized to a clock signal conveyed 
over a lead 15 from a clock synchronization circuit 14. Presently 
available diode lasers can produce light pulses having pulse widths of 
less than 10 picoseconds, at a repetition rate of up to 16 GHz, and with a 
pulse jitter of 0.1 to 0.5 picoseconds-fully adequate for this 
application. 
Light pulses 16 produced by mode-locked diode laser 12 are conveyed over an 
optical fiber 18 to the input of an optical signal splitter 20. Splitter 
20 may comprise a planar waveguide with a single-mode input and a 
plurality of "Y-branches" interconnected to split incoming light pulses 
for propagation through five (or more) single-mode output channels. The 
optical splitter may also comprise a plurality of single-mode polarization 
maintaining fiber couplers with a fixed splitting ratio. A suitable fiber 
coupler is the Model 904P, available from Canadian Instrumentation and 
Research Limited. Again, several such devices connected in a tree 
structure (not shown) can provide the replicate light pulse outputs. 
Splitter 20 divides each incoming light pulse between five output optical 
fibers 22, 24, 26, 28, and 30. Only optical fibers 22 and 30 are of equal 
length, optical fibers 24, 26, and 28, each being successively longer so 
as to develop successively longer time delays for light pulses propagating 
through optical fibers 24, 26, and 28, respectively. Specifically, optical 
fiber 24 is sufficiently long to produce one nominal unit of time delay, 
while the length of optical fiber 26 produces two nominal units of time 
delay, and optical fiber 28, three units of time delay. Each unit of time 
delay is much less than the time interval between the pulses produced by 
mode-locked laser 12. As a result of the different time delays, the light 
pulses propagating through optical fibers 24-28 reach a silicon V-groove 
coupler 34, to which the ends of optical fibers 22-30 are joined, at 
different times. Silicon V-groove coupler 34 interfaces the optical fibers 
to a monolithic array of modulators 32, each of optical fibers 22-28 being 
connected respectively of one of the modulators 32a, 32b, 32c, and 32d, 
which comprise the array. Alternatively, a channel waveguide splitter 
could be incorporated onto the same substrate as waveguide modulator array 
32, and each channel provided with an appropriate delay path. If the 
splitter were thus constructed, optical fiber 18 would be coupled to the 
substrate. 
Waveguide modulator array 32 is of the 1.times.2 directional-coupler type. 
Alternatively, Mach-Zehnder interferometer, acousto-optic, or reversed 
delta-beta directional-coupler linear intensity modulators could also be 
used. Although the waveguide modulator array is illustrated as comprising 
an electrically lumped element circuit, a traveling wave modulator array 
configuration may also be used, possibly improving bandwidth and/or 
efficiency. Each of the modulators in the preferred embodiment modulator 
array 32 are thus shown connected in parallel to an analog signal via 
leads 38. The analog signal is produced by a source 36, representing any 
one of a number of different possible high frequency signal sources 
producing an analog signal for which analog-to-digital conversion is 
required. Modulators 32a-32d each modulate the succession of light pulses 
passing therethrough as a function of the intensity of the analog signal 
applied to the modulator array over leads 38 at a particular point in 
time. However, since a given light pulse produced by mode-locked diode 
laser 12 is split into pulses of light that pass through each of 
modulators 32a-32d at progressively later points in time, each of the 
modulators vary the intensity of the light pulses at a slightly different 
part of the analog signal. Using this arrangement, each pulse produced by 
mode-locked laser 12 samples the analog signal at a plurality of different 
points in time, due to the various delays introduced in the replications 
of the source pulse passing along optical fibers 22-28. 
The intensity modulated light pulses are output from waveguide modulator 
array 32 through a silicon V-groove coupler 40. In addition, an 
unmodulated reference channel 42 is provided within the waveguide 
modulator array, and is operative to carry the light pulse input over 
optical fiber 30. The plurality of modulated light pulses are conveyed in 
channels 44 to optical fibers 46, 48, 50, and 52, while the unmodulated 
reference channel 42 is conveyed to an optical fiber 54. Each of optical 
fibers 46-54 are connected in V-groove coupler 40. 
Optical fibers 46, 48, 50, and 54 optionally are cut to an appropriate 
length to provide time delays that compensate for time delays introduced 
by optical fibers 24-28. (A waveguide channel combiner with appropriate 
compensating time delays could also be incorporated on the substrate of 
waveguide modulator array 32.) Specifically, optical fiber 46 may include 
three units of time delay, optical fiber 48, two units, optical fiber 50, 
one unit, and optical fiber 54, three units. By introducing appropriate 
compensatory time delays, pulses propagating through optical fibers 46-54 
arrive at photodetectors 56a, 56b, 46c, 56d, and 56e, substantially at the 
same time. The compensation equalizes the total propagation time of all 
replicate pulses, offsetting time delays introduced on the input side of 
waveguide modulator array 32. As the light pulses propagating through each 
of optical fibers 46-54 reach photodetectors 56, each of the 
photodetectors produce an electrical signal corresponding to the intensity 
of the light pulses input thereto. The photodetectors preferably comprise 
photodiodes. The electrical signals are output over leads 58a, 58b, 58c, 
58d, and 58e, which are connected to the inputs of amplifiers 60a, 60b, 
60c, 60d, and 60e, respectively. 
The output signals from amplifiers 60 are conveyed over leads 62a, 62b, 
62c, 62d, and 62e to the inputs of electrical analog-to-digital converters 
64a, 64b, 64c, 64d, and 64e, respectively. Not only do amplifiers 60 
increase the amplitude of the signal, but they also broaden the pulse 
width of the signal so that electrical analog-to-digital converters 64 can 
more readily produce digital signals representative of the amplitude of 
the electrical pulses input thereto. Clearly, it is important that 
amplifiers 60 either have equal gains, so as not to introduce errors in 
the amplitude of the signal monitored on each channel, or that any 
differences in the gain of each amplifier be corrected by calibrating each 
channel of the system. 
If the compensatory time delays are omitted from optical fibers 46-54, the 
signals input to electrical analog-to-digital converters 64 are staggered 
in time by intervals corresponding to the delays introduced at the input 
to waveguide modulator array 32. However, in some applications, 
simultaneous digital outputs from electrical analog-to-digital converters 
64 for each pulse generated by mode-locked diode laser 12 are preferable 
and the compensatory time delays should be provided. 
A synchronization signal is produced by clock synchronization circuit 14, 
for input to each of the electrical analog-to-digital converters 64a-64e 
over leads 65. The synchronization signal triggers the electronic A-D 
converters to initiate an A-D conversion process at an appropriate time 
interval after mode-locked diode laser 12 produces a light pulse. This 
time interval includes the total time for a plurality of light pulses to 
transit waveguide modulator array 32, and for corresponding electrical 
signals from photodetectors 56 to reach the electrical A-D converters. 
Electrical analog-to-digital converters 64a-64e produce output signals on 
electrical lines 66a-66e, each with 8 bits of resolution or precision. 
Each modulated light pulse input to one of the photodetectors 56a-56e 
causes a digital signal corresponding to the amplitude of the light pulse 
and thus corresponding to the analog value of the analog signal to be 
digitized with that degree of precision. Further, each of the digitized 
signals represents the value of the analog signal at a particular instant 
in time at which one of the replicated light pulses traveling through a 
specific waveguide modulator 32a-32d was modulated. 
Turning to FIG. 5, the time relationship between the replicate pulses in 
each of the channels of electro-optical analog-to-digital converter 10 are 
graphically illustrated. At the top of the graph are shown the pulses 
P.sub.1 -P.sub.12, as they enter modulators 32a-32d. Pulses P.sub.1 
-P.sub.12 are of equal amplitude and are spaced apart in time as a 
function of the time delays introduced by optical fibers 22-28. The 
interval .DELTA.T shown in FIG. 5 represents the delay between successive 
pulses output from mode-locked diode laser 12. In the lower portion of 
FIG. 5, the space-division demultiplexed modulated pulses P.sub.1 
'-P.sub.12 ' are illustrated as they exit waveguide modulator array 32. 
Each of the pulses P.sub.1 '-P.sub.12 ' has been modulated so that its 
amplitude or intensity corresponds to the amplitude of the analog signal 
(shown in the center of FIG. 5) at the time the corresponding pulses, 
P.sub.1 -P.sub.12, pass through modulators 32a-32d. It should be apparent 
that channel 1 through channel 4 in FIG. 5 correspond to modulators 
32a-32d. Using the space-division demultiplexing method of this 
embodiment, the interval between successive samples of the analog signal 
is equal to T.sub.s, as shown on the horizontal axis of the graph. 
If the maximum specified sample rate of each of electrical 
analog-to-digital converters 64a-64e is equal to S, clock synchronization 
circuit 14 can be set to drive mode-locked diode laser source 12 with a 
pulse repetition rate up to S. The incremental nominal units of time delay 
introduced by optical fibers 24, 26, and 28 is optimally set equal to 
1/M*S, where M is equal to the number of electrical analog-to-digital 
converters used in connection with the modulated light pulse signals, 
i.e., M=4 in the preferred embodiment shown in FIG. 1. As a general rule, 
the rate at which optical pulses sample the input signal is equal to M*S. 
The signal output from the electrical analog-to-digital converter 64e is 
used as a reference to correct for variations in the amplitude of light 
pulses produced by mode-locked diode laser 12 over time. Variations in the 
amplitude of such pulses at the laser affect the intensity of each 
modulated light pulse. Although the disclosed preferred embodiment 
includes only four electrical analog-to-digital converters for digitizing 
the modulated pulse signals, it is expected that up to about ten 
electrical analog-to-digital converters 64 could be used with a 
corresponding increase in the number of modulated light pulse channels to 
achieve even higher effective sample rates. However, if the analog signal 
produced by source 36 is much greater than one gigahertz, it is likely 
that it would be difficult to drive much more than four channels in 
modulator array 32. In addition, if the value for M is much more than 4, a 
loss in power of the laser pulse as it is split between each of the 
modulator channels may become significant with respect to signal-to-noise 
ratio at the input of the electrical analog-to-digital converters 64. In 
addition, as the bandwidth is increased, and the number of bits of 
resolution are increased, the jitter requirement for the mode-locked diode 
laser begins to approach 0.1 pico seconds. As a result, accuracy in the 
optical path length differences provided by optical fibers 22-30 becomes a 
small fraction of a millimeter. If an optical fiber distribution network 
is used for input and output of light signals to and from waveguide 
modulator array 32, this requirement for precision length optical fibers 
becomes somewhat impractical. If a computer generated, photolithographic 
mask were used to provide a monolithic waveguide distribution network 
having an input from optical fiber 18 for performing the function of 
splitter 20 on an integral waveguide device, it may be practical to 
provide up to 10 modulation channels. The second embodiment of the 
electro-optical analog-to-digital converter avoids many of these problems. 
Turning now to FIG. 2, a second embodiment involving time-division 
demultiplexing of optical pulses is shown generally at reference numeral 
100. With respect to this and the other embodiments described below, the 
same reference numerals are used to identify elements of each of the 
electro-optical analog-to-digital converters, where those elements provide 
the same function in each embodiment. For example, in electro-optical 
analog-to-digital converter 100, mode-locked diode laser 12 is again a 
source of light pulses 16 that are conveyed along optical fiber 18. 
Mode-locked diode laser 12 is triggered by a synchronization signal 
produced by clock synchronization circuit 14, also just as described in 
the previous embodiment. However, in electro-optical analog-to-digital 
converter 100, a single channel waveguide modulator 102 is used that 
includes an intensity modulator 104, which modulates light pulses 16 as a 
function of the amplitude of the electrical analog signal produced by 
analog signal source 36 at the point in time in which each pulse passes 
through modulator 102. Pulses of light that have been modulated are 
conveyed from modulator 102 over an optical fiber 106 to an array of 
serially connected optical switches 108a-108m. Each light pulse 
propagating along optical fiber 106 is modulated at a different time, so 
that its intensity corresponds to the amplitude of the analog signal 
produced by analog signal source 36 at the time that the light pulse 
passed through modulator 102. Accordingly, optical switch 108a provides a 
first point at which the modulated light pulse propagating down optical 
fiber 106 may be diverted to one of photodetectors 56a-56m. Optical switch 
108a is controlled in response to an electrical signal input over a lead 
110a from a divide-by-M pulse generator 112. 
In the embodiment shown in FIG. 2, divide-by-M pulse generator 112 receives 
a clock synchronization signal from clock synchronization circuit 14 over 
a lead 114. Where there are M electrical analog-to-digital converters 
64a-64m, optical switch 108a receives every Mth synchronization pulse 
produced by clock synchronization circuit 14. In response to a 
synchronization pulse input over lead 110a, optical switch 108a 
momentarily diverts the incoming light pulse to photodetector 56a. 
Accordingly, as described above, the photodetector produces an electrical 
signal corresponding to the intensity of the light pulse, which is 
amplified by amplifier 60a and converted to a digital signal having 8 bits 
of resolution by electrical analog-to-digital converter 64a. 
The next optical pulse propagating along optical fiber 106 passes through 
optical switch 108a onto an optical fiber 116b, through which it is input 
to an optical switch 108b. As the pulse reaches optical switch 108b, 
divide-by-M pulse generator 112 provides another synchronization signal 
that is input to optical switch 108b over a lead 110b. In response to the 
synchronization signal, optical switch 108b diverts the incoming light 
pulse through an optical fiber 116b into photodetector 56b. A digital 
signal is produced by the downstream electronic analog-to-digital 
converter 66b as a function of the intensity of electrical pulse developed 
in response to this light pulse, as already described. Similarly, 
successive light pulses are directed by successive optical switches 
108c-108m to the corresponding photodetector, until after M light pulses 
have been processed, the synchronization signal is again input to optical 
switch 108a to begin the sequence over again. 
In order to ensure that a light pulse reaches each of the optical switches 
108 in succession at the same instant that a synchronization signal causes 
an optical switch to divert a light pulse toward the appropriate 
photodetector, the length of each of the optical fibers 116 connecting 
successive optical switches 108 must be precisely controlled to match the 
intraswitch light pulse transit times to the intervals between 
synchronization signals. Optical switches 108 preferably comprise 
waveguide switches that respond to an applied voltage to divert an 
incoming light pulse between one of two output channels. 
Any disadvantages in the design and operation of electro-optical 
analog-to-digital converter 100 relate to the accumulative losses incurred 
by light pulses passing through each successive junction between optical 
fibers 116 and optical switches 108. Such losses can be at least partially 
compensated by calibration of the system; however, as the losses increase, 
the system becomes incapable of monitoring meaningful variations in the 
intensity of light pulses at the Mth photodetector. The effective sample 
rate of the time-division demultiplexing technique used in electro-optical 
analog-to-digital converter 100 is equal to M times the rate of the 
individual electrical analog-to-digital converters. Typically a 
mode-locked diode laser is easily capable of providing the required 
jitter, sample rate, and aperture time for this application. 
Turning now to FIG. 3, a more preferred embodiment of a time-division 
demultiplexing type of electro-optical analog-to-digital converter is 
shown, generally at reference numeral 150. In this embodiment, the 
discrete serially connected optical switches 108 shown in the preceding 
embodiment are replaced with a branching network comprising a waveguide 
optical switch array 158. As in the preceding embodiment, mode-locked 
diode laser 12 produces a series of optical pulses having substantially 
continuous amplitude, which propagate along an optical fiber 18. In FIG. 
3, however, an optional optical limiter 152 is shown disposed between 
mode-locked diode laser 12 and modulator 102. The purpose of optical 
limiter 152 is to ensure that optical pulses input to the optical 
modulator have a continuous amplitude, so that variations in the intensity 
of the light pulse produced by the mode-locked diode laser are filtered 
and not interpreted as changes in the amplitude of the analog signal being 
used to modulate each of the light pulses in modulator 102. Optical 
limiter 152 can comprise a diode laser amplifier having a bistable output. 
Alternatively, a nonlinear optical etalon can be used for this purpose. 
The optical limiting capability of CuCl etalon has been demonstrated at 
wavelengths other than those obtainable from a diode laser; however, 
etalons constructed from different materials should provide the same 
optical limiting capability at diode laser wavelengths and power levels. 
(See Peyghambarian, N. Gibbs, H. M., Rushford, M. C. and Weinberger, D. 
A., "Observation of Biexcitonic Optical Bistability and Optical Limiting 
in CuCl," Phys. Rev. Lett. 51, pg. 1692 (1983)). The optical limiter may 
also be used in the preceding time-division multiplexed type 
electro-optical analog-to-digital converter 100. If a reference channel is 
not provided in electro-optical analog-to-digital converter 10 (see FIG. 
1), the optical limited could also be inserted at some point along optical 
fiber 18 in that embodiment. 
Modulated signals output from modulator 102 have an intensity corresponding 
to the amplitude of the analog signal produced by signal source 36 at the 
time each light pulse passes through modulator 102. These modulated 
optical pulses are conveyed over an optical fiber 156 to the input of 
optical switch array 158. 
Optical switch array 158 includes a plurality of integral optical switches 
160, 162, and 164, disposed in a series/parallel branching tree 
arrangement. Optical switch 160 is controlled by an electrical signal 
conveyed to it over leads 166a from a switch control circuit 168. 
Similarly, optical switches 162 and 164 are controlled by signals produced 
by the switch control circuit and conveyed to the optical switches over 
leads 166b and 166c, respectively. Switch control circuit 168 produces the 
signals to control the optical switches in response to a clock 
synchronization signal produced by clock synchronization circuit 14, which 
is connected to the switch control circuit by a lead 170. 
Optical switch 160 has two branching output channels 172a and 172b. A light 
pulse entering optical switch 160 is diverted into one of the two channels 
in response to the switch control signal provided by switch control 
circuit 168. A light pulse propagating along optical fiber 156 is thus 
diverted to either optical switch 162 or 164 by optical switch 160. 
Similarly, optical switches 162 and 164 each include two output channels 
174a and 174b, and 176a and 176b, respectively. A light pulse entering 
either of optical switches 162 or 164 is diverted into one of these four 
channels in response to the switch control signal provided by switch 
control circuit 168. Channels 174a, 174b, 176a, and 176b are respectively 
connected to optical fibers 178a-178d. 
Optical fibers 178a-d may optionally be cut to different lengths to provide 
for successive delay times equal to integer multiples of the time interval 
between pulses produced by mode-locked diode laser 12, to ensure that 
successive light pulses arrive at photodetectors 56a-56d at the same point 
in time. For example, the first of four successive light pulses produced 
by mode-locked diode laser 12, can be selected to propagate into optical 
fiber 178d, which has an added transit time delay equivalent to three 
periods of the mode-locked diode laser. The next light pulse is selected 
by the optical switch array to propagate along optical fiber 178b, which 
has an added transit time delay equivalent to two periods of the 
mode-locked diode laser. Similarly, the third and fourth pulses in 
succession are selected to propagate along optical fibers 178c and 178a, 
and are subjected to added transit time delays equal to one period of 
mode-locked laser 12 in optical fiber 178c and to no additional delay in 
the other optical fiber, ensuring that all four pulses arrive at 
photodetectors 56a-56 d at the same instant. The outputs of photodetectors 
56a-56d are connected through amplifiers 60a-60d to electrical 
analog-to-digital converters 64a-64d so that the intensity of each of the 
four light pulses is converted to digital signals, which in the preferred 
embodiment are output with 8-bits of resolution over leads 66a-66d. 
Additional numbers of optical switches can of course be included in optical 
switch array 158. The 1.times.4 demultiplexing capability of optical 
switch array 158 has been demonstrated at up to 16 gigabits per second. 
One of the advantages of using a waveguide optical switch array is that 
optical-throughput losses are minimized, compared to using discrete 
optical switches, as in electro-optical analog-to-digital converter 100. 
Furthermore, losses in each of the channels are equalized. The highest 
switching rate for the electro-optical switches in the optical switch 
array never exceeds the repetition rate of mode-locked diode laser 12, 
which is anticipated to be within a 1-10 gigapulse/second range--easily 
within the capability of waveguide type optical switches. For 
electro-optical analog-to-digital converter 150, the effective sample rate 
is equal to M times the rate of each of the electrical analog-to-digital 
converters 64a-64d, wherein the preferred embodiment shown in FIG. 3, M 
equals 4. 
The time relationship for light pulses propagating through electro-optical 
analog-to-digital converters 100 and 150 is illustrated in FIG. 6. A 
plurality of pulses P.sub.1 -P.sub.8, having a constant amplitude, enter 
modulator 104. Modulated light pulses output from the modulator have an 
amplitude or intensity that corresponds to the amplitude of the analog 
signal being digitized at the time each light pulse passed through the 
modulator. The modulated pulses are identified by P.sub.1 '-P.sub.8 '. 
Again, .DELTA.T1 indicates the time delay between the input of a pulse to 
the modulator and its output. Similarly, .DELTA.T2 represents the delay 
between the time a pulse is output from the modulator and the time it is 
output from the demultiplexer, which corresponds to optical switches 108 
in respect to electro-optical analog-to-digital converter 100, and to 
optical switch array 158 with respect to electro-optical analog-to-digital 
converter 150. The demultiplexed and modulated light pulses are output 
from the demultiplexer on each of channels 1-4. With respect to 
electro-optical analog-to-digital converter 100, M such channels should be 
illustrated to provide proper correspondence to the more general case 
illustrated in FIG. 2; however, if M= 4, the two figures corresponds. The 
pulses identified by P.sub.1 "-P.sub.8 " correspond to the amplitudes of 
the optical pulses output from output channels 176b, 174b, 176a, and 174a, 
respectively, as shown in FIG. 3. T.sub.S is again defined as the interval 
between successive light pulse samples of the analog signal. 
A further embodiment of the electro-optical analog-to-digital converter, 
which uses wavelength-division demultiplexing, is shown generally at 
reference numeral 200, in FIG. 4. In electro-optical analog-to-digital 
converter 200, a clock sync circuit 202 produces reference synchronization 
pulses with a period .tau., which are input to a plurality of electrical 
time delay circuits 206a-206n that provide time delays, which are 
successively longer by equal amounts. For example, time delay circuit 206a 
provides a time delay equal to one nominal unit, time delay circuit 206b 
provides a time delay equal to two nominal units, and so on through time 
delay circuit 206n, which provides a time delay of N nominal units, where 
N&lt;.tau.. 
The time delay circuits are connected by leads 208a-208n to a plurality of 
mode-locked diode lasers 210a-210n. Each of the mode-locked diode lasers 
operates at a different frequency, producing pulses having different 
wavelengths, .lambda..sub.1 -.lambda..sub.N, which are output over optical 
fibers 212a-212n. Instead of using electrical time delay circuits 
206a-206n to delay the synchronization signals that initiate light pulses 
output from mode-locked diode lasers 210, time delays can alternatively be 
provided by cutting optical fibers 212a-212n to appropriate different 
lengths. 
The light pulses produced by mode-locked diode lasers 210 are conveyed to a 
wavelength-division multiplexer 214. Wavelength-division multiplexer 214 
combines the separate synchronous pulse trains from each of mode-locked 
diode lasers 210 for input to modulator 104 over an optical fiber 216. The 
analog signal output from analog signal source 36 modulates the intensity 
of each successive light pulse propagating through modulator 104 at 
different points in time. N such successive light pulses have different 
wavelengths. The modulated light pulses are conveyed to a 
wavelength-division demultiplexer 220 through an optical fiber 218. 
Wavelength-division demultiplexer 220 separates the successive incoming 
light pulses into different output channels as a function of the 
wavelength of the light pulse. A specific output channel is associated 
with each wavelength light pulse produced by mode-locked diode lasers 210. 
The demultiplexed and modulated light pulses output from 
wavelength-division demultiplexer 220 are conveyed through optical fibers 
222a-222n to photodetectors 56a-56n, which convert the incoming light 
pulses into an electrical signal having an amplitude proportional to the 
intensity of the light pulses. Optionally, optical fibers 222 may be cut 
to appropriate different lengths to introduce time delays in the 
propagation of the optical pulses input to the photodetectors to 
compensate for the time delays provided with respect to the input of the 
clock synchronization signals at the input of mode-locked diode lasers 210 
(or introduced in the optical fibers conveying light pulses from the 
mode-locked diode lasers to wavelength-division multiplexer 214). 
In the embodiment shown in FIG. 4, the optional time delays are not 
included, and the light pulses input to photodetectors 56a-56n arrive at 
different points in time. The electrical signals produced by 
photodetectors 56a-56n in response to the light pulses are amplified by 
amplifiers 60a-60n, producing signals that are input to electrical 
analog-to-digital converters 64a-64n. As explained with respect to the 
preceding embodiments, electrical analog-to-digital converters 64 produce 
digital signals on electrical lines 66a-66n corresponding to the amplitude 
of the analog electrical signals input thereto. Electrical lines 66a-66n 
are represented as each having 8-bit resolution in the preferred 
embodiment illustrated in FIG. 4. Each group of digital output lines 
associated with the electrical analog-to-digital converters conveys a 
digital signal corresponding to the amplitude of the analog signal 
produced by analog signal source 36 at a different point in time. The 
effective sample rate of electro-optical analog-to-digital converter 200 
is thus equal to N times the pulse rate of the synchronization signal 
produced by clock synchronization circuit 202. 
Electro-optical analog-to-digital converter 200 represents the most 
preferred embodiment, since it enjoys benefits of both the space 
demultiplexed and the time demultiplexed embodiments described above. Like 
time demultiplexed-type electro-optical analog-to-digital converters 100 
and 150, the wavelength-division technique used in respect to 
electro-optical analog-to-digital converter 200 requires only a single 
modulator 104. And, like space-division electro-optical analog-to-digital 
converter 10, this last most preferred embodiment demultiplexes each of 
the modulated light pulses in a passive manner, without the optical 
switching circuits and complexity of switch control required in the time 
demultiplexed embodiments. 
Turning now to FIG. 7, a timing diagram for one channel of the electrical 
analog-to-digital conversion portion of each of the embodiments is 
illustrated. Only two demultiplexed modulated pulses are represented in 
FIG. 7, and they are identified by P.sub.1 and P.sub.2. The interval 
.DELTA.T1 is the delay between the time at which each of the pulses enters 
the photodetector for that channel and an electrical signal is output, 
corresponding to the intensity of the light pulse, as represented by 
E.sub.1 and E.sub.2. Electrical pulses E.sub.1 and E.sub.2 enter the 
electrical analog-to-digital converter and are converted into digital data 
when the electrical analog-to-digital converter for that channel receives 
a synchronization signal, identified in FIG. 7 by the positive going edges 
of C.sub.1 and C.sub.2. The delay between positive and negative going 
edges of the synchronization signal defines the interval .DELTA.T2, which 
is the delay between the electrical analog-to-digital converters receiving 
the electrical pulses and the output of valid data therefrom in the form 
of the digitized signals. Valid data is present on the digital signal 
lines 66, with respect to each of the embodiments, during the time that 
the synchronization signal is between pulses C.sub.1 and C.sub.2. The 
interval between N successive samples of the analog signal being digitized 
is represented on the horizontal axis by NT.sub.S, wherein N is the number 
of output channels used in the preferred embodiment. 
While the present invention has been described with respect to several 
preferred embodiments and modifications thereto, those of ordinary skill 
in the art will appreciate that further modifications may be made to this 
invention within the scope of the claims that follow below. Accordingly, 
the scope of the invention should not be limited by the description of the 
preferred embodiments, but instead, is to be determined entirely by 
reference to the claims.