U.S. patent application number 09/836604 was filed with the patent office on 2001-09-13 for high-speed serial-to-parallel and analog-to-digital conversion.
This patent application is currently assigned to The Trustees of Princeton University. Invention is credited to Prucnal, Paul R..
Application Number | 20010020908 09/836604 |
Document ID | / |
Family ID | 25476346 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010020908 |
Kind Code |
A1 |
Prucnal, Paul R. |
September 13, 2001 |
High-speed serial-to-parallel and analog-to-digital conversion
Abstract
An optical-to-electrical converter includes an input port
configured to receive an optical signal. The converter further
includes a splitter configured to split the received I optical
signal into a plurality of optical signals. An optical stage has a
plurality of parallel stages, and each parallel stage receives a
corresponding one of the plurality of identical signals and outputs
a corresponding one of a plurality of sampled optical signals
within a corresponding sampling window. A plurality of delay
circuits receive a clock signal having a plurality of clock pulses
separated by a clock period. The delay circuits respectively output
a plurality of control pulses at a plurality of delayed timings
with respect to each clock pulse of the clock signal. An electrical
stage receives the plurality of sampled optical signals and
processes the optical signals at a sampling rate corresponding to
the clock period of the clock signal.
Inventors: |
Prucnal, Paul R.;
(Princeton, NJ) |
Correspondence
Address: |
PENNIE AND EDMONDS
1155 AVENUE OF THE AMERICAS
NEW YORK
NY
100362711
|
Assignee: |
The Trustees of Princeton
University
|
Family ID: |
25476346 |
Appl. No.: |
09/836604 |
Filed: |
April 16, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09836604 |
Apr 16, 2001 |
|
|
|
08941364 |
Sep 30, 1997 |
|
|
|
6265999 |
|
|
|
|
Current U.S.
Class: |
341/137 ;
359/238; 359/350; 398/43 |
Current CPC
Class: |
H04L 7/0075 20130101;
H03M 1/121 20130101 |
Class at
Publication: |
341/137 ;
359/238; 359/123; 359/350 |
International
Class: |
H03M 001/00; G02B
001/00; H04J 004/00; H04J 014/00; G02B 026/00; G02F 001/01 |
Claims
What is claimed is:
1. An optical-to-electronic convertor, comprising: an input port,
which receives an optical signal that is to be sampled; an optical
sampler stage including at least one high-speed optical shutter,
which samples the optical signal at a first sampling rate by
opening and closing the at least one high-speed optical shutter and
which provides a plurality of sampled optical signals as a result
thereof, the optical sampler stage including a plurality of
parallel channels, each different channel providing a different one
of said plurality of sampled optical signals, the optical sampler
stage including a splitter for splitting the optical signal into a
plurality of identical optical signals prior to the sampling at the
first sampling rate; and an electronic analog-to-digital converter
stage, which converts the plurality of sampled output signals into
a plurality of digital signals, the electronic analog-to-digital
converter stage performing the converting at a second sampling rate
slower than the first sampling rate.
2. The optical-to-electronic converter as recited in claim 1,
wherein the electronic analog-to-digital converter stage comprises
a single analog to digital converter which receives the outputs
from each of the plurality of parallel channels in a time
sequential manner.
3. The optical-to-electronic converter as recited in claim 1,
wherein the electronic analog-to-digital converter stage comprises
a plurality of electronic converters disposed in a plurality of
parallel channels, wherein the plurality of channels of the
electronic analog to digital converter stage respectively receive
the sampled optical signals output from the plurality of parallel
channels of the optical sampler stage.
4. The optical-to-electronic converter as recited in claim 1,
further comprising a conversion stage disposed between the optical
sampler stage and the electronic analog-to-digital converter stage,
the conversion stage performing a conversion of the plurality of
sampled optical signals into a plurality of electronic sampled
signals.
5. The optical-to-electronic converter as recited in claim 4,
wherein the conversion stage includes a plurality of photodetectors
respectively disposed in the plurality of parallel channels of the
optical sampler stage.
6. The optical-to-electronic converter as recited in claim 5,
wherein the optical sampler stage comprises: a plurality of
terahertz optical asymmetric demultiplexer assemblies (TOADs)
respectively disposed in the plurality of parallel channels of the
optical sampler stage and each configured to sample a corresponding
one of the plurality of identical optical signals at different
times, wherein the plurality of detectors are respectively disposed
between the plurality of TOADs and the electronic analog-to-digital
converter stage.
7. The optical-to-electronic converter as recited in claim 3,
wherein the plurality of parallel channels of the optical sampler
stage corresponds in number to n, and wherein n multiplied by the
second sampling rate is approximately equal to the first sampling
rate.
8. An optical to electronic converter, comprising: a first input
port configured to receive an analog optical waveform; a splitter
connected to the first input port and configured to split the
analog optical waveform into a plurality of approximately identical
waveforms; a second input port configured to receive a clock signal
having a predetermined clock period; a delay circuit configured to
receive the clock signal and to output a plurality of delayed clock
signals each having a different delay with respect to other of the
delayed clock signals; a plurality of terahertz optical asymmetric
demultiplexers configured to respectively receive the plurality of
approximately identical waveforms on an input port thereof, and
configured to receive a corresponding one of the plurality of
delayed clock signals on a control port thereof, each of the
plurality of terahertz optical asymmetric demultiplexers having an
output port for outputting the corresponding one of the plurality
of approximately identical waveforms within a fixed time period; a
plurality of photodetectors, each different photodetector connected
to a different one of said output ports of the plurality of
terahertz optical asymmetric demultiplexers and configured to
convert an input optical signal into an output electrical signal;
and a plurality of electrical analog-to-digital converters
respectively connected to the output ports of the plurality of
terahertz optical asymmetric demultiplexers and configured to
perform an analog-to-digital conversion of the corresponding
electrical signal into a digital signal, wherein a number
corresponding to said plurality of electrical analog-to-digital
converters is a value such that a conversion time of the plurality
of electrical analog-to-digital converters divided by the number of
said plurality of electrical analog-to-digital converters is
approximately equal to the time period of the plurality of
terahertz optical asymmetric demultiplexers.
9. A method of optical sampling, comprising the steps of: receiving
an optical signal that is to be sampled; splitting the received
optical signal into a plurality of approximately identical optical
signals; and respectively providing the approximately identical
optical signals to a plurality of channels of an optical stage; and
sampling, at the optical stage, the optical signal at a first
sampling rate by opening and closing a high-speed optical shutter
and providing a plurality of sampled optical signals as a result
thereof, each of the plurality of parallel channels of the optical
stage providing a different one of said plurality of optical
signals.
10. The method as recited in claim 9, further comprising the step
of converting the plurality of sampled optical signals into a
plurality of electronic sampled signals.
11. The method as recited in claim 10, further comprising the steps
of: splitting the received optical signal into a plurality of
approximately identical optical signals; and respectively providing
the approximately identical optical signals to a plurality of
channels of the optical stage so as to perform the sampling
step.
12. An analog optical sampler, comprising: a first input port that
is capable of receiving an analog optical waveform that is to be
sampled; a second input port that is capable of receiving a clock
signal; at least one Terahertz Optical Asymmetric Demultiplexer
(TOAD) that is configured to receive the analog optical waveform
and the clock signal; and an output port, from which is output a
sample of the analog optical waveform, the sample being provided to
the output port based on an output of at least one TOAD; wherein
the analog optical waveform is sampled when said TOAD is triggered
by the clock signal.
13. The analog optical sampler as recited in claim 12, wherein at
least one of the first input port, the second input port, and the
output port includes an optical waveguide or an optical fiber.
14. The analog optical sampler as recited in claim 13, wherein
distinct polarizations or colors are used to multiplex the first
and second input ports on a same optical waveguide or optical
fiber.
15. The analog optical sampler as recited in claim 12, wherein at
least one TOAD includes a Mach-Zehnder interferometer.
16. An optical sampler system, comprising: a first input port,
which receives an optical analog waveform that is to be sampled; a
second input port, which receives an optical clock signal at a
clock rate; a plurality of optical samplers, which produce a
plurality of sampled optical waveforms from said optical analog
waveform at a rate dependent on said clock signal, at least one of
said plurality of optical samplers including a Terahertz Optical
Asymmetric Demultiplexer (TOAD); and a plurality of output ports,
which output said plurality of sampled optical waveforms.
17. An optical sampler system, comprising: a plurality of first
input ports for receiving an optical analog waveform; a plurality
of second input ports for receiving an optical clock signal at a
clock rate; a plurality of optical sampler nodes for respectively
producing a plurality of sampled optical waveforms from said
optical analog waveform at a rate dependent on said clock signal; a
plurality of output ports for respectively outputting said
plurality of sampled optical waveforms; a first optical
distribution network for replicating and delivering a plurality of
copies of said optical clock signal; and a second optical
distribution network for replicating and delivering a plurality of
copies of said optical analog waveform, said second optical
distribution network including delay lines of differing optical
lengths so as to provide each of said plurality of optical sampler
nodes with a substantially unique timeslice of said analog optical
waveform upon receipt of a corresponding one of said plurality of
copies of said optical clock signal.
18. The optical sampler system as recited in claim 17, wherein said
plurality of copies of said optical clock signal are delayed with
respect to each other by a preset delay amount.
19. The optical sampler system as recited in claim 17, wherein at
least one of said plurality of optical sampler nodes includes at
least one Terahertz Optical Asymmetric Demultiplexer.
20. The optical sampler system as recited in claim 17, further
comprising an optical-to-electronic converter stage connected to at
least one of said plurality of output ports, and configured to
receive said sampled optical waveform and to convert said sampled
optical waveform into an electronic waveform.
21. The optical sampler system as recited in claim 20, further
comprising an analog-to-digital converter stage connected to said
optical-to-electronic converter stage, and configured to convert
said electronic waveform into a digital signal.
22. The optical sampler system as recited in claim 21, wherein said
plurality of optical sampler nodes produce said plurality of
sampled waveforms on a plurality of parallel channels,
respectively, the plurality of parallel channels corresponding to n
in number, n being an integer greater than one, wherein n
multiplied by a first sampling rate of the conversion by said
plurality of optical sampler nodes is approximately equal to a
second sampling rate of said analog-to-digital converter stage.
23. The optical sampler system as recited in claim 17, wherein
distinct signal levels of at least one of said copies of said
optical clock signal and said copies of said optical analog
waveform through distinct paths in said first and second
distribution networks are provided as correction signals used to
correct a calibration of subsequent magnitude measurements of said
optical sampler system.
24. An optical sampler system, comprising: a plurality of
high-speed optical shutters, each having a signal input port
capable of receiving an optical input signal, a trigger input port
capable of receiving an optical clock signal, and an output port; a
first optical distribution network, which receives an optical clock
signal and delivers a corresponding optical clock signal to the
trigger input port of each optical shutter; and a second optical
distribution network, which receives an optical analog waveform and
delivers a corresponding optical analog waveform to the signal
input port of each optical shutter; wherein, for each optical
shutter, the corresponding optical clock signal is synchronized
with the corresponding optical analog waveform to open and close a
gating window during which the optical shutter produces an optical
waveform sample on its output port.
25. The optical sampler system of claim 24, wherein the high-speed
optical shutter is a TOAD.
26. The optical sampler system of claim 24, wherein the high-speed
optical shutter is open for a duration in the range from about 0.01
psec to about 100 psec.
27. The optical sampler system of claim 24, wherein the optical
waveform sample produced by each optical shutter is substantially
unique.
28. The optical sampler system of claim 24, wherein the second
optical distribution network comprises delay lines of differing
optical lengths.
29. The optical sampler system of claim 24, wherein the first
optical distribution network comprises delay lines of differing
optical lengths.
30. The optical sampler system of claim 24, further comprising an
optical-to-electronic converter stage connected to the output ports
of the plurality of high-speed optical shutters.
31. The optical sampler system of claim 24, wherein the first and
second optical distribution networks each include an optical path
over which correction signals are provided to correct a calibration
of subsequent magnitude measurements of the optical waveform
samples.
32. A method of sampling an optical signal, comprising the steps
of: receiving a continuous optical analog signal having an
intensity that may be any value within two predetermined intensity
limits; receiving an optical triggering signal; providing the
continuous optical analog signal to an optical shutter; providing
the optical triggering signal to the optical shutter; and
outputting from the optical shutter, a discrete sample segment of
the continuous optical analog signal.
33. The method of claim 32, wherein the optical shutter is a
high-speed optical shutter.
34. The method of claim 33, wherein the high-speed optical shutter
is a TOAD.
35. The method of claim 32, wherein the duration of the discrete
sample segment is within arange of about 0.01 psec to about 100
psec.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to the field of
optoelectronic devices and, more particularly, to devices which
function as extremely high speed optical shutters, and to
applications of such devices.
[0003] 2. Description of the Related Art
[0004] A number of important commercial, scientific, medical and
military signal processing or sampling applications require high
speed conversion of time-varying analog waveforms into digital
form. Such higher digitization speed is useful because, among other
reasons, it provides better spatial resolution for lidar and
range-finding, better time resolution for clock synchronization
protocols, better instrumental resolution for sampling
oscilloscopes, and better channel separation for wideband
receivers. Higher speed analog-to-digital (A/D) converters are
additionally sought because there is often a threshold conversion
rate below which the application requiring the samples is
infeasible, such as digital receivers operating in a particular
microwave band.
[0005] It is known that an analog serial-to-parallel (S/P)
converter can be used to parcel out portions of a time-varying
waveform to parallel A/D converters working in parallel. In such
systems, the quality of the S/P converter stage bounds the
subsequent fidelity of the overall converter, so manufacturability
and control of noise are crucial considerations. Unfortunately,
today's all-electronic S/P converters operate well below 60
gigasamples per second (GSPS), which is not much faster than
today's state-of-the-art electronic A/D converters (e.g., 6 GSPS
claimed by Rockwell in the laboratory, for which, to applicants'
knowledge, no publicly-available enabling disclosure exists), so
there is little fan-out by the S/P converter. By way of comparison,
all-optical switching/sampling phenomena can occur at intrinsically
higher speeds than analogous electronic phenomena, since electron
mobility in the solid state is up to 10,000-fold slower than the
speed of light; the advantage is lessened by the ratio of device
sizes.
SUMMARY OF THE INVENTION
[0006] Accordingly, it is among the objects of the instant
invention to provide one or more of the following: (i) an improved
A/D converter operating at optical speeds yet benefitting from
progress in the speed of electronic devices (ii) an integrated,
fast A/D converter; (iii) an improved A/D converter with calibrated
outputs and servo-controlled input range; (iv) an improved A/D
converter with a parallel output signal; (v) an improved A/D
converter in a circuit; (vi) an A/D converter embedding a fast
analog S/P converter (vii) an integrated fast analog S/P converter;
(viii) a fast analog optical S/P converter employing a TOAD (as
defined below); (ix) an improved, fast analog S/P converter with
adjustable input dynamic range; (x) an improved S/P converter with
physical clocking; and (xi) a device capable of converting an
optical waveform into piecewise portions employing a TOAD.
[0007] In accordance with these and other objects, there is
provided an analog-to-digital converter, which includes a first
input port configured to receive an analog optical waveform. The
converter also includes a splitter connected to the first input
port and configured to split the analog optical waveform into a
plurality of identical waveforms. The converter further includes a
second input port configured to receive a clock signal having a
predetermined clock period. The converter also includes a delay
circuit configured to receive the clock signal and to output a
plurality of delayed clock signals each having different a
different delay with respect to others of the delayed clock
signals. The converter still further includes a plurality on
optical shutters configured to respectively receive the plurality
of identical waveforms on an input port thereof, and configured to
receive a corresponding one of the plurality of delayed clock
signals on a control port thereof, each of the plurality of optical
shutters having an output port for outputting the corresponding one
of the identical waveforms within a time period in a range of 0.01
psec to 100 psec. The converter also includes a plurality of
photodetectors respectively connected to the output ports of the
plurality of optical shutters and configured to convert an input
optical signal into an output electrical signal. The converter
further includes a plurality of electrical analog-to-digital
converters respectively connected to the output ports of the
plurality of optical shutters and configured to perform an
analog-to-digital conversion of the corresponding electrical signal
into a digital signal. The number of electrical analog-to-digital
converters in a fully populated configuration is such that a
conversion time of said analog-to-digital converters divided by the
number of electrical analog-to-digital converters is slightly less
than the time period of the optical shutters.
[0008] Aspects of the system and method according to the invention
exploit and improve upon recent advances in high speed optical
shutter technology, notably the Terahertz Optical Asymmetric
Demultiplexer (TOAD), disclosed in U.S. Pat. No. 5,493,433, which
is incorporated in its entirety herein by reference. Whereas most
optical shutters require impractically long interaction lengths,
high power, or both; the TOAD is compact and low power.
[0009] The TOAD exploits a strongly non-linear optical effect which
allows a gating pulse to cause either 0 or pi radians of phase
delay in a signal introduced into an interferometer. The phase
delay switches off after a brief interval, of the order of 1
picosecond (psec), so a signal beam meeting with itself in a TOAD
interferometer will emit only the waveform sampled by the 1 psec
shutter window. In contrast to conventional semiconductor logic
gates, the TOAD does not try to switch and reset in a small
multiple of the shutter cycle time. Rather, the TOAD can undergo an
Open/Off cycle only once before it needs to recover, typically for
100 psec. When combined with a precise optical delay line, each
TOAD can be used to sample (read) or inject (write) a signal's
amplitude or intensity in the shutter window time. The TOAD is then
latent, waiting for the electronics to invoke it again (e.g. every
4 nanoseconds (nsec) for 250 MH.sub.z CMOS).
[0010] Utilizing such high speed optical shutters, in accordance
with the system and method according to the invention, it is
possible to provide high speed, high quality S/P conversion, and
therefore high speed A/D converter systems embedding a fast analog
optical S/P converter with fan-out to whatever degree is necessary
to support operation beyond 1000 gigasamples per second (GSPS).
Also, in accordance with the invention, systems may include
all-electrical A/D converter devices, thus providing a back-end
with optimal price/performance. Preferably, the balance between
more fan-out in the S/P converter and more, lower-cost slow
electrical A/D converters in the back-end is optimally selected,
based upon overall price, performance, manufacturability, or other
criteria. While the present invention may be, and preferably is,
practiced using the TOAD, the concepts, teachings, and applications
described herein below are by no means limited to TOAD-based
systems, and will work with other optical shutters, other sequences
of functional units, and with novel electrical A/D converters.
[0011] In accordance with the invention, the TOADs will preferably
be solid state and formed by mass production processes. For
instance, integrated semiconductor optical amplifiers (SOAs under
development at Princeton University, British Telcom, NEC, and
elsewhere can be combined with integrated optical paths to form
complete TOAD devices. SOAs can advantageously be formed using
materials from columns III and V in the periodic table (e.g. GaAs),
or II-VI materials; a plurality of optical paths can be formed on a
low cost substrate; and the two monolithic constructions bonded
together. Additionally or alternatively, a plurality of optical
paths may be formed on the same substrate as the TOADs by additive,
subtractive, or even selective phase-change direct-write processes.
One may apply light, charged particle beam, heat, or other known
methods to cure the waveguide material or optical
interconnects.
[0012] In accordance with a preferred embodiment of the instant
invention, the time-varying magnitude of an input signal is
modulated by amplitude or intensity. In more general embodiments,
modulation of the magnitude comprises a combination of amplitude
modulation, polarization modulation, phase modulation, and
frequency modulation. These can all be exploited in their own right
by appropriate TOAD embodiments, or reduced to amplitude or
intensity modulation and treated as in the preferred embodiment.
For instance, a polarized signal can be converted into an intensity
modulated signal by being passed through a cross-polarizer. A
frequency modulated signal can be converted into an amplitude
modulated signal in a number of ways, including filtering with a
color-sensitive transmitter or reflector; or diffracting or
refracting and then using position, time of flight, or phase
information. A frequency modulated signal can be converted into an
intensity modulated signal by interfering it with a coherent
reference beam of comparable intensity and sampling the beat
frequency. In general, modulation can be permitted to occur within
the TOAD, instead of just prior to it. For instance, a phase
modulated signal can be converted into an intensity modulated
signal by being superposed on a coherent reference signal of
comparable intensity within the TOAD interferometer itself, thus
replacing the input splitter described in the '433 patent with a
bypass on one side and a reference beam for differential
measurement on the other side. This brief description is exemplary
and not comprehensive. As those skilled in the art will appreciate,
other methods are known, and it is anticipated that there will be
future embodiments of optical shutters which will be appropriate
for use in this invention.
BRIEF DESCRIPTION OF THE FIGURES
[0013] Various aspects, features, applications, and advantages of
the instant invention are depicted in the accompanying set of
drawings, which are intended to be exemplary and not limiting or
exhaustive, and in which, briefly stated:
[0014] FIG. 1 illustrates a preferred embodiment of an analog
optical S/P converter in block form.
[0015] FIG. 2 illustrates an analog optical S/P converter in
greater detail.
[0016] FIG. 3 illustrates a beam splitter used in a clocking
source.
[0017] FIG. 4 illustrates a clocking source including a modulator
unit.
[0018] FIG. 5 illustrates a "physically clocked" embodiment of a
clocking source.
[0019] FIG. 6 illustrate the use of delay lines to retard clock
signals.
[0020] FIG. 7 illustrate the use of delay lines to retard data
signals.
[0021] FIG. 8 illustrates an optical shutter unit implemented in
Terahertz Optical Asymmetric Demultiplexer.
[0022] FIG. 9 shows a measured TOAD waveform.
[0023] FIG. 10 plots the shape of a fast analog waveform using an
optical correlator.
[0024] FIG. 11 shows a measurement of the same analog signal by a
TOAD acting as an optical shutter.
[0025] FIG. 12 illustrates an apparatus in the manner of a sampling
oscilloscope.
[0026] FIG. 13 illustrates a sampling oscilloscope using a TOAD
acting as an optical shutter.
[0027] FIG. 14 illustrates an optical A/D converter system
embedding an analog optical S/P converter.
[0028] FIG. 15 illustrates an alternative embodiment of the slow
A/D converter back-end.
[0029] FIG. 16 illustrates a preferred slow O/E converter unit.
[0030] FIG. 17 illustrates a preferred slow electrical A/D
converter unit.
[0031] FIG. 18 illustrates a preferred processing unit.
[0032] FIG. 19 illustrates an A/D converter system with analog
electrical input and digital electrical output.
[0033] FIG. 20 summarizes an alternative A/D converter
embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0034] Reference is now made to FIG. 1, which illustrates a
preferred embodiment of an analog optical S/P converter in block
form. Optical input signal (1) enters an optical S/P converter (10)
by means of an optical path (2). Signal (1) is preferably coherent
light, such as 1.5 .mu.m light, and comprises a waveform, such as
shown in FIG. 9. Preferably, but not necessarily, signal (1) has
already been amplitude or intensity modulated with certain data of
interest. Path (2) is preferably an integrated waveguide, although
a variety of designs for optical paths through free-space and
guided paths (e.g. optical fibers) can be substituted, as would be
appreciated by persons skilled in the art, e.g., optical
engineers.
[0035] A plurality of output signals emerge from S/P converter
(10), illustrated as signal (4A) on path (5A), signal (4B) on path
(5B), and so forth. The reader should interpret references to
"signal (4)" without a suffix as applying to a plurality of (4A),
(4B), etc., and similarly with "path (5)" as applying to paths
(5A), (5B), etc. Output signals (4) contain information from the
input signal (1), and are preferably optical replicas of input
signal (1), although a plurality of them may be scaled to different
amplitudes from the input signal (1) and/or from each other.
Optionally, output signals (4) may comprise functional
modifications of input signal (1), such as linearization
corrections, and/or may mix in other signal streams, such as a
monochromatic reference beam.
[0036] Reference is now made to FIG. 2, which illustrates an
optical analog S/P converter in greater detail. In this embodiment,
a beam splitter (12) replicates an input signal (1) into a
plurality of output signals (14), which exit on their optical paths
(15). As persons skilled in the art will appreciate, one can
purchase beam splitters with high uniformity and low parasitic
losses commercially from a number of vendors, and beam splitters
can also be integrate with optical or optoelectronic subsystems
using well-known lithographic or casting techniques.
[0037] Each of a plurality of optical shutters (40) is fed both by
a signal (14) along an optical path (15) from the beam splitter
(12) and also by a clock signal (32) carried by an optical path
(31). Each optical shutter (40) emits an output signal (4) on a
path (5).
[0038] Clock signals (32) are illustrated schematically as
originating from a single clocking source (20), but use of multiple
clocking sources is permissible. The signal path (17) output of the
input master clock (20) is used to synchronize the electrical A/D
stages and subsequent latching circuitry.
[0039] Reference is now made to FIG. 3, which illustrates a beam
splitter (30) used in a clocking source (20). In this embodiment, a
master clock signal (28) of periodic pulses is introduced along
path (29) and distributed as clock signals (32) among paths (31).
Optionally, output clock signals (32) may comprise functional
modifications of master clock signal (28), for instance by
pulse-sharpening, clock-doubling, or polarization. Other signals
may optionally be mixed in with output clock signals (32), such as
complex waveforms. As one skilled in the art will appreciate, a
number of clock distribution topologies are known to electronics
engineers, with different topologies appropriate for different
embodiments, such as a star-of-stars hierarchy for obtaining
sufficient fan-out with tight timing synchrony. Some of these
topologies also include an amplification unit within the clocking
source (20).
[0040] Reference is now made to FIG. 4, which illustrates a
clocking source (20) including a modulator unit (24) providing an
optical signal (25) along optical path (26) to a beam splitter
(30). A light signal (27) input from path (29) is preferably
constant brightness from a coherent laser. The beam splitter could
preferably be similar to the one illustrated in FIG. 3. If the
modulator (24) is electro-optical, then it will preferably be fed
by an RF electrical trigger signal (21) from an RF input path (22).
If the modulator unit (24) is optoelectronic, then it will
preferably be fed by an optical trigger signal (21) from an optical
input path (22). Mixtures of electrical and optical inputs may also
be imposed as a trigger signal (21) along a suitable plurality of
paths (22).
[0041] Modulator unit (24) may require a power source, but such
sources are well known to those skilled in the art, and are not
depicted in the drawings in order to avoid distracting from the
description of the present invention.
[0042] Reference is now made to FIG. 5, which illustrates a
"physically clocked" embodiment of a clocking source. Use of a beam
splitter with one-more-than-2.sup.N outputs is preferred, but not
required. The function of modulator (24) in this embodiment is to
convert a "flyback" trigger signal (35) into a suitable input for
beam splitter (30), preferably by amplification and
pulse-sharpening, so that a preferred clocking source (20) emits
clocking waveforms (e.g., pulses) in parallel at precise periodic
intervals that do not depend on the coherence of an external clock.
An optical clock signal (32C) is provided by optical path (31C) to
optional phase correction unit (33), which emits signal (35) after
that time interval necessary to ensure that the periodicity of
pulses arriving at beam splitter (30) is a constant. There can be
one or a number of pulses in transit traversing the circuit
encompassing (34), (24), (26), (30), (32), and optionally (33); the
circuit serves as a delay line. If optional phase correction unit
(33) is omitted, path (31C) and path (34) number the same item.
[0043] The optional phase correction unit (33) can serve to adjust
the timing of the arrival of clock signal (35) at modulator unit
(24), and can be internally controlled by a clock or externally
controlled along a path (37), such as by "trimming" feedback from
elsewhere in the S/P converter or a system embedding it. A number
of units for phase correction are well-known, including stretching
optical fibers and tight temperature control.
[0044] A starting signal is used for the clocking source (20), and
the clocking can run thereafter without requiring an external
clock. There are a number of units for providing a starting signal.
An optional input (36) may be provided through phase correction
unit (33) or directly into modulator unit (24) to provide a pulse
from outside as the starting signal; or, the starting signal can be
generated internally, for instance by compounding its amplification
through repeated cycles until effective.
[0045] Reference is now made to FIG. 2, FIG. 6 and FIG. 7, which
illustrate the use of delay lines to relate the relative timing of
clock signals (32) and signal waveforms (4). Note that an optical
path length is determined by a path's index of refraction as well
as its physical length. In the embodiment shown in FIG. 6, the
optical lengths of the signal paths (15) are all the same, but each
clock path (31A), (31B), (31C), and so forth has an optical length
chosen to provide timing delays in successive integer units of
.tau.. In an alternative embodiment shown in FIG. 7, the optical
lengths of the clock paths (31) are all the same, but each signal
path (15A), (15B), (15C), and so forth has an optical length chosen
to provide timing delays in successive integer units of .tau.. More
complicated patterns can be utilized while remaining within the
scope of the invention as described herein. For example, in
embodiments of most value in certain applications,
manufacturability considerations make it preferable for the
ensemble of clock signals (32) and the ensemble of waveforms (14)
to intersect at a variety of times and places, in a matrix-like
configuration (e.g., clustered on chips in a module). Nor must the
intersections be regular; in some embodiments for applications
described below, they need not all have the same time intervals
.tau., nor comprehensive coverage, nor sequential ordering.
[0046] Reference is now made to FIG. 8, which illustrates an
optical shutter unit (40) implemented in a TOAD, such as the
Mach-Zehnder version disclosed in the '433 patent. Optical shutters
can also be built using an amplitude-modulating TOAD constructed
from a Mach-Zehnder, Michelson, or Sagnac interferometer by means
of techniques well-known to those ordinarily skilled in the art.
There have been attempts to build a TOAD using a loop mirror. These
and other forms of TOAD are all appropriate for use as optical
shutter units in connection with the instant invention, where each
embodiment of an optical shutter (40) is preferably a TOAD (41)
which gates a time-dependent input signal (14) with its
time-dependent waveform (42) to an output signal (4) with a
modulated magnitude.
[0047] Referring now to FIG. 8, the Terahertz Optical Asymmetric
Demultiplexer (TOAD) comprises a fiber or waveguide loop joined at
its base by the top half of a symmetric 2.times.2 splitter (18).
The bottom side of the splitter (18) receives the signal input and
transmits it to a photodetector at a receiver. In general
operation, light from the input is split by the splitter into two
identical waveforms which travel through the loop in opposing
directions. Because the two signal components will have traversed
exactly the same distance when they meet again in the splitter
(18), purely constructive interference occurs; the splitter (18)
therefore reflects all light back out the original input fiber and
passes no light to the detector. This loop-mirror also contains a
nonlinear element (e.g. a semiconductor optical amplifier (SOA))
located slightly off-center from the midway point. The asymmetric
placement is the reason fox the letter "A" in the word TOAD.
[0048] If injected into the loop before the signal pulse, a clock
or pulse will change the nonlinear element's index of refraction
for a brief period of time (the dwell time). This means that the
light traversing the loop after the gating pulse has passed through
the SOA will encounter a different propagation delay than light
traversing the loop before the gating pulse pumped the SOA. An
important feature of the TOAD is that this delay is engineered to
be exactly one-half of a wavelength, or exactly enough to change
the interference condition from constructive to destructive. The
coupler therefore expels light to the photodetector instead of back
out the input.
[0049] In accordance with Nyquist's Theorem, for a system with
maximum frequency component 2/.tau., no information is available
about the shape (roughly speaking, .sigma.) of the shutter waveform
so long as .sigma.<.tau. In practice, the shutter waveform (32)
has a finite rise time and fall time, so cross talk is reduced
significantly but not eliminated by making .sigma. somewhat smaller
than .tau. (e.g. 50%).
[0050] Reference is now made to FIG. 9, which shows a measured TOAD
waveform; in this case, 4 psec wide. The measurement was made by
duplicating the output of a single TOAD device, delaying the second
waveform by a variable interval (shown as the abcissa) and
measuring the amplitude of the convolution of the first against the
second (shown as the ordinate). The delay was swept from 0 to 10
psec and the results plotted.
[0051] Reference is now made to FIG. 10, which plots the shape (44)
of a fast analog waveform (actually, a laser pulse) as measured in
the standard way by an optical correlator. FIG. 10 is prior
art.
[0052] Reference is now made to FIG. 11, which shows a measurement
of the same analog signal by a TOAD acting as an optical shutter
(41) in the manner of FIG. 9. The measurement was performed in the
inventor's laboratory by creating a rudimentary sampling
oscilloscope using approximately 43 psec-wide pulses from a 1.3
.mu.m wavelength laser. The repetition rate was chosen as 10 nsec
so that the output of the TOAD could be measured with a 100
MH.sub.z photodetector. The clocking train was shortened to 1 psec
wide pulses at 500 femtojoules of energy per pulse, and was
injected into the control port of the TOAD as gating pulses. A
weaker branch of the same train was split from the first, acting as
the lower amplitude signal to be measured, and was time-lagged
along a precisely varied delay path before being injected into the
input port of the TOAD as signal pulses. The delay path was then
changed and the TOAD output measured again. The overall shape was
obtained by sweeping the delay path of the signal relative to the
control over a range of 100 psec. The FWHM .sigma..sub.--=43 psec,
and the area under the curve corresponds to the intensity of the
input signal integrated over the time when the TOAD acted as an
open shutter. It is clear by inspection that the measurement
provided at least 6 bits of resolution on the ordinate axis.
[0053] The practiced sampling oscilloscope method indicates a
useful analog application of the TOAD. The embodied sampling
oscilloscope apparatus proves an alternative embodiment of the S/P
converter wherein the fan-out feeds a plurality of branches-in this
example, 1--and the conversion is repeated temporally at precisely
known phase offsets, not necessarily sequentially, in order to
measure the shape of a repeated signal. The signal must be
reproducible in a finite time domain. A number of means for
reproducible signal delay lines with sub-psec precision are known
and could have been used instead to create the densely sampled
phase offsets.
[0054] Reference is now made to FIG. 12, which illustrates an
apparatus in the manner of a sampling oscilloscope which relies
upon analog data measured by a TOAD optical shutter. A coherent
optical signal (101) is fed on optical path (102) to a splitter
(103) which feeds an output signal (104) to optical path (105) and
an output signal (110) to optical path (111). Signal (110) on
optical path (111 ), even if weakened, enters device-under-test
(114) and emerges as signal (115) on optical path (116). There may
optionally be a plurality of amplifying units (106) along optical
path (105); and/or there may optionally be a plurality of
attenuating units (112) along the optical path traversing (103),
(111), (114), and (116); in any case, at least one of these options
must be implemented since the signal (115) must be weaker than the
non-linear regime of the TOAD device (117) and signal (104) must be
stronger than the switching threshold and must arrive at the input
port (109) prior to the arrival of signal (115) at input port
(119). The plurality of attenuation elements is depicted explicitly
as a single unit (112) but could in practice be anywhere along said
path, including merged with other units; likewise with the
plurality of amplification units depicted as a single unit (106).
There must also be a plurality of variable delay units (107) and/or
(108) which introduce the phase offsets needed to sample the
interval of interest. The control units can be internally
controlled and/or externally controlled.
[0055] External control signal (127) would be introduced through
path (126) and control signal (123) would be introduced through
path (122). The TOAD (117) emits a signal (120) on optical path
(121), said signal being representative of the convolution of the
time aperture of optical shutter (117) with the amplitude of signal
(115). Note that it is not strictly necessary for the elements
mentioned-including the device-under-test, splitter, delay units,
amplification units, attenuation units, and optical paths-to
precisely preserve the color of signal (115) with respect to signal
(104), since the clocking signal entering (109) is not interfered
against the data signal (115) entering (119). Therefore, optical
to-electrical and electrical-to-optical converters may be used
advantageously in device-under-test (114).
[0056] Reference is now made to FIG. 13, which illustrates an
alternative embodiment in the manner of a sampling oscilloscope
which relies upon analog data measured by a TOAD optical shutter
and uses the waveform being measured additionally as its own clock
signal. An optical source (129), which may advantageously be an
electrical-to-optical converter, introduces a signal (130) onto
optical path (131). A splitter (136) divides the signal (130) into
signal (134) on optical path (135) and signal (132) on optical path
(133). The embodiment attenuates (137) and/or amplifies (138) the
signal. There must be at least one variable delay unit (not
depicted) along the upper or lower path, as in FIG. 12. The clock
signal is required to arrive at port (143) before the data signal
arrives at port (141). The TOAD (144) emits a signal (146) on
optical path (147).
[0057] Other embodiments of sampling oscilloscopes are known in
industry and their reimplementation with TOAD-based optical
shutters would be self-evident for one of ordinary skill in the
art.
[0058] Reference is now made to FIG. 14, which illustrates an
optical A/D converter system (99) embedding an analog optical S/P
converter. Preferably, an O/E converter (50) accepts an optical
signal (14) from each of the paths (15) emitted from a S/P
converter (10), and converts said optical signal into an electrical
signal (54). An A/D converter (60) accepts the electrical signal
(54) from electrical lines (55) and produces a digital electrical
representation (64). The electrical lines are likely to be
transmission lines implemented as wires for low-speed operation and
microwave plumbing or coaxial paths for high speed operation.
Optionally, a processing unit (70) accepts said digital electrical
signal (64) along electrical lines (65) and delivers a corrected,
buffered version. In an alternative embodiment, the A/D converter
may incorporate the O/E converter integrally, so (54) and (55)
would be omitted.
[0059] Reference is now made to FIG. 15, which illustrates an
alternative embodiment of the slow A/D converter back-end: an
all-optical A/D converter (98). An optical sample-and-hold unit
(45) accepts the optical signal (4) from its optical path (5), and
slowly digitizes it, such as by the method disclosed in U.S. Pat.
No. 4,947,170, incorporated herein by reference. The optical signal
(4) may comprise a waveform encompassing a plurality of copies
provided to a common input line in time sequence. The time sequence
can be constructed, for example, by merging optional delay lines
(290) of differing lengths, with the delay lines originating from a
common output of (10).
[0060] Reference is now made to FIG. 16, which illustrates a
preferred O/E converter unit (50) for converting an analog optical
signal (5) into an analog electrical signal (54) representative of
it. Techniques for increasing the range and sensitivity and
limiting the noise introduced by optical-to-electrical converters,
such as photodetectors, are well-known. Common commercially
available optical-to-electrical converters include avalanche
photodetectors, photomultiplier tubes, charge-coupled devices, PIN
junctions, photosensitive resistors, photovoltaic devices, and
photosensitive capacitors, among others. Certain O/E converter
implementations can advantageously be implemented with
time-dependent control signals (56), such as a reset signal; with a
powering unit (57); and/or with scaling unit (58), such as for
dynamic range, bias and gain, linearity, or sensitivity settings,
possibly entailing connections to other units in the system (not
depicted). Each input path (5) may in fact comprise a plurality of
optical paths, each carrying its own coded version of optical
signal (4); each output path (55) may in fact comprise a plurality
of electrical lines, each conveying its own coded electrical signal
(54). The coding may be trivial (e.g. none) or complicated for
reasons arising from fault tolerance, power reduction, noise
reduction, and packaging, among others.
[0061] Reference is now made to FIG. 17, which illustrates a
preferred A/D converter (60) for converting an analog electrical
signal (54) into a digital electrical signal (64) representative of
it. In general, each A/D converter requires a time-dependent
control signal (66), preferably a clocking signal related to the
clocking signal of its optical particular optical shutter (40). A
time-dependent control signal will also be provided to its
processing unit (70) in order to know when to latch the signal, and
analog sample-and-hold units or digital latches may advantageously
be added at other points in accordance with well-known engineering
practices. Certain implementations of A/D converter (60) can
advantageously be implemented with additional time-dependent
control signals (66), such as a reset signal; with a powering unit
(67); and/or with scaling unit (68), such as for dynamic range,
bias and gain, linearity, or sensitivity settings, possibly
entailing connections to other units in the system (not depicted).
Each input path (55) may in fact comprise a plurality of electrical
lines, each carrying its own coded version of electrical signal
(54); the coding may be trivial (e.g. none) or complicated for
reasons arising from fault tolerance, power reduction, noise
reduction, and packaging, among others. Each output path shown as
(65) will generally comprise a multiplicity of electrical lines,
each conveying its own coded electrical signal (64), where the
coding will preferably be binary.
[0062] Reference is now made to FIG. 18, which illustrates a
preferred processing unit (70) for converting a plurality of
digital electrical signals (64) into a different plurality of
electrical signals (74) representative of it. In the preferred
embodiment, processing unit (70) is optional. A powering unit (77)
will advantageously be required for (70). Certain implementations
of processing unit (70) can advantageously be implemented with
time-dependent control signals (76), such as a reset signal or
output bus system clock signal; and/or with scaling unit (78), such
as for dynamic range, bias and gain, linearity, or sensitivity
settings, possibly entailing connections to other units in the
system (not depicted). For each stage (70), custom calibrations can
be measured through the individual path through (10), (40), and
(50). The calibrations can be applied locally or later; a lookup
table may advantageously be employed with or without calibration
data using memory unit (71) in order to expedite remapping signals
to appropriate (e.g. linear) representations of the original input
waveform's magnitude at a given time. Examples of the other units
in the system would be from an input stage to serve the dynamic
range. Each input path (65) generally comprises a plurality of
electrical lines, each carrying its own coded version of electrical
signal (64). The coding may be trivial (e.g. none) or complicated
for reasons arising from fault tolerance, power reduction, noise
reduction, and packaging, among others.
[0063] In the preferred embodiment, the ensemble of electrical
signals (74) will be buffered, latched, and clocked onto a system
bus by way of lines (75), in which case a memory unit (71) is also
provided for the processing unit (70). For this reason, the
synchrony of presenting the ensemble of electrical signals (74) to
a system bus restricts the architecture in which the processing
unit (70) is implemented. Simple digital systems require a master
clock which will advantageously be the cycle time of the flyback
signal (35), so all the latched bits will be injected onto the
system bus simultaneously. Such an embodiment produces a large
current impulse dI/dt, hence large simultaneous switching noise
which must be decoupled with a large, expensive capacitance between
power and ground. More complicated but affordable digital systems
splay the latched bits onto the bus over a distribution of time
slots, reducing the requirement for decoupling capacitance in
proportion to the increase in the rise time window, dt.
[0064] Each output path (75) will generally comprise a plurality of
electrical lines, each conveying its own coded electrical signal
(74), where the coding will advantageously be binary and optionally
for error detection/correction. In alternative embodiments, the
output path (75) may convey data other than binary electrical
signals, such as multistate digital, accept/reject/don't-care
evaluations, alarms, acoustic signals, or mechanical action.
[0065] Reference is now made to FIG. 19, which illustrates an A/D
converter system with analog electrical input and digital
electrical output. Note that in the preferred embodiment,
processing unit (70) is optional. In the preferred embodiment, an
input stage (80) is implemented as a laser modulator (90), such as
the module sold by United Technologies Photonics (UTP). Other input
stages may be advantageous, such as the method and apparatus
disclosed in U.S. Pat. No. 4,681,449, incorporated in its entirety
herein by reference. Laser modulator (90) receives an electrical
input signal (7) and a constant, coherent light source (81); its
output signal (83) is the modulation of light signal (81) by the
amplitude or intensity of the electric input signal (7). It may be
advantageous to embed the A/D converter system in other circuitry,
including front-end circuitry, in order to enable applications
described herein below.
[0066] The dynamic range of the TOAD devices (41) is finite, so
resealing the input signal that reaches the S/P converter (10)
advantageously broadens the dynamic range of operation for the
system comprising (10) and (99). Attenuation can avoid non-linear
and time-dependent saturation effects while amplification can avoid
a noise floor. In the preferred embodiment, an optional electrical
filter (97) can be employed to bias, attenuate or amplify the
electrical input signal (7) from a microwave guide (8) into
electrical signal (85) along path (86). In the preferred
embodiment, the electrical filter (97) is a dynamically variable
voltage divider and traveling wave amplifier. In an alternative
embodiment, an optional optical filter (95) optically amplifies or
attenuates a constant coherent signal (81) from path (82) as signal
(87) on path (88). In an additional alternative embodiment, an
optional optical filter (96) optically amplifies or attenuates a
time-varying optical signal (83) from path (84) as signal (1) on
path (2). Note that path (84) is marked twice since it may
advantageously be accessible from outside unit (80). For instance,
if an optical input signal is available instead of an electrical
input signal, the filter (96)--which is depicted as part of
optional unit (80)--may still be advantageously employed as
front-end to the A/D converter system (99) without using electrical
inputs. An optional reference beam (94) may also be fed into the
system for various reasons, notably to convert a phase modulated
signal (83) into an amplitude modulated signal (1), or to bias a
signal (83).
[0067] Typically, a first control unit (not shown) would provide a
first control signal (91) to filter (97), a second control unit
(not shown) would provide a second control signal (92) to filter
(95), and a third control unit (not shown) would provide a third
control signal (93) to filter (96). The control unit would be set
by the downstream processing unit (70); processing unit (70) may
establish the need to set control signals (91), (92), and/or (93)
from data reported to it by (78) from externally or from units
(50), (60), or (80). It may be advantageous to servo the input
filters (95), (96), and/or (97) and performance characteristics of
the back-end units (50), (60), and/or (70), for instance to
diagnose problems or optimize performance. Some, none or all of
these optional filters and connections to (70) may be employed. In
the absence of unit (97), electrical path (86) is electrical path
(8), and electrical signal (85) is electrical signal (7). In the
absence of unit (95), optical path (88) is optical path (82), and
optical signal (87) is optical signal (81). In the absence of unit
(96), optical path (2) is optical path (84), and optical signal (1)
is optical signal (83).
[0068] Reference is now made to FIG. 20, which concisely depicts an
alternative embodiment employing N=250-fold fan-out in the S/P
converter, a 1 psec external clock waveform, .tau..sub.--=4 psec
TOAD apertures, and B.sub.electrical=1 GHz electronics to sample a
B.sub.optical=250 GHz optical waveform at 250 GSPS. This embodiment
synchronously distributes a high-speed serial data stream into a
large number of low-speed parallel data streams. Slower data
streams are photoconverted, then sampled by all-electrical A/D
converters, and then latched into a memory unit.
[0069] An optical waveform (201) enters at the upper left-hand
corner of the diagram. The optical waveform is distributed N-ways
by an optical splitter, where the fan-out N is given by
N=B.sub.optical.div.B.sub.elect- rical. The larger the electrical
bandwidth B.sub.electrical, the smaller the requisite minimum size
of the splitter. For example, if the optical (input) bandwidth were
20 GHz and the A/D converter leaf nodes each operated at 1 GHz,
then at least a 20-way splitter would be required. The repetition
rate of the mode-locked laser driving the system is chosen to
correspond to the bandwidth of the electronics, that is,
1/T=B.sub.electrical=250 GHz. Here, the fan-out is 250-fold by a
star splitter hierarchy (202) into 250 equivalent taps, each
feeding an equal length delay line (203) followed by a TOAD
(204).
[0070] Each TOAD has two inputs, one for the signal (205) and one
for the control or gating pulse (206). The gating pulses will cause
the TOAD to open a 4 psec sampling window (209), allowing that
portion of the analog waveform from the signal input to pass to the
output of the TOAD and onward. The remainder of the analog waveform
outside the 4 psec window is suppressed and does not pass to the
output of the TOAD.
[0071] Each TOAD connects its gating input to a delay line (207) a
unique distance from a mode-locked laser pulse source, most readily
through a hierarchical clock distribution tree (210). Timing of the
control pulses at each TOAD is staggered by a .tau._=.sub.--4 psec
increment from its neighbors, so that the N samples produced at the
outputs of the N adjacent TOADs monitor 4Npsec. The N samples are
taken during a time period
T=N.tau._=NB.sub.optical=1/B.sub.electrical. Some time later, when
the N samples have been taken, the next mode-locked laser pulse
(211) enters the array of TOADs and another series of N samples is
taken. A continuous sample is composed piecewise as each TOAD takes
a snapshot through its time-limited window on the waveform,
shoulder to shoulder with the other TOADS. In this way the sampling
of the analog waveform continues round-robin without interruption.
In practice, the repetition rate of mode-locked lasers can be from
kHz to THz.
[0072] The pulse energy required after the splitter is
approximately 500 femtojoules. This is readily achieved by using a
conventional Nd:YLF laser if N<100. Here, with N=250, an
amplified mode-locked fiber ring laser or a mode-locked
semiconductor laser may be used. If insufficient power is available
from the laser, then several lasers may be synchronously
mode-locked.
[0073] Note that timing jitter within the A/D is negligible:
arbitrarily smaller than the Nyquist limit. The lengths of the
(fiber optic or solid state waveguide) delay lines are set at
manufacturing time and can be compensated for foreseeable or
measurable variations in temperature, humidity, vibration, or
voltage supply. The system's spurious free dynamic range will
therefore be limited by drift in the timing of the externally
supplied control pulse and should preferably exceed 120 dB within
the A/D module.
[0074] The timing control pulse can be physically clocked as in
FIG. 5 with a precision well below the Nyquist limit, leaving no
measurable jitter within the specified operating rate.
[0075] Each of the TOADS then feeds a dedicated photoconverter
(215) (e.g. a semiconductor avalanche photodiode or PIN junction,
depending on energetics). Note that the photodetectors under
consideration can be filled at any rate (e.g. over 1 psec or 1
.mu.sec) and can recover faster than the A/D converters (216) they
feed, so final low-cost A/D conversion remains the rate-limiting
event.
[0076] The output is subsequently electrical, and is digitized by
each A/D converter (216) at, for example, 1 GSPS with 8-bit
resolution. The 1 GHz cycle time is sufficiently slow that the
photodetector and A/D converter electronics can respond and recover
completely and be ready for the next sample.
[0077] In the preferred embodiment, local control logic (220)
remaps each A/D converter's output channel against a calibration
table to ensure linearity, and then either stores the data (221)
for later downloading at leisure, or else writes the electrical
data to a very wide synchronous bus (222). The bus (222) does not
need to operate at the same speed as the (rate-limiting) A/D
converters (216), and making the bus (222) faster or reading a
slower input signal would permit the bus (222) to be
proportionately narrower.
[0078] While the foregoing description has touched upon various
preferred embodiments and applications of the instant invention,
those skilled in the art, having read the foregoing, will
immediately recognize that the concepts detailed therein can be
implemented and/or used in numerous obvious alternative structures
and applications. Accordingly, it is understood that the scope of
applicants' invention shall not be limited to those preferred
and/or exemplary embodiments described herein, but instead, shall
be defined exclusively by the finally-issued claims (which claims
are intended to be read in the broadest reasonable manner), and any
and all equivalents thereto.
* * * * *