U.S. patent application number 09/837795 was filed with the patent office on 2001-09-13 for time-interleaved optical pulses for signal processing.
This patent application is currently assigned to California Institute of Technology, a California non-profit organization. Invention is credited to Yariv, Amnon.
Application Number | 20010021059 09/837795 |
Document ID | / |
Family ID | 22061388 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010021059 |
Kind Code |
A1 |
Yariv, Amnon |
September 13, 2001 |
Time-interleaved optical pulses for signal processing
Abstract
Devices and techniques for processing an analog signal by using
optical pulses at a high sampling rate. Parallel analog-to-digital
conversion channels can be implemented to achieve high-speed
analog-to-digital conversion.
Inventors: |
Yariv, Amnon; (San Mareno,
CA) |
Correspondence
Address: |
FISH & RICHARDSON, PC
4350 LA JOLLA VILLAGE DRIVE
SUITE 500
SAN DIEGO
CA
92122
US
|
Assignee: |
California Institute of Technology,
a California non-profit organization
|
Family ID: |
22061388 |
Appl. No.: |
09/837795 |
Filed: |
April 17, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09837795 |
Apr 17, 2001 |
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09193551 |
Nov 17, 1998 |
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6219172 |
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60065249 |
Nov 18, 1997 |
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Current U.S.
Class: |
359/264 ;
359/238; 359/245 |
Current CPC
Class: |
H04B 10/505 20130101;
G02F 7/00 20130101; H04B 10/508 20130101 |
Class at
Publication: |
359/264 ;
359/245; 359/238 |
International
Class: |
G02F 001/03; G02F
001/07; G02F 001/01; G02B 026/00 |
Claims
What is claimed is:
1. A device, comprising: an optical pulse generator configured to
produce a sampling optical pulse train with a temporally repetitive
sequence of optical pulses at a sampling pulse repetition rate,
wherein optical pulses in each sequence have pulse signatures
different from one another; and an optical modulator disposed to
receive said sampling optical pulse train and configured to
modulate a selected property of said optical pulses, in response to
an analog signal, to produce a modulated optical pulse train with
modulated optical pulses at said sampling repetition pulse rate to
carry information in said analog signal.
2. The device as in claim 1, wherein said pulse signatures include
optical wavelengths of said optical pulses.
3. The device as in claim 1, wherein said pulse signatures include
states of polarization of said optical pulses.
4. The device as in claim 1, wherein said selected property of said
optical pulses includes an amplitude of each optical pulse and
wherein said optical modulator includes an amplitude modulator.
5. The device as in claim 1, wherein said selected property of said
optical pulses includes a phase of each optical pulse and wherein
said optical modulator includes a phase modulator.
6. The device as in claim 1, wherein said optical modulator
includes an electro-optical modulator.
7. The device as in claim 1, wherein said optical pulse generator
includes at least one mode-locked laser that locks laser modes
according to an oscillator.
8. The device as in claim 1, further comprising a pulse separation
module disposed to receive said modulated pulse train from said
optical modulator and configured to separate said modulated pulse
train into N pulse trains where N is an integer greater than 1,
wherein each of said N pulse trains has a repetition pulse rate
less than said sampling pulse repetition rate by a factor of N and
is centered at a different wavelength.
9. The device as in claim 8, further comprising N electronic
channels connected in parallel with respect to one another, wherein
each electronic channel is connected to receive one of said N pulse
trains from said pulse separation module and to electrical
signals.
10. The device as in claim 9, wherein each electronic channel
includes an optical detector to receive a respective pulse train
and an analog-to-digital converter coupled to convert a detector
output from said optical detector into a digital form.
11. The device as in claim 9, wherein said sampling pulse
repetition rate is higher than a response speed of each electronic
channel and said repetition pulse rate of each of said N pulse
trains is not greater than said response speed of each electronic
channel.
12. The device as in claim 1, wherein said analog signal is an
optical signal.
13. The device as in claim 1, wherein said analog signal is an
electrical signal.
14. The device as in claim 1, wherein a wavelength of each optical
pulse in each sequence is different from wavelengths of any other
optical pulses in said each sequence, and wherein said pulse
generator includes: a mode-locked laser producing optical pulses in
response to an external oscillator that oscillates at a frequency
less than said sampling pulse repetition rate; a plurality of drop
filters connected relative to one another in series to filter said
optical pulses from said mode-locked laser to produce a plurality
of pulse trains centered at different wavelengths; and a plurality
of add filters connected relative to one another in series to
respectively receive said pulse trains from said drop filters, said
add filters operable to interleave pulses of different wavelengths
from said pulse trains to produce said sampling optical pulse
train.
15. The device as in claim 1, wherein a wavelength of each optical
pulse in each sequence is different from wavelengths of any other
optical pulses in said each sequence, and wherein said pulse
generator includes: a plurality of mode-locked lasers operating to
produce pulse trains at different wavelengths based a mode-lock
operation according to a common oscillator at said sampling pulse
repetition rate; and a plurality of add filters connected relative
to one another in series to respectively receive said pulse trains
from said mode-locked lasers, said add filters operable to
interleave pulses of different wavelengths from said pulse trains
to produce said sampling optical pulse train.
16. The device as in claim 15, further comprising an optical delay
element between each mode-locked laser and a respective add filter
to produce a proper delay for said interleaving pulses of different
wavelengths.
17. A method, comprising: generating a sampling optical pulse train
with a temporally repetitive sequence of optical pulses at a
sampling pulse repetition rate, wherein optical pulses in each
sequence have pulse signatures different from one another; and
modulating a selected property of said optical pulses in said
sampling optical pulse train according to a signal variation in an
analog signal to produce a modulated optical pulse train at said
sampling pulse repetition rate so that optical pulses in said
modulated optical pulse train are samples of the analog signal.
18. The method as in claim 17, wherein said pulse signatures
include optical wavelengths of said optical pulses.
19. The method as in claim 17, wherein said pulse signatures
include states of polarization of said optical pulses.
20. The method as in claim 17, wherein said selected property of
said optical pulses includes an amplitude of each optical
pulse.
21. The method as in claim 17, wherein said selected property of
said optical pulses includes a phase of each optical pulse.
22. The method as in claim 17, further comprising separating said
modulated optical pulse train into a plurality of optical pulse
trains centered at different wavelengths and at a pulse repetition
rate less than said sampling pulse repetition rate.
23. The method as in claim 22, further comprising: converting each
optical pulse train into a train of electronic pulses; and
converting said train of electronic pulses into a digital form to
represent information in the analog signal.
Description
[0001] This application is a continuation application of U.S.
patent application Ser. No. 09/193,551, filed Nov. 17, 1998 and
issued as U.S. Pat. No. 6,219,172 on Apr. 17, 2001, which claims
the benefit of U.S. Provisional Application No. 60/065,249,
entitled "Time Interleaved Sampling by Optical Techniques and Its
Application to Ultra High Speed A/D conversion" and filed Nov. 18,
1997.
TECHNICAL FIELD
[0002] The present invention relates to sampling and processing of
analog signals, more specifically, to analog-to-digital conversion
using optical techniques.
BACKGROUND
[0003] Analog electronic signals can be represented or
reconstructed by a certain set of discrete sampled values or
samples. Many electronic applications use samples of analog signals
instead of the analog signals. For example, digital data processing
and communication systems use digital data converted from samples
of an analog signal to achieve improved noise immunity and
processing flexibility in data processing and transmission.
Conversion of analog signals to digital data can be accomplished by
first sampling the analog signals into sampled values and then
digitizing the sampled values in a desired form.
[0004] A sampling rate or sampling frequency describes the number
of samples taken from an analog signal per unit time (e.g., one
second). The minimum sampling rate should be equal to or greater
than the Nyquist rate, which is double the highest frequency in an
analog signal, in order to preserve the minimum information content
in the original analog signal. Thus, a high sampling rate is
desirable in converting an analog signal with signal components at
high frequencies into digital form. In addition, an analog signal
may be oversampled at a sampling rate much higher than the Nyquist
rate to improve the signal-to-noise ratio and/or precision of a
subsequent analog-to-digital conversion.
[0005] Sampling of analog electronic signals is usually
accomplished electronically by using electronic circuits.
Electronic sampling techniques and circuits are well developed. The
maximum sampling rate achievable by an electronic circuit is
generally limited by the response times of the electronic
components and the circuit configuration. This further limits the
conversion speeds of many electronic analog-to-digital
converters.
[0006] Such speed limitation in electronic digital-to-analog
conversion can be an obstacle to many applications that require
high-speed analog-to-digital conversion. Real-time digital video in
applications such as telecommunication and machine vision is one
such example. The performance of the real-time digital video in the
existing video delivery on the Internet and in video conferencing
systems is not only limited by the bandwidth limitation in the data
transmission channels but also limited by the analog-to-digital
conversion rates.
SUMMARY
[0007] Therefore, there exists a need for devices and techniques
that provide high-speed sampling and analog-to-digital
conversion.
[0008] The present disclosure includes devices and techniques for
sampling analog signals at high sampling rates by using optical
pulses in a special way. It further provides devices and techniques
for converting such high-rate samples into digital data.
[0009] One aspect of the disclosure describes generation of a train
of dense pulses comprising a sequence of pulses that have different
pulse signatures so that one pulse is distinguishable from adjacent
pulses and different pulses with a common pulse signature can be
separated from other pulses to form a new pulse train. The pulses
are "dense" in a sense that the pulse repetition rate is higher
than the upper switching rate of many electronic devices.
[0010] One example of such dense pulse trains may include a
sequence of pulses that are centered at different wavelengths. One
device for generating this pulse train includes a mode-locked laser
for producing optical pulses with a known pulse repetition period,
a plurality of optical demultiplexer (e.g., "drop" filters)
connected in series and each configured to separate adjacent
oscillation modes in each pulse near a different center wavelength
and to transmit remaining modes in each pulse such that each
demultiplexer produces a train of pulses of the same pulse
repetition period at a different center wavelength, and a plurality
of optical multiplexers (e.g., "add" filters) connected in series
to form an optical path and configured to respectively couple the
plurality of pulse trains at different center wavelengths to the
optical path with a delay relative to one another so as to form an
interleaved dense pulse train. This interleaved dense pulse train
has a shortened pulse repetition period and a sequence of pulses at
different center wavelengths within one pulse repetition period of
the mode-locked laser.
[0011] Another device for generating the above interleaved dense
pulse train comprises a plurality of mode-locked lasers
respectively producing optical pulses at different center
wavelengths with the same pulse repetition period, and a plurality
of optical multiplexers (e.g., "add" filters) connected in series
to form an optical path and configured to respectively couple the
plurality of pulse trains from the mode-locked lasers to the
optical path to form the interleaved pulse train.
[0012] A second aspect of the disclosure includes sampling an
analog signal by using the above train of dense pulses. An optical
modulate is used to modulate a property of the pulses in the
interleaved dense pulse train in response to an analog signal and
therefore impose the information in the analog signal onto the
optical pulses. The analog signal may be in various forms such as
an electrical analog signal that drives the optical modulator or an
optical analog signal that interacts with the optical pulses to
produce the modulation (e.g., a wave-mixing device). The property
of the pulses may be the amplitude, phase, polarization, or other
parameters of the optical pulses. One simple implementation is
amplitude modulation by using an optical amplitude modulator that
is driven by an analog electrical signal. This converts the
information in the analog signal into amplitude variation of the
dense pulses at a sampling rate equal to the repetition rate of the
dense pulses.
[0013] A third aspect of the disclosure involves separating the
information-bearing dense pulse train into a plurality of pulse
trains according to their pulse signatures. In the above wavelength
interleaved pulse train, pulses at different center wavelengths are
separated into different pulse trains each with the same center
pulse wavelength. Each pulse train has less pulses per unit time
than the original pulse train. Hence, the pulse repetition rate is
reduced to the low pulse repetition rate of the original
unmodulated pulse trains. One or more optical demultiplexers, such
as a set of optical "drop" filters respectively corresponding to
the different center wavelengths, may be connected in series in the
optical path of the dense pulse train to perform the pulse
separation and to generate a plurality of parallel signal channels
at different center wavelengths and different time delays. Each
signal channel may further include a photosensor for converting the
modulated optical pulses into analog electrical pulses, and an
electronic analog-to-digital converter for converting the analog
pulses into digital data. The reduced pulse repetition rate of the
pulse trains in the signal channels can be set to accommodate for
the processing speed of the electronic analog-to-digital
converters. The analog-to-digital conversion of all signal channels
is performed electronically in parallel. The combined digital data
from all signal channels includes all the information content of
the analog signal if the repetition rate of the dense pulse train
is equal to or greater than the Nyquist rate of the analog signal
and hence can be used for subsequent digital signal processing.
[0014] One advantage of the systems in the disclosure is their
unique optical sampling by dense pulses to achieve desired high
sampling rates. A train of dense pulses comprising a sequence of a
plurality of pulses at different center wavelengths can be formed
to have a high pulse repetition frequency that is in general
difficult, if not possible to achieve with electronic sampling
devices. An increase in the sampling rate by a factor up to and or
greater than 10.sup.2 may be achieved.
[0015] Another advantage is an increased speed in analog-to-digital
conversion. This is at least in part due to the optical processing
in sampling and separating the modulated dense pulse train into
multiple pulse trains at different center wavelengths and in part
due to the parallel analog-to-digital conversion in the multiple
signal channels.
[0016] These and other aspects and advantages will become more
apparent in light of the accompanying drawings, the detailed
description, and the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a diagram of a signal sampling device based on
optical pulses with a high repetition rate.
[0018] FIG. 2 shows one implementation of the device of FIG. 1.
[0019] FIGS. 3A, 3B, 3C, and 3D show one embodiment of a pulse
generator used in the device of FIG. 2.
[0020] FIG. 4 shows an alternative embodiment of a pulse generator
used in the device of FIG. 2.
[0021] FIG. 5 illustrates amplitude modulation on optical pulses to
sample an analog signal based on operation of an optical
modulator.
[0022] FIG. 6A shows one embodiment of a drop filter module for
separating time-wavelength interleaved pulses.
[0023] FIG. 6B illustrates operation of the analog-to-digital
conversion by parallel electronic channels.
DETAILED DESCRIPTION
[0024] Signal sampling is conventionally achieved by using
electronic pulses that are usually effected by electronic switches
or modulators. The repetition rate of the electronic pulses is the
sampling rate.
[0025] An optical pulse train is not subject to many of the
physical barriers that limit electrical signals, e.g., line
capacitance and resistance. Hence, an optical signal in general can
have a repetition rate much higher than what is possible for an
electronic pulse train and can have sharp pulse edges. An optical
pulse train can be modulated by using an analog signal, which may
be electrical, optical or of other forms, to drive an optical
modulator. The modulated pulses then become samples of the analog
signal in the optical domain. A higher sampling rate, which may not
be possible by electronic sampling techniques, can therefore be
achieved.
[0026] However, such optical samples may not be easily converted
into electronic pulses for further electronic processing since the
repetition rate of the optical pulses can be too high for
electronic circuits to respond or detect.
[0027] This difficulty can be overcome by using a plurality of
parallel electronic channels to process the data contained in an
optical pulse train. Each electronic channel is designated to
process only a fraction of the data carried in the optical pulse
train. An opto-electronic signal processing device can be
implemented to combine the above high-speed optical sampling and
parallel electronic processing to achieve sampling rates and
processing speeds higher than the state of art electronic
circuits.
[0028] FIG. 1 shows an embodiment of an opto-electronic device 100
that is based on the high-speed optical sampling and parallel
electronic processing. An analog electronic signal 121 is to be
sampled and may be converted into digital form. An optical pulse
generator 110 produces an optical pulse train 112 with a high
repetition rate, f.sub.s. An optical modulator 120, e.g., an
electro-optical modulator, responds to the analog electronic signal
121 to modulate the optical pulses in the optical train 112 and
hence produces a modulated optical train 122. A property of an
optical pulse, e.g., phase or amplitude, is modulated to encode the
information of the analog signal 121 onto the optical pulse to
represent a sample of the analog signal 121.
[0029] An optical pulse separation module 130 separates the
modulated optical pulse train 122 into a plurality of pulse trains
132P.sub.1, 132P.sub.2, . . . , 132P.sub.N (e.g., N number of pulse
trains) each having a common reduced repetition rate of f.sub.s/N.
A series of N sequential pulses (indicated by P.sub.1, P.sub.2, . .
. , and P.sub.N) in the modulated optical pulse train 122 are
separated into the pulse trains 132P.sub.1, 132P.sub.2, . . . ,
132P.sub.N, respectively. Hence, the first pulse train 132P.sub.1
has pulses P.sub.1, P.sub.1+N, P.sub.1+2N, and so on; the second
pulse train 132P.sub.2 has pulses P.sub.2, P.sub.2+N, P.sub.2+2N,
and so on; and the last pulse train 132P.sub.N has pulses P.sub.N,
P.sub.2N, P.sub.3N, and so on. The separated pulse trains
132P.sub.1, 132P.sub.2, . . . , 132P.sub.N are then routed to N
parallel optical detectors 140 (D.sub.1, D.sub.2, . . . , and
D.sub.N) and are converted into N parallel electronic channels
142.
[0030] One way to implement above pulse separation is to generate
the dense pulse train 112 with different pulse signatures, e.g.,
pulse wavelength or pulse polarization. Each pulse in a sequence of
pulses is assigned a pulse signature different from adjacent pulses
or any other pulses in the sequence. The pulse separation module
130 separates these pulses according to the signatures to produce
different pulse trains as the parallel optical channels.
[0031] Since each separated optical pulse train has a reduced
repetition rate of f.sub.s/N, each parallel electronic channel 140
can operate at this reduced rate f.sub.electronic=f.sub.s/N rather
than the high rate f.sub.s. Each electronic channel may include an
analog-to-digital converter ("ADC") so that the analog information
in the modulated optical pulse train 122 is converted into digital
form by N parallel ADCs. For given f.sub.electronic and f.sub.s,
the number N may be adjusted to match the total processing rate of
parallel electronic channels and the optical sampling rate of the
optical pulse train 122: f.sub.electronic=f.sub.s/N. An improvement
of the processing speed by a factor of N can therefore be achieved
over the state-of-art electronic devices.
[0032] For many practical applications, N may be chosen between 10
and 100. Hence, for f.sub.electronic=10 GHZ and N=10, a sampling
rate of f.sub.s=100 GHZ can be achieved by using the system
100.
[0033] FIG. 2 shows one implementation 200 of the system 100 in
FIG. 1. Certain aspects of this system is disclosed by Yariv and
Koumans in "Time interleaved optical sampling for ultra-high speed
A/D conversion," Electronic Letters, vol. 34 (21) pp.2012-2013
(1998), which is incorporated herein by reference. An optical pulse
generator 210 is configured to produce an optical pulse train 212
with sequentially repetitive sets of pulses where each set includes
N sequential pulses centered at different wavelengths
.lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.N at a
repetition rate f.sub.s. An optical amplitude modulator 220
modulates the amplitudes of the pulses in the pulse train 212 to
produce an amplitude-modulated pulse train 222. Hence, the
amplitude variation in the pulse train 222 represents the
information in the analog signal 121.
[0034] The optical pulse separation module 130 in the FIG. 1 is
implemented by using an optical filter module 230 to separate the
pulses in the pulse train 222 according to their wavelengths into N
different pulse trains 230.lambda..sub.1, 230.lambda..sub.2,
230.lambda..sub.3, . . . , and 230.lambda..sub.N. Each pulse train
230.lambda..sub.i(i=1, 2, . . . N) has pulses only of the
wavelength .lambda..sub.i and at a reduced common repetition rate
of f.sub.s/N. N parallel electronic channels, each including an
optical detector 240 and an ADC, converts the analog samples in the
N different pulse trains 230.lambda..sub.1, 230.lambda..sub.2,
230.lambda..sub.3, . . . , and 230.lambda..sub.N into digital
data.
[0035] The optical pulse generator 210 may be formed of a
mode-locked laser 310, a set of demultiplexers 330 such as "drop"
filters connected in series, and a set of optical multiplexers 350
such as "add" filters connected in series as shown in FIG. 3A. Many
optical add and drop filters may be used. See, for example,
Kewitsch et al., "All-fiber zero-insertion-loss add-drop filter for
wavelength-division multiplexing," Optics Letters, Vol.23(2), pp.
106-108 (1998) and Agrawal, "Fiber-optic communication systems",
2nd edition, Chapter 7, John Wiley & Sons, New York (1997),
which are incorporated herein by reference.
[0036] The mode-locked layer 310 is preferably actively mode-locked
by using an external electronic oscillator at an oscillation
frequency f.sub.osc=f.sub.s/N. Both f.sub.osc and N may be adjusted
so as to achieve a desired f.sub.s. One example of the mode-locked
laser is shown in FIG. 3B where an edge-emitting semiconductor
laser 312 is driven by a current source 314 and is mode-locked by a
mode-locking oscillator 316. Other mode-locked lasers may also be
used, including other types of semiconductor lasers and fiber
lasers.
[0037] The laser output from the laser 310 is a pulse train 320
with a repetition rate of f.sub.s/N. Each pulse includes spectral
components of different wavelengths emitted by the laser medium.
Hence, the pulse train 320 can be filtered to produce one or more
optical trains centered at desired wavelengths selected from the
spectral components in the laser output. The drop filters 330 are
specifically designed to achieve this filtering.
[0038] Referring to FIG. 3C, N drop filters 330.lambda..sub.1,
230.lambda..sub.2, . . . , and 330.lambda..sub.N at different
wavelengths .lambda..sub.1, .lambda..sub.2, . . . , and
.lambda..sub.N are connected in series in a fiber 332 to form an
optical path that receives the optical pulse train 320 from the
laser 310. Each drop filter is configured to select a spectral
component at a wavelength .lambda..sub.i (i=1, 2, . . . , N) to
produce an output and transmits the remaining spectral components
down the fiber 332. Hence, the selected output 340.lambda..sub.i
from a drop filter 330.lambda..sub.1 is a pulse train centered at
the wavelength .lambda..sub.i and has the same pulse repetition
rate f.sub.s/N as the unfiltered pulse train 320. Since the
spectral components at wavelengths other than .lambda..sub.i are
eliminated, the pulse width of the selected output
340.lambda..sub.i is broadened compared to the pulse width of the
unfiltered pulse train 320.
[0039] The add filters 350 are designed to interleave pulse trains
330.lambda..sub.1, 330.lambda..sub.2, . . . , and 330.lambda..sub.N
to form a dense pulse train 212 with a repetition rate of f.sub.s.
One implementation is illustrated in FIG. 3D. N add filters
350.lambda..sub.1, 350.lambda..sub.2, . . . , and 350.lambda..sub.N
are connected in series in a fiber 352. Each add filter
350.lambda..sub.i(i=1, 2, . . . , N) is connected to receive a
respective output 340.lambda..sub.i from the drop filter
330.lambda..sub.i and add the output 340.lambda..sub.i to the fiber
352. A constant temporal delay is introduced (by using an fiber
delay line or other optical delay element) to each add filter
350.lambda..sub.i so that the outputs 340.lambda..sub.1,
340.lambda..sub.2, . . . , and 340.lambda..sub.N are sequentially
added to the fiber 352 and the temporal separation between the
first output 340.lambda..sub.1 and the last output
340.lambda..sub.N is equal to the pulse separation produced by the
laser 310.
[0040] Thus, the newly generated pulse 212 in the fiber 252 is a
special pulse train of a repetitive pulse sequence which is
sequentially formed of N pulses of different wavelengths. In every
period of 1/f.sub.s, a pulse at a different wavelength arrives.
[0041] In a period of T=(N-1)/Nf.sub.s, the spectral pattern of the
pulses repeats, i.e., the wavelength of a pulse repeats after every
N pulses. This pulse train has the unique property that the
wavelength of each pulse in the train is different from those of
its neighbors in a controlled fashion. The pulses are essentially
identical in shape and equally spaced in time.
[0042] FIG. 4 shows another alternative implementation 210B of the
pulse generator 210 in FIG. 2. N different mode-locked lasers
400.lambda..sub.1, 400.lambda..sub.2, . . . , and 400.lambda..sub.N
are used to generate N pulse trains of the same repetition rate at
f.sub.s/N but at different wavelengths .lambda..sub.1,
.lambda..sub.2, . . . , and .lambda..sub.N. The mode-locked lasers
400.lambda..sub.1, 400.lambda..sub.2, . . . , and 400.lambda..sub.N
are mode-locked by a common mode-locking oscillator 316. The
outputs of the lasers are then interleaved in a fiber 352 to form
the dense pulse train. The output of each laser is properly delayed
by using a delay element 410. The multilayers can be integrated
monolithically to be compact and the delay between pulses of
different wavelengths may also be electronically controlled by
phase delays of RF drivers to the individual lasers.
[0043] The optical amplitude modulator 220 in FIG. 2 modulates the
amplitude of each pulse in the dense time-wavelength interleaved
pulse train 212 to obtain samples of an analog signal 121 v(t)
(e.g., a rf signal). The sampling frequency is f.sub.s, the pulse
repetition rate. The analog signal 121 is applied as a voltage (or
current) to the modulator 220 such that the instantaneous
transmitted optical intensity is proportional to v(t) or some
representation of v(t). This is illustrated in FIG. 5.
[0044] Since each pulse undergoes a multiplication by the optical
modulator 220 by a factor kv(t) where k is a constant of the
modulator 220. The result is a sampling of the signal by the
optical pulses. The variation of v(t) during the passage of an
individual pulse is assumed to be negligible.
[0045] The modulated pulse train 222 from the modulator 220 has a
repetition rate f.sub.s which may be fast for conventional
electronics to process. One aspect of the invention is to separate
the pulse train 222 into N pulse trains of a reduced pulse rate so
that N optical detectors 240 can be used to convert the pulses into
N parallel electronic channels and therefore to process the data in
parallel.
[0046] FIG. 6A shows one embodiment of the drop filter module 230
for separating the dense pulse train 222. The construction is
similar to the device shown in FIG. 3C used in the pulse generator
shown in FIG. 3A. The modulated pulses centered at a common
wavelength .lambda..sub.i (i=1, 2, . . . , N) are selected out by a
drop filter at that wavelength .lambda..sub.i to form a new
modulated pulse train 230.lambda..sub.i with a reduced pulse
repetition rate at f.sub.s/N, since the spectral pattern of the
pulses repeats every N pulses in the pulse train 222. N modulated
pulse trains are generated among which the information of the
analog signal 121 are distributed. The pulse trains are further
converted into N electronic channels of digital data as shown by
FIG. 6B.
[0047] Although the present invention has been described in detail
with reference to the preferred embodiments, various modifications
and variations may be possible without departing from the spirit of
the invention. For example, pulse trains with different wavelengths
can also be generated in other ways using a single laser, for
example, by spectral slicing. See, Knuss et al., "Scalable 32
channel chirped-pulse WDM source," Electronic Letters, vol.32(14),
pp.1311-1312 (1996). Other mode-locked lasers can also be used to
generate the dense pulse trains such as lasers disclosed by Sanders
et al. in "Timing jitter and pulse energy fluctuations in a
passively mode-locked 2-section quantum-well laser coupled to an
external cavity," Applied Physics Letters, Vol.59(11), pp.1275-1277
(1991), by Arahira et al. in "Transform-limited optical short-pulse
generation at high repetition rate over 40 GHz from a monolithic
passively mode-locked DBR laser diode," IEEE Photonics Technology
Letters, Vol.5(12), pp.1362-1365 (1993), and by Salvatore et al. in
"Wavelength tunable source of subpicosecond pulses from CW
passively mode-locked 2-section multiple-quantum-well laser," IEEE
Photonic Technology Letters, vol.5(7), pp.756-758(1993). Each
disclosure of the above references is incorporated herein by
reference.
[0048] These and other variations and modifications are intended to
be encompassed by the following claims.
* * * * *