U.S. patent application number 09/802751 was filed with the patent office on 2002-02-07 for wdm system that uses nonlinear temporal gratings.
Invention is credited to Hakimi, Farhad, Hakimi, Hosain.
Application Number | 20020015206 09/802751 |
Document ID | / |
Family ID | 27499290 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020015206 |
Kind Code |
A1 |
Hakimi, Farhad ; et
al. |
February 7, 2002 |
WDM system that uses nonlinear temporal gratings
Abstract
A method of generating a signal including: generating a first
sequence of coherent optical pulses which are sufficiently close in
spacing so as to overlap and interfere upon traveling a
predetermined length down an optical fiber to form a first
interference pattern with a first central lobe having a
characteristic wavelength, wherein the energy of the pulses of the
first sequence of pulses is within the non-linear regime of the
optical fiber; manipulating the first sequence of optical pulses;
in a similar manner generating a second sequence of coherent
optical pulses; manipulating the second sequence of optical pulses;
and introducing both the first and second manipulated sequences of
pulses into the optical fiber, wherein the manipulating of the
pulses of the first and the second sequence of pulses shifts the
characteristic wavelengths of the first and second central lobes,
respectively, to first and second transmission signal
wavelengths.
Inventors: |
Hakimi, Farhad; (Watertown,
MA) ; Hakimi, Hosain; (Watertown, MA) |
Correspondence
Address: |
ERIC L. PRAHL
Fish & Richardson P.C.
225 Franklin Street
Boston
MA
02110-2804
US
|
Family ID: |
27499290 |
Appl. No.: |
09/802751 |
Filed: |
March 8, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09802751 |
Mar 8, 2001 |
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09722080 |
Nov 22, 2000 |
|
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60222708 |
Aug 3, 2000 |
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60244298 |
Oct 30, 2000 |
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Current U.S.
Class: |
398/147 |
Current CPC
Class: |
H04B 10/25077
20130101 |
Class at
Publication: |
359/161 ;
359/173 |
International
Class: |
H04B 010/00; H04B
010/12 |
Claims
What is claimed is:
1. A method of generating a signal for transmission down an optical
fiber characterized by dispersion and a non-linear regime of
operation, said method comprising: generating a first sequence of
coherent optical pulses each of which has an associated energy,
wherein the pulses in the first sequence of optical pulses are
sufficiently close in spacing so that upon traveling a
predetermined length down the optical fiber, the pulses of the
first sequence of pulses will overlap and interfere to form a first
interference pattern with a first central lobe having a
characteristic wavelength, and wherein the associated energy of at
least one of the pulses of the first sequence of pulses is within
the non-linear regime of the optical fiber; manipulating the pulses
of the first sequence of optical pulses to generate a first
manipulated sequence of pulses; generating a second sequence of
coherent optical pulses each of which has an associated energy,
wherein the pulses in the second sequence of optical pulses are
sufficiently close in spacing so that upon traveling a
predetermined length down the optical fiber, the pulses of the
second sequence of pulses will overlap and interfere to form a
second interference pattern with a second central lobe having a
characteristic wavelength, and wherein the associated energy of at
least one of the pulses of the second sequence of pulses is within
the non-linear regime of the optical fiber; manipulating the pulses
of the second sequence of optical pulses to generate a second
manipulated sequence of pulses; and introducing both the first
manipulated sequence of pulses and the second manipulated sequence
of pulses into said optical fiber, wherein the manipulating of the
pulses of the first and the second sequence of pulses shifts the
characteristic wavelengths of the first and second central lobes,
respectively, to first and second transmission signal
wavelengths.
2. The method of claim 1 wherein each of the pulses of said first
sequence of pulses has energy that is within the non-linear regime
of the optical fiber.
3. The method of claim 2 wherein each of the pulses of said second
sequence of pulses has energy that is within the non-linear regime
of the optical fiber.
4. The method of claim 1 wherein the step of manipulating the
pulses of the first sequence of optical pulses comprises changing
at least one of amplitude, phase, and timing of the pulses of the
first sequence of pulses relative to each other.
5. The method of claim 4 wherein the step of manipulating the
pulses of the second sequence of optical pulses comprises changing
at least one of amplitude, phase, and timing of the pulses of the
second sequence of pulses relative to each other.
6. The method of claim 1 wherein the first sequence of pulses
includes only two pulses.
7. The method of claim 6 wherein the second sequence of pulses
includes only two pulses.
8. The method of claim 1 wherein generating said first sequence of
coherent optical pulses comprises delivering a first coherent laser
beam at a first predetermined wavelength and producing said first
sequence of pulses from the first coherent laser beam.
9. The method of claim 8 wherein generating said second sequence of
coherent optical pulses comprises delivering a second coherent
laser beam at the first predetermined wavelength and producing said
second sequence of pulses from the second coherent laser beam.
10. The method of claim 1 further comprising encoding first data
onto the first sequence of pulses before manipulating the pulses of
the first sequence of pulses.
11. The method of claim 10 further comprising encoding second data
onto the second sequence of pulses before manipulating the pulses
of the second sequence of pulses.
12. The method of claim 1 wherein introducing both the first
manipulated sequence of pulses and the second manipulated sequence
of pulses into said optical fiber comprises passing both the first
manipulated sequence of pulses and the second manipulated sequence
of pulses through a wavelength multiplexer.
13. The method of claim 1 wherein generating said first sequence of
optical pulses comprises: supplying a first continuous single
coherent wave laser beam at a first preselected wavelength; and
chopping the first continuous wave laser beam to produce the first
sequence of optical pulses.
14. The method of claim 13 wherein generating said second sequence
of optical pulses comprises: supplying a second continuous single
coherent wave laser beam at a second preselected wavelength; and
chopping the second continuous wave laser beam to produce the
second sequence of optical pulses.
15. The method of claim 14 wherein the first and second preselected
wavelengths are the same.
16. The method of claim 1 wherein generating said first sequence of
optical pulses comprises: supplying a first single coherent optical
pulse at a first preselected wavelength; and producing the first
sequence of optical pulses from the first single coherent optical
pulse.
17. The method of claim 16 wherein generating said second sequence
of optical pulses comprises: supplying a second single coherent
optical pulse at a second preselected wavelength; and producing the
second sequence of optical pulses from the second single coherent
optical pulse.
18. The method of claim 17 wherein the first and second preselected
wavelengths are the same.
19. An apparatus for sending optical signals down an optical fiber
characterized by dispersion and a non-linear regime of operation,
said apparatus comprising: a first optical signal source which
during operation generates a first sequence of coherent optical
pulses each of which has an associated energy, wherein the pulses
in the first sequence of optical pulses are sufficiently close in
spacing so that upon traveling a predetermined length down the
optical fiber, the pulses of the first sequence of pulses will
overlap and interfere to form a first interference pattern with a
first central lobe having a characteristic wavelength, and wherein
the associated energy of at least one of the pulses of the first
sequence of pulses is within the non-linear regime of the optical
fiber; a frequency shifter which during operation manipulates the
pulses of the first sequence of optical pulses to generate a first
manipulated sequence of pulses; a second optical signal source
which during operation generates a second sequence of coherent
optical pulses each of which has an associated energy, wherein the
pulses in the second sequence of optical pulses are sufficiently
close in spacing so that upon traveling a predetermined length down
the optical fiber, the pulses of the second sequence of pulses will
overlap and interfere to form a second interference pattern with a
second central lobe having a characteristic wavelength, and wherein
the associated energy of at least one of the pulses of the second
sequence of pulses is within the non-linear regime of the optical
fiber; a second frequency shifter which during operation
manipulates the pulses of the second sequence of optical pulses to
generate a second manipulated sequence of pulses; and a wavelength
division multiplexer system which during operation receives both
the first manipulated sequence of pulses and the second manipulated
sequence of pulses and introduces them into said optical fiber,
wherein the first and second frequency shifters shift the
characteristic wavelengths of the first and second central lobes,
respectively, to first and second transmission signal wavelengths.
Description
[0001] This application is a continuation of U.S. Ser. No.
09/722,080, entitled "Nonlinear Temporal Grating As A New Optical
Solitary Wave" filed Nov. 22, 2000 and which claims the benefit of
U.S. Provisional Application No. 60/222,708, filed Aug. 3, 2000 and
of U.S. Provisional Application No. 60/244,298, filed Oct. 30,
2000.
TECHNICAL FIELD
[0002] This invention relates generally to transmitting optical
signals in optical fibers, and more particularly, to reducing pulse
broadening in a nonlinear operating region of optical fiber
transmission and ultra-fast optical switching.
BACKGROUND
[0003] FIG. 1 shows that an initial optical pulse 2 becomes a
broader pulse 3 after traveling through an optical fiber 4. One
source of broadening of pulse 2 results from dispersion. One cause
of dispersion is a variation in a fiber's refractive index with
wavelength. The fiber's refractive index is defined as the ratio of
speed of light in vacuum to speed of light in the fiber. The
refractive index variations make longer and shorter wavelength
components of pulse 2 travel at different speeds in optical fiber
4. After traveling through a certain length of optical fiber 4, the
speed variations produce broader pulse 3. Another cause of
dispersion is waveguide dispersion, which is induced by the
geometric configuration of fiber 4.
[0004] Pulse broadening can affect the quality of digital data
transmission in optical fiber 4. Digital data is transmitted as a
series of optical pulses. Each temporal interval for a pulse may
represent one binary bit. For example, a data format called On-Off
Keying (OOK) indicates the binary states "1" and "0" corresponding
to the presence and absence of a pulse, respectively. As pulses
broaden and overlap, a receiver may not be able to determine
whether a pulse is present in a particular time interval or whether
a detected optical signal is the tail of a previous or subsequent
pulse. Inserting an amplifier 5 into optical fiber 4 can help to
reduce receiver errors due to propagation weakening of pulse
intensities. But, amplifier 5 does not help to reduce receiver
errors caused by the dispersion-generated pulse broadening and
overlap.
[0005] Present optical fiber communications typically use optical
pulses having wavelengths of about 1.5 microns, because
erbium-doped fibers can provide quality optical amplification at
1.5 microns. Unfortunately, many older optical fibers produce
significant chromatic dispersion in optical signals at 1.5 microns.
This chromatic dispersion produces significant pulse broadening,
which limits transmission wavelengths and distances in contemporary
optical networks.
[0006] The refractive index of fiber 4 also varies with the
magnitude of electric field .epsilon. of pulse 2. For symmetric
molecules, such as silica glasses of which most optical fibers are
made, the first-order .epsilon. dependent term in the refractive
index vanishes. The higher-order terms of .epsilon.,
.epsilon..sup.2 in particular, in the refractive index produce most
of the nonlinear effects in optical fibers. When the intensity in
pulse 2 is low, the higher-order terms of .epsilon. in the
refractive index only have negligible effects, and therefore pulse
2 is in a linear operating region of fiber 4. When the intensity of
pulse 2 is sufficiently high, the higher-order terms of .epsilon.
become non-negligible and cause pulse 2 to enter a nonlinear region
of operation of fiber 4.
[0007] A notable manifestation of the nonlinear operation of fibers
is self-phase modulation (SPM). SPM generally causes a pulse to
broaden in spectrum while the pulse is propagating in the nonlinear
operation region of a fiber. However, the effects of spectral
broadening caused by SPM may counterbalance the effects of
chromatic dispersion with the result that the pulse retains its
shape.
[0008] The chromatic dispersion is characterized by a second order
chromatic dispersion parameter .beta..sub.2, which is a function of
the pulse's wavelength and derivatives of the fiber's refractive
index with respect to the wavelength. If .beta..sub.2 is negative,
the pulse is said to be propagating in an anomalous dispersion
regime of the fiber. In the anomalous dispersion regime, the SPM
causes the leading edge of the pulse to travel slower than its
trailing edge, thus effectively compressing the pulse and balancing
out the pulse broadening induced by the second order chromatic
dispersion.
[0009] A pulse propagating in the fiber with balanced SPM and
chromatic dispersion is a form of solitary wave called a soliton.
Ideally, a soliton may travel a long distance while retaining its
shape and spectrum. However, a soliton is susceptible to amplitude
fluctuations, which may be caused by, for example, the amplifiers
that are required along the fiber. The amplitude fluctuations
generate frequency shifts, which in turn cause Gordon-Haus time
jitters due to different frequencies traveling at different
velocities. The frequency shifts and Gordon-Haus time jitters are
detrimental to a data transmission system. In a wave-length
division multiplexing (WDM) system, frequency shifts produce
undesired emissions outside of the allotted frequency band assigned
to each channel, and the undesired emissions may interfere with
other channels or other systems; while time jitters create problems
of data clock recovery at a receiver or regenerator site, because
data bits represented by the optical pulses may not be synchronized
due to the timing uncertainties.
[0010] Time jitters can be reduced by inserting sliding filters in
strategically chosen locations along the fiber span. Another method
to reduce time jitters is a dispersion-managed soliton technique
that uses dispersion compensating fibers, which have dispersion
characteristics tuned to compensate for the time jitters along the
fiber span. The overall average dispersion characteristicis, on the
other hand, is designed to counterbalance the SPM.
[0011] Even with dispersion management, any soliton, when traveling
far enough into a fiber, surrenders to an effect called third order
dispersion (TOD). TOD causes the soliton to spread unsymmetrically
in the temporal domain into a widened, non-symmetrical pulse. FIGS.
2 illustrates the TOD effects on a soliton. FIG. 2 shows a soliton
pulse after being unsymmetrically spread by TOD.
[0012] Some implementations allow a soliton to travel over long
distance with optical regenerators. The design of regenerators, for
example, optical 2R (re-shape and re-time) or 3R (re-shape, re-time
and re-amplify), involves complicated issues such as polarization
sensitivities, cost and complexities.
SUMMARY
[0013] In general, in one aspect, the invention is a method of
generating a signal pulse in an optical fiber characterized by
dispersion and a refraction index that has a nonlinear regime of
operation. The method involves generating a sequence of coherent
optical pulses each of hich has an associated energy; and
introducing the sequence of pulses into the optical fiber, wherein
the pulses in the sequence of pulses are sufficiently close in
spacing so that after traveling a predetermined length down the
optical fiber, the pulses of the sequence of pulses overlap and
interfere to form an interference pattern. The associated energy of
at least one of the pulses of the sequence of pulses is within the
nonlinear regime of the optical fiber.
[0014] In general, in another aspect, the invention is a method of
generating a signal pulse in an optical fiber that involves
generating a sequence of coherent optical pulses each of which has
an associated energy; and introducing the sequence of pulses into
the optical fiber, wherein the pulses in the sequence of pulses are
sufficiently close in spacing so that after traveling a
predetermined length down the optical fiber, the pulses of the
sequence of pulses overlap and interfere to form an interference
pattern having a central lobe and multiple side lobes. The
interference pattern is characterized by a contrast ratio, and the
associated energy of each pulse of the sequence of pulses is
sufficiently high relative to characteristics of the optical fiber
so as to cause the contrast ratio of the interference pattern to
increase as the interference pattern propagates further along the
optical fiber.
[0015] In general, in still another aspect, the invention is a
method of generating a signal pulse in an optical fiber that
involves generating a sequence of coherent optical pulses each of
which has an associated energy; and introducing the sequence of
pulses into the optical fiber, wherein the pulses in the sequence
of pulses are sufficiently close in spacing so that after traveling
a predetermined length down the optical fiber, the pulses of the
sequence of pulses overlap and interfere to form an interference
pattern having a central lobe and multiple side lobes. The
associated energy of each pulse of the sequence of pulses is
sufficiently high relative to characteristics of the optical fiber
so as to cause energy from the side lobes to transfer into the
central lobe as the interference pattern propagates further along
the optical fiber.
[0016] Preferred embodiments include one or more of the following
features. Each of the pulses of the sequence of pulses has energy
that is within the nonlinear regime of the optical fiber. The
sequence of pulses may include only two pulses or it may include
more than two pulses. The generating of a sequence of coherent
optical pulses involves supplying a continuous wave laser beam; and
chopping the continuous wave laser beam to produce the sequence of
optical pulses. Alternatively, the method of generating the
sequence of coherent optical pulses involves supplying a single
coherent optical pulse; and producing the sequence of optical
pulses from the single optical pulse.
[0017] In general in still another aspect, the invention is a
system for generating a signal pulse in an optical fiber
characterized by dispersion and a refraction index that has a
nonlinear regime of operation. The system includes a source of
coherent laser energy; and a transmitter for coupling to the
optical fiber and which during operation, receives the laser energy
from the source and outputs a sequence of coherent optical pulses.
The transmitter is configured to generate the pulses in the
sequence of pulses with sufficiently close spacing so that after
traveling a predetermined length down the optical fiber, the pulses
of the sequence of pulses overlap and interfere to form an
interference pattern. The transmitter is also configured to
generate at least one pulse of the sequence of pulses to have an
energy that is within the nonlinear regime of the optical
fiber.
[0018] Preferred embodiments include one or more of the following
features. The transmitter is configured to generate each of the
pulses of the sequence of pulses to have an energy that is within
the nonlinear regime of the optical fiber. The sequence of pulses
may include only two pulses or it may include more than two pulses.
If the source of coherent laser energy provides a continuous wave
optical beam, the transmitter might then include an optical shutter
that during operation chops the continuous optical beam to produce
the sequence of optical pulses. Alternatively, it the source of
coherent light supplies a single coherent optical pulse, then the
transmitter might include a splitter that receives the single
pulse, a plurality of optical paths connected to an output of the
splitter, each of the plurality of optical paths characterized by a
different delay, and a combiner receiving each of the plurality of
optical paths and during operation outputting the sequence of
optical pulses.
[0019] In general, in yet another aspect, the invention is a method
of generating a signal for transmission down an optical fiber
characterized by dispersion and a non-linear regime of operation.
The method includes: generating a first sequence of coherent
optical pulses each of which has an associated energy, wherein the
pulses in the first sequence of optical pulses are sufficiently
close in spacing so that upon traveling a predetermined length down
the optical fiber, the pulses of the first sequence of pulses will
overlap and interfere to form a first interference pattern with a
first central lobe having a characteristic wavelength, and wherein
the associated energy of at least one of the pulses of the first
sequence of pulses is within the non-linear regime of the optical
fiber; and manipulating the pulses of the first sequence of optical
pulses to generate a first manipulated sequence of pulses. It
further includes generating a second sequence of coherent optical
pulses each of which has an associated energy, wherein the pulses
in the second sequence of optical pulses are sufficiently close in
spacing so that upon traveling a predetermined length down the
optical fiber, the pulses of the second sequence of pulses will
overlap and interfere to form a second interference pattern with a
second central lobe having a characteristic wavelength, and wherein
the associated energy of at least one of the pulses of the second
sequence of pulses is within the non-linear regime of the optical
fiber; and manipulating the pulses of the second sequence of
optical pulses to generate a second manipulated sequence of pulses.
The method also includes introducing both the first manipulated
sequence of pulses and the second manipulated sequence of pulses
into the optical fiber, wherein the manipulating of the pulses of
the first and the second sequence of pulses shifts the
characteristic wavelengths of the first and second central lobes,
respectively, to first and second transmission signal
wavelengths.
[0020] In general, in still yet another aspect, the invention
features an apparatus for sending optical signals down an optical
fiber characterized by dispersion and a non-linear regime of
operation. The apparatus includes: a first optical signal source
which during operation generates a first sequence of coherent
optical pulses each of which has an associated energy, wherein the
pulses in the first sequence of optical pulses are sufficiently
close in spacing so that upon traveling a predetermined length down
the optical fiber, the pulses of the first sequence of pulses will
overlap and interfere to form a first interference pattern with a
first central lobe having a characteristic wavelength, and wherein
the associated energy of at least one of the pulses of the first
sequence of pulses is within the non-linear regime of the optical
fiber; a frequency shifter which during operation manipulates the
pulses of the first sequence of optical pulses to generate a first
manipulated sequence of pulses; a second optical signal source
which during operation generates a second sequence of coherent
optical pulses each of which has an associated energy, wherein the
pulses in the second sequence of optical pulses are sufficiently
close in spacing so that upon traveling a predetermined length down
the optical fiber, the pulses of the second sequence of pulses will
overlap and interfere to form a second interference pattern with a
second central lobe having a characteristic wavelength, and wherein
the associated energy of at least one of the pulses of the second
sequence of pulses is within the non-linear regime of the optical
fiber; a second frequency shifter which during operation
manipulates the pulses of the second sequence of optical pulses to
generate a second manipulated sequence of pulses; and a wavelength
division multiplexer system which during operation receives both
the first manipulated sequence of pulses and the second manipulated
sequence of pulses and introduces them into the optical fiber,
wherein the first and second frequency shifters shift the
characteristic wavelengths of the first and second central lobes,
respectively, to first and second transmission signal
wavelengths.
[0021] Embodiments may have one or more of the following
advantages. A new optical solitary wave, the hyper-soliton, is
discovered for transmitting in the non-linear operating region of
an optical fiber. The hyper-soliton does not spread as it travels
down the fiber, and carries digital signals over a broad frequency
range. Further aspects, features and advantages will become
apparent by the following.
[0022] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
DESCRIPTION OF DRAWINGS
[0023] FIG. 1 illustrates pulse broadening in a prior art optical
fiber;
[0024] FIG. 2 shows a soliton pulse after being spread by TOD;
[0025] FIG. 3 shows a system, which uses interference to reduce
pulse broadening in a linear operating region of an optical
fiber;
[0026] FIGS. 4 illustrates an interference pattern produced from
the system of FIG. 3;
[0027] FIG. 5 illustrates diffraction broadening caused by a wide
slit;
[0028] FIG. 6 illustrates multi-slit interference;
[0029] FIG. 7 shows a system, which uses interference to reduce
pulse broadening in a nonlinear operating region of an optical
fiber;
[0030] FIGS. 8 illustrates an interference pattern produced from
the system of FIG. 7;
[0031] FIG. 9 shows an optical transmitter for use in the system of
FIG. 7;
[0032] FIGS. 10A-C illustrate Mach-Zehnder interferometers;
[0033] FIG. 10D illustrates how the variable delay component of the
interferometer adjusts the delay between pulses;
[0034] FIGS. 11A-D illustrate how the interferometer adjusts the
pulse amplitude;
[0035] FIG. 12 shows another optical transmitter for use in the
system of FIG. 7;
[0036] FIG. 13 shows another optical transmitter containing a pulse
splitter for use in the system of FIG. 7
[0037] FIG. 14 shows a 1.times.5 optical beam splitter for use in
the transmitter of FIG. 12;
[0038] FIGS. 15A and 15B illustrate temporal shiftsproduced by the
systems of FIG. 3 and FIG. 7, respectively;
[0039] FIG. 16 shows a wavelength division optical transmission
system; and
[0040] FIG. 17 shows a wavelength division optical transmission
receiver.
[0041] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0042] The present invention is based, in part, on a multi-pulse
technique which we developed to reduce the pulse broadening effects
of dispersion on a transmitted pulse. Our multi-pulse technique
involves generating for each pulse to be transmitted a series of
closely spaced coherent pulses and then sending that series of
closely spaced pulses down the fiber. According to the multi-pulse
technique, the individual pulses within the series of pulses are
selected to have amplitudes that are within the linear operating
region of the fiber and they are spaced closely enough to interfere
with each other as they broaden. The technique is described in
detail in U.S. patent application Ser. No. 09/282,880, entitled
"Quasi-Dispersionless Optical Fiber Transmission, Dispersion
Compensation and Optical Clock," incorporated herein by reference.
To lay the groundwork for discussing the present invention, we will
first present an overview of the multi-pulse technique.
[0043] FIG. 3 schematically illustrates a system 50 which
implements the multi-pulse technique. It includes an optical
transmitter 54 connected to a receiver 65 at a distant location 15
via an optical fiber 52. Transmitter 54 generates a sequence of
coherent, closely spaced pulses 56-59 and sends them into optical
fiber 52. Each of the pulses 56-59 has a nonzero time delay with
respect to the preceding pulse in the sequence of pulses. As each
of the pulses 56-59 moves down the fiber 52, it will broaden due to
dispersion in the fiber. If the original pulses in the sequence are
spaced closely enough, as they broaden, they will overlap and will
interfere with each other. For example, pulses 57 and 58 broaden to
become overlapping pulses 61 and 62 after a certain propagation
distance.
[0044] FIG. 4 shows an example of the resulting interference
pattern 80 that is produced by the overlapping coherent pulses.
Note that the interference pattern typically has a narrow central
lobe 69 and multiple side lobes 70 and 71 of lower amplitude. The
peaks of central lobe 69 and side lobes 70 and 71 fall under an
envelope 18, which matches the shape of a broadened pulse that
would result from sending one of the pulses 56-59 down fiber 52.
Effectively, the sequence of coherent pulses 56-59 forms a single
pulse (i.e., central lobe 69) that is able to broaden less rapidly
when propagating through fiber 52. The reason for the slower rate
of spreading is because the pulse sequence acts as a spectral
filter on the resulting single pulse. However, distortions
introduced by the spectral filter can have an irreversible effect
on the spectrum of the transmitted pulse, which is inversely
proportional to its temporal duration. Therefore, with spectral
filtering, it may be not possible to recover the temporal duration
of the transmitted pulse, which is a drawback that can reduce the
potential data rate of the transmission system.
[0045] Referring to FIGS. 3 and 4, central lobe 69 is extracted or
detected by a nonlinear device at receiver 65, e.g., an intensity
discriminator. The nonlinear device, using a threshold that is
higher than the side lobes but lower than the central lobe, detects
the existence of the higher central lobe and ignores the lower
amplitude side lobes. It should be clear that effective operation
of this system depends on there being sufficient contrast between
the central lobe and the other side lobes and any undesired noise
that might be present. Also note that if only a single pulse had
been transmitted down the fiber, the pulse broadening effect of
dispersion would probably have produced a low amplitude, wide pulse
by the time it reached the receiver. The wider, lower pulse would
have been difficult if not impossible to detect over the noise.
[0046] We also note that when the sequence of pulses is in the
linear region of the fiber, the contrast ratio of the central lobe
to the side lobes remain the same as the pulse sequence travels
down the fiber (as shown in FIG. 4), even though temporal waveform
of the pulse sequence is being stretched due to dispersion. We make
this observation at this point because as will become apparent
later when the signal is in the non-linear region of the fiber, the
contrast ratio of the pulse sequence no longer remains the same
when it propagates down the fiber.
[0047] It is useful to observe that interference pattern 80 is
similar to the intensity pattern that is produced by coherent plane
wave light after passing through a multiple aperture grating and
the envelope 18 is similar to the intensity pattern produced by a
coherent light beam after passing through a single slit aperture.
The diffraction effect of a single aperture is illustrated in FIG.
5 which shows an intensity pattern 20, which a coherent light beam
22 makes on a screen 24 located in the far field behind a wide slit
26. If the slit 26 is not too wide, diffraction broadens the
intensity pattern 20 to beyond the width of the slit 26.
[0048] The interference pattern produced by a multiple aperture
grating is illustrated in FIG. 6 which shows an intensity pattern
28, which the coherent light beam 22 produces on the screen 24 when
located behind multiple narrow slits 30-34. The intensity pattern
28 has lobes 36-38 and minima 42-44 due to interference between
light waves from the different slits 30-34. The interfering light
impinging on screen 24 is characterized by a central lobe 36 and
multiple side lobes 37, 38. Central lobe 36, as in the case of the
sequence of coherent pulses traveling down a fiber, is much
narrower than the diffraction-widened pattern 20 from the wide slit
26 of FIG. 5. And the envelope 40 of the peaks of the multiple
lobes of the interference pattern 28 matches the pattern 20 from
the wide slit 26 of FIG. 5.
[0049] In other words, the sequence of closely spaced coherent
pulses that produce the multi-lobe interference pattern in the
fiber might be viewed as having associated therewith a temporal
grating (TG) much like the spatial grating that also produces a
multi-lobe interference pattern.
[0050] By increasing the energy in pulses 56-59 so that they are in
the nonlinear region of the fiber, significant performance
enhancements are achieved. In essence, when the system is operated
in the nonlinear region, the central lobe of the resulting
interference pattern increases in height and the side lobes are
significantly suppressed in comparison to the interference pattern
that is produced when operating within the linear region.
Continuing the grating analogy mentioned above, the nonlinear mode
of operation is like associating with the sequence of pulses a
nonlinear temporal grating (NTG). This results in a new type of
solitary wave that we call a hyper-soliton and that is
fundamentally different from temporal grating pulse. Unlike the
central lobe of temporal grating, the central lobe of the nonlinear
temporal grating (i.e., hyper-soliton) would not spread as the
pulse travels down the fiber. In that respect, it is similar to a
soliton. The transformation from temporal grating to hyper soliton
(nonlinear temporal grating) is akin to the transformation from
return-to-zero (RZ) pulse transmission to soliton transmission in a
fiber optic link. We will refer hereinafter to the sequence of
pulses that operate within the nonlinear region of the fiber as HS
(hypersoliton) pulses.
[0051] A system which operates in the nonlinear region is shown in
FIG. 7. Like the previously described system, it includes a
transmitter 154 connected to a receiver 165 over an optical fiber
52. In this case, however, transmitter 154 generates a series of
coherent pulses 156-159 that are within the nonlinear region of
fiber 52. Because operation in the nonlinear region produces an
interference pattern with an enhanced central lobe, receiver 165
does not require and therefore does not include an intensity
discriminator to isolate the central pulse of the interference
pattern from the side lobes.
[0052] For this mode of operation, the energy in each of the pulses
156-159 needs to be above an energy threshold which defines a lower
boundary for the nonlinear operating region of fiber 52. The energy
threshold of a pulse in a given fiber can be determined by a
nonlinear phase shift that is produced within the pulse. When the
energy of a pulse causes the nonlinear phase shift to be .pi. or
larger, the energy of the pulse has exceeded the energy threshold
and the pulse has entered the nonlinear region of the fiber. More
specifically, the energy threshold can be derived from an equation
that defines the relationship between the energy E and the
non-linear phase shift .phi.. The equation generally includes
factors such as fiber and pulse characteristics. The equation is
.phi.=2.pi.n.sub.2 EL/(.lambda..tau.A), where n.sub.2 is the
non-linear Kerr coefficient (n.sub.2=3.2.times.10.sup.-20 meter
square/watts), E is the pulse energy in watts.times.second, L is
the length of the fiber (meter), .lambda. is the wavelength
(meter), .tau. is the pulse width (second), and A is the effective
fiber core area (meter square) in the propagating mode field.
[0053] As in the case of the lower energy pulses, the HS pulses
after traveling a sufficient distance overlap and produce an
interference pattern such as is shown in FIG. 8. That sufficient
distance is several times the characteristic length which is equal
to .tau..sup.2/.beta..sub.- 2, where .tau. is the pulse duration,
and .beta..sub.2 is the second order chromatic parameter of the
system.
[0054] The nonlinearity acts to concentrate the energy of the
multiple pulses into central lobe 67 of the interference-generated
pattern 64 and draws energy away from the side lobes 66, 68 thereby
suppressing the side lobes and increasing the central lobe, which,
in turn, increases the contrast between central lobe 67 and side
lobes 66,68, as previously noted. Compared with the interference
pattern resulting from linear operation, the HS interference
pattern has a significantly higher contrast ratio. Moreover, the
narrow central lobe of the interference pattern retains its shape
as it propagates down the fiber. The HS pulses not only balance the
SPM and the second-order chromatic dispersion like a soliton, but
also remain substantially unaffected by the presence of TOD and
higher-order dispersions. Unlike a soliton that must be transmitted
in the anomalous dispersion regime to retain its shape, the HS
pulses may be transmitted in either the normal dispersion regime or
the anomalous dispersion regime.
[0055] Referring again to FIG. 8, it should be noted that in
nonlinear regime the shape of waveforms in both time and frequency
domains evolve in a similar manner as the pulse sequence
travels.
[0056] The advantages of transmitting HS pulses are numerous. For
example, HS pulses are robust and resistant to polarization mode
dispersion (PMD) of any installed optical fiber link. PMD is a
result of random birefringence in the fiber, and is a major problem
in a high-speed long haul system. Furthermore, HS pulses are immune
to frequency shifts and time jitters, both of which, as explained
in the background, are detrimental to a transmission system.
Because HS pulses effectively result from applying a spectral
filter that travels with a soliton, the filter removes any
frequency shifts and time jitters that would otherwise have been
present in the HS pulses.
[0057] There are a number of different ways to generate the HS
pulses, some of which will now be described. In general,
transmitter 154 includes a laser source and one or more
programmable or configurable components that produce sequences of
pulses with desired spacing and amplitude. If the pulses are to
carry encoded digital data, an encoder will also be included in
transmitter 154. The laser source may be a continuous-wave (CW)
laser that generates a monochromatic coherent light beam In that
case, a programmable optical shutter chops the light from the CW
laser to produce a sequence of mutually coherent pulses.
[0058] Alternatively, referring to FIG. 9, a pulse laser source
174, e.g. a 10 GHz pulse laser, may be used to generate a sequence
of equally-spaced narrow pulses that may be spaced more closely
than desired. In that case, a programmable optical shutter 177
allows some of the pulses to pass (178 and 178') while blocking out
other pulses in order to generate a sequence of more widely spaced
narrow pulses. The distance between the pulses, e.g., pulses 178
and 178', should be great enough to avoid one pulse from
interfering with a neighboring pulse after dispersion widening has
taken place.
[0059] In one embodiment of transmitter 154, there is a
bi-directional, multi-stage Mach-Zehnder interferometer 179 which
generates a sequence of closely-spaced mutually coherent pulses
from each single pulse 178 coming from shutter 177. Additionally,
interferometer 179 is also capable of adjusting the delays and
amplitudes for the sequence of pulses to optimize performance, e.g.
to control the shape of the interference pulse that the sequence of
closely-space pulses produce after broadening.
[0060] Referring to FIG. 10A, a single-stage Mach-Zehnder
interferometer includes two arms, a splitter 161, and a combiner
162. Splitter 161 splits an incoming pulse into two equal intensity
pulses, each of which is sent down a corresponding one of the two
arms. One arm of the interferometer has a variable delay element
160A for adjusting the delay on the pulse traveling in that arm.
The other arm has no variable delay element. Thus, the pulse in one
arm is delayed relative to the pulse in the other arm. By varying
the delay introduced by the variable delay element, the delay
between the two pulses can be changed thereby changing the relative
phases of the two pulses. Thus, variable delay element 160A may
also be referred to as a variable phase shifter. Combiner 161
recombines the two pulses to form a sequence of two pulses that are
delayed relative to each other.
[0061] FIGS. 10B and 10C illustrate a two-stage and a three-stage
Mach-Zehnder interferometer, respectively. The multistage
Mach-Zehnder interferometers are constructed by serially linking
multiple single stages. However, in the embodiments we have shown,
all stages prior to the last one have a fixed delay element instead
of the variable delay element, as described for the single stage
Mach-Zehnder interferometer of FIG. 10A and only the last stage
includes the variable delay element. Of course, one could also
construct multistage Mach-Zehnder interferometers in which more
than the last stage includes a variable delay element, and the
amount of delay introduced by each delay element may be different
from the ones illustrated in FIG. 10. But for purposes of
explaining the operation of them, the embodiments we have chosen to
illustrate are easier to understand.
[0062] The amount of delay introduced by each delay element is a
factor of two greater than the delay introduced by the previous
stage. That is, if a interferometer has N stages that are numbered
from 0 to N-1, the delay in stage N-n is 2.sup.(N-n)T and the
variable delay element in the last stage has the longest delay
equal 2.sup.(N-1)T plus an adjustment. It should be readily
apparent that since each stage introduces progressively longer
delay and the last stage introduces the longest delay that the last
stage essentially controls the delay between two groups of pulses.
For example, referring to FIG. 10D which shows a two-stage
Mach-Zehnder interferometer, the variable delay element 160D
controls the separation in time between the pulses in the first
group (i.e., pulses b1 and b2) and the pulses in the second group
(i.e., pulses b3 and b4). If the adjusted delay is .delta.=-0.2 t,
the delay between the group of two pulses (b1-b2 and b3-b4) is
changed from 2 t to 2 t+.delta., i.e., 1.8 t, and therefore the
delays between the adjacent pulses of the four pulse sequence
become t, 0.8 t and t, respectively. Similarly, in the three-stage
Mach-Zehnder interferometer of FIG. 10C, the delay between the two
groups of four pulses (i.e., pulses c1-c4 in the first group and
pulses c5-c8 in the second group) can be changed from 4 t to 4
t+.delta. by adjusting the variable delay component 160C in the
last stage.
[0063] FIGS. 11A-D illustrate four alternative ways to adjust the
amplitudes of the generated pulses. The pulses with adjusted
amplitudes form an apodized pulse array. FIGS. 11A and 11B show,
respectively, a single-stage and a two-stage interferometer with an
additional attenuator in one arm of one of the stages. The
attenuator changes the amplitude of the pulse passing through that
arm. FIG. 11C illustrates a way of adjusting the amplitude without
using an attenuator. If the delay introduced by variable delay
element 160A is T instead of 2 T as in FIG. 10B, the two pulses
that are combined will overlay each other and form a pulse with
approximately twice the amplitude. FIG. 1 ID illustrates that the
amplitude may be adjusted with a splitter 161' that puts different
weights on the two arms. In general, an X/Y splitter puts a weight
of X in one arm and a weight of Y in the other arm, therefore, a
70/30 splitter splits an incoming pulse into two pulses with an
amplitude ratio of 70 to 30.
[0064] With the delay and amplitude adjustability of the
interferometer, the performance of transmitter 154 can be readily
fine-tuned for optimal non-linear transmissions.
[0065] Another embodiment of transmitter 155 shown in FIG. 12
contains only pulse laser 175 and programmable shutter 177. Pulse
laser 175 generates closely spaced coherent pulses having
separations of the desired amount for producing the multiple pulses
in a group which eventually interferes to form the interference
pulse. Shutter 177 carves out pulses from he stream of pulses 176
generated by pulse laser 175. For example, shutter 177 may allow
the first two pulses in every five pulses to pass through. Thus,
shutter 177 creates groups of pulses with uniform spacing between
the groups. For a pulse laser that produces a sequence of pulses
with 20 ps pulse duration, the spacing between any adjacent groups
of five is 100 ps. Thus, transmitter 154 transmits digital data bit
at a rate of 10 Gb/s, with each bit carried by the two pulses in
each group of five.
[0066] Another embodiment of transmitter 153 is illustrated in FIG.
13. Transmitter 154 includes a CW laser source 74 that produces a
monochromatic coherent light beam 76. Programmable high-speed
shutter 177 in transmitter 153 chops light beam 76 to generate a
sequence of source pulses 78. Each source pulse 78 enters a pulse
splitter 79, which produces a series of N delayed and coherent
pulses from the source pulse and sends the series of pulses to
optical fiber 52.
[0067] Still referring to FIG. 13, pulse splitter 79 uses a
1.times.N beam splitter 88, e.g., a 1.times.N fiber coupler, to
produce N mutually coherent pulses from each source pulse. The
1.times.N beam splitter 88 has an optical output along each of N
directions, and each output couples to an optical waveguide
P.sub.1-P.sub.N, e.g., optical fibers. Each optical waveguide
P.sub.1-P.sub.N has an optical length measured to produce one of
the temporal delays of the series of pulses 156-159 of FIG. 7.
Optical waveguides P.sub.1-P.sub.N couple to an inverted 1.times.N
beam splitter 82 that recombines the delayed pulses to produce the
series of pulses 156-159 shown in FIG. 7. Pulse splitter 79 may
also include optical amplifiers (not shown) either in the separate
waveguides P.sub.1-P.sub.N or at its output.
[0068] Referring to FIG. 14, an embodiment of the 1.times.N optical
beam splitter is a planar integrated optical splitter 90, which can
function as the 1.times.N optical beam splitter 88 (for N=5).
Optical splitter 90 has an input hole 92. Hole 92 diffracts each
received source pulse into five mutually coherent pulses, which are
directed along different directions. Each mutually coherent pulse
is collected by a separate optical waveguide 94-99, which carries
the pulse to an optical conduit P.sub.1-P.sub.5. Optical waveguides
P.sub.1-P.sub.5 can be continuations of waveguides 94-99 or optical
fibers of various lengths. Various other embodiments of the beam
splitter are described in U.S. patent application Ser. No.
09/282,880, filed Mar. 31, 1999, and incorporated herein by
reference.
[0069] By changing the phasing and/or amplitude of the pulses
within the sequence of closely-spaced coherent pulses, it is
possible to shift the central lobe of the interference signal. The
central lobe enhancement effect and the suppression of side lobes
persists even when the central lobe of the temporal signal is
shifted to either side of the center of the envelope. FIG. 15B
shows the interference signal produced by a system operating in the
nonlinear region and in which the phasing of the pulses has been
adjusted to shift the central lobe toward the left side of the
envelope.
[0070] The leftward shift of the center lobe of pattern 238 implies
a change in the time at which the pulse will arrive at the
receiver. In other words, shifting the center lobe is a way of
introducing a delay in the pulse. In addition, it is also the case
that shifting the center lobe in the time domain produces a shift
towards the same direction in the center lobe in the frequency
domain. In other words, the spectral content of the pulse can be
altered.
[0071] A similar phenomenon occurs in the case of liner operation.
However, the presence of the side lobes in the linear case
seriously limits the amount of shift that can be introduced without
producing an ambiguous signal. For example, FIG. 15A shows a
shifted pattern 202 generated through linear operation. Notice that
as the center lobe shifts to the left, the side lobe on the right
also shifts to the left and grows in amplitude. Soon the side lobe
will be of comparable amplitude to the shifted center lobe and it
will not be possible to discriminate between the two. In contrast,
the HS pulses yield a much larger dynmaic range as compared to
operation in the linear region. That is, the center lobe can be
shifted much farther before the much lower amplitude side lobes
present a problem. Because of the wider dynamic range, a system
that transmits HS pulses is truly a broadband system.
[0072] As we noted above, the frequency of the HS pulses can also
be steered by adjusting the relative phases (i.e., time delays)
between the pulses in the sequence of closely-spaced coherent
pulses. Thus, for example, interferometer 179 of FIG. 9 and pulse
splitter 79 of FIG. 14 may each be used as frequency shifters to
convert the frequency of source pulses into a pre-selectable
frequency for transmission.
[0073] The time and frequency steering features can be used to
transmit digital data on an optical fiber. For example, the time
steering feature may be used for a digital data transmission format
called Pulse Position Modulation (PPM). According to the PPM
format, in a temporal interval during which a digital bit is
transmitted, the binary states "1" and "0" are indicated by the
presence of a pulse in the first and second half of the interval,
respectively; or vice versa. The frequency steering feature may be
used for another data format called Frequency Shift Keying (FSK).
According to the FSK format, the binary states "1" and "0" are
indicated by a pulse transmitted in the temporal interval with
frequency f.sub.1 and f.sub.2, respectively.
[0074] The frequency steering feature is also highly useful for
providing multiple channels in a multi-access digital network. FIG.
16 shows an embodiment of a nonlinear optical transmission system
252 with a transmitter 253. Each channel of transmitter 253
includes a pulse laser source (S1-S5), an optical programmable
shutter 177, an encoder 280, and a channelizer 250. Encoder 280
encodes the pulses from the output of shutter 177 according to a
pre-defined data format, e.g., PPM or FSK. Channelizer 250 then
shifts the frequency of the pulses to a pre-determined output
frequency for that channel. Both encoder 280 and channelizer 250
may be implemented by Mach-Zehnder interferometers 179, as
previously described.
[0075] All of the laser sources (S1-S5) may generate pulses with
the same center frequency f.sub.c. Channelizer 250 shifts the
frequency of the pulses for each of the input sources (S1-S5) to a
distinct output frequency. As illustrated in FIG. 16, the
frequencies and wavelengths generated at the output of the five
transmitters are f1-f5 and .lambda.1-.lambda.5, respectively.
[0076] Transmitter 253 further includes a wavelength-division
multiplexing (WDM) device 251 to multiplex signals from multiple
sources into the same fiber 52. Fiber 52 carries multiple WDM
channels, with each of the channels characterized by a unique
wavelength. Each channel carries a data stream that can be encoded
independent of other data streams. Thus, the WDM technique
increases transmission capacity without requiring electronics of
higher speed to process each channel.
[0077] WDM device 251 and frequency shifters 250 are in general
bi-directional. Therefore, the same WDM device 251 and frequency
shifters 250 used in transmitter 253 may be used in a receiver 290.
Referring to FIG. 17, WDM device 251 at receiver 290 directs the
multiplexed data streams in fiber 52 towards five decoders 254,
each of the decoders receiving a data stream characterized by a
unique wavelengths. Decoder 254 then decodes received data format
(e.g., PPM or FSK). Similar to transmitter 253, decoder 254 may be
implemented by Mach-Zehnder interferometers 179.
[0078] A number of embodiments of the invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. Accordingly, other embodiments are within
the scope of the following claims.
* * * * *