U.S. patent application number 15/062310 was filed with the patent office on 2017-09-07 for high capacity transmission system with full nonlinear penalty cancellation.
The applicant listed for this patent is Verizon Patent and Licensing Inc.. Invention is credited to Glenn A. Wellbrock, Tiejun J. Xia.
Application Number | 20170257172 15/062310 |
Document ID | / |
Family ID | 59723797 |
Filed Date | 2017-09-07 |
United States Patent
Application |
20170257172 |
Kind Code |
A1 |
Xia; Tiejun J. ; et
al. |
September 7, 2017 |
HIGH CAPACITY TRANSMISSION SYSTEM WITH FULL NONLINEAR PENALTY
CANCELLATION
Abstract
An optical device includes a light source and diffuser, such as
non-linear material, to form a supercontinuum of light energy of
different wavelengths. An optical channel generator forms channels
from the supercontinuum and forwards a multiplexed signal carrying
the channels. The signal travels to an optical receiver through an
optical fiber. The optical receiver identifies a non-linear penalty
associated with forwarding the multiplexed signal on the optical
fiber. The optical receiver modifies attributes of the received
channels, such as increasing the magnitude of one of the channels,
to cancel out the non-linear penalty.
Inventors: |
Xia; Tiejun J.; (Richardson,
TX) ; Wellbrock; Glenn A.; (Wylie, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Verizon Patent and Licensing Inc. |
Arlington |
VA |
US |
|
|
Family ID: |
59723797 |
Appl. No.: |
15/062310 |
Filed: |
March 7, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 10/506 20130101;
H04J 14/02 20130101; H04B 10/2557 20130101; H04B 10/6163
20130101 |
International
Class: |
H04B 10/61 20060101
H04B010/61; H04J 14/02 20060101 H04J014/02 |
Claims
1. A method comprising: generating, by a light source included in
an optical device, a light pulse; amplifying, by an amplifier
included in the optical device, the light pulse to form an
amplified light pulse; modifying, by a diffuser included in the
optical device, the amplified light pulse to form a supercontinuum;
forming, by an optical channel generator included in the optical
device, channels from the supercontinuum; combining, by the optical
channel generator, the channels to form a multiplexed optical
signal; forwarding, by the optical channel generator, the
multiplexed optical signal to an optical receiver via an optical
fiber, determining, by the optical receiver, a nonlinear penalty
associated with transmitting the multiplexed optical signal to the
optical receiver via the optical fiber; and cancelling, by the
optical receiver, the nonlinear penalty.
2. The method of claim 1, wherein the optical channel generator
includes: a tone separator configured to wavelength division
demultiplex the supercontinuum to form the channels; a plurality of
optical modulators configured to modulate the channels based on
data to be transmitted; and a tone combiner configured to multiplex
the modulated channels into the multiplexed optical signal.
3. The method of claim 1, wherein the diffuser includes highly
nonlinear fiber (HNLF), and wherein the HNLF includes at least one
of: a narrow-core fiber with silica cladding; a tapered fiber with
air cladding; a microstructured fiber that includes air holes
introduced within a cladding; or a non-silica fiber.
4. The method of claim 1, wherein the amplifier includes an erbium
doped fiber amplifier.
5. The method of claim 1, wherein the nonlinear penalty attenuates
one of the channels relative to other ones of the channels, and
wherein canceling the nonlinear penalty includes: increasing a
magnitude of the attenuated one of the channels relative to the
other ones of the channels.
6. The method of claim 1, wherein the nonlinear penalty modifies a
phase of one of the channels relative to other ones of the
channels, and wherein canceling the nonlinear penalty includes:
changing the phase of the attenuated one of the channels relative
to the other ones of the channels.
7. The method of claim 1, wherein the light source includes a
picosecond soliton laser.
8. A device comprising: a light source configured to generate a
light pulse; an amplifier configured to receive and amplify the
light pulse to form an amplified light pulse; a diffuser configured
to receive the amplified light pulse and form a supercontinuum; an
optical channel generator configured to: form channels from the
supercontinuum, combine the channels to form a multiplexed optical
signal, and forward the multiplexed optical signal via an optical
fiber; and an optical receiver configured to: receive the
multiplexed optical signal through the optical fiber, determine a
nonlinear penalty associated with transmitting the multiplexed
optical signal via the optical fiber, and cancel the nonlinear
penalty.
9. The device of claim 8, wherein the optical channel generator
includes: a tone separator configured to wavelength division
demultiplex the supercontinuum to form the channels; a plurality of
optical modulators configured to modulate the channels based on
data to be transmitted; and a tone combiner configured to multiplex
the modulated channels into the multiplexed optical signal.
10. The device of claim 8, wherein the diffuser includes highly
nonlinear fiber (HNLF), and wherein the HNLF includes at least one
of: a narrow-core fiber with silica cladding; a tapered fiber with
air cladding; a microstructured fiber that includes air holes
introduced within a cladding; or a non-silica fiber.
11. The device of claim 8, wherein the amplifier includes an erbium
doped fiber amplifier.
12. The device of claim 8, wherein the nonlinear penalty attenuates
one of the channels relative to other ones of the channels, and
wherein the optical receiver, when canceling the nonlinear penalty,
is further configured to increase a magnitude of the attenuated one
of the channels relative to the other ones of the channels.
13. The device of claim 8, wherein the nonlinear penalty modifies a
phase of one of the channels relative to other ones of the
channels, and wherein the optical receiver, when canceling the
nonlinear penalty, is further configured to change the phase of the
attenuated one of the channels relative to the other ones of the
channels.
14. The device of claim 8, wherein the light source includes a
picosecond soliton laser.
15. A non-transitory computer readable medium configured to store
instructions executed by an optical device, wherein the optical
device includes: a light source configured to generate a light
pulse; an amplifier configured to receive and amplify the light
pulse to form an amplified light pulse; a diffuser configured to
receive the amplified light pulse and form a supercontinuum; and an
optical channel generator configured to: form channels from the
supercontinuum, combine the channels to form a multiplexed optical
signal, and forward the multiplexed optical signal to an optical
receiver via an optical fiber, and wherein the instructions further
cause the optical receiver to: determine, based on the received
multiplexed optical signal, a nonlinear penalty, and cancel the
nonlinear penalty.
16. The non-transitory computer readable medium of claim 15,
wherein the optical channel generator includes: a tone separator
configured to wavelength division demultiplex the supercontinuum to
form the channels; a plurality of optical modulators configured to
modulate the channels based on data to be transmitted; and a tone
combiner configured to multiplex the modulated channels into the
multiplexed optical signal.
17. The non-transitory computer readable medium of claim 15,
wherein the diffuser includes highly nonlinear fiber (HNLF), and
wherein the HNLF includes at least one of: a narrow-core fiber with
silica cladding; a tapered fiber with air cladding; a
microstructured fiber that includes air holes introduced within a
cladding; or a non-silica fiber.
18. The non-transitory computer readable medium of claim 15,
wherein the amplifier includes an erbium doped fiber amplifier.
19. The non-transitory computer readable medium of claim 15,
wherein the nonlinear penalty attenuates one of the channels
relative to other ones of the channels, and wherein instructions
further cause the optical receiver, when canceling the nonlinear
penalty, to: increase a magnitude of the attenuated one of the
channels relative to the other ones of the channels.
20. The non-transitory computer readable medium of claim 15,
wherein the light source includes a picosecond soliton laser.
Description
BACKGROUND
[0001] In an optical network, dense wavelength division
multiplexing (DWDM) permits the multiplexing of multiple optical
carriers onto a single optical fiber by using different wavelengths
of laser light. In DWDM, each transport channel has only one
optical carrier that occupies a fixed optical bandwidth. Since the
total usable optical bandwidth of an optical fiber is fixed, a DWDM
system may have a fixed number of total optical channels. For
example, a typical total usable bandwidth of an optical fiber may
be in the range of 5-10 THz, and an associated DWDM system may have
a fixed number of optical channels, such as 76 or 128 channels,
that share the total usable bandwidth.
[0002] In DWDM systems, a nonlinear transmission penalty may arise
due to optical nonlinear effects, such as self-phase modulation
(SPM) and/or crosstalk between different channels, such as cross
phase modulation (XPM). The nonlinear transmission penalty may
impact the signal integrity of phase modulated optical signals and
contribute to phase noise. Various techniques have been developed
to reduce the nonlinear transmission penalty and/or its negative
effects. For example, SPM effects may be canceled using nonlinear
Schrodinger equation back propagation techniques in calculations
after a signal is received coherently so that frequency components
within a channel may be corrected for certain frequency and phase
changes. However, XPM effects cannot be minimized through back
propagation techniques without knowing frequency and phase
relationships between the channels. Using a combed light source
(i.e., a source that locks together channel frequencies) may reduce
frequency uncertainty, but a nonlinear transmission penalty may
remain due to phase uncertainty in the channels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0003] FIG. 1 is a diagram that depicts an exemplary optical
network in which data is transmitted across the optical network
using light extracted from a supercontinuum;
[0004] FIG. 2 depicts further details of components of an optical
transmitter and an optical receiver included in the optical network
of FIG. 1;
[0005] FIG. 3 is a diagram that depicts functions performed at the
optical transmitter of FIG. 1 when transmitting optical signals
extracted from a supercontinuum, and functions performed at the
optical receiver of FIG. 1 when receiving and processing the
transmitted optical signals after traversal of the optical
fiber;
[0006] FIG. 4 is a diagram that depicts attributes of an optical
output generated by the optical transmitter in the optical network
of FIG. 1;
[0007] FIG. 5 depicts attributes of received light, received at the
optical receiver of FIG. 1, corresponding to the optical output
shown in FIG. 4 after traversing an optical fiber in the optical
network of FIG. 1;
[0008] FIG. 6 is a diagram illustrating exemplary components of a
computing device that may be included in the optical network of
FIG. 1;
[0009] FIG. 7 is a flow diagram that illustrates an exemplary
process for transmitting optical signals from a supercontinuum
generated from a light source using a nonlinear material; and
[0010] FIG. 8 is a flow diagram that illustrates an exemplary
process for cancelling non-linear effects in received optical
signals transmitted in the process of FIG. 7.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0011] The following detailed description refers to the
accompanying drawings. The same reference numbers in different
drawings may identify the same or similar elements. The following
detailed description does not limit the invention.
[0012] In certain implementations, to eliminate a nonlinear
transmission penalty in transmitting optical signals, an optical
system described herein may use an optical transmitter system that
includes a light source, an amplifier, and a diffuser to generate a
supercontinuum that represents a wide optical spectrum. Signals are
extracted from the supercontinuum and modulated and multiplexed for
transmission across the optical system. For example, the optical
system may be a Dense Wavelength Division Multiplexing (DWDM)
system. The multiplexed signal is received by an optical
transmitter, and the optical transmitter may perform back
propagation to cancel any non-linear effects in the received
signals to recover the originally transmitted signals due to known
characteristics (e.g., known phase differences) in the wavelengths
of the supercontinuum. The optical receiver may then process the
signals to extract any transmitted data.
[0013] FIG. 1 is a diagram that depicts an optical network 100 in
which data is transmitted across optical network 100 using light
energy generated by a seed light source. As shown in FIG. 1,
optical network 100 may include an optical transmitter 110, an
optical receiver 120, optical fiber 130, a network(s) 140, and a
system management node 150.
[0014] Optical transmitter 110 may include light system 160 and
optical channel generator 170. In one implementation, described
below with respect to FIG. 2, light system 160 may include a seed
light source that generates a pulsed (e.g., having a short
duration) optical output having a narrow spectrum) and optical
elements that convert the pulsed optical output into a wide
spectrum light, referred to as a "supercontinium." Optical channel
generator 170 may include, among other components described below
with respect to FIG. 2, multiple modulators (shown in FIG. 2 as
modulators 230) for modulating light of multiple optical channels 1
through n, with each channel 1 through n of optical network 100
having a corresponding optical wavelength .lamda..sub.1 through
.lamda..sub.n. Each of the multiple modulators changes (i.e.,
modulates) input light of a certain wavelength .lamda. based on
received data to be transmitted. In one implementation, each of the
multiple modulators includes components for applying differential
phase modulation to the input light of wavelength .lamda. based on
the received data to be transmitted.
[0015] Optical receiver 120 may include coherent receivers 180 and
a Digital Signal Processing (DSP) unit 190 (referred to herein as
"DSP 190"). Coherent receivers 180 may include multiple coherent
receivers, with each of the multiple coherent receives receiving,
coherently detecting, and analog-to-digital converting modulated
light signals of a particular wavelength .lamda.. Coherent
receivers 180 may pass the resulting digital signals to DSP 190 for
digital signal processing. DSP 190 performs, for example, forward
error correction (FEC) upon the digital signals for each channel
having one of the wavelengths .lamda..sub.1-.lamda..sub.n.
[0016] Network(s) 141 may include one or more networks of various
types including, for example, a public land mobile network (PLMN)
(e.g., a Code Division Multiple Access (CDMA) 2000 PLMN, a Global
System for Mobile Communications (GSM) PLMN, a Long Term Evolution
(LTE) PLMN, and/or other types of PLMNs), a satellite network, a
telecommunications network (e.g., Public Switched Telephone
Networks (PSTNs)), a local area network (LAN), a wide area network
(WAN), a metropolitan area network (MAN), an intranet, the
Internet, and/or a cable network (e.g., an optical cable
network).
[0017] System management node 150 may include one or more network
devices that connect to network(s) 140 and which receives
notifications of conditions associated with optical transmitter
110, optical receiver 120 and/or other components of network(s)
140. In one implementation, for example, the notifications may
identify, to system management node 150, a nonlinear transmission
penalty associated with the channels carried within optical network
100 due to optical nonlinear effects within each channel, such as
self-phase modulation (SPM) and/or crosstalk between different
channels, such as cross phase modulation (XPM). System management
node 150 may forward instructions to optical receiver 120 to cancel
out any detected nonlinear effects. Because the transmitted signals
are generated from a supercontinuum and, therefore, share common
attributes (e.g., a common phase or common phase differences), a
nonlinear penalty in the received signals can be virtually
eliminated. For example, if nonlinear effects cause a given signal
at a given wavelength to be received at a lower magnitude relative
to other signals at other wavelengths, system management node 150
may direct optical receiver 120 to boost the magnitude of the given
signal to cancel the nonlinear effects. Similarly, if the nonlinear
effects cause a given signal at a wavelength to be received at a
different phase relative to other signals at other wavelengths,
system management node 150 may direct optical receiver 120 to
modify the phase of the given signal to counter the nonlinear
effects.
[0018] The configuration of the components of optical network 100
depicted in FIG. 1 is for illustrative purposes only, and other
configurations may be implemented. Therefore, optical network 100
may include additional, fewer and/or different components, that may
be configured differently, than depicted in FIG. 1. For example,
though only a single optical transmitter 110, a single optical
fiber 130, and a single optical receiver 120 are depicted in FIG.
1, optical network 100 may include multiple different optical
transmitters 110 interconnected with multiple different optical
receivers 120 via multiple optical fibers 130
[0019] FIG. 2 depicts further details of exemplary components of
optical transmitter 110 and optical receiver 120. As shown in FIG.
2, light system 160 of optical transmitter 110 may include a light
source 200, an optical amplifier 210, and a diffuser 220.
[0020] Light source 200 may generate an optical output that is used
to form channels in optical network 100. Light source 200 may
correspond to a seed source, such as a laser that transmits
ultrashort (e.g., of picosecond duration) light pulses. For
example, light source 200 may be a 28 Gbaud (gigabaud) soliton
laser. A soliton laser may send light pulses that will not change
their shape because nonlinear effects in the light pulses balance a
dispersion of the light pulses. For example, the soliton laser may
include a mode-locked color center laser coupled to a second cavity
that contains a fiber. The soliton laser may form short pulses
through the interaction of the laser (or main) cavity with the
fiber (or control) cavity, and an output intensity may be
stabilized by controlling the control cavity. Because the soliton
laser uses of the nonlinear (i.e. intensity-dependent) refractive
index of the fiber, pulse shape and width may also be
stabilized.
[0021] Optical amplifier 210 may receive and amplify light energy
generated by light source 200. For example, optical amplifier 210
may include, for example, to: (1) a doped fiber amplifier amplifier
(e.g., an Erbium doped fiber amplifier (EDFA)) that uses stimulated
emission in the amplifier's gain medium to amplify received light
energy; (2) a semiconductor optical amplifier (SOAs) that uses
electron-hole recombination to amplify received light energy; (3) a
Raman amplifier that uses Raman scattering of incoming light with
phonons in the lattice of the gain medium to produce addition
(e.g., amplifying) photons coherent with the incoming photons from
light source 200; or (4) a parametric amplifier that uses
parametric amplification to amplify received light energy. Optical
amplifier 210 may output light pulses that have similar wavelengths
bands of higher intensity than the light pulses generated by light
source 200. For example, optical amplifier 210 may be a high-power
optical amplifier, such as an amplifier that has more than a 30 dBm
(decibel-milliwatts) of output power.
[0022] Diffuser 220 may receive and diffuse (e.g., spread out the
wavelengths of) the amplified light pulses outputted by optical
amplifier 210. For example, diffuser 220 may be a nonlinear medium,
such as highly nonlinear fibers (HNLFs). Types of HNLFs include,
for example, (1) narrow-core fibers with silica cladding; (2)
tapered fibers with air cladding in which fibers are stretched to
produce thin cladding diameters (approximately 2 .mu.m) and the
surrounding air acts as the cladding; (3) microstructured fibers
(also referred to as "holey" fibers or photonic crystal fibers
(PCFs)) that include air holes introduced within cladding of the
fibers; and (4) non-silica fibers, such as fibers produced from
lead silicates, chalcogenides, tellurite oxide, bismuth oxide,
etc.
[0023] The ultrashort pulses from amplifier 210 may be affected by
dispersion and a multitude of nonlinear effects when passed through
diffuser 220, and these nonlinear effects may include, for example,
SPM, XPM, four-wave mixing (FWM) whereby interactions between two
or three wavelengths produce two or one new wavelengths, stimulated
Raman scattering (SRS) whereby photons of the pulse interact with
optical phonons, etc., together with dispersion. The dispersion and
the nonlinear effects may generate frequencies outside the input
pulse spectrum, and for sufficiently intense pulses, the HNLFs can
extend the pulse spectrum over a wide frequency range referred to
as a supercontinuum.
[0024] The resulting supercontinuum, generated by diffuser 220 from
the received amplified light pulses, may have a sufficiently large
bandwidth to cover the desired optical bands for the channels in
optical network 100. As used herein, the term "supercontinuum" is
intended to include a broadening of spectral input (e.g., from
light source 200) around a peak wavelength generated by light
source 200. Furthermore, the power (or brightness) of the
wavelengths included in a supercontinuum may have a spectral
flatness such that the magnitudes of the respective wavelengths
differ by less than a desired amount (e.g., between 5 dB to 40
dB).
[0025] As described in greater detail below, bands of wavelengths
included in the supercontinuum may share certain optical
characteristics, such as a common phase and/or known phase
differences. Thus, even if phase changes occur in the light output
of light source 200 (or is introduced by amplifier 210), this phase
change may occur consistently throughout the wavelengths in the
supercontinuum. In this way, fixed phase coupling relationships
exist in the different wavelengths included in the supercontinuum.
Similarly, if a peak wavelength (or frequency) generated by light
source 200 changes and/or a wavelength shift is introduced by
amplifier 210, the supercontinuum continues to provide a full
spectrum for channels to be transmitted through optical system 100.
Consequently, even if phase and/or frequency changes occur in light
source 200, optical transmitter 110 may continue to output channels
having known frequencies and known phase relationships. As
described below, this type of channel relationship enables full
non-linear penalty cancellation by optical receiver 120 that cannot
be achieved if different channels are generated using respective
lasers since the exact frequency and phase relationships of the
respective lasers cannot be reliably determined.
[0026] Furthermore, HNLFs may enable large Raman-induced frequency
shifts (RIFS) that enable tuning of the peak wavelength generated
by light source 200. For example, if diffuser 220 include an HNLF,
diffuser 220 may be used to adjust a peak (or center) wavelength so
that the supercontinuum includes a desired wavelength or range of
wavelengths.
[0027] If the input pulses generated by light source 200 are of
sufficiently short duration (e.g., about a picosecond), then
self-phase modulation in diffuser 220 may lead to significant
spectral broadening that is also temporally coherent. However, if
the pulses from light source 200 have too long of a duration (e.g.,
longer than a picosecond), then stimulated-Raman scattering tends
to dominate, causing a series of cascaded discrete Stokes lines to
appear until a zero dispersion wavelength is reached. When the zero
dispersion wavelength is reached, a soliton Raman continuum may
form, causing the generation of the supercontinuum to be more
inefficient.
[0028] As further shown in FIG. 2, optical channel generator 170 of
optical transmitter 110 may include a tone separator 225, multiple
modulators 230-1 through 230-n, a tone combiner 235, and an output
optical amplifier 240.
[0029] Tone separator 225 may couple to diffuser 220 to receive the
generated supercontinuum. The received supercontinuum may include
multiple wavelengths .lamda..sub.1 through .lamda..sub.n,
corresponding to channels 1 through n. Tone separator 225
wavelength division directs light of the multiple wavelengths
.lamda..sub.1 through .lamda..sub.n, included in the
supercontinuum, into multiple outputs, with each output of tone
separator 225 being associated with a different one of wavelengths
.lamda..sub.1 through .lamda..sub.n. The multiple outputs of tone
separator 225 connect to modulators 230-1 through 230-n, with each
of modulators 230-1 through 230-n receiving a respective wavelength
of wavelengths .lamda..sub.1 through .lamda..sub.n. For example,
the first output of tone separator 225 may include demultiplexed
light having a wavelength of .lamda..sub.1 and that may be inputted
to modulator 230-1. As a further example, the second output of tone
separator 225 may include a demultiplexed light having a wavelength
of .lamda..sub.2 and that may be inputted to modulator 230-2. As
yet another example, the nth output of tone separator 225 may
include demultiplexed light having a wavelength of .lamda..sub.n
and that may be inputted to modulator 230-n. Even if a wavelength
of the output generated by light source 200 changes and the
resulting supercontinuum shifts (e.g., includes a different range
of frequencies), the supercontinuum may still include wavelengths
.lamda..sub.1 through .lamda..sub.n. Thus, tone separator 225 may
continue to extract wavelengths .lamda..sub.1 through .lamda..sub.n
for use in generating output channels despite frequency shifts in
the output of light source 200.
[0030] Modulators 230-1 through 230-n (generically referred to
herein as "modulator 230" or "modulators 230") may each receive
demultiplexed light of a respective wavelength from tone separator
225. For example, modulator 230-1 may receive demultiplexed light
of wavelength .lamda..sub.1 from tone separator 225, modulator
230-2 may receive demultiplexed light of wavelength .lamda..sub.2
from tone separator 225, and modulator 230-n may receive
demultiplexed light of wavelength .lamda..sub.n. Each modulator 230
may apply a differential phase modulation to the received light
from tone separator 225 in accordance with data to be transmitted
on the particular channel corresponding to the wavelength .lamda.
handled by the modulator 230. For example, modulator 230-1 may
receive demultiplexed light of wavelength .lamda..sub.1 from tone
separator 225 and may apply a differential phase modulation to the
light based on a first stream of data (not shown in FIG. 2)
received at modulator 230-1. As another example, modulator 230-n
may receive a demultiplexed light of wavelength .lamda..sub.n from
tone separator 225 and may apply a differential phase modulation to
the light based on an n.sup.th stream of data (not shown in FIG. 2)
received at modulator 230-n.
[0031] Tone combiner 235 may receive the modulated light of
wavelengths .lamda..sub.1 through .lamda..sub.n from modulators
230-1 through 230-n and may multiplex the light into a single
multi-wavelength optical output comprising modulated optical
signals of the multiple wavelengths .lamda..sub.1 through
.lamda..sub.n. Tone combiner 235 may supply the multi-wavelength
output to optical amplifier 240. Optical amplifier 240 may amplify
the signal amplitudes of the various wavelengths .lamda..sub.1
through .lamda..sub.n for transmission over optical fiber 130.
Optical amplifier 240 may include, for example, one or more erbium
doped fiber amplifiers. Optical fiber 130 is depicted as a single
optical fiber span connecting optical transmitter 110 and optical
receiver 120. In other implementations, however, optical fiber 130
may include multiple interconnecting fibers, possibly including
optical switches or routers, for switching/routing optical signals
from optical transmitter 110 to optical receiver 120. In one
implementation, optical fiber 130 is a low polarization mode
dispersion (PMD) fiber. PMD is a form of modal dispersion in which
optical signals of different polarizations spread while travelling
through a waveguide because random imperfections and asymmetries
within the waveguide cause the optical signals to travel at
different speeds through the waveguide.
[0032] FIG. 2 additionally depicts coherent receivers 180 of
optical receiver 120 as including an input optical amplifier 245, a
tone separator 250, multiple coherent receivers 255-1 through
255-n, and DSP 190.
[0033] Input optical amplifier 245 may amplify the signal
amplitudes of the various wavelengths .lamda..sub.1 through
.lamda..sub.n of optical signals transmitted over optical fiber 130
and received at optical receiver 120. Optical amplifier 245 may
include, for example, one or more erbium doped fiber amplifiers.
Tone separator 250 may perform wavelength division demultiplexing
of the single multi-wavelength optical output of optical
transmitter 110. As previously described, the optical output of
optical transmitter 110 may include modulated optical signals of
the multiple wavelengths .lamda..sub.1 through .lamda..sub.n, and
tone separator 250 may extract the multiple signals into multiple
outputs, with each output of tone separator 250 being associated
with a different wavelength of wavelengths .lamda..sub.1 through
.lamda..sub.n. The multiple outputs of tone separator 250 may be
connected to coherent receivers 255-1 through 255-n.
[0034] Each of the coherent receivers 255-1 through 255-n may
receive the demultiplexed optical signals of a respective one of
wavelengths .lamda..sub.1 through .lamda..sub.n, and may coherently
detect and perform analog-to-digital conversions of the optical
signals of the respective wavelength .lamda.. Coherent receivers
255-1 through 255-n each pass the resulting digital signals to DSP
190 for digital signal processing.
[0035] DSP 190 performs, for example, forward error correction
(FEC) upon the digital signals for each channel having one of the
wavelengths .lamda..sub.1-.lamda..sub.n. In one exemplary
implementation, the FEC algorithm performed by DSP 190 includes
Reed-Solomon forward error correction. Other types of FEC, however,
may alternatively be used. In addition, DSP 190 may apply a
nonlinear compensation algorithm, described further below, to
compensate for propagation nonlinearities induced during traversal
of optical fiber 130 in order to achieve full nonlinear penalty
cancellation. For example, DSP 190 may leverage
[0036] The configuration of the components of the optical network
depicted in FIG. 2 is for illustrative purposes only, and other
configurations may be implemented. Therefore, the optical network
may include additional, fewer and/or different components, that may
be configured differently, than depicted in FIG. 2.
[0037] FIG. 3 is a diagram that depicts functions performed at
optical transmitter 110 when transmitting optical signals generated
using a generated supercontinuum, and functions performed at
optical receiver 120 when receiving and processing the transmitted
optical signals after traversal of optical fiber 130. As shown in
FIG. 3, optical transmitter 110 (e.g., tone separator 225) may
generate an optical output 310 that includes multiple channels of
light. FIG. 4 depicts further details of the multiple wavelengths
of optical output 310 generated by tone separator 225. As shown in
FIG. 4, optical output 310 may include multiple different
wavelengths .lamda..sub.1 through .lamda..sub.n, each wavelength
.lamda. being associated with a respective one of channels 1
through n, with the frequency/wavelength of each of the channels 1
through n of optical output 310 being locked to each other. For
example, the frequency difference between the channels may be
constant across all the multiple channels of optical output 310.
Light of each of the multiple channels of optical output 310 may be
modulated by modulators 230 based on received input data that is to
be transmitted, and the modulated optical output 310 may then be
combined, or multiplexed, by tone combiner 235 for transmission
through optical fiber 130. For example, modulators 230 may perform
a differential phase modulation in which each optical signal
extracted by tone separator is represented by a phase difference
between two adjacent bits of data within the stream of the input
data. In the example shown in FIG. 4, each of the channels included
in modulated optical output 310 may be transmitted at a
substantially equal magnitude (or power) level within optical fiber
130.
[0038] Referring again to FIG. 3, as the modulated, multiplexed
optical output 310 traverses optical fiber 130, nonlinearities may
be introduced in optical output 310 due to the fiber medium,
environmental factors, and the effect of neighboring channels in
optical output 310. Optical receiver 120 (e.g., tone separator 250)
may receive and demultiplex a distorted version 320 that reflects
changes in optical output 310 caused by transmission through
optical fiber 130. FIG. 5 depicts further details of the nonlinear
distorted version 320 as received at optical receiver 120 after
traversing optical fiber 130.
[0039] As seen in FIG. 5, a nonlinear effect 500 is evidenced in
the multiple channels of nonlinear distorted version 320, with the
various wavelengths including a magnitude distortion relative to
one another, and relative to optical output 310. Although nonlinear
effect 500 is presented in the context of magnitude differences, it
should be appreciated that nonlinear effect 500 may also include
other factors, such as changes in frequency, phase, etc.
Furthermore, although nonlinear effects 500 are presented in the
context of constant magnitude differences, nonlinear effect 500 may
include periodic and/or varying magnitude levels in the different
wavelengths (e.g., a magnitude for signals at a given wavelength
may vary during different time periods). Nonlinear effects 500 may
be measured at optical receiver 120 by comparing attributes of
nonlinear distorted version 320 to known attributes of optical
output 310 (e.g., attributes associated with the supercontinuum
such as magnitude, phase, etc.).
[0040] Referring again to FIG. 3, DSP 190 at optical receiver 120
performs forward error correction (FEC) 330 upon the received,
differentially modulated signals. In one implementation, FEC 330
may include using a FEC algorithm such as the Reed-Solomon error
correction algorithm. FEC 330 corrects any isolated phase errors
introduced in the transmitted optical signals.
[0041] In addition, DSP 190 may apply non-linear compensation 340
to the forward error corrected digital signals to compensate for
propagation nonlinearities induced during traversal of optical
fiber 130. Field propagation in optical fiber 130 may be governed
by the nonlinear Schrodinger equation, and the optical signals
received at optical receiver 120 can be used as an initial
condition. Using the nonlinear Schrodinger equation and nonlinear
distorted version 320 received at optical receiver 120 as the
initial condition, the nonlinear Schrodinger equation may be used
to computer model the signal transmitted at optical transmitter 110
prior to propagation across optical fiber 130. The result of the
computer modeling, using the Schrodinger equation and the known
initial condition of the optical channels, is the original
transmitted optical signals, from modulators 230-1 through 230-n,
transmitted from optical transmitter 110. The computer modeling,
using the nonlinear Schrodinger equation, thus, cancels
nonlinearities induced in the optical signals 310 during its
traversal of optical fiber 130.
[0042] For example, as previously described, the different
wavelengths included in the supercontinuum may have known phase and
frequency relationships. Consequently, even if phase and/or
frequency changes occur in light source 200, optical transmitter
110 may continue to output channels at known frequencies and known
phase relationships. This characteristic enables full non-linear
penalty cancellation that cannot be achieved if different channels
are generated using respective lasers since the exact frequency and
phase relationships of the respective lasers cannot be reliable
determined. For example, the nonlinear Schrodinger equation
includes phase coupling relationship terms that can be canceled in
channels generated from the supercontinuum due to the fixed phase
relationship in the supercontinuum. Thus, DSP 190 may determine
that any detected phase or other differences in received light 130
are introduced through non-linear effects within optical fiber 130
and should be cancelled since these differences are not present in
optical output 310 (due to the supercontinuum). In contrast, the
nonlinear Schrodinger equation cannot be used to fully correct for
phase and/or frequency separation differences in received signals
generated by multiple lasers since the phase and/or frequency
separation differences may be characteristics of the originally
transmitted signals (e.g., due to differences in the multiple
lasers) and not introduced by an optical fiber carrying the
signals. Because the receiver cannot differentiate between
transmitter-induced signal anomalies and signal anomalies caused by
non-linear effects, cancelling the signal anomalies in received
signals generated by multiple lasers may lead to inaccurate data
transmission.
[0043] In this way, complete nonlinear penalty cancellation may be
achieved by optical receiver 120, allowing fiber capacity to be
significantly improved. For example, total cancelling of nonlinear
effects allows that more channels of closer spaced wavelengths kto
be carried on optical fiber 130 because wavelength spacing is no
longer needed to manage nonlinear effects.
[0044] FIG. 6 is a diagram illustrating exemplary functional
components of a computing device 600 according to an implementation
described herein. For example, system management node 150 and/or an
element of network 140 (e.g., a hot spot, a node, router, blade,
optical transmitter 110, optical receive 120, etc.) may include one
or more computing devices 600. As shown in FIG. 6, device 600 may
include a bus 610, a processing unit 620, a memory 630, an input
device 640, an output device 650, and a communication interface
660.
[0045] Bus 610 may include a path that permits communication among
the components of device 600. Processing unit 620 may include any
type of single-core processor, multi-core processor,
microprocessor, latch-based processor, and/or processing logic (or
families of processors, microprocessors, and/or processing logics)
that interprets and executes instructions. In other embodiments,
processing unit 620 may include an application-specific integrated
circuit (ASIC), a field-programmable gate array (FPGA), and/or
another type of integrated circuit or processing logic.
[0046] Memory 630 may include any type of dynamic storage device
that may store information and/or instructions, for execution by
processing unit 620, and/or any type of non-volatile storage device
that may store information for use by processing unit 620. For
example, memory 630 may include a random access memory (RAM) or
another type of dynamic storage device, a read-only memory (ROM)
device or another type of static storage device, a content
addressable memory (CAM), a magnetic and/or optical recording
memory device and its corresponding drive (e.g., a hard disk drive,
optical drive, etc.), and/or a removable form of memory, such as a
flash memory.
[0047] Input device 640 may allow an operator to input information
into device 600. Input device 640 may include, for example, a
keyboard, a mouse, a pen, a microphone, a remote control, an audio
capture device, an image and/or video capture device, a
touch-screen display, and/or another type of input device. In some
embodiments, device 600 may be managed remotely and may not include
input device 640. In other words, device 600 may be "headless" and
may not include a keyboard, for example.
[0048] Output device 650 may output information to an operator of
device 600. Output device 650 may include a display, a printer, a
speaker, and/or another type of output device. For example, device
600 may include a display, which may include a liquid-crystal
display (LCD) for displaying content to the customer. In some
embodiments, device 600 may be managed remotely and may not include
output device 650. In other words, device 600 may be "headless" and
may not include a display, for example.
[0049] Communication interface 660 may include a transceiver that
enables device 600 to communicate with other devices and/or systems
via network 140 using, for example, wireless communications (e.g.,
radio frequency, infrared, and/or visual optics, etc.), wired
communications (e.g., conductive wire, twisted pair cable, coaxial
cable, transmission line, fiber optic cable, and/or waveguide,
etc.), or a combination of wireless and wired communications.
Communication interface 660 may include a transmitter that converts
baseband signals to radio frequency (RF) signals and/or a receiver
that converts RF signals to baseband signals.
[0050] Communication interface 660 may include and/or may be
coupled to an antenna for transmitting and receiving RF signals.
For example, communication interface 660 may be coupled to an
antenna assembly that includes one or more antennas to transmit
and/or receive RF signals. The antenna assembly may, for example,
receive data from communication interface 660 and transmit RF
signals associated with the data, or the antenna assembly may
receive RF signals and provide them to communication interface 660
to be processed.
[0051] Communication interface 660 may include a logical component
that includes input and/or output ports, input and/or output
systems, and/or other input and output components that facilitate
the transmission of data to other devices. For example,
communication interface 660 may include a network interface card
(e.g., Ethernet card) for wired communications and/or a wireless
network interface (e.g., a Wi-Fi) card for wireless communications.
Communication interface 660 may also include a universal serial bus
(USB) port for communications over a cable, a Bluetooth.RTM.
wireless interface, a RFID interface, a NFC wireless interface,
and/or any other type of interface that converts data from one form
to another form.
[0052] As will be described in detail below, device 600 may perform
certain operations, and device 600 may perform these operations in
response to processing unit 620 executing software instructions
contained in a computer-readable medium, such as memory 630. A
computer-readable medium may be defined as a non-transitory memory
device. A memory device may be implemented within a single physical
memory device or spread across multiple physical memory devices.
The software instructions may be read into memory 630 from another
computer-readable medium or from another device. The software
instructions contained in memory 630 may cause processing unit 620
to perform processes described herein. Alternatively, hardwired
circuitry may be used in place of, or in combination with, software
instructions to implement processes described herein. Thus,
implementations described herein are not limited to any specific
combination of hardware circuitry and software.
[0053] Although FIG. 6 shows exemplary components of device 600, in
other implementations, device 600 may include fewer components,
different components, additional components, or differently
arranged components than those depicted in FIG. 6. Additionally, or
alternatively, one or more components of device 600 may perform one
or more tasks described as being performed by one or more other
components of device 600.
[0054] FIG. 7 is a flow diagram that illustrate an exemplary
process 700 for transmitting optical signals using a supercontinuum
generated by a nonlinear material. Process 700 is described below
with reference to FIGS. 1-6. Furthermore, process 700 may be
implemented by optical transmitter 110. In other implementations,
one or more steps of process 700 may be performed by another
component, such as optical receiver 120 and/or another component
that is not shown in FIGS. 1-6.
[0055] As shown in FIG. 7, process 700 may include generating a
short pulse (block 710) and amplifying the short pulse (block 720).
For example, short optical pulses, centered at a wavelength
.lamda., may be generated by light source 200, such as a
short-pulse (e.g., picosecond) soliton laser. In block 720, the
pulses may be amplified by one or more optical amplifier 210 so
that the short pulses have a greater brightness (or magnitude) but
are still centered on wavelength .lamda.. For example, the pulses
may be passed through a series of optical amplifier 210 until a
desired power (or intensity) level is achieved.
[0056] The amplified pulses may then be passed through diffuser 220
to generate a supercontinuum (block 730). For example, diffuser 220
may be a nonlinear material that causes light energy of other
wavelengths from the amplified pulses due to nonlinear effects. The
supercontinuum may be separated into different output channels of
individual wavelengths .lamda..sub.1, .lamda..sub.2, .lamda..sub.3,
. . . , .lamda..sub.n (block 740). For example, the output (e.g.,
the supercontinuum) from diffuser 220 may be directed to tone
separator 225 of optical channel generator 170, and tone separator
225 may use wavelength division to separate the supercontinuum into
different wavelength outputs, with each output corresponding to a
single channel (e.g., a single wavelength of light).
[0057] Modulators 230 may then modulate the separate channels to
carry data (block 750). For example, each modulator 230, associated
with a particular wavelength of the outputs, may apply a
differential modulation to modulate the outgoing wavelength in
accordance with the data to be transmitted. Referring to FIG. 2,
modulators 230-1 through 230-n each receive wavelength division
demultiplexed light corresponding to a respective wavelength
.lamda..sub.1 through .lamda..sub.n. Each modulator 230 receives an
input data stream (not shown in FIG. 2) and performs differential
modulation upon the demultiplexed light to generate a modulated
optical signal output. In one implementation, the differential
modulation may include differential phase modulation applied by
each modulator 230 based on a respective stream of data received at
that modulator. For example, modulator 230-2 receives demultiplexed
light of wavelength .lamda.2 from tone separator 225, and
sequential bits of a stream of input data, and differentially phase
modulates the light of wavelength .lamda.2 based on the sequential
bit values of the stream of input data.
[0058] Tone combiner 235 may multiplex the modulated channels and
transmit the multiplexed, modulated channels toward optical
receiver 120 (block 760). For example, tone combiner 235 wavelength
division multiplexes multiple wavelengths from modulators 230 into
a multi-wavelength multiplexed optical output. Tone combiner 235
receives the multiple modulated optical signal outputs from
modulators 230-1 through 230-n, including optical signals
transmitted via wavelengths .lamda..sub.1 through .lamda..sub.n,
and multiplexes the light of different wavelengths into a single
multi-wavelength optical output, which is provided to optical
amplifier 240. Optical amplifier 240 amplifies the
multi-wavelengths of light to boost the signal power for
transmission over optical fiber 130. The amplified multiplexed
signal may then be forwarded via optical fiber 130 toward optical
receiver 120.
[0059] FIG. 8 is a flow diagram that illustrates an exemplary
process 800 for processing optical signals generated from a
supercontinuum and received over an optical fiber(s). Process 800
may be implemented by optical receiver 120.
[0060] Optical receiver 120 may receive a version of multiplexed
signal sent from optical transmitter 110 and may extract signals
carried in the received signal (block 810). For example, tone
separator 250 of optical receiver 120 may perform wavelength
division demultiplexing of the multi-wavelength multiplexed input
signals received from optical transmitter 110 over optical fiber
130. Referring to FIG. 2, modulated multi-wavelength light signals
received over optical fiber 130 from optical transmitter 110 are
amplified by optical amplifier 245 and provided to tone separator
250. Tone separator 250 wavelength division demultiplexes the
single input, comprising the differentially modulated signals from
the generated super supercontinuum, into multiple outputs, with
each of the outputs corresponding to one of the multiple
wavelengths .lamda..sub.1 through .lamda..sub.n.
[0061] In block 810, each of coherent receivers 255-1 through
255-n, associated with a particular wavelength of the signals,
converts input optical signals of the particular wavelength into a
digital signal output. Each of the coherent receivers 255-1 through
255-n receives demultiplexed optical signals of a respective one of
wavelengths .lamda..sub.1 through .lamda..sub.n from tone separator
250, and coherently detects and analog-to-digitally converts the
optical signals of the respective wavelength .lamda.. Coherent
receivers 255-1 through 255-n each pass the resulting digital
signals to DSP 190 for digital signal processing. Coherent
receivers 255 may use known techniques for coherently detecting and
converting modulated light signals of a respective wavelength
.lamda. to a digital signal output.
[0062] Optical receiver 120 may perform back propagation to
identify non-linear effects in the channels included in a received
multiplexed signal (block 820). For example, DSP 190 may perform
forward error correction (FEC) on the digital signal outputs from
coherent receivers 255-1 through 255-n to correct errors. The
errors may include, for example, phase error that may occur when
the timing of optical transmitter 110 and optical receiver 120 go
out of alignment. In one exemplary implementation, DSP 190 employs
a FEC algorithm such as, for example, Reed-Solomon error
correction. Other types of FEC, however, may alternatively be used.
Use of FEC by DSP 190 may include resetting the reference phase to
handle any optical interruptions. Optical receiver 120 may then
identify differences between the received signals, after FEC.
[0063] DSP 190 may cancel the non-linear effects to recover the
originally transmitted channels (block 830). The propagation
nonlinearities are induced in the multi-wavelength signals during
traversal of the optical fiber. As previously described, DSP 190
may apply non-linear compensation 340 to the digital signals, based
on the nonlinear Schrodinger equation, to cancel nonlinearities
induced in the signals while traversing optical fiber 130. Using
the nonlinear Schrodinger equation, and the received optical
signals at optical receiver 120 as an initial condition, the
Schrodinger equation may be used to computer model the signal
transmitted at optical transmitter 110 prior to propagation across
optical fiber 130. The result of the computer modeling, using the
Schrodinger equation and the initial condition of the received
optical signals, should be the original optical signals,
reconstructed via the computer modeling, transmitted by optical
transmitter 110.
[0064] The foregoing description of implementations provides
illustration and description, but is not intended to be exhaustive
or to limit the invention to the precise form disclosed.
Modifications and variations are possible in light of the above
teachings or may be acquired from practice of the invention. For
example, while a series of blocks has been described with respect
to FIGS. 7 and 8, the order of the blocks may be varied in other
implementations. Moreover, non-dependent blocks may be performed in
parallel.
[0065] Certain features described above may be implemented as
"logic" or a "unit" that performs one or more functions. This logic
or unit may include hardware, such as one or more processors,
microprocessors, application specific integrated circuits, or field
programmable gate arrays, software, or a combination of hardware
and software.
[0066] No element, act, or instruction used in the description of
the present application should be construed as critical or
essential to the invention unless explicitly described as such.
Also, as used herein, the article "a" is intended to include one or
more items. Further, the phrase "based on" is intended to mean
"based, at least in part, on" unless explicitly stated
otherwise.
[0067] In the preceding specification, various preferred
embodiments have been described with reference to the accompanying
drawings. It will, however, be evident that various modifications
and changes may be made thereto, and additional embodiments may be
implemented, without departing from the broader scope of the
invention as set forth in the claims that follow. The specification
and drawings are accordingly to be regarded in an illustrative
rather than restrictive sense.
* * * * *