U.S. patent application number 12/696957 was filed with the patent office on 2010-06-03 for optical orthogonal frequency division multiplexed communications with nonlinearity compensation.
This patent application is currently assigned to CeLight, Inc.. Invention is credited to Jacob Khurgin, Isaac Shpantzer.
Application Number | 20100135656 12/696957 |
Document ID | / |
Family ID | 46332154 |
Filed Date | 2010-06-03 |
United States Patent
Application |
20100135656 |
Kind Code |
A1 |
Khurgin; Jacob ; et
al. |
June 3, 2010 |
OPTICAL ORTHOGONAL FREQUENCY DIVISION MULTIPLEXED COMMUNICATIONS
WITH NONLINEARITY COMPENSATION
Abstract
The present invention discloses a transmitter and receiver for
optical communications system, which provide compensation of the
optical link nonlinearity. M-PSK modulating is used for data
embedding in an optical signal in each WDM channel using orthogonal
frequency division multiplexing (OFDM) technique. At the receiver
side electrical output signals from a coherent optical receiver are
processed digitally with the link nonlinearity compensation. It is
followed by the signal conversion into frequency domain and
information recovery from each subcarrier of the OFDM signal. At
the transmitter side an OFDM encoder provides a correction of I and
Q components of a M-PSK modulator driving signal to compensate the
link nonlinearity prior to sending the optical signal to the
receiver.
Inventors: |
Khurgin; Jacob; (Baltimore,
MD) ; Shpantzer; Isaac; (Bethesda, MD) |
Correspondence
Address: |
CELIGHT, INC.
12200 TECH RD.
SILVER SPRING
MD
20904
US
|
Assignee: |
CeLight, Inc.
|
Family ID: |
46332154 |
Appl. No.: |
12/696957 |
Filed: |
January 29, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12418060 |
Apr 3, 2009 |
7693428 |
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12696957 |
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12045765 |
Mar 11, 2008 |
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12418060 |
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11679376 |
Feb 27, 2007 |
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12045765 |
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11695920 |
Apr 3, 2007 |
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11679376 |
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61045783 |
Apr 17, 2008 |
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Current U.S.
Class: |
398/43 ; 375/260;
398/158; 398/192; 398/208 |
Current CPC
Class: |
H04L 2027/0018 20130101;
H04J 14/02 20130101; H04B 10/613 20130101; H04B 10/6163 20130101;
H04L 27/18 20130101; H04B 10/65 20200501; H04B 10/63 20130101; H04J
14/06 20130101; H04L 27/2697 20130101; H04B 10/61 20130101; H04B
10/614 20130101; H04B 10/6165 20130101; H04L 27/2096 20130101 |
Class at
Publication: |
398/43 ; 398/208;
398/192; 375/260; 398/158 |
International
Class: |
H04B 10/00 20060101
H04B010/00; H04B 10/06 20060101 H04B010/06; H04B 10/04 20060101
H04B010/04; H04J 14/00 20060101 H04J014/00; H04L 27/28 20060101
H04L027/28 |
Claims
1. An optical receiver to receive a data modulated beam from an
optical link, comprising: at least a first coherent receiver for
receiving the data modulated beam and a local oscillator beam; the
data modulated beam being one channel of orthogonal frequency
division multiplexed (OFDM) communication system, the OFD
multiplexing having N subcarrier in each channel, a set of
photodetectors receiving output beams from the coherent receiver
and producing electrical signals I and Q; a digital signal
processing unit receiving the electrical signals I and Q,
converting them into digital signals, calculating an expected phase
shift caused by the signal distortion for each sampling interval,
performing the distortion compensation by multiplying the instant
signals on a distortion coefficient depending on the expected
distortion phase shift, transforming the digital signals into
frequency domain, forming a set of spectral signals each having its
own digital frequency, demodulating the data encoded in each
spectral signal, and outputting a received information.
2. The receiver of claim 1, wherein the distortion is associated
with linear and nonlinear transmission impairments.
3. The receiver of claim 1, wherein the transmission is in optical
fiber.
4. The receiver of claim 1, wherein the distortion phase shift is
expected nonlinear phase shift caused by the fiber nonlinearity
.phi..sub.1(t.sub.i).sub.i=G.sub.1 P.sub.1 (t.sub.i) and G.sub.1 is
a parameter being proportional to a link nonlinearity .gamma.,
G.sub.1=M .gamma.EL.sub.1, wherein L.sub.1 is a length of a
compensating part of the optical link, E is an average optical beam
power and M is a power coefficient in unit of Watt indicating the
launch power in the optical link at the front end per voltage level
corresponding to one digitization bit;
P.sub.1(t.sub.i)=Q(t.sub.i).sup.2+I(t.sub.i).sup.2is an instant
power of the signal.
5. The receiver of claim 4, wherein L.sub.1 is a second half-length
of the optical link.
6. The receiver of claim 1, wherein the coherent receiver is based
on a 90-degrees optical hybrid.
7. The receiver of claim 1, wherein the optical receiver is adapted
for operation with the optical signal of two polarization
states.
8. The receiver of claim 1, wherein the modulation format is
selected from QPSK or QAM or M-QAM.
9. The receiver of claim 1, wherein the distortion is caused by the
nonlinearity of the electro-optical modulation of the beam in the
channel.
10. An optical transmitter to transmit a data encoded beam over an
optical link, comprising: a digital data stream entering an
orthogonal frequency division multiplexed (OFDM) encoder, the
encoder outputting I and Q analog signals driving an optical
modulator, the modulator modulating separately each OFDM subcarrier
of each channel of an initial optical beam from a light source, the
modulator outputting a modulated optical beam to be transmitted in
the optical link, wherein the OFDM encoder performs a compensation
of the optical link transmission distortions by multiplying each
subcarrier on a compensation coefficient.
11. The transmitter of claim 10, wherein the distortion is
associated with linear and nonlinear transmission impairments.
12. The transmitter of claim 10, wherein the distortion the
compensation is performed by estimating instant power
P.sub.2(t.sub.i)=Q(t.sub.i).sup.2+I(t.sub.i).sup.2, where a
sampling interval .DELTA.t.sub.i=t.sub.i+1-t.sub.i is equal or less
than a symbol interval, calculating an expected distortion phase
shift .phi..sub.2 (t.sub.i).sub.i for the i-th sampling interval,
performing a distortion compensation by multiplying each subcarrier
on a distortion compensation coefficient depending on the expected
distortion phase shift.
13. The transmitter of claim 12, wherein the expected distortion
phase shift is .phi..sub.2(t.sub.i).sub.i=G.sub.2 P.sub.2 (t.sub.i)
and G.sub.2 is a parameter being proportional to a link
nonlinearity .gamma., G.sub.2=M .gamma.EL.sub.2, wherein L.sub.2 is
a length of a compensating part of the optical link, E is an
average optical beam power and M is a power coefficient in unit of
Watt indicating the launch power in the optical link at the front
end per voltage level corresponding to one digitization bit.
14. The transmitter of claim 13, wherein L.sub.2 is a first
half-length of the optical link.
15. A system for a data transmission via an optical communication
link, comprising: a digital data stream entering an orthogonal
frequency division multiplexed (OFDM) encoder, the encoder
outputting I and Q analog signals driving an optical modulator, the
modulator modulating separately each OFDM subcarrier of each
channel of an initial optical beam from a light source, the
modulator outputting a modulated optical beam to be transmitted in
the optical link, wherein the OFDM encoder performs a compensation
of the optical link transmission distortions by multiplying each
subcarrier on a compensation coefficient; the modulated beam being
received by an optical receiver, comprising: at least a first
coherent receiver for receiving the data modulated beam and a local
oscillator beam; a set of photodetectors receiving output beams
from the optical hybrid and producing electrical signals I and Q; a
digital signal processing unit receiving the electrical signals I
an d Q, converting them into digital signals, an expected
distortion phase shift for the i-th sampling interval, performing a
distortion compensation by multiplying the signals I(t.sub.i) and
Q(t.sub.i) on a coefficient depending on the expected distortion
phase shift, transforming the digital signals into frequency
domain, forming a set of spectral signals each having its own
digital frequency, demodulating the data encoded in each spectral
signal, and outputting a received information.
16. The system of claim 15, wherein the OFDM encoder compensates
transmission distortions of a first half of the optical link, and
the digital signal processing unit compensates transmission of a
second half of the optical link.
17. The system of claim 15 adapted to operate with data
transmission using an optical beam having two polarization
states.
18. The system of claim 15, wherein a data transmission rate is 100
Gb/s.
19. The system of claim 15, wherein the modulation format is
selected from QPSK or QAM or M-QAM.
20. The system of claim 15, wherein the channel is one of many WDM
channels of the transmission system.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of Ser. No.
12/418,060, it claims priority to U.S. provisional application
61/045,783 filed Apr. 17, 2008, and this application is a
continuation-in-part of U.S. patent application Ser. No. 12/045,765
filed Mar. 11, 2008 No, Ser. No. 11/679,376 filed Feb. 27, 2007 and
Ser. No. 11/695,920 filed Apr. 3, 2007, all of which applications
are fully incorporated herein by reference.
FIELD OF INVENTION
[0002] This invention relates generally to optical communications
systems and methods that utilize coherent detection technique, WDM
M-PSK transmission and optical orthogonal frequency division
multiplexing (OFDM). In particular the present invention addresses
a method and system for digital compensation of nonlinearity in
optical signals received via optical communication link. The
invention discloses communication typically up to 100 Gb/s
transmission rate that can be implemented for various information
exchange structures including data transmission and video
broadcasting.
BACKGROUND OF THE INVENTION
[0003] Orthogonal frequency division multiplexing is widely used
technique of transmission in the RF domain where it allows
mitigating signal fading in multi-path propagation. The present
invention discloses the use of orthogonal frequency division
multiplexing in optical links and, in particular, in fiber
communications.
[0004] In optical OFDM systems each WDM channel the optical carrier
is directly modulated by a complex RF signal that can be construed
as a linear combination of M separate digitally modulated RF
signals at frequencies f.sub.m such that f.sub.m=m/Th power where T
is the period of modulation. Thus the total symbol rate of the
transmitted information is M/T. In the text we shall refer to the
frequencies f.sub.m as "subcarriers".
[0005] In modern optical communication systems, a coherent
detection technique is implemented, which provides improved
sensitivity compared with traditional direct detection schemes.
Typically coherent detection is used with phase-shift-keying (PSK)
data transmission. The present invention is also focused on M-PSK,
and in the preferred embodiment, QPSK (quadrature PSK) data
transmission. However this does not limit the scope of the
invention, and various types of data modulation can benefit from
the disclosed invention.
[0006] In a coherent receiver, the QPSK incoming optical signal is
mixed with a strong local oscillators to produce in-phase (I) and
in-quadrature (Q) outputs. I and Q components of the output optical
signal are converted into electrical signals by a set of
photodetectors. In the preferred configuration four balanced
photodetectors are used to recover QPSK encoded data.
[0007] Data transmission multiplexing light of two orthogonal
polarizations via the same optical channel allows doubling the data
rate. At the receiver side, the orthogonal polarizations are split
by a polarization beam splitter, and the light of each orthogonal
polarization is detected separately.
[0008] U.S. patent application Ser. No. 10/405,236 by Roberts et
al. discloses a nonlinearity compensation system applicable to WDM
optical transmission. It considers many WDM channels and
essentially performs numerically operations of complex amplitudes
of the signals in all channels. However it is completely
impractical to assure perfect control of the relative optical phase
shifts between different WDM channels as they travel through their
respective fibers (shown as 10a in FIG. 2 of '236) and through the
MUX. The latency of the system is quite long, it includes travel
time through the link, plus processing, which is typically a few
milliseconds. Over that time the relative phases of different
channels significantly shift. Such system requires the adjustment
of their parameters at a rate of GHz. Alternatively such system may
be used with a look-up table (LUT). The calculations show that the
size of such LUT and the power consumptions make this solution
impractical. Furthermore, since the whole link is dispersive in the
system described in '236, the disclosed compensation does not
provide sufficient link performance.
[0009] High capacity optical signal transmission is affected by the
channel nonlinearity and dispersion, which leads to the limitations
in the channel capacity, transmission distance and error rates. The
present invention addresses this problem of the signal distortion
caused by nonlinear effects.
SUMMARY OF THE INVENTION
[0010] The present invention provides a system and method for
optical communications with a high throughput. The system and
method are disclosed for optical transmission with M-PSK modulating
of an optical signal in each WDM channel using orthogonal frequency
division multiplexing (OFDM) technique. The improved performance is
achieved by the link non-linearity compensation at the transmitter
and/or at the receiver side.
[0011] Each subcarrier of OFD-multiplexed channel of WDM
communication system is individually QPSK encoded with data. The
data modulated beam is transmitted towards receiver, which
comprises at least one optical hybrid and a set of photodetectors
outputting electrical signals I and Q. A digital signal processing
(DSP) unit receives the electrical signals I and Q, converts them
into digital signal and multiplies by a parameter compensating the
link nonlinearity. Then the signals I and Q are transformed in
frequency domain forming a set of spectral signals each having its
own digital frequency. The data encoded in each spectral signal is
demodulated, and the received information is displayed or used for
further processing.
[0012] In the preferred embodiment the DSP unit compensates
nonlinearity of the second half-length of the optical link; however
it may compensate the whole link or any portion of it.
[0013] In the preferred embodiment the optical hybrid is a
90-degrees optical hybrid, an integrated device made of an
electro-optical material or thermo-optical material or a
combination of thereof.
[0014] In the preferred embodiment the transmitter also performs
nonlinearity compensation procedure. It operates in the following
manner: a digital data stream enters an orthogonal frequency
division multiplexed (OFDM) encoder, the encoder outputs I and Q
analog signals driving an optical modulator. The modulator
modulates separately each OFDM subcarrier of each WDM channel of an
initial optical beam from a light source. The modulator outputs a
M-PSP modulated optical beam that is transmitted in the optical
link, and the OFDM encoder performs a compensation of the optical
link nonlinearity by multiplying each subcarrier on a compensation
coefficient.
[0015] In the preferred embodiment the DSP unit compensates
nonlinearity of the first half-length of the optical link; however
it may compensate the whole link or any portion of it.
[0016] An optical system for data transmission using the described
above transmitter and receiver with a transmission rate up to 100
Gb/s. The system may be adapted to operate with light having two
polarization components.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 A block diagram of an OFDM QPSK communications
system.
[0018] FIG. 2 A block diagram of an OFDM QPSK communications system
operating in two polarizations.
[0019] FIG. 3 An optical modulator unit structure.
[0020] FIG. 4 A data encoding block in OFDM QPSK communications
system.
[0021] FIG. 5 A coherent optical receiver for OFDM communications
system: (a) with 90-degrees optical hybrid, (b) with 120-degrees
optical hybrid.
[0022] FIG. 6. An adaptive feedback link in OFDM communications
system.
[0023] FIG. 7 A block diagram of an WDM OFDM QPSK communications
system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0024] This disclosure describes a number of embodiments of one or
more optical transmission systems and elements. Within this
disclosure, the term "optical" indicates electromagnetic range at
or near optical frequencies; this includes visible light and
so-called "near-visible" light such as near infrared, infrared, far
infrared and the near and far ultra-violet spectra. The preferred
operating range is around 1.5 micron.
[0025] FIG. 1 illustrates a point-to-point OFDM data transmission
system using coherent detection. In a transmitter 1 a digital data
stream 2 enters an OFDM encoder 3, which outputs two analog signals
4 and 5 (I and Q) driving an optical modulator 6. The modulator 6
applies the modulation to a light beam 7 emitted by a light source
8. The signal 9 transmitted via an optical link 10 is received by
coherent receivers 11. Local oscillator optical signal 12 coming
from a light source 13 enters the coherent receiver 11 and
interferes with the optical signal 14. The receiver 11 includes an
optical hybrid 15, which is a 90-degrees optical hybrid in the
preferred embodiment. In another embodiment it is a 120-degrees
optical hybrid. Output optical signals 17-20 from the optical
hybrid enter a photodetector unit 16 with at least four balanced
photodetectors. I and Q electrical outputs 21, 22 from the
photodetector unit enter a set of A/D converters 23, followed by a
digital signal processing (DSP) unit 26. The output signal 27 can
be used for the further processing or display. A control line 28
provides a control signal for the OFDM encoder to adjust the
modulation signal to comply with the transmission characteristics.
The components of the optical receiver 11 will be described in more
details in the following paragraphs.
[0026] In another embodiment, the system operates with the light
transmission in two polarization states, and the receiver 11 is a
polarization diversity receiver (FIG. 2), and it further comprises
the following elements. The signal is received by coherent
receivers 11H and 11V after splitting by a polarization beam
splitter 29 into two beams 30H and 30V with orthogonal
polarization. Local oscillator optical signals 12H and 12V having H
and V polarization state coming from a local oscillator light
source 13 enter the coherent receivers 11H and 11V and interfere
with optical signals 30H, 30V having the corresponding H and V
polarization states. Each of the receivers 11H and 11V includes an
optical hybrid and a set of photodetectors; it will be described in
more details in the following paragraphs. Each of the receivers
outputs two electrical signals 21H, 22H and 21V, 22V, converted
into digital signals in 23, followed by a digital signal processing
unit 26. Output signals 27 represent a series of the decoded data
streams that can be displayed or transformed into any format for
further presentation and use. In the preferred embodiment each of
10 data streams of 27 has a data rate of 10 Gb/s (100 Gb/s
total).
[0027] Obviously the system can operate in bi-directional
configuration with data transmission in both directions. In this
case light sources, located at each end of the link, have double
functions. Each light source generates the beam for the data
transmission by the transmitter 1 and, at the same time, it
provides the local oscillator signal for the receiver 11.
[0028] A variety of the M-PSK data modulation formats can be used
in the system and method disclosed in the present invention: QAM,
M-QAM, QPSK, BPSK, etc. In one embodiment a quadrature phase shift
keying modulation format (QPSK) is implemented. In the preferred
embodiment the modulator 6 is a Mach-Zehnder Interferometer (MZI)
electro-optic modulator. In the preferred embodiment shown in FIG.
3 QPSK data is embedded in the system using two separate data
modulators, which are the parts of the optical modulator 6. One
modulator 31 is used for I component and another modulator 32 is
for Q component of the data stream. The optical beam 7 is split by
the splitter 33 into two beams 34 and 35, modulated and then
combined together by the combiner 36 forming the output beam 9. A
phase shift of 90-degrees is introduced by a phase shifter 37 in
one of the beams 38 or 39. The output beam 9 is transmitted to the
receiver via optical link. The optical link can be a fiber link or
a free-space link.
[0029] In the preferred embodiment the QPSK modulator is an
integrated device as disclosed in U.S. patent application Ser. Nos.
11/679,378 and 10/613,772 by the same inventive entity.
[0030] FIG. 4 shows an embodiment of the OFDM encoder 3. This type
of encoder was described in details in U.S. patent application Ser.
No. 12/045,765 filed Mar. 11, 2008 by the same team of inventors.
In the present invention the OFDM encoder further includes a module
for the link nonlinearity compensation, which was not previously
disclosed.
[0031] A serial data stream 2 (FIG. 4) is converted into a parallel
sub-carrier data stream 46 in a serial-to-parallel converter 47. In
OFDM, the sub-carrier frequencies are chosen so that the
sub-carriers are orthogonal to each other, meaning that cross-talk
between the sub-channels is eliminated and inter-carrier guard
bands are not required. Parallel output data stream 46 enters a
QPSK data encoder 48. Two parallel output signals 49 and 50
correspond to I and Q parts of the QPSK signals of each subcarrier.
Inverse Fast Fourier Transform is applied in an IFFT unit 51 to the
data streams 49 and 50. Then the phase shift is introduced to the
signals 52 and 53 in a nonlinearity compensation unit 54. The
nonlinearity unit operation will be discussed in more details in
the following paragraphs. A cyclic prefix is added to the signals
56, 57 at a prefix unit 58; the cyclic prefix takes a few last
symbols of each data block and repeats them at the beginning of the
next block. The purpose is to make the scheme resistant to
chromatic dispersion. Two sub-carriers may experience differential
delay up to the length of prefix, but the orthogonality between the
sub-carriers will be preserved and the data will be recovered at
the receiver. The data streams 59, 60 are converted in an
parallel-to-serial converter 61, followed by convertion of 62,63
into analog signals in a D/A converter 64. The analog I and Q
signals 4 and 5 are applied to the optical modulator 6 as shown in
FIG. 1.
[0032] In our system the compensation is achieved within one WDM
channel modulated using M-PSK format with OFD multiplexing. Since
OFDM signal is resilient to chromatic dispersion we do not have
dispersion compensation (or at least we significantly
undercompensate it) with this dispersion XPM and FWM between WDM
channels is not an issue. Within WDM channel we compensate
nonlinear crosstalk between OFDM sub-channels where we measure
amplitude instantly even before optical fiber. So no need for long
feedback.
[0033] It is important to emphasize that pre-compensation for the
nonlinear effects works perfectly only in the dispersionless link.
In the link with dispersion the instant power varies with distance,
and it is practically impossible to predict it at each and every
point. It is desirable, therefore, to sample the instant power as
often as possible. While instant power inside the link might be
inaccessible one can still sample it at two ends and thus provide
the improved compensation.
[0034] The nonlinearity compensation at the transmitter side in
unit 54 is performed by introducing additional phase shift into the
signal whose purpose is to compensate for the expected nonlinear
shift in the optical link. The compensation is achieved by first
estimating the instant input power
P.sub.1(t.sub.i)=Q(t.sub.i).sup.2+I(t.sub.i).sup.2, where a
sampling interval .DELTA.t.sub.i=t.sub.i+1-t.sub.i is equal or less
than a symbol interval, then calculating expected nonlinear phase
shift as .phi..sub.1(t.sub.i).sub.i=G.sub.1P.sub.1(t.sub.i) and
finally performing an operation
I(t.sub.i)=I(t.sub.i)cos(.phi.(t.sub.i))-Q(t.sub.i)sin(.phi.(t.sub.i))
and
Q(t.sub.i).sub.i=I(t.sub.i)sin(.phi.(t.sub.i).sub.i)+Q(t.sub.i).sub.i
cos(.phi.(t.sub.i)).
[0035] Here G.sub.1 a is the input parameter that is proportional
to the compensating portion of the optical link (in the preferred
embodiment it is a half-length L/2) and the fiber nonlinearity
parameter gamma .gamma. (in units of 1/(W*km)
[0036] G.sub.1=M.gamma.E(L/2), where E is an average laser power, M
is the coefficient (in unit of W) indicating the launch power in
the optical fiber at the front end per voltage level corresponding
to one digitization bit in our system. In includes the laser power,
insertion loss of the multiplexer, MZI modulator transfer
parameters and all other system components characteristics.
[0037] In another embodiment an adjustment of the parameter G.sub.1
is performed periodically to follow slow changes of the optical
link properties. The parameter is adjust by using a feedback 28
from the receiver side as shown in FIGS. 1 and 2. The adjustment
occurs on a very long time scale (seconds).
[0038] FIG. 5 illustrates two embodiments of the coherent receiver
11 to be used to recover QPSK data: (a) with 90-degrees optical
hybrid, (b) with 120 degrees optical hybrid. The incoming signal 14
enters an optical hybrid 15, which is a 90-degrees optical hybrid
in the preferred embodiment. The 90-degrees hybrid has four
couplers 71, 72, 73, 74 and a phase shifter 75. The structure of
the 90-degrees optical hybrid 15 is disclosed in detail in
co-pending U.S. patent application Ser. No. 11/695,920 and parent
patents for that application, incorporated herein by reference. The
incoming signal 14 is mixed with the local oscillator optical
signal 12 producing four output optical signals 17-20. A set of
four balanced photodetectors 80-83 is used to convert the signals
17-20 into electrical domain. I and Q electrical outputs 21 and 22
are digitized in the A/D converter 23.
[0039] In another embodiment the optical hybrid is a 120-degrees
optical hybrid shown in FIG. 5 (b). The structure and performance
of the 120-degrees optical hybrid is disclosed in details in U.S.
Patents No. 4,732,447 by Wright and in U.S. Pat. No. 7,085,501 by
Rickard. 120-degrees optical hybrid 90 has three inputs 24, 91, 21
and three outputs 92, 93, 94. The output signals 92-94 pass through
three detector diodes 95, 96, and 97 as illustrated. In the signal
processing unit 34 the electrical signals 98,99, and 100 are split
into two signal paths each. Each of these six signals is mixed with
a signal from a local oscillator so as to create phase differences
between said six signal paths. These six signals are combined in
two groups of three so as to create an in phase and a quadrature
channels in a 120-degrees hybrid processing unit 101. The
transmitted data is recovered from the in-phase and quadrature
signals.
[0040] The above description of the 120-degrees optical hybrid is
presented as an illustration of its possible structure and
performance. Obviously various modifications can be made by a
person skilled in the art. The present invention is not limited to
one particular example, but comprises a variety of possible
embodiments.
[0041] The DSP unit 26 is shown in more detail in FIG. 6. I and Q
serial digital signals 24 and 25 are parallelized in a
serial-to-parallel converter 110. The obtained signals 11 and 112
are used as an input to the FFT block 113 which generates the
parallel data symbol streams 114 and 115, comprising the signals of
each of the sub-carriers. The FFT block is followed by a receiver
nonlinearity compensation unit 116, generating streams 117 and 118,
which enter a parallel-to-serial unit 119 for grooming the parallel
data back to serial or perhaps as a set of serial signals of a
lower rate (a parallel-to-serial converter and data demultiplexer).
Parameters of the output signal 120 are measured periodically at an
evaluation unit 121, which provides a first control signal 28 to
the transmitter and a second control signal 122 to the receiver
non-linearity compensation unit 16. The output signal 27 is
presented as a parallel set of 10 Gbps serial signals. The
operation of OFDM signal recovery is disclosed in more details in
co-pending U.S. patent application Ser. No. 12/045,765 by the same
team of inventors.
[0042] The nonlinearity compensation at the transmitter side in
unit 116 is performed by introducing additional phase shift into
the signal whose purpose is to compensate for the expected
nonlinear shift in the optical link. It is carried out similarly to
the nonlinear compensation at the receiver. G.sub.2 is the input
parameter for the compensating portion of the optical link, which
is in the preferred embodiment the second half of the optical link,
and in general case it differs from the parameter G.sub.1 for the
first half of the link.
[0043] The system performance is measured in 121. In one embodiment
it measures the signal BER, in another embodiment--eye diagram
opening, and there is variety of parameters that may be used to
characterize the system performance. Since the power levels may
differ in different WDM channels, it is desirable to use
performance in each channel to close the loop by adaptively
adjusting G.sub.1 and G.sub.2 parameters in slow regime (seconds)
following the link changes due to the environment or other
reasons.
[0044] The main advantage of the proposed system for the long haul
communications consist of its resiliency in the presence of
chromatic dispersion. With each sub-channel essentially occupying
only a narrow frequency band in the vicinity of f.sub.c+f.sub.m
(f.sub.c is an optical carrier) the effect of chromatic dispersion
is mitigated. Furthermore, with integration period of T one can add
a guard-band equal to a certain fraction of .alpha.T of T (for
example .alpha.=0.1) which will allow one to tolerate much larger
group delay. In essence the OFDM system with M sub-carriers
operating at symbol rate B can tolerate the chromatic dispersion as
a conventional system with symbol rate B/.alpha.M.
[0045] An embodiment of WDM transmission link which employs OFDM is
shown in FIG. 7. A light source 130 generates radiation 131 having
multiple wavelengths. In the preferred embodiment the light source
130 an optical comb generator as disclosed in U.S. Pat. No.
7,123,800 by the same inventive entity, incorporated herein by
reference. A wavelength demultiplexer 132 separates each tooth of
the spectral comb. Optical beams 133-135 . . . N form a series of
optical channels. Each channel is modulated by QPSK signal using
OFDM system as shown in FIG. 1. A multiplexer 139 combines all
channels for the transmission via the optical link 10. At the
receiving end each channel is separated by a demultiplexer 140 and
processed as shown in FIG. 1.
[0046] The foregoing description of a preferred embodiment of the
invention has been presented for purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed, and obviously many
modifications and variations are possible in the light of the above
teaching. The described embodiment was chosen and described in
order to best explain the principles of the invention and its
practical application to thereby enable others skilled in the art
to best utilize the invention in various embodiments and with
various modifications as are suited to the particular use
contemplated. It is intended that the scope of the invention be
defined by the claims appended hereto.
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