U.S. patent application number 16/532433 was filed with the patent office on 2020-06-04 for transmission of subcarriers having different modulation formats.
The applicant listed for this patent is Infinera Corporation. Invention is credited to Ahmed AWADALLA, Abdullah KARAR, Han Henry SUN, Kuang-Tsan WU.
Application Number | 20200177282 16/532433 |
Document ID | / |
Family ID | 57205300 |
Filed Date | 2020-06-04 |
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United States Patent
Application |
20200177282 |
Kind Code |
A1 |
AWADALLA; Ahmed ; et
al. |
June 4, 2020 |
TRANSMISSION OF SUBCARRIERS HAVING DIFFERENT MODULATION FORMATS
Abstract
Consistent with the present disclosure, an optical communication
system is provided in which data is carried over optical signals
including subcarriers. The subcarriers may be modulated with the
standard modulation formats noted above, but the modulation formats
are selectively assigned to the subcarriers, such that some
subcarriers are modulated with different standard modulation
formats than others. As used herein, a "standard modulation format"
is one of BPSK, and n-QAM, where n is an integer greater than one.
Such n-QAM modulation formats include of 3-QAM, 4-QAM (QPSK),
8-QAM, 16-QAM, 64-QAM, 128-QAM, and 256-QAM. By selecting the
number of subcarriers and the types of modulation formats employed,
an optical signal with an effective SE that is between that of the
standard modulation formats can be generated for transmission over
a distances that more closely matches the link distance. Such
custom or intermediate SE signals can be tailored to a particular
optical link SNR to provide data transmission rates that are higher
than the low order modulation formats that would otherwise be
employed for optical signals carried by such links. As a result,
more efficient data transmission can be achieved.
Inventors: |
AWADALLA; Ahmed; (Gatineau,
CA) ; KARAR; Abdullah; (Kingston, CA) ; SUN;
Han Henry; (Ottawa, CA) ; WU; Kuang-Tsan;
(Kanata, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Infinera Corporation |
Sunnyvale |
CA |
US |
|
|
Family ID: |
57205300 |
Appl. No.: |
16/532433 |
Filed: |
August 5, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14986521 |
Dec 31, 2015 |
10374721 |
|
|
16532433 |
|
|
|
|
62154150 |
Apr 29, 2015 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04J 14/026 20130101;
H04J 14/0298 20130101; H04B 10/5161 20130101 |
International
Class: |
H04B 10/516 20060101
H04B010/516; H04J 14/02 20060101 H04J014/02 |
Claims
1-45. (canceled)
46. An apparatus, comprising: a laser that supplies an optical
signal; a modulator that receives the optical signal; and a
transmission circuit that supplies at least one electrical signal
to the modulator, the modulator modulating the optical signal based
on said at least one electrical signal to provide a plurality of
optical subcarriers, one of said plurality of optical subcarriers
having a first modulation format and a second one of the plurality
of optical subcarriers having a second modulation format different
than the first modulation form, each of the plurality of optical
subcarriers being a Nyquist pulse-shaped optical subcarrier, such
that each of the plurality of optical subcarriers does not
spectrally overlap with one another.
47. An apparatus in accordance with claim 46, wherein the first
modulation format is a binary phase shift keying (BPSK) modulation
format, and the second modulation format is an n-quadrature
amplitude modulation (QAM) modulation format, where n is an
integer.
48. An apparatus in accordance with claim 46, wherein the first
modulation format is a quadrature phase shift keying (QPSK)
modulation format, and the second modulation format is an
n-quadrature amplitude modulation (QAM) modulation format, where n
is an integer.
49. An apparatus in accordance with claim 46, wherein the first
modulation format is a first quadrature amplitude modulation (QAM)
modulation format and the second modulation format is a second QAM
modulation format.
50. An apparatus in accordance with claim 46, wherein the first and
second ones of the plurality of subcarriers have first and second
frequencies, respectively, and third and fourth ones of the
plurality of subcarriers having third and fourth frequencies,
respectively, the third and fourth frequencies are both higher than
the first frequency and less than the second frequency, such that
third and fourth subcarriers have modulation formats that are a
higher order than both the first and second modulation formats.
51. An apparatus in accordance with claim 46, wherein each of a
first group of the plurality of subcarriers has a respective one of
a first plurality of modulation formats, the first modulation
format being one of the first plurality of modulation formats, and
a second group of the plurality of subcarriers has a respective one
of a second plurality of modulation formats, the second modulation
format being one of the second plurality of modulation formats,
wherein each of the first group of the plurality of subcarriers has
a respective one of a first plurality of frequencies and each of
the second group of the plurality of subcarriers has a respective
one of a second plurality of frequencies, each of the plurality of
first frequencies being less than a frequency associated with the
optical signal supplied by the laser, and each of the plurality of
second frequencies being greater than the frequency associated with
the optical signal supplied by the laser.
52. An apparatus in accordance with claim 46, wherein the first and
second ones of the plurality of subcarriers have first and second
frequencies, respectively, the first and second frequencies being
less than a frequency of the optical signal supplied by the
laser.
53. An apparatus in accordance with claim 46, wherein the first and
second ones of the plurality of subcarriers have first and second
frequencies, respectively, the first and second frequencies being
greater than a frequency of the optical signal supplied by the
laser.
54. An apparatus in accordance with claim 46, wherein the first and
second ones of the plurality of subcarriers have first and second
frequencies, respectively, the first frequency being greater than a
frequency of the optical signal supplied by the laser, and the
second frequency being less than the frequency of the optical
signal supplied by the laser.
55. An apparatus in accordance with claim 54, wherein the first
modulation format has a higher order than the second modulation
format.
56. An apparatus in accordance with claim 54, wherein the first
modulation format has a lower order than the second modulation
format.
57. An apparatus in accordance with claim 50, wherein a power level
of the third and fourth ones of the plurality of subcarriers is
greater than a power level of the first and second ones of the
plurality of subcarriers.
58. An apparatus in accordance with claim 46, wherein the modulator
includes a plurality of Mach-Zehnder modulators.
59. An apparatus in accordance with claim 46, further including a
forward error correction encoder circuit that supplies encoded
data, wherein the electrical signal is based on the encoded
data.
60. An apparatus, comprising: a laser that supplies an optical
signal; a modulator that receives the optical signal; and a
plurality of engine circuits, each of which supplying a
corresponding one of a plurality of outputs, each of the plurality
of outputs being indicative of a corresponding one of a plurality
of modulation formats, a first one of the plurality of modulation
formats being different than a second one of the plurality of
modulation formats; a plurality of filter circuits, each of which
receiving a corresponding one of a plurality of inputs, each of the
plurality of inputs being indicative of a respective one of the
plurality of outputs, such that, based on each of the plurality of
inputs, each of the plurality of filter circuits supplies a
respective one of a plurality of filter outputs; and a plurality
driver circuits supplying a plurality of drive signals to the
modulator based on the plurality of filter outputs, the modulator
providing a plurality of optical subcarriers based on the plurality
of drive signals, one of said plurality of optical subcarriers
having the first one of the plurality of modulation formats and a
second one of the plurality of optical subcarriers having the
second one of the plurality of modulation formats, each of the
plurality of optical subcarriers being a Nyquist pulse-shaped
optical subcarrier, such that each of the plurality of optical
subcarriers does not spectrally overlap with one another.
61. An apparatus in accordance with claim 60, wherein the first
modulation format is a binary phase shift keying (BPSK) modulation
format, and the second modulation format is an n-quadrature
amplitude modulation (QAM) modulation format, where n is an
integer.
62. An apparatus in accordance with claim 60, wherein the first
modulation format is a quadrature phase shift keying (QPSK)
modulation format, and the second modulation format is an
n-quadrature amplitude modulation (QAM) modulation format, where n
is an integer.
63. An apparatus in accordance with claim 60, wherein the first
modulation format is a first quadrature amplitude modulation (QAM)
modulation format and the second modulation format is a second QAM
modulation format.
64. An apparatus in accordance with claim 60, further including a
forward error correction encoder circuit that supplies encoded
data, wherein the electrical signal is based on the encoded
data.
65. An apparatus, comprising: a demultiplexer circuit having a
plurality of first outputs; a plurality of engine circuits, each of
which supplying a corresponding one of a plurality of second
outputs based on a corresponding one of the plurality of first
outputs, each of the plurality of second outputs being indicative
of a corresponding one of a plurality of modulation formats, a
first one of the plurality of modulation formats being different
than a second one of the plurality of modulation formats; a
plurality of fast Fourier transform (FFT) circuits, each of which
supplying a corresponding one of a plurality of frequency domain
outputs based on a respective one of the plurality of second
outputs; and a plurality of filter circuits, each of which
supplying a corresponding one of a plurality of filter outputs
based on a respective one of the plurality of frequency domain
outputs.
66. An apparatus in accordance with claim 65, further comprising: a
plurality driver circuits supplying a plurality of drive signals
based on the plurality of filter outputs; a laser that supplies an
optical signal; and a modulator providing a plurality of optical
subcarriers based on the plurality of drive signals, a first one of
said plurality of optical subcarriers having the first one of the
plurality of modulation formats and a second one of the plurality
of optical subcarriers having the second one of the plurality of
modulation formats, each of the plurality of optical subcarriers
being a Nyquist pulse-shaped optical subcarrier, such that each of
the plurality of optical subcarriers does not spectrally overlap
with one another.
67. An apparatus in accordance with claim 65, wherein the first
modulation format is a binary phase shift keying (BPSK) modulation
format, and the second modulation format is an n-quadrature
amplitude modulation (QAM) modulation format, where n is an
integer.
68. An apparatus in accordance with claim 65, wherein the first
modulation format is a quadrature phase shift keying (QPSK)
modulation format, and the second modulation format is an
n-quadrature amplitude modulation (QAM) modulation format, where n
is an integer.
69. An apparatus in accordance with claim 65, wherein the first
modulation format is a first quadrature amplitude modulation (QAM)
modulation format and the second modulation format is a second QAM
modulation format.
70. An apparatus in accordance with claim 65, further including a
forward error correction encoder circuit that supplies encoded
data, wherein the demultiplexer receives a demultiplexer input, the
demultiplexer input being based on the encoded data.
71. An apparatus in accordance with claim 65, further including: a
multiplexer circuit that provides a multiplexed output based on the
plurality of filter outputs.
72. An apparatus in accordance with claim 71, further including: an
inverse FFT circuit that provides a time domain output based on the
multiplexed output.
73. An apparatus, comprising: a plurality of engine circuits, each
of which supplying a corresponding one of a plurality of outputs
based on a corresponding one of a plurality of inputs, the
plurality of inputs being based on data input to the apparatus,
each of the plurality of outputs being indicative of a
corresponding one of a plurality of modulation formats, a first one
of the plurality of modulation formats being different than a
second one of the plurality of modulation formats, the plurality of
modulation formats including: a binary phase shift keying (BPSK)
modulation format, a quadrature phase shift keying (QPSK)
modulation format, and at least one of n-quadrature amplitude
modulation (QAM) formats, where n is an integer, the first one of
the plurality of modulation formats and a second one of the
plurality of modulation formats for modulating first and second
optical subcarriers, respectively, each of the first and second
optical subcarriers being a Nyquist pulse-shaped optical
subcarrier, such that the first and second optical subcarriers do
not spectrally overlap with one another; and a plurality of filter
circuits, each of which supplying a corresponding one of a
plurality of filter outputs based on a respective one of the
plurality of outputs from the plurality of engine circuits.
Description
[0001] This application claims priority under 35 U.S.C. .sctn. 119
to U.S. Provisional Patent Application No. 62/154,150, filed on
Apr. 29, 2015, 2015, the content of which is incorporated by
reference herein in its entirety.
[0002] Optical communication systems are known in which data is
carried over amplitude/phase modulated optical signals, which are
transmitted along an optical fiber link to a receiver node. Such
optical signals may be transmitted in accordance with a variety of
standard modulation formats using polarization multiplexing (also
known as dual polarization), such as binary phase shift keying
(BPSK), 3-quadrature amplitude modulation (3-QAM), quadrature phase
shift keying (QPSK, or 4-QAM), 8-QAM, 16-QAM, 32-QAM, and 64-QAM,
with spectral efficiency (SE) of 2, 3, 4, 6, 8, 10, and
12b/dual-pol-symbol, respectively. Higher order modulation formats
have a high SE, as well as an associated constellation with points
that are relatively close to one another. Accordingly,
distinguishing such constellation points may be difficult,
especially if the high SE signal is transmitted over an optical
link that has an associated low signal-to-noise ratio (SNR) or has
other impairments. As a result, high SE signals are more
susceptible to noise and may have higher bit error rate for a given
SNR. On the other hand, low order modulation format signals have a
low SE, with associated constellation points that are relatively
far apart. Thus, transmission of such low SE (i.e., low order
modulation format) signals over a link with an associated SNR will
incur fewer errors than if high SE signals were transmitted. Put
another way, for a given SNR, high SE signals will incur more
errors and have a higher bit error rate (BER) than low SE
signals.
[0003] Thus, there is a tradeoff between capacity and reach. Lower
order modulation formats having a low SE can be transmitted farther
because fewer errors are incurred, but such lower order modulation
formats have fewer bits per symbol and thus less capacity. Higher
order modulation formats, on the other hand, have a higher SE, such
that more bits per symbol can be transmitted to provide greater
capacity. However, such higher order modulation formats are more
susceptible to errors, and thus, for a given power level, cannot be
transmitted over longer distances because the farther an optical
signal is transmitted the more errors will be incurred.
[0004] Thus, a given modulation format can be transmitted a certain
distance, which as noted above, is longer for lower order
modulation formats, and shorter for higher order modulation
formats. If the length of a particular link, however, is between
the transmission distances associated with one of the standard
modulation formats, the standard modulation format having a
transmission distance closest to that of the link, but having an
associated BER less than that associated with the link will be
selected. However, although relatively few errors will occur
because the selected standard modulation format transmission
distance is significantly more than the link distance, the capacity
of the selected standard modulation format will be less than that
which could be realized if an intermediate modulation format having
a transmission distance close to that of the link were
employed.
[0005] Although transmission with intermediate modulation formats,
such as 5-QAM (5b/dual-pol-symbol) and 7-QAM (7b/dual-pol-symbol),
other than the standard modulation formats, can be used, the
circuitry required to generate/decode such modulation formats is
complex and requires relatively high gate counts and high power in
implementation.
[0006] Thus, there is a need for optical communication systems that
can generate, without complex circuitry, optical communication
signals that have SEs between those of the standard modulation
formats, such as BPSK, QPSK, 3-QAM, 8-QAM, 16-QAM, 32-QAM and
64-QAM for example.
SUMMARY
[0007] Consistent with an aspect of the present disclosure, an
apparatus is provided that comprises a laser that supplies light
and a modulator that receives the light. In addition, a
transmission circuit is provided that supplies an electrical signal
to the modulator, the modulator modulating the light based on the
electrical signal to generate a modulated optical signal having
first and second pluralities of subcarriers. Each of the first
plurality of subcarriers has an associated first modulation format
and each of the second plurality of subcarriers has an associated
second modulation format, which is different than the first
modulation format.
[0008] Consistent with a first aspect of the present disclosure, an
apparatus is provided that includes a laser that supplies light,
and a modulator that receives the light. The apparatus further
includes a transmission circuit, such that, based on a plurality of
control signals, electrical signals are supplied to the modulator,
the modulator modulates the light to supply first and second
pluralities of subcarriers based on the electrical signals, each of
the first plurality of subcarriers having an associated first
modulation format and each of the second plurality of subcarriers
having an associated second modulation format, which is different
than the first modulation format.
[0009] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only and are not restrictive of the invention, as
claimed.
[0010] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate one (several)
embodiment(s) of the invention and together with the description,
serve to explain the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 illustrates a block diagram of an optical
communication system consistent with an aspect of the present
disclosure;
[0012] FIG. 2 illustrates a diagram of a transmit block, including
a transmission circuit, consistent with an additional aspect of the
present disclosure;
[0013] FIG. 3a illustrates a first portion of the transmission
circuit in greater detail;
[0014] FIG. 3b illustrates a Tx Engine circuit of FIG. 3a in
greater detail;
[0015] FIG. 4 illustrates a second portion of the transmission
circuit in greater detail;
[0016] FIG. 5a illustrates a portion of transmit photonic
integrated circuit consistent with the present disclosure;
[0017] FIG. 5b illustrates a spectrum of an optical signal output
from the transmit photonic integrated circuit shown in FIG. 5a;
[0018] FIG. 5c illustrates a spectrum of another optical signal
output from the transmit photonic integrated circuit shown in FIG.
5a in which the powers of individual sub-carriers are
non-uniform;
[0019] FIGS. 5d and 5e illustrates plots of bit error rate (BER)
and Q Factor, respectively, as a function of subcarrier power
ratios;
[0020] FIG. 6 illustrates a block diagram of a receive block
consistent with an aspect of the present disclosure;
[0021] FIG. 7 illustrates a portion of a receiver photonic
integrated circuit consistent with the present disclosure;
[0022] FIG. 8 illustrates a first portion of the receive block
shown in FIG. 6;
[0023] FIG. 9 illustrates a second portion of the receive block
shown in FIG. 6;
[0024] FIG. 10 illustrates a series of bit error rate (BER) vs.
signal-to-noise ratio (SNR) plots consistent with an aspect of the
present disclosure; and
[0025] FIG. 11 illustrates an additional example of a portion of a
transmission circuit consistent with the present disclosure.
DESCRIPTION OF THE EMBODIMENTS
[0026] Consistent with the present disclosure, an optical
communication system is provided in which data is carried over
optical signals including subcarriers. The subcarriers may be
modulated with the standard modulation formats noted above, but the
modulation formats are selectively assigned to the subcarriers,
such that some subcarriers are modulated with different standard
modulation formats than others. As used herein, a "standard
modulation format" is one of BPSK, and n-QAM, where n is an
integer. Such n-QAM modulation formats include of 3-QAM, 4-QAM
(QPSK), 8-QAM, 16-QAM, 64-QAM, 128-QAM, and 256-QAM. By selecting
the number of subcarriers and the types of modulation formats
employed, an optical signal with an effective SE that is between
that of the standard modulation formats can be generated for
transmission over a distances that more closely matches the link
distance. Such custom or intermediate SE signals can be tailored to
a particular optical link SNR to provide data transmission rates
that are higher than the low order modulation formats that would
otherwise be employed for optical signals carried by such links. As
a result, more efficient data transmission can be achieved.
[0027] Various circuits and techniques for generating and
processing optical signals including subcarriers are described in
the following: U.S. Patent Application Publication No. 2014/0092924
titled "Channel Carrying Multiplexed Digital Subcarriers"; U.S.
Patent Application Publication No. 2015/0280834 titled "Frequency
And Phase Compensation For Modulation Formats Using Multiple
Sub-Carriers"; U.S. Patent Application Publication No.
2015/0280853, titled "Configurable Frequency Domain Equalizer for
Dispersion Compensation of Multiple Sub-Carriers"; U.S. patent
application Ser. No. 14/788,564, filed Jun. 30, 2015, and titled
"Feedback Carrier Recovery Device"; and U.S. patent application
Ser. No. 14/754,521, filed Jun. 29, 2015, and titled "Frequency
Domain Coded Modulation With Polarization Interleaving For Fiber
Nonlinearity Mitigation In Digital Sub-Carrier Coherent Optical
Communication Systems." The entire contents of each of the
foregoing are incorporated herein by reference. Reference will now
be made in detail to the present exemplary embodiments of the
present disclosure, which are illustrated in the accompanying
drawings. Wherever possible, the same reference numbers will be
used throughout the drawings to refer to the same or like
parts.
[0028] FIG. 1 illustrates an optical link of optical communication
system 100 consistent with an aspect of the present disclosure.
Optical communication system 100 includes a plurality of
transmitter blocks (Tx Block) 12-1 to 12-n provided in a transmit
node 11. Each of transmitter blocks 12-1 to 12-n receives a
corresponding one of a plurality of data or information streams
Data-1 to Data-n, and, in response to a respective one of these
data streams, each of transmitter blocks 12-1 to 12-n may output a
group of optical signals or channels to a combiner or multiplexer
14. Each optical signal carries an information stream or data
corresponding to each of data streams Data-1 to Data-n. Multiplexer
14, which may include one or more optical filters, for example,
combines each of group of optical signals onto optical
communication path 16. Optical communication path 16 may include
one or more segments of optical fiber and optical amplifiers, for
example, to optically amplify or boost the power of the transmitted
optical signals.
[0029] As further shown in FIG. 1, a receive node 18 is provided
that includes an optical combiner or demultiplexer 20, which may
include one or more optical filters, for example, optical
demultiplexer 20 supplies each group of received optical signals to
a corresponding one of receiver blocks (Rx Blocks) 22-1 to 22-n.
Each of receiver blocks 22-1 to 22-n, in turn, supplies a
corresponding copy of data or information streams Data-1 to Data-n
in response to the optical signals. It is understood that each of
transmitter blocks 12-1 to 12-n has the same or similar structure
and each of receiver blocks 22-1 to 22-n has the same or similar
structure.
[0030] FIG. 2 illustrates one of transmitter blocks 12-1 in greater
detail. Transmitter block 12-1 may include a digital signal
processor (DSP) 202 including circuitry or circuit blocks CB1-1 to
CB1-10, each of which receiving, for example, a corresponding
portion of Data-1 and supplying a corresponding one of outputs or
electrical signals 202-1 to 202-10 to a circuit, such as
application specific integrated circuit (ASIC) 204. ASIC 204
include circuit blocks CB2-1 to CB2-10, which supply corresponding
outputs or electrical signals 204-1 to 204-10 to optical sources or
transmitters OS-1 to OS-2 provided on transmit photonic integrated
circuit (PIC) 205. As further shown in FIG. 2, each of optical
sources OS-1 to OS-2 supplies a corresponding one of modulated
optical signals having wavelengths .lamda.1 to .lamda.10,
respectively. The optical signals are combined by an optical
combiner or multiplexer, such as arrayed waveguide grating (AWG)
208, for example, and combined into a band or group of optical
signals supplied by output 206-1. Alternatively, a known optical
power multiplexer may be provided to combine the optical signals.
Optical sources OS-1 to OS-10 and multiplexer 208 may be provided
on substrate 205, for example. Substrate 205 may include indium
phosphide or other semiconductor materials. Although FIG. 2
illustrates ten circuit blocks CB1-1 to CB1-10, ten circuit blocks
CB2-1 to CB2-10, and ten optical sources OS1-1 to OS-10, it is
understood that any appropriate number of such circuit blocks and
optical sources may be provided. Moreover, it is understood, that
optical sources OS-1 to OS-10, as well as multiplexer 208, may be
provided as discrete components, as opposed to being integrated
onto substrate 205 as PIC 206. Alternatively, selected components
may be provided on a first substrate while others may be provided
on one or more additional substrates in a hybrid scheme in which
the components are neither integrated onto one substrate nor
provided as discrete devices.
[0031] DSP and ASIC 202 collectively constitute transmission
circuit 1 that supply drive signals (electrical signals) to the
modulators in optical source OS-1 as well as the remaining optical
sources.
[0032] FIG. 3a illustrates a portion of transmission circuit 1,
namely, circuit block CB1-1 of DSP 202 in greater detail. Circuit
block CB-1 includes a Forward Error Correction (FEC) encoder 302
that receives data stream Data-1 and encodes the data stream to
provide input data Data In at a rate R. The same FEC encoder engine
is used to encode data associated with each subcarrier. Data In is
supplied to demultiplexer circuit 304 which includes a plurality of
switches that supply portions of Data In at respective rates R0 to
Rm-1 to a corresponding one of engine circuits 306-1 to 306-m.
Typically, the average of R0 to Rm-1 (namely, (R0+Rm-1)/m) is equal
to rate R.
[0033] Each engine circuit (collectively referred to as "306")
supplies a digitized analog signal (SC0 to SCm-1) that is
representative of a respective one of a plurality of subcarriers of
an optical signal which is ultimately output from an optical
source, as discussed in greater detail below. Each of the digitized
analog signals is next fed to a corresponding one of fast Fourier
transform (FFT) circuits 308-1 to 308-m to convert each such signal
into a respective frequency domain signal. Each frequency domain
signal is then subject to filtering by a corresponding one of
filters 310-1 to 310-m. In one example, a power associated with
each frequency domain signal is adjusted for optimal performance to
provide greater energy or power to high order modulation format
subcarriers which may be more prone to errors and less power to low
order modulation format subcarriers which are less susceptible to
errors. Put another way, particular powers are assigned to each
subcarrier, such that the average bit error rate (BER) of all
subcarriers is optimized to have a lowest value possible. That is,
each higher order subcarrier and each lower order subcarrier, for
example, has a respective one of a plurality of bit error rates
(BERs). The average BER, i.e., an average of the BERs over all the
high and lower order subcarriers, has a lower value than if the
powers of the subcarriers were not optimized as described above.
Such minimum average BER can be obtained when the BER of each
subcarrier is substantially the same or uniform.
[0034] In one example, digitized analog signals corresponding to
lower order modulation formats, such as BPSK, preferably have an
associated power that is less than a power associated with
digitized analog signals corresponding to higher order modulation
format, such as 8-QAM. The filtered or power adjusted frequency
domain signals are next input to a multiplexer 312 that distributes
the power adjusted frequency domain signals over each of four
outputs 314. These frequency domain signals are then converted back
to the time domain by inverse fast Fourier transform circuit (IFFT)
316 and, the resulting time domain signals are next supplied on
outputs 318, each of which being coupled or connected to a
respective one of digital to analog converters (DACs) 410, 412,
414, and 416. Spectrum 317 is a representation of digitized analog
subcarrier signals SC0 to SCm-1 in the frequency domain prior to
input to IFFT 316, and output waveform 319 form IFFT 316 is a
representation of the subcarrier signals in the time domain.
[0035] As further shown FIG. 3a, each of the Tx Engine circuits
receives a corresponding one of a plurality of control signals.
Selection of a desired modulation format by the Tx Engine circuits
will next be described with reference to FIG. 3b, which shows Tx
Engine circuit 306-1 in greater detail. Remaining Tx Engine
circuits 306-2 to 306m-1 have the same or similar structure as Tx
Engine circuit 306-1.
[0036] As shown in FIG. 3b, a control signal may be supplied to
switch 320, which may be implemented in either firmware or
software. In response to the control signal one, switch 320 directs
the data to one of modulation format circuit 321 to 327, and the
selected modulation format circuit is activated to supply a
digitized analog signal associated with a selected modulation
format, which in this example, is one of BPSK, 3-QAM, QPSK, 8-QAM,
16-QAM, 32, QAM, and 64-QAM). The control signal may further be
employed to control switch 328 to direct the digitized analog
signal to FFT 308-1 from the selected one of modulation format
circuits 321 to 327. Accordingly, for example, if an optical link
has a particular SNR which causes a given number of bit errors to
occur during propagation along the length of the link, a
combination of subcarrier modulation formats can be selected, such
that the effective BER associated with an optical signal carrying
such subcarriers approximates the BER of the link, and thus the
transmission distance of an optical signal including such
subcarriers more closely approximates the link distance. As such,
an intermediate SE that provides maximum data transmission rate can
be obtained for the link. As noted above, in the conventional
approach, a standard transmission format would be employed for such
link having a low SE that yields a capacity that is less than that
associated with the intermediate SE described above. An expression
for determining an intermediate SE of an optical signal including m
subcarriers will next be presented. If M is the total number of
subcarriers, A is the number of subcarriers with modulation format
1, SE=X1 for modulation format 1, B is the number of subcarriers
with modulation format 2, and SE=X2 for modulation format Y, an
average SE (SEavg) of the optical signal satisfies:
SEavg=(A/M)*X1+(B/M)*X2 (Eq. 1),
where M is a sum of A+B, A being a number of the first plurality of
subcarriers and B being a number of the second plurality of
subcarriers. Accordingly, there are M-1 additional SEs between the
two available ones (X1 and X2). Such additional SEs can be realized
without complex hardware requiring high gate counts or a higher
power in implementation.
[0037] The above expression for SEavg is for when there are two
modulation formats. A general expression for the average SE is:
SEavg=(A1/M)*X1+(A2/M)*X2+(A3/M)*X3+ . . . (An/M)*Xn, (Eq. 2)
where M is the total number of subcarriers, A1 is the number of
subcarriers with modulation format 1, SE=X1 for modulation format
1, A2 is the number of subcarriers with modulation format 2, SE=X2
for modulation format 2, etc., An is the number of subcarriers with
the nth modulation format, and SE=Xn for the nth modulation format.
Put another way,
SEavg = 1 M n = 1 M Xn . ( Eq . 3 ) ##EQU00001##
[0038] If all the subcarriers are passed to one single FEC circuit,
as noted above, the equivalent BER is the mean BER of all the
different subcarriers.
[0039] Turning to FIG. 4, DACs 410 and 412 receive a respective one
of a pair of the outputs 318 from IFFT 316. DACs 410 and 412, in
turn, output corresponding analog signals, which are filtered by
low-pass or roofing filters 418 and 420 to thereby remove, block or
substantially attenuate higher frequency components in these analog
signals. Such high frequency components or harmonics are associated
with sampling performed by DACs 410 and 412 and are attributable to
known "aliasing." The analog signals output from DACs 414 and 416
are similarly filtered by roofing filters 422 and 424,
respectively. The filtered analog signals output from roofing
filters 418, 420 422, and 424 may next be fed to corresponding
driver circuits 426, 428, 430, and 432, which supply modulator
driver signals that have a desired current and/or voltage for
driving modulators present in PIC 206 to provide optical signals
with the desired subcarriers noted above. PIC 206 is discussed in
greater detail below with reference to FIG. 5a.
[0040] FIG. 5a illustrates transmitter or optical source OS-1 in
greater detail. It is understood that remaining optical sources
OS-1 to OS-10 have the same or similar structure as optical source
OS-1.
[0041] Optical source OS-1 may be provided on substrate 205 and may
include a laser 508, such as a distributed feedback laser (DFB)
that supplies light to at least four (4) modulators 506, 512, 526
and 530. DFB 508 may output continuous wave (CW) light at
wavelength .lamda.1 to a dual output splitter or coupler 510 (e.g.
a 3 db coupler) having an input port and first and second output
ports. Typically, the waveguides used to connect the various
components of optical source OS-1 may be polarization dependent. A
first output 510a of coupler 510 supplies the CW light to first
branching unit 511 and the second output 510b supplies the CW light
to second branching unit 513. A first output 511a of branching unit
511 is coupled to modulator 506 and a second output 511b is coupled
to modulator 512. Similarly, first output 513a is coupled to
modulator 526 and second output 513b is coupled to modulator 530.
Modulators 506, 512, 526 and 530 may be, for example, Mach Zehnder
(MZ) modulators. Each of the MZ modulators receives CW light from
DFB 508 and splits the light between two (2) arms or paths. An
applied electric field in one or both paths of a MZ modulator
creates a change in the refractive index to induce phase and/or
amplitude modulation to light passing through the modulator. Each
of the MZ modulators 506, 512, 526 and 530, which collectively can
constitute a nested modulator, are driven with data signals or
drive signals supplied via driver circuits 426, 428, 430, and 432,
respectively. The CW light supplied to MZ modulator 506 via DFB 508
and branching unit 511 is modulated in accordance with the drive
signal supplied by driver circuit 426. The modulated optical signal
from MZ modulator 506 is supplied to first input 515a of branching
unit 515. Similarly, driver circuit 328 supplies further drive
signals for driving MZ modulator 512. The CW light supplied to MZ
modulator 512 via DFB 508 and branching unit 511 is modulated in
accordance with the drive signal supplied by driver circuit 428.
The modulated optical signal from MZ modulator 512 is supplied to
phase shifter 514 which shifts the phase of the signal 90.degree.
(.pi./2) to generate one of an in-phase (I) or quadrature (Q)
components, which is supplied to second input 515b of branching
unit 515. The modulated data signals from MZ modulator 506, which
include the remaining one of the I and Q components, and the
modulated data signals from MZ modulator 512, are supplied to
polarization beam combiner (PBC) 538 via branching unit 515.
[0042] Modulator driver 430 supplies a third drive signal for
driving MZ modulator 526. MZ modulator 526, in turn, outputs a
modulated optical signal as either the I component or the Q
component. A polarization rotator 524 may optionally be disposed
between coupler 510 and branching unit 513. Polarization rotator
524 may be a two port device that rotates the polarization of light
propagating through the device by a particular angle, usually an
odd multiple of 90.degree.. The CW light supplied from DFB 508 is
rotated by polarization rotator 524 and is supplied to MZ modulator
526 via first output 513a of branching unit 513. MZ modulator 526
then modulates the polarization rotated CW light supplied by DFB
508, in accordance with drive signals from driver circuit 430. The
modulated optical signal from MZ modulator 526 is supplied to first
input 517a of branching unit 517.
[0043] A fourth drive signal is supplied by driver 432 for driving
MZ modulator 530. The CW light supplied from DFB 508 is also
rotated by polarization rotator 524 and is supplied to MZ modulator
530 via second output 513b of branching unit 513. MZ modulator 530
then modulates the received optical signal in accordance with the
drive signal supplied by driver 432. The modulated data signal from
MZ modulator 530 is supplied to phase shifter 528 which shifts the
phase the incoming signal 90.degree. (.pi./2) and supplies the
other of the I and Q components to second input 517b of branching
unit 517. Alternatively, polarization rotator 536 may be disposed
between branching unit 517 and PBC 538 and replaces rotator 524. In
that case, the polarization rotator 536 rotates both the modulated
signals from MZ modulators 526 and 530 rather than the CW signal
from DFB 508 before modulation. The modulated data signal from MZ
modulator 526 is supplied to first input port 538a of polarization
beam combiner (PBC) 538. The modulated data signal from MZ
modulator 530 is supplied to second input port 538b of polarization
beam combiner (PBC) 538. PBC 538 combines the four modulated
optical signals from branching units 515 and 517 and outputs a
multiplexed optical signal having wavelength .lamda.1 to output
port 538c. In this manner, one DFB laser 508 may provide a CW
signal to four separate MZ modulators 506, 512, 526 and 530 for
modulating at least four separate optical channels by utilizing
phase shifting and polarization rotation of the transmission
signals. Although rotator 536 and PBC 538 are shown on the PIC, it
is understood that these devices may instead be provided
off-PIC.
[0044] In another example, splitter or coupler 510 may be omitted
and DFB 508 may be configured as a dual output laser source to
provide CW light to each of the MZ modulators 506, 512, 526 and 530
via branching units 511 and 513. In particular, coupler 510 may be
replaced by DFB 508 configured as a back facet output device. Both
outputs of DFB laser 508, from respective sides 508-1 and 508-2 of
DFB 508, are used, in this example, to realize a dual output signal
source. A first output 508a of DFB 508 supplies CW light to
branching unit 511 connected to MZ modulators 506 and 512. The back
facet or second output 508b of DFB 508 supplies CW light to
branching unit 513 connected to MZ modulators 526 and 530 via path
or waveguide 543 (represented as a dashed line in FIG. 5a). The
dual output configuration provides sufficient power to the
respective MZ modulators at a power loss far less than that
experienced through 3 dB coupler 510. The CW light supplied from
second output 508b is supplied to waveguide 543 which is either
coupled directly to branching unit 513 or to polarization rotator
524 disposed between DFB 508 and branching unit 513. Polarization
rotator 524 rotates the polarization of CW light supplied from
second output 508b of DFB 508 and supplies the rotated light to MZ
modulator 526 via first output 513a of branching unit 513 and to MZ
modulator 530 via second output 513b of branching unit 513.
Alternatively, as noted above, polarization rotator 524 may be
replaced by polarization rotator 536 disposed between branching
unit 517 and PBC 538. In that case, polarization rotator 536
rotates both the modulated signals from MZ modulators 526 and 530
rather than the CW signal from back facet output 508b of DFB 508
before modulation.
[0045] The polarization multiplexed output from PBC 538, may be
supplied to multiplexer 208 in FIG. 2, along with the polarization
multiplexed outputs having wavelength .lamda.2 to .lamda.10 from
remaining optical sources OS-2 to OS-m. Multiplexer 208, which, as
noted above, may include an AWG 204, supplies a group of optical
signals to multiplexer 14 (see FIG. 1). It is understood that PICs
present in transmitter blocks 12-2 to 12-n operate in a similar
fashion and include similar structure as PIC 206 shown in FIGS. 2
and 5 to provide optical signal including subcarriers having
different modulation formats to provide a desired SE, as noted
above.
[0046] Thus, by selecting digitized analog signal corresponding
different modulation formats by applying appropriate control
signals to Tx Engines 306-1 to 306-m, respective drive signals are
applied to the nested MZ modulator shown in FIG. 5a, such that an
optical signal including subcarriers modulated in accordance with
the modulation formats associated with the selected digitized
analog signals is output from the PIC. The optical signal thus
generated also has a desired SE, as noted above. As further noted
above, in one example, certain subcarriers may have a first
modulation format while others have a second modulation format in
response to first control signals. Consistent with an aspect of the
present invention, the first and second modulation formats are
different from one another and are selected from the group of
standard modulation formats: BPSK and n-QAM, where n is an integer
greater than 1, such that n-QAM modulation formats includes 3 QAM,
4 QAM (QPSK), 8 QAM, 16 QAM, 32 QAM, 64-QAM, 128-QAM, and 256-QAM.
In another aspect of the present disclosure, the first modulation
format is an N-QAM modulation format, where N is a first integer,
and the second modulation format is an M-QAM modulation format,
where M is a second integer that is less than the first integer.
The number of subcarriers, such as SC0 to SCm-1, having the first
modulation format may be the same or different than the number of
subcarriers having the second modulation format. Consistent with an
additional aspect of the present disclosure, third and fourth
control signals may also be applied to the Tx Engines 306-1 to
306-m so that third and fourth modulation formats may be applied to
third and fourth groups of the subcarrier. Here also, the third and
fourth modulation formats may be selected from the standard
modulation formats, and number of subcarriers in the third group
may be the same or different than the number of subcarriers in the
fourth group. In addition, the third and fourth modulation formats
may be different from one another, as well as different from the
first and second modulation formats. Further, the subcarriers may
include first, second, third and fourth groups of subcarriers
having different modulation formats from one another based on
appropriate application of control signals. It is understood, that
the combinations of modulation formats discussed above is exemplary
only, and that any appropriate combination of modulation formats
and number of subcarriers can be employed.
[0047] FIG. 5b illustrates an examples of a spectrum associated
with an optical signal output from optical source OS-1. Here, the
optical signal includes four subcarriers (SC0-SC3). Subcarriers SC0
and SC3 may have a first modulation format, such as, QPSK, and
second subcarriers, such as subcarriers SC1 and SC2, may have a
second modulation format, such as 8 QAM to provide an effective or
average SE that is between that associated with QPSK and 8 QAM. As
generally understood, subcarriers may be generated by modulating a
carrier frequency (e.g., f0 in FIG. 5b having a zero baseband
frequency), which is the frequency of the light output from the
laser to the modulator discussed above with reference to FIG. 5a,
as opposed to modulating individual carriers supplied from
respective lasers. In addition, subcarriers associated with the
same carrier frequency may be encoded with a common or shared FEC
encoder engine or circuit, as well as decoded with a shared FEC
decoder engine or circuit.
[0048] FIG. 5c illustrates an example in which power of certain
subcarrier has been adjusted to optimize or obtain a lowest average
bit error rate (BER) for a particular combination of subcarriers
and modulation formats. In this example, inner subcarriers SC1 and
SC2 are modulated in accordance with a 16 QAM modulation format,
and outer subcarriers SC0 and SC3 are modulated in accordance with
a QPSK modulation format. As shown in FIG. 5c, inner subcarriers
having a higher order modulation format have a higher power, in
this example, relative to the outer subcarriers. It is understood,
however, that various devices described herein may impart excessive
loss or noise, for example, even to subcarriers having a lower
order modulation format. In such instances, such subcarriers having
a lower order modulation format may have a power that exceeds that
of subcarriers having a higher order modulation format and
transmitted with the lower order modulation format subcarriers.
[0049] In another example, the following modulation formats and
corresponding SEs are available for polarization multiplexed (PM)
subcarriers: PM-BPSK(SE=2), PM-3 QAM(SE=3), PM-QPSK(SE=4), PM-8
QAM(SE=6), and PM-16 QAM(SE=8). Various combinations of subcarriers
modulated in accordance with two or more of these modulation
formats can yield optical signals that can have one of (12) SEs
without extra hardware or power. The optical signal SE (obtained
from particular combinations of subcarrier modulation formats) can
be selected from one of: 2.25 2.5 2.75 3.25 3.5 3.75 4.5 5 5.5 6.5
7 7.5. These SE values are calculated based on the SE expression
noted above.
[0050] An example of power optimization of the subcarriers shown in
FIG. 5c will next be described with reference to FIGS. 5d and 5e.
Preferably, a ratio of the power of the subcarriers having a first
modulation format, such as 8 QAM, to the power of the subcarriers
having a second modulation format, such as QPSK, is adjusted so
that the average BER (BERavg) of the optical signal is at a minimum
(see FIG. 5d, which shows the Optimum Power Ratio and corresponding
Minimum BER). It is noted that for a fixed total power for the
optical signal, at lower power ratios, such as at 0 dB, the power
of 8 QAM subcarriers is the same as the QPSK subcarriers, resulting
in the 8 QAM subcarriers having a higher BER. Accordingly, the
average BER is higher for such low power ratios. On the other hand,
if the power ratio is high (to the right in FIG. 5d), the 8 QAM
subcarriers have high power and the QPSK subcarriers have
relatively low power. As such, the QPSK subcarriers incur a
significant number of bit errors, and the average BER is high under
these circumstances as well. In one example, the optimum power
ratio may be 3.7 dB and, in another example, the optimum power
ratio is 3.3 dB. As noted above, the system SNR may impact the
optimum power ratio.
[0051] Put another way, the modulator is capable of outputting each
of the n pluralities of subcarriers (n being greater than 1) at a
plurality of powers, such that the modulator supplies a first one
of the plurality of n pluralities of subcarriers at a first one of
the plurality of powers and a second one of the n pluralities of
subcarriers at a second one of the plurality of powers. A ratio of
the first and second powers is one of a plurality of ratios of each
of the plurality of powers to one another and is associated with a
Q value of the optical signal that is greater than Q values
associated with remaining ones of the plurality of ratios, or an
average BER that is less than an average BER associated with
remaining ones of the plurality of ratios.
[0052] Preferably, the modulator is controlled to output an optical
signal with subcarriers having power levels and a corresponding
power ratio that yields a minimum BERavg.
[0053] In one example, BERavg satisfies:
BERavg = 1 n = 1 M Xn n = 1 M Xn BERn ( Eq . 4 ) ##EQU00002##
where BERn is the bit error rate of subcarriers having the nth
modulation format, and Xn, as noted above, is the spectral
efficiency of such nth modulation format. In a case in which first
and second pluralities of subcarriers are provided, each such
plurality being modulated in accordance with first and second
modulation formats, respectively, Eq. 4 can be written as:
BERavg=[X1*BER1+X2*BER2]/(X1+X2) (Eq. 5)
where BER1 and BER2 are the bit error rates of the first and second
pluralities of subcarriers, respectively, X1 is the spectral
efficiency (SE) of the first modulation format, and X2 is the SE of
the second modulation format. In the example discussed above with
respect to FIG. 5c in which the first and second subcarriers are
modulated based on 8QAM (e.g., first) and QPSK (e.g., second)
modulation formats, respectively,
[0054] BER1 and BER2 are the bit error rates of the polarization
multiplexed (PM)-8 QAM and PM-QPSK subcarriers, respectively, the
spectral efficiency of PM-8 QAM is 6, and the spectral efficiency
of QPSK or 4. Substituting these values into Equation 5:
BERavg=[6*BER1+4*BER2]/(6+4) (Eq. 6)
or
BERavg=[3*BER1+2*BER2]/5 (Eq. 7)
[0055] As generally understood, BER is inversely related to a
Quality (Q) Factor. For example, for binary modulation formats,
such as BPSK and QPSK, BER=0.5*erfc(Q/ 2), where Q (in dB) is
20*Log 10(Q).
[0056] Accordingly, as shown in FIG. 5e, at the optimum power
ratio, the Q Factor associated with the optical signal is at a
maximum.
[0057] In the above examples, a given signal-to-noise ratio (SNR)
is assumed. Different SNRs may result in different optimal power
ratios and different maximum Q Factors, as well as corresponding
minimum values for BERavg.
[0058] In another example, minimum BERavg may be obtained when the
BER of each individual subcarrier is substantially the same. In
addition, the powers of the subcarriers may be set or adjusted so
that the BER of each subcarrier is preferably within a range of
.+-.20% of the average BER (BERavg), as calculated in accordance
with equations Eq. 4-7. In another example, the powers are selected
so that the BER of each subcarrier is more preferably within a
range of .+-.15% of the average BER, and, in another example, the
subcarrier powers are such that the BER of each subcarrier is
preferably within a range of .+-.15%. And, in further preferred
embodiments, the subcarrier powers are such that the BER of each
subcarrier is preferably within a range of .+-.10% of the average
BER and even more preferably within a range of .+-.5% of the
average BER. By adjusting the subcarrier powers to be close to the
average BER or close to having the same powers, the average BER is
reduced and can approximate the minimum BER. Improved performance
can thus be achieved compared to an optical signal in which the
BERs fall outside a range of 20% of the average BER, for example.
In addition, the powers of the subcarriers discussed above, such as
in regard to FIG. 5c, may yield BERs that fall within the above
noted 5% to 20% ranges about the average BER.
[0059] It is noted that the optical signals disclosed herein are
typically not orthogonal frequency division multiplexed (OFDM)
optical signals. The spectra of subcarriers in such OFDM optical
signals typically overlap with one another (due to modulation based
on time-domain rectangular pulses that ensure orthogonality). The
subcarriers disclosed herein, however, do not spectrally overlap
(due to Nyquist pulse shaping used in generating the subcarriers),
as shown in FIGS. 5b and 5c, for example.
[0060] As noted above, optical signals output from transmitter
block 12-1 are combined with optical signals output from remaining
transmitter blocks 12-2 to 12-n onto optical communication path 16
and transmitted to receive node 18 (see FIG. 1). In receive node
18, demultiplexer 20 divides the incomings signal into optical
signal groupings, such that each grouping is fed to a corresponding
one of receiver blocks 22-1 to 22-n.
[0061] One of receiver blocks 22-1 is shown in greater detail in
FIG. 6. It is understood that remaining receiver circuitry or
blocks 22-2 to 22-n have the same or similar structure as receiver
block 22-1.
[0062] Receiver block 22-1 includes a receive PIC 602 provided on
substrate 604. PIC 602 includes an optical power splitter 603 that
receives optical signals having wavelengths .lamda.1 to .lamda.10,
for example, and supplies a power split portion of each optical
signal (each of which itself may be considered an optical signal)
to each of optical receivers OR-1 to OR-10. Each optical receiver
OR-1 to OR-10, in turn, supplies a corresponding output to a
respective one of circuit blocks CB3-1 to CB3-10 of ASIC 606, and
each of circuit blocks CB3-1 to CB3-10, supplies a respective
output to a corresponding one of circuit blocks CB4-1 to CB4-10 of
DSP 608. DSP 608, in turn, outputs a copy of data Data-1 in
response to the input to circuit blocks CB4-1 to CB4-10.
[0063] Optical receiver OR-1 is shown in greater detail in FIG. 7.
It is understood that remaining optical receivers OR-2 to OR-10
have the same or similar structure as optical receiver OR-1.
Optical receiver OR-1 may include a polarization beam splitter
(PBS) 702 operable to receive polarization multiplexed optical
signals .lamda.1 to .lamda.10 and to separate the signal into X and
Y orthogonal polarizations, i.e., vector components of the optical
E-field of the incoming optical signals transmitted on optical
communication path 16, which may include an optical fiber, for
example. The orthogonal polarizations are then mixed in 90 degree
optical hybrid circuits ("hybrids") 720 and 724 with light from
local oscillator (LO) laser 701 having wavelength .lamda.1. Hybrid
circuit 720 outputs four optical signals O1a, O1b, O2a, O2b and
hybrid circuit 724 outputs four optical signals O3a, O3b, O4a, and
O4b, each representing the in-phase and quadrature components of
the optical E-field on X (TE) and Y (TM) polarizations, and each
including light from local oscillator 701 and light from
polarization beam splitter 702. Optical signals O1a, O1b, O2a, O2b,
O3a, O3b, O4a, and O4b are supplied to a respective one of
photodetector circuits 709, 711, 713, and 715. Each photodetector
circuit includes a pair of photodiodes (such as photodiodes 709-1
and 709-2) configured as a balanced detector, for example, and each
photodetector circuit supplies a corresponding one of electrical
signals E1, E2, E3, and E4. Alternatively, each photodetector may
include one photodiode (such as photodiode 709-1) or single-ended
photodiode. Electrical signals E1 to E4 are indicative of data
carried by optical signals .lamda.1 to .lamda.10 input to PBS 702
demodulated with LO 701 (.lamda.1). For example, these electrical
signals may comprise four base-band analog electrical signals
linearly proportional to the in-phase and quadrature components of
the optical E-field on X and Y polarizations.
[0064] FIG. 8 shows circuitry or circuit blocks CB3-1 and CB4-1 in
greater detail. It is understood that remaining circuit blocks
CB3-2 to CB3-10 of ASIC 606 have a similar structure and operate in
a similar manner as circuit block CB3-1. In addition, it is
understood that remaining circuit blocks CB4-2 to CB4-10 of DSP 608
have a similar structure and operation in a similar manner as
circuit block CB4-1.
[0065] Circuit block CB3-1 includes known transimpedance amplifier
and automatic gain control (TIA/AGC 802) circuitry 802, 804, 806,
and 808 that receives a corresponding one of electrical signals E1,
E2, E3, and E4. Circuitry 802, 804, 806, and 808, in turn, supplies
corresponding electrical signals or outputs to respective ones of
anti-aliasing filters 810, 812, 814, and 816, which, constitute low
pass filters that further block, suppress, or attenuate high
frequency components due to known "aliasing". The electrical
signals or outputs form filters 810, 812, 814, and 816 are then
supplied to corresponding ones of analog-to-digital converters
(ADCs) 818, 820, 822, and 824.
[0066] ADCs 818, 820, 822, and 824, may sample at the same or
substantially the same sampling rate as DACs 310, 312, 314, and 316
discussed above. Preferably, however, circuit block CB4-1 and DSP
608 have an associated sampling rate that is less than the DAC
sampling rate, as described in greater detail in U.S. patent
application Ser. No. 12/791,694 titled "Method, System, And
Apparatus For Interpolating An Output Of An Analog-To-Digital
Converter", filed Jun. 1, 2010, the entire contents of which are
incorporated herein by reference.
[0067] Turning to FIG. 9, outputs from ADCs 818, 820, 822, and 824
are supplied to FFT 902, which converts these outputs to the
frequency domain. The frequency domain signals correspond to
subcarriers SCO to SCm-1 are input to a demultiplexer, which
provides each subcarrier representation to a corresponding one of
chromatic dispersion (CD) and polarization mode dispersion (PMD)
equalization circuits 903-1 to 903-m. Each output of CD and PM Eq
circuits 903-1 to 903-m is fed to a respective one of IFFTs 904-1
to 904-m. Based on the received inputs to the IFFTs, each IFFT
supplies a corresponding time domain signal to a respective one of
Rx Engine circuits 906-1 to 906-m. Each of the Rx Engine circuits,
in turn, may decode the received time domain signal in accordance
with the modulation format associated with such signal. Each Rx
Engine circuit may also be controlled by a user to accommodate an
associated modulation format. The outputs of each Rx Engine circuit
is a copy of a portion of Data In, at a respective one of data
streams R0 to Rm-1. The data portions are supplied to a multiplexer
908, which combines the portions to provide a copy of the Data In
at rate R (shown as Data Out in FIG. 9). Data Out is supplied to
FEC decoder circuit 910, which performs error correction on Data
Out and supplies a copy of the data stream input to the system in
FIG. 1. As noted above, the same FEC engine or a common FEC engine
is used to decode data carried by each subcarrier.
[0068] FIG. 10 illustrates a series of plots 1000 of BER vs. SNR
for various SEs. SEs of 2, 4, and 8 [b/dual-pol-symbol] correspond
to standard modulation formats noted above. As further seen in FIG.
10, plots between those labeled SE=2, SE=4, and SE=8 are associated
with intermediate modulation formats and SEs that are obtained by
combining combinations of the standard modulation format
subcarriers, as discussed above. One such plot is associated with
an SE of 6, for example.
[0069] It is noted that electrical signals associated with the
subcarriers may experience more loss at higher frequencies than
lower frequencies as such electrical signals propagate in
transmission lines and traces in DSPs 202 and 608, ASICs 204 and
606, as well as in electrical connections to various devices on the
transmit and receive PICs discussed above. In order to compensate
for such losses (i.e., electrical transmission impairments) higher
frequency subcarriers may be modulated in accordance with low order
modulation formats, which are less susceptible to noise and may
incur fewer errors, and lower frequency subcarriers may be
modulated in accordance with higher order modulation formats,
because the lower frequency subcarriers do not experience as much
loss and thus will have less noise. Such lower frequency
subcarriers can, therefore, carry data with fewer errors and can be
modulated in accordance with a higher order modulation format.
[0070] Thus, in the examples shown in FIGS. 5b and 5c, higher
frequency subcarriers SC0 and SC3 may be modulated in accordance
with a lower order modulation format, such as QPSK, as noted above,
or BPSK, whereas lower frequency subcarriers SC1 and SC2 may be
modulated in accordance with a higher order modulation format, such
as 8 QAM or 16 QAM.
[0071] Other transmission impairments in the electrical domain as
well as those in the optical domain may also be compensated by
appropriate choice of subcarrier modulation formats. For example,
subcarriers can be modulated at lower order modulation formats at
frequencies that are more susceptible to optical loss, polarization
mode dispersion (PMD), chromatic dispersion (CD), or other optical
transmission impairments. However, those subcarriers that
experience fewer optical transmission impairments can be modulated
at higher order modulation formats.
[0072] FIG. 11 illustrates another example of a portion of
transmission circuit 1 similar to discussed above in connection
with FIG. 3a. In FIG. 11, however, one Tx Engine circuit, such as
Tx Engine 1106, receives pairs of Data In portions having rates R0
and R1, respectively. This is because certain modulation formats
are advantageously encoded across two subcarriers. Here, Tx Engine
circuit 1106 supplies digitized analog signals representative of
spectrally adjacent ones of the first plurality of subcarriers,
such as SC0 and SC1. The digitized analog signals SC0 and SC1 are
further processed by FFT 308-1 and 308-2, respectively, in a manner
similar to or the same as that discussed above with reference to
FIG. 3a. Other circuits shown in FIG. 11 also operate in a manner
similar to that described above with reference to FIG. 3a.
[0073] As noted above, subcarriers in an optical signal are
modulated with different modulation formats to provide a variety of
SEs that facilitate efficient data transmission over a variety of
optical link distances.
[0074] Other embodiments will be apparent to those skilled in the
art from consideration of the specification. It is intended that
the specification and examples be considered as exemplary only,
with a true scope and spirit of the invention being indicated by
the following claims.
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