U.S. patent number 9,270,379 [Application Number 14/218,857] was granted by the patent office on 2016-02-23 for optical nyquist superchannel generation using microwave low-pass filtering and optical equalization.
This patent grant is currently assigned to NEC Laboratories America, Inc.. The grantee listed for this patent is NEC Laboratories America, Inc.. Invention is credited to Yue-Kai Huang, Dayou Qian, Fatih Yaman, Shaoliang Zhang.
United States Patent |
9,270,379 |
Huang , et al. |
February 23, 2016 |
Optical nyquist superchannel generation using microwave low-pass
filtering and optical equalization
Abstract
Disclosed are structures and methods for generating a Nyquist
superchannel.
Inventors: |
Huang; Yue-Kai (Princeton,
NJ), Zhang; Shaoliang (Plainsboro, NJ), Yaman; Fatih
(Monmouth Junction, NJ), Qian; Dayou (Princeton, NJ) |
Applicant: |
Name |
City |
State |
Country |
Type |
NEC Laboratories America, Inc. |
Princeton |
NJ |
US |
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Assignee: |
NEC Laboratories America, Inc.
(Princeton, NJ)
|
Family
ID: |
51729085 |
Appl.
No.: |
14/218,857 |
Filed: |
March 18, 2014 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20140314411 A1 |
Oct 23, 2014 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61802795 |
Mar 18, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B
10/506 (20130101); H04B 10/25073 (20130101); H04J
14/0298 (20130101) |
Current International
Class: |
H04J
14/02 (20060101); H04B 10/50 (20130101); H04B
10/2507 (20130101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Liu; Li
Attorney, Agent or Firm: Kolodka; Joseph
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Patent
Application Ser. No. 61/802,795 filed Mar. 18, 2013 for all
purposes as if set forth at length herein.
Claims
The invention claimed is:
1. A method of generating a Nyquist superchannel output signal
comprising the steps of: performing Nyguist shaping in two steps:
one in a baseband frequency and one in an optical frequency
including: filtering an electrical baseband signal with finite
rise-time symbols by high-order microwave low pass filter (LPF)
such that the baseband signal energy is confined to a small
fraction above a Nyquist rate, and no digital-to-analog converters
are used for signal generation; upconverting the filtered signal to
optical frequencies by applying the filtered signal to and driving
an optical modulator substantially in its linear region;
demultiplexing a multi-tone optical signal and applying a
demultiplexed output to the optical modulator; passively coupling
an output from the optical modulator to other outputs from other
modulators such that multiple subcarriers are multiplexed at a
spacing above their Nyquist bandwidth; and optically equalizing the
coupled multiple subcarriers such that the Nyquist superchannel is
generated.
Description
TECHNICAL FIELD
This disclosure relates generally to the field of optical
communications and in particular to optical Nyquist superchannel
generation using microwave low pass filtering and optical
equalization.
BACKGROUND
Optical superchannel is an emerging technology that supports
optical transport data rates in excess of 100-Gb/s by combining
multiple optical subcarriers to create a composite optical signal
exhibiting a desired capacity. Advantageously, optical superchannel
technologies may provide increased capacity sufficient to support
the ever-increasing video and mobile traffic demands imposed on the
Internet. Accordingly, methods, systems or structures that
facilitate the development and/or deployment of optical
superchannel technologies would represent a welcome addition to the
art.
SUMMARY
An advance in the art is made according to an aspect of the present
disclosure directed to a method and structures for generating an
optical Nyquist superchannel utilizing microwave low pass filtering
and optical equalization. According to one aspect of the present
disclosure, a method of generating a Nyquist superchannel output
signal comprises the steps of: filtering an electrical baseband
signal with finite rise-time symbols by high-order microwave low
pass filter (LPF) such that the baseband signal energy is confined
to a small fraction above a Nyquist rate; upconverting the filtered
signal to optical frequencies by applying the filtered signal to
and driving an optical modulator substantially in its linear
region; demultiplexing a multi-tone optical signal and applying a
demultiplexed output to the optical modulator; passively coupling
an output from the optical modulator to other outputs from other
modulators such that multiple subcarriers are multiplexed at a
spacing above their Nyquist bandwidth; and optically equalizing the
coupled multiple subcarriers such that the Nyquist superchannel is
generated.
Viewed from another aspect, a method according to the present
disclosure performs Nyquist shaping on standard QAM signals with a
NRZ waveform at both baseband and optical frequencies. In
particular microwave LPFs are utilized at baseband to confine
signal energy close to the Nyquist rate. Multiple filtered baseband
signals are used to modulate laser(s) to generate optical PDM QAM
signal. Modulator drive voltage swing is adjusted to limit
operation in a linear region and optical subcarriers are separately
modulated and passively combined at a spacing slightly above
Nyquist bandwidth to form optical Nyquist superchannel. The signal
so generated is optically equalized to improve OSNR sensitivity and
advantageously is performed with a single optical equalization
device with repetitive transmission profile such that Nyquist
shaping is performed on multiple subcarriers. Finally, Fabry-Perot
etalon-based deviced are used to generate repetitive OEQ profile
for fixed operation or LCoS based optical shaping modules may be
employed to generate OEQ profile with flexible wavelength
operation.
BRIEF DESCRIPTION OF THE DRAWING
A more complete understanding of the present disclosure may be
realized by reference to the accompanying drawing in which:
FIG. 1 depicts a schematic block diagram depicting the generation
of a Nyquist superchannel according to an aspect of the present
disclosure;
FIGS. 2(a)-2(b) depicts graphs of (a): intensity vs. wavelength of
the filtering performed at Nyquist bandwidth by the microwave LPFs
have achieved very steep signal edge roll-offs; and (b) intensity
vs. wavelength of a three-subcarrier Nyquist superchannel after OEQ
is applied using an LCoS optical shaping module according to an
aspect of the present disclosure; and
FIG. 3 depicts a graph showing a back-to-back bit-error-rate vs.
signal OSNR measurement according to an aspect of the present
disclosure;
DETAILED DESCRIPTION
The following merely illustrates the principles of the disclosure.
It will thus be appreciated that those skilled in the art will be
able to devise various arrangements which, although not explicitly
described or shown herein, embody the principles of the disclosure
and are included within its spirit and scope.
Furthermore, all examples and conditional language recited herein
are principally intended expressly to be only for pedagogical
purposes to aid the reader in understanding the principles of the
disclosure and the concepts contributed by the inventor(s) to
furthering the art, and are to be construed as being without
limitation to such specifically recited examples and
conditions.
Moreover, all statements herein reciting principles, aspects, and
embodiments of the disclosure, as well as specific examples
thereof, are intended to encompass both structural and functional
equivalents thereof. Additionally, it is intended that such
equivalents include both currently-known equivalents as well as
equivalents developed in the future, i.e., any elements developed
that perform the same function, regardless of structure.
Thus, for example, it will be appreciated by those skilled in the
art that the diagrams herein represent conceptual views of
illustrative structures embodying the principles of the
invention.
In addition, it will be appreciated by those skilled in art that
any flow charts, flow diagrams, state transition diagrams,
pseudocode, and the like represent various processes which may be
substantially represented in computer readable medium and so
executed by a computer or processor, whether or not such computer
or processor is explicitly shown.
In the claims hereof any element expressed as a means for
performing a specified function is intended to encompass any way of
performing that function including, for example, a) a combination
of circuit elements which performs that function or b) software in
any form, including, therefore, firmware, microcode or the like,
combined with appropriate circuitry for executing that software to
perform the function. The invention as defined by such claims
resides in the fact that the functionalities provided by the
various recited means are combined and brought together in the
manner which the claims call for. Applicant thus regards any means
which can provide those functionalities as equivalent as those
shown herein. Finally, and unless otherwise explicitly specified
herein, the drawings are not drawn to scale.
Thus, for example, it will be appreciated by those skilled in the
art that the diagrams herein represent conceptual views of
illustrative structures embodying the principles of the
disclosure.
By way of some additional background, we begin by noting that
optical superchannel is a promising technology for increasing fiber
channel capacity in next generation optical networks, i.e. 400 Gb/s
or 1 Tb/s per channel. As may be readily appreciated, by "packing"
multiple subcarriers into a tighter spacing, optical superchannel
can dramatically improve spectral efficiency of the transmission
without increasing signal constellation advantageously avoiding
high transmission penalties.
Nyquist optical superchannel, as its name suggests, allows the
subcarriers within the superchannel to be multiplexed at a
frequency spacing equal to or slightly larger than the individual
subcarrier baud-rate, thereby enabling "Nyquist-rate"
transmission.
In order to avoid any crosstalk between subcarriers, pulse shaping
techniques are required to confine signal energy of each subcarrier
to a certain bandwidth equal to, or slightly higher than its signal
baud-rate. To achieve the best transmission performance in terms of
receiver OSNR sensitivity and fiber nonlinearity tolerance, it is
desirable to produce a flat-top subcarrier signal spectrum. As may
be readily appreciated, a rectangular shaped signal spectrum will
exhibit a broader impulse response (ideally sine pulses) in the
time domain which can be recovered by an adaptive time delay
estimator (TDE) in the receiver DSP.
As is known, the so called "Nyquist" pulse shaping may be performed
either digitally or optically. In the case of digital pulse
shaping, digital generated data are either convoluted with a sine
pulse function, or passed through a sharp rectangular low-pass
filter (LPF), such that the signal spectrum is confined while
maintaining orthogonality of neighboring symbols in time domain. As
may be readily understood, Nyquist-shaped signal(s) cannot be used
for optical signal modulation without digital-to-analog converters
(DAC). For fiber channel capacity beyond 100 Gb/s requiring Nyquist
bandwidth above 16 GHz, it is difficult and costly to design and
build a DAC with such high-speed characteristics.
Another pulse shaping method involves "quasi-Nyquist" shaping in
the optical domain. With this quasi-Nyquist pulse shaping method,
the signal is first generated by driving optical modulators with
rectangular waveforms exhibiting a finite rise and fall time, and
producing optical bandwidth much larger than the Nyquist bandwidth
(equivalent to the signal baud-rate). The generated signal is then
optically filtered thereby reducing the signal bandwidth before
multiple subcarriers are multiplexed thereby avoiding large
cross-talk in reduced subcarrier spacing.
Optical equalization may then be applied to the multiplexed
subcarriers using a pre-defined optical profile, in which the
maximum transmittance is at the edge of each subcarrier, and the
center region of each subcarrier is attenuated such that a more
flattened spectrum is produced thereby increasing the receiver OSNR
sensitivity. As may be understood, the optical filtering and
equalization may be performed using fixed optical components such
fiber Bragg gratings (FBGs) and Faby-Perot (FP) etalons. To add
flexibility, programmable liquid-crystal-on-silicon (LCoS) based
optical wave-shaping modules may be employed--along with their much
higher cost. Even with these techniques, contemporary optical
filtering technology does not provide enough resolution to produce
a sharp roll-off at signal band edge to sufficiently eliminate
crosstalk between adjacent subcarriers.
With this additional background in place we note that--according to
the present disclosure--by performing the Nyquist shaping process
in two steps, one in a baseband frequency and one in an optical
frequency, our method according to the present disclosure presents
several advantages over prior-art methods. First, it is easier to
obtain a high-order filter response exhibiting sharp filter edges
using microwave filters in the baseband frequencies (compare to
optical filtering). Consequently, better performance is achieved by
reducing any cross-talk between subcarriers. Secondly--since the
filtering is done in the baseband--there is little risk of chopping
off too much signal energy due to signal/filter frequency mismatch
that could be caused--for example--by laser drifting.
Notably--and as compared to the implementations of digital Nyquist
shaping--methods according to the present disclosure do not require
high-speed DACs for signal generation--thereby dramatically
reducing the system complexity and cost. Finally, optical
equalization (OEQ) can be performed on multiple subcarriers or even
multiple superchannels using only one single optical device with
repetitive profile thereby reducing implementation cost even
further.
As may be appreciated by those skilled in the art, by separating
Nyquist shaping into two steps, one in baseband filtering and
another in optical equalization, methods according to the present
disclosure achieve better performance as compared to doing it all
optically. Moreover, the cost of implementing these two steps
separately relaxes the requirements quite significantly for
generating Nyquist subcarriers with large data rate. For example,
to generate subcarriers using digital Nyquist shaping with larger
than 100-Gb/s data rate, high speed DAC with sampling rate larger
than 30 GSa/s is required. As compared to both purely digital
Nyquist shaping or optical Nyquist shaping, methods according to
the present disclosure may be applied to current standard
transmission technologies with Non-Return-To-Zero (NRZ) waveforms
to obtain similar or better performance with much lower cost and
complexity.
More specifically, a number of distinct aspects and advantages of
methods according to the present disclosure become apparent. In
particular, i) (a) Using microwave LPF instead of optical filtering
or digital filtering to either improve performance or reduce
implementation cost and complexity; and (b) using single OEQ
devices to perform Nyquist shaping over multiple subcarriers reduce
the cost significantly; and ii) the adjustment of the modulator
drive voltage is crucial in maintaining the linear signal
up-conversion to optical frequencies without signal distortion. As
may be readily appreciated, this step is different than standard
modulation method where standard NRZ waveforms are used
Turning now to FIG. 1, there it shows a schematic block diagram of
the generation a of Nyquist superchannel according to an aspect of
the present disclosure. As depicted in that FIG. 1, the process
begins with the generation of multiple optical tones. The tones,
exhibit a free spectral range (FSR) of f.sub.sc which represents
the spacing between subcarriers, advantageously can be optical
combs generated by a fundamentally mode-locked laser, a gain
switched laser, or through wide-band phase modulation.
An optical demultiplexer (Tone DeMUX) is then used to separate each
optical tone for individual subcarrier modulation. As may be
understood, if f.sub.sc is large as compared to the range of laser
frequency drifting, than separate lasers can also be used for
subcarrier modulation.
For each subcarrier--assuming that quadrature amplitude modulation
(QAM) is used on both polarizations, a total of four electrical
data signals will be generated. These signals can be standard,
non-return-to-zero (NRZ) binary waveforms for DP-QPSK modulation,
or rectangular, multi-level waveforms for high-order QAM modulation
such as 16-QAM or 64-QAM. A NRZ signal waveform will exhibit
frequency null at f.sub.sym (the modulation symbol rate) and
several high frequency side-lobes.
The first step of Nyquist shaping according to the present
disclosure is to apply the four signal lanes through four separate
microwave LPFs (MWLPF) each exhibiting a cut-off frequency slightly
above f.sub.sym/2 (less than 10%) to remove the frequency contents
above the Nyquist bandwidth, as shown in the inset in FIG. 1.
Advantageously, commercial microwave LPFs having a high-order
filter design are readily available to produce a sharp frequency
roll-off that is required. The four LPF outputs are then applied to
and used to drive four ports of a polarization division multiplexed
(PDM) in-phase and quadrature (I/Q) modulator to up-covert the
baseband QAM signal to optical frequency in two polarizations.
Notably, and instead of driving the modulator port(s) at a full
range V.sub.D like standard NRZ waveforms, appropriate measures
must be taken such that only a linear region of the optical
modulator is used thereby avoiding signal distortion. Therefore
appropriately valued electrical attenuators (ATT) must be inserted
between the LPFs and the PDM I/Q Modulator. The resulting PDM QAM
modulated subcarriers output by the PDM I/Q Modulator are combined
using--for example--passive optical couplers.
As shown graphically in FIG. 2(a), filtering performed at Nyquist
bandwidth by the microwave LPFs have achieved very steep signal
edge roll-offs, which advantageously will achieve better
performance than optical filtering when cross-talk between
subcarriers is considered. In the example depicted in FIG. 2(a), a
standard 127-Gb/s DP-QPSK signal, which normally requires 50-GHz in
DWDM transmission, can be confined to about 36-GHz wide, roughly
13% above the Nyquist bandwidth.
Returning to our discussion of FIG. 1, it is noted that a second
step of Nyquist shaping according to the present disclosure may
advantageously be performed over the whole superchannel after
subcarrier multiplexing using one single optical equalization (OEQ)
module which may advantageously be based upon--for example--FP
etalon or LCoS technologies. Preferably, the OEQmodule exhibits a
repetitive transmission profile, in which the maximum transmittance
is at an edge of each subcarrier as shown in FIG. 1 inset, and a
center region of each subcarrier is attenuated.
As may be appreciated, the output spectrum of the superchannel
exhibits a much more uniformed energy distribution across its
occupied band. This uniform spectral distribution advantageously
improves receiver OSNR sensitivity since the ratio between the
transmitted signal and noise can be maintained for both low and
high frequency contents. An example, consider the graph shown in
FIG. 2(b). There it graphically demonstrates a three-subcarrier
Nyquist superchannel after OEQ is applied using an LCoS optical
shaping module. Nyquist shaping creates long impulse responses as
compared to NRZ waveforms. Using adaptive time-domain equalizers
(TOE) with adequate tap length, which are already implemented for
compensating other fiber impairments, the impulse response can be
handled by the receiver DSP without incurring penalty.
Finally, FIG. 3 graphically shows a back-to-backbit-error-rate vs.
signal OSNR measurement. At a soft-decision forward error
correction (SD-FEC) limit--which is the maximum error rate before
decoding to maintain error-free transmission--adding the microwave
LPF, the first step of the Nyquist shaping, only degrades the
performance of the 128-Gb/s DP-QPSK subcarrier by about 0.5 dB.
(Alternatively stated, one needs 0.5 dB more OSNR to achieve the
same performance). However, because of the sharp filtering created
by the microwave filters, the subcarriers can be multiplexed at
37.5-GHz spacing for 400G transmission, only 17% higher than
Nyquist rate, without incurring any more penalties from cross-talk.
After performing OEQ--the second step of Nyquist shaping--the OSNR
performance improves significantly--about 1-dB better--than the
original NRZ signal.
At this point, the foregoing is to be understood as being in every
respect illustrative and exemplary, but not restrictive, and the
scope of the invention disclosed herein is not to be determined
from the Detailed Description, but rather from the claims as
interpreted according to the full breadth permitted by the patent
laws. It is to be understood that the embodiments shown and
described herein are only illustrative of the principles of the
present invention and that those skilled in the art may implement
various modifications without departing from the scope and spirit
of the invention. Those skilled in the art could implement various
other feature combinations without departing from the scope and
spirit of the invention.
* * * * *