Note: Descriptions are shown in the official language in which they were submitted.
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SYSTEM AND METHOD FOR MEASURING IN-BAND
CROSS-TALK IN
OPTICAL COMMUNICATION SYSTEMS
Technical Field
The present invention is directed to a system and method for measuring in-band
cross-
talk in an optical communication system, and particularly for using such
measurement in
conjunction with other measurements to estimate bit error rate (BER).
Background Art
Optical routing in optical communication systems, such as wavelength-division-
multiplexed (WDM) optical systems, requires a wavelength and polarization
insensitive
optical switch. Determining a bit error rate (BER) after each of these
switches is useful for
determining and maintaining the health of a WDM network. The BER is defined as
the ratio
of the number of erroneous bits received to the total number of bits received
per second.
One way of characterizing the performance of a transmission system is to
measure the
BER level to form eye diagrams. Eye diagrams are a known technique to track
channel
power as a function of time. These diagrams are generated by plotting the
received signal as
a function of time, and then shifting the time axis by one bit interval and
plotting again. The
superimposed bits define most probable (constructive and destructive)
interference events due
to transmission in the channels adjoining the particular channel plotted.
Thereby, the eye
diagram depicts the worst-case impairment as measured by the greatest ordinate
value clear
of traces (by the vertical dimension of the clear space between a peak and a
null). A system
that is not excessively impaired shows clear discrimination between "1's" and
"0's" in a
digital signal, with an "eye opening" in the center of the diagram. A truly
unimpaired system
is considered to have an eye opening of 1Ø
Generally, the impairments that limit the system's performance cause two types
of
degradation in the received eye pattern; random fluctuations in the bit energy
(caused by
noise) and non-random pulse shape distortions. Non-random pulse shape
distortions are
sometimes referred to as Inter-Symbol Interference (ISI). As bit rates
increase to the gigabit
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per second range and higher it becomes useful to manage the impairments that
affect the
shape of the received pulses, and to limit the ISI. While compensation of ISI
has met with
some success, compensation of random fluctuations remains difficult.
Ultimately, these
random fluctuations may significantly impact the BER of the optical system.
S Ideally, the BER of each channel would be measured independently of the type
of
modulation present. This is typically done in the laboratory by sending a
pseudo-xandom bit
stream through the system and comparing data at both ends of the system.
However, since
the desired systems have a very low BER, it may be difficult to directly
measure the BER
practically. Further, the processes affecting the BER could vary significantly
over the
extended period of time required to measure the BER. Thus, if the BER
increased
significantly above the desired BER even for a relatively short period of
time, the mean BER
would most likely be below a desired threshold BER, making this measurement
unreliable.
Moreover, when attempting to assess the BER of deployed systems, direct
measurement is
even more impractical. As such, techniques have been developed to estimate the
BER using
1 S parameters such as the optical signal-to-noise ratio (OSNR) as well as
other electrical noise
sources.
Typically, monitoring the BER of a system is conducted using spectrum
analyzers to
look at the primary noise source, such as amplified spontaneous emission
(ASE). However,
sources of noise other than ASE may be present which are not apparent from the
optical
signal-to-noise ratio (OSNR), but which still affect the BER. Efforts to find
a metric of BER
typically entail demodulating the transmitted signal, measuring the power
spectral density
(i.e., Garner signal power to noise floor), or channel sampling. While the
accuracy of this
inferential technique may increase with each additional accurate assessment of
noise
parameters, there is still a need to improve techniques for estimating the
BER.
2S
Disclosure of the Invention
The present invention is directed to measuring an additional metric of BER,
which
may be used to enhance the estimation of BER.
It is an object of the present invention to provide determination of features
of in-band
cross-talk in an optical communication system and use this as a metric in
estimating the BER.
According to an exemplary embodiment of the present invention, a system for
estimating in-band cross-talk in an optical communication system may include a
selective
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element for separating a signal in a desired channel from a plurality of
channels in the optical
communication system; a filter, which passes the signal at a rate proportional
to the time rate
of change of a phase of an optical source generating the signal; a digital
signal processor,
which r°ceives the signal from the filter and converts the signal into
a frequency domain; and
a spectrum analyzer, which measures at least one feature of the signal in the
frequency
domain to quantify the in-band cross-talk.
According to another exemplary embodiment of the invention, a method for
estimating in-band cross-talk in an optical communication system includes
separating a signal
in a desired channel from a plurality of channels in the optical communication
system;
passing the signal in proportion to the time rate of change of a phase of an
optical source
generating the signal; converting the signal into a frequency domain; and
analyzing at least
one feature of the signal in the frequency domain to quantify the in-band
cross-talk.
It is further an object of the present invention to provide a more accurate
estimate of
BER in optical communication systems which are sensitive to in-band cross-
talk, for example
in systems employing wavelength division multiplexing, independently of
transmitted data
format.
According to another exemplary embodiment of the invention, estimating bit
error
rate (BER) in an optical communication system includes a selective element,
which separates
a signal in a desired channel from a plurality of channels in the optical
communication
system; a filter, which passes the signal at a rate proportional to the time
rate of change of a
phase of an optical source generating the signal; a digital signal processor,
which converts the
signal into a frequency domain; a spectrum analyzer which measures at least
one feature of
the signal in the frequency domain to quantify the in-band cross-talk; and a
post processor
which combines at least one feature measured by the spectrum analyzer with at
least one
noise feature to estimate BER.
According to another exemplary embodiment of the present invention, a method
for
estimating bit error rate (BER) in an optical fiber includes separating a
signal in a desired
channel from a plurality of channels in the optical communication system;
passing the signal
at a rate proportional to the time rate of change of a phase of an optical
source generating the
signal; converting the signal into a frequency domain; analyzing the signal in
the frequency
domain to quantify the in-band cross-talk; and combining at least one feature
from the
analyzing with at least one noise feature to estimate BER.
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Brief Description of the Drawings
The invention is best understood from the following detailed description when
read
with the accompanying drawing figures. It is emphasized that the various
features are not
necessarily drawn to scale. In fact, the dimensions may be arbitrarily
increased or decreased
for clarity of discussion.
Figure 1 is a block diagram of the system for measuring in-band cross-talk in
accordance with an exemplary embodiment of the present invention.
Figures 2a-2h are plots of gain versus frequency for varying levels of power
in the
low frequency spectrum measured according to an exemplary embodiment of the
present
invention.
Modes For Carrying Ont the Invention
In the following detailed description, for purposes of explanation and not
limitation,
exemplary embodiments disclosing specific details are set forth in order to
provide a
thorough understanding of the present invention. However, it will be apparent
to one having
ordinary skill in the art that the present invention may be practiced in other
embodiments that
depart from the specific details disclosed herein. In other instances,
detailed descriptions of
well-known devices and methods may be omitted so as not to obscure the
description of the
present invention.
The amount of noise determines the BER a channel can attain. Briefly, the
present
invention is directed to recognizing that one metric of BER is the in-band
cross-talk. In-band
cross-talk is an extraneous optical field, which interferes with the
communications signal
upon optical-to-electrical conversion resulting in noise having a spectrum,
which falls within
the electrical bandwidth of the receiver system. Illustratively, in-band cross-
talk may be
cross-talk within a single channel, which arises from any pair of back
reflections generated in
an optical communication system. In an optical communication system, if a
signal is
reflected twice, that erroneous signal is then traveling in the same direction
as the desired as
the desired signal and may interfere with the desired signal. Even if the
wavelength output
by the optical source is stable, the time delay between the input signal and
the reflected signal
may interfere, and this may lead to beating. To this end, the relative phase
is random and
temporally varying, leading to a time varying interference (e.g., beating).
The importance of
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measuring in-band cross-talk has increased with the rise of optical
networking, in which the
network may be reconfigured by the flip of a switch. '
Since there may be numerous sources of noise in an optical system, it is
difficult to
discern which portions of the noise spectrum are due to which sources. For
example, in-band
cross-talk does not normally change the overall optical signal-to-noise ratio
(OSNR), since
the in-band cross-talk occurs in a much narrower spectrum than the OSNR and is
not
resolved in the OSNR measurements. However, in-band cross-talk normally will
be
concentrated in a spectral region in proportion to the time rate of change of
the relative
phases between the signal and the cross-talk components. For some devices such
as
semiconductor lasers, this type of noise will be most prevalent at low (e.g.,
radio)
frequencies. By taking the ratio of the noise spectral densities within this
band and outside
this band, the noise due to in-band cross-talk may be determined. While the
absolute value of
the in-band cross-talk is difficult to ascertain, the relative values may be
useful in estimating
the BER, especially when used with other metrics of BER to improve these
estimates. The
measurement of phase noise in an optical source is known from the study of
laser noise.
A configuration for determining the low frequency features of in-band cross-
talk in a
single WDM channel in an optical communication system 2$ according to an
exemplary
embodiment of the present invention is shown in Fig. 1. All incoming WDM
channels on an
optical waveguide such as an optical fiber 10 are passed through a selective
element 12.
After passing through the selective element 12, the signals are incident on a
photodetector 14.
A single channel from the plurality of WDM channels is selected based on the
corresponding
illuminated pixel for the deflected wavelength or the location of the tunable
filter.
According to the illustrative embodiment of the present invention shown ion
Fig.l, .
the optical communication system 28 incorporates an optical waveguide such as
an optical
fiber and/or a planar optical waveguide. However, the invention of the present
disclosure
may be used in optical communication systems incorporating other types of
optical
wavguides. Moreover, the invention of the present disclosure may be used in
optical
communication systems, which include "free-space" portions as well. These free-
space
portions include, but are not limited to, micro-optic devices such as filters,
isolators and
switches. Finally, in the exemplary embodiment shown in Fig.l, selective
element 12 may be
a dispersive element, or a tunable filter. If a tunable filter is used, the
filter location may be
dithered to ensure optimal channel overlap with the filter passband. This
dithering frequency
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may then be filtered out by post-processing. The input signal is demultiplexed
(and spatially
separated) into component wavelengths by the selective element 12. The
selective element
12 may be any conventional demultiplexer, such as a grating, a blazed grating,
an arrayed
waveguide grating, or a prism; a micro-optic based filter; a thin-film based
filter; or a
waveguide based filter such as a fiber Bragg grating (FBG). Of course, this
list is illustrative
and not exhaustive and other optical elements within the purview of the
artisan of ordinary
skill may be used for selective element 12.
The signal from the selected channel is passed from the photodetector 14
through a
low-noise pre-amplifier 16 and low frequency filter 18, which is
illustratively an anti-aliasing
filter. The low frequency filter 18 is selected in proportion to the time rate
of change of the
phase in the optical source (not shown) and according to well known radio
frequency (rf)
techniques. The signal is then sampled by an analog-to-digital converter (ADC)
20 at a
frequency high enough to prevent signal degradation due to aliasing;
illustratively this
frequency is equal to or greater than the Nyquist frequency (fN) of the
previous analog filter
18. In this embodiment, the dynamic range of these elements is illustratively
greater than 30
dB.
A digital signal processing (DSP) unit 22 recovers the low frequency signature
across
the phase noise spectrum of the optical source (e.g. laser) spectrum by
converting the signal
from the ADC 20 to the frequency domain via windowing and either a Discrete
Fourier
Transform (DFT) or a Fast Fourier Transform (FFT). If the levels of in-band
cross-talk are
relatively low (illustratively on the order of -30dB), additional signal
averaging in the
frequency domain with a finite impulse response (FIR) filter is usefully
performed.
The resultant signal is then provided to a spectrum analyzer 24 which can be
used to
determine the magnitude, location, number and width of the in-band cross-talk
features;
particularly the peak of the spectrum, for a more accurate picture.
Alternatively or
simultaneously, the noise spectral density of the in-band cross-talk spectrum
can be averaged
over an appropriate frequency range and then compared with a spectrum outside
this
frequency range to estimate the contribution of the in-band cross-talk to the
BER. The
appropriate frequency range is determined by the speed with which the phase
noise of the
optical source changes. However, the lower frequencies of this range, where
1/f noise is
prevalent, should not be included. An upper end should cut off well after any
such noise is
expected to be present. For example, the appropriate frequency range may be
from about 3/4
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of where the phase noise maxima occur to about twice this frequency. This
value is
dependent on the phase noise spectrum of the source. Illustratively, for DFB
lasers, this
frequency range is on the order of approximately 50 MHz.
Once these in-band cross-tally features have been quantified, these features
may be
combined with other measurements to provide a more accurate estimate of BER in
a post
processing unit (PPU) 26. Illustratively, the in-band cross-talk features may
be combined
with the received signal's power spectral density (PSD). The PSD is the
Fourier transform of
the autocorrelation of the noise amplitude, i.e., the degree to which any the
noise random
variables at different times depend on one another.
Additional information may be included in the PPU to increase the accuracy of
the
BER estimate. Such information may include but is not necessarily limited to
the ASE noise
floor, the number of add/drops the channel has undergone, the width of the
main lobe of the
PSD, and the location of the wavelength band of the channel. By converting the
phase noise
of in-band cross-talk into amplitude noise, the metric of the in-band cross-
talk may be readily
included with the other metrics to more accurately estimate BER. Moreover, the
effect of
phase-to-intensity noise conversion by multiple reflection on Gigabidsec DFB
laser
transmission systems is known.
Figures 2a-2h illustrate the in-band cross-talk features measured by the
illustrative
system of Fig. 1. As can be seen therein, the in-band cross-talk features
increase with
increasing levels of power. For the plots shown in Figures 2a-2h, the data was
processed
with sixty-four averages to reduce the influence of other noises.
The invention being thus described, it would be obvious that the same may be
varied
in many ways by one of ordinary skill in the art having had the benefit of the
present
disclosure. Such variations are not regarded as a departure from the spirit
and scope of the
invention, and such modifications as would be obvious to one skilled in the
art are intended
to be included within the scope of the following claims.
