Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02675914 2009-08-20
SYSTEMS AND METHODS FOR CHARACTERIZING THE AMPLITUDE AND
DIFFERENTIAL PHASE OF AN OPTICAL SIGNAL
FIELD OF THE INVENTION
The present invention relates to the analysis of optical signals and more
particularly concerns schemes for characterizing the amplitude and
differential
phase of high bit-rate optical data signals.
BACKGROUND
io Advanced modulation schemes are being increasingly used in high bit-rate
optical
communications. For most such systems, both the amplitude and the relative
phase of the electric field are used to permit more than one bit of
information to be
carried by an digital optical symbol. Normally, the phase is encoded
differentially,
that is, information is encoded by the difference in phase between successive
symbols. It is convenient to display such amplitude and relative phase
information
in the form of a constellation diagram, representing these digital optical
signals in
the complex field space. Hence, each point in a constellation diagram will
normally
correspond to the amplitude and differential phase of at least a sample of the
optical signal. The imparted phase and amplitude modulation, as well as phase
or
2o amplitude noise or drifts arising from either the transmitter or the
intervening
propagation medium, will then be visible on the constellation diagram.
Measured constellation diagrams for example find application in optical
communication to monitor phase drift laser source in an optical transmitter,
by
comparing the relative phase between consecutive pulses. They can also be used
to measure the effects of any or all of chromatic dispersion, polarization
mode
dispersion, and nonlinear fiber behavior on the detected optical signal.
The amplitude and phase information necessary to display a constellation
diagram
can be obtained by a coherent detection scheme, whereby the optical signal-
under-test (SUT) is mixed with an optical local oscillator. Referring to FIG.
1
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(PRIOR ART), there is shown a constellation viewer according to prior art. The
signal to be analyzed is mixed with a local oscillator in the coherent
detection
optics. Preferably, but not necessarily, this optical local oscillator emits a
phase-
stable continuous wave (cw) output. As a result of this mixing, the initial
signal is
decomposed into four different portions, each corresponding to respective four
linearly-independent states in the complex field space. Coherent detection
optical
means, also referred to as an optical hybrids, are known in the art and
exemplary
configurations therefore are shown in FIGs. 2A and 2B (PRIOR ART). A receiver
generally detects the four quadrature signals in pairs through a balanced
detection
io scheme, thereby converting the optical signals into electrical signals,
which are
amplified and digitally converted. The resulting signals are then processed
and the
resulting constellation diagram displayed.
Digitally encoded optical signals are currently transmitted with an
increasingly high
bit rate. In order to detect and characterize a stream of consecutive pulses
of the
signal to be analyzed, as described by Leven et al ["Coherent Receivers for
Practical Optical Communication Systems", Proceedings OFC/NFOEC 2007,
Anaheim CA, March 2007], the speed of the receiver and of the signal
processing
unit needs to be very high, i.e. greater than the symbol speed itself, and
hence
2o expensive components are required. This increases substantialiy the cost of
constellation viewing systems.
One known solution to reduce the demand for receiving and processing speeds is
to use optical sampling, where the short (typically 1 - 5 ps) pulses from a
phase-
stable repetitive pulsed laser (such as a mode-locked fiber laser, for
instance) are
used as the local oscillator, thus sampling the signal-under-test in a
stroboscopic
manner.
An alternative means for optical sampling, described by Westlund et al
[Journal of
3o Lightwave Technology, vol 23, no.6, pp-2012-2022 (2005)], uses a fiber
parametric optical amplifier that is pumped by a short phase-stable repetitive
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pulsed laser. The resulting idler signal faithfully reproduces the relative
amplitude
and phase information of the input SUT, sampled stroboscopically as determined
by the pump pulses. Advantageously, as described in the cited reference, this
FOPA can be rendered essentially insensitive to the state-of-polarization of
the
input SUT.
FIG. 3 (PRIOR ART) illustrates a case where such optical sampling is
undertaken
where the pulse rate of the pulsed local oscillator corresponds to the symbol
rate
of the signal to be analyzed. In such a system the receiving and processing
need
io only to be equal to the symbol speed, not greater, but components meeting
this
requirement for current data bit rates are nevertheless still fairly
expensive.
The requirement for such high-speed electronics can be overcome. In one known
solution, the pulse rate is reduced below the symbol rate so as to undersample
the
1s signal. The weakness of this approach is that phase noise or phase drift in
the
transmitter laser occuring on a time scale shorter than the sampling rate may
lead
to aliasing that could render the measurement unreliable.
There is therefore still a need for a system and apparatus to characterize the
2o amplitude and differential behaviour of an optical signal, which does not
require
very high speed detection electronics and is substantially insensitive to
aliasing
caused by phase noise variations in the transmitter.
SUMMARY OF THE INVENTION
25 Generally, there is provided a method for extracting the optical signal
parameters
necessary to display constellation diagram of a digitally encoded optical
signal,
comprising separating the digitally encoded optical signal into two signal
components, and comparing, through a coherent detection scheme, information
from one signal component with information from the other signal component to
30 which a delay is imposed. Differential phase information can therefore be
obtained
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for two consecutive data pulses without requiring receiving and processing
speeds
matching the data bit rate.
In accordance with one aspect of the invention, there is provided a system for
extracting the optical signal parameters necessary to display a constellation
diagram of an optical signal-under-test, comprising :
- a phase-stable repetitive pulsed laser for generating reference laser
pulses;
- an optical assembly for separating the signal-under-test into two signal
portions, and for separating the reference laser pulses into two reference
pulsed laser portions;
- optical delay means for introducing a relative delay between the two
reference pulsed laser portions;
- two optical hybrid interferometers, each receiving as input a respective one
of said signal portions, and a respective one of said reference pulsed laser
portions; and
- two receiving means, each configured to detect and transform into electrical
signals outputs from a corresponding one of said optical hybrid
interferometers.
In accordance with another aspect of the invention, there is also provided a
system for extracting the optical signal parameters necessary to display a
differential constellation diagram of an optical signal-under-test, comprising
:
- A phase stable repetitive pulse laser for generating reference laser
pulses;
- An optical assembly for separating the signal-under-test into two signal
portions, and for separating the reference laser pulses into two
reference pulsed laser portions;
- Optical delay means for introducing a relative delay between said signal
portions corresponding to one symbol duration of said optical signal-
under-test;
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- Two Fiber Optical Parametric Amplifier (FOPA) means, respectively and
synchronously pumped by said reference pulsed laser portions, each
FOPA means having a sampling gate receiving a respective one of said
signal portions and operable to form sampled replicas thereof at an idler
s wavelength, the idler wavelength being different than wavelengths of
said reference laser pulses and said signal-under-test;
- An optical hybrid interferometer receiving as input one said replicas of
the signal portions; and
- Receiving means configured to detect and transform into electrical
signals outputs of the optical hybrid interferometer.
In one embodiment of the invention (shown in Fig 4A), there is provided a
system
for extracting the optical signal parameters necessary to display a
constellation
diagram of a digitally encoded optical signal-under-test (SUT), comprising:
a phase stable repetitive pulse laser generating a reference pulsed laser
signal at a predetermined pulse rate (pulsed LO);
an optical assembly, comprising coupler means, for separating each of the
SUT and the pulsed LO into two portions, the optical assembly further
comprising a delay line in either or both of the pulsed LO or SUT optical
paths introducing a net delay corresponding to one symbol duration;
a pair of hybrid optical means each receiving one said portion from the SUT
and the pulsed LO; and
a pair of receivers, each associated with one of the coherent detection
means.
In addition, this embodiment may also comprise polarization-control means to
ensure that the signal-under-test and the local oscillator have substantially
the
same state of polarization at the point where they are superposed.
Such a system can be associated with lower speed signal processing and display
electronics since the differential phase information with respect to the
pulsed LO
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is obtained from a respective one of the two receivers, for two consecutive
data
pulses. The required speed of the receiving and processing electronics is
therefore
dictated by the sampling rate, which can be set low enough so that low speed
and
low cost receiving and processing components are sufficient.
In a second embodiment of the invention (shown in Fig 4B), there is provided a
system for extracting the optical signal parameters necessary to display a
constellation diagram of a digitally encoded optical signal-under-test (SUT),
comprising:
a phase stable repetitive pulse laser generating a reference pulsed laser
signal at a predetermined pulse rate (pulsed LO);
an optical assembly, comprising coupler means, for separating each of the
SUT and the pulsed LO into two portions, the optical assembly further
comprising a delay line in either or both of the pulsed LO or SUT optical
is paths introducing a net delay corresponding to one symbol duration;
a pair of polarization-diversity hybrid optical means each receiving one said
portion from the SUT and the pulsed LO; and
a pair of receivers associated with one of the said polarization-diversity
hybrid optical means, each receiver comprising eight detectors.
In a third embodiment of the invention, there is provided a system for
extracting
the optical signal parameters necessary to display a constellation diagram of
a
digitally encoded optical signal-under-test at a first wavelength, comprising:
An optical assembly comprising an optical spiitting means for separating the
input SUT into two portions, the optical assembly further comprising a delay
line introducing a delay corresponding to one symbol duration in one of the
SUT portions,;
Two substantially-polarization independent optical parametric amplifiers
(OPA), preferentially pumped by the output of a common phase-stable
repetitive pulsed laser at a second wavelength, each receiving a respective
one of said SUT portions at its input, whereby each OPA produces a
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respective signal at a third wavelength, each said respective signal
conserving relative phase and amplitude properties of the corresponding
one of said SUT portions, thereby defining delayed and undelayed signals
at the third wavelength;
Hybrid optical means comprising an optical assembly serving as a coherent
detection mixer, receiving the delayed and undelayed signals at the third
wavelength, combining parts of said portions and imparting a phase shift to
at least one said part thereof;
a receiver comprising four detectors associated with the coherent detection
mixer.
Such a system can be associated with lower speed signal processing and display
electronics receiving the differential amplitude and phase information for two
consecutive data pulses directly from the receiver. Again, the required speed
of
1s the receiving and processing electronics is therefore dictated by the
sampling
speed.
In a fourth embodiment of the invention, there is provided a system for
extracting
the optical signal parameters necessary to display a constellation diagram of
a
2o digitally encoded optical signal-under-test at a first wavelength, the
system being
suitable for characterizing polarization-multiplexed signals-under-test, said
system
comprising:
An optical assembly comprising an optical splitting means for separating the
input SUT into two portions, the optical assembly further comprising a delay
25 line introducing a delay corresponding to one symbol duration in one of the
SUT portions;
Two substantially-polarization independent optical parametric amplifiers
(OPA), preferentially pumped by the output of a common phase-stable
repetitive pulsed laser at a second wavelength, each receiving a respective
30 one of the SUT portions at its input, whereby each OPA produces a
respective signal at a third wavelength, each said respective signal
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conserving relative phase and amplitude properties of the corresponding
one of said SUT portions, thereby defining delayed and undelayed signals
at the third wavelength;
Polarization-diversity hybrid optical means (as shown in Fig. 2B) comprising
polarization separating means, serving as a coherent detection mixer,
receiving the delayed and undelayed signals at the third wavelength,
combining parts of said portions and imparting a phase shift to at least one
said part thereof;
a receiver comprising eight detectors associated with the coherent detection
mixer.
Other features and advantages of the invention will be better understood upon
reading of preferred embodiments thereof with reference to the appended
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 (PRIOR ART) is a schematic representation of a system for extracting
the
optical signal parameters necessary to display a constellation diagram of a
digitally
encoded optical signal according to prior art.
FIGs. 2A and 2B (PRIOR ART) are schematic representations of exemplary hybrid
optical means enabling coherent detection. FIG 2B respresents a variant of FIG
2A, which includes polarization diversity optics.
FIG. 3A (PRIOR ART) is a schematic representation of a system for extracting
the
optical signal parameters necessary to display a constellation diagram of a
digitally
encoded optical signal according to prior art, where the sampling rate is
equal to
the symbol rate of the SUT.
3o FIG. 3B (PRIOR ART) is a schematic representation of a system for
extracting the
optical signal parameters necessary to display a constellation diagram of a
digitally
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encoded optical signal according to prior art, where the sampling rate is much
less
than the symbol rate of the SUT.
FIG. 4A is a schematic representation of a system for extracting the optical
signal
s parameters necessary to display a constellation diagram of a digitally
encoded
optical signal according to a first embodiment of the invention, where a
polarization-control means (not shown) is used to control the state of
polarization
of the SUT before it is input to the system.
io FIG. 4B is a schematic representation of a system for extracting the
optical signal
parameters necessary to display a constellation diagram of a digitally encoded
optical signal according to a second embodiment of the invention, including
polarization diversity means
15 FIG. 5A is a schematic representation of a system for extracting the
optical signal
parameters necessary to display a constellation diagram of a digitally encoded
optical signal according to another embodiment of the invention.
FIG. 5B is a schematic representation of a system including polarization-
diversity
20 means, for extracting the optical signal parameters necessary to display a
constellation diagram of a digitally encoded optical signal according to
another
embodiment of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
25 According to embodiments of the present invention, there are provided
methods
and systems for extracting the optical signal parameters necessary to display
a
constellation diagram of an optical signal under test (SUT). It will be
understood by
one skilled in the art that the methods and systems disclosed herein and
equivalents thereof may be used in any application where such amplitude and
3o differential phase information are desired, and may be particularly useful
where
the signal to be analyzed contains data digitally encoded at a high bit rate.
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In a first embodiment shown in FIG. 4A, there is shown a system for extracting
the
optical signal parameters necessary to display a constellation diagram of a
SUT
according to a first embodiment of the invention. The system first includes a
5 phase-stable repetitive pulse laser (pulsed LO), such as a mode-locked fiber
ring
laser, which generates reference laser pulses at a predetermined pulse rate.
These reference laser pulses will be used to optical sample the signal-under-
test
in a stroboscopic manner. An optical assembly separates each of the digitally
encoded optical signal and the reference pulsed laser signal into two signal
io portions, preferably, but not necessarily, of approximately equal amplitude
for each
pair. Any appropriate optical elements may be used for this purpose, as will
be
readily understood by one skilled in the art. In one embodiment, all optical
signals
propagate in optical paths that comprise optical waveguides (the relative path
length has to be well controlled) and are split into portions by splitting
means, such
1s as an optical waveguide couplers. The optical waveguides may be embodied by
optical fibers, planar waveguides or any other appropriate optical structure.
The
optical assembly further includes a delay line introducing a delay
corresponding to
one symbol duration in one of the signal components of the reference pulsed
laser
signal, this delay being adjustable to correspond to the symbol rate of the
SUT.
2o By the expression "one symbol duration", it is understood that the delay is
set
substantially equal to the time period between the onset of one optical pulse
period and the onset of the next optical pulse period. Preferentially, the
delay line
may be embodied by a free-space delay line using linear displacement. As an
example, a typical delay-inducing path difference for a DQPSK signal at 40
Gbps
25 is about 15mm. The delay line is shown in FIG. 4A in the path of one of the
signal
components of the pulsed laser, but in an alternative embodiment could be
provided in the path of a signal component of the SUT. Alternatively, multiple
delays could be induced in a combination of signal components and arranged to
create the desired net delay.
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A pair of hybrid optical means enabling coherent detection, embodied by
optical
hybrids as shown in FIGs. 2A or by any other appropriate coherent detection
optical assembly. are provided, and each receives as input one signal
component
from the digitally encoded optical signal and one signal component from the
reference pulsed laser signal. The upper hybrid optical means shown in FIG. 4A
will output at a given time one pulse of the data signal mixed with the four
quadrature states of the reference signal, whereas the lower hybrid optical
means
will output the next (i.e. delayed) pulse of the data signal mixed with the
four
quadrature states of the reference signal. The output of each mixer therefore
io includes differential phase information between the digital data signal and
the
reference pulsed signal, for two consecutive data pulses, respectively. A pair
of
receivers is further provided, each receiver being associated with one of the
coherent detection devices. The receivers preferably include balanced
detectors
for converting the optical signals into electrical signals, and digital
conversion
means to output the detected information digitally. Four detectors are
required to
connect to the fiber outputs of each hybrid optical means; hence a total of
eight
detectors are required for this embodiment. Signal processing and displaying
means can receive the digital information from each receiver, and for a given
pair
of consecutive data pulses compare the information from both receivers to
deduce
therefrom differential phase information between these data pulses. As will be
appreciated by one skilled in the art, the required speed of the receivers is
only
dictated by the predetermined pulse rate of the repetitive pulse laser, if
analysis of
consecutive pulses therefrom is required, and is independent of the data bit
rate.
Referring to FIG. 4B, there is shown a variant to the embodiment of FIG. 4A
where
the hybrid optical means enabling coherent detection include polarization
splitting
means to render them polarization diverse, as shown in FIG. 2B. The
sensitivity of
this embodiment is not dependent upon the relative alignment of the respective
states of polarization of the input SUT and the pulsed LO. As well, this
3o embodiment is suitable for characterization of polarization-multiplexed
signals.
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Eight detectors are required to connect to the fiber outputs of each hybrid
optical
means; hence a total of sixteen detectors are required for this embodiment.
Referring to FIG. 5A, there is shown a system according to another embodiment
of
the invention, suitable for measuring a non-polarization-multiplexed SUT. In
this
case, the SUT is split via an optical splitter into two portions, one of the
portions
being delayed by a delay line by one symbol period. Each of these two portions
is
then input into a substantially polarization-independent Fiber-optic
parametric
amplifier (FOPA), these FOPA being synchronously pumped, preferably with the
io same phase-stable repetitive pulse laser. A suitable such repetitive pulse
laser
may be a mode-locked fiber ring laser. Details of a suitable such an FOPA are
described in commoly owned patent U.S. no 7,199,870 (WESTLUND et al.). Each
FOPA outputs an amplified copy of the stroboscopically-sampled corresponding
input signal at a wavelength ("idler" wavelength) different from both the SUT
and
the pump, at the repetition rate of the FOPA. Each copy preserves the relative
amplitude and phase information of the corresponding input signal. The
resulting
two idler outputs are then input into the hybrid optical means shown in Fig.
2A (or
functionally equivalent coherent detection optical means), the
stroboscopically-
sampled idler signal corresponding to the delayed SUT portion serving as the
local
oscillator for the other (non-delayed) stroboscopically-sampled idler.
It should be appreciated that the relative delays between the signals could be
imparted by a relative delay in the pump signals between the two FOPAs, rather
than by an optical delay in a portion of the SUT optical path, as described
above.
Optical filtering means may be used to ensure that negligible residual light
at the
initial signal wavelength or pump wavelength is detected.
The hybrid optical means therefore outputs the undelayed signal mixed with the
four quadrature states of the delayed signal, yielding directly the
differential phase
information between two consecutive pulses. Only four detectors are required
for
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this embodiment. The receiving and processing components necessary to analyze
the resulting signals therefore need only be adapted to the repetition rate of
the
FOPA.
Refering to Fig. 5B, there is shown a system according a fourth embodiment of
the
invention, suitable for measuring a polarization-multiplexed ("Pol-MUX")
signal. As
with the preceeding embodiment, the SUT is split via an optical splitter into
two
portions, one of the portions being delayed by a delay line by one symbol
period.
Each of these two portions is then input into a substantially polarization-
io independent Fiber-optic parametric amplifier (FOPA), these FOPA being
synchronously pumped, preferably with the same phase-stable repetitive pulse
laser. A suitable such repetitive pulse laser may be a mode-locked fiber ring
laser.
Each FOPA outputs an amplified copy of the stroboscopically-sampled
corresponding input signal at a wavelength ("idler" wavelength) different from
both
the SUT and the pump, at the repetition rate of the FOPA. Each copy preserves
the relative amplitude and phase information of the corresponding input
signal.
The resulting two idler outputs are then input into the hybrid optical means
shown
in Fig. 2B (or functionally equivalent coherent detection optical means), the
stroboscopically-sampled idler signal corresponding to the delayed SUT portion
serving as the local oscillator for the other (non-delayed) stroboscopically-
sampled
idler.
It should be appreciated that the relative delays between the signals could be
imparted by a relative delay in the pump signals between the two FOPAs, rather
than by an optical delay in a portion of the SUT optical path, as described
above.
Optical filtering means may be used to ensure that negligible residual light
at the
initial signal wavelength or pump wavelength is detected.
3o The hybrid optical means of Fig. 2B is polarization diverse, and therefore
outputs
the undelayed signal mixed with the four quadrature states of the delayed
signal
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for two orthogonal states of polarization, yielding directly the differential
phase
information between two consecutive pulses. Eight detectors are required for
this
embodiment. The receiving and processing components necessary to analyze the
resulting signals therefore need only be adapted to the repetition rate of the
FOPA.
Of course, numerous modifications could be made to the embodiments described
above without departing from the scope of the present invention.
