Note: Descriptions are shown in the official language in which they were submitted.
CA 02466850 2004-05-11
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention relates to a method and apparatus for measuring polarization-
mode dispersion in optical devices such as optical fibers or components.
DISCUSSION OF THE PRIOR ART
Polarization-mode dispersion (PMD) is a fundamental property of optical fiber
and components by which any lightwave signal is split into two polarization
modes
that travel at different speeds on the basis of polarization state. The two
polarizatio n
modes experience a difference in propagation time known as differential group
delay
(DGD). The PMD of an optical fiber or component is simply the average DGD.
Existing PMD measuring techniques [see, for' example Y. Namihira et al,
Electron
Lett., 1992, 28, No. 25, 2265-2266 (1992) and A. Galtarossa et al, J. of
Lighwave
Technol., 14, 42 - 42 {1996)] utilize the fixed-analyzer method, the Jones-
matrix
method, and the Poincare-sphere method. A major limitation of these methods is
that any motion of the measuring apparatus, especially at the end of fibers,
can
totally destroy the measured results. Maintaining a motionless condition is
often
difficult, especially with field measurements. One motionless dispersion
measuring
technique based on measuring four-wave mixing (FWM) products in a iow-
dispersion, low PMD measurement fiber has been proposed [see S. Song et a(, J.
of
Lightwave Technol., 17, 2530-2533 (1999)]. The polarization-mode dispersion
measurement accuracy of this technique is limited by an additional PMD added
by
the measuring fiber itself, which is used to generate four-wave mixing
signals.
Moreover, the PMD measurement accuracy of the method is also limited by the
wavelength tuning range of the variable signal because the four-wave
conversion
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CA 02466850 2004-05-11
efficiency drops rapidly with wavelength detuning, causing a reduction in the
intensity of the FW'M signal and a degradation in the optical signal-to-noise
ratio
(OSNR).
GENERAL DESCRIPTION OF THE INVENT10N
The object of the present invention is to provide a method and apparatus for
determining polarization-mode dispersion in optics! devices which are
insensitive to
mechanical vibration and instability in the test equipment or apparatus, and
which
significantly reduce the cost and time of PMD testing in the running of dense
wavelength-division multiplexed (DWDM) networking systems
Thus, according to one aspect, the invention relates to a method of
determining the polarization-mode dispersion in an optical device comprising
the
steps of:
generating a first pump laser beam with a fixed wavelength;
generating a probe laser beam with a second fixed wavelength and the same
input polarization direction as said first pump laser beam;
generating a second pump laser beam having a variable wavelength and a
polarization direction orthogonal to the first pump laser beam and the probe
laser
beam;
launching the first and second pump laser beams and said probe laser beam
into an optical device to generate three output signals;
inputting the three output signals into a semiconductor optical amplifier
(SOA)
to generate four-wave mixing products dependent upon the polarization-mode
dispersion of the device; and
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CA 02466850 2004-05-11
computing the average polarization-mode dispersion of the device by
measuring the power ofi the four-wave mixing products versus the wavelength of
the
second pump laser beam.
According to another aspect, the invention relates to an apparatus for
determining the polarization-mode dispersion in an optical device comprising:
first laser means for generating a first pump laser beam with a fixed
wavelength;
second, probe laser means for generating a probe laser beam with a second
fixed wavelength and the same input polarization direction as said first pump
laser
~ 0 beam;
third laser means for generating a second pump (seer beam having a variable
wavelength and a polarization direction orthogonal to the first pump laser
beam and
the probe laser beam;
polarization beam splitter means for receiving said first pump, said probe and
said second pump beam, and relaying said beams through an optical device under
test to generate three output signals;
semiconductor optical amplifier means for receiving said output signals and
generating four-wave mixing products dependent upon 'the polarization-mode
dispersion of the device; and
analyzer means for computing the polarization-mode dispersion of the device
under test by measuring the power of the four-wave mining products versus the
wavelength of the second pump laser beam.
The above defined method is insensitive to mechanical vibrations and
instabilities in the apparatus, because the polarization-mode dispersion
measured
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CA 02466850 2004-05-11
by the apparatus depends only on the related states of polarization (SOP)
change
between the second pump Laser beam and the first pump laser beam or the probe
laser beam in the device under test (DUT), and not the position coordinates of
the
device or the apparatus.
SRlEF DESCRIPTION OF THE DRAWiNC~S
The invention is described below in greater detail with reference to the
accompanying drawings, wherein:
Figure 1 is a block diagram showing the spectrum of the traditional four-wave
mixing process in a semiconductor optical amplifier based on using two co-
polarized
laser beams;
Figure 2 is a block diagram showing a polarization-mode dispersion
measurement setup using a broad-band orthogonal-pumps four-wave mixing
technique in a semiconductor optical amplifier according to an embodiment of
the
present invention;
Figure 3 is a block diagram showing the spectrum of the broad-band
orthogonal-pumps four-wave mixing process used in the present invention;
Figure ~ is a block diagram showing a polarization-mode dispersion
measuring apparatus in accordance with the present invention;
Figure 5 is a block diagram showing a polarization-mode dispersion
measuring apparatus of a dense wavelength-division multiplex (DWDM)
transmission link in accordance with the present invention;
Figure 6 is a block diagram of another polarization-mode dispersion
measuring apparatus of a DWDM transmission link in accordance with the present
invention; and
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Figure 7 is a block diagram showing an in-field polarization-mode dispersion
measuring apparatus in accordance with the present invention in a running DWDM
networking system.
DESCRIPTiOi~ ~F THE PREFERRED EMBODIMENT
In a nonlinear optical material, wave-mixing arises from a nonlinear optical
response when more than one wave (signal and pump) is present. The outcome of
this effect is generation of another wave with an amplitude proportional to
the
product of the signs! and the pump amplitudes. The phase and the frequency of
the
generated waves are the sum and difference of those of the interacting waves.
Fig.
1 shows a traditional schematic of an optical spectrum generated by four-wave
mixing in a semiconductor optical amplifier based on using two co-polarized
laser
beams. In this scheme the amplitude and the optical signal-to-noise ratio of
the
FWM products decrease quickly with increasing frequency shift between the pump
and the probe signal frequencies [see !~. Uskov et al, IEEE J. Quantum
Electron 30,
1769 - 1781 (1994)]. In this case, there is described a broad-band orthogonal-
pump
(BOP) scheme based on using two orthogonally polarized pumps which give
constant amplitude and optical signal-to-noise ratio of the FWM products over
a
large range of frequency shifts [see M.W.K. Mark et ai, IEEE Photon Technoi.
Lett.,
10, 1401-1403 (1998)], which will significantly enhance the accuracy of the
dispersion. measurement in comparison with the current existing technology.
Fig. 2 shows the configuration of the broad band orthogonal pump scheme for
measuring the polarization-mode dispersion in a device such as an optical
fiber or
component. The optical frequency difference ~(~ between the first pump laser
beam
c~~ (optical power P,) and the probe input beam cep (optical power PP) is
fixed. The
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CA 02466850 2004-05-11
optical frequency difference bt~2 between the first laser pump beam c~1 and
the
second pump laser beam w2 can be changed. The first pump laser beam (~,~1 and
the probe beam t~P have the same state of polarization (SOP) and the second
pump
beam c~2 is polarized orthogonally to their SOP.
Fig. 3 shows an optical spectrum at the output of the amplifier far four-wave
mixing using the broad-band orthogonal pump scheme. The FWM output beam of
interest is c~2S, which is a replica of the input probe beam c~P shifted in
frequency by
bc~2. To understand the intensity of the wave w2s, we can use a simple lumped
model for FWM in semiconductor optical amplifiers, in which the amplifier is
regarded as a lumped element providing saturable gain followed by third-order
nonlinearity and amplified spontaneous emission noise see J.P.R. Lacey et al,
J.
Lightwave Technol. 16, 2419 - 2427, (1998)]. The three input beams undergo
four-
wave mixing in the gain medium of the amplifier and produce wave-mixed signals
at
several wavelengths. The FWM optical field at frequency t~2s satisfies the
equation:
C~2S = w2 -(~1- wP) - (1 )
and the corresponding field E2S is given by
E2S E2(E1 ~ EP)Y P1(~P ' ~1) expllL(wP ' ~1 ~ ~2)t +~P - ~1 ~ ~2]~
~EP(E1 ~ E2)~21(~2 - ~1)exp~~~~~2 ' ~1 ~ wP)t ~ ~2 ' ~1 + ~P]~ 2
So,
2~ E2S = E2(E1 . Ep)YP1 (wP - ~1 )expC~(~zs~ ~' ~~)]
~EP(E1 ~ ~2)~21~w2 - ~1)expL~(~2sf + ~~)l (3)
where cal, c~2, wP, c~2s, E1, E2, EP, and E2S are the frequencies and the
field
amplitudes of the first pump laser beam, the second pump laser beam, the input
probe beam; and the FWM signal of interest, respectively.
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CA 02466850 2004-05-11
~~ = tpP - t~1 + ~2 is the phase difference between the interacting waves. The
complex coupling coefficient yP1(~P - ~1) is produced by the same carrier
grating as
in conventional FWM and decreases rapidly as ~ wP - w1 ~ increases [see A.
Uskov et.
a(, supra].
When a photo detector is used to measure optical output, the power at w2s is
given
by
P2S ~E2~2~~1 ~ EP~2IYP1(~P-~1)~2 ~~~P~2~~1 - E2~2~~21(~2 ~1)~2 4'
since in the inventors' experiments tr~1 - wP = ~c~ is fixed and the input
polarization of
these two beams are parallel, the first term in the above equation gives a
background as C~2 varying. The second term will fluctuate due to the existence
of the
polarization-mode dispersion of the device under test. The contribution of
this term
can be written as
F2S (°SOP2,SOP1) = 1I2 I EP I2, Y21(~2 ° ~1) ~ 2[1 ~ S2 . ~1]
(5)
where s1 = [S1~1~ S2~1~ 531)]T and s2 - [slt2f s2c2> s3c2~]T are the two
vectors of the first
pump beam c~1 (or the probe tx~P) and the second pump beam c~2 representing
the
polarization states of the input signals on the Poincare sphere.
To see how FWM power P2S generated in a semiconductor optical amplifier can be
used to measure the polarization-mode dispersion in an arbitrary test fiber,
it should
first be noted that, according to the fixed polarizer method [see Craig D.
Poole et al,
Journal of Lighfinrave Technol. 12, 917 (1994)], the first order PMD of a
fiber can be
measured by launching a fixed state of polarization beam into the test fiber
and then
passing the output through a fixed polarizer. The output power from the
polarizer is
given by the expression,
T = 112[1 + s (c~).P] (6)
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CA 02466850 2004-05-11
where s(c~) is the SOP of the light incident on the polarization analyzer and
P is the
unit vector specifying the transmission state (i.e. the pass axis of the
polarization
analyzer): First order polarization-mode dispersion is then estimated using
the
formula
<~T>_K~<Ne~ (7)
~GJ
where < ~T> is the mean PMD, < Ne > is the mean number of maxima and minima
of the T curve in the frequency band ~cz~, and ~c is the palarization coupling
factor.
Comparing equation (5) with equation (6), it is seen that they are the same
function, except that the polarization state of the first pump laser beam c~~
with the
fixed frequency in (5) replaces the polarizer transmission state P in (6).
This
suggests that an alternative to the fixed polarizer method would be to launch
two
fixed-states of polarization signals into the test fiber and pass the output
through a
semiconductor optical amplifier. According to (5), the four-wave mixing power
generated in a SOA will vary with frequency changes of the test signal exaefly
as
would be the output of the test signaP atone passing through a fixed
polarizer. This
means that the four-wave mixing transfer function (5) can be used in place of
the T
function (6) when calculating the first order PMD using (7). The advantage of
calculating PMD using the four-wave mixing power produced in a semiconductor
optical amplifier is that no special care need be taken to maintain a strict
spatial
orientation between the test fiber and the measurement equipment (such as a
polarizer). This is because the probe wave follows the signal wave through
both the
test and the amplifier and, therefore, automatically establishes the
polarization
reference in the amplifier.
CA 02466850 2004-05-11
Referring again to Figure 1, when the pump laser beams c~, and the probe
laser beams (~P have been input via a polarization beam spiitter 2, an
isolator ~ and
a 2 x 2 switch 4. through a device 1 under test to a nonlinear optical
material such as
a semiconductor optical amplifier, FWM signals t~s and ccc~A are created at
the end
output of the SOA. The four-wave mixing signals exiting the SOA 5 are fed to
an
optical spectral analyzer 6. The amplitude and the optical signal-to-noise
ratio of the
FWM signals decrease quickly with increasing frequency shift ~c.~ between the
pump
and the probe signal frequencies. Thus, using the four-wave mixing scheme in
Fig.
1, the frequency shift ~c~ is limited to the very small range. To overcome
this
problem, we have made use of a broad-band orthogonal-pump (BOP) scheme
based on using two orthogonaily polarized pumps that gives constant amplitude
and
optical signal-to-noise ratio of the four-wave mixing products to achieve a
large
range of frequency shifts, which will significantly enhance the accuracy of
the PMD
measurement compared with current existing technology.
Fig. 2 shows the configuration of the broad-band orthogonal pump apparatus
for measuring the PMD in a device 1 such as an optical fiber or component. The
optical frequency difference bw between the first pump laser beam c~~ (optical
power P~) and the probe input beam c~P (optical power Pp) is fixed. The
optical
frequency difference bt~2 between the first pump beam c~~ and the second pump
beam t~2 can be changed. The first pump beam c~~ and the probe beam r~~ have
the same states of polarization and the second pump beam t~2 is polarized
orthogonaliy to their SOP.
Fig. 3 shows an optical spectrum at the output of the SOA for four-wave
mixing using the BOP scheme. The FWM output beam of interest is c~2s, which is
a
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CA 02466850 2004-05-11
replica of the input probe beam c~P, shifted in frequency by ~c~z. Based on
the
schemes of Fig. 2 and Fig. 3, the amplitude and the OSNR of the beam t~2S are
much more less insensitive on the frequency shift ~c~2 see Z. O. Lu et al, The
Proceedings of the Third Canadian Conference on Broadband Research, 3, 238
249 (1999)].
Based on our above , the inventors have produced the apparatus shown in
Fig. 4, which can be used for measuring PMD in the optical fibers or
components by
using a BOP-FWM technique in a semiconductor optical amplifier , in this
apparatus, the cavity consists of two interconnecting fiber rings. Two 1.55-pm
commercial semiconductor optical amplifiers 8 and 9 are biased at a certain
interjection current and are placed in the two rings. The typical fiber-to-
fiber small
signal gain of the amplifiers 8 and 9 is about 10 dB with the 10-dBm
saturation
output power. The TE/TM differential gain of the amplifiers 8 and 9 is less
than 0.5
dB. To ensure unidirectional operation and to prevent back reflections, a
polarization-independent optical isolator 10 is employed at the intersection
of the two
rings. A dual-wavelength filter 11 and a tunable filter 12 are used in
combination
with the amplifiers 8 and 9, polarization controllers 14, isolator 10 and the
beam
splitters 15 to created the three different lasing beams to act as the first
pump laser
beam w,, the probe laser beam t~P and the second pump laser beam w2,
respectively, in the BOP-FWM apparatus. A polarization beam splitter 15 was
used
to ensure that both,the pump laser beam t~~ and the probe signal beam coP have
the
same state of polarization and the second pump laser pump t~2 is polarized
orthogonally to their SOP. The ring laser output is obtained from a 3-dB beam
coupler 16. The three laser beams have gone through the isolator 3 and the 2x2
CA 02466850 2004-05-11
optics! switch 4 before they are launched into the semiconductor optical
amplifier 5
to generate four-wave mixing signals. If the three laser beams go through the
device under test 1 such as optical fibers or components, the FWM signal
intensities
will depend on the polarization mode dispersion of the DUT. in the proposed
measurement set-up, a high-speed 2x2 optical switch 4 can be used to obtain
the
interested four-wave mixing signal intensity in the following two situations:
with DIJT
and without such device. Then the frequency dependence of the FWM signal in
the
device can be subtracted, so the measurement errors of the polarization-mode
dispersion can be significantly reduced.
(n the set-up shown in Fig. 4, the measuring process comprises the steps of
scanning the tunable filter 12 and the other tunable fitter 6 to make sure
that the
intensity of the four-wave mixing beam eu2s can be continuously monitored and
recorded, which is equal to t~2-(c~1~c~P), at an exit end of the SOA 5 when
the tunable
filter 12 is changed. The output of the filter 6 is fed via a detector 18 to a
digital
signal processor and analyzer 19, which analyzes the output of the SOA 5 and
controls the switch 4 and the filter 6. After the measured FWM signal power of
the
beam t~2s has been obtained as a function of the frequency of the beam c~2,
the
formula (7) can be used to calculate the mean PMD of the device under test.
When
the tunable filter6 has been stopped at each value for the certain time period
T, the
high-speed 2x2 switch 4 is controlled with the speed V to obtain the VT/2-
times
average four-wave mixing intensities with and without the device under test.
The
frequency dependence of the FWM signal in the device under test can be
subtracted, so that the measurement errors and accuracy of the polarization-
mode
dispersion in the DUT can be significantly reduced. The embedded software is
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CA 02466850 2004-05-11
important to the apparatus, because sophisticated software is required to
individually
control the various elements of the apparatus of Fig. 4. For example, when the
tunable filter 12 values are changed, the tunable filter 6 must also be
changed to
achieve the corresponding values. Before the tunable filters are changed to
other
values, the 2x2 switch 4 must record the FWM signal power of the two
situations
many times with and without the device under test. All the test results and
control
are automatically processed using the digital signal processor (DSP) and a
signal
analyzer 19.
Reference is made to Figs. 5 and 6 which show two apparatuses for
polarization-mode dispersion measurement of dense wavelength-division
multiplexed (DWDM) networking systems. No any laser sources are required
because DWDM transmitter laser sources 20 are used as PMD measuremeni: laser
sources, which significantly reduces the testing equipment cost and testing
times as
compared with current PMD measurement technology. The laser beams c~, ....cr~n
are fed through a wavelength-division multiplexer 21 and an erbium doped fiber
amplifier (EDFA) 22 to high-speed 2x2 optical switch 24 to achieve the FWM
intensities of interest with or without a DUT. The switch 24 is connected by
two
variable optical attenuators 26i and optical couplers 26 to a second
multiplexer 27.
The attenuators 25 control the expected power level of the first pump laser
beam,
the probe laser beam and the second pump beam before going through an
i:~olator
29 to an semiconductor optical amplifer 30. The wavelengths of the first pump
laser
beam cam and the probe laser beam t.~m+, are fixed. The wavelength of the
second
pump laser beam Ce~K is changeable from c~~ to c~~. But t~k~c,~m and c~k~m+~,
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In the apparatus ofi Fig. 5, two 3-dB couplers 26, the 1x(N-2) optical switch
31
and a Nx1 multiplexer 27 have been used to select signal ~~, as the fiirst
pump laser
beam, (gym+, as the probe laser beam and t,~k as the second pump laser beam,
where
k = 1, 2,....., N, but k#m and k~Gm~-1. The measuring process comprises the
steps
of scanning the tunable filter 32 to make sure that the intensity of the FWM
ouput
beam G~2S, which is equal to cz~k-(G.jm czy+~), at an exit end of the SOA 30,
can be
continuously monitored when the 1 x(N-2) switch 31 is changed. After the
measured
FWM signal power of the beam w2S as a function of the f~reguency c~k is
obtained,
the formula (7) is used to calculate the mean polarization-mode dispersion of
the
device 1. For each second pump laser beam c~k, the 1 x(N-2) switch 50 and the
tunable filter 32 can be stopped for time period T, and the high-speed and the
2x2
switch 24 can be controlled at ;;peed V to obtain the VT/2-times average four-
wave
mixing intensities in the both cases with and without the device 1.
The apparatus of Fig. 5 includes two 1x3 optical couplers 36, two fixed
filters
37 and a tunable filter 38 to select c.~am as the first pump laser beam,
c~m.~, as the
probe laser beam and c~k as the second pump laser beam, where k = 1, 2, .....,
N,
but k#m and k#m+1. The me:~suring process includes the steps of scanning the
tunable filter 38 to make sure that the intensity of the FWM output beam c~2s,
'which
iS equal t0 C~k-((~m'Wm+1), at an exit end of the SOA 30, can be continuously
monitored and recorded when the tunable filter ~k has been changed. After the
measured FWM signal power of the beam c~2s as a function of the frequency t~k
is
obtained, the formula (7) is used to calculate the mean dispersion of the
device 1.
For each second pump laser pump ~k the tunable filter 32 is stopped for the
certain
time period T, and the high-speed 2x2 switch 4 is controlled at the speed V to
obtain
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CA 02466850 2004-05-11
the VT12-times average FWM intensities with and without the DUT. The frequency
dependence of the four-wave mixing signal in the device 1 can be subtracted,
so the
measurement errors and accuracy of the dispersion in the device 1 can be
significantly reduced.
Referring to Fig. 7, an in-field apparatus for measuring and monitoring
polarization-mode dispersion in a span of optical fibers in a running DWDM
networking system, using a SCUP-FWM technique in a semiconductor optical
amplifier and the optical transmitters of the networking systems, includes two
tap
couplers 40 connected by transmission links 41 to obtain the DWDM signals from
transmitters 20 and 42 before dnd after the transmission links 41,
respectively. The
apparatus of Fig. 5 or 6, indicated generally at 44 can be used to obtain the
intensity
of the FWM output beam of interest before and after transmission links. The
frequency dependence of the FWM signal in the transmission links 41 as a
function
of the frequency c~k, is subtracted, and the formula (7) is used to calculate
the mean
PMD of the transmission links 41. All the processing and data analyzing steps
are
controlled by the embedded software in a digital signal processor (DSP).
14
