Download Development of a high precision interferometric set-up for the

Development of a high precision interferometric set-up for the
measurement of polarization mode dispersion
M. Zafrullah, M. Aleem Mirza, M. Waris and M. K. Islam
Abstract: — The measurements of different parameters
remained always a prime feature of the practical
development of any accurate system. In connection with the
optical communication, Phenomenon of PMD in single
mode optical fibers takes place due to enormous polarization
cross couplings throughout a specific length of the fiber and
becomes a deteriorating factor for high speed fiber optic
networks. We have quantified the benefits of applying
different polarization mode dispersion measuring schemes,
which leads to the ultimate outcome of the system
refinement. A precise optical test set-up is developed in lab,
which provides the exact picture of the state of polarization
through a certain fiber length. The set-up is used to
measure PMD for single mode and hi-bi specialty fibers,
thus making the set-up useful for both telecom and sensor
applications of optical fibers.
Keywords-component; (PMD, high speed optical networks)
INTRODUCTION
The dispersion is broadly categorized in two major
impairments as the Group-Velocity-Dispersion (GVD) and
Multipath Interference Dispersion (MPID) [1]. Polarization
Mode Dispersion (PMD) falls under the second category of
dispersion and occurs due to the asymmetry in refractive index
of the optical fiber, a phenomenon known as birefringence. A
single mode optical fiber can be modeled as having two
orthogonal polarization modes traveling along the two
birefringent axes of the fiber. In hi-bi (highly birefringent)
fiber, there is a very weak coupling between the two States Of
Polarization (SOP) of the light wave traveling through the fiber
and PMD is the delay in the two SOP caused by the specific
value of the birefringence over a certain length of the fiber.
However, in standard single mode fiber, with small and
random value of the birefringence, there occurs a strong cross
coupling between the two SOP of the traveling light wave. In
this case the value of PMD is statistically measured, as the
delay caused by the two orthogonal modes is a random
function over a certain length of the fiber.
Of course, It was no until recently, when the single channel bit
rates reached beyond 10 G bit /s, that PMD became interesting
to a larger technical community. PMD is now regarded as a
M. Zafrullah and M. Aleem Mirza are with University of Engineering &
Technology, Taxila, Pakistan.
M. Waris is with Univ. College of Engg. & Tech. Mirpur AK, Pakistan.
M.K.Islam is heading the Department of Computer Sc. & IT, Mirpur,
Univ. of AJ&K, Pakistan.
Email: [email protected]
major limitation in optical transmission systems in general, and
an ultimate limitation for ultra-high speed single channel
systems based on standard single mode fibers. In the future, the
PMD obstacle could be ultimately solved by using polarization
maintaining fibers and polarizers [5]. Dispersion-shifted fibers
(DSF) were developed as the solution to the dispersion
problem. However, fiber manufacturers had little concern
about PMD, and as DSFs have a smaller core area than
conventional single-mode fibers, they are more sensitive to
core ellipticities. This resulted in fibers with relatively high
birefringence and consequently, high PMD. The installation of
DSF-links was widespread in he early 1990’s and some of
these links are entirely limited by PMD even at a low data-rate
of 2.5 G bit /s [6]. As installation of new fiber often represents
a major cost in a complete transmission link, particularly in
highly populated areas, the utilization of already installed fiber
is preferable, especially when a large part of the installed fibers
is still not utilized. Therefore, ways to reduce or overcome the
PMD obstacle are desirable. One way to reduce the PMD of
installed fiber links could be to replace only the parts that have
large PMD, but rather, as installed links often consist of many
parallel fibers, the traffic could be rerouted in he hubs along the
link, so that a low-PMD “path” would be selected by simply
switching fiber connectors [5,6].
Techniques for measuring the distribution of PMD along a
fiber cable would thus be very useful in encountering the set
backs in the high bit rate communication networks, where we
have dealt this problem with practical measurements carried
out in the lab set up to a high accuracy level. Though there
does exist PMD measurement techniques in optical fibers
[2,3] like Poincar´e sphere method, Jones matrix method,
Fixed analyzer technique, but our focus has been the
Interferometric method, which provides direct measurement of
PMD in time domain. Over last few years there has been
significant work carried out using interferometric method to
measure PMD in single mode ordinary optical fiber widely
used in telecommunication applications [3,4]. The designed
interferometric set-up is not only useful to measure the PMD
of standard telecommunication fibers but equally well capable
of measuring the birefringence (or beat-length) and PMD of
hi-bi (mostly called as Polarization Maintaining) optical
fibers.
PMD THEORY
Differential group delay: PMD is actually the delay caused
by the two orthogonal polarization components of the single
mode traveling through an optical fiber. The presence of
birefringence between these two orthogonal axes creates a time
delay (∆τ) between the two degenerated polarization modes,
called polarization mode delay. In case of hi-bi fibers, where
weak cross coupling occurs between two orthogonal SOP, the
weighted average of ∆τ is calculated to find the effective
polarization mode delay i.e. differential group delay (DGD) as:
= Amplitude cross coupling of E field from
polarization axis X to Y.
x( ) = Autocorrelation function of light source
along polarization axis X.
DGD = (P. ∆τ) / P, ------------------- (1)
where P is the intensity of the cross-coupled light energy at a
particular point in fiber. However in ordinary single mode fiber
there is a strong cross coupling between the two polarization
modes and fiber can be modeled as succession of randomly
oriented birefringent crystals connected in series.
The
weighted average of ∆τ is calculated through Gaussian fit and
the second moment of the Gaussian fit gives differential group
delay as:
DGD = { (P. ∆τ2) / P}1/2 -----------------(2)
PMD coefficient: In case of weak cross coupling between
polarization modes, the PMD coefficient is given as DGD per
unit length of the fiber, and the typical units are pico second
per meter. On the other hand in case of strong polarization
cross coupling the coefficient is DGD divided by the square
root of the fiber length [3,4]. This is due to the statistical
nature of DGD in ordinary single mode fiber and the typical
units in this case are pico seconds/√kilo meter.
White Light Interferometry: This technique measures ∆τ
when the two orthogonal polarization modes are crosscorrelated in time domain. The cross correlation is attained
through an analyzer placed at 45° to both the axes (fig.1). The
delay is induced in the cross correlator by a scanning mirror of
the Michelson interferometer. When the delay caused by the
path difference of the scanning mirror is tuned to the ∆τ, the
two orthogonal states become coherent and interfere. The
intensity of the interference is proportional to the energy
coupled from one axis to the other (ε). Fig.1 explains the
function of cross correlator for a fiber with weak polarization
cross coupling at certain points. The output interferogram thus
identifies the precise location of such points and also assists us
in evaluating the birefringence and h-parameter of such fiber.
In a fiber with strong and random polarization cross coupling
the interferogram contains closely spaced fringes and PMD can
be calculated through Gaussian fit. The use of broadband
source (hypothetically called a white light source) increases the
precision, at each point where path difference of interferometer
is matched to ∆τ, the image of source auto-correlation function
is produced The following expression for the output intensity
of scanning white light interferometer (complete derivation
provided in the annex-I, handouts), depicts this mechanism:
I det =
1
∆nbl
∆L
∆L + ∆nbl
∆L − ∆nbl
Γx (0) + 2εΓx
+ Γx
+ εΓx
+ εΓx
2
c
c
c
c
----------------------------------------(3)
where
nb
= Birefringence of Optical Fiber.
l
= Distance between point of polarization
cross coupling and fiber end.
c
= Speed of light in free space
L = Scan length of WLI from its balance point
This also shows that smaller the coherence length of the light
source better is the measurement accuracy of the system.
PMD MEASUREMENT
Design Methodology: The design practicality of the PMD
measurement has been standardized by the simulations
conducted for the conformity of the results obtained through
the careful handling of the interferometry set-up. The
flexibility finds much space for the added features of the
followed research techniques. The accuracy is the robust trait
of this set-up over the techniques discussed earlier. However,
the more practical approach, as adapted by many other
researchers and test equipment manufacturers in the world, is
definitely the interferometry as it measures the PMD directly in
time domain. Our choice for this methodology is mainly due
to the same particular reason, nevertheless the following
specific design features are incorporated in addition to the
standard design of white-light interferometer:
1.
2.
3.
4.
5.
6.
7.
Large scan length is possible, thus resulting in
successful testing of hi-bi fibers of length upto 1km.
Germanium detector is used with pW sensitivity.
Signal-to-noise ratio is improved by using the phase
modulation technique.
Phase modulation is
implemented by vibrating the stationary mirror
through a piezo actuator. Demodulation is done
through a lock-in amplifier, which has sensitivity in
µvolts.
Further improvement in signal-to-noise ratio is
attained through servo controlled highly precise
scanning of mirror with an accuracy of 0.1 µm.
The whole set-up is operated on a vibration-isolated
platform to achieve repeatability of testing.
ELED light source (center wavelength at 1310 nm) is
used to excite the fiber under test. The coherence
length of source is smaller than normal SLD source.
This feature is important to measure the low values of
PMD in either types of the fiber.
In-line fiber optic polarization controller is also used
to measure the PMD when input state of polarization
(SOP) is changed.
EXPERIMENTAL SET-UP
The schematic of the set-up established in lab is shown in
Fig.2. A broadband ELED source is used with center
wavelength at 1.31 µm and FWHM of 70nm. Such broadband
source has a coherence length of 24.5 um, thus setting a
measurement accuracy of 0.08 ps for our set-up. An all fiber
light launch system is used to enhance the measurement
accuracy for the cross-coupled intensity signal. This features
helps in enhanced contrast of the interferogram. A nonpolarizing pellicle type beam-splitter (BS) with 50:50 splitting
ratio is used. Sensitivity of the system is further increased by
the use of the critical components as mentioned in design
methodology. The servo controlled translational stage can
provide a scan length of 50 cm easily, thus making it possible
to measure PMD of larger lengths of hi-bi (or PM) fibers. In
case of standard single mode fiber with strong cross coupling
and large lengths (>2km), the need for input polarization
controller and output analyzer is not significant. However for
hi-bi fibers or short lengths of ordinary fiber the PMD is
dependent on the SOP of input light, which can further be
controlled by the polarization controller.
Salient Features: Some important features
applications of our white-light Interferometry set-up are:
1.
2.
3.
4.
5.
6.
7.
8.
and
The white light interferometric set-up provides direct
measurement of P and ∆τ from every point of
polarization cross coupling, contrary to other PMD
measurement techniques which need post analysis
after the measurements.
It can measure DGD or effective delay ∆τ as low as
0.1 ps.
It has a wide dynamic range for measuring fiber
lengths of more than 100 km (ordinary single mode
fiber).
It can also measure PMD, birefringence and hparameter of hi-bi fibers (upto 1 km length).
The set-up can measure the coherence length and thus
spectral properties of a light source, and also can
determine the cavity length of laser diode and SLD by
evaluating its multimodal structure present in
interferogram.
The set up can measure the polarization cross
coupling at the fiber joints e.g., thus making it a good
tool to characterize the angular errors in PM fiber
splices and fiber pigtails for the sources and devices.
Matlab program to calculate PMD in ordinary single
mode fiber (SMF) is developed using the statistical
techniques [3].
Since the output interferogram of any device under
test (DUT) is recorded in PC, an intensive analysis of
the results is possible through post processing of the
data.
this as 1.5 ps/m. The values thus measured are quite
comparable to those given by the manufacturer of PM fiber. A
few other types of PM fibers are also characterized on this set
up and their measured values of PMD and birefringence are
also found similar to those mentioned in their specifications.
PMD in Ordinary SMF: Single mode fibers are tested
through Scanning Interferometer to evaluate their PMD value
and PMD coefficient. A software program, using the ITUG650 standard as baseline, has been written to fully automate
the measurement of PMD. Fig.4 shows interferometric data
used to compute the PMD for a sample length of 25km of
SMF. The value measured through our lab set up has also been
compared with the values measured by the industrial PMD
measurement equipment, and found similar. Quite a number of
SMF spools of different lengths are tested, and it is found that
accuracy in measuring PMD is again dependent upon the
coherence length of light source and also on the accuracy of
Scanning (especially for shorter fiber lengths).
CONCLUSIONS
White light Interferometry set-up developed is a multiple-use
instrument for variety of research and development activities in
the fields of fiber optic communications and sensors. It has
been used to measure PMD for telecommunication grade single
mode fiber, and hi-bi polarization maintaining fiber. The latter
types of fibers are considered specialty fibers and there is no
off-the-shelf equipment available to characterize their
birefringence or PMD. The lab based set developed fulfils this
purpose also. Successful results have been taken for both
ordinary and PM types of single-mode optical fibers. The set
up can also be used extensively to characterize the spectral and
coherence related properties of light sources, as well as various
other fibers and integrated optical devices.
REFERENCES
[1].
N.M. Breton, G.W. Schinn, “PMD or Multipath interference
dispersion:
which measurement is of more practical
importance”, Journal of Wave Review, EXFO, 2000.
[2].
B.L. Heffner, “Single-mode propagation of mutual temporal
coherence: equivalence of time and frequency measurements
of polarization-mode dispersion”, Optics Letters, Vol.19,
No.15, August, 1994.
[3].
ITU-T Recommendations G650: Definition and Test
Methods for the relevant parameters of single-mode fiber.
[4].
TIA/EIA-455-124 Standard, FOTP-124: Polarization-Mode
Dispersion Measurement
for Single-Mode Optical Fibers
by Interferometry, 1999.
[5].
C.J.Nielsen, “Influence of polarization mode coupling on the
transmission-bandwidth
of
a
single-mode
fiber,”
J.Opt.Soc.Am., vol.72,pp 1142-1146, 1982.
[6].
A.Hasegawa and F.Tappert,‘Transmission of stationary
nonlinear optical pulses in dispersive fibers - Anomalous
dispersion,”Appl.Phys.Lett.,vol.23, pp.142-144,1973.
RESULTS
Various measurements are carried out on the white light
interferometric set up developed in the lab. The typical
interferograms obtained for various tests are shown in Figures
3 and 4. The effect of fiber pigtail with ELED source is also
obvious from the interferogram and secondary peaks are
accompanying the main autocorrelation function of the light
source [1].
PMD in PM Fiber: In order to measure the birefringence,
the light is launched into both axes of the PMF and phase delay
is measured through Scanning Interferometer. In order to
ensure the calculated value of birefringence, tests may be
performed on different lengths of same PMF. Fig.3 shows the
interferograms to measure birefringence values for 5m and
11m of the fiber lengths. The calculated average value is
4.5x10-4. The PMD value can also be calculated directly from
I
I
ε2I
Interference contrast
I
ε 2I
x
Analyzer
I
ε2I
y
M
B
45°
M
Fig.1. Measurement of ∆τ through an unbalanced Michelson interferometer
P ower
M e te r
D e t e c to r
In t e r f e r o m e t e r
M o tio n
C o n t r o lle r
CFP
P o la r iz a t io n
C o n t r o lle r
DUT
ELED
M2
BS
C o llim a t in g
O p tic s
T .S .
C F P : C o ile d F ib e r P o la r iz e r
P T : P ie z o T r a n s d u c e r
B S : B e a m S p lit t e r
D U T : D e v ic e U n d e r T e s t
M 1 , M 2 : M ir r o r s
T S : T r a n s la t io n S t a g e
A / D : A n a lo g t o D ig it a l
L o c k - In
Amp
Ref
M1
PT
A /D
O s c illa t o r
C o m p u te r
Fig 2. Schematic of white light interferometric set-up
0
0
-5
-5
9
-10
-10
8
-15
-20
-25
-30
-35
-40
7
-20
Interference Signal
Interferenc e S ignal (dB)
-15
Interferenc e S ignal (dB )
10
-25
-30
-35
-40
5
4
3
-45
-45
6
2
-50
-50
1
-55
-55
0
0.5
1
1.5
2
2.5
3
3.5
Mirror Displacement (mm)
4
4.5
5
Fig.3a : Interferogram For
the Birefringence Test of 5m PMF
0
0
0.5
1
1.5
Mirror Displcement (mm)
2
2.5
Fig.3b : Interferogram For the
Birefringence Test of 11m PMF
0
1
2
3
4
Interferometer Delay (ps)
5
6
7
Fig.4: Interferogram of 25Km
SMF spool, PMD = 0.26 PS/(Km)1/2