This paper presents the eect of light polarization in a hybrid ber optic sensor. The hybrid sensor is dened as a combination of interferometer sensor and modalmetric sensor. Hybrid sensor system is based on the eect of outputting interferometric sensor of interference as a result of changes made by the modalmetric system. Balance of polarization in the arms of classical interferometer sensor leads to improvement of contrast of interference pattern at the output of the interferometer. Aim of this study is to present a balance light polarization eects in a hybrid system

M Karol

M Szustakowski

M Yczkowski

P Markowski

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Vol. 124 (2013) ACTA PHYSICA POLONICA A No. 3
Optical and Acoustical Methods in Science and Technology 2013
Balance of Polarization in a Hybrid Fiber Optic Sensor
M. yczkowski∗, M. Szustakowski, P. Markowski and M. Karol
Military University of Technology, Institute of Optoelectronics, S. Kaliskiego 2, 00-908 Warsaw, Poland
This paper presents the eect of light polarization in a hybrid ber optic sensor. The hybrid sensor is dened
as a combination of interferometer sensor and modalmetric sensor. Hybrid sensor system is based on the eect of
outputting interferometric sensor of interference as a result of changes made by the modalmetric system. Balance of
polarization in the arms of classical interferometer sensor leads to improvement of contrast of interference pattern
at the output of the interferometer. Aim of this study is to present a balance light polarization eects in a hybrid
system.
DOI: 10.12693/APhysPolA.124.610
PACS: 42.81.Pa, 42.30.Wb, 42.79.Qx
1. Design and operation of examined system
This paper describes the eect of light polarization
in the function of the hybrid MachZender interfero-
meter. Hybrid interferometers systems are the subject
of research of many groups of researchers from dierent
disciplines. Hybrid system combines the features of in-
terference ber optic sensor and modalmetric ber optic
sensor [1]. Modalmetric ber sensor in hybrid system
replaces the entire or a part of one arm of the inter-
ferometer. At the output of the hybrid system, interfer-
ence pattern is recorded in the same way as in the classi-
cal interferometer. The relationship between the optical
path dierence between the arms of the interferometer
can be expressed according to the following formula (1):
∆Φ =
2π
λ0
∆L, (1)
where ∆Φ  phase shift, ∆L optical path length shift,
λ0  wavelength.
In the ber optic interferometry, interference fringes
are observed as a change of the intensity of the radiation
in time. This change is a result of two waves, and the
intensity of the radiation at the output of the interfero-
meter can be described by Eq. (2):
IOUT = 〈EE∗〉, (2)
where
E = E1 + E2, (3)
E1, E2  amplitude vectors of light propagating in the
rst and second arm of the interferometer.
A measure specifying the interference is contrast of in-
terference fringes. Interference contrast can be described
by the formula (4):
V =
Imax − Imin
Imax + Imin
, (4)
where Imax maximum light intensity, Imin minimum
light intensity.
∗corresponding author; e-mail: mzyczkowski@wat.edu.pl
Maximum possible radiation intensity is obtained if
two waves of two interferometer arms are in phase while
the minimum possible intensity is achieved when the two
waves are out of phase. In other cases, the radiation
intensity is a function of both waves amplitude vectors.
In the case of light waves critical dependence on obtain-
ing any interference is the state of polarization of the
two waves. Polarization is a measure of the oscillation of
propagating wave. Polarization states of light can be vi-
sualized in many ways, one of them is to determine point
of the sphere known as the Poincaré sphere [2]. Each
point on the sphere represents a distinct state of polar-
ization, which can be described by two angular values.
The Poincaré sphere can contain several characteristic
points. The poles represent the linear polarization state,
while the equator of sphere represents the circular po-
larization states. All the remaining points on the sphere
are all possible intermediate states of elliptical polariza-
tion. In the case of ber-optic interferometry, critical
value of the polarization are orthogonal states for which
it is impossible to obtain an interference image. Interfer-
ence fringes with a maximum amplitude can be achieved
for the waves with two similar polarization states. If the
polarization of the two waves is not uniform and dier-
ent from the orthogonal interference, obtained pattern
has a reduced amplitude in proportion to the dierence
in polarization states of both waves.
The above description applies to the interference sys-
tem. In the hybrid system output signal is also dependent
on the response of the modalmetric system. Modalmet-
ric part of sensor is a system integrating lateral distribu-
tion of radiation. The connection between the multimode
ber and single-mode ber is performed by an integrator
of the specle pattern of radiation. Specle pattern of the
radiation propagating in the ber may be described by
the formula (5):
I(x, y, z) = lim
T→∞
1
T
∫ T/2
−T/2
|u(x, y, z, t)|2 dt = |A(x, y, z)|2 ,
(5)
where x, y, z  geometric dimensions of the ber.
When the external disturbance occurs, the transverse
distribution of radiation changes and the transfer of prop-
(610)
Balance of Polarization in a Hybrid Fiber Optic Sensor 611
agated optical power between excited modes or excitation
of higher order modes proceeds. Observation of the seg-
ment of transverse distribution at the time causes that
when the disorder occurs, the intensity of radiation on the
part of the cross-section also changes (quantitatively).
This type of event is observed at the output, as a di-
rect change in the amplitude of radiation depends on the
occurrence of disturbances.
The combination of the interference system with the
modalmetric system causes that the output signal car-
ries information consistent with the operation of the in-
terferometric and modalmetric sensor. The operation of
the modalmetric sensor signicantly aects the interfer-
ence pattern by introducing the direct amplitude changes
corresponding with the nature of external disturbances.
Direct change of the amplitude of the signal propagating
in one interferometer arm caused by a disorder, deteri-
orates the visibility of interference pattern giving direct
information about a disturbance. In this way, in a hy-
brid system, it is possible to precisely measure changes
in the signal generated by the modalmetric system. This
leads to increased sensitivity and speed of operation of
the entire system.
2. Examined sensor system
The system was built using standard optical compo-
nents used in telecommunication. For construction pur-
poses, the following components were used: single mode
of 9 µm core and 125 µm cladding and multimode of
62.5 µm core and 125 µm cladding. As a light source,
a laser-C5-1550 Series of multi quantum well (MQW) dis-
tributed feedback (DFB) was applied, and as a detector
standard telecommunications diode. The electrical signal
was processed by the oscilloscope DLM 2054 Yokogawa.
As disorder generator, a piezoelectric modulator with 8 m
of multimode coiled ber was used, and the modulator
was triggered by the generator AFG3021 Tektronix. The
MachZehnder interferometer system consisted of two
arms  reference arm with a length of 800 m made with
single-mode ber and a measuring arm with the length
of 800 m made with three sections of single-mode ber
at 250 m and 50 m section of a multimode optical ber.
A diagram of the system is presented in Fig. 1.
Fig. 1. The experimental setup.
Part of sensing structure for this type of sensor is a part
of the measurement arm built with a multimode ber.
The test system was equipped with a General Photon-
ics electric polarization controller in order to compensate
changes in polarization between the arms of the interfero-
meter.
3. Experimental details and results
The system was tested for response to various types of
signals applied to the piezo modulator. The test signals
was:
• 1 Hz sine wave, presented in Fig. 2;
• 1 Hz rectangular wave, presented in Fig. 3;
• 1 Hz triangular wave, presented in Fig. 4;
• 1 Hz Lorentz wave, presented in Fig. 5.
The system was tested in terms of response for the
matched polarization states between the two arms of
the interferometer, and the lack of matched polarization.
The results are presented in Figs. 25.
Fig. 2. The system response (red) to force a 1 Hz
sine wave, (a) without matching polarization, (b) with
matching polarization in both arms of the interfero-
meter. Reference signal is marked with blue colour.
Fig. 3. As in Fig. 2, but for rectangular wave.
Fig. 4. As in Fig. 2, but for triangular wave.
The tested system was examined in terms of sensitiv-
ity to the frequency range 1 Hz1 kHz. Characteristics
of sensitivity as a function of frequency are presented
in Fig. 6.
As presented in Fig. 6, polarization matching in both
arms of interferometer is essential for high sensitivity of
the hybrid system.
612 M. yczkowski et al.
Fig. 5. As in Fig. 2, but for a Lorentz wave.
Fig. 6. Characteristics of sensitivity as function of fre-
quency.
The nal stage of the investigations was to measure
the response time of the system. For this purpose, as a
disorder a square wave signal was used. The measured
parameter was the rise time dened as the time between
the achievement of 10% and 90% of the maximum ampli-
tude of the signal. The results are presented in graphic
form in Fig. 7.
Fig. 7. Sensor response to rectangular stimulation.
Numerical results for the measurement of the response
time are presented in Table.
4. Conclusions
Analyzing the results, especially the characteristics of
the sensitivity of the hybrid system one can conclude
that matching polarized waves propagating in the two
arms of the interferometer is essential for the entire sys-
tem. In the hybrid system an interferometric measure-
ment leads to intensication of the response of modal-
metric system. In the output signal, direct amplitude
response of modalmetric sensor is modulated by interfer-
ence image (fringes). This fact is a reason that during the
TABLE
The timing parameters of hybrid and classical
interferometer.
Amplitude
Time [ms]
Hybrid Classic
10% of max 3.888 3.89
90% of max 7.352 7.904
rise time T90%T10% 3.464 4.014
occurrence of disturbances, an interference image is im-
mediately changed and the change is proportional to the
appearing disturbance. However, matching of the state
of polarization waves of the two interferometer arms also
increases the speed of system response. In this case, the
interferometer generates better quality interference im-
age to provide more accurate response of modalmetric
system. Applying hybrid systems to known ber optic
sensors structures [311] with matching of polarization
causes quicker reaction and identication of the external
disturbance. This method of improving the output sen-
sor signal can provide even more accurate localization of
the disturbance and identication of the type of disor-
der. These improvements can be used to eliminate false
alarms caused by environmental sources.
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Balance of Polarization in a Hybrid Fiber Optic Sensor

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Balance of Polarization in a Hybrid Fiber Optic Sensor

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