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IEEE SENSORS JOURNAL, VOL. 12, NO. 8, AUGUST 2012
Temperature and Strain Independent Modal Interferometric Torsion
Sensor Using Photonic Crystal Fiber
Sandipan M. Nalawade, Sagar S. Harnol, and Harneet V. Thakur, Member, IEEE
Abstract— An all-fiber modal interferometric torsion sensor
based on simple splicing configuration is presented wherein a
photonic crystal fiber is spliced in between multimode fiber and
single mode fiber. Appreciable torsion sensitivity of ∼79.83 pm/°
is obtained in the long dynamic range of 180°. It is also found that
there is no effect of strain and temperature on torsion sensitivity
in the range of 0-4500 με and 30–200 °C, respectively, leading
to zero cross-sensitivity of torsion with strain and temperature.
Index Terms— Modal interferometer, photonic crystal fiber,
strain, temperature, torsion.
Fig. 1.
Schematic of the experimental set up.
In the present work, we have focused our attention on
fabricating a torsion sensor having negligible cross sensitivity
to both strain and temperature.
I. I NTRODUCTION
I
N RECENT years, modal interferometric sensors using
PCF have been established due to their inherent advantages
namely its ease of fabrication, cost effectiveness along with
unique characteristics of PCFs. In past, these interferometers
have been widely used for measurement of various physical
parameters viz. strain, temperature, displacement, vibration,
refractive index and torsion [1]–[5]. In structural health monitoring (SHM), accurate measurement of various physical
parameters play an important role. For such applications,
measurement of one parameter should be ideally independent
of the other parameters. Till date not much attention has
been given to torsion measurement using fiber optic sensors,
although it plays an important role in such applications.
In recent years, torsion sensors based on Sagnac interferometer using different PCF geometries have been explored
[6]–[8]. In general, researchers have focused on improving
the torsion sensitivity with exploring its dependence only on
temperature. Zu et al. [7] have reported the maximum torsion
sensitivity of 1 nm/° with ultralow temperature sensitivity
−0.5 pm/°C in the range of 30–100 °C. Chen et al. [6] have
observed slightly lower torsion sensitivity ∼0.94 nm/° with
comparatively higher temperature sensitivity of 50.8 pm/°C.
Both temperature and strain independent torsion sensor was
demonstrated by Frazao et al. [9] using Sagnac interferometer.
PCF based modal interferometer for torsion sensing has
only been exploited by Frazao et al. [5] wherein amplitude
of FFT peaks has been monitored. This set up is critical due
to interference of two polarized spatial modes of the PCF.
Manuscript received October 3, 2011; revised March 5, 2012; accepted
April 25, 2012. Date of publication May 25, 2012; date of current version
June 6, 2012. This work was suppored in part by the Defense Institute
of Advanced Technology (DU) and Defense Research and Development
Organization, India. The associate editor coordinating the review of this letter
and approving it for publication was Prof. Julian C. C. Chan.
The authors are with the Department of Applied Physics, Defense
Institute of Advanced Technology, Pune 411025, India (e-mail:
[email protected]; [email protected]; [email protected]).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JSEN.2012.2198809
II. E XPERIMENT
All-fiber modal interferometer is fabricated by splicing PCF
PM-1550-01 (Crystal Fibre A/S, Denmark) of length 16 cm
in between standard MMF and SMF fibers. The schematic of
the set up is as shown in Fig. 1.
The core mode and cladding modes of PCF have been
excited by using lead-in MMF fiber. MMF eases the coupling
of light into PCF unlike SMF at lead-in, while SMF at the leadout helps in getting the interference pattern at the splice region.
An optimized core offset of 4.23 μm has been given at the
input splice between MMF and PCF with 11 dB insertion loss
of the sensor. Also the length dependency of fringe spacing is
highly minimized due to lead-in MMF in the proposed configuration giving a more stable interference assisting strain insensitivity as compared to previously reported SMF-PCF-SMF
configuration [1]. In past, different structures using different
fibers have been explored as modal interferometers for various
sensing applications [10]–[13]. The optical sensing analyser,
Si-720 of Micron Optics Inc. has been used for measurements. Torsion has been applied to the fiber using rotational
mounts fitted on translation stages whereas temperature and
strain responses are studied using cylindrical oven with digital
controller and translation stages, respectively. For all the
measurements the dip around 1552 nm has been tracked.
III. R ESULTS AND D ISCUSSION
Transmission spectrum obtained is as shown in Fig. 2
and its Fast Fourier transform (FFT) is shown in inset of
the figure. The figure revealed the understanding of modes
involved in the interference. The four peaks obtained in the
FFT corresponds to two core and two cladding modes, out
of which only dominant core mode and a cladding mode
resulted in the obtained interference pattern. Torsion effects
on the fiber have been observed and is as indicated in Fig. 3,
it clearly shows the wavelength shift with twist angles at 0°,
10° and 20°. Appreciable torsion sensitivity of ∼79.83 pm/°
has been obtained in large dynamic range of 180° with very
1530–437X/$31.00 © 2012 IEEE
NALAWADE et al.: TEMPERATURE AND STRAIN INDEPENDENT MODAL INTERFEROMETRIC TORSION SENSOR USING PCF
Fig. 2.
Transmission spectrum. Inset: spatial FFT.
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temperature. The reasons for temperature insensitivity lie in
the fiber composition and geometry [1], [14] whereas ultralow
strain sensitivity of the sensor is due to inverse relation of
ne f f (effective refractive index difference between core
mode and excited cladding mode) with PCF length, as also
mentioned by Zu et al. [15].
It is worth mentioning that, although the obtained torsion
sensitivity is less than that obtained by Kim et al. [8], it is
totally independent of temperature and strain effects.
To conclude, torsion sensor with appreciable torsion sensitivity over a long dynamic range has been demonstrated.
The obtained sensitivity is not affected by strain and temperature variations in significantly high ranges of strain
(0–4500 με) and temperature (30-200 °C), respectively, and
hence can play critical role in applications like SHM.
R EFERENCES
Fig. 3.
Transmission spectra at different twist angles.
Fig. 4.
(a)
Fig. 5.
Torsion response of the sensor.
(b)
(a) Strain and (b) temperature response of the sensor.
good linearity of 99.2%, as indicated by Fig. 4. Resolution of
the sensor has been limited to 0.05° due to 4 pm resolution
of the sensing analyser.
Strain and temperature characteristics of the modal interferometer have also been explored. Shift in the same wavelength dip has been observed for both strain and temperature responses and is represented by Figures 5, respectively.
Temperature response has been taken for two torsion values.
Figures 5 clearly indicate that there are slightly random
variations in the wavelength shift with both strain and
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