Highly sensitive twist sensor based on tilted fiber

Optical Fiber Technology 20 (2014) 491–494
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Optical Fiber Technology
www.elsevier.com/locate/yofte
Regular Articles
Highly sensitive twist sensor based on tilted ﬁber Bragg grating of
polarization-dependent properties
Yanfang Lu a,⇑, Changyu Shen a,b, Debao Chen a, Jinlei Chu a, Qiang Wang c, Xinyong Dong a
a
Institute of Optoelectronic Technology, China Jiliang University, Hangzhou 310018, China
Department of Electronics, Carleton University, Ottawa, Ontario K1S 5B6, Canada
c
College of Quality and Safety Engineering, China Jiliang University, Hangzhou 310018, China
b
a r t i c l e
i n f o
Article history:
Received 3 February 2014
Revised 16 May 2014
Available online 25 June 2014
Keywords:
Fiber grating
Polarization-dependent properties
Tilted ﬁber Bragg grating (TFBG)
Twist sensing
a b s t r a c t
The transmission intensity of the tilted ﬁber Bragg grating (TFBG) is strongly dependent on the polarization properties of the TFBG. The polarization characteristic of the cladding modes can be used for twist
measuring. In this paper, a highly sensitive ﬁber twist sensor is proposed. The transmission intensity
on the strong loss wavelength showed a quasi-sin h changing with the twist angle ranging from 0° to
180° for S- or P-polarized input. A high sensitivity of 0.299 dB/° is achieved, which is almost 17.9 times
higher than that of the current similar existing twist sensor. The twist angle can be measured precisely
with the matrix.
Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction
Twist sensing is important to monitor the health condition of
engineering structures such as bridges and buildings. The most
common approach during the designing twist ﬁber sensors is to
measure change in circular birefringence caused by ﬁber twist,
such as circular birefringence in Sagnac loops twist sensing [1–
3]. So far, a number of optical ﬁber twist sensors based on speciﬁc
ﬁbers and ﬁber gratings have been demonstrated, integrated polarization-maintaining ﬁbers (PMF) birefringence [4], square coreless
ﬁbers [5], ﬁber Bragg grating in-ﬁber twist sensor[6,7], and so on.
However, the Sagnac-based twist sensors usually suffer from temperature dependence and instability. (Long period ﬁber gratings)
LPFGs present cross sensitivities between different physical
parameters such as temperature, pressure, strain and so on [8].
Wang [7] improve the sensitivity by using polarization dependent
loss (PDL) multimeter with the sensitivity of 0.955 dB/°. Both the
interferometric and grating sensors usually suffer from temperature dependence and require relatively complex signal interrogation. Recently, the tilt ﬁber Bragg gratings (TFBGs), which
enhance the coupling of light from the core mode to a large
number of counter propagating cladding modes, have been utilized
[9–15]. Such as Chen [11] proposed a novel twist sensor based on a
ﬁber Bragg grating with 81° tilted structure and Zhen [14]
proposed a magnetic ﬁeld sensor by using tilted ﬁber grating.
⇑ Corresponding author.
E-mail address: [email protected] (Y. Lu).
http://dx.doi.org/10.1016/j.yofte.2014.05.011
1068-5200/Ó 2014 Elsevier Inc. All rights reserved.
This paper presents a highly sensitive TFBG twist sensor based
on the polarization-dependent properties of the cladding modes.
Owing to the polarized properties of the TFBG, the twist angle
changes can be obtained by measuring the variation of the transmission intensity. A high sensitivity of 0.299 dB/° is achieved,
which is almost 17.9 times higher than that of the current similar
existing twist sensor.
2. Sensor fabrication and operation principle
A TFBG of 10 mm long was written in hydrogen-loaded photosensitive Corning SMF-28 ﬁber with a pulsed KrF excimer laser
by using the phase-mask method. A tilt angle of 10° leads to a large
number of high order cladding mode resonances. Because of the
existence of tilted angle, besides the mode coupling between forward propagating and counter-propagating core modes, the core
mode also couples a multitude of cladding modes (each at a different wavelength) shown up as loss peaks in the transmission spectrum. The Bragg wavelength and the ith order cladding mode
resonance wavelengths of the TFBG can be expressed as [15]:
kBragg ¼ 2Neff ðcoreÞ K= cosðhÞ
ð1Þ
kiclad ¼ ðNeff ðcoreÞ þ Nieff ðcladÞÞ K= cosðhÞ
ð2Þ
N ieff ðcladÞ
where Neff ðcoreÞ and
are the effective indices of the ﬁber
core and cladding.
Fig. 1 shows the schematic diagram of the twist sensor. An
ampliﬁed spontaneous emission (ASE) source of 1450–1650 nm
492
Y. Lu et al. / Optical Fiber Technology 20 (2014) 491–494
Fig. 1. Schematic diagram of the experimental setup.
wavelength range is used as the light source. The output spectrum
is detected with an optical spectrum analyzer (OSA, AQ6370,
Advantest, Japan). The maximum resolution of the OSA is 20 pm.
A polarization controller (PC) is used to adjust the polarization
states of the input light in order to obtain a high fringe visibility.
The principle of the proposed sensor can be explained by Fig. 2.
The tilted grating planes break the azimuthally symmetry of the
ﬁber and two orthogonal polarization states (P-polarized and
S-polarized states) of the electrical ﬁeld input light can be deﬁned.
The P-polarized light parallels to the x–z plane and S-polarized
light perpendicular to the x–z plane. Assumed that the input E-ﬁeld
vector Ei is P-polarized for the TFBG when the ﬁber has no twist
shown in Fig. 2(a), the excited cladding mode of Ec is P-polarized
because of the cladding mode characteristic of the TFBG. And consequently, the S-polarized signal is minimized. When the TFBG is
twisted, the input light Ei will have a twist angle shown in
Fig. 2(b). As a result, the cladding mode of Ec also has a twist angle
with the x–z plane decomposed as Ecx and Ecy .The Ecx is P-polarized
Fig. 2. Polarization of the cladding mode for the TFBG: (a) no twist, (b) twist.
Fig. 3. The measured transmission spectra of a TFBG sensor at P-polarized states on the different angles in clockwise twist, inset: detailed spectra from 1544 nm to 1548 nm.
Y. Lu et al. / Optical Fiber Technology 20 (2014) 491–494
493
Fig. 4. The intensities response to the twist angle for the selected resonance wavelength.
and the Ecy is S-polarized with the equations of Ecx ¼ Ec cos h and
Ecy ¼ Ec sin h; when the twist angle is h. From the equations, we
can see that the transmission spectrum of the P-polarized state
and S-polarized state of the TFBG can be expressed as quasi-sin h
changing spectrum. On the contrary, when the input light is
the S-polarized state, the results are just the opposite of the
analysis.
Before the ﬁber was twisted, we set the pre-polarization state at
P-polarization by adjusting the PC. We applied a small axial tension to the ﬁber maintaining it straight to eliminate measurement
errors from axial-strain and bending effects. The transmission
spectrum of the TFBG with clockwise twist used for the experiments reported is shown in Fig. 3. The inset shows the detailed
intensities evolutions of the cladding modes from 1544 nm to
1548 nm with the twist angle. In Fig. 3, the P-polarized states
become weak gradually and the S-polarized states increase
simultaneously with the TFBG sensor twist in clockwise range of
0–90°. When the input light is the S-polarized, the results were just
opposite to that of the P-polarization input light.
Fig. 4 shows the intensities response to the twist angle for the
selected resonance wavelength of the cladding mode. It can be
seen that with the increasing of the twist angle, the intensity of
P-polarized signal became weak. In order to obtain the twist sensitivity of the sensor, the transmission spectra were recorded by
increasing the twist angle from 0° to 180° with an interval of 9°.
It shows the resonance wavelength’s intensity periodically and
the variation trend is similar to Sinc function as shown in Fig. 5.
It is now clear that the resonance wavelength’s intensities of the
P- and S-polarization signal are changing with twist angle in a Sinc
function. However, in practical application, we hope to obtain a linear relationship. Therefore, as we plot the intensities of the absolute of the P-minus S-polarized signal, the linear relationship of
the twist and the transmission loss in the spectrum was obtained
in clockwise and anti-clockwise directions, as Fig. 6 shows. The lin-
Fig. 5. Resonance wavelength’s intensity shifts with the twist angle for P- and Spolarized input.
Fig. 6. Intensities of the absolute of the P- minus S-polarized signal shifts with the
twist angle.
3. Experiment and discussions
494
Y. Lu et al. / Optical Fiber Technology 20 (2014) 491–494
Fig. 7. (a) Intensity variations against temperature change, (b) wavelength shift against temperature change.
ear ﬁt with a high value of 0.999 for clockwise and anti-clockwise
were obtained. And the sensitivity of 0.299 dB/° was obtained.
Beneﬁted from the using of intensity demodulation method, the
proposed sensor has an application advantage in marked contrast
with other grating based twist sensors which usually employ
high-cost wavelength-shift means.
To conﬁrm the expected temperature insensitivity of the twist
sensor, the changes of intensity versus temperature dependences
were tested. In experiment, the TFBG was mounted in a temperature controller and the temperature was varied stepwise between
20 °C and 80 °C. Fig. 7 shows the changes of intensity with the variation of the temperature. Fig. 7(a) shows the intensity variation
with a slope of 0.00374 dB/°C. The intensity keeps nearly stable
with different temperature. The TFBG wavelength shifts follow
approximately linear temperature dependence with a slope of
7.95 pm/°C over the temperature range between 20 °C and 80 °C,
as shown in the Fig. 7(b). This is mainly because of that the water’s
RI changed by the temperature.
If we want get the precise twist angle without considering the
temperature, the method follows based on the matrix is needed.
The twist angle and temperature can be measured simultaneously
by using the following matrix equation
De
bT
1
¼
ae bT aT be be
DT
aT
ae
DI
Dk
ð3Þ
where De and DT are the variation of the twist angle and temperature, DI is the variation of the intensity, Dk is the wavelength shifts
corresponding to the TFBG. ae , be are the twist coefﬁcient of the
TFBG for the intensity and wavelength shifts. aT , bT are the temperature coefﬁcient of the TFBG for the intensity and wavelength shifts.
The twist and temperature coefﬁcient are obtained by linear ﬁt
of the measured data have been presented in Figs. 6 and 7. For
example, in the range of 0–45°, the twist angle can be measured
by the matrix as follows,
0:00795 0:00374 DI
De
1
¼
0:00238
0
0:299
DT
Dk
ð4Þ
Based on the above matrix, the twist angle and temperature
variation can be calculated out after measuring the intensity variation and wavelength shifts. In addition, the Bragg wavelength of
the FBG can be used as the temperature compensation of optical
ﬁber sensor.
4. Conclusions
In conclusion, we have presented a compact and effective TFBG
twist sensor in this paper. The length of the 10° TFBG is 10 mm. For
the measurement range of ±180°, a sensitivity of 0.299 dB/° is
obtained, which is almost 17.9 times higher than that of the current similar existing twist sensor. In this work, the sensing scheme
shows very low temperature dependence, though temperature
results in the wavelength shifts. Based on the matrix, the twist
angle can be measured precisely without considering the
temperature.
Acknowledgments
The authors would like to acknowledge most appreciatively
Professor Jacques Albert of the Carleton University for the providing of the TFBG. This work was supported by National Natural Science Foundation of China (No. 51374188) and Zhejiang Provincial
Natural Science Foundation for Distinguished Young Scientists
(No. LR13E040001).
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