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920 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 13, JULY 1, 2011
A Temperature-Insensitive Twist Sensor by Using
Low-Birefringence Photonic-Crystal-Fiber-Based
Sagnac Interferometer
Peng Zu, Chi Chiu Chan, Yongxing Jin, Tianxun Gong, Yifan Zhang, Li Han Chen, and Xinyong Dong
Abstract—An optical fiber twist sensor is proposed by using
solid core low birefringence photonic crystal fiber (LB-PCF)-based
Sagnac interferometer. The twist effects on the fiber are theoret-
ically analyzed. The results show that the dip wavelength of the
transmission spectrum shifts with the twist angle with a high sen-
sitivity and resolution of 1.00 nm and 0.01 , respectively. The
sensor is also insensitive to environmental temperature change
with an ultralow thermal dependent coeffiecient of 0.5 pm C.
Index Terms—Birefringence, low-birefringence photonic crystal
fiber, optical fiber twist sensor, Sagnac interferometer.
I. INTRODUCTION
SAGNAC interferometer (SI) has been intensively inves-
tigated as sensors for various parameters sensing such as
strain, pressure, temperature, twist and so on [1]–[5]. Recently,
with the development of photonic crystal fibers (PCFs), per-
formance of the sensors based on SI with PCF are greatly im-
proved such as sensitivity [1], [5], flexibility [1] and thermal
dependence [4]. High-birefringence fibers (HBFs) or polariza-
tion-maintaining fibers (PMFs) are commonly used in the SI to
introduce birefringence for producing a wavelength dependent
output for various measurements. For the sensors based on SI,
most attentions were paid on linear birefringence effect caused
by strain [4], pressure [2], core deformation [3] and so on. How-
ever, circular birefringence effect is also important and worthy
for attention. Twist effect on the fiber is one of the major causes
of the circular birefringence in some situations. Various high
birefringent photonic crystal fibers (HB-PCFs) were used in the
Sagnac loop to realize the twist sensors. A linear relationship be-
tween the spectrum shift and the twist angle was obtained with
a sensitivity of 0.06 nm [5] and nm [6]. Low-bire-
fringence fiber (LBF) was also employed in the Sagnac loop.
J.M. Estudillo-ayala analyzed the twist effects on the LBF in the
SI [7]. Y.X. Jin mentioned the twist induced spectrum shift in
single mode fiber (SMF) based SI was a sinusoidal function [8],
Manuscript received January 25, 2011; revised March 22, 2011; accepted
April 09, 2011. Date of publication April 19, 2011; date of current version June
15, 2011.
P. Zu, C. C. Chan, Y. Zhang, and L. H. Chen are with the School of Chem-
ical and Biomedical Engineering, Nanyang Technological University, Singa-
pore 637459, Singapore (e-mail: eccchan@ntu.edu.sg).
Y. Jin and X. Dong are with the Institute of Op toelectronic Technolog y, China
Jiliang University, Hangzhou 310018, China.
T. Gong is with the School of Electrical and Electronic Engineering, Nanyang
Technological University, Singapore 637553, Singapore.
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/LPT.2011.2143400
which was different from the experimental result of the sensor
based on HBF. In addition, various fiber gratings were also used
to perform twist sensing [9], [10]. Besides, twist sensors based
on two modes operation in HBF [11] and in-fiber polarimeters
were also reported [12].
In this letter, a LB-PCF based SI is used to demonstrate the
twist sensing. The principle, sensor scheme, results and conclu-
sion are discussed in the following sections.
II. ANALYSIS TWIST EFFECTS ON THE FIBER
In a real optical fiber, the core area is often not perfectly cir-
cular symmetry due to residual stress which leads to the intrinsic
linear birefringence .Whenthefiber is twisted, core defor-
mation and shear strain give rise to a component of linear bire-
fringence and circular birefringence, respectively. The total ef-
fects of the twisted fiber can be treated as a retarder and a rotator
[13]. By fixing one end of the fiber and twisting on the other
end, the retardance between the two orthogonal guided
modes is expressed as a function of twist angle which is given
as [13]:
(1)
where
(2)
(3)
is the twisted fiber length. is the strain induced optical rota-
tion which is proportional to and given as:
(4)
where is a constant which depends on the photoelastic coef-
ficients of the material. For a single mode fiber, .If
the centerofthefiber is twisted while both ends of the fiber are
fixed, the twisted effects are doubled and twice of the twist angle
()is used in the equations. The simulation results are shown
in Fig. 1.
For the case of HBF, the linear birefringence is greater than
circular birefringence induced by the twist effect [13]. Little
amount of light couples between the two polarized modes, thus
the twisted fiberactsasarotator[13]. Then retardance
varies linearly with the twist angle periodically in every linear
region (Fig. 1).
1041-1135/$26.00 © 2011 IEEE
ZU et al.: TEMPERATURE-INSENSITIVE TWIST SENSOR BY USING LB-PCF-BASED SAGNAC INTERFEROMETER 921
Fig. 1. Calculated fiber retardance as a function of twist angle .
Fig. 2. Sensing scheme based on LB-PCF for twist measurement. (ASE: Am-
plified spontaneous emission light source.) Inset: SEM image of the cross sec-
tion of the PCF (LMA-10).
For the case of LBF, the intrinsic linear birefringence is
smaller than the circular birefringence [13]. The retardance
becomes
(5)
which can be describedbyaSincfunction( ) approxi-
mately (Fig. 1).
III. SENSOR STRUCTURE AND OPERATION PRINCIPLE
The schematic diagram of the twist sensor is shown in Fig. 2.
The configurationwasbasedonaSIwhichwasobtainedby
splicing a section of 50-cm-length solid core LB-PCF (LMA-10,
NKT Photonics A/S) with the two output ports of a 3-dB SMF
coupler (Fig. 2). The scanning electron graph (SEM) in Fig. 2
shows a hexagonal arrangement of air-holes around the solid
core in the cross section of the PCF which is approximately cir-
cular symmetric. The core, cladding, and mode field diameters
are 10 m, 125 m, and 7.5 m, respectively. The total splicing
loss between SMF and PCF is about 4 dB.
The 3-dB coupler splits the light into two beams counter-
propagating along the loop and recombines them again. Sub-
sequently, the two beams interfere at the 3-dB coupler. If the
insertion loss in the loop is neglected, the relative transmission
ratio is given as [4]:
(6)
In the absence of birefringence of the loop, the total phase differ-
ence ,so . All the input light of all the wavelengths
would reflect back completely to the input port of the ideal 3-dB
coupler and no signal will appear on the port which is connected
to optical spectrum analyzer (OSA).
A transverse force is applied on the LB-PCF for introducing
an initial linear birefringence which will lead to a constant phase
difference .is the linear birefringence induced
Fig. 3. Variation trend of measured transmission spectra at different twist an-
gles. Inset: measured transmission spectra at twist angles of 0 and 60 .
by the transverse force over the length of on the PCF. is the
operating wavelength. If a broadband light is launched, a wave-
length-dependent sinusoidal interference fringe is observed (the
inset in Fig. 3). When the LB-PCF is twisted, the fringe will be
shifted due to the extra circular birefringence and phase differ-
ence caused by the twist. In this case, the total phase
difference can be expressed as . Considering
is a constant, the spectrum shift is given by
(7)
For the case of HBF, the simulation result (Fig. 1) and
the experimental results in [4], [5] show that, the coefficient
is a constant, so is linearly proportional to the
twist angle . On the other hand, for the case of LBF, is
following a pattern of Sinc function.
The sensitivity of the sensor depends on the coefficient
.Thus,theuseoftheLB-PCFisoneofthebest
choices for the proposed sensor to improve the torsion sensi-
tivity. It is because of its air-hole structure, the sensor based
on PCF is more sensitive to fiber twist effects than traditional
fibers [5], [6]. Moreover, if the HBF is used for this proposed
sensing scheme, the circular birefringence caused by the twist
effects will be greatly swamped by the large linear birefrin-
gence of HBF itself [13]; hence the twist sensor based on LBF
will achieve a higher torsion sensitivity than the one based on
HBF. Finally, the ultralow thermal dependence property of the
PCF will decrease the influence of the ambient temperature
variation and increase the torsion sensitivity.
IV. EXPERIMENTAL RESULTS AND DISCUSSION
A length of 20-mm PCF, together with a balanced fiber, was
clamped between two metal plates in order to introduce the nec-
essary initial birefringence. The force applied on the PCF was
adjusted until a dip on the transmission spectrum appeared on
the OSA (AQ6370) within the range of the amplified sponta-
neous emission (ASE, 1520–1620 nm) light source. The curve
indicated by “0” in Fig. 3 shows the initial transmission spec-
trum with the wavelength of the dip at 1578.1 nm.
While the PCF was twisted from its center clockwise (CW)
or counterclockwise (CCW), the transmission spectrum shifted
to the shorter or longer wavelength side, respectively, which
was completely reversible and repeatable. The inset in Fig. 3
shows the measured transmission spectra at the twist angles of
0and 60 . In order to obtain the twist sensitivity of the sensor,
the transmission spectra were recorded by increasing the twist
angle from 0 to 360 with an interval of 10 . It shows the dip
wavelength shifts periodically and the variation trend is similar
922 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 13, JULY 1, 2011
Fig. 4. Dip wavelength shift with the twist angle.
to Sinc function by the 3-D diagram in Fig. 3. The visibility of
the transmission spectrum varies totally about 10 dB during the
applied twist effect. The reason is that the splitting ratio of the
coupler depends on the wavelength and polarization state [14],
so the practical splitting ratio slightly deviates from 3 dB, which
causes the decrease in the visibility of the interference fringes.
Moreover, the change of the dip wavelength in the spectrum of
the proposed sensor is used to measure the twist effect, so the
performance of the sensor is not affected by the variation on the
visibility of the transmission spectrum.
The relationship between the dip wavelengths and twist an-
gles is shown in Fig. 4. The dip wavelength shifted 52.7 nm
from the minimum wavelength 1545.2 nm at the twist angle of
60 to the maximum wavelength 1597.9 nm at the twist angle of
140 in the first period, which is 3 times larger than the wave-
length shift range of the twist sensor by employing SMF [8].
Comparing the results of the sensors based on PCF and SMF,
the variation trends were similar, but PCF was more sensitive
to the twist effect [8]. The curve is fit with a Sinc function by
ahigh value of 0.9898, which means the experimental data
are in accordance with the simulation results.
Taking the linear range from 75 to 140 as an example, linear
fit was applied to the curve with a high value of 0.9937,
which means the dip wavelength increased with a good linearity.
The sensitivity in this linear region is 1.00 nm . In practical
application, the sensor can be pretwisted to this linear range for
achieving a high sensitivity measurement. The achieved sensi-
tivity of this proposed twist sensor is 17 times and 12.5 times
higher than that of the twist sensor by use of HB-PCF [5] and
PM- side hole fiber [6] based SI, respectively, 521 times higher
than that of the polarization mode interferometer by means of
fabrication two in-line polarizers on the hollow core PCF [12],
255 times higher than that of ultra long-period LPG [15]. More-
over, the resolution of the twist sensor is measured as 0.01 at
the limit resolution of the OSA of 10 pm.
Another distinct advantage of this twist sensor based on PCF
is the ultralow temperature sensitivity, which is confirmed in
the experiment. Fig. 5 shows the dip wavelength moved about
40 pm to the shorter wavelength side when the temperature in-
creased from 30 Cto100 C. The temperature coefficient is ob-
tained as pm C which can be neglected comparing to the
high twist sensitivity of 1.00 nm .
V. C ONCLUSION
A twist sensor by employing a section of LB-PCF inserted in
the SI is proposed and experimentally demonstrated. The sensor
has a sinusoidal wavelength-dependence output with the assis-
tance of the transverse force applied on the LB-PCF which is
used for introducing initial necessary linear birefringence. The
Fig. 5. Dip wavelength shift at different temperatures.
transmission spectrum shift is a Sinc function form which is dif-
ferent from the result of the twist sensor by employing HBF. The
sensor achieved a large enhanced sensitivity of 1.00 nm and a
high resolution of 0.01 with a good repeatability. The temper-
ature-insensitive property was also confirmed experimentally
with an ultralow temperature coefficient of pm C.
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