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In this paper, we experimentally studied group polarimetric pressure sensitivity of a fabricated elliptical core side-hole fiber (EC-SHF). We investigated the polarimetric behavior of this fiber under three steps of chosen pressure intervals of 100psi to 104psi, 1000psi to 1004psi and 2000psi to 2004psi, respectively. We obtained high group polarimetric sensitivity from 2.6×10-7 psi −1 (3.7×10-5 MPa −1) to 3×10-7 psi −1 (4.3×10-5 MPa −1) under 100psi to 104psi pressure interval which decreased under upper pressure intervals. We carried out a simulation to understand the reason for change of polarimetric sensitivity at telecommunication wavelengths (1550nm region) and also the relationship between the group birefringence variation and increasing pressure. For this purpose, by using finite element method (FEM), we demonstrate the effects of structural deformation and stress variation of EC-SHF cross section on spectral transformation of fundamental polarization modes. Our simulation and experimental results are in agreement with each other and indicate that under different pressure intervals, shorter wavelengths have more birefringence variation and pressure sensitivity than longer wavelengths. These results show that changing the group birefringence and pressure sensitivity in lower pressure are higher than the upper ones. Index Terms—elliptical core side-hole fiber, finite element method, group polarimetric sensitivity, telecommunication wavelengths. J.
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Group Polarimetric Pressure Sensitivity
of an Elliptical-Core Side-Hole Fiber
at Telecommunication Wavelengths
Jalal Sadeghi, Hamid Latifi, Michal Murawski, Farnood Mirkhosravi, Tomasz Nasilowski,
Pawel Mergo, and Krzysztof Poturaj
Abstract—In this paper, we experimentally studied group polari-
metric pressure sensitivity of a fabricated elliptical-core side-hole
fiber (EC-SHF). We investigated the polarimetric behavior of this
fiber under three steps of chosen pressure intervals of 100 to 104
lbf/in2, 1000 to 1004 lbf/in2, and 2000 to 2004 lbf/in2, respectively.
We obtained high group polarimetric sensitivity from 2.6×107
(3.7×105MPa1)to3×107(lbf/in2)–1 (4.3×105MPa1)
under 100 to 104 lbf/in2pressure interval, which decreased under
upper pressure intervals. We carried out a simulation to under-
stand the reason for change of polarimetric sensitivity at telecom-
munication wavelengths (1550-nm region) and also the relationship
between the group birefringence variation and increasing pres-
sure. For this purpose, by using finite-element method, we demon-
strate the effects of structural deformation and stress variation of
EC-SHF cross section on spectral transformation of fundamental
polarization modes. Our simulation and experimental results are
in agreement with each other and indicate that under different
pressure intervals, shorter wavelengths have more birefringence
variation and pressure sensitivity than longer wavelengths. These
results show that changing the group birefringence and pressure
sensitivity in lower pressure are higher than the upper ones.
Index Terms—Elliptical core side-hole fiber, finite element
method, group polarimetric sensitivity, telecommunication wave-
IN RECENT years,microstructure optical fibers have been
widely used as pressure sensors because of their special
structure [1]. The sensitivity of high birefringence microstruc-
ture optical fibers (HiBi-MOFs) to hydrostatic pressure has
drawn attention of scientific community as a feasible pres-
sure sensor. For instance, various types of HiBi-MOFs pressure
sensors have been exploited, to further improve the polarimet-
ric response to hydrostatic pressure. Among these, elliptical
core side-hole fibers (EC-SHFs) as an important subgroup of
Manuscript received March 24, 2015; accepted May 17, 2015.
J. Sadeghi and F. Mirkhosravi are with the Laser and Plasma Institute, Shahid
Beheshti University, Tehran 1983963113, Iran (e-mail:;
H. Latifi is with the Department of Physics and Laser and Plasma Institute,
Shahid Beheshti University, Tehran 1983963113, Iran (e-mail:
M. Murawski and T. Nasilowski are with Inphotech Ltd., 02-676 Warsaw,
Poland, and also with the Military University of Technology, 01-476 Warsaw,
Poland (e-mail:;
P. Mergo and K. Poturaj are with the Laboratory of Optical Fiber Tech-
nology, Maria Curie-Sklodowska University, 20-031 Lublin, Poland (e-mail:;
Color versions of one or more of the figures in this paper are available online
Digital Object Identifier 10.1109/JSTQE.2015.2435896
HiBi fibers were introduced to the standard fiber. Since then,
side-hole fibers have been attracting much interest for lateral
and radial pressure sensing [2]–[4]. Clowes et al. reported two
kinds of side-hole fibers and showed that the larger side hole
has higher sensitivity. In their work, a polarimetric pressure
sensitivity of 7.32 ×106MPa1at 1300 nm was presented
[5]. Urbanczyk et al. studied two different core-shaped side-
hole fibers and achieved a polarimetric pressure sensitivity of
8.86 ×106MPa1at 826 nm [6]. Recently, many pressure
sensors based on PM-PCFs have been reported at telecommuni-
cation wavelengths. In addition, the highest polarimetric pres-
sure sensitivities of 1.06 ×105MPa1by Martynkien et al.
and 2.30 ×105MPa1by Wu et al. were demonstrated at
1550 nm [7], [8]. The instability of the higher order mode pat-
tern limits the practicality of the multimode fiber optic devices.
One approach to resolve this problem is to use a single mode
EC-SHF at telecommunication wavelengths. In this case, the in-
tensity distribution of the fundamental mode is well-defined and
stable. At telecommunication wavelengths, only a single mode
is guided, whereas at shorter wavelengths multimode behavior
is dominant. Therefore, we are looking for an EC-SHF which
possesses the advantages of low loss single mode fibers, po-
larization maintaining fibers and flexible structure fibers, which
are extremely suitable for pressure sensing applications. Herein,
we investigated the dependence of the pressure sensitivity of
side-hole fiber upon the fiber geometry using the finite element
method (FEM). For this purpose, we briefly studied four types
of the EC-SHF fiber with different side-hole cross sections to
find the most effective focus of radial pressure and stress trans-
portation into the fiber core located between the two air holes.
Finally, we simulated and fabricated a low loss single mode
EC-SHF with high group birefringence at telecommunications
We utilized the EC-SHF as a pressure sensor head with the
aims of exploring group polarimetric dependence on telecom-
munication wavelengths and studying the spectral behavior of
the fiber subjected to different pressure conditions in a sagnac
interferometric configuration. We have experimentally demon-
strated the behavior of dispersion and group polarimetric pres-
sure sensitivities of one section of EC-SHF to the radial pres-
sure. These experimental results showed a complicated behavior
in different pressure intervals and also in different wavelength
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Fig. 1. (a) The SEM micrograph of EC-SHF. (b) The experimental setup of
high pressure sensor in the sagnac configuration.
intervals. We observed an increase in birefringence with apply-
ing pressure and simultaneously a decrease in the pressure sen-
sitivity under upper pressures. These experimental results also
showed different group polarimetric pressure sensitivities of dif-
ferent pressure intervals. To understand the effects of different
radial pressures on the sensitivity and dispersion variations, we
simultaneously examined the impact of deformation and stress
variation on effective mode index transformation and finally the
group birefringence changing in 1550 nm region by using FEM.
Fig. 1(a) shows a scanning electron microscope (SEM) micro-
graph of an EC-SHF fabricated and used in our simulations and
experiments. As indicated in Fig. 1(a), the slow and fast diam-
eters (Dsand Df) of the elliptical core are 4.41 and 1.72 μm,
respectively. This EC-SHF has two quasi-rectangular air chan-
nels located parallel to the elliptical core. The length (L) and
width (W) of these channels are 33 and 25.32 μm, respectively.
The slow axis of the core ellipse is perpendicular to the line that
connects the centers of both holes. The distance between the
core edges and the hole edges is 2 μm.
In order to characterize its polarimetric response to high
radial pressure, we utilized the sagnac loop interferometer.
For polarization maintaining fibers, the relationship between
fiber length (L), operation wavelength (λ), group birefringence
(Bg=λ2/2LΔl), wavelength spacing of the peaks (Δλ) and
the group polarimetric sensitivity (KPg) in the sagnac loop
interferometer is defined by [3]:
The EC-SHF was subjected to radial pressure by injecting
water with a high pressure stainless steel unit. The pressure
was measured by using a digital pressure gauge with 2 lbf/in2
pressure accuracy. Temperature stability of the pressure unit was
lower than ±0.5 °C/h. The experimental configuration based on
a sagnac interferometer loop is shown in Fig. 1(a). This loop
was composed of a broadband light source (SLD-1560 nm),
a conventional 3-dB coupler, an isolator, an optical spectrum
analyzer (OSA-Agilent-86142B with a maximum resolution of
10 pm), a polarization controller, and a segment of EC-SHF
with length of 50 cm as the sensing head.
In Fig. 2(a)–(c) we summarize our results, which show the
shift direction of the spectral transmission and sensitivities of
Fig. 2. The shift direction of spectral transmission of three different pressure
intervals. (a) 100 to 104 lbf/in2, (b) 1000 to 1004 lbf/in2, (c) 2000 to 2004
lbf/in2, (d) the group pressure sensitivity of EC-SHF under 4 lbf/in2pressure in
different intervals.
different wavelengths under different pressures. We sampled
three arbitrary 4 lbf/in2pressure intervals to study spectral vari-
ation under increasing pressure: 100 to 104 lbf/in2, 1000 to
1004 lbf/in2and 2000 to 2004 lbf/in2. At 100 lbf/in2, there
were seven peaks in the interference pattern; by increasing the
pressure to 1000 and 2000 lbf/in2, 12 and 19 peaks, respec-
tively, were apparent. As applied radial pressure increased, the
wavelength spacing of the peaks decreased, which indicates an
increase in the group modal birefringence. This increase is more
apparent when we investigate the red shift of every peak in dif-
ferent pressure intervals. As shown in Fig. 2(a) and (d) (the black
line), in the interval of 100 to 104 lbf/in2, the pressure sensitiv-
ity was in the range of 5.9 to 1.96 nm/lbf/in2on an exponential
decay curve. According to Fig. 2(b) and (d) (the red line), the
pressure sensitivity at 1000 to 1004 lbf/in2was in the range of
2.72 to 1.27 nm/lbf/in2with a slower exponential decay behav-
ior. In the case of 2000 to 2004 lbf/in2pressure (the blue line),
the sensitivity was in the range of 1.36 to 0.75 nm/lbf/in2which
decays more slowly compared to the lower pressure intervals.
To understand the impact of radial pressure on different wave-
length changes and also the effects of different pressure inter-
vals on polarimetric properties, we simulated our EC-SHF by
coupling the Mechanics and RF modules of the Comsol Multi-
physics commercial software [3]. With these modules, we were
able to determine the changes in spectral transformation and
mode confinement of EC-SHF’s cross section resulting from ra-
dial pressure changes. First, we determined the elastic modulus
and Poisson’s ratio of different parts of EC-SHF. The GeO2-
doped elliptical core was prepared by a modified chemical-vapor
deposition process. The GeO2concentration in the core region
Parameter Value
ESiO272.5 GPa
ESiO2/GeO2 61 GPa
νSiO2 0.165
νSiO2/GeO2 0.197
Fig. 3. (a) The stress distribution of four types of the EC-SHF fiber with
different side-hole cross sections. (b) The stress distribution and structural de-
formation of SC-SHF with quasi-rectangular air channels under three pressure
100, 1000 and 2000 lbf/in2.
was 20 mol%. According to the Sellmeier dispersion relation,
this concentration corresponds to refractive index contrast of
0.027 at 1550 nm region. Current literature data concerning
the dependence of elasticity modulus (E) and Poisson ratio (ν)
on dopant concentration for SiO2/GeO2glasses is limited. One
experimental evaluation of the dependence of E on dopant con-
centration indicates that E decreases 0.8% for 1 mol% of GeO2
concentration. In addition, νincreases with dopant concentra-
tion according to a linear model given by [9]:
where mis the dopant concentration expressed in mole percent
and νSiO2 and νGeO2 are material constants for pure silica and
pure germanium oxide, respectively [10], [11]. The value of the
elasticity modulus and Poisson ratio parameters of the pure and
doped silica are listed in Table I.
To have a good understanding about the role of holes in stress
distribution, our next step of simulation modeled four types of
EC-SHF with different side-hole cross sections. Fig. 3(a)-a1il-
lustrates the EC-SHF with two square cross section holes. This
geometry indicates that maximum stress is produced on the cor-
ner of square air holes. Fig. 3(a)-a2and (a)-a3also indicate
that circular holes do not transport optimal radial pressure to
the elliptical core region. Hence, we were required to construct
an optimal geometry to increase the ratio of stress transport.
To do this, we modeled a structure with quasi-rectangular holes.
Fig. 3(a)-a4shows two quasi-rectangular air channels with round
corners. The simulations showed that maximum stress transport
occurs by the use of this design of channels. Therefore, this
comparison indicates that EC-SHF with quasi-rectangular air
channels would be a good candidate for future applications of
high radial pressure measurement. In the third step, we inves-
tigated the stress distribution and the structural deformation of
longer axis of the core in comparison with shorter axis under
radial pressure.
The distributed load within the EC-SHF was transferred to
the elliptical core which was joined to the two adjacent quasi-
rectangular air channels. The air channel walls were deformed,
which prevented the exertion of horizontal pressure on the core.
The stress distribution and deformation under three pressure in-
tervals 100, 1000, and 2000 lbf/in2are illustrated in Fig. 3(b).
At this point, the shape and thickness of the silica walls and
the value of the E and νparameters of the pure and doped sil-
ica play the main roles in the structural deformation and the
stress distribution. Because of the especial shape and size of the
air channels, stiffness and flexural rigidity of this structure do
not have a linear form. In other words, by increasing pressure,
the dimensions of air channels decrease and the stress distri-
bution increases. This increases the stiffness of the structure
and decreases structural flexibility, which affects birefringence
changing. In the next set of simulations, the mode analysis mod-
ule was coupled with the mechanics module, which allowed us
to elicit more details about events occurring between the two
polarizations of the fundamental or higher order core modes at
telecommunication wavelengths and shorter wavelengths. We
present a brief theory of spatial modes in elliptical core fiber
(ECF) from 400 nm to 2000 nm wavelengths. The number of
guided modes of this fiber depends on the optical wavelength
(see Fig. 4(a)). For telecommunication wavelengths, our results
showed a single guided mode in which all spatial modes with the
exception of the fundamental mode are cut off. One approach
to the modal birefringence changing that we propose is to use a
spectral transformation in the two orthogonal polarizations.
Our simulation results show that this fiber guides only one
spatial mode (LP01) for wavelengths longer than 1250 nm.
Therefore in 1550 nm region an interference is implemented be-
tween the two orthogonal polarizations of the LP01xand LP01 y
modes [12].
Also, our results show that within wavelengths longer than
critical value (1250 nm), for the two orthogonal polarization
modes of LP01, increasing mode enlargement is observed (see
Fig. 4(b)).
In the case of intensity expansion, the evolution of mode field
diameter (MFD) of the fundamental modes as a function of
wavelength (the five wavelengths) is shown in Fig. 4(b). Also,
the effective mode index (neff ) of different modes has differ-
ent wavelength dependence. The modal group birefringence
Fig. 4. (a) The effective refractive mode index of EC-SHF’s spatial modes
from 400 to 2000 nm wavelengths. (b) The evolution of MFD and effective mode
index of the two fundamental modes for five wavelengths at telecommunication
wavelengths (1300, 1400, 1500, 1600 and 1700 nm).
is obtained by: Bgsim =n
eff (LP01y)neff (LP01 x). Here,
eff (LP01x)and neff (LP01 y)are the group birefrin-
gence of simulation result and the effective mode index of two
polarizations, respectively.
In addition, to understand the interaction between the LP01x
and LP01ymodes occurring during exerting pressure, we si-
multaneously examined inseparable effects of deformation and
stress variation on the group birefringence changing in 1550 nm
region. Deformation along the fast and slow axis of elliptical
core causes a residual decrease of core asymmetry, which indi-
cates the actual interaction between the LPx
01 and LPy
01 modes
and a decrease of birefringence on wavelengths.
Stress changes along the two axes are associated with increas-
ing the difference effective mode index between LPx
01 and LPy
modes and increasing the group birefringence. As indicated in
Fig. 5(a), for five different pressures of 0, 500, 1000, 1500 and
2000 lbf/in2in Fig. 5(a), the birefringence increases directly with
pressure. Additionally, Fig. 5(a) shows Langmuir curve behav-
Fig. 5. (a) Simulation results of group birefringence under five pressure steps
from 0 to 2000 lbf/in2. (b) The experimental group birefringence at three pres-
sure 100, 1000 and 2000 lbf/in2. (c) Experimental results of group polarimetric
pressure sensitivity.
ior with increasing pressure. In other words, the lower pressure
intervals deform the EC-SHF’s cross section more than higher
intervals. In our simulation results, the impact of the Lang-
muir behavior of deformation on birefringence variation is ob-
vious in the gaps between group birefringence (Bgsim) curves
(see Fig. 5(a)). Gaps at shorter wavelengths are longer than the
gaps at larger wavelengths. This translates into exponentially
decaying curves of experimental pressure sensitivity which are
more sensitive at shorter wavelengths (see Fig. 2(d)).
In order to measure KPg, we measured the experimen-
tal group birefringence (Bgexp). We obtained the Bgexp of
EC-SHF, as shown in Fig. 5(b) for pressures of 100, 1000,
and 2000 lbf/in2. As indicated in this figure, for 100lbf/in2,
the group birefringence with an additive linear fit was in the
range of 6.5×105to 2.62 ×104, increased to the range of
1.4×104to 3.2×104for 1000 lbf/in2, and increased again
to the range of 2.25 ×104to 3.63 ×104for 2000 lbf/in2.
There are a few other features apparent in Fig. 5(b). First,
there is a larger gap between the 100 and 1000 lbf/in2lines
than the 1000 and 2000 lbf/in2lines, which indicates greater
birefringence variation. Second, the gaps between curves in-
crease at shorter wavelengths which shows shorter wavelengths
have more birefringence variation under pressure. Finally, we
can determine KPgusing (1), our pressure sensitivity results,
and our group birefringence results (see Fig. 5(c)). In the 100
to 104 lbf/in2pressure interval, at 1460 nm, we obtained a
KPgof 2.6×107(lbf/in2)1(3.7×105MPa1), whereas
at 1680 nm, this value changed to 3×107(lbf/in2)1(4.3×
105MPa1). The values of the group polarimetric sensitiv-
ity within the 1000 to 1004lbf/in2interval decreased from
2.3×107(lbf/in2)1(3.3×105MPa1)at 1440 nm to
2.1×107(lbf/in2)1(3.1×105MPa1) at 1642.5 nm. Also,
we obtained KPgvalues in the 2000 to 2004 lbf/in2in-
terval ranging from 2×107(lbf/in2)1(2.9×105MPa1)
at 1440 nm to 1.6×107(lbf/in2)1(1.5×105MPa1)at
1640 nm.
In summary, this paper experimentally illustrates group
polarimetric sensitivity enhancement by an EC-SHF in the
1550 nm region. The group birefringence increases with pres-
sure, whereas pressure sensitivity decays exponentially in
different pressure intervals. These results indicate that shorter
wavelengths were more sensitive to radial pressure than longer
ones. We studied group polarimetric pressure sensitivity for 4
lbf/in2pressure changes in three intervals: 100 to 104 lbf/in2,
1000 to 1004 lbf/in2, and 2000 to 2004 lbf/in2. To understand the
role of holes in stress distribution and the impact of radial pres-
sure on wavelength-dependent changes we modeled four types
of EC-SHF fiber with differing side-hole cross sections. We
present a brief description of an EC-SHF’s spatial modes to find
the single mode region in the 400–2000 nm wavelength interval.
Finally, we investigated the effects of different pressure intervals
on polarimetric properties of the produced EC-SHF at single
mode regions. Our simulation and experimental results agree
and indicate that shorter wavelengths feature more birefrin-
gence variation than longer wavelengths under different pressure
intervals. Finally, we obtained group polarimetric sensitivities
within various pressure intervals. For the 100 to104 lbf/in2inter-
val, this sensitivity changed from 2.6×107(lbf/in2)1(3.7×
105MPa1)to3×107(lbf/in2)1(4.3×105MPa1), re-
spectively. For the 1000 to1004 lbf/in2interval, the sensi-
tivity decreased to 2.3×107(lbf/in2)1(3.3×105MPa1)
and 2.1×107(lbf/in2)1(3.1×105MPa1), respectively.
Within the 2000 to 2004 lbf/in2interval, the sensitivity ranged
from 2×107(lbf/in2)1(2.9×105MPa1)to1.6×107
(lbf/in2)1(1.5×105MPa1), respectively. Consequently, our
results clarify the role of the shape and dimensions of the
core ellipticity, two quasi-rectangular air channels, and dopant
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Jalal Sadeghi was born in Malayer, Iran, in 1982. He received the B.S. degree
in solid state physics from Kharazmi University, Tehran, Iran, in 2010, and
the M.S. degree in atomic, molecular, and laser physics from Shahid Beheshti
University, Tehran, in 2012, where he is currently working toward the Ph.D.
degree at Laser and Plasma Research Institute. His current research interests
include optical microstructured fibere, optical measurements, and integrated
microstructured waveguides for optofluidic applications.
Hamid Latifi was born in Tehran, Iran, in 1958. He received the B.S. degree
in physics from California State University at Hayward, Hayward, CA, USA,
in 1982, and the M.S. and Ph.D. degrees in physics from New Mexico State
University, Las Cruces, NM, USA, working on interaction of high-energy laser
pulses with aerosols in 1989. He was a Postdoctoral Researcher with Colorado
State University at Fort Collins working on measurement of mesosphere sodium
temperature and density using Na Lidar. He has been a Faculty Member at the
Department of Physics, Shahid Beheshti University, Tehran, Iran, since 1992,
where he also joined Laser and Plasma Research Institute in 1998. He has
published more than 100 papers on various fields of optics and laser, such as gas
lasers, fiber optics sensors, nondestructive optical measurements, and optical
microfluidic measurements.
Michal Murawski was born in Warsaw, Poland, in 1985. He received the M.Sc.
Eng. and Ph.D. degrees from the Military University of Technology, Warsaw,
in 2009 and the 2014, respectively, both in material science. In 2010, he joined
InPhoTech Ltd., Warsaw, where he is currently an R&D Technology Manager.
His research interest includes photonics, since 2007, concentrating mainly on
microstructured fibers, optical measurements, and sensors. His main research
interest includes splicing of optical fibers. He has been working in several
industrial and research projects related to photonics.
Farnood Mirkhosravi was born in Tehran, Iran, in 1989. He received the B.S.
degree in theoretical physics from Tabriz University, Tabriz, Iran, in 2012. He
is currently working toward the M.S. degree in photonics at Shahid Beheshti
University, Tehran. His research interests include development of interrogation
techniques in optical sensor measurement, fabrication and calibration of pressure
and strain optical fiber sensors for industrial applications, and optimization of
dielectric barrier discharge arrangement in an air-flow control.
TomaszNasilowski was born in Warsaw, Poland, in 1971. He received the M.Sc.
degree and Engineer diploma in optoelectronics from the Warsaw University
of Technology, Warsaw, Poland, and the University of Quebec at Hull, Hull,
QC, Canada, in 1995. He received the Ph.D. degree in physics from the Warsaw
University of Technology in 2000.
He is working with the Free University in Brussels (VUB) since 1998,
where he has got a Guest Professorship in 2002. Since 2010, he has been
with the Military University of Technology, Warsaw, developing his interest in
photonics, since 1995 he is concentrating mainly on photonic crystals, optical
fibers, optical measurements, and sensors. He is currently the CEO at InPhoTech
Ltd., Warsaw. He has been leading severalindustrial and research projects related
to these topics supported by the Belgium, Polish, and EC funds.
Pawel Mergo was born in 1973. He received the M.Sc. degree in chemistry and
optical fiber technology and the Ph.D. degree in physical chemistry from Maria
Curie-Sklodowska University, Lublin, Poland, in 1997 and 2003, respectively.
He is currently the Head at the Laboratory of Optical Fibers Technology, Maria
Curie-Sklodowska University. His research interests include design, fabrication,
and measurements of optical fibers for both telecommunication and special
applications. He has published more than 60 papers on fiber optics especially
on optical fiber technology.
Krzysztof Poturaj is currently working on optical fiber technology. His main
research interests include optical fiber performs and optical fiber drawing tech-
nology. He has been working in several industrial and research projects related
to photonics technology.
... wing to the overwhelming advantages over the electrical sensors, optical fiber sensors have been widely used to measure temperature, pressure, refractive index, optofluidics, acoustic wave and magnetic field [1][2][3][4][5][6][7]. In the past few decades, a typical distributed fiber sensor of phase-sensitive optical time domain reflectometry (Φ-OTDR) attracts much attention and has been intensively studied due to its characteristics of high sensitivity, accurate localization, fast response and quantitative measurement [8][9][10][11][12]. ...
An approach of phase-shift transform is proposed and demonstrated to suppress the interference fading in phase sensitive optical time domain reflectometry (-OTDR). The phase-shift transform is executed in the digital domain and the complicated frequency modulation is not required. The operation principle is carefully analyzed and a signal with complementary amplitude can be obtained with this method. Through synthesizing the complementary signals, the false phases can be corrected. Benefiting from the flexibility of this approach, multiple transforms are implemented to make sure that all phases are retrieved accurately. Experimental results show that the intensity fluctuation over 60 dB is reduced to 15 dB, which decreases the standard deviation of differential phase to 0.0224. Besides, 30 dynamic measurements are tested and all distorted phases have been corrected, indicating the high reliability of this method.
... One way to improve the fiber sensitivity is to investigate elliptical core fibers and their dependence on the pressure sensitivity in the side hole upon the fiber geometry. Different side-hole cross-sections to find the most effective focus of radial pressure and stress transportation [9]. ...
This paper presents a new methodology for measuring vibration and temperature using fiber Bragg grating (FBG) sensors embedded in carbon fiber polymer attached directly on the iron core of a power transformer. The device has a quick-coupling clip design and is fabricated from a carbon fiber reinforced composite (CFRP) for robustness and flexibility. Temperature and vibration FBG sensors are embedded in the carbon fiber clip, which is easily and quickly coupled directly to the transformer iron core. In the low frequency ranges, from 0 to 500 Hz, vibrations are mainly generated in the core. Rigidity is controlled by clip thickness and geometry, since the system is designed to have a wide operating frequency range. The embedded strain FBG sensor sensitivity has been improved due to its spring-shaped design static and dynamic simulations applying the Finite Element Method (FEM) were performed to determine the optimal dimensions and structure of the part. The clip’s frequency response function (FRF) was assessed and tested by using three techniques: FEM harmonic analysis, mechanical impact testing, and optical FBG sensing. The results demonstrate a 3.94 % differential among the methods. A comparison was made of the power transformer’s FRF when fitted with a glued free FBG sensor and an embedded FBG sensor. In both cases, the transformer was subjected to varying electrical load conditions (no-load, linear, and nonlinear load). The results showed a 19 % average sensitivity improvement when the embedded sensor was compared to the free sensor.
To address the problem of decreased recognition accuracy of event samples in practical Φ-OTDR monitoring scenarios due to external environmental interference, this paper proposes a feature correction algorithm based on sample feature weighting method. By establishing a correlation evaluation method and a weight allocation scheme based on sample feature correlation, combined with the BP algorithm, an average recognition rate of 99.50% for four types of events (climbing, strong wind, koncking and background, 6000 samples) in strong wind environments was achieved, which is 3% higher than the algorithm using BP classifier. The results demonstrate that the proposed algorithm can effectively enhance the performance of Φ-OTDR in complex environments.
For FPI sensor demodulation systems to be used in actual engineering measurement, they must have high performance, low cost, stability, and scalability. Excellent performance, however, necessitates expensive equipment and advanced algorithms. This research provides a new absolute demodulation system for FPI sensors that is high-performance and cost-effective. The reflected light from the sensor was demultiplexed into distinct channels using an array waveguide grating (AWG), with the interference spectrum features change translated as the variation of the transmitted intensity in each AWG channel. This data was fed into an end-to-end neural network model, which was utilized to interrogate multiple interference peaks’ absolute peak wavelengths simultaneously. This architecturally simple network model can achieve remarkable generalization capabilities without training large-scale datasets using an appropriate data augmentation strategy. Experiments show that in simultaneous multi-wavelength and cavity length interrogations, the proposed system has the precision of up to ± 14 pm and ± 0.07 µm, respectively. The interrogation resolution can theoretically reach the pm level benefit from the neural network method. Furthermore, the system’s outstanding demodulation repeatability and suitability were demonstrated. The system is expected to provide a high-performance and cost-effective, reliable solution for practical engineering applications.
A high-sensitivity, low-cost, ultrathin microbubble structure based on an Unsymmetrical air-microbubble Fabry-Pérot interferometer (UAFPI) is proposed. The UAFPI, which has an ultrathin wall of ∼6 μm, is constructed by splicing chemically-etched erbium-doped fiber (EDF) to form an air microbubble and applying a taper to one side of the air microbubble. Compared with the air-microbubble Fabry-Pérot interferometer (AFPI), the UAFPI has a high strain sensitivity of 10.15 pm/με, which is approximately three times higher than that of the AFPI, and a low-temperature sensitivity of 2.4 pm/℃ in the range of 25℃-80℃. The proposed sensor has several benefits, including high strain sensitivity, low-temperature sensitivity, a simple fabrication process, and low cost. Therefore, such a UAFPI-based strain sensor has broad prospects for mass production and practical applications.
A compact U-shaped interferometer with high refractive index (RI) sensitivity based on bent single-mode fiber is proposed and demonstrated experimentally. The sensor is fabricated by roasting a balloon-shaped single-mode optical fiber to form a U-shaped modal interferometer. Since the spectral dips show different sensitivities to both the surrounding RI and temperature, simultaneous measurement of RI and temperature can be achieved by demodulating the shifts of the different resonance dips without cascading additional components. The sensitivity of RI reaches up to -1705.66 nm/RIU in the range of 1.3330-1.3581, which is about 10 times as the reported arts. The proposed compact U-shaped interferometer is a potentially effective sensor for chemical and biological applications where an accurate RI measurement of a temperature-changing liquid is required.
Microfiber sensor was regarded as a promising salinity measurement approach in marine surveying. However, the stability and adaptability of microfiber sensors to the complicate and variable moving seawater environment are usually poor. In this paper, a well-encapsulated miniaturized microfiber-optic sensor is proposed for salinity measurement in dynamic seawater environment. The sensor shows a fast response time of about 80 ms, good repeatability, good stability in water current, and good long-term working ability, which indicates that the sensor is a promising device in marine detection. After calibrated, the sensor is applied to the in-situ detection for the actual internal solitary wave (ISW), including the vertical salinity profile and the salinity measurement during the ISW propagation process. Results show that the propagation time of ISW captured by the microfiber sensor and the amplitude of ISW measured by the sensor are both in good agreement with those captured by CCD camera. This paper shows a microfiber salinity sensor with good performance, which will offer useful reference for promoting the practicality of microfiber sensors.
This paper describes the design, fabrication, and characterization of the smallest pressure sensors with the required sensitivity, bandwidth, and robustness for turbulent airflow measurements. These fiber-facet monolithic silicon optical pressure sensors consist of a silicon diaphragm above a sealed cavity contained within a 125 μm wide by 5 μm thick silicon disk mounted directly onto a single-mode optical fiber facet. The fiber-mounted sensors form pressure-sensitive interferometric cavities with 1.6 MHz bandwidth, 4 nm/kPa deflection, and up to 0.07 V/kPa sensitivity. The short length of the optical cavity makes the sensors optically efficient and optically broadband, allowing multiplexing using standard optical communications equipment. This is demonstrated experimentally by optically-multiplexing three fiber-facet sensors with up to 161 kPa dynamic ranges and 2 Pa noise floors. These balanced-detection pressure-sensing arrays provide a small-footprint, high-resolution alternative to traditional turbulence measurement equipment.
In this paper, an optical fiber sensor is proposed for measurement of the boiling point and boiling range of liquids. Since boiling point and/or boiling range of liquids correlate directly to chemical composition of liquids, the proposed sensor provides opportunity for on-line and in-situ chemical characterization of various liquids. The proposed sensor is manufactured on the tip of a standard optical fiber, and consists of a Fabry-Perot temperature sensor and short section Vanadium doped fiber, which serves as a microheater. The latter is cyclical and rapidly heated by application of high-power fiber-coupled laser diode, while the Fabry-Perot temperature sensor is used to detect appearance of boiling process and to measure the absolute temperature. The proposed sensor is less than 1 mm long and allows for characterizing samples in quantities of 140 nL in as short time as ten seconds. All-silica/all-fiber design of the sensor provides high chemical inertness, dielectric design and possibility for remote operation. Two sensor types are proposed; the first is intended for measurement of very small samples (140nL). This measurement takes only about 10 seconds. The second sensor type is suitable for online monitoring of binary or multicomponent mixtures, and provides accurate boiling range data of a given mixture.
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Optical fibers, the enablers of the Internet, are being used in an ever more diverse array of applications. Many of the rapidly growing deployments of fibers are in high-power and, particularly, high power-per-unit-bandwidth systems where well-known optical nonlinearities have historically not been especially consequential in limiting overall performance. Today, however, nominally weak effects, most notably stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) are among the principal phenomena restricting continued scaling to higher optical power levels. In order to address these limitations, the optical fiber community has focused dominantly on geometry-related solutions such as large mode area (LMA) designs. Since such scattering, and all other linear and nonlinear optical phenomena including higher order mode instability (HOMI), are fundamentally materials-based in origin, this paper unapologetically advocates material solutions to present and future performance limitations. As such, this paper represents a 'call to arms' for material scientists and engineers to engage in this opportunity to drive the future development of optical fibers that address many of the grand engineering challenges of our day.
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Pressure fiber sensors play an important role in downhole high pressure measurements to withstand long term operation. The purpose of this paper is to present an application of hollow core photonic crystal fiber (HC-PCF) as a high pressure sensor head for downhole application based on dispersion variation. We used a high pressure stainless steel unit to exert pressure on the sensor. The experimental results show that different wavelengths based on sagnac loop interferometer have additive sensitivities from 5 × 10−5 nm/psi at 1480 nm to 1.3 × 10−3 nm/psi at 1680 nm. We developed a simulation to understand the reason for difference in sensitivity of wavelengths and also the relationship between deformation of HC-PCF and dispersion variation under pressure. For this purpose, by using the finite element method, we investigated the effect of structural variation of HC-PCF on spectral transformation of two linear polarizations under 1000 psi pressure. The simulation and experimental results show exponential decay behavior of dispersion variation from −3.4 × 10−6 1/psi to −1.3 × 10−6 1/psi and from −5 × 10−6 1/psi to −1.8 × 10−6 1/psi, respectively, which were in a good accordance with each other.
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We propose a side-hole polarization-maintaining photonic crystal fiber (PM-PCF) with ultrahigh polarimetric sensitivity to hydrostatic pressure. The proposed fiber has a very simple structure with a core surrounded by a double row of large air holes and a central row of small air holes. A pair of ultralarge side holes was symmetrically introduced into the silica cladding of the fiber to enhance the polarimetric response to hydrostatic pressure. Modal birefringence B as large as 2.34 × 10-3 and polarimetric pressure sensitivity dB/dp as high as -2.30 × 10-5 MPa - 1 were achieved at 1.55 μm for the proposed fiber. Combining the advantages of both side-hole fibers and PM-PCFs, it is believed to be an excellent candidate for future applications of hydrostatic pressure measurement.
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We designed, manufactured and characterized two birefringent microstructured fibers that feature a 5-fold increase in polarimetric sensitivity to hydrostatic pressure compared to the earlier reported values for microstructured fibers. We demonstrate a good agreement between the finite element simulations and the experimental values for the polarimetric sensitivity to pressure and to temperature. The sensitivity to hydrostatic pressure has a negative sign and exceeds -43 rad/MPa x m at 1.55 microm for both fibers. In combination with the very low sensitivity to temperature, this makes our fibers the candidates of choice for the development of microstructured fiber based hydrostatic pressure measurement systems.
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We report the results of our preliminary investigation on the use of hollow-core photonic bandgap fiber (HC-PBF) for hydrophone application. The response of the commercial HC-1550-02 fiber to acoustic pressure, in terms of normalized responsivity (NR), is measured to be - 334.4 dB re 1 microPa(-1). This agrees well with the theoretically predicated value of -331.6 dB re 1 microPa(-1) and is about 15 dB higher than that of the conventional fiber (HNSM-155). With straightforward fiber structure modifications (thinner outer silica cladding and higher air-filling ratio of inner microstructured cladding), the NR could be further enhanced to - 310 dB re 1 microPa(-1).
The composition dependences of molar volume and elastic constants were investigated for sodium and potassium germanate glasses of composition xR2O·GeO2 (O < x < 0.65 in the sodium-glasses and O < x < 0.75 in the potassium-ones). The variation of the molar volume could be approximated by straight lines crossing at 0.18 and 0.16 in x in the sodium- and potassium-glasses, respectively. At these compositions the elastic constants showed maxima. With reference to the structures of several alkali germanate crystals, it was concluded that the fraction of GeO6 units, N6, reaches a maximum at 0.18 in x in the sodium-glasses (15.6) mol.% Na2O) and at 0.16 in x in the potassium-glasses (13.8 mol.% K2O). It was also considered that N6 is not null at 0.5 but is null probably at 1.0 in x in the glasses. A network model of the germanate glasses was discussed.
We present fiber Bragg grating pressure sensors in air-hole microstructured fibers for high-temperature operation above 800 degrees C. An ultrafast laser was used to inscribe Type II grating in two-hole optical fibers. The fiber Bragg grating resonance wavelength shift and peak splits were studied as a function of external hydrostatic pressure from 15 psi to 2000 psi. The grating pressure sensor shows stable and reproducible operation above 800 degrees C. We demonstrate a multiplexible pressure sensor technology for a high-temperature environment using a single fiber and a single-fiber feedthrough.
Elastic and elastooptic coefficients used to predict acoustic response sensitivity for two single-mode optical fibers have been determined from Brillouin scattering measurements. These measurements were made on two ITT single-mode fiber preforms currently of interest in the fabrication of fiber-optic acoustic sensors. Previous predictions of acoustic sensitivity assumed the optical fiber waveguides as homogeneous fused silica cylinders. It was found that this assumption introduces no more than a 5% error in the pressure sensitivity for a low numerical aperture (N.A.) fiber and a 30% error for a high N.A. fiber.
A simple technique is used to measure the wavelength dependence of the birefringence in highly birefringent fibers. In stress-induced fibers, the stress and shape birefringences reinforce each other, whereas in elliptical-core fibers the stress and shape birefringences can either reinforce each other or partially cancel, depending on the wavelength.
Polarization and modal birefringence of elliptical-core two-mode fibers are investigated. Wavelengths corresponding to zero group delay difference (GDD) between the two spatial modes and between the orthogonal polarizations are computed when the fiber parameters, i.e., the relative core/cladding index difference and the ratio of major over minor axis, are varied. Simple relationships between the zero GDD wavelengths and fiber parameters are obtained. With proper fiber design, zero GDD between the two spatial modes and the two orthogonal polarizations can be achieved at the same wavelength.