Pressure sensor realized with polarization-maintaining photonic crystal fiber-based Sagnac interferometer.
ABSTRACT A novel intrinsic fiber optic pressure sensor realized with a polarization-maintaining photonic crystal fiber (PM-PCF) based Sagnac interferometer is proposed and demonstrated experimentally. A large wavelength-pressure coefficient of 3.42 nm/MPa was measured using a 58.4 cm long PM-PCF as the sensing element. Owing to the inherently low bending loss and thermal dependence of the PM-PCF, the proposed pressure sensor is very compact and exhibits low temperature sensitivity.
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Article: Fiber ring interferometer.[show abstract] [hide abstract]
ABSTRACT: A ring interferometer rotation detector (gyroscope) using optical fiber waveguide is designed. The sensitivity of the device is enhanced via multiple traverses of counterrotating beams around an area, but restrictions on the optimum fiber length are imposed by the photon noise limit. A well-defined wavefront and efficient coupling of light into the fiber are required. Laser light divided by a beam splitter is focused on the terminations of single-mode fibers. After exiting, the light is returned through the fibers in the opposite direction and the beams are recombined at the beam splitter, with the image magnified and displayed. Displacement of one end of the fiber from the focal point of the converging lens produces growing fringes as an error signal. The system is sufficiently sensitive for navigation applications, is free from pulling and lock-in characterizing ring laser systems (and hence can detect very low angular velocities), but is inferior to the ring laser in ultimate theoretical sensitivities.Applied Optics 05/1976; 15(5):1099-100. · 1.69 Impact Factor
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ABSTRACT: A theoretical and experimental analysis of a push-pull acoustic transducer incorporated in a Sagnac interferometer is presented. The transducer is optimized for detection of ultrasonic signals in the frequency range 0.4-1.5 MHz. Two different configurations are investigated. In one, the sensing fiber forming the transducer constitutes the total loop length, and in the second an additional delay coil is included in the middle section of the Sagnac loop. In the latter we demonstrate experimentally noise-equivalent pressures of 36-43 dB re. 1 μPa/Hz<sup>1/2</sup> in the frequency range 0.4-1.0 MHz. An approximate theoretical model is presented, and we obtain reasonable agreement between theory and experiment for both the frequency-dependent noise equivalent pressure and the directional responsivity of this push-pull acoustic fiber-optic sensorJournal of Lightwave Technology 10/1994; · 2.56 Impact Factor
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ABSTRACT: A modified Sagnac interferometer-based fiber temperature sensor is proposed. Polarization independent operation and high temperature sensitivity of this class of sensors make them cost effective instruments for temperature measurements. A comparison of the proposed sensor with Bragg grating and long-period grating fiber sensors is derived. A temperature-induced spectral displacement of 0.99 nm/K is demonstrated for an internal stress birefringent fiber-based Sagnac interferometer. © 1997 American Institute of Physics.Applied Physics Letters 01/1997; 70(1):19-21. · 3.79 Impact Factor
Pressure sensor realized with polarization-maintaining
photonic crystal fiber-based Sagnac interferometer
H. Y. Fu,1,* H. Y. Tam,1Li-Yang Shao,1Xinyong Dong,1
P. K. A. Wai,2C. Lu,2and Sunil K. Khijwania3
1Photonics Research Centre, Department of Electrical Engineering, Hong Kong Polytechnic University,
Hung Hom, Kowloon, Hong Kong
2Photonics Research Centre, Department of Electronics and Information Engineering,
Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
3Department of Physics, Indian Institute of Technology, Guwahati, India
*Corresponding author: firstname.lastname@example.org
Received 3 March 2008; accepted 16 April 2008;
posted 22 April 2008 (Doc. ID 93390); published 14 May 2008
A novel intrinsic fiber optic pressure sensor realized with a polarization-maintaining photonic crystal fiber
(PM-PCF)basedSagnacinterferometeris proposed anddemonstratedexperimentally.Alargewavelength–
pressure coefficient of 3:42nm=MPa was measured using a 58:4cm long PM-PCF as the sensing element.
Owing to the inherently low bending loss and thermal dependence of the PM-PCF, the proposed pressure
sensor is very compact and exhibits low temperature sensitivity.
060.2370, 060.2420, 060.5295.
© 2008 Optical Society of America
Optical fiber Sagnac interferometers have been devel-
oped for gyroscopes and other sensor applications due
of manufacture, and lower susceptibility to environ-
mental pickup noise in comparison to other types of fi-
ber optic sensors [1,2]. Polarization-maintaining fiber
(PMF) is usually used in Sagnac interferometerstoin-
troduce optical path difference and cause interference
loop [3–7]. However, conventional PMFs (e.g., Panda
and bow-tie PMFs) have a high thermal sensitivity
due to the large thermal expansion coefficient differ-
ence between boron-doped stress-applying parts and
the cladding (normally pure silica). Consequently,
conventional PMFs exhibit temperature-sensitive bi-
refringence. Therefore, conventional PMF based Sag-
nac interferometer sensors exhibit relatively high
temperature sensitivity, which is about 1 and 2 orders
ing (LPG) and fiber Bragg grating(FBG) sensors[3,4].
When they are used for sensing other measurements
rather than temperature, such as pressure, the tem-
perature change and fluctuation will cause serious
cross-sensitivity effects and would affect the measure-
ment accuracy significantly.
In recent years, polarization-maintaining photonic
crystal fiber (PM-PCF) has become commercially
available and subsequently has attracted a lot of
research interest in investigating its potential in
communications and sensing applications [8–10].
PM-PCF possesses very low bending loss due to
the large numerical aperture and small-core dia-
meter. This feature is crucial for the realization of
practical sensors. It is also significantly less tem-
perature dependent than conventional PMFs due
to its pure silica construction without any doped ma-
terials in the core or cladding, except with air holes
running along the entire length of the fibers. Pre-
vious reports showed that the thermal sensitivity
55–164 times smaller than that of conventional
© 2008 Optical Society of America
20 May 2008 / Vol. 47, No. 15 / APPLIED OPTICS2835
cross-sensitivity effects can thus be neglected for sen-
sing applications in which the temperature variation
is not too large. Furthermore, owing to the flexible
fabrication design of PM-PCF, its birefringence can
be much higher than that of conventional PMFs
. This helps to reduce the length of the sen-
Recently, a PM-PCF-based Sagnac interferometer
employed as a strain sensor that demonstrated high
sensitivity of 0:23pm=με and measurement range of
up to 32εm has been reported . A PM-PCF-based
pressure sensor with polarimetric detection has also
been proposed and demonstrated . Polarimetric
sensors are complicated and are generally not pre-
ferred in most applications. In this paper, we propose
and demonstrate a pressure sensor based on a PM-
PCF Sagnac interferometer. The Sagnac loop itself
acts as a sensitive pressure sensing element, making
it an ideal candidate for pressure sensor. Other
reported fiber optic pressure sensors generally re-
quired some sort of modification to the fiber to in-
crease their sensitivity . The proposed pressure
sensor does not require polarimetric detection and
the pressure information is wavelength encoded.
The theoretical analysis for the pressure-induced
spectral shift is briefly presented in this paper. Pres-
sure measurement results show a sensing sensitivity
of3:42nm=MPa, which isachieved byusing a 58:4cm
PM-PCF-based Sagnac interferometer. The demon-
strated measurement range is 0:3MPa, which is lim-
ited by the test apparatus available in our laboratory.
Important features of the pressure sensor are the low
thermal coefficient and the exceptionally low bend-
ing loss of the PM-PCF, which permits the fiber to
be coiled into a 5mm diameter circle. This allows
the realization of a very small pressure sensor.
Figure 1 shows the experimental setup of our pro-
posed pressure sensor with the PM-PCF based Sag-
nac interferometer. It includes a conventional 3dB
single-mode fiber coupler and a 58:4cm PM-PCF.
The PM-PCF (PM-1550-01, Blaze Photonics) has a
beat length of <4mm at 1550nm and a polarization
extinction ratio of >30dB over 100m. The scanning
electron micrograph (SEM) image of the transverse
Experimental Setup and Operating Principle
cross section of the PM-PCF is shown in the inset
of Fig. 1. Mode field diameters for the two orthogonal
polarization modes are 3.6 and 3:1μm, respectively.
The combined loss of the two splicing points is
∼4dB. Low splicing loss could be achieved by re-
peated arc discharges applied over the splicing
points to collapse the air holes of the small-core
PM-PCF . Collapsing enlarges the mode field
at the interface of the PM-PCF so as to match the
mode field of the single-mode fiber (SMF). The Sag-
nac interferometer is laid in an open metal box and
the box is placed inside a sealed air tank. The tank is
connected to an air compressor with adjustable air
pressure that was measured with a pressure meter.
The input and output ends of the Sagnac interferom-
eter are placed outside the air tank. When a broad-
band light source (amplified spontaneous emission
source with pumped erbium-doped fiber) is con-
nected to the input, an interference output, as shown
in Fig. 2, can be observed. By measuring the wave-
length shift of one of the transmission minimums
with an optical spectrum analyzer (OSA), the applied
pressure to the PM-PCF can be determined.
In the fiber loop,the two counterpropagating lights
split by the 3dB SMF coupler interfere again at the
coupler and the resulting spectrum is determined by
the relative phase difference introduced to the two
orthogonal guided modes mainly by the PM-PCF.
Ignoring the loss of the Sagnac loop,the transmission
spectrum of the fiber loop is approximately a periodic
function of the wavelength and is given by
T ¼ ½1 ? cosðδÞ?=2:
The total phase difference δ introduced by the PM-
PCF can be expressed as
δ ¼ δ0þ δP;
where δ0and δPare the phase differences due to the
intrinsic and pressure-induced birefringence over
the length L of the PM-PCF and are given by
δ0¼2π · B · L
sensor constructed with PM-PCF based Sagnac interferometer.
(Color online) Schematicdiagramoftheproposedpressure Fig. 2.
based Sagnac interferometer.
(Color online) Transmission spectrum of the PM-PCF
2836 APPLIED OPTICS / Vol. 47, No. 15 / 20 May 2008
δP¼2π · ðKPΔPÞ · L
B ¼ ns− bf is the birefringence of the PM-PCF; ns
and nf are effective refractive indices of the PM-
PCF at the slow and fast axes, respectively. ΔP is
the applied pressure and the birefringence-pressure
coefficient of PM-PCF can be described as 
The spacing S between two adjacent transmission
minimums can be approximated by
S ¼ λ2=ðB · LÞ:
The pressure-induced wavelength shift of the trans-
mission minimum is Δλ ¼ S · δP=2π. Thus, the rela-
tionship between wavelength shift and applied
pressure can be obtained by
Equation (7) shows that for a small wavelength shift
the spectral shift is linearly proportional to the ap-
Figure 2 shows the transmission spectrum ofthe PM-
PCF-based Sagnac interferometer at atmospheric
pressure, i.e., at zero applied pressure. The spacing
between two adjacent transmission minimums is
∼5:3nm and an extinction ratio of better than
20dB was achieved. The intrinsic birefringence of
the PM-PCF used in our experiment is 7:8 × 10−4
The air compressor is initially at one atmospheric
pressure (about 0:1MPa). In our experiment, we can
increase air pressure up to 0:3MPa; thus, the max-
imum pressure that can be applied to the PM-PCF-
based Sagnac interferometer sensor is ∼0:4MPa. At
one atmospheric pressure one of the transmission
minimums occurs at 1551:86nm and shifts to a long-
er wavelength with applied pressure. When the ap-
plied pressure was increased by 0:3MPa, a 1:04nm
wavelength shift of the transmission minimum
was measured, as shown in Fig. 3. Figure 4 shows
the experimental data of the wavelength–pressure
variation and the linear curve fitting. The measured
wavelength–pressure coefficient is 3:42nm=MPa
with a good R2value of 0.999, which agrees well
with our theoretical prediction. From Eq. (7), the
birefringence–pressure coefficient is ∼1:7 × 10−6
MPa−1. The resolution of the pressure measurement
is ∼2:9kPa when using an OSA with a 10pm wave-
length resolution. Because of the limitations of our
equipment, we have not studied the performance
ofthis pressuresensorfor high pressure atthis stage.
However, we found that the PM-PCF can stand pres-
Experimental Results and Discussions
sure of 10MPa without damage to its structure. This
part of the work isongoing andwill be reported in our
Although the length of PM-PCF used in our experi-
ment is 58:4cm, it is important to note that the PM-
PCF can be coiled into a very small diameter circle
with virtually no additional bending loss so that a
compact pressure sensor design can be achieved.
The induced bending loss by coiling the PM-PCF fi-
ber into 10 turns of a 5mm diameter circle, shown in
the inset of Fig. 4, is measured to be less than 0:01dB
with a power meter (FSM-8210, ILX Lightwave Cor-
poration). The exceptionally low bending loss will
simplify sensor design and packaging and fulfills
the strict requirements of some applications where
small size is needed, such as in down-hole oil well ap-
plications. To investigate the effects of coiling, we
have studied two extreme cases in which the PM-
PCF was wound with its fast axis and then its slow
axis on the same plane of the coil. There were no
measurable changes for either the birefringence or
the wavelength–pressure coefficient when the fiber
was coiled into 15 and 6mm diameter circles with
both of the orientations coiling. The coiling of the
PM-PCF into small diameter circles makes the entire
sensor very compact and could reduce any unwanted
environmental distortions, such as vibrations.
(Color online) Measured transmission spectra under dif-
mum at 1551:86nm against applied pressure with variation up to
0:3MPa based on one atmospheric pressure.
(Color online) Wavelength shift of the transmission mini-
20 May 2008 / Vol. 47, No. 15 / APPLIED OPTICS2837
The wavelength–pressure coefficient is indepen-
dent of the length of the PM-PCF, as described in
Eq. (7). Figure 5 shows the wavelength-pressure coef-
ficients are 3.46 and 3:43nm=MPa for PM-PCFs with
lengths of 40 and 79:6cm, respectively. After compar-
ing the two wavelength–pressure coefficients with
that of the pressure sensor with a 58:4cm PM-PCF
(Fig. 4), we observed that the wavelength–pressure
coefficient is constant around 1550nm; this agrees
well with our theoretical prediction. However, the
cause this would result in broad attenuation peaks in
reading accuracy of the transmission minimums.
Temperature sensitivity of the proposed pressure
sensor is also investigated by placing the sensor into
an oven and varying its temperature. Figure 6 shows
the wavelength shift of a transmission minimum ver-
sus temperature linearly with a good R2value of
0.9984. The measured temperature coefficient is
?2:2pm=°C, which is much smaller than the
10pm=°C of fiber Bragg grating. The temperature
may be neglected for applications that operate over
a normal temperature variation range.
Based on the small size, the high wavelength–
ity characteristic, and other intrinsic advantages of
fiber optic sensors, such as light weight and electro-
sensor is a promising candidate for pressure sensing
even in harsh environments. Considering the whole
pressure sensing system, we can also replace the light
tection at the sensing signal receiving end. Since the
power fluctuation is very small even when the PM-
photodiode, the entire system can be made into a very
portable system. Furthermore, the use of intensity de-
tection instead of wavelength measurement would
makes the system much more attractive.
A novel fiber Sagnac interferometer pressure sensor
realized by using a PM-PCF as the sensing element
has been proposed and demonstrated. Experimental
results and simplified theoretical analysis of the
pressure sensor have been presented. The sensitivity
of the pressure sensor is 3:42nm=MPa. The proposed
pressure sensor exhibits the advantages of high sen-
sitivity, compact size, low temperature sensitivity,
and is potentially low cost.
The authors thank Dr. Limin Xiao and Dr. S. Y. Liu
for fruitful discussions and timely help. This work
was supported in part by the Hong Kong Polytechnic
University under project G-U263 and in part by the
Central Research Grant of the Hong Kong Polytech-
nic University under project G-YX77.
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