Pressure sensor realized with polarization-maintaining
photonic crystal fiber-based Sagnac interferometer
H. Y. Fu,
1,
* H. Y. Tam,
1
Li-Yang Shao,
1
Xinyong Dong,
1
P. K. A. Wai,
2
C. Lu,
2
and Sunil K. Khijwania
3
1
Photonics Research Centre, Department of Electrical Engineering, Hong Kong Polytechnic University,
Hung Hom, Kowloon, Hong Kong
2
Photonics Research Centre, Department of Electronics and Information Engineering,
Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
3
Department of Physics, Indian Institute of Technology, Guwahati, India
*Corresponding author: freddy.fu@polyu.edu.hk
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) 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. © 2008 Optical Society of America
OCIS codes: 060.2370, 060.2420, 060.5295.
1. Introduction
Optical fiber Sagnac interferometers have been devel-
oped for gyroscopes and other sensor applications due
to their uniqueadvantages, such as simple design, ease
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 interferometers to in-
troduce optical path difference and cause interference
betweenthe two counterpropagating waves in the fiber
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
of magnitude higher thanthat of long-periodfibergrat-
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
of PM-PCF based Sagnac interferometers is
55–164 times smaller than that of conventional
0003-6935/08/152835-05$15.00/0
© 2008 Optical Society of America
20 May 2008 / Vol. 47, No. 15 / APPLIED OPTICS 2835
PMF-based ones [11,12]. Temperature-induced
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
[13]. This helps to reduce the length of the sen-
sing fiber.
Recently, a PM-PCF-based Sagnac interferometer
employed as a strain sensor that demonstrated high
sensitivity of 0:23 pm=με and measurement range of
up to 32 εm has been reported [ 9]. A PM-PCF-based
pressure sensor with polarimetric detection has also
been proposed and demonstrated [10 ]. 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 [14]. 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
of 3:42 nm=MPa, which is achieved by using a 58:4 cm
PM-PCF-based Sagnac interferometer. The demon-
strated measurement range is 0:3 MPa, 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 5 mm diameter circle. This allows
the realization of a very small pressure sensor.
2. Experimental Setup and Operating Principle
Figure 1 shows the exp erimental setup of our pro-
posed pressure sensor with the PM-PCF based Sag-
nac interferometer. It includes a conventional 3 dB
single-mode fiber coupler and a 58:4 cm PM-PCF.
The PM-PCF (PM-1550-01, Blaze Photonics) has a
beat length of <4 mm at 1550 nm and a polarization
extinction ratio of >30 dB over 100 m. The scanning
electron micrograph (SEM) image of the transverse
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 3:1 μm, respectively.
The combined loss of the two splicing points is
∼4 dB. 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 [ 15 ]. 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 3 dB 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 ¼½1 cosðδÞ=2: ð1Þ
The total phase difference δ introduced by the PM-
PCF can be expressed as
δ ¼ δ
0
þ δ
P
; ð2Þ
where δ
0
and δ
P
are the phase differences due to the
intrinsic and pressure-induced birefringence over
the length L of the PM-PCF and are given by
δ
0
¼
2π · B · L
λ
; ð3Þ
Fig. 1. (Color online) Schematic diagram of the proposed pressure
sensor constructed with PM-PCF based Sagnac interferometer.
Fig. 2. (Color online) Transmission spectrum of the PM-PCF
based Sagnac interferometer.
2836 APPLIED OPTICS / Vol. 47, No. 15 / 20 May 2008
δ
P
¼
2π · ðK
P
ΔPÞ · L
λ
: ð4Þ
B ¼ n
s
− b
f
is the birefringence of the PM-PCF; n
s
and n
f
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 [10]
K
P
¼
∂n
s
∂P
∂n
f
∂P
: ð5Þ
The spacing S between two adjacent transmission
minimums can be approximated by
S ¼ λ
2
=ðB · LÞ: ð6Þ
The pressure-induced wavelength shift of the trans-
mission minimum is Δλ ¼ S · δ
P
=2π. Thus, the rela-
tionship between wavelength shift and applied
pressure can be obtained by
Δλ ¼
K
P
· λ
B
· ΔP: ð7Þ
Equation (7) shows that for a small wavelength shift
the spectral shift is linearly proportional to the ap-
plied pressure.
3. Experimental Results and Discussions
Figure 2 shows the transmission spectrum of the PM-
PCF-based Sagnac interferometer at atmospheric
pressure, i.e., at zero applied pressure. The spacing
between two adjacent transmission minimums is
∼5:3 nm and an extinction ratio of better than
20 dB was achieved. The intrinsic birefringence of
the PM-PCF used in our experiment is 7:8 × 10
−4
at 1550 nm.
The air compressor is initially at one atmospheric
pressure (about 0:1 MPa). In our experiment, we can
increase air pressure up to 0:3 MPa; thus, the max-
imum pressure that can be applied to the PM-PCF-
based Sagnac interferometer sensor is ∼0:4 MPa. At
one atmospheric pressure one of the transmission
minimums occurs at 1551:86 nm and shifts to a long-
er wavelength with applied pressure. When the ap-
plied pressure was increased by 0:3 MPa, a 1:04 nm
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 3:42 nm=MPa
with a good R
2
value 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 ∼2:9 kPa when using an OSA with a 10 pm wave-
length resolution. Because of the limitations of our
equipment, we have not studied the performance
of this pressure sensor for high pressure at this stage.
However, we found that the PM-PCF can stand pres-
sure of 10 MPa without damage to its structure. This
part of the work is ongoing and will be reported in our
further studies.
Although the length of PM-PCF used in our experi-
ment is 58:4 cm, 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 5 mm diameter circle, shown in
the inset of Fig. 4, is measured to be less than 0:01 dB
with a power meter (FSM-8210, ILX Lightwave Cor-
poration). The exceptionally low bending loss will
simplify sensor design and packaging and fulfills
the str ict 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 6 mm 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.
Fig. 3. (Color online) Measured transmission spectra under dif-
ferent pressures.
Fig. 4. (Color online) Wavelength shift of the transmission mini-
mum at 1551:86 nm against applied pressure with variation up to
0:3 MPa based on one atmospheric pressure.
20 May 2008 / Vol. 47, No. 15 / APPLIED OPTICS 2837
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 3:43 nm=MPa for PM-PCFs with
lengths of 40 and 79:6 cm, respectively. After compar-
ing the two wavelength–pressure coefficients with
that of the pressure sensor with a 58:4 cm PM-PCF
(Fig. 4), we observed that the wavelength–pressur e
coefficient is constant around 1550 nm; this agrees
well with our theoretical predicti on. However, the
length of the PM-PCF cannot be reduced too much be-
cause this would result in broad attenuation peaks in
the transmission spectrum and that would reduce the
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 R
2
value of
0.9984. The measured temperature coefficient is
2:2 pm=°C, which is much smaller than the
10 pm=°C of fiber Bragg grating. The temperature
may be neglected for applications that operate over
a normal temperatur e variation range.
Based on the small size, the high wavelength–
pressure coefficient, the reduced temperature sensitiv-
ity characteristic, and other intrinsic advantages of
fiber optic sensors, such as light weight and electro-
magnetically passive operation, the proposed pressure
sensor is a promising candidate for pressure sensing
even in harsh environments. Considering the whole
pressure sensing system, we can also replace the light
source with laser and use a photodiode for intensity de-
tection at the sensing signal receiving end. Since the
power fluctuation is very small even when the PM-
PCF is bent, intensity detection is practical for real ap-
plications. Because of the compact size of the laser and
photodiode, the entire system can be made into a very
portable system. Furthermore, the use of intensity de-
tection instead of wavelength measurement would
greatly enhance interrogation speed and consequently
makes the system much more attractive.
4. Conclusion
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 pressur e sensor is 3:42 nm=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.
References
1. V. Vali and R. W. Shorthill, “Fiber ring interferometer,” Appl.
Opt. 15, 1099–1103 (1976).
2. S. Knudsen and K. Blotekjaer, “An ultrasonic fiber-optic hy-
drophone incorporating a push–pull transducer in a Sagnac
interferometer,” J. Lightwave Technol. 12, 1696–1700 (1994).
3. A. N. Starodumov, L. A. Zenteno, D. Monzon, and E. De La
Rose, “Fiber Sagnac interferometer temperature sensor,”
Appl. Phys. Lett. 70,19–21 (1997).
4. E. De La Rose, L. A. Zenteno, A. N. Starodumov, and
D. Monzon, “All-fiber absolute temperature sensor using an
unbalanced high-birefringence Sagnac loop,” Opt. Lett. 22,
481–483 (1997).
5. Y. J. Song, L. Zhan, S. Hu, Q. H. Ye, and Y. X. Xia, “Tunable
multiwavelength Brillouin–erbium fiber laser with a polariza-
tion-maintaining fiber Sagnac loop filter,” IEEE Photon. Tech-
nol. Lett. 16, 2015–2017 (2004).
6. M. Campbell, G. Zheng, A. S. Holmes-Smith, and P. A. Wallace,
“A frequency-modulated continuous wave birefringent fibre-
optic strain sensor based on a Sagnac ring configuration,”
Meas. Sci. Technol. 10, 218–224 (1999).
7. Y. Liu, B. Liu, X. Feng, W. Zhang, G. Zhou, S. Yuan, G. Kai, and
X. Dong, “High-birefringence fiber loop mirrors and their ap-
plications as sensors,” Appl. Opt. 44, 2382–2390 (2005).
8. G. Kakarantzas, A. Ortigosa-Blanch, T. A. Birks, P. St. Russell,
L. Farr, F. Couny, and B. J. Mangan, “Structural rocking filters
in highly birefringent photonic crystal fiber,” Opt. Lett. 28,
158–160 (2003).
Fig. 5. (Color online) Wavelength shift of the transmission mini-
mum against applied pressure for PM-PCFs with length of 40
(circles) and 79:6 cm (triangles); the wavelength–pressure coeffi-
cients are 3.46 and 3:43 nm=MPa, respectively.
Fig. 6. (Color online) Wavelength shift of the transmission
minimum at 1551:86 nm against temperature.
2838 APPLIED OPTICS / Vol. 47, No. 15 / 20 May 2008
9.X.Dong,H.Y.Tam,andP.Shum,“Temperature-insensitive
strainsensorwithpolarization-maintainingphotoniccrystalfiber
basedSagnacinterferometer,” Appl. Phys. Lett.90,151113(2007).
10. H. K. Gahir and D. Khanna, “Design and development of a
temperature-compensated fiber optic polarimetric pressure
sensor based on photonic crystal fiber at 1550 nm,” Appl.
Opt. 46, 1184–1189 (2007).
11. C.-L. Zhao, X, Yang, C. Lu, W. Jin, and M. S. Demokan, “Tem-
perature-insensitive interferometer using a highly birefrin-
gent photonic crystal fiber loop mirror,” IEEE Photon.
Technol. Lett. 16, 2535–2537 (2004).
12. D.-H. Kim and J. U. Kang, “Sagnac loop interferometer based
on polarization maintaining photonic crystal fiber with re-
duced temperature sensitivity,” Opt. Express 12, 4490–
4495 (2004).
13. T. P. Hansen, J. Broeng, S. E. B. Libori, E. Knudsen, A. Bjarklev,
J. R. Jensen, and H. Simonsen, “Highly birefringent index-
guiding photonic crystal fibers,” IEEE Photon. Technol. Lett.
13, 588–590 (2001).
14. Y. Zhang, D. Feng, Z. Liu, Z. Guo, X. Dong, K. S. Chiang, and B.
C. B. Chu, “High-sensitivity pressure sensor using a shielded
polymer-coated fiber Bragg grating,” IEEE Photon. Technol.
Lett. 13, 618–619 (2001).
15. L. Xiao, W. Jin, and M. S. Demokan, “Fusion splicing small-
core photonic crystal fibers and single-mode fibers by repeated
arc discharges,” Opt. Lett. 32, 115–117 (2007).
20 May 2008 / Vol. 47, No. 15 / APPLIED OPTICS 2839
