High pressure sensor based on photonic
crystal fiber for downhole application
H. Y. Fu,
1
Chuang Wu,
2
M. L. V. Tse,
1
Lin Zhang,
2
Kei-Chun Davis Cheng,
1
H. Y. Tam,
1,
*
Bai-Ou Guan,
2
and C. Lu
3
1
Photonics Research Centre, Department of Electrical Engineering, The Hong Kong Polytechnic University,
Hung Hom, Kowloon, Hong Kong SAR, China
2
School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian, China
3
Photonics Research Centre, Department of Electronics and Information Engineering,
The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China
*Corresponding author: eehytam@polyu.edu.hk
Received 25 January 2010; revised 11 April 2010; accepted 14 April 2010;
posted 14 April 2010 (Doc. ID 123283); published 4 May 2010
We demonstrate a polarization-maintaining (PM) photonic crystal fiber (PCF) based Sagnac interfero-
meter for downhole high pressure sensing application. The PM PCF serves as a direct pressure sensing
probe. The sensor is transducer free and thus fundamentally enhances its long-term sensing stability. In
addition, the PM PCF can be coiled into a small diameter to fulfill the compact size requirement of down-
hole application. A theoretical study of its loss and birefringence changes with different coiling diameters
has been carried out. This bend-insensitive property of the fiber provides ease for sensor design and
benefits practical application. The pressure sensitivities of the proposed sensor are 4.21 and 3:24 nm=
MPa at ∼1320 and ∼1550 nm, respectively. High pressure measurement up to 20 MPa was achieved with
our experiment. It shows both good linearity in response to applied pressure and good repeatability with-
in the entire measurement range. The proposed pressure sensor exhibits low temperature cross sensi-
tivity and high temperature sustainability. It functions well without any measurable degradation effects
on sensitivity or linearity at a temperature as high as 293 °C. These characteristics make it a potentially
ideal candidate for downhole pressure sensing. © 2010 Optical Society of America
OCIS codes: 060.2370, 280.5475, 060.5295, 120.5790, 060.2420.
1. Introduction
Fiber optic sensors for down hole application ha ve at-
tracted much research interest in both aca demia and
industry [1–5]. Conventional electrical gauges used
for in-well parameter measurements have a high fail-
ure rate when implemented in a high temperature
environment. Fiber optic sensors have an estimated
lifetime of 5–10 years, function up to 250 °C [4], are
electrically passive, and are suitable for use in explo-
sive and/or corrosive environments. Fiber optic sen-
sors for downhole application are focused mainly on
the measurement of temperature, pressure, strain,
and flow [5]. Most fiber optic pressure sensors for
downhole application are based on conventional
fibers. However, because of the ultralow pressure sen-
sitivity of these fibers, elaborate transducers or com-
plicated structures need to be introduced to gain and
enhance such functionality. On the other hand, photo-
nic crystal fibers (PCFs) have been less exploited for
downhole sensing application. As a new class of opti-
cal fiber, PCF guides light by a periodic array of micro-
structure running down the entire fiber length [6,7].
Owing to the novel structure with flexible design, nu-
merous applications, including sensing applications,
have been proposed for PCFs [8–19]. One of the most
attractive characteristics provided by PCFs is an
0003-6935/10/142639-05$15.00/0
© 2010 Optical Society of America
10 May 2010 / Vol. 49, No. 14 / APPLIED OPTICS 2639
airhole structure that is sensitive to pressure and
thus has the potential for direct pressure sensing.
Recently, a polarization-maintaining (PM) PCF was
made commercially available. The PM PCF has two
large airholes that surround a solid core in one of
the orthogonal directions. The PM PCF exhib its high
birefringence, low bending loss, and reduced tempera-
ture sensitivity [11,12,15]. A PM PCF used as a
sensing element for strain, pressure, torsion, and cur-
vature measurements has been reported [13–18]. The
multiplexing capability of the PM PCF based sensors
with a Sagnac interferometer configuration has also
been investigated [20]. Together with our proposed
multiplexing techniques, we believe the proposed sen-
sors can be further enhanced in a real application.
We report on the high pressure sensing capability of
the PM PCF based Sagnac loop interferometer [16].
Characteristics of the sensor have been investigated
with regard to downhole application. High birefrin-
gence and bend insensitivity of the PM PCF enabled
us to use and to coil a relatively short length of fiber,
resulting in a compact sensor that can be used to ad-
vantage in space-limited downhole application. The
pressure achieved during our experiment reached
20 MPa. The pressure sensitivities of the proposed
pressure sensor are 4.21 and 3:24 nm=MP a at ∼1320
and ∼1550 nm, respectively. The pressure sensor re-
sponds to the applied pressure with good linearity and
is highly repeatable. It is less sensitive to tempera-
ture, owing to the low thermal coefficient of the PM
PCF, which is made entirely of pure silica and thus
reduces the requirement of temperature compensa-
tion when it is used in a harsh environment where
temperature fluctuation is inevitable [15]. In addi-
tion, its pressure sensing performance has been
tested to function well to as high as 293 °C.
2. Principle and Fabrication of the Pressure Sensing
Element
A. Experimental Setup
The experimental setup of our proposed high pres-
sure sensor based on PM PCF with a Sagnac loop
configuration is illustrated in Fig. 1. The operating
principle of the PM PCF based pressure sensor has
been reported previously [15], so we summarize only
the essential details here. As shown in Fig. 1, the
3 dB coupler splits the input signal equally into two
signals with a π=2 phase difference between them.
The two signals counterpropagate through the PM
PCF before they interfere again at the coupler. With
a broadband light source at the input, the output
transmission spectrum is approximately a periodic
function of the wavelength. The pressure-induced bi-
refringence change of the PM PCF results in a phase
difference between the two signals and then causes a
shift of the output interference spectrum. Applied
pressure variation can then be determined by mea-
suring the wavelength shift of the interference spec-
trum. Alternatively, intensity-based detection can
also be implemented if we use a laser at the input
and a photodetector to measure the output. This
could decrease the cost of the system and make it ea-
sier to achieve a highly portable system [15]. We used
a broadband light source based on a superlumines-
cent light-emitting diode (SLED) that combines
two SLEDs centered at 1320 and 1550 nm. The
Sagnac loop consists of a 3 dB fiber optic coupler and
a piece of PM PCF spliced to the two ports of the
coupler on one side. The PM PCF (PM-1550-01, by
Blaze Photonics, Bath, UK) has a beat length of
<4 mm at 1550 nm. The total loss of the two splicing
points was ∼4 dB, and the splicing points had good
mechanical strength [21]. The length of the PM PCF
was 60 cm and coiled circularly to a diameter of
1:8 cm. Thus, it is compact and suitable for use in
space-limited downhole applications. The PM PCF
here serves as a direct pressure sensing element
without the use of a transducer. An optical spectrum
analyzer was used to record the sinelike inte rference
output signal from the Sagnac loop in the wavelength
domain. The coiled PM PCF was placed inside a pres-
sure chamber. Together with the 3 dB coupler, they
were kept stable during the experiment. The high
pressure chamber was filled with oil. Its pressure
can be adjusted by a high pressure compressor and
measured with a pressure gauge, as illustrated in
Fig. 1. The pressure chamber is fitted with a feed-
through sealed by glue to extend the fiber outside the
chamber for measurement. Although the maximum
pressure value of the pressure compressor is 30 MPa,
the glued feedthrough failed at such high pressure,
and thus the measurement results presented here
are all limited to 20 MPa. In addition, the chamber
was positioned inside a tempe rature-controlled fur-
nace. Hence, by using this setup we can adjust both
pressure and temperature to verify the performance
of our proposed pressure sensor.
B. Compact Size
One of the most important requirements for down-
hole sensors is compact size because of the space-
limited environment. Our proposed PM PCF based
Sagnac interferometric pressure sensor fulfills this
requirement. The PM PCF has a low bending loss
and can be coiled into a small diameter to the centi-
meter scale. The PM PCF with a length of 60 cm was
coiled to a diameter of 1:8 cm. In our experiment, no
obvious changes were observed with regard to loss
and birefringence, even when the fiber was coiled
to the small diameter of 6 mm [15]. To have a com-
plete understanding of its limitation, we present a
Fig. 1. (Color online) Experimental setup of the PM PCF based
Sagnac interferometer for high pressure sensing application.
2640 APPLIED OPTICS / Vol. 49, No. 14 / 10 May 2010
theoretical study of the confinement loss and bire-
fringence change versus the coil diameter. A finite-
element method based vectorial optical mode solver
(Mode Solutions by Lumerical Solutions, Vancouver,
British Columbia, Canada) was used for the simula-
tions. Figure 2 shows the numerical results for the
confinement loss and birefringence of the PM PCF
with different coiling diameters for two perpendicu-
lar orientations. The orientation along the two large
holes of the PM PCF is denoted as 0 deg, and the
other orientation is denoted as 90 deg. A 6 mm bend-
ing diameter was simulated for all the cases, which is
close to the physical limitation of the fiber. The con-
finement loss increases rapidly with the decrease in
radius of curvature below an onset value of 10 mm
under both bending orientations. However, a low
bending loss was found under the extreme bending
orientation. A loss of ∼0:2 dB was accumulated when
a 1 m fiber was coiled 53 times with a radius of 3 mm.
In comparison with conventional single mode fiber,
its bending loss is quite small. On the other hand,
the birefringence begins to increase significantly at
a bending radius of 10 mm. The birefringence splits
away and indicates a strong effect on different bend-
ing orientations. However, only an ∼5% change in
birefringence was introduced at a 10 mm bending
radius. This relatively bend-insensitive character-
istic can benefit sensor implementation in real
applications.
3. High Pressure Sensing
Figure 3(a) shows the output optical spectrum of the
high pressure sensor at the 1550 nm wavelength
band. The extinction ratio between the transmission
maxima and the transmission minima is ∼30 dB
around 1550 nm, which is approximately the center
wavelength of the light source. The spacing between
two adjacent transmission minima is 5:7 nm, denot-
ing a birefringence of 7:0 × 10
−4
at 1550 nm. A fiber
Bragg grating (FBG) centered at 1544:68 nm is
installed together with the PM PCF sensor head to
calibrate the applied temperature. When applied
pressure increases, the whole spectrum shifts toward
longer wavelengths. By measuring the wavelength
shift of one of the transmission minima in the wave-
length spectrum, the pressure variation can be deter-
Fig. 2. (Color online) Numerical results of (a) confinement loss and (b) birefringence of the PM PCF under bending with a 6 mm diameter.
Fig. 3. (Color online) (a) Output optical spectrum of the PM PCF based Sagnac interferometric pressure sensor; the peak shows the
reference FBG. (b) Output optical spectra of the pressure sensor under applied pressure from 0 to 20 MPa at room temperature. One
of the transmission minima shifts from 1509.8 to 1574:8 nm, and the peak in the third spectrum indicates the reference FBG.
10 May 2010 / Vol. 49, No. 14 / APPLIED OPTICS 2641
mined, as illustrated in Fig. 3(b). Initially, one of the
transmission minima is at 1509: 8 nm. It shifts to-
ward a longer wavelength by 65 nm when the applied
pressure increases from 0 to 20 MPa. According to
the measured experimental data, the pressure sensi-
tivity can be calculated to be 3:24 nm=MPa by apply-
ing linear curve fitting. The pressure response of the
sensor is highly linear with a good R
2
value of 0.9997.
If a wavelength meter with 10 pm resolution is used,
it leads to a pressure resolution of 3 kPa. For compar-
ison, we also performed the experiment with a light
source at the 1320 nm wavelength band. The corre-
sponding pressure sensitivity is 4:21 nm=MPa with
an R
2
value of 0.9995, as shown in Fig. 4(a). These
results agree well with our previous theoretical pre-
diction that the amount of the wavelength shift is
approximately proportional to applied pressure at
a small wavelength range [15]. For a larger wave-
length range, the pressure sensitivity is different
because of its corresponding birefringence and the bi-
refringence-pressure coefficient changes at particu-
lar wavelengths. Because of the limitation set by
the glue we used to seal the fiber feedthrough, the
highest applied pressure achieved was 20 MPa.
However, there was no obvious evidence showing
any physical limitation by the PM PCF itself, thus,
higher pressure sensing is possible with this sensor.
The repeatability of the sensor is one of the impor-
tant issues. To test the repeatability of our proposed
sensor, we performed three separate measurements
from0to20 MPa. From the experimental results we
observed that the proposed sensor is highly repeata-
ble, as illustrated in Fig. 4(b). The calculated average
standard deviation for wavelength shift variations
is only 0:024 nm, which corresponds to a small pres-
sure variation of 7:55 kPa. This small variation is
negligible for high pressure sensing such as down-
hole application.
4. Performance in a High Temperature Environment
The high temperature sustainability of fiber optic sen-
sors is an important prerequisite for downhole appli-
cation. In practice, when designing a sensor system,
the variation of an ambient temperature on the sensor
response is always a critical issue. The effect of ambi-
ent temperature fluc tuations on the sensor perfor-
mance was investigated. The temperature of the
furnace was measured with a thermocouple and cali-
brated by a FBG installed inside the pressure cham-
ber. The pressure sensitivity of the FBG was also
Fig. 4. (Color online) Wavelength shift of the transmission minimum (a) at approximately 1320 and 1550 nm under applied pressure from
0 to 20 MPa and (b) under applied pressure from 0 to 20 MPa; good repeatability is demonstrated.
Fig. 5. (Color online) Wavelength shift of the transmission minimum under applied pressure from 0 to 20 MPa at temperatures of
(a) 20 °C, 66 °C, 90 °C, and 120 °C and (b) 293 °C.
2642 APPLIED OPTICS / Vol. 49, No. 14 / 10 May 2010
taken into account in the measurements. We tested
the pressure sensor at temperatures of 20 °C, 66 °C,
90 °C, and 120 °C. Figure 5(a) illustrates the mea-
sured wavelength shift of the transmission minimum
for different temperatures. There is no significant ef-
fect on the pressure sensing performance with a
100 °C temper ature variation. The corresponding
average pressure shift is only −0:19 MPa. The pres-
sure sensor has a low temperature cross sensitivity
that agrees with that reported in previous publica-
tions [11–15]. Furthermore, the sensor can sustain
a temperature as high as 293 °C, as shown in Fig. 5(b);
there is no observable degradation in sensor perfor-
mance. The pressure sensor maintains a linear
response with the same pressure sensitivity at a tem-
perature as high as 293 °C, which is well above the
typical high temperature of 250 °C for downhole
pressure sensing application.
5. Conclusion
The potential of a high pressure sensor for downhole
application by use of a PM PCF based Sagnac inter-
ferometer has been demonstrated experimentally.
The pressure sensor performance has been investi-
gated under high pressure and at a high temperature,
both of which are the two main considerations for in-
well application. The pressure sensitivity is 3:24 nm=
MPa at ∼1550 nm under applied pressure from 0 to 20
MPa. The sensor has a good linear response to applied
pressure, is highly repeatable, and functions well
without any degradation at a temperature as high
as 293 °C. The high pressure sensitivity and the tem-
perature insensitivity as well as the high temperature
sustainability of the PM PCF distinguish the pro-
posed high pressure sensor from other fiber optic
pressure sensors for downhole applications. The sen-
sor has a direct sensing PM PCF head that is simple to
design, compact in size, and easy to manufacture, all
of which make it an ideal candidate for high pressure
sensing in real applications.
The funding support of this research by the Uni-
versity Grants Council Matching Grant of the Hong
Kong Special Administrative Region Government
under the Niche Areas project J-BB9J is gratefully
acknowledged.
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