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JOURNAL OF ELECTRONIC SCIENCE AND TECHNOLOGY OF CHINA, VOL. 6, NO. 4, DECEMBER 2008
Miniaturized Fiber-Optic Fabry-Perot Interferometer
for Highly Sensitive Refractive Index Measurement
Ming Deng, Tao Zhu, Yun-Jiang Rao, and Hong Li
Abstract⎯This paper presents a novel miniaturized
fiber-optic Fabry-Peort interferometer (FPI) for highly
sensitive refractive index measurement. This device was
tested for the refractive indices of various liquids
including acetone and ethanol at room temperature. The
sensitivity for measurement of refractive index change
of ethanol is 1138 nm/RIU at the wavelength of 1550 nm.
In addition, the sensor fabrication is simple including
only cleaving, splicing, and etching. The signal is stable
with high visibility. Therefore, it provides a valuable tool
in biological and chemical applications.
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Index Terms⎯Fiber optic sensors, Fabry-Perot,
miniature sensor, photonic crystal fiber, refractive index
measurement.
range of the RI measurement is also limited. In addition,
many existing devices have shown large temperature
cross-sensitivity. As a result, temperature variation induced
errors need to be corrected in real time.
Fiber-optic Fabry-Perot interferometers (FPI) have been
successfully commercialized and widely used for
measurement of temperature, strain, pressure, displacement,
ultrasonic waves, and RI [7] . Especially, the measurement of
RI using the FPI sensors has attracted great attentions as
they can offer a number of desirable advantages over other
optical fiber sensors, such as high sensitivity, large
measurement range, ability for multi-parameters measurement, rapid response, etc. [8] . Xiao, et al. [9] reported a FPI
refractive-index sensor formed by two polished fiber
end-faces hosted in a holey sleeve. A resolution of 10 −5 was
estimated in monitoring the changes in the refractive index
of gases. However, the sensor assembly was complicated
and required the use of epoxy and various components
made of different materials. As a result, the device had a
strong dependence on temperature. Recently, Wei, et al. [10]
described a FPI refractive-index sensor by fabricating an
open micro-notch FP cavity in a single-mode fiber. This
device is insensitive to temperature with the temperature
cross-sensitivity of 1.1×10 −6 RIU/ ° C. But in this scheme,
the mechanical strength of this device is significantly
reduced due to stress concentrations resulting from its nonsymmetrical structure. Moreover, this sensor is too
expensive in fabrication, the open FP cavity is very easy to
be contaminated, lowering its measurement accuracy and
reliability.
In this paper, we present a novel in-line fiber optic FP
structure with a micrometric tip that are used to precisely
measure the refractive indices of different liquids.
Compared with other FP refractive-index sensors, the
sensor fabrication is simple, including only cleaving,
splicing, and etching. The signal is stable with high
visibility. Therefore, it provides a valuable tool in
biological and chemical applications.
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1. Introduction
Miniaturized and robust optical sensors capable of
accurate and reliable measurement of refractive index (RI)
have attracted considerable attentions in biological and
chemical applications since density, concentration, temperature, and stress, and so on [1] can be detected through
measurements of the refractive index. Many optical RI
sensors have been employed in recent years, such as
grating-based RI sensors [2]-[4] , fiber surface plasmon
resonance (SPR) RI sensors [5] , photonic crystal fibers [6] ,
and so on. These devices have shown excellent advantages
of small size, corrosion resistance, immunity to electromagnetic interference, high accuracy, high sensitivity, and
fast response time, etc. However, they are based on
evanescent field, which resulted in a nonlinear response
wavelength to refractive index, meaning that the sensitivity
varies at different refractive index ranges. The dynamic
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Manuscript received September 18, 2008; revised October 20, 2008. This
work was supported by the Key Project of Natural Science Foundation of
China under Grant No. 60537040 and the Natural Science Foundation
Project of CQ CSTC under Grant No. 2007BB3125.
M. Deng and H. Li are with the Key Laboratory of Optoelectronic
Technology and Systems, Chongqing University, Chongqing, 400044,
China (e-mail: [email protected] ).
Y.-J. Rao and T. Zhu are with the Key Laboratory of Optoelectronic
Technology and Systems, Chongqing University, Chongqing, 400044,
China and the Key Lab of Broadband Optical Fiber Transmission &
Communication Networks Technology (Ministry of Education of China),
University of Electronic Science& Technology of China, Chengdu,
610054, China (e-mail: [email protected] and [email protected] ).
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2. Sensor Fabrication and
Operating Principle
Three types of commercial fibers are used in the
experiment: single-mode fiber (Corning Inc.), graded-index
multimode fiber (Lucent Inc.), and photonic crystal fiber
JOURNAL OF ELECTRONIC SCIENCE AND TECHNOLOGY OF CHINA, VOL. 6, NO. 4, DECEMBER 2008
366
(Crystal Fiber Inc.). Here, the graded-index multimode
fiber (MMF) is germanium-doped in the core with pure
fused silica cladding. The core diameter of the MMF is 62.5
μm and the cladding diameter is 125 μm. The structure of
the photonic crystal fiber (PCF) is shown in Fig. 1. The
cladding of the fiber is composed of elementary triangle
cells, each having an air channel in the center. The central
cell has no holes and it continues the fiber core. The pith
length ( Λ) of the PCF is 8μm, the diameter of its air hole
(d) is 3.68 μm and its external diameter is 125 μm.
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Fig. 3. The novel FPI with a cavity length of 60 μm.
Due to the low reflectivity of the cavity surfaces, the
sensor can be described as low finesse FPI and therefore the
total reflective intensity I R (λ) can be derived using the
two-beam optical interference equation:
B
B
⎡⎛ 4π nL
⎞⎤
I R (λ )= I1 +I 2 +2 I1 I 2 cos ⎢⎜
+Φ0 ⎟ ⎥
⎠⎦
⎣⎝ λ
(1)
where I 1 and I 2 are the reflections at the cavity surfaces, n
is the refractive index of the medium filling the cavity, L is
the cavity length, λ is the optical wavelength in vacuum,
and Φ0 is the initial phase of the interference.
B
MMF PCF
I1
B
Light
B
I2
B
B
B
B
According to (1), we can see that the interference signal
reaches its maximum when the phase of the cosine term
becomes an even number of 2π , namely:
Fig. 1 The structure of PCF.
SMF
B
Filling
liquid (n)
I R (λ )= I max (λ ) ,
Fig. 2. The schematic diagram of novel FPI.
Since the doped silica will be etched much faster than
pure region, a section of the graded-index MMF is
immersed in the hydrofluoric acid (HF 50%) to remove the
core of the fiber. One end of the etched graded-index MMF
is cleaved and spliced to a single-mode fiber (SMF),
forming the first interference mirror, the other end of the
graded-index MMF is also cleaved with a desirable length
under a microscope and then spliced to a section of PCF,
forming the second interference mirror. The pigtailed PCF
is cleaved as thin as possible and cleaved with an angle.
Hence, the reflective light from the end face is too weak to
interfere with other light from the former two reflecting
surfaces. Fig. 2 shows the schematic of the FP structure. Fig.
3 shows the scanning electron microscope (SEM) image of
the novel FP structure with a micrometric tip. The cavity
length is about 60 μm as estimated from the SEM image.
λm
+Φ0 = 2mπ
(2)
where m is an integer and λ m is the wavelength corresponding to the mth FPI reflection peak.
The two adjacent interference maximums have a phase
difference of 2π. Therefore the optical length of the cavity
can be given by
λm1λm 2
(3)
Ln =
2 λm1 − λm 2
B
Filling
liquid (n)
4π nL
when
B
where λ m1 and λ m2 are the peak wavelengths of the adjacent
fringes of the FPI.
The optical length of the cavity can also be calculated
by using the Fast Fourier Transform (FFT) of the reflection
spectrum, which is given as
B
B
B
Ln =
B
k
,
2 N δν
k = 0,1, 2," N − 1
(4)
where N is the sample point number of FFT, k is the
abscissa corresponding the maximum FFT value, and
δν = δλ / λ 2 . In theory, (3) and (4) can be used to calculate
either the absolute refractive index n or the absolute cavity
length L if one of them is known.
In many cases, only the relative refractive index change
is of interest and the range of refractive index variation is
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DENG et al.: Miniaturized In-Line Fabry-Perot Interferometer for Highly Sensitivity Refractive Index Measurement
small so the phase shift is less than 2π. In this case, the
phase ambiguity issue can be avoided. Rearranging (2), we
obtain the changes in the refractive index of the cavity of
the fiber optic FPI, given by
Δλm
Δn =
λm 0
(5)
n0
The interrogation of the FPI sensor is schematically shown
in Fig. 5. A broadband source was used to excite the FPI
sensor through a 3 dB fiber coupler. The reflected
interference signal from the sensor was detected by a
high-accuracy optical spectrum analyzer (OSA, Si720,
Micron Optics, USA). The wavelength resolution and
precision of OSA are 2.5 pm and 1 pm, respectively.
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where Δn is the change in the refractive index of the cavity,
n0 is the initial refractive index of the cavity, λm 0 is the
3dB Coupler
n0 changes to n0 + Δn . It can be seen that the relative
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3. Experiments and Discussion
The interference spectrum of the fabricated novel FPI in
air at room temperature is shown in Fig. 4. The fringe
visibility of this particular device is about 10 dB higher than
that of a FPI formed by femtosecond laser [10] . The loss of
the FPI was about 34 dB, which was sufficient for most
sensing applications. The length of the FP cavity was found
to be 61.262 μm using (4) with n set to be 1.0003 for air at
the wavelength of 1550 nm [9] . The calculated value was
very close to the cavity length estimated by the SEM image.
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air
Air
Acetone
acetone
Ethanol
ethanol
Intensity(dBm)
−15
−25
−35
−45
−55
−65
1520
1530
1530
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FPI sensor
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Fig. 5. Experimental setup for RI measurement.
The interference spectra of the FPI sensor immersed into
various liquids are also shown in Fig. 4 for comparison. The
signal intensity decreases with the increment of liquid
refractive index, which is mainly because the power
reflective coefficients at the surfaces decreased and thus
lowered the reflective intensity when the device was
immersed in liquids. The spectral distance between the two
adjacent peaks also decreased, indicating the increase of
refractive index of the medium inside the cavity. Calculating
(4), we obtain the refractive indices of the liquids:
n acetone =1.3571 and n ethnol =1.3619, which are close to the
commonly accepted values.
B
B
B
B
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−5
−75
1520
Optical
spectrum
analyzer
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Broadband
source
wavelength corresponding to the mth reflectance maximum
being monitored, and Δλm is the drift of λm 0 , when
index change is proportional to the spectral shift of the
interfeogram. When monitoring the relative index change
of ethanol with a nominal refractive index of 1.362, the
sensitivity of the measurement is 1138 nm/RIU at the
wavelength of 1550 nm according to (5). The actual
detection limit depends on the resolution with which the
spectral shift of the interferogram can be determined. If a
resolution of 2.5 pm is achieved in the determination of
interferogram shift, a detection limit of 2.2×10 −6 RIU is
attained.
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1540
1550
1540
1550
Wavelength
(nm)
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1560
1560
1570
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Fig. 4. Interference spectrum of the FPI sensor in air, acetone, and
ethanol.
To evaluate its capability for refractive index
measurement, the fiber FPI sensor was tested using different
liquids including acetone and ethanol at room temperature.
4. Conclusions
We have presented a novel in-line Fabry-Perot
interferometer with a micrometric tip for highly sensitive
refractive-index measurement. By demodulation of the
interference spectrum, the change of optical path distance
caused by the environment can be obtained. This effect can
be very useful in a broad range of biomedical applications,
such as refractive index measurement and concentration
monitor. The sensor was evaluated for refractive index
measurement of various liquids and the results agreed on
with the reported data. More importantly, the device
provides advantages such as small size, all-fiber inline
structure, easy fabrication, linear response, high sensitivity,
and high resolution; therefore, it is possible to stably work
in harsh environments.
Acknowledgment
The authors would like to thank the support by the Key
Teacher Foundation of Chongqing University.
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JOURNAL OF ELECTRONIC SCIENCE AND TECHNOLOGY OF CHINA, VOL. 6, NO. 4, DECEMBER 2008
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Ming Deng was born in Shannxi Province, China, in 1979.
She received the B.S. and M.S. degrees from the Xi’an University
of Science and Technology, Shannxi, in 2003 and 2006,
respectively, both in communication and information engineering.
She is currently pursuing the Ph.D. degree with the Department of
Optoelectronic Engineering, Chongqing University. Her research
interests include fiber-optic passive and active optical sensors.
Tao Zhu was born in Sichuan Province, China, in 1976. He
received the M.S. and Ph.D. degrees from the Chongqing
University, Chongqing, in 2003 and 2008, respectively, both in
optical engineering. He is a member of the Optical Society of
America. His research focuses on passive and active optical
components and optical sensors.
Yun-Jiang Rao received his M.Eng and Ph.D. degrees in
optoelectronic engineering from Chongqing University in 1986
and 1990, respectively, where he led a team to develop the first
fully-automatic optical fiber fusion splicing machine in China. He
joined the Optoelectronics Division of Electric and Electronic
Engineering Department in Strathclyde University, U.K., as a
post-doctoral research fellow in 1990. He was employed by Kent
University in U.K. as a research fellow and then a senior research
fellow during 1992 to 1999, where he made important
contributions to fiber-optic low-coherence interferometry and
in-fiber Bragg grating sensors. During 1999 to 2004, he was a
Chang-Jiang Chair Professor in optical engineering with
Department of Optoelec-tronic Engineering, Chongqing
University, China, and established the Optical Fiber Technology
Group with strong support of the Ministry of Education of China
under the Program of Chang-Jiang Scholar Professorship. He is
currently the Head of Optical Fiber Technology Research Centre
and the Dean of School Communication and Information
Engineering, University of Electronic Science and Technology of
China and Chang-Jiang Chair Professor in Optical Engineering.
Hong Li was born in Sichuan Province, China, in 1982. He
received the B.S. degree from the Chengdu University of
Information and Technology, Chengdu, in 2006. He is currently
pursuing the M.S. degree with the Department of Optoelectronic
Engineering, Chongqing University. His research interests focuse
on novel fiber-optic communication and sensing devices.