Fiber in-line Fabry–Pérot interferometer for
simultaneous measurement of reflective
index and temperature
Xiaoqi Ni (倪小琦), Ming Wang (王 鸣)*, and Dongmei Guo (郭冬梅)
Jiangsu Key Laboratory on Opto-Electronic Technology, School of Physical Science and Technology,
Nanjing Normal University, Nanjing 210023, China
*Corresponding author: wangming@njnu.edu.cn
Received December 11, 2015; accepted February 23, 2016; posted online April 6, 2016
A fiber in-line Fabry–Pérot interferometer is presented. The sensing head consists of a micro ellipsoidal air cavity
and a small section of solid-core photonic crystal fiber. The reflective index (RI) and temperature can be
interrogated simultaneously through a fast Fourier transform and by tracing the dip wavelength shift of the
reflective spectrum. Experimental results show that the RI amplitude and wavelength sensitivities are 5.30/
RIU and 8.46 × 10
−1
nm∕RIU in the range from 1.34 to 1.43, and the temperature amplitude and wavelength
sensitivities are 6.8 × 10
−4
∕°C and 2.48 × 10
−3
nm∕°C in the range from 15°C to 75°C, respectively. Easy
fabrication, a simple system, and simultaneous measurement make it appropriate for dual-parameter sensing
application.
OCIS codes: 280.4788, 060.2370.
doi: 10.3788/COL201614.050601.
The measurement of reflective index (RI) has played an
important role in physical, chemical, and biomedical
areas. The optical fiber-based RI sensors have been
intensively investigated and widely applied due to their
unique advantages such as simplicity, immunity to
electromagnetic interference, corrosion resistance, and re-
mote sensing capability. Most of them are based on fiber
gratings, Fabry–Pérot (FP) or Mach–Zehnder (MZ) inter-
ferometer, micro fiber, selectively infiltrated photonic
crystal fiber (PCF) coupler, deposition of a thin film over
the sensing element, and many other structures
[1–11]
. The
FP interferometer (FPI)-based sensors have been much
more extensively studied owing to their characteristics
of simple configurations, compactness, stable perfor-
mances, and endurance for high temperature and high
pressure environments, etc. RI sensing cannot be carried
out reliably without the simultaneous measurement of
temperature. In recent years, optical fiber-based RI
sensors with temperature compensation have been
extensively investigated
[5,7,8,12–14]
.
In this Letter, an FPI is presented . The sensing head of
the interferometer consists of a closed air cavity and a sec-
tion of PCF. The former responds to the envelope of the
reflective spectrum, and the latter contributes to the high-
frequency fringe spectrum. The spectral response of the
interferometer with different lengths of PCF is analyzed.
The frequency characteristics of reflective spectra when
the interferometer is dipped into solutions with different
RIs are studied. The influence of temperature on reflective
spectra is also studied. Taking interaction into consider-
ation, a matrix equation is used to demodulate RI and
temperature simultaneously.
The FPI was made by splicing a standard single mode
fiber (SMF) with a solid-core PCF whose cross section is
shown in Fig.
1(a) (core diameter: 7.0 μm, cladding diam-
eter: 125 μm). The splicing was performed on a fusion
splicer (Fujikura 80S) and the discharge power and time
were −40 dB and 1000 ms, respectively. The discharge
position was on the SMF, about 100 μm from the end,
in order not to destroy the end face of the PCF. Such fu-
sion parameters ensure complete collapsing of the air holes
in a limited region. After three discharge times, air holes in
the cladding of the PCF collapsed and an elliptical air
cavity was formed in the fiber. The collapsed region
can be regarded as pure silica. Subsequently, the PCF
was cut off with a small section remaining, as shown in
Fig.
1(b). An optical sensing analyzer si720 provided by
Micron Optics was used to act as both the light source
and the spectral receiver. The scanning laser ranging from
1510 to 1590 nm was launched to the sensor from the
SMF. The reflective spectra before cleaving (depicted with
a black solid line), and when the remaining PCF are 1 and
9 mm after cleaving (dotted in blue and red, respectively),
are shown in Fig.
2.
We found out that before cleaving the reflective spec-
trum is influenced only by the closed air cavity. Taking
Fig. 1. Microscopic image of (a) cross-sectional image of PCF
and (b) sensing head.
COL 14(5), 050601(2016) CHINESE OPTICS LETTERS May 10, 2016
1671-7694/2016/050601(5) 050601-1 © 2016 Chinese Optics Letters