Optics and Photonics Journal, 2013, 3, 265-267
doi:10.4236/opj.2013.32B062 Published Online June 2013 (http://www.scirp.org/journal/opj)
A Smart Graded-index Multimode Fiber Based Sensor
Unit for Multi-parameter Sensing Applications
Shuo Fang, Baoyong Li, Dawei Song, Jianzhong Zhang*, Weimin Sun, Libo Yuan
Key Laboratory of In-Fiber Integrated Optics, Ministry of Education, Harbin 150001, China
Received 2013
ABSTRACT
We demonstrate a smart optical fiber sensor unit to realize a multi-parameter sensing, including temperature, curvature
and strain or displacement. The sensor unit is composed of a Bragg grating in graded-index multimode fiber and a
Fizeau cavity.
Keywords: Fiber Optic Sensor; Fiber Bragg Grating; Fiber Fizeau Cavity
1. Introduction
Fiber optic sensors offer lots of advantages including elec-
tromagnetic interference immunity, multiplexing capabil-
ity of several sensors in series, and resistance to harsh
environment such as high temperature and big pressure.
Multi-parameter sensing is also an important trend for
optical fiber sensor. Many techniques are based on mul-
tiplexing different FBGs or combining different types of
sensors, such as hybrid FBG/Fizeau interferometer, hybrid
FBG/long-period gratings and so on [1-3]. On the other
hand, multimode fiber (MMF) based sensors [4-7] have
attracted much attention recently because of their high
sensitivity and low price. A step-index MMF-based tem-
perature and strain sensor, a refractive index sensor and a
curvature sensor based on MMF, and a strain and tem-
perature sensor based on a graded-index (GI) MMF are
all demonstrated. Here we propose and demonstrate a GI
MMF based sensor unit to realize a multi-parameter
sensing, including temperature, curvature and strain or
displacement.
2. Setup and Experiments
The Sensor Unit is composed of several parts, shown in
Figure 1. A short Bragg grating is written in a section
(~3 cm) of GI MMF with cleaved end faces and spliced
with a single mode fiber. Then this structure is inserted
into a micro-silicon tube of ~500 μm internal diameter.
Another cleaved fiber of ~300 μm in diameter is inserted
into the micro-silicon tube from the other side and a
Fizeau cavity is realized by the cleaved end-faces. Ultra-
violet curing glue is used to fix the positions of fibers,
finally creating the sensor unit. The sensor unit is con-
nected with a circulator. The light source is a broadband
source ASE, and an OSA and personal computer (PC)
based system is used to acquire and demodulate the sig-
nals from the sensor unit.
Figure 1. A GI MMF based sensor unit and its demodulation sche me .
*Corresponding author.

S. FANG ET AL.
266
A signal from the sensor unit is shown in Figure 2.
Interference signal spectrum under different Fizeau cav-
ity length is shown in Figure 3(a).We can use the track-
ing bimodal techniques to demodulate the Fizeau cavity-
based sensor. We test the Fizeau cavity length by using a
movable stage with 20 μm resolution and the tracking
bimodal method. There is a good linear relationship be-
tween the stage shift and the length of Fizeau cavity by
the tracking bimodal method, shown in Figure 3(b). This
proves the feasibility of the demodulation scheme for the
Fizeau cavity length, and the Fizeau cavity length can be
increased to 5000 μm because of GI MMF used here. It is
well known that the Fizeau cavity-based sensor has low
crosstalk with temperature because of the minimum cav-
ity length.
The GI MMF-Bragg grating based sensor is free from
strain because of the packaging method shown in Figure
1. Its spectrum, shown in Figure 4(a), can be used to
monitor environmental temperature based on the shift of
the wavelength. It has 3 main reflection peaks and their tem-
perature responses are ~18.4 pm/C, ~18 pm/C, ~18.3pm/C
respectively, shown in Figure 4(b). We also test its re-
sponse to curvature. By sticking this structure on a flexi-
ble steel bar and increasing the curvature of the steel bar,
we found that the center wavelengths of the main peak
almost do not shift, shown in Figure 4(c). However, their
intensities are changed, shown in Figure 4(d), and the
intensity differential between the different peaks could be
used as a sensing demodulation scheme, which immune
to the power vibration of light sources. So we can use GI
MMF- Bragg grating measures temperature and curvature
based on the wavelength shift and the intensity change,
respectively.
1520 1540 15601580 1600
-25
-20
-15
-10
-5
0
wave l eng t h（nm）
relative intensity（dB）
part2
part1
Figure 2. A signal of the GI MMF based sensor unit.
Figure 3. (a) Interference signal spectrum(part 1 in Figure
1), (b) Relationship between the stage shift and the length of
Fizeau cavity.
Figure 4. (a) The signal of MMF-FBG (part 2 in Figure 2), (b) The temperature response of the MMF-FBG, (c)center wave-
length with the change of curvature, (d) the curvature response of the GI MMF-FBG.

S. FANG ET AL. 267
The whole measurement proves that the GI MMF
Bragg grating based temperature and curvature sensor can
be used to monitor the temperature individually without
being affected by the applied strain, and the Fizeau cavity
based sensor can simply give the strain value when re-
ducing the thermal expansion effect of monitored struc-
tures.
3. Discussion and Conclusions
A single mode fiber lead-in based GI MMF Bragg grat-
ing show the different characteristics, comparing with
normal Bragg gratings, and could be used to distinguish
the temperature and curvature based on its wavelength
and intensity. The GI MMF based Fizeau cavity is used
as strain or displacement sensing by reading the interfer-
ence fringes. The GI MMF based sensor unit can be mul-
tiplexed readily based on a 2 by N coupler. The GI MMF
Bragg gratings could be distinguished by their wave-
lengths. The Fizeau cavity with different lengths could be
discriminated by the Fast Fourier transform of the Fizeau
interference spectra. We expect to multiplex much more
sensor units because the long cavity range (~5mm) could
be realized based on GI MMF fiber comparing with the
single mode fiber based Fizeau sensor. We are trying to
build a system for a civil engineering application and
further detail experiments are going on.
In conclusion, we demonstrate a GI MMF based sensor
unit to realize a multi-parameter sensing, including tem-
perature, curvature and strain or displacement, which
expect to have applications in the civil engineering area.
4. Acknowledgements
Authors thank for the support by National Science foun-
dation projects (60907034, 61077063, 11178010 and
LBH-Z10195), China Postdoctoral Science Foundation
funded project (20100480965), Harbin Science founda-
tion (2011RFLXG004) and the Fundamental Research
Funds of the Central University, China.
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