A Smart Graded-index Multimode Fiber Based Sensor Unit for Multi-parameter Sensing Applications

doi:10.4236/opj.2013.32B062

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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

Email: *[email protected]

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

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.

REFERENCES

[1] Y. J. Rao, M. R. Cooper, D. A. Jackson, C. N. Pannell

and L. Reekie “Simultaneous Measurement of Displace-

mentandTemperatureUsingIn-Fibre-Bragg-Grating-Based

Extrinsic Fizeau Sensor,” Electronics Letters, Vol. 36,

2000, pp. 1610-1612.

doi:10.1049 /el:20001134

[2] A C. L. Wong, Paul A. Childs and G. D. Peng, “Simulta-

neous Demodulation Technique for a Multiplexed Fiber

Fizeau Interferometer and Fiber Bragg Grating Sensor

System,” Optics Letters, Vol. 31, 2006, pp. 23-25.

doi:10.1364 /OL.31.000023

[3] D. Zhou, L. Wei, W. K. Liu, Y. Liu and J. W. Y. Lit,

“Simultaneous Measurement for Strain and Temperature

Using Fiber Bragg Gratings and Multimode Fibers,” Ap-

plied Optics, Vol. 47, 2008, pp. 1668-1672.

doi:10.1364 /AO.47.001668

[4] Mohammed W S, Mehta A and Johnson E G “Wave-

length Tunable Fiber Lens Based on Multimode Interfer-

ence,” Journal of Lightwave Technology, Vol. 22, 2004,

pp. 469-477. doi:10.1109/JLT.2004 .824379

[5] E. Li, X. Wang and C. Zhang, “Fiber-Optic Temperature

Sensor Based on Interference of Selective Higher-Order

Modes,” Applied Physics Letters, Vol. 89, No. 9, 2006,

pp. 091119-091121.

doi:10.1063 /1.2344835

[6] Q.Wang, G. Farrell and W. Yan, “Investigation on Sin-

gle-Mode-Multimode-Single-Mode Fiber Structure,”

Journal of Lightwave Technology, Vol. 26, No. 5, 2008,

pp. 512-519.

[7] S. J. Peng, A. N. Zhang, J. Z. Zhang, W.M. Sun and L. B.

Yuan, “SMS FBG and Its Multi-Parameters Sensing,”

Application Proceedings of SPIE - The International So-

ciety for Optical Engineering, 2010, p. 7853.