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Temperature-Independent Fiber Bragg Grating Liquid-Level Sensor

Based on Reflection Optical Power Detection

Tuan Guo , Qida Zhao, Lifang Xue, Guiling Huang, Shiyu Gao, Yan Yu, Luming Zhao, Lihui LiuInstitute of Modern Optics, Nankai University, Tianjin, China 300071

ABSTRACT

Design and construction of temperature-insensitive fiber Bragg grating (FBG) liquid level sensor based on bending

cantilever beam (BCB) is proposed and demonstrated. The BCB induces spatially gradient strain on the unique sensing

FBG, resulting in a Bragg bandwidth modulation. The broadening of FBG spectrum bandwidth and the reflected optical

power are corresponded to liquid level changes, insensitive to spatially uniform temperature variations. In the liquid-level

range of 500 mm and temperature change from 20 Cto 80 oC, the liquid level measurement fluctuates less than 2%

without any temperature compensation. By a pin-photodiode (PD) optical power detecting, the liquid-level sensor avoids

expensive and complex demodulation techniques and potentially costs low.

Keywords: Fiber Bragg gratings, liquid-level sensor, temperature-insensitive, bending cantilever beam.

1. INTRODUCTION

Fiber Bragg gratings (FBGs) have been increasingly studied as smart optical sensors in a variety of applications. Great

deals of interests have being focused on liquid-level sensing for the numbers of requirements in current modern industry.

Wide range of liquid-level sensing techniques based around mechanical [1], electrical [2] and ultrasonic [3] methods have

been reported. However, their applicability is compromised if the liquid to be monitored is conductive or if the

environment is potentially explosive. The use of optical fiber technology, for its intrinsical advantage of dielectric and

immunity to electromagnetic interference, is particularly appropriate for potentially explosive environment. Several

optical fiber liquid-level sensors have been developed during the past few years[4-15]. For example, a liquid-level sensor

based on the refractive-index sensitivity of long period gratings with a high precision but very short distances in the

micrometer range [12], microbending optical fiber sensors with a resolution of 10 mm in meter range measuring and a

temperature measurement is needed for temperature compensation [13], intensity modulation using a pair of fibers as

transmitting source light and receiving the partial reflection light off the liquid surface through a glass prism, restricted by

the fluctuation of source light power and the cleanness of prism surface[14,15].The ultimate goal of developing newsensing devices with better qualities and possible lower costs encourages research in this field.

In this letter, a novel method for liquid-level sensor using fiber Bragg grating based on spectrum bandwidth modulation

is proposed and demonstrated. The bending cantilever beam (BCB) is designed to induce spatially gradient strain on the

originally unique sensing FBG. The broadening of Bragg spectrum bandwidth is in response to the liquid level changes,

but is immune to spatially uniform temperature changes. A pin-photodiode (PD) is utilized to detect reflection optical

Email: [email protected] / [email protected]; Phone: 86-22-23509849; Fax: 86-22-23508770

Passive Components and Fiber-based Devices II, edited by Yan Sun, Jianping Chen, Sang Bae Lee, Ian H. WhiteProc. of SPIE Vol. 6019, 60191W, (2005) 0277-786X/05/$15 doi: 10.1117/12.637060

Proc. of SPIE Vol. 6019 60191W-1


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power avoiding the expensive using of optical spectrum analyzer (OSA). The simple and low-cost liquid-level sensor has

a considerable application potential, particularly in which temperature changes need to be considered.

2. PRINCIPLE

Fig.1 shows the structure of the bending cantilever beam (BCB). BCB can be divided into three parts with different

functions: the above horizontal beam where the force is applied, the side beam used to induce spatially gradient strain and

the beam base with which the whole BCB is firmly fixed on experimental vessel. Optical fiber is attached on the outer

surface of side beam along its central axes and the area of the grating is symmetrically parallel to the top of half round

hole.

Fig. 1. Schematic diagram of fiber Bragg grating liquid-level sensing and the structure of the bending cantilever beam.

For different points on the FBG, when force is applied, the elongation of spatial period of the grating varies from

the maximum at arc center to the minimum on both end sides. Thus, the shifts of Bragg reflection sub-wavelength are

separated in response to different positions of the FBG. However, the shifts of each separated sub-wavelength are

comparatively smaller than that of the bandwidth. As a result, most parts of every sub-reflection spectrum overlaps with

others. Then, one reflection spectrum with wholly broadened bandwidth generally comes into being. Because most

intensity of reflection light is convergent upon the center of FBG where the spatial period is elongated most and the

longest wavelength is reflected, the Bragg reflection spectrum presents a gradient up-edge. The results of subsequent

experiments confirm the above analysis, which is shown in Fig.2.

As to temperature influence, because the thermal expansion coefficient is only related to the material which is uniform

in the whole cantilever beam, thus it keeps its value of ~23.6m/mk no matter where the position is. Axial strains of

different parts of the FBG are equal when the temperature changes. Therefore, temperature changes only result in the

shifts of Bragg reflection wavelength and do not affect the bandwidth[16].



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Fig. 2. Reflection spectrum of fiber Bragg grating at three liquid level: (a):-300mm; (b) 150mm; (c) 0mm; (d) 200mm.

3. EXPERIMENTAL RESULTS AND DISCUSSION

Fig.1 shows the schematic figure of the experimental measurement system. Light from a broadband source (BBS)

illuminates the grating via a 3 dB coupler, an optical spectrum analyzer (OSA) is used to measure the Bragg wavelength

shift and bandwidth broadening by monitoring the reflection spectrum of the Bragg grating, and a pin-photodiode (PD) is

used to measure the reflection power. The FBG used in this work is UV-written in a hydrogenate B/Ge fiber using a

uniform-period phase mask, with Bragg reflectionwavelength of 1550 nm at a temperature of 28 C.A properly designed

column buoy is vertically fixed under the horizontal beam of the BCB to transfer liquid level variations into vertical force.

With the liquid change of -300~200mm, the measured bandwidth broadening BWand wavelength shift B ofFBG in response to liquid level change is shown in Fig.3. Here the bandwidth threshold is set to 15 dB in order to cover

the whole broadened bandwidth. Fig.4 shows the response relationship between FBG total reflection power and liquid

level change through a PD optical power detecting.

It is difficult to determine the direction of liquid level variation simply by monitoring the optical power. One effective

solution is to pre-weigh a buoy to ensure that the BCB works toward only one direction with one sign and that zero

pressure corresponds to zero liquid level.

Reflection spectrum shows a stable figure through repeated measurement. The liquid level measurement fluctuates less

than 2% without any temperature compensation. Based on the structure of BCB, the liquid level sensitivity and

measurement range can be flexibly adjusted by changing the thickness of arc beam and the position where force is applied.



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Liquid level (mm)

-300 -200 -100 0 100 2001553

2.0Bandwidth (15dB)

Peak wavelength1552

1.6

1551

1.2

15500.8

0.41549

-6 -4 -2 0

Force (N)

2 4

Fig. 3. Bandwidth broadening BW and wavelength shift B of FBG in response to liquid level change

Liquid level (mm)

-300-24

-200 -100 0 100 200

-25

-26

-27

-28

-29

Total reflection power

-6 -4 -2 0 2 4

Force (N)

Fig. 4. FBG total reflection power in response to liquid level change

4. CONCLUSION

In this letter, a novel method for liquid-level sensor using fiber Bragg grating based on spectrum bandwidth modulation

is proposed and demonstrated. The bending cantilever beam (BCB) is designed to induce spatially gradient strain on the

unique sensing FBG. The broadening of Bragg spectrum bandwidth is in response to the liquid level changes, but is

immune to spatially uniform temperature changes. A PD is utilized to detect reflection optical power avoiding the

expensive using of optical spectrum analyzer (OSA). The simple and low-cost liquid-level sensor has a considerable

application potential, particularly in which temperature changes need to be considered.


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Re fle ction po we r(d Bm )

Wa ve le ng th (n m)Ba nd width (n m )



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Acknowledgments: This work was supported in part by the National Natural Science Foundation under Grant 60572018,

by the Doctoral Programme Foundation of Institution of Higher Education under Grant 20020055036, by the Tianjin

Development Project of Science and Technology under Grant 05YFGPG04900, by the Key Laboratory of Optoelectronic

Information Technical Science, Ministry of Education under Grant 2003-21 China.

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