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Report Focs sensors

Report Focs sensors

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Published by: bilalkhan107 on Nov 14, 2010
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Temperature-Independent Fiber Bragg Grating Liquid-Level SensorBased 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
Design and construction of temperature-insensitive fiber Bragg grating (FBG) liquid level sensor based on bendingcantilever beam (BCB) is proposed and demonstrated. The BCB induces spatially gradient strain on the unique sensingFBG, 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-levelrange of 500 mm and temperature change from 20
to 80
, the liquid level measurement fluctuates less than 2%without any temperature compensation. By a pin-photodiode (PD) optical power detecting, the liquid-level sensor avoidsexpensive and complex demodulation techniques and potentially costs low.
Fiber Bragg gratings, liquid-level sensor, temperature-insensitive, bending cantilever beam.
Fiber Bragg gratings (FBGs) have been increasingly studied as smart optical sensors in a variety of applications. Greatdeals 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 theenvironment is potentially explosive. The use of optical fiber technology, for its intrinsical advantage of dielectric andimmunity to electromagnetic interference, is particularly appropriate for potentially explosive environment. Severaloptical 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 themicrometer range [12], microbending optical fiber sensors with a resolution of 10 mm in meter range measuring and atemperature measurement is needed for temperature compensation [13], intensity modulation using a pair of fibers astransmitting source light and receiving the partial reflection light off the liquid surface through a glass prism, restricted bythe 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 modulationis proposed and demonstrated. The bending cantilever beam (BCB) is designed to induce spatially gradient strain on theoriginally 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
guotuan2001@163.com / guotuan@mail.nankai.edu.cn; 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: 10.1117/12.637060Proc. of SPIE Vol. 6019 60191W-1
 power avoiding the expensive using of optical spectrum analyzer (OSA). The simple and low-cost liquid-level sensor hasa considerable application potential, particularly in which temperature changes need to be considered.
Fig.1 shows the structure of the bending cantilever beam (BCB). BCB can be divided into three parts with differentfunctions: the above horizontal beam where the force is applied, the side beam used to induce spatially gradient strain andthe 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 roundhole.
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 fromthe maximum at arc center to the minimum on both end sides. Thus, the shifts of Bragg reflection sub-wavelength areseparated in response to different positions of the FBG. However, the shifts of each separated sub-wavelength arecomparatively smaller than that of the bandwidth. As a result, most parts of every sub-reflection spectrum overlaps withothers. Then, one reflection spectrum with wholly broadened bandwidth generally comes into being. Because mostintensity of reflection light is convergent upon the center of FBG where the spatial period
is elongated most and thelongest wavelength is reflected, the Bragg reflection spectrum presents a gradient up-edge. The results of subsequentexperiments 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 uniformin the whole cantilever beam, thus it keeps its value of ~23.6µm/m·k 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 theshifts of Bragg reflection wavelength and do not affect the bandwidth[16].
Proc. of SPIE Vol. 6019 60191W-2
Fig. 2. Reflection spectrum of fiber Bragg grating at three liquid level: (a):-300mm; (b) 150mm; (c) 0mm; (d) 200mm.
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 wavelengthshift and bandwidth broadening by monitoring the reflection spectrum of the Bragg grating, and a pin-photodiode (PD) isused 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
.A properly designedcolumn 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
and wavelength shift
of FBG 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 liquidlevel 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 effectivesolution 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 lessthan 2% without any temperature compensation. Based on the structure of BCB, the liquid level sensitivity andmeasurement range can be flexibly adjusted by changing the thickness of arc beam and the position where force is applied.
Proc. of SPIE Vol. 6019 60191W-3

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