High Placement Effect of Fibre Bragg Grating Sensor

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1 High Placement Effect of Fibre Bragg Grating Sensor Suzairi Daud a,b*, Muhammad Safwan Abd Aziz a,b, Ahmad Fakhrurrazi Ahmad Noorden a and Jalil Ali a,b a Laser Center, Ibnu Sina Institute for Scientific & Industrial Research, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia. b Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia. suzairidaud@utm.my Abstract A practical pass-through type fibre Bragg grating (FBG) temperature sensor system have been designed and tested for the determination of high placements effect towards the performance of FBG sensor head. By varying of FBG s high, the output spectra were measured for all transmission and reflection systems. Findings shows that the shift in Bragg wavelength, λλ BB increased proportionally with temperature changes independent to the high of FBG. The sensitivity of FBG measured in transmission and reflection systems were recorded to be and pm cm -1 respectively. Keywords: High Placement Effect, FBG, Sensor 1. Introduction Fibre Bragg grating (FBG) technology in temperature sensing have progress rapidly. Hill and group [1] were the first who s developed the FBG technology in 1978 based on the principle of Bragg reflection. It became the best fibre optics sensor device as compared with other fibre optics sensors due to its unique advantages [2]. It is useful in measuring the small changes in temperature and strain accurately in variety of environment [3] includes in underwater, harsh environment, hill and mountain, and disaster places. By practical, Bragg wavelength changed correspond with the changes in strain and/or temperature of the gratings. With the present of the propagation light in fibre core, the characteristics of light wave will change with the changes in external fields [4]. This includes the amplitude, phase, and polarization of light. As for FBG, the reflected light affect by the small changes of wavelength, strain, and temperature applied onto the gratings. In this paper, the performance of FBG was tested in harsh environment, where the sensor head of FBG exposed directly into the sunlight. The high placements of FBG were set to be in different level to determine the effect of the high placement of FBG sensor toward their performance in temperature based sensor. This is very important especially for the application in the high building such as hotels or towers. 2016(1):6 eissn

2 2. Theory The light propagated in fibre core will effect or change the characteristics of light wave. The light travels through the FBG will pass through it and some will be reflected back. Optical waves from broadband laser source are partially reflected from one end grating when it pass through the FBG. Certain directions might observed where wavelets created at each plane are in phase. Resonant condition is satisfied if these directions corresponds to a mode of fibre. The optical waves that partially reflected constructively interfere with each other only for a specific wavelength called Bragg wavelength [5]. Hence, for a broadband source, only a narrow spectrum at the Bragg wavelength is reflected and the rest will be transmitted. Theoretically, the Bragg wavelength, λλ BB of an FBG is given by the equation: λλ BB = 2nn eeeeee ΛΛ (1) where λλ BB is the Bragg wavelength, nn eeeeee is the effective refractive index, and ΛΛ is the grating period of the fibre core. This equation forms the basis equation for any wavelength-modulated FBG sensors. Assuming an isothermal condition, the Bragg wavelength change or the shift in Bragg wavelength, λλ BB upon strain, temperature, and wavelength changes [6] can be expressed as: ΔΔΔΔ BB = 2 ΛΛ eeeeee + nn eeeeee ΔΔΔΔ + 2 ΛΛ eeeeee + nn eeeeee ΔΔΔΔ + 2 ΛΛ eeeeee + nn eeeeee ΔΔΔΔ (2) where LL is the change in strain, TT is the change in temperature, and λλ is the change in wavelength of the fibre respectively. In reality, the variation of refractive index due to the changes in wavelength is negligible [7]. The effect of changes in wavelength is very small as compared with the effect of temperature and strain. In addition, the periodic spacing of the index modulations in fibre is unaffected by the wavelength change. Thus, by neglecting the wavelength effects, Equation (2) can be rewritten as: ΔΔΔΔ BB = 2 ΛΛ eeeeee + nn eeeeee ΔΔΔΔ + 2 ΛΛ eeeeee + nn eeeeee ΔΔΔΔ (3) The change in physical spacing between successive index modulations caused a shift in Bragg wavelength. The strain-optic effect induced a change in refractive index, causing shift in Bragg wavelength [8]. From Equation (3), the changed in center wavelength of the Bragg grating for a given changes in strain is given as: λλ BB = λλ BB (1 pp ee )εε zz (4) where λλ BB is the Bragg wavelength change, εε zz is the strain applied and pp ee = for germanosilicate optical fibre. Thermal expansion (or contraction) changed the grating period and affect the optical response of FBG [9]. The Bragg wavelength, λλ BB and the effective refractive index, nn eeeeee of the fibre are temperature dependent (thermo-optic effect). The changed in Bragg wavelength for a given changes in temperature is given as: λλ BB = λλ BB (εε + αα) TT (5) where λλ BB is the Bragg wavelength change, λλ BB is the Bragg wavelength, εε is the thermo-optic coefficient, αα is the thermal-expansion coefficient, and TT is the temperature change (in C). For germanium-doped silica fibre, the values of pp ee, εε, and αα are 0.213, 8.60 x 10-6 C -1, and 0.55 x 10-6 C - 1 respectively [10].

3 3. Experimental Set-up In this research, a commercial optical FBG sensor with 1550 nm wavelength been used for the purposes. One end of the FBG was connected with tunable laser source (TLS) while the other end was connected with optical spectrum analyzer (OSA) through a single-mode optical fibre SMF-28 optics cable. Broadband light from TLS launched into the fibre core and transmitted through the FBG in the system. As the broadband light propagates through it, the light source with the wavelength matched to the Bragg condition will be reflected, while the rest will be transmitted [11]. These induced a significant power dip at the Bragg wavelength. Figure 1 shows the schematic diagram of the experimental set-up for measuring the transmission spectra of the FBG system. The present of optical coupler is necessary for the measuring of reflection spectra. Figure 1 Schematic diagram of experimental set-up for transmission system 4. Result and Discussion Set-up for measuring the sensitivity of FBG in temperature responds were prepared in an opn area, under direct sunlight. Broadband laser source from TLS were launched into the fibre core. From here, both transmission and reflection spectra of FBG can be measured and calculated through the OSA. The changes of Bragg wavelength with external temperature were observed for both transmission and reflection spectra. This is due to the perturbations of the gratings resulting a shift in the Bragg wavelength for both transmitted and reflected spectra. As the outdoor temperature change to environmental conditions, thermal expansion in the grating occurred. Due to the thermal expansion, the effective refractive index, nn eeeeee of the FBG changed. This caused the variation in FBG wavelengths. The variation in λλ BB monitored based on transmission and reflection spectra from the OSA. The FBG were placed in a perspex box holder to ensure the fibre was in the static place (high). It is then measured the transmission and reflection spectra for each levels tested. The shift in Bragg wavelength for different temperature were recorded. It is then be repeated in different high placement (2.0 cm, 4.0 cm 10.0 cm, 20.0 cm, and 30.0 cm respectively).

4 Figure 2 shows the graph of Bragg wavelength shift against temperature for the transmission system in the specific high. From the graph plotted, the sensitivities or performance of FBG for all the given high are almost same, which is pm cm -1. The sensitivity or performance of FBG for the reflection systems were recorded to be pm cm -1 for all high placements. The graph of Bragg wavelength shift against temperature for the reflection system in the specific high is shown in Figure 3. It can be seen that there is a good agreement between the experimental results and theoretical analysis, and the typical temperature sensitivity [12]. Bragg wavelength shift, λ B (nm) Temperature, T ( C) 2.0 cm 4.0 cm 10.0 cm 20.0 cm 30.0 cm Figure 2 Graph of Bragg wavelength shift against temperature for transmission system Bragg wavelength shift, λ B (nm) Temperature, T ( C) 2.0 cm 4.0 cm 10.0 cm 20.0 cm 30.0 cm Figure 3 Graph of Bragg wavelength shift against temperature for reflection system The application in reflection method offers advantages over the transmission method. Only the light matched with the Bragg condition of the grating is measured over relatively small background intensity in reflection spectrum. Figure 4 shows the comparison of Bragg wavelength shift, λλ BB against temperature for the reflection spectra and theoretical analysis. Results show the linearity of the FBG sensing system. There is a good correlation between temperature changes and Bragg wavelength shift

5 obtained from the experiments done. A linear response observed between the temperature change and Bragg wavelength shift throughout the measured region. Bragg wavelength shift, λ B (nm) Graph of Bragg wavelength shift vs. Temperature Temperature, T ( C) Experimental results Theoretical analysis Figure 4 Comparison of experimental results and theoretical analysis 5. Conclusion The sensitivity and performance of FBG for the temperature sensor systems have been designed, developed, and tested. An excellent linear response observed between the temperature changes and the Bragg wavelength shift, λλ BB throughout the outdoor temperature ranged from 25 to 42 C respectively. Acknowledgement Authors would like to thank Laser Center, Ibnu Sina Institute for Scientific & Industrial Research, Universiti Teknologi Malaysia, Faculty of Science, and Ministry of Higher Education Malaysia for the support in research facilities. This research work was supported by UTM s GUP grant and Ministry of Higher Education Malaysia. References [1] Neil, J. G Development of Temperature Compensated Fiber Optic Strain Sensors Based on Fiber Bragg Gratings. University of Toronto, Institute for Aerospace Studies. [2] Hill, K. O., Fujii, Y., Johnson, D. C. and Kawasaki, B. S Photosensitivity in Optical Fiber Waveguides: Application to Reflection Filter Fabrication. Applied Physics Letter. 32(10): [3] Daud, S., Jalil, M. A., Najmee, S., Saktioto, Ali, J. and Yupapin, P. P Development of FBG Sensing System for Outdoor Temperature Environment. Procedia Engineering. 8: [4] Daud, S., Ueamanapong, S., Srithanachai, I., Poyai, A., Niemcharoen, S., Ali, J. and Yupapin, P. P Particle Accelerator using Optical Tweezers for Photodetector Performance Improvement. IEEE Transaction on Nanotechnology. 11(6): [5] Bawei, Z. and Mojtaba, K High-Temperature Resistance Fibre Bragg Grating Temperature Sensor Fabrication. IEEE Sensors Journal. 7: [6] Xia, M., Jiang, M., Sui, Q. M. and Lei, J Theoretical and Experimental Analysis of Interaction from Acoustic Emission on Fiber Bragg Grating. Optik. 126: [7] Othonos, A. and Kalli, K Fiber Bragg Grating; Fundamentals and Applications in Telecommunications and Sensing. Applied Optics. 45(8).

6 [8] Hee, C. and Lee, J Characteristics of a Fiber Bragg Grating Temperature Sensor using the Thermal Strain of an External Tube. Journal of the Korean Physical Society. 59(5): [9] Daud, S., Noordeen, A. F. A Fibre Bragg Grating Sensor System for Temperature Application. Jurnal Teknologi, 78(3): [10] Daud, S., Chaudary, K. T., Bahadoran, M. and Ali, J Z-Transform Method for Optimization of Add-Drop Configuration System. Jurnal Teknologi. 74(8): [11] Daud, S., Aziz, M. S., Chaudary, K. T., Bahadoran, M. and Ali, J Sensitivity Measurement of Fibre Bragg Grating Sensor. Jurnal Teknologi, 78(3): [12] Zhang, B. and Kahrizi, M High-Temperature Resistance Fiber Bragg Grating Temperature Sensor Fabrication. IEEE Sensors Journal. 7(4).

CHIRPED FIBER BRAGG GRATING (CFBG) BY ETCHING TECHNIQUE FOR SIMULTANEOUS TEMPERATURE AND REFRACTIVE INDEX SENSING

CHIRPED FIBER BRAGG GRATING (CFBG) BY ETCHING TECHNIQUE FOR SIMULTANEOUS TEMPERATURE AND REFRACTIVE INDEX SENSING Siti Aisyah bt. Ibrahim and Chong Wu Yi Photonics Research Center Department of Physics,