Optical Fiber Sensor For Smart Structure Monitoring

C S S R 0 8’ 0 9

14 - 15 March 2009

C O N F E R E N C E ON S C I E N T I F I C & S O C I A L R E S E A R C H

Optical Fiber Sensor for Smart Structure Monitoring Nor Jannah Muhd-Satar and Mohd Kamil Abd-Rahman Faculty of Applied Science, UiTM Shah Alam [email protected], [email protected]

ABSTRACT An optical fiber displacement sensor for structural monitoring using an optical time domain reflectometer (OTDR) and optical power meter are demonstrated. The sensor probe, which is in the shape of figure-of-eight, is placed in-between a length of standard single mode optical fiber SMF-28. The sensor introduces power loss into the system with reduced fiber loop diameter when displacement is applied. Displacement will induce strain and microbending effects which degrade the signal quality. A gradual drop of the light intensity with displacement variation at a particular location of the sensor within the fiber length is described. Optical losses of 9.41 dB for a 140 mm displacement with a sensitivity 0.0185 dB/mm were measured using OTDR. The results measured from OTDR instrument were compared with a simpler and cost effective power meter system and it matches well with one another. The sensor system which is compact, robust and simple can be made to be used for dynamic monitoring of civil structure and the system is cost effective and can be easily installed in large buildings, bridges, highways, tunnels, dams and geo-technical sites. Keywords: Optical Fiber Sensor, Smart Structure Monitoring, Distributed Displacement Sensor

1. INTRODUCTION A smart structure is a system containing multifunctional parts that can perform sensing, control, and actuation; it is a primitive analogue of a biological body. Smart materials are used to construct these smart structures, which can perform both sensing and actuation functions. Most advanced countries have these smart structures installed in strategic locations as in multi-storey buildings, bridges, and other civil structures. These sensors have been placed and embedded in the structures during the construction of the structures. Sensors can also be placed externally on the existing structure which previously does not have sensors installed in them. Recently there has been intense research in the development of optical fiber sensors which are useful to civil structural monitoring and industries in the determination of stress, pressure, strain, vibration and crack on the structure. An optical sensors based on microbending effect have been studied with the use of various method. The sensors are simple, reliable, low cost, can be multiplexed and used in distributed applications. Fibers often exhibit excess loss when they are spooled as a result of small deflections of the fiber axis that are random amplitude and are randomly distributed along the fiber. Because of these small random bends and stress, the loss induced in optical fiber is called as microbending loss. Microbending is the mechanical perturbation of a multimode fiber waveguide causes a redistribution of light power among the many modes in the fiber. Bending in fiber can result in increased attenuation which can degrade system performance. As more bending occurs in the fiber, the loss induced in the fiber is higher. This effect leads to devising optical fiber sensors, which are of particular interest to civil structure monitoring and industries because of their robust and simple construction. Furthermore, these sensors do not require complex signal processing techniques, such as for interferometric or phase modulated sensors.

Paper number: 5599447

C S S R 0 8’ 0 9

14 - 15 March 2009

C O N F E R E N C E ON S C I E N T I F I C & S O C I A L R E S E A R C H

In this paper, we demonstrate a displacement fiber sensor for applications in distributed structural monitoring using optical time-domain reflectometer. The sensor which is in the shape of figure-of-eight does not require complex techniques in processing its signal. 2. LITERATURE REVIEW Several researches proposed distributed and displacement sensing probe using different measurement method with the aimed to determine any disturbance. Pinto et al proposed a quasi-distributed displacement sensor to monitor power loss through the decreased in fiber loop radius. Four displacement sensing head were developed. They used OTDR YOKOGAWA-AQ7260 to collect the data. In their research, they found the sensitivity of the each sensor head is 0.027 dB/mm. Kwon et al suggested multiplexed fiber optic OTDR sensors for monitoring of soil sliding. This sensor has been developed to monitor slope stability in soil layers by measuring the displacements of the structure. Binu et al reported vibration amplitudes ranging from 0.008 to 0.74 mm within a frequency range of 75 to 275 Hz. Zhang and Bao demonstrated a distributed optical fiber vibration sensor based on spectrum analysis of polarization-OTDR system. The experiment detected up to 5 Hz vibrating events in a 1 km fiber-link with 10 m spatial resolution. Tang et al reported a distributed optical fiber sensor with the principle of microbend loss for damage measurement in concrete or rock sliding. Their research recorded that the sliding range of dynamic sensing response reaches 0 – 3.6 mm. 3. METHODOLOGY Figure 1 show the sensor probe which was shaped into a figure-of-eight using standard single mode optical fiber SMF-28. The sensor head was placed in-between a 0.5 km length of the SMF-28. The sensor introduces power loss into the system with decreasing fiber loop diameter (Ø) and sensing head length s when displacement (D) is applied to the fiber. The fiber was bonded at one end and was pulled at the other end for the occurrence of the displacement. The schematic of an OTDR (Anritsu-MT9081D) sensing method is illustrated in Figure 2. The OTDR with a sensing resolution of 1.0 meter used 10.0 ns optical pulse signal at 1550 nm wavelength. Without fracturing the fiber, a maximum displacement was obtained with minimum loop diameter of about 16.0 mm. The intensity of the return pulse is measured and integrated as a function of time and is plotted as the function of the fiber length. The return loss created by the sensor was measured at the precise location in the fiber length as a step drop in signal intensity. Another measurement technique, which is intensity-based, used continuous-wave broadband light source as the input signal and the sensor outputs to a power meter (PM), as shown in Figure 3. Two types of PM were used that is ILX Lightweight optical multimeter (OM) and fiber meter FM8515c (FM). Amplified spontaneous emission (ASE) from an EDFA acts as the input light source to the sensor. The average input power was set to less than 1.0 mW. Higher power levels would contribute unnecessary nonlinear effect into the SMF-28 and the measurement would not represent the actual sensing signal of the sensor probe. Optical loss in the sensor system was derived from the input and output power measurement when displacement was applied. 20 mm D

Ø1 Ø2

s Figure 1: Sensor probe in the shape of figure-of-eight

Paper number: 5599447

C S S R 0 8’ 0 9

14 - 15 March 2009

C O N F E R E N C E ON S C I E N T I F I C & S O C I A L R E S E A R C H

OTDR

Figure 2: Measurement using OTDR

ASE OM/ FM Figure 3: Measurement using Optical Power Meter

4. RESULTS AND DISCUSSIONS For the first part of the experiment, data were collected using OTDR and the induced losses were analyzed. With the optical signal wavelength setting at 1550 nm and pulse width of 10 ns, data and plots were converted to traces shown in Figure 4 and 5 below. In Figure 4, the step drop in return loss, due to the decreased in fiber loop diameter, shows that microbending incurred at a particular location, 0.3806 km where the sensor was placed along the fiber length. Continuous increased in the fiber loss with displacement is shown in Figure 5 and it is consistent with the change in Ø and s.

Figure 4: Return loss shown as a step drop in optical signal intensity

Figure 5: Displacement increases the return loss at precise location of the sensor, 0.3806 km

The result shown in Figure 6 has a linear relationship between s and D for all measurement tools, where s decreases as D is applied to the fiber.

Paper number: 5599447

C S S R 0 8’ 0 9

14 - 15 March 2009

C O N F E R E N C E ON S C I E N T I F I C & S O C I A L R E S E A R C H

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Figure 6: Linear relationship of sensing length s and displacement D 12 FM

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Figure 7: Bending at the edges of the fiber loop induces strain which might cause the exponential behaviour Bending of the fiber by reducing the diameter of the fiber loop resulted in exponential increased in the return loss of the sensor. A maximum displacement of 140 mm was measured as the OTDR recorded a maximum attenuation at the sensor head location. By using PM, the maximum displacement recorded is 120 mm as there is high return loss in the system. Figure 7 shows high return loss of 9.41 dB induced by the sensor when the displacement of 140 mm was applied, with sensing head length, s (Figure 8) reduced to 36 mm while øavg was 16 mm. The exponential behaviour might be contributed from the strain of microbending at the edges of the fiber loop. The results measured from OTDR matches well with the result measured using PM with a different for 2 dB loss because PM traced the losses induced along the fiber meter while OTDR traced the loss at sensor head only. A measurement using OM recorded a high Paper number: 5599447

C S S R 0 8’ 0 9

14 - 15 March 2009

C O N F E R E N C E ON S C I E N T I F I C & S O C I A L R E S E A R C H

return loss of 9.75 dB induced by the sensor when the displacement of 120 mm was applied and s reduced to 39 mm while øavg was 14 mm. However, the measurement using FM recorded that high return loss induced is only 8.10 dB for a 120 mm displacement, s is 41 mm and øavg reduced to 15 mm. 12 FM

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Figure 8: Increase of loss with the decrease of sensing head length Initial loss recorded using OTDR was very small when the displacement was at 20 mm where the diameter of the loops was not small enough to create microbending effect. As reported by Pinto et al, the losses recorded in the experiment also begin to be significant for a displacement D larger than 40 mm [4] and the diameter of the loops was 35 mm. Figure 9 presents an exponential behaviour between loss and displacement. By applying an algorithm equation, a linear relationship between loss and displacement were obtained as in Figure 10. From the equation of the graph, the sensitivity of the sensor was found to be 0.0185 dB/mm.

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Figure 9: Increase of loss with displacement

Paper number: 5599447

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C S S R 0 8’ 0 9

14 - 15 March 2009

C O N F E R E N C E ON S C I E N T I F I C & S O C I A L R E S E A R C H

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Figure 10: Measurement using OTDR, linear plot of Log Loss with increasing displacement 12 FM

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Figure 11: Comparison increase of loss with displacement by using OTDR and PM From Figure 11, PM plot shows the same trend with the result obtained from OTDR and it shows that PM system can be an alternative system which can be installed at low cost. The average power measurement from PM has about ±0.3 dBm instrumental error. The PM readouts have about 2.5-dB more than the OTDR measurement because PM took into the consideration of the total power loss in the whole length of the fiber, which includes intrinsic fiber loss, Fresnel back-reflection, absorption and other back scattering loss in the fiber. 5. CONCLUSIONS An optical fiber sensor for smart structure monitoring with single sensor head was constructed using OTDR and power meter system. Optical return loss of 9.41 dB was measured using OTDR for a 140 mm Paper number: 5599447

C S S R 0 8’ 0 9

14 - 15 March 2009

C O N F E R E N C E ON S C I E N T I F I C & S O C I A L R E S E A R C H

maximum displacement with sensitivity 0.0185 dB/mm from a single sensor head. Several sensor heads can be easily placed within the fiber length and the location of these sensors can be precisely located exact fault location and determine the health condition of the structure. The advantage of this displacement fiber sensor system is that it is simple in developing the sensor and sensing the signal, furthermore several sensor heads can simply be distributed and detected anywhere within the line of detection without incurring additional cost. The results obtained from OTDR instrument match well with a simpler and cost effective power meter system. The application of the distributed fiber sensor is expected to have optimistic future for smart structure monitoring and sensing with the simple and compact configuration system. Using OTDR configuration sensor system, fault location in the structure can be easily determined. However, to be more portable, miniaturize and low cost, the detection of fault in the structure using optical power meter instruments was presented. The disadvantage of optical power meter sensor configuration is the location of the fault at precise point cannot be located because of the function ability of the power meter. Therefore, distributed sensor system cannot be applied for the measurement using optical power meter. However, with pulsed input signal, the intensity or power of the return pulse can be measured and analyzed as a function of time and furthermore, it can be derived as the function of the fiber length. Hence, the position of the sensor probes, thus the fault locations can be precisely identified. ACKNOWLEDGEMENT We acknowledge the Faculty of Applied Science, Universiti Teknologi MARA, Shah Alam for the facilities and support provided. REFERENCES Berthold, J.W. (1995). Historical Review of Microbend Fiber-Optic Sensors. Journal of Lightwave Technology, 7. Bjarklev, A., and Andreasen, S.B. (1988). Microbending Characterization of Optical Fibres From Artificially Induced Deformation. Journal of Electronic Letters, 6. Binu, S., Mahadevan Pillai, V.P., and Chandrasekaran, N. (2007). Fibre Optic Displacement Sensor for the Measurement of Amplitude and Frequency of Vibration. Journal of Optics and Laser Technology, 39. Hageman, H.J., Ungelek, J., and Wiechert, D.U. (1990). Optical Time Domain Reflectrometry (0TDR) of Diameter Modulations in Single-Mode Fibers. Journal of Lightwave Technology, 11. Kobayashi, T., Kasashima, M., Iwashima, H., and Cao, J. (2003). Microbend Optical Fiber Sensor for High-Spatial Resolution Measurement of Strain Distribution, Proceedings of the SICE Annual Conference, Japan. Kwon, I.B., Kim, C.Y., Seo, D.C., and Hwang, H.C. (2006). Multiplexed Fiber Optic OTDR Sensors for Monitoring Oil Sliding, Proceedings of the XVIII IMEKO Conference of the World Congress, Brazil. Lo, Y.L., and Xu, S.H. (2007). New Sensing Mechanism Using an Optical Time Domain Refletometry with fiber Bragg Gratings. Journal of Sensors and Actuators, 136. Mendez, A., and Morse, T.F. (2007). Microbending Loss: Specialty Optical Fiber Handbooks. Academic Press. Pinto, M.P., Frazao, O., Baptista, J.M., and Santos, J.L. (2006). Quasi-distributed Displacement Sensor for Structural Monitoring Using a Commercial OTDR. Journal of Optics and Lasers in Engineering, 44. Tang, T.G., Wang, Q.Y., and Liu, H.W. (2008). Experimental Research On Distributed Fiber Sensor for Sliding Damage Monitoring. Journal of Sensors and Actuators, 136. Zhang, Z., and Bao, X. (2008). Distributed Optical Fiber Vibration Sensor Based On Spectrum Analysis of Polarization-OTDR System. Journal of Sensors and Actuators, 136.

Paper number: 5599447