Fiber optic distributed pressure sensor for structural monitoring applications S. Binu *a, V.P. Mahadevan Pillai a, N. Chandrasekaran b a Department of Optoelectronics, University of Kerala, Thiruvananthapuram-695581, Kerala, India. b Faculty of Applied Physics, Institute of Armament Technology, Pune-411025, India. ABSTRACT The state of the art in optical sensing is achieved with fiber optic distributed sensors. Such sensors permit the measurement of a desired parameter along length of the fiber. This is clearly of particular advantage for applications such as smart skins, as a sensor can measure the variation of, for example, temperature or pressure over significant areas of structures such as bridges, dams, aircraft etc. This article describes the principle of operation, the design and performance of a single mode fiber based multipoint microbend pressure sensor architecture which may be utilized in distributed measurement. The sensor element is a single mode fiber coupled to the measurand field through the usual microbend inducing structures. The combinations of multiple microbend sensors can form a sensor array for the distributed sensing applications in the monitoring of local pressure or deformation along structure and OTDR can be conveniently used for interrogation of each sensor unit. The sensor sensitivity can set a specific value according to the requirements of the measurement condition. Connected with multiplexed sensing processing schemes, the sensor array may find an application in the real time monitoring and damage detection of large and critical engineering structures. This feature greatly increases the information from a single instrument and hence decreases the cost-benefit ratio. Keywords: OTDR, single mode fiber, distributed sensing, pressure, Rayleigh backscattering, microbend phenomena. 1. INTRODUCTION Sensing, and the need for ever more sensitive and reliable sensors to measure a large range of parameters of interest, is one of the most active areas of optoelectronics in recent times. The well known benefits of fiber optics for data transmission apply equally well for fiber optic sensor systems. An important advantage of fiber optic sensor is that, by combining fiber sensing with fiber telemetry, unique forms of sensors and sensor networks can be created. Here we refer to a specific class of fiber sensors, known as distributed sensors, in which the measurand field acting on a length of fiber can be monitored in both the temporal and spatial domains. Distributed sensing is a technique whereby one sensor can collect data that is spatially distributed over many thousands of individual measurement points. The following advantages of distributed sensing over conventional sensing are clearly apparent in applications where strain, pressure and temperature profile are required over long distances or large areas. It is cost effective Many measurement points are addressed simultaneously. Data acquisition is performed by a single processor. Installation and maintenance are simplified. Distributed sensors are particularly attractive for use in applications where monitoring of a single measurand is required at a large number of points or continuously over the path of the fiber. Examples of application areas include stress monitoring in the evaluation of structural integrity of buildings, bridges, dams, storage tanks etc or mobile platforms (ships, oil platforms, aircraft, space craft etc.) temperature profiling in electrical power transformers, generators, reactor systems, furnaces, process control systems and simple fire detection systems. Distributed sensors can also be used for leakage detection in pipelines, fault diagnostics and detection of magnetic/electrical field anomalies in power distribution * binuopto@yahoo.com : 0471-2412167, 020-24389550 (extn: 249)
systems, continuity testing of transportation networks and to form simple intrusion alarm systems. Of particular interest in the area of distributed sensing is the development of sensors embedded in composite materials for use in the real time evaluation of stress, vibration and temperature in structures and shells especially in the aerospace industry(1,2,3). The most basic form of distributed sensor relies on the detection of regions of localized excess loss due to for example, micro bending in a length of fiber by direct OTDR analysis of the Rayleigh backscattered light. In the case of measurand dependent loss, a region of localized high loss due to perturbation of the fiber by the measurand field (pressure, temperature, external refractive index etc) causes a change in the slope of the detected backscatter signal versus time delay curve (OTDR curve) at a time delay corresponding to the spatial position of the perturbation(4,5,6). There are some very distinct advantages to be obtained from the OTDR method. In summary these are: it is a non-destructive method, permitting many measurements on the irradiated sample to define recovery mechanisms; it is a self-referenced method; thus overcoming problems with the long-term stability of the instrumentation; it permits simultaneous length and loss measurement giving the technique a large dynamic range for the measurement of the radiation effect; it requires access to one end of the fiber only; and it gives a direct indication of loss uniformity and mode distribution effects. 2. METHODOLOGY The ability to poll a large number of passive optical sensors or sensing regions form a single optoelectronics terminal greately enhance the commercial viability of any optical sensing technology. Particularly attractive are systems that permit the monitoring of a physical parameter or measurand, but also its variation along the length of a continuous uninterrupted optical fiber. The OTDR comprises what is effectively, a one dimensional optical radar (lidar!). The OTDR characterizes a fiber by transmitting an optical pulse from an internal laser down the fiber; the width of the pulse can be controlled. As the pulse travels down the fiber, some of the light within the pulse is scattered in all directions signal reflection occurs due to Rayleigh and Fresnel back scattering events. Microscopic density fluctuation within the core of the fiber material give rise to refractive index impurities, which in turn are responsible for Rayleigh, back scattering. Fresnel reflections originate from many points along the fiber where abrupt and discrete discontinuities occur in the index of refraction. The back scattered signal in time domain is used for detection of location as well as the intensity of anomalies. The light that is scattered in the backward direction travels back to the OTDR in which an optical splitter directs the backscattered light to a detector within the OTDR. The detector converts the light into an electrical signal that is analysed, with the resulting data displayed as a trace on the screen of the OTDR. If the delay between the launch of the pulse and the time at which backscattered light is received is τ, then the fiber section from which backscattering occurred is identified as that which lies at a distance s from the launching end of the fiber, where s = cτ/2, where c is the velocity of light in the fiber. With this technique, the entire length of the fiber is sensitive and forms a quasi distributed sensor with the Fresnel reflections and a distributed sensor with the Rayleigh back scattering (7,8,9). To demonstrate the feasibility of distributed sensing of pressure with single mode fibers and with as instrument that is applied in real world applications, we have adapted sensing fibers to a portable OTDR (CMA4000i, Net Test) which is normally used for testing of optical fibers, e.g. in telecommunication networks. The CMA 4000 i, is a dual wavelength OTDR capable of performing measurements at wavelength of 1310 and 1550 nm, that can be applied for testing of single mode fiber. The pulse width of the laser diode can be varied between 10-1000 ns. The instrument was combined with sensing fibers of upto 4.9 km length, whose fiber ends were fixed in standard connectors. The schematic experimental set up of the prototype distributed optical fiber sensor system for localization of pressure is showing in fig. 1 (a). It consists of the protected sensing fiber that is adapted to the OTDR and a laptop computer for automatic evaluation and documentation of the measurements. The sensing fiber is perturbed by four serially placed microbending plates from the input end. Pressure is applied by placing weight to the upper plate, which in turn forces the lower plate. The output from the laser diode (wavelength = 1550 nm, pulse width 100 ns) is launched on to the sensing fiber. A portion of the
power back scattered from the 100 ns width forward travelling pulses, which propagates back to the OTDR in which an optical splitter directs the back scattered light to a detector within the OTDR. The OTDR reference trace shows the excitation pulse and the Fresnel reflection from the far end of the fiber, as well as decrease in the backscatter light level with increasing distance. This decrease depends on the fiber attenuation properties, and from the slope of the trace the loss of the sensing fiber at the 1550 nm laser diode wavelength can be determined. If the fiber is under pressure at different positions along the fiber, discrete stepdrop signals occur at the corresponding section of the OTDR trace, which are an indicator for the position of pressure applied. A series of OTDR measurements were performed with increasing pressure on sensing fiber corresponding increasing weight from 50 gm to 140 gm. 3. RESULTS Figure 1(b) shows typical OTDR traces taken on a sensing fiber perturbed by four microbending plates and figure 1(c) illustrates the experimental results of a four sensor calibration curve. Loss (db) Length (km) Fig 1(b) Typical OTDR trace taken on a sensing fiber perturbed by four microbending plates.
0.8 Loss (db) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Series1 Series2 Series3 Series4 Pressure acting at points 1.0264 km (seri.1) 1.1607 km (seri.2) 1.1937 km (Seri.3) & 2.4520 km (seri.4) 0 40 50 60 70 80 90 100 110 120 130 140 150 Mass (gram ) Fig. 1(c) :Plot of the measured loss of four microbending plates versus pressure applied to the microbending plates Here the experimental result is the relation between excess induced loss and the effective microbending period parameterized by the pressure applied to the plates. It is clear that the loss of a given mode should increase linearly with length and linearly with applied pressure. Therefore, although each mode has losses which are linear in the applied pressure, the amount of energy coupled out of the fiber increases more rapidly with pressure, as with increasing pressure more and more modes reach the threshold for coupling out. For early reception times, the curves have a small slope whose magnitude represents the intrinsic fiber loss. As can be seen from the figure, this slope is essentially constant throughout the fiber except at the locations of the four stressed microbending plates. The constancy of the slope indicates that the fiber is essentially perturbation free. The jumps at the microbending plates are indicative of the loss induced by the pressure. This loss increase is already related to the pressure applied to the microbending plates. The height of the obtained step drop signal (sensitivity) is strongly dependent on the applied pressure. The back scattering technique offer on attractive method for read out of seriously placed microbending plates. This experimental fact is explainable in terms of a theory of the propagation of the modal power distribution through the sensor system. For the type of semiconductor laser often used in backscattering measurements, the launched modal power distribution closely approximates the microbending steady state. Optical fiber sensors of a new type have been proposed, they are able to monitor remotely the spatial distribution of the physical parameter of interest. The sensor use OTDR to detect externally induced changes in the back scatter signature of single mode fiber. The device which has been described has a sufficient sensitivity and range to be useful in many situations in which the pressure must be monitored at many points simultaneously. 4. CONCLUSIONS When the unique compatibility of optical fiber sensors and composite structures is fully implemented to create smart structures, many benefits will be realized. Ideally, the simple and low-cost installation of fiber optic sensors will provide (1) a fabrication cure monitor (2) a built-in sensor for testing suitability of components (3) an in-service monitor for tracking structure response to characterize fatigue and to assess damage and (4) a response feedback sensor for implementation of an active damping or control reconfiguration. Demonstrations reported here indicate that fiber optic distributed sensor system based on OTDR may provide all these desirable sensing functions. The system described
herein provides a simple, potentially low -cost distribution pressure measurement method that will permit the use of commercial communication single mode optical fibers as sensors in smart materials and structures as well the development of fiber optic distributed pressure sensors for a wide range of engineering applications. Distributed optical fiber sensor systems undoubtedly will have a large part to play in the monitoring and diagnostics of critical extended structures. This is especially true for the new generation of self adjusting, self monitoring intelligent or smart structures. To conclude then the possibilities for passive, interference free, insulating, easily installed distributed measurement sensors are attractive. The applications prospectus are good the technical problems are interesting and challenging, but the economic questions are complex and as yet unresolved. ACKNOWLEDGEMENTS S. Binu would like to acknowledge Prof N. K. Sanyal, former Head, Department of Physics, University of Gorakhpur, and Chairman, U.P. State Higher Education Series Commission, Allahabad, India, Prof. V.P.N. Nampoori and Prof. P. Radhakrishnan, International School of Photonics, Cochin University of Science & Technology, Cochin-22, India for their valuable guidance and constant encouragement. REFERENCES 1. G. Zhou, L.M. Sim, Smart Materials and Structures, 11,925 (2002). 2. H. Murayama, K. Kageyama, H. Naruse, A. Shimada, Journal of Intelligent Material Systems and Structures, 5,17 (2004). 3. E. Bocherens, S. Bourasseau, V.Dewynter-Marty, S. Py, M. Dupont, P. Ferelinand and H. Berenger, Smart Materials and Structures, 9, 310 (2000). 4. J. M. Berthold III, Journal of Light Wave Technology, 13,1193 (1995). 5. M. K. Barnoski, S. M. Jensen, Applied Optics, 15,2112 (1976). 6. A. D. Kersey, A. Dandridge, Journal of the Institution of Eleectonic and Radio Engineers, 58,599 (1988). 7. J. P Dakin, J. Phys. E: Sci. Instrum., 20,954 (1987). 8. M. D. Rourke, Optics Letters, 6, 440 (1981). 9. B. L. Danielson, Applied Optics, 24, 2313 (1985).