Advanced developments in composite structures using innovative fiber architecture has stimulated interest in novel characterization methods for 3-D braided composite materials/structures. In this paper, a compatible technique suitable for 2-D and 3-D braided structures is presented. Several samples of smart structures were designed, fabricated and tested. Results show that the developed sensing technique is sensitive and applicable to complicated composite structures. In addition, the sensors can be incorporated into the structure during the braiding process, and can be used for in situ monitoring and feedback control of the process.

Determination of the state of cure of epoxy resin based systems is of considerable interest to manufacturers of large carbon fiber reinforced plastic and glass reinforced plastic structures. Optical methods designed to indicate the cure state have been developed using a loss mechanism which is a function of the refractive index of the curing system. Such techniques are however subject to corruption from losses arising from other influences and consequently are limited in their measurement resolution. In this paper, two techniques which are able to provide a high degree of accuracy of measurement of refractive index are investigated as a means of performing cure measurements. The methods investigated involve the interaction of the evanescent field of a side polished optical fiber with an overlay waveguide or a surface plasmon. Coupling between the fiber and the overlay waveguide (or plasmon) is strongly influenced by the refractive index of the bulk superstrate above the overlay (in this case the curing resin system). Both sensing schemes are self referencing and are not influenced by loss.

In this paper, we report the results of a series of experiments undertaken to quantitatively evaluate the accuracy of a fiber optic Fabry-Perot strain sensor embedded in a material system. The optical fiber sensor is embedded in three material systems containing different physical attributes to simulate a variety of local stress fields. The material systems, which included a homogeneous material, a model system, and a composite laminate, are subjected to tensile and compressive loads to evaluate the performance of the sensor under each of these conditions. In each test, the data obtained from the optical fiber sensor is compared to data from externally attached resistance strain gages for validation purposes. Results demonstrate that optical fiber sensors provide highly accurate results if sufficient chemical and/or mechanical load transfer mechanisms are present.

In this paper, recent advances in the extrinsic Fabry-Perot interferometric (EFPI) optical fiber sensors are reported. We present a theoretical analysis of the output from an optical fiber extrinsic Fabry-Perot interferometer (EFPI) based on Kirchhoff diffraction theory. The resulting equation is solved using numerical integration. The results of the theoretical analysis predict the fringe period and contrast typically seen for the EFPI. Experimental verification of the theoretical analysis is presented. An extrinsic Fabry-Perot interferometric (EFPI) fiber sensor with a micro-lens is demonstrated. The micro-lens is formed at the output endface of the lead-in fiber by fusing this fiber endface into a spherical surface. Experiments for the lensed EFPI and standard EFPI fiber sensors indicate that the decay rate of the fringe contrast versus the air gap in the lensed EFPI sensor is much smaller than that in a standard EFPI sensor.

A numerical model has been developed to calculate the modal phase shift of circular step index profile weakly guiding fibers under axial strain. Whenever an optical fiber is under stress, the optical path length, the index of refraction, and the propagation constants of each mode change. In consequence, the phase of each mode is also modified. A relationship for the modal phase shift is presented. This relation is applied to both single mode and two-mode fibers in order to determine the sensitivity characteristics of strained fibers. It was found that the phase shift is strongly dependent on the core refractive index, n(co). It was also found that it is possible to design fibers which are insensitive to axial strain. Practical applications of strain insensitive fibers are discussed.

We describe results obtained with a two-channel fiber optic grating sensor system using interferometric determination of strain-induced Bragg-wavelength shifts. The system provides high resolution to quasi-static and dynamic strain perturbations of the gratings in the system. Results demonstrating the detection of low frequency (about 1 Hz) strain levels of about 6 nanostrain/sq rt Hz in a 3-point bending flexural beam experiment are presented.

We describe the development and testing of two sapphire fiber sensor designs intended for use in high temperature environments. The first is a birefringence-balanced polarimetric sapphire fiber sensor. In this sensor, two single crystal sapphire rods, acting as the birefringence sensing element, are connected to each other in such a way that the slow axis of the first rod is aligned along with the fast axis of the second rod, and the fast axis of the first rod is along the slow axis of the second rod. This sensor has been demonstrated for measurement of temperature up to 1500 C. The second is a sapphire-fiber-based intrinsic interferometric sensor. In this sensor, a length of uncoated, unclad, structural-graded multimode sapphire fiber is fusion spliced to a singlemode silica fiber to form a Fabry-Perot cavity. The reflections from the silica-to-sapphire fiber splice and the free endface of the sapphire fiber give rise to the interfering fringe output. This sensor has been demonstrated for the measurement of temperature above 1510 C, and a resolution of 0.1 C has been obtained.

The effects of such parameters as exposure time, exposure power, and elevated temperature exposure on the growth and performance of photoinduced two-mode gratings are presented. We also present the results of an investigation into the possibility of utilizing the two polarization axes of an elliptical-core two-mode fiber to monitor two vibration modes simultaneously with one optical fiber. Two separate gratings are written onto a separate axis of a single two-mode fiber. Each polarization will be used to monitor a different vibration mode of a thin cantilever beam. The effects of the formation of a second grating on the performance of the first grating are investigated, and results are presented. These results will determine the feasibility of using the two polarization axes to monitor two vibrational modes of a structure.

An interrupt immune technique for the two-mode sensor is designed and developed. Preliminary results indicate a minimum resolution of half of one fringe for a 15-cm gauge length sensor in transmission. Also, a new derivation of the phase-strain relationship for a two-mode elliptical core fiber sensor has been carried out which shows an improvement of 5 percent when compared to the previous models. Finally, a procedure for single-ending and localizing the two-mode sensor is given in detail. A single-ended, localized sensor has been used in a cantilever beam experiment and yields a strain sensitivity of 0.1 rad/micro-epsilon m.

The sensing mechanism of an optical fiber accelerometer based on the interaction between the frequency shifted two modes in a two-mode fiber with an off-center core is presented. When the fiber is bent, by the action of an acceleration in a specific direction, due to the asymmetry induced by the off-center core, there exists a preferential net strain. The bending causes a tension or a compression field in the fiber and provides a positive or negative acceleration, respectively. The frequency shift is achieved by an collinear acousto-optic interaction in the two-mode fiber. We show both theoretical analysis and experimental results. Several approaches which can enhance the sensitivity are also provided.

In this paper, we present a weighted Mach-Zehnder interferometric sensor that is used to filter vibration modes in a one-dimensional beam placed in a clamped-free arrangement. The sensitivity of the fiber has been tailored for a specific mode shape of an excited beam by tapering a single mode circular core fiber. Mode suppressions on the order of 4 dB have been demonstrated.

The applicability of an embedded fiber-optic acoustic sensor to detect internal cracks and flaws in polymeric materials has been experimentally demonstrated. The sensor is based on a remote polarimetric technique. It has been shown that proper control of the polarization and phase of the optical beam is required to obtain meaningful results. This sensor has shown promising results in determining acoustical properties of plexiglass and obtaining information regarding the location and the extent of the flaw from the amplitude of the fiber-optic sensor signal. The sensing fiber of this sensor is not modified and is mechanically intact. Therefore it is attractive for embedded fiber-optic sensing applications.

The use of an embedded interferometric fiber optic sensor with an active homodyne demodulation scheme is demonstrated for the detection of propagating ultrasonic pulses in epoxy-matrix composites. Surface and embedded generation of ultrasound was accomplished by special transducers. Also reported are efforts directed towards the implementation of this in situ ultrasonic detection technique for cure monitoring in Hysol EPK 907, a room-temperature cured epoxy, and Hercules AS4/1919 composites. Similarities between the measured cure parameter, tau, and the viscosity of the epoxy during the cure process are discussed.

Extrinsic Fabry-Perot interferometric sensors have been used to obtain calibrated, quantitative measurements of the in-plane displacement components associated with the propagation of ultrasonic elastic stress waves on the surfaces of solids. The frequency response of the sensor is determined by the internal spacing between the two reflecting fiber endface surfaces which form the Fabry-Perot cavity, a distance which is easily controlled during fabrication. With knowledge of the material properties of the solid, the out-of-plane displacement component of the wave may also be determined, giving full field data.

In this paper we described an optic fiber Mach-Zehnder interferometer constructed with metal coating fiber. The metal coating fiber is designed specially for monitoring the deformation in composite material. It has high sensitivity for stress and vibration applied on it and has low sensitivity for temperature. And it can be used to measure the force applied on it in a high temperature environment, more than 300 degree(s)C. The sensitivity for deformation measurement is better than 1 micrometers . The design of the metal coating fiber and its characteristics are given in this paper. And the measurement results of a fiber Mach-Zehnder interferometer for composite material are given too.

The development of an Optoelectronic Smart Structure Interface (OSSI) will be required for the practical implementation of Smart Structure Technology. Such an interface will permit an extremely user-friendly interconnection to any Smart Structure component permitting the optical signals from an embedded optical fiber sensor array to be transmitted from the structure in a form most acceptable for that specific application. We have developed a simple, passive, fast response wavelength demodulation system for intracore Bragg grating sensors and have demonstrated real time strain measurements. Currently, we are exploring the use of this wavelength demodulation system with Bragg grating tuned fiber laser sensors and intend to show that such a system can form the basis of an OSSI.

Temperature may be determined by monitoring the modal phase shift of an optical fiber. In this paper we present the results of a numerical model that has been developed to calculate the phase shift of a weakly guiding optical fiber due to thermal strain. Whenever an optical fiber is subjected to temperature changes, the optical path length, the index of refraction and the propagation constants of each fiber mode change. In consequence, the modal phase term, (beta) _inL, of the fields is also modified. A relationship for the modal phase shift is presented. This relation is applied to both single mode and two mode fibers in order to determine the sensitivity characteristics of fibers that are subjected to temperature changes.

For simple interferometric fiberoptic sensors, the effects of strain and temperature are indistinguishable. This paper addresses that problem by demonstrating a single wavelength, two-mode polarimetric fiberoptic sensor capable of simultaneously measuring temperature and strain. The sensor consists of two interferometers, a polarimetric and a two-mode, formed in a single elliptical core fiber. The interferometers respond with different sensitivities to strain and temperature like simultaneous functions of the two variables. A technique is developed by which the outputs of the interferometers are used simultaneously to measure strain and temperature with rms errors less than 30 (mu) (epsilon) and 1 degree(s)C. Finally, measurement results for a real-time system are presented.

Fiber optic cables have long since held the promise of providing low cost, widespread sensing capabilities. The use of fiber optic sensors within a large civil structure could allow for multiple sensing capabilities providing information as to the health of a structure. The Stafford Emerging Technologies Research Complex is a five-story, 65,000 square foot building currently under the final phases of construction on the campus of the University of Vermont. Over the course of the eight months approximately seventy fiber optic sensors have been installed within the concrete frame work of the building. The intrinsic and extrinsic fiber sensors are comprised of various types of singlemode and multimode cables. Since this project is the first major installation of it's kind, very little was known as to what techniques should be implemented to maximize fiber survivability. While installing the sensor network at the Stafford building site many lessons have been learned that would aid in future fiber installations. The techniques developed while installing fiber optic sensors are presented.

Stress distribution inside pavements can be determined by embedding single-mode optical fibers in the pavement. The stress experienced by the fiber, changes the relative index of refraction between two orthogonal components of a polarized light traveling in the fiber. This phenomenon causes a phase delay, which is known as birefringence, between the two orthogonal components. This paper represents the birefringence caused by lateral pressure on concrete and soil pavements.

In this paper an analysis is presented on the use of embedded optical fiber sensors in concrete elements and structures for the non-destructive measurement of internal stress and strain as well as for the assessment of structural integrity. A discussion of the fundamental materials and micro-mechanical aspects regarding the fiber/matrix interaction is given. In addition, a summary of the experimental results obtained to date is made along with the applications sought for this new technology.

In this study three extrinsic Fabry-Perot interferometers are arranged in a rosette configuration and embedded in a homogeneous neat resin and a composite material to measure an arbitrary in-plane strain field. The data obtained from the embedded optical gauges are compared to external resistance strain gauges to assess the accuracy of the internal sensors. Tests are also conducted to evaluate the degradation in compression strength due to the presence of the optical sensor. Data suggests that the embedded rosette configured fiber optic sensor degrades the composite's compressive strength by approximately 65%. The measured strength reduction is compared with classical models to predict this value.

The area of embedded sensors for performance and health monitoring of critical structures such as dams represents a logical extension of earlier efforts in applications of fiber optic sensors. We have been most fortunate in having a 7.5+ MW hydroelectric dam beginning construction on the Winooski River here in Vermont. Based on our work in embedding sensors into the Stafford Building, the idea of embedding sensors into the concrete superstructure of a hydroelectric dam seemed most noteworthy. We are modifying photoelastic (or polarization) based fiber optic pressure sensors while allowing the multiplexing of up to 10 sensors onto each of separate multimode fibers. The modifications entail varying the physical packaging of the sensor's components for better meshing with the dam's rebar-concrete configuration. The individual sensors will be interrogated via optical frequency domain (chirped) techniques to provide a total of 50 discrete pressure readings along the dam's 15 m (high) by 160 m (long) surface. A number (probably 12, chosen because of materials costs) of fewmode and multimode embedded fiber optic vibration sensors are also being developed for embedding in the immediate area surrounding the hydroelectric turbines. We will then be able to determine the dam structure's frequency response as the turbines are subjected to varying electrical and water loads. By relying on our prior experience with using embedded sensors for communications and sensing, we will attempt to also analyze the multiplexed upstream- surface embedded fibers to determine if we can also use those fibers to perform vibration studies. The results and/or plans for this project will also be presented.

An embedded fiber-optic (F-O) sensor has been developed for translaminar monitoring of the structural integrity of composites, with a view to application in composite helicopter flexbeams for bearingless main rotor hubs. This through-thickness strain sensor is much more sensitive than conventional in-plane embedded F-O sensors to ply delamination, on the basis of a novel insertion technique and innovative Bragg grating sensor. Experimental trials have demonstrated the detection by this means of potential failures in advance of the edge-delamination or crack-propagation effect.

We present an impact detection and location system which uses fiber optic extrinsic Fizeau interferontric sensors embedded in a graphite/epoxy composite laminate. The acoustic signals generated by the impact events are detected by four fiber optic sensors. The fiber sensor and the embedding process are described. Also developed are a mathematical method and computer program that allow calculations of unambiguous impact location from the sensor data. The impact location can be determined with a 0.5 millimeter resolution and an accuracy typically less than five millimeters.

In-fiber Bragg reflectors produced with coherent, transverse UV illumination promise to be versatile optical elements for spectral selection and distributed sensing. These fiber devices were generated by repetitive exposures to a moderate intensity, frequency-doubled dye laser under holographically stable conditions until sufficient periodic index modulation had accumulated to produce a strong reflection. Fluences in the range of 1000 J/sq cm were delivered over the course of many seconds to minutes to obtain an index modulation depth of about 1 x 10 exp -4. In an alternate approach, we used single about 1 J/sq cm pulses from a line-narrowed KrF excimer to produce gratings with a modulation depth of about 2 x 10 exp -5, and more recently have achieved values above 1 x 10 exp -4. An unanticipated benefit of the single-pulse preparation method appears to be the markedly enhanced thermal stability of the grating structure, as compared to that reported for gratings produced with multiple exposures.

A new cylindrical coil configuration for polyvinylidene flouride (PVF2) film based fiber optic phase modulator is studied for the frequency response and nonlinearity of phase shift at the resonance frequency. This configuration, hitherto unapproached for PVF2 film modulators, offers resonance at well defined, controllable and higher frequencies than possible for the flat-strip configuration. Two versions of this configuration are presented that differ strongly in both the resonance frequency and the phase shift nonlinearity coefficient.

A new configuration of the extrinsic Fabry-Perot interferometer that eliminates the need for biasing the phase difference at the quadrature point is presented. This new version based on four-beam interference utilizes two sensor heads on a single directional coupler in a split- cavity cross-coupled extrinsic fiber interferometer (SCEFI) arrangement. The spectrum analysis scheme devised for signal demodulation allows a linear readout of the phase shift. Both dynamic as well as static phase shifts are considered. Significant applications of the SCEFI are also discussed.

The development of a fiber laser sensor which permits efficient interrogation of Bragg grating sensors is reported. The fiber laser is linewidth-narrowed and tuned by a remotely located, sensing Bragg grating that is surface adhered to a structure under test. The Bragg grating- tuned fiber laser is used in conjunction with a passive wavelength demodulation system (WDS) to form a fiber laser strain sensor system (FLS³), which was used to track both static and dynamic strains on an aluminum beam. The FLS³ could measure strains with a resolution of approximately 4 (mu) (epsilon) and a bandwidth of 13.0 kHz. The viability of the laser strain sensor concept lends itself to the development of a compact, potentially embeddable smart sensor that would output demodulated sensing data directly to the user.

A piezoelectric polyvinylidene fluoride (PVF₂) film with transparent indium tin oxide electrode metallization is placed directly in the path of a single mode fiber output to form an extrinsic optical interferometer. This device can be used concurrently with another extrinsic interferometer on a fiber directional coupler to generate a carrier phase modulation on which the signal phase shift is superimposed. Experimental results of the induced phase shifting coefficient are presented for two arrangements of the piezofilm differing in their boundary clamping conditions.

We describe a fiber ring strain sensor configuration which incorporates an active fiber gain section to allow lasing within the ring. Analysis of the beating between modes of this ring laser provides a measure of the optical path length in the ring, and thus strain applied to the fiber. The sensor provides an output frequency which can be tracked to determine the integrated strain over the length of the fiber ring. Results are presented showing the sensor response up to a fiber strain of approximately 4000 micro-strain.

Fiber optic Bragg gratings were used and evaluated as strain gauges. An expression describing the relationship of the strain to relative spectral shift was derived from the phase index of refraction of the waveguide and it was experimentally verified. The application of these sensors in large scale concrete structures was evaluated.

Initial flight tests of a fiber optic strain gauge based on a balanced double-polarization Michelson interferometer have been performed. The sensor was fixed on a carbon fiber reinforced plastic plate which, in turn, was screwed to the main wing spar of a Cessna C207A aircraft. Strain was monitored under different flight conditions and compared to the readout of a conventional resistive strain gauge.