Fiber optic sensors offer a series of important advantages with respect to alternative technologies including immunity to electromagnetic interference, small size, light weight, long and short gage length options, the ability to be multiplexed in large numbers and environmental ruggedness. These characteristics make them ideal for many aerospace and natural smart structure applications that are overviewed in this paper.

The advances in the area of fiber optic sensors have led to applications in various niche areas. Within the past few years, researchers have seemingly led the way in the application of such fiber optic sensors within the civil engineering arena. Specifically, various large civil structures have had differing types of fiber optic sensors installed within and upon these structures leading to measurements not previously available. A review of this `smart structures' research is presented in this paper.

Composite utility poles have the potential to overcome many of the limitations of wooden poles that are currently widely used. Significant advantages include superior strength and uniformity, light weight for ease of deployment, the ability to be recycled reducing hazardous waste associated with chemically treated wooden poles, and compatibility with embedded fiber optic sensors allowing structural loads to be monitored. This paper describes the usage of fiber optic grating sensors to support structural testing of a 22 foot composite pole.

This paper discusses the different methods of fabricating fiber gratings, the different types of fiber gratings and fiber grating devices, and their connection with laser diodes and fiber lasers. We then discuss the fiber grating as a sensor, its sensitivity to different measurands, methods of decoding the sensor response, multiplexing the sensors, and finally their high temperature limitations. Specific references to the materials presented are very numerous and have been omitted. A short list of review papers on fiber gratings is given at the end of the paper. These review papers have more extensive reference lists for many of the developments covered here.

This report presents the results of experiments performed to verify the performance of the fiber-optic absolute extrinsic Fabry-Perot interferometer (AEFPI) for strain measurements. In these experiments, AEFPI sensors are surface mounted and embedded in various materials and subjected to mechanical and thermal strains. Strains measured by the AEFPI are compared to analytical predictions and to metallic foil strain gage measurements where possible. The AEFPI sensors and demodulation equipment were purchased from Fiber and Sensor Technologies (F&S) in Virginia, and all experiments were performed at the Composites Laboratory of Sandia National Laboratories in Livermore, California. The results of the tests indicate that these sensors are suitable for static and quasi-static strain measurements in both surface mounted and embedded configurations; however, they have a resolution of 100 (mu) (epsilon) , which limits their potential applications. A brief explanation of the theory behind the operation of the AEFPI sensor is presented along with the manufacturer's specifications for the particular model used in thee experiments. The details of the experiments are then described, and a summary of the results presented. Finally, conclusions regarding the accuracy, resolution, linearity, and repeatability of the AEFPI are extracted from the data.

Since the well-known demonstration of a fiber-optic strain gage by Butter and Hocker in 1978, significant refinements have been made in the area of fiber optic sensing, enabling the measurement of many different physical quantities, including strain, displacement, linear and circular acceleration, temperature, degree of cure in plastics, chemical compositions, pressure, acoustic waves, and fluid flow rates. Both analytical and experimental efforts have contributed to our current understanding of the relationship between the elongation of a host medium and phase changes in the light passing through an optical fiber. This paper describes research which partially fills in the remaining gap by quantifying the influence of shear strains on the phase change of light passing through an embedded optical fiber. In this experiment, optical fibers were embedded in 18-inch long by 2.25-inch diameter composite tubes. Three tubes were fabricated with axial fibers and one with a helical fiber, using a hand layup fabrication technique. These tubes were also instrumented with two strain gage rosettes. The tubes were subjected to pure torsional loads while the surface strains and the fiber-optic phase changes were measured. A modified all-fiber Mach-Zehnder interferometer with active homodyne feedback was used to determine the phase changes in the optical fibers due to the applied strains. The phase changes were also predicted using fundamental concepts of structural mechanics and existing phase-strain models.

Ultrasonic sensors and generators based on fiber-optic systems are described. It is shown that intrinsic fiber optic Fabry-Perot ultrasound sensors that are embedded in a structure can be stabilized by actively tuning the laser frequency. The need for this method of stabilization is demonstrated by detecting piezoelectric transducer-generated ultrasonic pulses in the presence of low frequency dynamic strains that are intentionally induced to cause sensor drift. The actively stabilized embedded fiber optic Fabry-Perot sensor is also shown to have sufficient sensitivity to detect ultrasound that is generated in the interior of a structure by means of a high-power optical fiber that pipes energy from a pulsed laser to an embedded generator of ultrasound.

There is currently a need for new technologies that are designed specifically for the economical field monitoring of toxins, explosives, and chemical contaminants. The United States has, for example, implemented five regulatory acts to protect its ecologies and its citizens from environmental pollution, and these acts all mandate the monitoring of various chemical contaminants. It is generally accepted that the number of analyses that would be required to meet these new standards would exceed the capacity of all the certified testing labs in the country. New field-portable equipment is needed that can supplement lab-based diagnostic analytical instrumentation, but a continuing problem has been the development of field hardware that can identify and quantify with high specificity a particular species of interest. One of the most promising strategies for performing such narrowly targeted field assays is based on sensors that harness natural immune and protective responses of animals and humans to hone in on a specific compound. This paper discusses the design of a new solid-state portable fluorometer that can be used for the interrogation of a wide range of multimode fiber optic biosensors.

Pacific Northwest Laboratory is developing a large-area chemical sensor that combines chemically coatings and optical spectroscopy to detect target compounds. The chemically selective material is incorporated into the cladding of an optical fiber waveguide. The material is interrogated using optical spectroscopic techniques to determine the concentration of target compounds. The optical interrogation method includes two spectroscopies: visible-near infrared absorption spectroscopy and Raman spectroscopy. This work develops the physical and mathematical models of such a sensor and provides a set of tools with which to make design predictions for the large-area chemical sensors. The theoretical relationships derived herein allow the use of bulk absorption parameters and bulk Raman coefficients to predict sensor performance.

Many chemical sensors rely on a sorbent material to collect and concentrate analyte molecules at the sensor's surface where they can be detected. Ideally, this sorbent material will impart the chemical sensor with both sensitivity and selectivity for the target species. If the sensor is to be reversible, then the species must also desorb from the material or be actively removed by some process such as catalytic destruction. Polymer materials offer many attractive features for chemical sensing. Organic compounds are readily sorbed in a reversible fashion, selectivity can be altered by varying the chemical structure, and polymer materials can be processed into thin films. In this paper, we discuss the factors that govern the sorption of vapors by organic polymers. The approach described has been applied in the past for the design and selection of polymers for acoustic wave sensors. However, the principles apply equally well to the sorption of vapors by polymers used on optical chemical sensors. For example, the polymer could be applied as a thin film to a planar waveguide as the cladding along the length of an optical fiber, or to the end of an optical fiber. Species sorbed into the polymer could then be detected by a change in an optical signal.

The initial feasibility demonstration of a reagentless chemiluminescence based fiber optic sensor technology for use in advanced regenerative life support applications in space and planetary outposts is described. The primary constraints for extraterrestrial deployment of any technology are compatibility with microgravity and hypogravity environments; minimal size, weight, and power consumption; and minimal use of expendables due to the great expense and difficulty inherent to resupply logistics. In the current research, we report the integration of solid state flow through modules for the production of aqueous phase reagents into an integrated system for the detection of important analytes by chemiluminescence, with fiber optic light transmission. By minimizing the need for resupply expendables, the use of solid phase modules makes complex chemical detection schemes practical. For the proof of concept, hydrogen peroxide and glucose were chosen as analytes. The reaction is catalyzed by glucose oxidase, an immobilized enzyme. The aqueous phase chemistry required for sensor operation is implemented using solid phase modules which adjust the pH of the influent stream, catalyze the oxidation of analyte, and provide the controlled addition of the luminophore to the flowing aqueous stream. Precise control of the pH has proven essential for the long-term sustained release of the luminophore. Electrocatalysis is achieved using a controlled potential across gold mesh and gold foil electrodes which undergo periodic polarity reversals. The development and initial characterization of performance of the reagentless fiber optic chemiluminescence sensors are presented in this paper.

A novel fiber optic-based probe for the identification of pure and contaminated solids has been developed. Ultimately, this probe is to be used to characterize the make-up and degree of contamination of hazardous waste sites as a preliminary step in the clean-up process. The transmission range of zirconium fluoride fibers allows for the measurement of fundamental stretching frequencies in the mid-infrared (MIR) region as well as overtone and combination bands in the near-infrared (NIR) region. This provides a substantial increase of the measurable concentration range of this MIR/NIR system with respect to systems which can measure only MIR or NIR information. The spectra of samples measured using this fiber optic system is compared to spectra measured in a standard in-compartment diffuse reflectance accessory. The spectra are evaluated in terms of signal-to-noise, measurement time, and detection limits.

Advances in technology have reshaped the fiber optic acoustic sensing landscape. The Sagnac interferometer configuration can be used to sense environmental parameters other than rotation simply by creating a path length difference. The output of the acoustic sensor contains information about the amplitude and location of an acoustic disturbance. The Sagnac interferometer has the ability to generate polarization effects. These effects are used to generate nonreciprocal phase shifts between counterpropagating beams in the fiber coil, which combine with variations in the different polarization states of the counter propagating beams in the fiber coil to generate intensity fluctuations that are used to monitor acoustic signals. The operating wavelength of the acoustic sensor is 1300 nm. The primary purpose of the acoustic sensor is to sense acoustic signals with frequencies of 0 - 50 kHz. The following are methods for improving the sensitivity and linearity of the acoustic sensor. At the center of the Sagnac loop, sensitivity is minimal. Thus, the sensing region is placed closer to one end of the loop. Also, introducing `teeth' in the sensing region, using different fiber coatings and shielding the sensor achieves better sensitivity. Additionally, a piezoelectric cylinder wrapped with a length of fiber is included in the Sagnac loop to take care of phase modulation. Choosing a light source and a light detector with linear operation will improve linearity. Also, effective signal processing is employed in our system to improve overall performance.

Optical fibers are finding increasing application in feedback and control systems, medical, industrial, illumination and imaging applications, discrete, as well as distributed sensors and networks. In addition to a course in optical fiber principles and components, a fiber optic systems and applications course designed for both electronics and optical engineering technology students has been created at the Oregon Institute of Technology, in the Laser Optical Engineering Technology (LOET) Department. The systems and applications course is designed to illustrate the benefits of using fibers in communications and sensing, as well as to provide hands-on exposure to concepts of test, measurement, and calibration of fiber-optic components and instrumentation. In the laboratories of both courses, optical fiber sensor experiments are used to teach such concepts as selection and testing of components, design and assembly of transducers, system integration, testing, characterization, and optimization. These two mandatory courses provide an excellent introduction to optical fiber technology and applications. The courses exist primarily due to an NSF-OIT grant, however, companies have made significant contributions of hardware to the program. As a result, over $350 K of fiber optics components and instrumentation now are available for senior-students' use, to develop unique, year-long projects.

The authors present a design concept describing a multifunctional data acquisition and analysis architecture for advanced power system monitoring. The system is tailored to take advantage of the salient features of low energy sensors, particularly optical types. The discussion of the system concept and optical sensors is based on research at BPA and PNL and on progress made at existing BPA installations and other sites in the western power system.

A fiber optic sensor for determining the type and condition of aircraft coatings was originally developed to provide adaptive control of automated coating removal. The sensor, based on analysis of optical reflectance spectra, has also been found useful for determining the condition of other materials. Investigations have shown that artificial neural networks can be trained to recognize specific materials or material conditions from the sensor signals.

The fundamental mode which propagates in a single mode fiber is actually a degenerate combination of two orthogonally polarized components. In standard single mode fibers, these components travel with the same velocity and so environmental disturbances can cause energy to couple from one component to the other, with the result that the polarization-state of the light varies unpredictably. High birefringence (HiBi) optical fibers are single mode fibers designed to maintain the polarization of the light launched into them to a high degree. This is achieved by introducing birefringence in the core of the optical fiber by prestressing the core or by fabricating the core with an asymmetry. Birefringence causes the two polarized components to travel with different velocities and thus prevents transfer of optical power from one to the other. If linearly polarized light is launched into these fibers along one of the principal stress axes, the state of polarization (SOP) is maintained. However, when linearly polarized light is launched at an angle with the principal axis, the SOP will periodically change from linear to elliptical to linear over a length (beat length) characteristic of the degree of birefringence. In recent years HiBi fibers are finding application in a variety of sensors based on the effect of external stress on the SOP of the output light. The phase difference between the two polarized modes in these fibers can be significantly changed by stretching the fiber. This phenomenon has formed the basis of strain gauge. Here we have evaluated the performance of three different HiBi fibers for sensing axial strain by mounting them on the surface of specimens. In most strain sensing applications, the protective coating over the fibers plays a crucial role in transfer of strain to the fiber core. The effect of the protective coating in each of these fibers is observed. Three fibers were used in our experiment.

Smart materials and structures are part of a rapidly evolving, multidisciplinary approach to using a material's intrinsic properties or combining materials to achieve inherent intelligence (Rogers 1989; Ahmad, et al. 1990). Smart materials may be defined as materials that possess intrinsic properties capable of responding and adapting to external stimuli. The material' s intelligence may be the result of its composition, processing, microstructure, presence of defects, or conditioning. Smart structures may be comprised of integrated smart materials and/or more discrete components such as actuators or sensors that, in combination, provide the required intelligence. Optical fibers have been the basis of advanced polymer composites to prepare intelligent structures for the past ten years (Claus 199 1). Optical fibers are small, immune to electromagnetic interference, and lightweight. They can be embedded in other materials, have an adjustable composition, and can operate in harsh environmental conditions. Optical fiber-based "smart structures" are able, via embedded or attached optical fiber (the "smart material") and the associated electronic circuitry, to monitor the polymer's physical integrity and structural behavior during use. The unique ability of optical fiber to act as a signal transmitter as well as to modulate a propagating optical signal as a response to external stimuli has led to numerous applications of optical fiber-based smart structures. Although capable of detecting electrical and chemical phenomena, optical fiber sensors have been developed primarily for determining strain, thermal expansion, and vibration of structural components. Non-optical glass or polymer fibers are typically embedded in polymer structures to enhance strength and toughness, for example, panels for the automobile and aircraft industry. Replacing a portion of the structural fiber with optically-conducting fiber permits fabricating robust, optically-active structures such as tamper-indicating secure containers. Secure containers are optical fiber-based smart structures that offer the ability to continually or passively monitor the integrity of the container walls. Continually monitored secure containers monitor in real time, with a container breach activating the smart structure. Smart structure activation can produce numerous consequences within the container, depending on the specific application of the container, the size of the container and the complexity of the accompanying electronics. At a minimum, smart structures can be given the ability to recognize and record container breaching. Difficulty in defeating the tamper-indicating secure container depends on the smart materials' stealth and the smart structure's complexity. Complexity can be enhanced by incorporating the smart material into the container walls using additional, non-active decoy material. In addition to tamper indication, the combination of optical fiber embedded in a polymer matrix lends itself to various sensing capabilities. Either the fiber can act as a buried sensor, for example, detecting radiation or temperature changes, or the polymer matrix can be made sensitive to pressure or specific chemical(s), causing the polymer to react and create a signal in the optical fiber. For example, chemical sensors can be prepared by embedding an optical fiber array into a polymer sheet that is then coated with another polymer sensitive to the specific chemical. Similar to the manner in which discrete optical fibers function (light travelling down a high refractive index core a Pacific Northwest Laboratory is operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract DE-ACO6-76RLO 1830. reflecting off a lower refractive index cladding), a continuous, linear region of high(er) refractive index can be created in a clear polymer such as poly(methylmethacrylate), creating an optical path or channel. These clear polymer "windows" are capable of tamper indication as well as providing sensing capabilities. Waveguide circuitry written into the polymer window can be designed so that, when the window experiences a stimulus such as a temperature or pressure change, the stimulus is manifested in an attenuation or phase shift between sensing and reference waveguide paths. The objective of this paper is to describe the design, construction, and potential applications of several optical-based smart structures. The properties of optical fiber/polymer matrix smart structures will be exemplified through the construction of a small, tamper-indicating secure container and tamper-indicating panels for a secure video system. Tamper-indicating and sensing capabilities of polymer windows containing channel waveguides, plus their integration into secure containers will be discussed. Problems associated with design, construction, and growth potential of optical-based smart structures to other technical areas will be addressed.

Fiber optic sensors have very attractive features for industrial applications. Because they use nonconducting glass instead of wires they can operate in high electromagnetic field environments and explosion hazard areas. The expectations of the 1980s were that fiber optic sensor usage would be over $100 M per year in 1993; actual sales were under $20 M. The order-of-magnitude error was in part due to forecasting methodology and in part due to users not accepting the new sensor technology to replace traditional sensors. To understand the difference between predicted and actual sales, 15 studies generated in the mid 1980s were examined and compared with actual industry revenues in 1993. General trends in sensor development were examined by looking at published papers. Fiber optic sensor papers now account for about 30% of all fiber optic papers published with the fraction growing. Industry needs were examined by surveying sensor applications engineers in chemical process control, industrial companies, and electric power. A 55% reply rate was achieved. Sensor characteristics most desired were reliability and stability; cost and size were not considered as important.

In fiber optic interferometric sensors, polarization noise and fading may occur due to environmental induced birefringence fluctuations. One way to overcome this problem is to depolarize the source light. For broadband sources such as light emitting diodes, the light can be easily depolarized by utilizing fiber implementations of conventional Lyot depolarizes. However, for highly coherent lasers, this approach is not viable. Several techniques for producing a depolarized source from coherent light have been demonstrated. However, they are only suitable for certain input polarization states, and often involve bulk optical components which may not be ideal for certain applications. In this paper, we report a simple all fiber depolarizer for coherent light which is suitable for any input polarization states.

A simple extrinsic fiber optic vibration sensor (FOVS) is described. Measurements are in the range of a maximum amplitude of 25 mm with frequencies up to 1 kHz. Evaluation of the natural frequencies and the damping coefficient are also demonstrated.

Pacific Northwest Laboratory (PNL) has fabricated cerium-activated lithium silicate glass scintillating fiber waveguide neutron sensors via a hot-downdraw process. These fibers typically have a transmission length (e^-1 length) of greater than 2 meters. The underlying physics of, the properties of, and selected devices incorporating these fibers are described. These fibers constitute an enabling technology for a wide variety of neutron sensors.

A novel technique of measuring liquid viscosity and air mass flow using forward light scattering from an optical fiber is being presented. The sensing principle is based on the fact that the frequency response of a partially submerged vibrating fiber probe is sensitive to viscosity and mass flow of the fluid. The viscosity and mass flow are determined by measuring the vibration of a sinusoidally excited taunted optical fiber under different flow conditions. The sensor is found to exhibit an excellent sensitivity for measuring other viscosity (liquid approximately 0.1 cP) and flow (air approximately 0.1 m/s). The sensor is also found to exhibit a high S/N ratio (> 50 dB) and stability without having any signal amplification or feedback in the system.