This paper provides a broad overview of the field of fiber optic chemical sensors. Several different types of fiber optic sensors and probes are described, and references are cited for each category discussed.

This paper describes a novel fiber optic-based hydrogen sensor. The sensor consists of a thin-film etalon, constructed on the distal end of a fiber optic. The exterior mirror of the etalon is palladium or a palladium-alloy, which undergoes an optical change upon exposure to hydrogen. Data is presented on fiber optic sensors constructed with palladium and several alloys of palladium. The linearity of the optical response of these sensors to hydrogen is examined. Etalons made with pure palladium are found to be desirable for sensing low concentrations of hydrogen, or for one-time exposure to high concentrations of hydrogen. Etalons made from palladium alloys are found to be more desirable in applications were repeated cycling in high concentrations of hydrogen occurs.

A novel low cost interferometric displacement sensor has been developed which tracks distance from the tip of a fiber optic probe. A unique interrogation technique is used which produces a 32-bit phase word, giving the system a dynamic range greater than 10⁹. Therefore, a displacement resolution of less than 0.01 nm can be achieved with a full range of 6 mm. The measurement range can be extended beyond 10 m by simply adjusting the digital fringe counter and sacrificing resolution yet maintaining the greater than 10⁹ dynamic range. Demodulation rates of 40 kHz have been achieved which facilitates dynamic measurements. Results from an application to hard disk (HD) profilometry are presented.

A twin-bridge fiber optic calorimeter has been built and is currently being tested at Los Alamos National Laboratory (LANL). The intrinsic optical phase shift induced by changes in temperature is measured in both a reference canister and a sample canister. This system incorporates two Michelson interferometers each with an optical path mismatch of 1.6 km. A digital demodulation scheme is used which produces a 32 bit phase word which tracks up to 500,000 fringes with a resolution of 10^-4 fringe, giving the system a dynamic range greater than 10⁹. Both interferometers are demodulated simultaneously at a rate of 83 kHz. The phase difference between the reference and sample interferometers is proportional to the temperature difference between the canisters and therefore correlates to the power produced by the sample in question. The optical system performance will be described along with preliminary calorimetric measurement results.

A novel design approach for a highly reliable buried intrusion detection sensor is described. The design involves the use of a low cost depolarized Sagnac fiber interferometer with a 'sensing loop' consisting of a delay line and buried fiber segment. The intrusion sensor is configured for an 'all fiber' remote deployment where active components (source, receiver, demodulator) are located separately and connected to the sensor through an insensitive fiber tether. A robust and cost effective buried sensor 'mat' design was developed. This design enabled high sensitivity as well as ease of deployment. Sensors were built and evaluated. Test results indicate an effective design.

Integrated-optic devices for use in high-voltage substations are reviewed. Specifically, two types of integrated-optic Mach-Zehnder and the integrated-optic Pockels cell are described and compared. A system for monitoring the condition of fluid-and-paper insulation systems, such as are used in many current transformers and power transformer bushings, is also described. This condition monitoring system measures the dissipation factor of an insulation system being monitored. It uses an integrated-optic Pockels cell to measure the phase of the voltage on the high-voltage transmission line to which the insulation system is connected. Preliminary results, showing that the system is capable of measuring the dissipation factor to an accuracy of 0.5%, are presented.

We have developed a hand-held fiber optic based optical coherence tomography (OCT) system for scanning of the oral cavity. We have produced, using this scanning device, in vivo cross-sectional images of hard and soft dental tissues in human volunteers. Clinically relevant anatomical structures, including the gingival margin, periodontal sulcus, and dento- enamel junction, were visible in all the images. The cemento- enamel junction and the alveolar bone were identified in approximately two thirds of the images. These images represent, or our knowledge, the first in vivo OCT images of human dental tissue.

We propose an alternative configuration for an interferometric distributed fiber optic sensor. The sensor uses a combination of Mach-Zehnder and Sagnac interferometers that share the same fiber. The output of the Sagnac interferometer is proportional to the product of the position where the disturbance was applied and the magnitude of the disturbance. The output of the Mach-Zehnder interferometer is a function of the disturbance magnitude only. The position and magnitude of a disturbance applied to the sensing fiber can thus be obtained from these two signals. Results obtained with a 200 m distributed fiber sensor are discussed.

This paper outlines improvements that have been made in a multiaxis fiber grating strain sensor that can also be used in certain cases to measure temperature. The current status and future prospects for these senors are outlined.

There are approximately 576,000 major bridges, 3.2 billion square feet of bridge deck, and 162,000 miles of highways making up a part of the civil infrastructure system in the United States. Fiber grating strain sensors have the ability to play an important role in the health monitoring of these structures. The advantages of fiber optic sensors including EMI resistance, unobtrusive size, multiplexing/distributed capabilities, and environmental ruggedness, make them a logical choice for structural monitoring.

A methodology is described for determining a relation (K- matrix) between wavelength shifts and (1) axial strain, (2) two transverse strains and (3) temperature change experienced by a multi-parameter Bragg grating sensor. The sensor is formed by writing gratings at two wavelengths in polarization maintaining fiber. The methodology is based on separate experimental calibrations of sensor response to transverse loading (diametral compression), axial loading and temperature changes. Strains produced in the core by the loadings or temperature changes used in the calibrations are determined by finite element analyses.

Pacific Northwest National Laboratory is developing a fiber optic grating sensor demodulator using a low cost static Fourier-transform interferometer. The spectrometer uses a fiber optic source and a plane mirror to form an interferogram in the spatial domain that is recorded by a linear photodiode array detector. Using this instrument with an interferogram fringe spacing of 20 microns provides fiber grating strain resolution of about 700-microstrain.

The security of civil engineering works demands a periodical monitoring of the structures. The current methods (such as triangulation, water levels, vibrating strings or mechanical extensometers) are often of tedious application and require the intervention of specialized operators. The resulting complexity and costs limit the frequency of these measurements. The obtained spatial resolution is in general low and only the presence of anomalies in the global behavior urges a deeper and more precise evaluation. There is therefore a real need for a tool allowing an automatic and permanent monitoring from within the structure itself and with high precision and good spatial resolution. In many civil structures like bridges, tunnels and dams, the deformations are the most relevant parameter to be monitored in both short and long-terms. Strain monitoring gives only local information about the material behavior and too many such sensors would therefore be necessary to gain a complete understanding of the structure's behavior. We have found that fiber optic deformation sensors, with measurement bases of the order of one to a few meters, can give useful information both during the construction phases and in the long term. In the case of beams and bridges, long-gage sensors can be used to evaluate the curvature variations and calculate the horizontal and vertical displacements by double integration of the curvatures.

A rugged telemetry system for coiled-tubing drilling and other hostile environment applications is briefly described. System performance before and after being tested in drilling operations (rotating, drilling, and reaming) was unchanged. However, in a laboratory test at 150 degrees Celsius, signal- to-noise ratio performance at higher carrier frequencies was degraded. Design improvements are also presented.

Fiber Bragg grating sensors generally consist of a single grating written in a low-birefringent optical fiber. The wavelength shift of the peak in the reflected spectrum from these sensors can be used to measure a single component of strain or a change in temperature [Lawrence, 1997]. Fibers are also available with a significant enough birefringence to maintain the polarization state along great lengths and through many turns. This 'polarization maintaining' fiber is commercially available through several companies and in several configurations (including different cladding material and wavelength shift). The grating usually extends approximately 3 mm - 5 m in length. Udd gives a detailed explanation of fiber optics, Bragg gratings and birefringence [Udd, 1991]. As light from an LED is passed through the fiber, only the wavelength consistent with the grating period will be reflected back towards the source. All other wavelengths will pass through. The reflected spectrum will shift as the fiber is strained along its axis at the grating location. Strain or temperature changes at any other location have negligible effect on the wavelength encoded data output. When the Fiber Bragg grating single-axis sensor (termed fiber hereafter) is strained transversely the wavelength will separate into two distinct peaks according to a mathematical relationship defined by Lawrence and Nelson [Lawrence, Nelson et al. 96]. Using these Fiber Bragg grating fibers a corrosion sensor which measures the rate of material was developed. The principle behind this newly developed corrosion sensor is to pre-stress the fiber with a known load. The load is applied by inducing a uniform hoop stress through pressure fitted cylinders around the fiber. This induced stress creates a broadening of the reflected spectrum until the bifurcation of the reflected intensity peaks is distinguishable. As the material from the outer cylinder corrodes away the applied stress will be relieved. Finally, when no load is achieved, the reflected spectrum will have a single peak centered around the nominal Bragg grating wavelength. If a polarizing-maintaining 3-axis grating is used then the sensor would be even more sensitive, having two distinct peaks in each wavelength regime which shift.

Traditional methods of measuring surface tension and viscosity of liquid/vapor interface are invasive systems making unavoidable mechanical contact with the interface that is under investigation. We have demonstrated a complete fiber optics based heterodyne sensor system that non-invasively measures the interfacial surface tension and viscosity. Measured values from this complete fiber optics system for some simple liquid vapor interfaces give consistent values with established values obtained by other invasive methods.

The use of fiber optic sensors for the internal state measurements of large civil structures has been increasing in recent years. In many instances, sensors are embedded into the reinforced concrete structure is an attempt to measure a single parameter of interest. Installation and preliminary measurements obtained from a suite of fiber optic sensors which were embedded into a 67 m steel truss bridge spanning the Winooski River in Vermont (USA) are presented.

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