This paper provides a description Navy's near term and far term efforts for shipboard fiber optic sensors. Fiber optic sensors for measuring parameters such as pressure, temperature, rotational speed, voltage and current are now being integrated into the Navy shipbuilding process. Sensing techniques include Fabry-Perot, Fluorescent Decay, Pockel's Effect, and Faraday Rotation. Multiplexing schemes for interconnecting different types of sensors are being evaluated.

The U.S. Navy has embarked on a program to develop and install fiber optic (FO) sensors on hull, mechanical, and electrical equipment for control and monitoring purposes. Sensor reliability goals are 100,000 hours mean-time- between-failure and a life time of 40 years. This paper presents results of a study of FO systems and sensors with the goal of estimating reliability and durability in the shipboard environment. Sources of information were technical reports and papers, and direct inputs from manufacturers, developers, and users of FO sensors. Little information was available on reliability tests for individual sensors. Results indicate that existing FO sensor components and systems potentially can achieve the required Navy reliability and durability.

Remote visual inspection equipment such as borescopes, fiberscopes and videoimagescopes provide Power Plant NDE groups with the necessary tools for establishing superior corrective and preventative plant maintenance programs. These scopes allow the user to evaluate the condition of pipes, boilers, turbines, heat exchangers and other critical machinery without costly and time consuming disassembly. Overall plant efficiency is increased and the likelihood of catastrophic failure is reduced significantly. Ancillary equipment such as video and digital processors provide a permanent record of the inspection while employing sophisticated 3-D measurement, trend analysis, documentation and image manipulation.

We present design considerations of a diaphragm-type fiber optic combustion pressure sensor employing a low-cost optoelectronic transceiver. The key transceiver element is a tapered optical fiber bundle-based coupler design that relaxes critical alignment requirements, and allows the use of low-cost components. Sensor system evaluation data are presented for high-engine-load and high-combustion-temperature conditions, and for known detection. The test results closely resemble outputs of a heat-sunk flame-shielded, instrumentation-grade piezoelectric reference transducer, and demonstrate better temperature stability.

We have developed a multichannel fiber-optic based radiation pyrometer for monitoring the temperature distribution across a moving web up to 4 meters wide. The system employs an array of up to 160 pick-ups whose outputs are delivered by optical fibers to the monitor where they are optically multiplexed onto a 16-channel germanium photodiode array. The pick-ups are deployed linearly across the web on 2.5 cm centers. A 32-channel prototype system achieved a noise limited precision of +/- 1 degree(s)C over the range 140 degree(s)C to 200 degree(s)C. The noise increased to +/- 5 degree(s)C at 100 degree(s)C.

In 1987 the first laser-based emissivity measuring pyrometer was introduced. This instrument is currently in worldwide use in industrial and research applications. In 1990 a fiber optic version was developed; this provided broader temperature ranges, smaller target sizes, lower cost and most importantly flexibility of sensor head size, shape and materials. Furthermore the fiber optic version meets the requirements for on-line IR temperature measurement and control in metallurgical, chemical, ceramic and electronics manufacturing where portable instrument access is prohibited and where the sensor environment is harsh.

A novel measurement system for 2D temperature distribution measurement is presented, which combines two color pyrometry with computer tomography techniques. Basic principles and measurement system are described. The system is specially designed for measuring transient flame temperature distribution in a closed combustor. Fuels such as butane, gasoline are used in experiments. Flame temperature during combustion are obtained.

An integrated analytical measurement system is being assembled for application to remote analysis in chemically and radiologically harsh environments. A single, compact, integrated probe will be the only section of the system in the hostile environment. The instrument array initially composed of a fiber optic Raman spectrophotometer, a fiber optic FTIR, gamma radiation spectrometer, beta radiation detector, and a CCD camera. Within the hostile region, a robotic positioning system or remote manipulators will provide mobility for the multisensor probe. The application of this array will be to characterize multiphase nuclear waste.

A study of real-time in situ monitoring of the chemical states of urethane cross-linked solid rocket propellant during cure and aging using an embedded fiber optic sensor and a Fourier transform IR spectrometer is presented. The aging study focused on the monitoring and identification of chemical species that migrate across the propellant-insulation bondline. In this work, a short length of tapered chalcogenide fiber was used as the sensor. The segment of fiber containing the taper sensor was mounted in a temperature controlled aluminum boat to which either inert or live propellant samples were added. The spectral data collected over a six month period indicated that the equipment and sensors were suitable for monitoring the degree of curve and contaminant migration in solid rocket propellant.

This paper presents the TB COLOR^TM fiber optic spectrocolorimeter, developed jointly by the french companies BERTIN & Cie, a private Research and Development company and TRAPIL (TRAnsports Petroliers par PIpeLine) the prime company in charge of transportation of oil products in France, to control the dispatching of petroleum oil products through multiproduct pipelines. The principle of operation, based on in line color analysis, is described then the TB COLOR^TM system configuration and technologies are presented. Emphasis is given to industrial implementation and exploitation results.

A novel optical transducer has been developed for measuring relatively slowly varying temperatures within inaccessible but mechanically stable environments. It is ideally suited to monitoring geological and industrial sites underground. As the transducer may be embedded for many years, it is based on technology known for stable long-term operation which has been optically adapted for novel use. The transducer uses a bimetallic strip in conjunction with an optical displacement sensor to modulate the spectrum of an incident optical signal. The spectral changes are detected chromatically to remove dependence of optical intensity fluctuations. The effect of displacement perturbations caused for instance by thermal expansion and mechanical stress was reduced by choosing the bimetal to have a large deflection.

A fiber optic flocculation sensor based on measuring the intensity of light reflected by solid particles in suspension (i.e. paper pulp) in a well defined measurement volume, was constructed. This sensor is designed for monitoring the flocculation state of paper pulp in the papermaking process. The flocculation determines to a great extent the quality of the final product, the paper. Tests with different types of pulp were performed in both a closed loop system and a small paper machine. In this investigation the flocculation state is expressed as a root mean square flocculation index. The flocculation index delivered by this fiber optic system shows a very high correlation with the flocculation index provided by a camera system `looking at' the same pulp, while the latter has a great resemblance with the human perception of the flocculation.

The digital fiber optic sensing system used for real-time remote measurement of the amount of oil in a tanker is described. The system consists of a fiber optic (FO) interface-temperature complex probe, a driving mechanism for the FO probe, and a microcomputer-based integrated data processing unit. It has been proven by the analogue experiment of the system that the gauging of the amount of oil is accurate within +/- 0.2%.

We report results from a program to develop fiber-optic sensor-based instrumentation methods to allow the in-situ analysis of ceramic barrier filters. The sensor used was an extrinsic Fabry-Perot cavity created between the ends of two longitudinally aligned fibers. Filters instrumented with these fiber sensors were tested in a combustor simulator at the Westinghouse Science and Technology Center. These tests were performed using silica optical fibers capable of withstanding the high temperature and harsh chemical environment of the combustor. The single-ended approach of the reflective Fabry-Perot sensors is well suited for high thermal strain measurements. The results from several tests are presented.

A multichannel Bragg grating fiber laser demodulation system capable of interrogating four or more Bragg grating sensors for strain and temperature monitoring has been designed and developed. System configuration and various practical considerations for a field deployable system are discussed. Preliminary data indicates a dynamic strain range in excess of 5000 (mu) (epsilon) at a resolution of 1 (mu) (epsilon) . Both the strain dynamic range and resolution are easily modified.

A high-temperature sapphire strain gage based on the in-line extrinsic fiber- optic Fizeau interferometer was used to measure strain imparted by a 4000 lb compressive load applied at a temperature of 1100 degree(s)C. Experimental strain sensitivities on the order of 1 (mu) (epsilon) were obtained.

We report on the use of optical low coherence reflectometry for silicon characterization. The measurement system uses a low coherence light source (edge-emitting LED) in conjunction with a fiber optic Michelson interferometer. This non-contact fiber optic measurement system has been used to measure silicon thickness and flatness to an accuracy of +/- 1.5 micrometers in the laboratory.

This paper describes the operating principle and construction of a new type of rotary motion sensor. This novel optical device is based on a patented evolution of the high precision optical shaft encoder. It integrates highly accurate measurement of the angular position and 2D center position of a rotating shaft. The current work described is aimed specifically at using this sensor for monitoring the condition of rolling element bearings. The process by which the output from four stationary, non-contact, fiber optic sensing heads can scan a coded disk coupled to a shaft and derive its center position to approximately 30 nanometers at an angular resolution of approximately 0.0005 min is explained. The design and implementation of the computer architecture to which the sensor is interfaced via noise immune fiber optic links and the algorithms which underlie the processing software are also examined.

Optical fiber thermometers offer several advantages over refractive optics thermometers. Besides eliminating the need for a direct line of sight to the measured surface, it is of special value for use in high electric or magnetic fields, or in strong rf or microwave fields due to the electrically nonconducting nature of the optical fibers. At present, fluorescent fiber thermometer has been developed and used for temperature measurement as low as -200 degree(s)C, but its contact modality limits the application in surface temperature measurement of moving objects. A noncontact fluoride fiber thermometer for near room temperature measurement is presented in this paper.

The results of measurements of the intrinsic phase-differences of titanium- indiffused lithium niobate waveguides, for use in integrated optics Pockels cell high-voltage sensors, are presented. The dependencies of the intrinsic phase-differences of these waveguides on their lengths and widths are investigated; a change of between 4.9 and 5.9 degree(s)/micrometers /mm was obtained. Also, the change in the intrinsic phase-difference as a function of both temperature and time was investigated; a typical change of 0.02 degree(s)/ degree(s)C/mm was measured and, following a small initial change, the bias was found not to drift with time. Some suggestions for possible post-processing of the output signals, of the integrated optics Pockels cell high-voltage sensors, to increase the dynamic range and to compensate for small changes in the bias, are presented.

A new microbend fiber-optic sensor has been developed and applied to static, and, dynamic fracture problems and impact detection. The static and dynamic calibration of this sensor is described. The sensor was made using a multimode fiber and the response of the sensor was studied and optimized. It has a strain sensitivity (gage factor) of over 28 with wide dynamic range and high frequency response. A single edge notched specimen was used with this sensor to determine both the static and dynamic stress intensity factors. The experimental results match well with the theoretical predictions and with the results obtained using electrical strain gages.

This paper reports the performance of low-profile multi-fiber connectors between sensor-embedded composite panels. The interconnection of such composite panels has been cited as a major limitation to optical fiber sensor- embedded `smart materials and structures' during the past fourteen years. Typically the leads of optical fiber sensors embedded in composites have been brought out of the material either at the sides through protective tubing or through the surface via standard connectors in recessed depressions in the material surface. The first of these options suffers from a lack of mechanical robustness, while the second serves to locally weaken the material and to expose the fiber sensor channel to the external environment. We report successful connector embedment at the panel edge with singlemode connector losses of less than 1.1 dB, and connectorized fiber sensors accessed through the fiber connector.

An area of concern for erbium doped Fiber Laser Sensors (FLS) has been the issue of `cost per sensor'. To address this issue and promote the FLS as a viable sensor system, techniques for multiplexing intracore Bragg gratings in serial and parallel fiber laser architectures are presented. These configurations allow multiple lasing lines to be supported using a single 980 nm pump. The architectures are examined in terms of power requirements, complexity, and strain sensitivity. Simultaneous tracking of multiple sensors subject to either temperature or strain variations has been achieved and the data is presented. Preliminary results indicate a low level of crosstalk between sensors.

The main objective of this study is the development of an embedded fiber optic sensor for testing ceramic composites in a very high temperature environment. The sensing element is an optical grade sapphire fiber operating on the principle of spatial modulation in a multimode waveguide. In order to employ this waveguide as a stress sensor, optomechanical testing has been performed to examine the optical response to external stresses. Several tests, including tension, micro-bending, and lateral compression, are in progress. These tests will establish the basis for using embedded optical sensors for characterization of ceramic composites in real environment. The principles of operation and experimental investigations on the microbending tests are presented in this paper. The results show that the developed sensor can be applied for stress monitoring as well as displacement measurements in a very high temperature environment.

This paper discusses a first step in the use of weighted optical fiber sensors to selectively determine the mode shape amplitudes of different vibration modes in a simply supported cable. A weighted fiber sensor has a sensitivity function that varies along the length of the fiber. To date, such weighted sensitivity fibers have been fabricated by either writing spatially chirped refractive index gratings in two-mode elliptical-core fiber or by geometrically tapering single mode or two-mode elliptical core fiber. The output signal from the fiber sensor system is proportional to the weighted integral of strain along the length of the fiber, and subsequent analysis indicates that this yields a measure of only one selected mode.

The bending sensitivity of two-mode fiber sensors is discussed. A phenomenological approach based on an apparent core offset is proposed and verified by an experiment where combined bending and extension are present in the fiber.

Demodulation of internal fiber Bragg grating sensors has been demonstrated. Methods are described by which remote fiber gratings can be interrogated, in order that the direction and magnitude of environmentally induced changes, such as temperature or strain, might be measured.

A theoretical waveguide model of elliptical fibers with the step-index is presented. From it the eigenvalue equation, the mode field distributions and the cutoff frequencies can be obtained. The effectiveness of the model is demonstrated in characterizing elliptical fibers with the step-index. The waveguide theory for circular fibers is just a special case of this model.

The paper discusses an application of the optical frequency-domain reflectometry method for damage and stress fiber sensors embedded in composites. Proposed sensor is based on an optical sweep generator with an external cavity laser diode. This diode provides a single frequency CW generation with 0.85-micron wavelength and maximum sweep rate up to 10,000 GHz/s. Under those conditions one can reach the optical frequency shift of 100 kHz/m in fiber. The spatial resolution of 10 cm in 100-m embedded fiber while determining a position of a damage was obtained.

This paper reports the use of embedded extrinsic Fabry-Perot interferometric (EFPI) optical fiber sensors for the evaluation of composite materials containing arrays of piezoelectric actuator elements. The EFPI sensors are used in both differential and absolute measurement configurations. The extended 2D array of many small rod actuators is electrically driven through a pair of conductive electrode plates placed on top of and below the laminate. By applying an electrical potential difference between the plates, the actuator elements may be made to elongate axially. Two-dimensional spatial control of the resulting actuation function may be achieved by the interconnection of multiple conducting electrode addressing elements across the laminate.

Four multimode fiber optic sensing units are combined to form a simple fiber optic silicon impact sensor. Three high resonant frequency sensing units are used to localize the impact point by detecting the impact induced acoustic waves and their relative arrival times. The impact location accuracy of 4 cm is achieved over a 1 m X 2 m area. The averaged signal output from four sensing units is utilized to compensate the acoustic attenuation effect of the real impact magnitude. The novel linear design of each sensing unit structure offers an excellent linearity for impact magnitude detection.

The ability of Raman spectroscopy to nondestructively evaluate thermal degradation in graphite reinforced epoxy composites was examined. A series of composite samples, exposed to temperatures ranging from 150 to 400 degree(s)C for periods of 2 to 20 minutes, were analyzed by Fourier transform Raman and reflectance IR spectroscopies. The intensity of the Raman and IR polymeric backbone vibration at 1600 cm^-1 diminishes with increasing thermal exposures and can be correlated to failure strain and flexural strength measured by four point bending tests, as well as acoustic emission events. These data, along with IR transmission spectra of species evolved from composite pyrolysis, suggest that thermal degradation occurs in three stages: (1) polymeric fragmentation (possibly microcracking), (2) advanced polymer degradation observed as delamination between the four ply layers, and (3) final composite failure with fiber fracture.

This paper describes the framework and objectives of the OSMOS (Optical fiber Sensing system for MOnitoring of Structures) project. OSMOS is a CEC funded BRITE project with the aim of demonstrating the industrial feasibility of manufacturing optical fiber smart structures for Civil Aeronautics and Civil Engineering. The sensor concepts and associated technological issues to be addressed are presented in connection with the specific applications investigated in the framework of this project.

Low cost, rugged and reliable fiberoptic sensors are being developed to meet the needs of geotechnical engineers. The primary emphasis has been on load and pressure sensors, including pore water pressure sensors. The microbend sensors developed have been tested in the laboratory up to water pressures of 100 psi and loads of 50 lb. Accuracy of sensor measurements are within 5% and is being improved upon. Sensors with larger range or more sensitivity can easily be built without changing the basic sensor design. A semi-automated calibration and testing system was developed to characterize the sensors. In this paper we describe some of the applications for the sensors, their construction, characterization system, and experimental performance.

This paper reports the performance of short gage length optical fiber sensors embedded in a reinforced concrete specimen for the quantitative measurement of periodic strain. We report the use of practical, short gage length relative and absolute strain sensors for the measurement of strain in a reinforced concrete specimen. Both types of fiber sensors were attached to steel reinforcement rods prior to filling with concrete, and were collocated with conventional foil strain gages to allow direct comparison of output signals. The relative fiber sensors were of the extrinsic Fabry-Perot interferometric type operating at a wavelength of 1300 nm and the absolute strain sensors used wavelength information to measure absolute strain. The results of this work show that such optical fiber elements may be considered for long term quantitative evaluation of civil structure components.

Experimental results are presented on the embedding of optical fiber cables into cement specimens in order to determine the feasibility and limitations of using said fibers as sensors for the measurement of internal stresses as well as the evaluation of structural integrity. Pull-out tests have revealed that the surface bonding between the plastic jacket of a fiber cable and the cement matrix is poor and inadequate for an effective load transfer. Experiments using loaded cement specimens with embedded fiber cables inside suggest that the various protective layers present in the cable's construction prevent the fibers from properly sensing any external perturbations up to the specimen's failure and, in some instances, even after failure. Therefore, use of optical fiber cables as direct sensing elements for stress/strain measurement is not recommended for most applications, due to the lack of an appropriate load transfer mechanism and sensitivity. However, they can instead be used as leads to communicate the actual embedded sensors with the outside world.

This paper describes an optical fiber interferometer that uses a short segment of silica hollow-core fiber spliced between two sections of single-mode fiber to form a mechanically robust in-line optical cavity. The hollow-core fiber is specifically manufactured to have an outer diameter that is equal to the outer diameter of the single mode lead fibers thereby combining the best qualities of existing intrinsic and extrinsic Fabry-Perot sensors. Uniaxial tension and pure bending strength tests are used to show that the new configuration does not diminish the axial strength of bare fiber, and reduces the bending strength by 17% at most. Similar tests confirm that the fiber sensor has 1.96% strain to failure. Axisymmetric finite element analysis is used to investigate the reliability of the in-line etalon during typical thermoset composite cure conditions, and parametric studies are performed to determine the mechanically optimal cavity length.

All-passive fly-by-light technology has been considered to replace the conventional fly-by-wire control systems in aircraft. Both the sensors and their associated interfaces providing data to the flight control computers have been developed. Primarily, two types of electro-optic interfaces and four different types of Fiber Optic technologies have been demonstrated for sensors/switches. Comparison data shows the development of passive TDM flight control technology is near production-ready, but the digital optical position sensors and switches are prohibitively expensive. Two innovative approaches are required for improving producibility and cost of the TDM sensors. Sensors based on other techniques require substantial development to meet system requirements and are still in their infancy. System, device and manufacturing engineers must work closely to implement system requirements and concurrent engineering approaches at early stages if the fiber optic technology is to move from the laboratory to production aircraft.

The U.S. Navy is developing fiber optic temperature sensors to detect shipboard fires for the damage control monitoring system. For fire detection, continuous measurements of temperature are required to a value of 1100 degree(s)C in every compartment, in addition to other measurements. Fire detection information is transmitted on a fiber optic data network for display to the ship's damage control officer. Fabry-Perot, black body, and microbend sensors were evaluated on a Navy ship, the ex-USS SHADWELL, to determine ability to detect fires. Results demonstrated the ability to measure temperatures up to 1100 degree(s)C with no deleterious effects from the shipboard environment of high humidity, vibration, and shock. Further research and development has revealed additional fiber optic sensor technology, based on other techniques, that is also capable of high temperature measurement onboard ship. Multiplexing is being used to reduce sensor cost.

In this study distributed fiber optic sensors using commercial telecommunication and special sensor fibers have been studied and tested. A PC-compatible OTDR-card is used to obtain a portable measuring system. Especially the measuring of strain and stress in steel plates and bars is studied in comparison with strain gage measurements. The sensing of ice temperature with a special sensing fiber is also reported. Applications of the fiber sensors studied here are strain and stress measurements and crack detection in steel chambers and pipes especially at high temperatures (> 300 degree(s)C), and freezing detection of road structures.