Extrinsic Fabry-Perot interferometric (EFPI) sensors have previously been demonstrated for relative strain and temperature measurements for smart structure applications. Inherent difficulties in the signal processing of these devices has created the need for absolute measurement capabilities. In this paper, we present an absolute measurement technique based upon white-light interferometric path matching. The system matches a reference gap to the sensing gap of an EFPI. When the difference of these two lengths is within the coherence length of the source, an intensity envelope is created in the system output. Determination of the corresponding path mismatch indicates the size of the sensor gap and hence strain can be determined. This measurement technique is capable of multiplexing an array of EFPI sensors and data will be presented demonstrating four multiplexed devices. Theoretical considerations for system optimization are also presented. As the only fiber-optic sensors subcontractor to Northrop Corporation on the Navy/Air Force-sponsored Smart Metallic Structures (SMS) program, Fiber & Sensor Technologies (F&S) is developing the optical fiber fatigue gage instrumentation for a multiplexed, in situ structural health monitoring system for aging aircraft. In March, 1995, F&S successfully demonstrated the system on a full-size F/A-18 wing-box spar fully instrumented with 12 of F&S' patented EFPI optical fiber strain gages. F&S is now in process of up-scaling the signal processing system in addition to the optics and intends to demonstrate a second generation multipoint sensor system capable of simultaneously monitoring strains at up to 60 different sites throughout the aircraft later in 1995 or early 1996.

We report on civil engineering applications of wavelength multiplexed optical-fiber Bragg grating arrays produced directly on the draw tower for testing and surveying advanced structures and material like carbon fiber reinforced concrete elements and prestressing tendons. We equipped a 6 m X 0.9 m X 0.5 m concrete cantilever beam reinforced with carbon fiber lamellas with fiber Bragg grating sensors. Static and dynamic strain levels up to 1500 micrometers /m were measured with a Michelson interferometer used as Fourier spectrometer with resolutions of about 10 micrometers /m for all sensors. Comparative measurements with electrical resistance strain gauges were in good agreement with the fiber optic results. We used the fiber sensors in two different arrangements: some Bragg grating array elements measured the local strain while others were configured in an extensometric way to measure moderate strain over 0.1-1 m.

The paper discusses two complimentary optical fiber sensing techniques which have been researched for structural monitoring applications. The short-gauge-length sensor system is based on fiber gratings and has achieved a strain resolution below 1 microstrain. The long-gauge-length sensor system has achieved 100 micron spatial resolution using a new OTDR technique. Results are presented for surface-mounted and embedded sensors. Both sensors systems can be multiplexed to make more efficient use of the interrogating unit. Their system designs should be capable of being developed to meet real engineering applications.

A deformation monitoring system, based on low-coherence interferometry in standard telecommunication fibers embedded in concrete structures, is presented. This measuring system, developed for the needs of civil engineering, features a precision of 10 micrometers over measuring length of up to 100 m. Extensive tests have been carried out on a number of 1 m X 5 m X 0.5 m high-performance reinforced concrete ties, within a research on seepage flow through cracked elements. The presented system allowed the internal measurement of the deformations during the whole setting process (including the thermal expansion phase), of the deformations during the tensile test and of the crack openings evolution. This paper presents the operating principle of the low- coherence interferometer, the lessons learned in installing optical fibers in concrete structures, as well as a comparison with the measurements performed by standard sensors installed on the structure surface.

The real advantage of an optical fiber displacement sensor lies in solving vibration testing problems for which the measured system need precise and various frequency response. Here we present a description of the experimental setup, test procedure, and data-processing method for measuring the vibration isolated effect of an air cushion table. The nonlinear vibrations of a table are discussed and investigated by optical fiber and Michelson set-up. From the experimental results we proved this measuring device is economical and feasible.

Optical coherence function can be synthesized in an interferometer by using direct frequency modulation characteristics of laser diodes. Recently we found that the additional phase modulation in one arm of an interferometer enables the synthesis of arbitrary shapes of the optical coherence function. In this presentation, at first, the principle of the synthesis of the coherence function is explained, and experimental results are shown. Next, phase-modulating optical coherence domain reflectometry by synthesis of the coherence function (p-OCDR), a distributed fiber-optic sensor by synthesis of the coherence function, and optical information processing by synthesis of the coherence function are mentioned, and recent experimental results are shown.

A prototype nanometer resolution multiplexing system, capable of multiplexing up to 32 fiber-optic-based extrinsic Fabry-Perot sensors with similar optical path differences for quasi-static absolute measurements has been developed. It is the first time that such a multiplexing system for use with a large number of point sensors has been demonstrated. Several prototype sensor have been incorporated into the network and demonstrated. A range to resolution of better than 10⁴:1 has been achieve. It was also demonstrated that the sensors in the network can be interchanged easily. This instrument is easy to use in practice for multiparameter measurement.

In this paper, the design of an optical fiber system for the monitoring of ventilation in a high way tunnel is presented. It employs a star topology for the sensing of dust and CO, at four levels of concentrations, 1.0-4.0 mg/cm³ and 100-400 ppm, respectively. A wavelength demultiplexer (employing an interference filter) is used for distinguishing between the signals from the dust/CO subsystems. A time division multiplexing scheme utilizing a single pulse technique is employed in detecting the dust signals. In the CO subsystem, a switching protocol permits the detection of the optical PCM signals from the sensors. A wind speed/dual direction sensor is spatially multiplexed with the above subsystems. A fiber optic system with eight sensor lines representing the dust/CO subsystems together with a single wind sensor line has been proposed. A laboratory model with four sensor lines has been constructed, and results from preliminary evaluation of the system are given.

We present a multiplexed fiber-optical sensor system for in-line monitoring of volatile organic compounds in air. The transducer consists of sensitive polymer films coated on glass substrates, a source of white light, a fiber-optical multiplexed and a photodiode array detector. It is based on the interferometric (phase sensitive) measurement of the change of optical parameters (thickness and refractive index) of the polymer film when exposed to volatile organic compound (VOCs) vapors. Polymer films swell fast and reversibly depending on the concentration of analyte in the vapor phase and the coefficient of distribution between ambient medium and polymer bulk. We shortly review the transducer's physical and chemical properties, and describe its applicability in sensor arrays, including internal referencing, and the perspectives in process control. This opens a way to rugged pattern recognition and multicomponent analysis of fairly complex mixtures of VOCs.

Time-domain (TDM) and frequency-domain (FDM) multiplexing techniques for near-IR diode laser spectroscopy based gas sensors are described. Three multiplexing techniques are demonstrated on a passive optical fiber network with four sensors. Absorption sensitivities varying from 2.5%, for a direct detection TDM systems, to 0.01% for a frequency modulation spectroscopy FDM system, are demonstrated.

This paper provides an overview of distributed strain and temperature sensing techniques that employ Brillouin scattering in single-mode optical fibers. These techniques are based on strain- and temperature- induced changes in the Brillouin frequency shift. Emphasis will be placed on recent progress in performance such as improvements in dynamic range, measurement accuracy, and spatial resolution.

A fiber optic distributed temperature sensor has been developed by using Raman scattering light in order to apply to the temperature monitoring systems in underground transmission lines. With the measuring capacity expanded to 10 approximately equals 30 km and the distance resolution upgraded to below 1 m, the application field of the sensor has been widened, and the sensor is currently employed for temperature monitoring in various fields such as buildings, plants, etc..

We propose the principle of a high-dynamic, quasi-distributed temperature sensor. We previously have demonstrated the possibility of realizing a high-dynamic point sensor based on the behavior of the green emission in erbium doped silica fibers. We present here the study of the 1.13 micrometers and 1.24 micrometers emission lines, coming from the same levels. Those lines present the same temperature dynamic as the green ones. As the lower level of these transitions is the ⁴I_11/2 level and not the fundamental one, the signal is absorption free. The signal wavelength also corresponds to a transparence region of the intermediate fibers. These arguments permit developing an efficient quasi-distributed configuration. In addition, the intensity ratio of the emission lines is only temperature dependant, so the measurement is self-calibrated. We also demonstrate that the emitting levels can be excited around 800 nm, by the excited state absorption process, which allow using a standard laser diode as pumping source.

A two-point optical fiber temperature sensor which makes use of a multimode 1 X 2 fiber switch has been developed. It exploits the fluorescence decay of a Nd-doped glass as temperature sensing mechanism. The fluorescence responses from the Nd:glass probes are alternatively routed, via the optical switch, to a DSP which converts the fluorescence decay time into a modulation frequency proportional to it. The system, which demonstrates the feasibility of multi-point temperature sensing with a decay time-based approach, could be easily expanded to multiple- sensor configuration by means of a binary-tree scheme. The probes were tested in the temperature range 0 degree(s)C to 250 degree(s)C. The results of the tests and the calibration of one probe are reported.

In this paper, a distributed optical fiber strain sensor using backward stimulated Brillouin scattering (BSBS) combining both the optic heterodyne and OTDR is presented.

Distributed optical-fiber sensing promises to be a powerful technique for the monitoring and diagnosis of large structures. The technique provides the means by which the spatial and temporal distribution of, for example, strain and temperature can be measured, with good accuracy, throughout the structure. Backscatter methods in fibers are limited, in the performance which they can provide, by a low signal level. Better performance can be achieved by using forward-scatter methods which use polarimetric techniques and/or nonlinear optical effects. Such methods are reviewed in this paper and three of them are described in detail. Prospects for application are very promising.

Results are reported from recent research on the use of the Brillouin gain/loss mechanism for distributed sensing. A theoretical model of the interaction of the pulsed and CW beams is described and compared with experiments. Results from a system with a 51 km sensing length are presented. Issues related to the variation within the sensing fiber of the polarizations of the two beams are investigated. The use of the system for simultaneous distributed sensing and communication is described along with a possible method to simultaneously measure temperature and strain.

The continued development of photon counting optical time domain reflectometry has been aided by the advent of more efficient and sensitive detectors. To date, photon counting has generally been reserved for specialized applications requiring sophisticated experimental equipment and instrumentation. This paper reviews recent developments which reduce both the cost and complexity of the equipment required and describes an applications of the technique to high resolution distribution sensing.

A new method of discriminating between temperature and strain effects in fiber sensing, using a single, conventionally written in-fiber Bragg grating, is presented. The technique is based on a dual wavelength scheme and uses wavelength information from the first and second order diffraction from a single grating element when illuminated with light coinciding with the primary and secondary reflecting wavelengths. It is often assumed that during the grating growth the fiber core responds linearly to the incident UV radiation, thus resulting in a sinusoidal refractive index profile and only one discrete wavelength being reflected. In practice the recording process is nonlinear with continued exposure of the grating resulting in saturation of the index perturbation, this results in higher order grating reflections at roughly integer multiples of the primary reflected optical frequency, i.e. 1/integer multiples of the incident wavelength. The determination of the wavelength dependent strain and temperature coefficients at the primary and secondary reflections can be used to give independent temperature and strain measurements.

A theoretical analysis of determination and improvement of the resolutions of distributed fiber optic temperature sensing system is discussed. A mathematical expression is derived to reveal the inherent relations between the resolutions. An integral sensing system is set up on the basis of analyzing in theory and the experimental result is also given.

The degree of light polarization propagating in a birefringent optical fiber diminishes along the propagating distance. This effect is of particular interest in optoelectronics since the quasi-monochromatic semiconductor sources are not perfectly coherent. Therefore, measurement of the degree of polarization of light propagating along the fiber can be directly applied to determine the coherence characteristics of laser diodes. The issue concerns also applications in polarimetric sensing with birefringence fibers. The paper analyzes the problem of polarization's degree fading in polarimetric fiber optic sensors in view of optimizing the performance of real polarimetric sensors with highly birefringent fibers. The influence of degree of polarization of partially polarized light passing through highly birefringent optical fiber on metrological parameters of polarimetric sensors is presented.

A method of determining a modulation transfer function (MTF) is proposed for fiber optic components with symmetrical spread function. A central irradiencies measured in the image of variable slit and MTF is calculated by using the results of this measurements and suggested formula with computer processing. Also an apparatus is presented.

A multiplexed, low coherence, dual LED, multimode fiber sensor processing system, for measurement of optical path changes generated within Fabry-Perot field sensors, has been constructed and tested. Differences between reference and field sensor coherence profile peak fringe positions were measured using simple signal thresholding techniques and a 10-channel timer-counter board, triggered and stopped by zero-crossover detection signals. S/N ratio at APD detectors allowed measurement update rates in excess of 40 Hz and resolution better than 1/100th fringe. Range to resolution capability was in excess of 12 X 10³:1.

Fiber Bragg grating written into the core by a sideways exposure to an ultra violet-laser interference pattern have shown great promise for use as practical strain sensors in large structures. One way to sense the strain of the grating is by using active interrogation whereby the fiber Bragg grating is used as the optical feedback element of a laser cavity, and the lasing wavelength is monitored as the system output. Compared to passive broadband techniques, the fiber Bragg laser sensor provides much stronger optical signals, thus leading to a much improved signal-to- noise ratio. In order to optimize the power output from this sensor, one wishes to model the output from the fiber laser in terms of the Er-doped fiber parameters, the pump characteristics, the cavity mirror reflectivities and losses in the cavity. In this paper we solve the rate and propagation equations for a Fabry Perot cavity to obtain explicit closed form equations for the output power, threshold pump power, as well as for the optimum length. Experiments where we used an electron cyclotron resonance plasma enhanced chemical vapor deposition apparatus to deposit dielectric thin films on one fiber end point in order to change the reflectivity of a cavity mirror, while monitoring the reflectivity in situ, verify the validity of the model.

A new method for the simultaneous interrogation of conventional two-beam interferometers and Bragg grating sensors is proposed and demonstrated. An unbalanced Mach-Zehnder interferometer illuminated by a low coherence source is used to act as a wavelength tunable source for the grating and as a path matched filter for a Fizeau cavity. The technique gives high phase resolution. The grating sensor gave a dynamic strain resolution of approximately equals 0.05 (mu) (epsilon) /(root)Hz at 20 Hz and the interferometric resolution was better than 1 mrad/(root)Hz at 20 Hz, corresponding to an rms mirror displacement of 0.08 nm.