Frequency-derived distributed optical-fiber sensing is a potentially-powerful technique for the distributed measurement of extended physical parameters, such as temperature and strain. The technique is capable, in principle, of high spatial resolution (better than 0.1 m) and accuracy (approximately 1%). This paper investigates the limitations on the performance of this system, and summarizes the experimental results achieved to date.

Given the obvious appeal of a distributed sensing system, it is perhaps surprising that so few distributed fiber optic sensors (DFOS) have so far appeared in the commercial marketplace, especially when considered in the light of the significant research effort that has been expended in this area. This paper describes an extensive review of the commercial prospects for DFOS which has recently been completed. Information on the suppliers and applications of commercially available DFOS systems--dominated by the optical fiber distributed temperature sensor--is presented. This is complemented by a review of the important trials of commercially available DFOS systems.

A new reflectometry, phase-modulating optical coherence domain reflectometry by synthesis of the coherence function (p-OCDR), is proposed in this paper. The optical coherence function synthesized by direct frequency modulation of a laser diode is scanned by phase modulation of a reference lightwave in an interferometer. This method has no mechanical moving part, and is easy to adjust a spatial resolution and a measuring range in optical circuits under test. Consideration of the influence of unideal factors to the performance of p-OCDR tells us that p- OCDR can realize a higher performance more easily than usual OCDR. Additionally p-OCDR can compensate a nonlinearity in the direct frequency modulation by the phase modulation. Finally basic experiments of p-OCDR are successfully carried out.

This paper describes initial results of two fiber optic sensor multiplexing techniques incorporating extrinsic Fabry-Perot interferometric sensors and applications of these sensors. The first multiplexing technique described is based upon path-matching techniques and the second is based upon optical time domain reflectometry.

Results of measurements of longitudinal and transverse crack growth using multimode fiber optical time domain reflectometry are presented. Crack detection thresholds less than 0.1 millimeter are readily achieved with OTDR. A sensor package design has been developed to provide controllable directional response characteristics. In particular, a method to eliminate the orientation angle problem and yield omnidirectional sensor response characteristics has been discovered. Results are reported on a novel all-fiber sensor capable of sensing 10 micrometers crack displacements while surviving and sensing 150% strains and displacements of 6 mm. Transmission measurements of the sensor using white light are also reported. The method is suitable for distributed sensing applications covering large areas of structures and adaptable to response enhancements required for real time structural monitoring at rates on the order of hundreds of Hertz. Applications include bridges, buildings, main-steam pipelines, and offshore platforms.

Corrosion of steel within bridges and other large concrete structures has become a serious problem and consequently there is growing interest in replacing the steel within such structures with carbon fiber based composite materials. The first highway bridge in the world to use carbon fiber composite prestressing tendons was constructed and opened to the public in the fall of 1993. This two span bridge was also unique in another respect, it is the first highway bridge in the world to have been built with a set of fiber optic Bragg grating sensors structurally integrated into several of its precast concrete deck support girders. A four-channel fiber laser sensor demodulation system that was rugged, compact and transportable was developed for this project. This demodulation system monitored the changes in the internal strain on all three types (steel and two types of carbon fiber composite) of prestressing tendons over several months. The same structurally integrated fiber optic sensing system has also been used to measure the change in the internal strain within the deck girders arising from both static and dynamic loading of the bridge with a large truck.

We analyze the measurement errors in fiber Bragg based strain sensors which can be induced by source spectral structure. The effect is numerically modeled, and compared to experimental observations.

The paper describes recent progress on developing a complimentary pair of fiber grating-based sensor methods for structural monitoring. Particular emphasis is placed, firstly, on interrogating arrays of point-sensors based on fiber gratings using an acousto-optic tunable filter and, secondly, on determining the optical path length between these point sensors using optical time domain reflectometry. The combined system is capable of measuring both local and spatially-averaged strain. A new approach which permits the simultaneous measurement of strain and temperature, using superimposed fiber gratings, is presented. Potential applications in aerospace composites are also discussed.

Fiber-optic intracore Bragg gratings have been widely used as strain sensors. In most applications, the strain along a fiber grating is uniform and the strain is measured by the shift of the reflective wavelength of the grating. If the grating is under a gradient strain field, however, its reflective spectrum will be not only shifted but also distorted due to the chirp of grating periods. The shape of this spectrum depends on the strain distribution. Therefore the location and the shape of the reflective spectrum together can provide information on the strain distribution over a small region (sub-centimeter). We have calculated the reflective spectra of gratings by using the T-matrix formalism which allows us to simulate gratings with any distribution of the refractive-index modulation depth and the chirp of the period length. The calculation results show a strong dependence of the reflective spectrum of a grating on the distributions of its refractive-index modulation depth and period length. Experimental results have verified the T-matrix calculations. These results indicate that fiber-optic gratings can be used as quasi-distributed strain sensors to detect strain gradients that is very useful for monitoring and studying the presence and the evolutionary process of fiber sensor malfunction or material fringe/damage.

A fiber optic sensor system capable of detecting the strain distribution in a long length of optical fiber containing a series of fiber Bragg gratings is described. The technique utilizes RF sub-carrier based determination of the optical propagation time between the grating elements, and wavelength division-based addressing of the individual FBGs in the system. The approach will have applications in structural strain monitoring where long gage length sensing is required at multiple locations.

Nonlinear transduction mechanism in electrostrictive and magnetostrictive materials is exploited to demonstrate multiplexed low frequency fiber optic electric and magnetic field sensors. The sensor utilizes a single interferometer and a single unmodulated laser while demonstrating extremely low cross-talk between the electric and magnetic channels. Low frequency electromagnetic signals from remotely located electric and magnetic dipoles are detected with the fiber optic sensor.

We report the first experimental demonstration of a frequency hopping strategy for use in a passive interferometric optical fiber sensor multiplexing network. The preliminary results successfully demonstrate the principle, and two interferometric sensors have been multiplexed with crosstalk levels less than -38 dB.

We describe an optical fiber interferometric sensor addressing technique which utilizes frequency modulation of a single laser source to control the visibility of individual sensors. Interrogation of the sensors is performed by matching the laser modulation frequency to the frequency response characteristics of each individual sensor. We show that improvements in system performance can be achieved through the use of pseudo-noise RF modulation applied to the laser.

The principles of operation of distributed fiber-optic seismoacoustic sensor based on intermode interference in low mode fibers are studied. A model of this sensor for seismo-acoustic vibrations registration was made and tested. This sensor has the following performances: working frequency band is 2 - 40 Hz, transformation coefficient is 1.5 mV(DOT)s²/m at frequency 20 Hz, sensitivity threshold is 1.6(DOT)10^-3 m/s², sensor length is 100 m. Interferometric procedures of signal forming and processing were researched.

Numerous fiber optic sensors have been developed that utilize wavelength modulation to encode the values of sensed parameters of interest. Two basic techniques have been used to process the return signals from these types of sensors: Fabry-Perot cavity modulation and diffraction grating plus CCD array. The first of these suffers from a limited spectral range while the second is limited in speed due to the serial readout of the CCD array. A thin film Faraday phase grating modulated by an external magnetic field in combination with a single large area photodetector is able to overcome these limitations. In this paper we develop a theoretical model for the performance of such a device and compare it with experimental results. Finally, the practical application of this technology is discussed.

The U.S. Navy is presently developing fiber optic sensor technology for machinery monitoring and control and damage control systems. We have developed a redundant, reflective star network of four fiber optic sensors (temperature, pressure, liquid level, and smoke) which can be monitored using either Time Division Multiplexing or Frequency Modulated Continuous Wave techniques. The sensors used are short cavity, low finesse Fabry- Perot sensors; processing utilizes the varying power reflectances from the sensors and is done in a desk top computer. The networks allow for the passive, remote monitoring of an area or of a machine using a single fiber and opto-electronics package. The paper presents the philosophies of the two designs and the results of network testing. Projections are made concerning the maximum number of sensors that are multiplexable using these approaches, and comparisons are drawn between the two methods. The sensors used have been qualified for shipboard application and thus compete directly with electrical equivalents.

A fiberoptic sensor, based on the extrinsic Fabry-Perot interferometer (EFPI) design, was developed for intelligent processing of epoxy matrix composite materials. The EFPI sensor is modified so that, when embedded in a composite material during fabrication, it will output a signal that is proportional to the degree of polymerization (cure state) of the epoxy. After cure of the epoxy, the sensor may be used to measure initial strains.

The fiber optic measuring network for scalar and vector physical field investigation is studied. The special method of network signals processing which allows minimizing the number of measuring lines is suggested. It is shown that by choosing of measuring line construction and trajectory one can reconstruct the distributions of scalar field amplitudes or distributions of vector field Decarts components or distributions of square of vector field gradient.