Distributed optical-fiber sensing (DOFS) utilizes the unique advantages of the optical fiber as a passive, dielectric, flexible and one- dimensional measurement medium. It offers full spatial and temporal information concerning the behavior of a large range of measurand fields. Among the many potential application areas of DOFS are those in aerospace, petrochemicals, electricity supply, mining and civil engineering. Methods for realizing DOFS have hitherto concentrated on linear backscatter techniques in the fiber. New explorations to be described in this paper give the emphasis to nonlinear, forward-scatter techniques, and especially to two of these which rely on the optical Kerr effect. The primary advantage of this approach is a significantly improved spatial resolution, down to approximately equals 0.1. m. A description also will be given of a promising quasi-distributed (forward-scatter or backscatter) arrangement which used the (nonlinear) photosensitivity of fibers to devise a DOFS system for simultaneous quasi-distributed measurement of strain and temperature. Prospects for the future of DOFS technology will be reviewed.

Techniques for manufacturing of Bragg fiber gratings as well as the sensor system based on in fiber Fabry-Perot interferometers are presented.

Polarimetric fiber optic sensors belong to phase sensors, in which phase shift between two electric field components of a light beam passing through a highly birefringent (HB) optical fiber is responsible for a change in polarization state of the output light. Since the output polarization can be easily affected by different stresses, axial strain, hydrostatic pressure of temperature acting on a measuring fiber that is the reason why polarimetric fiber optic sensors hold great potential in applications to many fields of modern industry and civil engineering. The paper presents a short theoretical overview of polarimetric sensors with various types of HB fibers on the basis of a research collaboration between two optoelectronics laboratories at Univ. of Quebec at Hull and at Warsaw Univ. of Technology. Current trends of developments and possible future applications are also discussed.

Development of the theoretical understanding of the principle of a new fiber optic polarimetric sensors based on deformation effects (such as those induced by hydrostatic pressure, longitudinal strain, twisting effect and temperature) on lowest-order mode propagation in highly birefringent polarization-maintaining fibers is discussed and the latest results of birefringence measurements in bow-tie and elliptical-core fibers influenced simultaneously by longitudinal strain and twisting effect are presented.

The paper describes the means of raising the sensitivity of fiber optical sensors involving single multimode and one-mode light guides. First, this is the use of a polarizational beam splitter which is installed in front of the inlet face of a multimode light guide and which fully transmits the polarized radiation from the source to the light guide and reflects half the power of the depolarized output beam to the photodetector. The scheme allows one to raise the output signal level by more than a factor of two as compared with the traditional scheme. Second, in an ordinary one-mode light guide, it is recommended to place a compensator between a polarizational beam splitter and the light guide face. And third, in the case of the use of a single polarized one-mode light guide the compensator should be replaced by a nonmutual magneto-optical element. It is shown that in the circuits with one-mode light guides one can virtually fully eliminate separation power losses of the output beam.

The Sagnac interferometer has been used to support fiber optic gyros to sense rotation which currently represents the largest single market for fiber sensors. The Sagnac fiber optic interferometer may also be used to sense time varying and very slowly varying environmental effects as well as instrumentation to measure the properties of light sources and optical fiber. This paper provides an overview of these and other applications of the Sagnac fiber optic interferometer.

The interference of light in single-mode optical fiber is presented in the report. Both the coherence and the polarization of interfered light beams are taken into account in the proposed description due to applying the Jones matrix calculus. The resulting formula for source of Gaussian or Lorentzian spectral distribution makes possible estimating the influence of each parameter on the interference of light in the optical fiber. It can be used also to explaining the Fresnel's and Arego's conditions for the interference of polarized light beams.

An application of a measuring system based on fiber optic Sagnac interferometer for measurements of phase characteristics of vibrating elements is described. Due to the application of a set of phase- polarization detectors, such a system may be applied to obtain phase characteristics of measuring heads used in fiber optic interferometry. Used instrumentation allows to study elements in the frequency range from 100 Hz to 1.4 MHz.

Unified physical interpretation of spatial and frequency characteristic of fiber optic loop interferometer is presented. The existence of insensitiveness regions of this interferometer and its periodic frequency characteristic is shown. It is also demonstrate that such an interferometer may be used as a distributed sensor for localization of a disturbance, because its response is linearly dependent on the position of this disturbance in relation to the loop.

A distributed fiber optic sensor for localization and identification of a disturbance along an optical fiber is presented. The sensor is based on a double-loop Sagnac interferometer. Any disturbance causing phase shift for a frequency up to 20000 Hz may be detected and localized by the sensor with a simple configuration of an optic system.

Recent research at NIST has greatly extended the capabilities of Faraday effect sensors for both magnetic field and electric current measurements. Current sensors using single-mode optical fiber show temperature stability near material limits, and are approaching commercial availability for applications in the power industry. The Faraday effect in iron garnets shows great promise for measureing current at low levels and/or high speeds. Sensors with noise equivalent currents of about 200 nA/(root)Hz have been demonstrated. Magnetic field sensors using iron garnets and flux concentration, have led to sensors with noise equivalent magnetic fields in the range of 1 pT/(root)Hz.

A further development of the fiber optic hydrostatic pressure sensor with a liquid crystal film acting as a sensing element is presented. To enhance the amount of the optical signal entering the liquid crystal optrode gradient index (GRIN) rod lenses have been used in the pressure environment. The paper discusses the effect of stress induced birefringence in a GRIN lens, its influence on defocusing phenomena under pressure and the optimized configuration of the fiber optic liquid crystal sensor preset for a required range of pressure.

The use of high-frequency pulsed diodes in laser Doppler anemometry allows the separation of multiple velocity components be means of time division multiplexing. An instrument based on such a technique is optically simpler than a multicomponent continuous-wave (CW) anemometer, but electronically much more complex. By including optical fibers and optical delay lines, the electronic requirements are reduced. In this paper, the practicality of a fiber delay network is demonstrated and the equivalence of pulsed and CW optical fringes is shown. Preliminary results for a 1-dimensional pulsed system are presented.

We report the description of a new kind of Mach-Zehnder interferometers integrated in lithium niobate (LiNbO₃), which can be used for sensor applications and coherence multiplexing transmission systems. These modulators are designed to introduce large optical delays, of some tens to some hundreds of micrometers. The orientation of the waveguides, parallel to the Z-axis of LiNbO₃, allows a quasi-polarization independent behavior.

A passive signal processing technique of miniature (approximately equals 20 micrometers ) low finesse Fibre-optic Fabry-Perot interferometric sensor with digital processing is demonstrated, where the quadrature is achieved using a single dual wavelength source.

A new fiber implementation of a grating interferometer for studying in- plane displacements of an object under load is presented. Applications of a fiber optic grating interferometers include laboratory and out of the laboratory tests of materials, construction elements and solving various mechanical problems, especially when a small sensor size is required. The system was tested for a specimen loaded in a four point bending configuration and phase methods for computer aided fringe analysis were implemented.

The concept of the smart structure integrates structural engineering, sensing, control systems and actuation to provide a mechanical assembly which is capable of responding to its environment and/or loading conditions. The realization of the smart structure requires integration of skills in a variety of scientific and engineering disciplines ranging from mechanical engineering through materials science into signal processing, data analysis, sensing and actuation. The sensing technology must have a number of key features of which the ability to take distributed measurements of various parameters throughout the structure is paramount. Fiber optics technology therefore promises to have a significant role to play in the evolution of the smart structures concept. This paper analyses this role in detail, presents an assessment of the current state-of-the art in fiber optic technology related to smart structures and presents a scenario for future developments.

Results of preliminary testing of two newly developed highly birefringent fibers, especially designed for hydrostatic pressure sensing employing a white-light interferometric technique are presented. In particular, measurements of pressure and temperature sensitivities, temperature-pressure cross-sensitivity, and mode coupling effect induced by pressure and temperature are reported.

As single mode fiber optic sensors become more complex there is an increasing need for efficient low loss components that are compatible with optical fibers. In this paper we investigate two methods of constructing in-line fiber optic channel dropping filters with bandwidths in the range 7 nm to 40 nm and insertion losses <EQ 0.2 dB. Both devices exhibit a degree of thermal tunability.

In order to demodulate optical fiber interferometric sensors in the coherence multiplexing scheme it is worthwhile using static interferomers. Measurement of a physical parameter is obtained by matching the optical path difference (OPD) of each interferometric sensor with a receiver. An attractive solution consists in using grating interferometer as a demodulator. Indeed, it permits one to record correlation peaks (each one corresponding to a sensor) on a photodiode array, thus making demultiplexing easier. On the other hand, it is possible to record only the peak envelope if the grating interferometer is adjusted in a well defined configuration. In this paper we concern ourselves with the theoretical determination of the conditions needed to simplify calculations and interpretation of the experimental results.

Birefringent filters, analogous to bulk-optic Solc filters, have been constructed from linearly birefringent optical fiber. Controlled twisting of portions of the fiber is used to rotate the polarization azimuth between retarder elements, and allows the devices to be made from continuous lengths of fiber. Filters having up to four elements have been made and evaluated, and a passband width of 10 nm has been demonstrated for a four-element filter.

The role played by optical interference in determining the mutual coherence function of an optical field propagating in few-mode fiber waveguides is examined under different conditions of the source coherence. By using the classical coherence formalism and guided-mode field representation, the mutual coherence function of the optical field guided in weakly-guiding fiber waveguides excited by stationary, cross- spectrally pure source is given in terms of the exciting coefficients, the source degree of coherence and guided-mode parameters that take also into account the first-order as well as the second-order modal dispersion. In particular, the optical interference is examined by evaluating the intensity distribution at the exit face and in the far- field of few-mode fiber waveguide as well as by evaluating the modulus of spatial and temporal coherence of an optical field at its output.115

A theoretical model has been developed to describe the bend loss characteristic of buffered single-mode optical fiber, based on interference between the core-guided mode and a 'whispering gallery' mode guided in the buffer coating. The theory is supported by experiments in which the relative phase of the modes was modulated by temperature or wavelength. As the arc length of the bend is increased, multiple-order interference is observed and predicted. The model was extended to over-moded fibers, and was consistent with experiments demonstrating coupling between the LP₁₁ and a whispering gallery mode.

The classical coherence formalism and guided-mode field representation is used to discuss the operation of few-mode fiber waveguide excited by a low-coherence, cross-spectrally pure, spatially coherent source in Michelson interferometer configuration as a sensor, even if a suppressed interference pattern at its exit face exists. In the case of a low- coherence excitation of few-mode fiber waveguide the principle of coherence modulation can be used, that is, the optical path difference between guided modes that exceeds the source coherence length can be compensated in Michelson interferometer configuration. The analysis of temporal coherence in a particular case of two-mode, weakly-guiding, step-index fiber waveguide takes also into consideration the effect of second-order modal dispersion; the potential applications to low- coherence source based interferometric sensors are discussed.

The results of theoretical and experimental studies on the construction of refractive index phaser transducer for the range below 1.45 are presented. This transducer may be applied in liquid refractometer. Presented results concern the technology of measurement head based on fiber optic taper and measurements of this device.

A method of demodulation of disturbance function used in fiber optic interferometers is presented. In the described demodulator a commonly used method of analog processing has been combined with digital processing. The obtained system exhibits high stability of working parameters.

Fiber optic techniques have extended the range of engineering applications of interferometry. Example applications of intrinsic, extrinsic and full-field systems and applications are described. Intrinsic interferometric sensors are those in which the optical signal is guided in the measurement volume, and the measurand modulates the properties of the waveguide. An example is described in which optical fiber interferometers are used for measurements of heat flux and unsteady temperature in turbomachinery test rigs. Examples of extrinsic systems are those in which the displacement of a remote target surface is detected interferometrically via an air-path. Two specific cases are discussed: the detection of acoustic emission in metal-cutting processes for tool wear monitoring, and surface-profiling. Electronic speckle pattern interferometry is an example of a full-field measurement technique, and fiber optic systems for deformation, shape and vibration measurement are described.

For the requirements of nanometrology, a new configuration of scanning interferometric profilometer, using a tapered fiber as emitting and detecting local probe, has been recently developed. The very large versatility allows a classical detection in far field, as well as a superresolution in near-field, for both transparent or nontransparent samples.

We demonstrate a real endoscopic moire metrology by using a fiberscope as an image transmitting system. For grating projection interference fringes from two single-mode fibers were used. The substraction of the projected grating from a reference was provided in real-time and moire fringes could be observed on a live image which is important for medical diagnostic.

A software for design of fiber optics laser Doppler anemometers is developed. The characteristics of four anemometer probes designed using this software are under consideration. A design of fiber optics laser knife for flow visualization is described.

A fiber optical sensor (FOS) for sensitive measurements of absolute angles has been developed. The sensor principle is based on the well defined angular dependence of the bending of a cantilever under the influence of its own weight. The cantilever bending is measured via a noncontact and high resolution method using two fiber optical sensors. These sensors are oriented perpendicular to each other and fixed opposite to the cantilever so that they are rotated synchronously during every angular movement. The current values of the rotation angle, the angular velocity, and the angular acceleration are determined in an analyzing and data processing unit and visualized on the computer monitor. This high precision absolute rotation-angle sensor has the advantages of being compact, insensitive to external influences such as electromagnetic fields, humidity, atmospheric density fluctuations, and nuclear radiation and is therefore applicable in nearly every kind of angular measurement problem and in unfavorable environmental conditions.