Interferometric distributed optical-fiber sensor

An interferometric technique is described for detecting and locating perturbations along an optical fiber. This distributed sensor, based on a modified fiber-ring interferometer, has a position-dependent response to time-varying disturbances such as strain or temperature. These disturbances cause a phase shift that is detected and converted to spatial information. The sensor consists of two parts, namely, a reflecting-fiber-ring interferometer and a differentiating-ring interferometer. The reflecting ring consists of a fiber ring with one port of the coupler connected to a reflector. Consequently the output port of the reflecting-ring interferometer is the same as the input port. Because it is an inherent zero-path-imbalanced system, a short-coherence-length source such as a light-emitting diode can be used. Any time-varying perturbation on the fiber in the ring results in a detector signal proportional to the product of the rate-of-phase change caused by the perturbation and the distance of the perturbation relative to the center of the fiber ring. The second part of the system, a differentiating-ring interferometer, consists of the same fiber-ring interferometer modified only slightly. The output of this part of the sensor is proportional only to the rate of phase change as a result of the unknown perturbation and contains no distance information. By dividing the output of the reflecting-ring interferometer by the output of the differentiating-ring interferometer, we determine disturbance location. Results obtained with a 155-m distributed fiber sensor are discussed.     

A physical interpretation of the equivalence theorem

A physical interpretation of the equivalence theorem is presented. A simple example is used to show that the external and internal equivalence cases are analogous to incident, reflected, and transmitted wave problems in the geometrical optics sense.     

An illustrative equivalence theorem example

Discusses the field equivalence principle where a fictitious cylindrical surface is involved. Field plots obtained from numerical results are given     

On the equivalence of geometrical-optics reflected fields and the stationary-phase solution of the associated radiation integrals

This tutorial shows how the geometrical-optics expression for the field reflected from a surface can be derived directly from the general first-order stationary-phase solution of the associated radiation integral. It will improve the understanding of how these two techniques are related to one another. The material presented is ideally suited for post-graduate courses on high-frequency electromagnetics techniques.     

Circular aperture pattern synthesis from collapsed equivalent line-source distributions

A novel circular aperture pattern synthesis technique is presented, which enables a linear line-source distribution to be converted to a rotationally symmetric circular aperture distribution, of which any /spl phi/-cut radiation pattern is ideally the same as the principal plane pattern of the line-source distribution. Line-source pattern synthesis techniques are numerous and versatile and the technique presented here allows these techniques to be applied to circular apertures as well. This new synthesis method is most compatible with line-source distributions which have zero edge illumination.     

Electromagnetic scattering by a two-dimensional wedge composed ofconductor and lossless dielectric

Electromagnetic scattering by two-dimensional wedges composed of perfectly conducting and lossless dielectric material is considered. Heuristic geometrical theory of diffraction type diffraction coefficients are presented and a hybrid moment-method/physical-optics technique is developed to verify the accuracy of the proposed diffraction coefficients. The purpose is to show that the heuristic approach can yield acceptably accurate results for a number of composite wedge geometries, rather than to present rigorous diffraction coefficients for the composite wedge per se. Calculated results are presented in which the results obtained by means of the two methods are compared. Very good agreement is achieved for a variety of wedge geometries     

Aperture theory and the equivalence principle

Aperture theory in antenna courses is a topic that is usually dealt with fairly superficially at undergraduate and graduate levels. The paper takes a fresh look at aperture theory and its relationship to the equivalence principle. Some subtleties and possible misconceptions are discussed. Emphasis is placed on the mathematical analysis of the process by which the aperture is short-circuited, and the subsequent interpretation of various approximations to the aperture fields.