| When a laser beam passes through an elastic medium, its optical path length is varied if the elastic medium is exposed to acoustic pressure. As a result, the interferometric fiber-optic sensing of sound waves via laser beam has been studied by many authors. In this study, emphasis is put on the sensing elements and their application to array design. The sensing element or array is a part of a single-mode interferometer.;The coil sensor theory has been applied to design a sensitive, broadband, omnidirectional sensor in which the fiber coils are wound in a equiangular-intersectional technique. To assure the omnidirectionality in a specified coordinate plane, this sensor must have an adequate number of coils in the equiangular-intersection arrangement, perpendicular to that coordinate plane.;A distributed line array is investigated. Its theoretical development is formulated on the bases of the single coil element theory and the conventional line array theory. In the low frequency region, the array pattern is similar to a single-coil element pattern. In the high frequency region, this array pattern behaves like that of a conventional discrete line array. Changing the number of fibers in each array element is recommended for shading the directivity pattern; this will not affect the resonance frequency of the coils in the array.;A continuous line array model is discussed. It has been found that the predominant force acting on the fiber is attributed only to the symmetric pressure. Two acoustically induced modes of vibration in the fiber are studied: the symmetric vibration, and the unsymmetric vibration. The total optical phase shift is caused mainly by the symmetric vibration. A space resonance phenomenon is predicted, which is associated with symmetric vibration. This space resonance is suppressed when a long interaction length is used.;By the use of a plane strain model, the normalized phase shift is calculated for the frequency range from 200 Hz to 45 kHz. Since the acousto-optic interaction is strongly dependent on the strain configuration of the fiber, the predicted directivity pattern and frequency response are explained in terms of the induced vibrations. |
