Abstract: This paper describes two optical fiber waveguide modification methods which allow the synthesis of effective modal filters and sensors for the analysis and active feedback control of vibrating structures. Both methods allow the sensitivity of the fiber to be varied along its length when used as a long gauge length integrating sensor. The performance of these sensors is similar to that of shaped piezoelectric film elements which have been demonstrated for several years for similar applications as modal filters, except the fiber sensors allow low- profile geometries in both dimensions orthogonal to the fiber axis. The first method uses modal birefringence between the LP$-01$/ and LP$-11$/ modes in two-mode fibers modified by tapering the waveguide during or subsequent to fabrication, with the sensitivity of a differential section of fiber along its length determined by the local core radius. By adjusting the taper geometry as a function of position along the fiber, individual vibration modes in a structure may be measured, with information concerning other orthogonal modes suppressed. The second method uses spatially weighted photorefractive index gratings in similar fibers to achieve modal selectivity. For both cases, we describe new methods for generating tapers and two-mode gratings, and present theoretical data and experimental results for such sensors embedded in composite laminates. !6
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