Optical fiber, vibration-mode filters incorporating photoinduced refractive-index gratings

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Abstract
The authors present an overview of results obtained with an optical fiber vibration-mode sensor, a new type of distributed, optical fibre sensor whose strain sensitivity varies as a function of length along the sensor. The change in the sensor's strain sensitivity is achieved via a chirped, refractive-index grating formed within the core of two-mode, germanium-doped, elliptical-core fiber. For a simple cantilever beam, the proper chirp variation for an nth-mode vibration-mode filter is theoretically shown to correspond to the shape of that vibration mode. To fabricate such a chirped grating, the fiber is exposed to intense light at 514 or 488 nm while attached to a thin, cantilever beam statically strained in the shape of the nth vibration. The grating chirp occurs when the beam is released and the grating period undergoes a differential change along the length of the fiber. The grating-based, vibration-mode sensor has been used to selectively detect both the first and second vibration modes of a flexible cantilever beam.