Permanent, photoinduced refractive-index gratings written along the longitudinal axis of
germanium-doped, two-mode, elliptical-core fibers are found to affect the differential phase
modulation between the co-propagating LP 0l and LP 11 even modes. The modal beat length
of a two-mode fiber grating is found to vary inversely with strain induced to the fiber and
leads to a new type of weighted optical fiber sensor whose sensitivity varies as a function
of length along the structure to which it is attached or embedded. A weighting function is
imparted to the fiber sensor by way of a chirped, two-mode grating written into the core,
the chirp adopting the shape of a specific vibration-mode amplitude. This novel type of
spatially-weighted, distributed fiber sensor is shown experimentally to function as an
efficient vibration-mode filter by enhancing either the first or second mode of a thin, one dimensional
cantilever beam. These results support future applications of spatially-weighted/
distributed optical fiber sensors for the evaluation and control of vibrating
structures.
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