Source of apparent attenuation-notch splitting in long-period fiber grating bend sensors

Ueyn L. Block; Vinayak Dangui; Michel J. F. Digonnet; Martin M. Fejer

doi:10.1117/12.647832

10 November 2005 Source of apparent attenuation-notch splitting in long-period fiber grating bend sensors

Ueyn L. Block, Vinayak Dangui, Michel J. F. Digonnet, Martin M. Fejer

Proceedings Volume 6004, Fiber Optic Sensor Technology and Applications IV; 60040G (2005) https://doi.org/10.1117/12.647832
Event: Optics East 2005, 2005, Boston, MA, United States

Abstract

Long-period fiber gratings (LPFGs) have recently been utilized as optical bend sensors by observing changes in their transmission spectra as the fiber is bent. One such spectral change reported is the "splitting" of each attenuation notch into two. To date, explanations given for this apparent notch splitting have proven unsatisfactory. In this communication, we show that the apparent notch splitting is due neither to a splitting of degenerate cladding modes nor to the phase-matching condition being satisfied at multiple wavelengths for a given cladding mode. In contrast, bending causes new notches to be formed at nearby wavelengths as a result of coupling to asymmetric cladding modes that are not coupled to in a straight UV-induced LPFG. With increased bending, these new notches' central wavelengths shift in the opposite direction as the original notches, thus causing the apparent splitting of the latter. We use a numerical analysis to show that the cladding modes of a fiber undergo large spatial changes when the waveguide is bent. These changes allow coupling in a bent fiber between modes with differing azimuthal symmetry even with a uniform UV-induced index perturbation. All of the primary experimental effects published thus far are well-described with this analysis. This improved understanding of bent LPFGs will be important for the development of devices and is also relevant whenever there is interaction with the cladding modes in a curved optical fiber.

Citation Download Citation

Ueyn L. Block, Vinayak Dangui, Michel J. F. Digonnet, and Martin M. Fejer "Source of apparent attenuation-notch splitting in long-period fiber grating bend sensors", Proc. SPIE 6004, Fiber Optic Sensor Technology and Applications IV, 60040G (10 November 2005); https://doi.org/10.1117/12.647832

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