Optical fibers are used in smart structures mainly as sensor elements and communication links. Fiber Bragg grating (FBG) based sensors have become the most popular ones among all the fiber sensors. For distributed measurement, the sensing network is required to cover as much area as possible. Also, the sensors are required to be placed as closely as possible to achieve a better surface-scanning. These together necessitate the need for a number of sensors to be incorporated in one fiber line, something restricted in practice by the individual sensor response and the source bandwidth. In this paper, a theoretical analysis of Bragg gratings, in isolation and in an array is presented. An existing theory of FBG has been revisited with a view to reduce the sensor's bandwidth. Effect of inclusion of second derivatives and cladding modes in the coupled mode theory used here is explained. For a typical 4% Ge doped single mode fiber, FWHM of a grating is found to be 1.7 nm as against 3.7 nm, when no such modification is done. Uniform grating array is preferred over the non-uniform type, from the fabrication and signal processing point of view. But for an array consisting of a large number of uniform gratings, addressing each sensor element becomes the problem. An array of non- uniform cluster of uniform gratings can be a good remedy for this. The number of gratings in one cluster is a trade-off between the measurement and signal-processing ease.

Date Published: 14 November 1997
PDF: 10 pages
Proc. SPIE 3241, Smart Materials, Structures, and Integrated Systems, (14 November 1997); doi: 10.1117/12.293508

Published in SPIE Proceedings Vol. 3241:
Smart Materials, Structures, and Integrated Systems
Alex Hariz; Vijay K. Varadan; Olaf Reinhold, Editor(s)