ture may be determined by monitoring the modal phase shift of an optical fiber. In this paper we present the results of a numerical model that has been developed to calculate the phase shift of a weakly guiding optical fiber due to thermal strain. Whenever an optical fiber is subjected to temperature changes, the optical path length, the index of refraction and the propagation constants of each fiber mode change. In consequence, the modal phase term, $beta$-in$/L, of the fields is also modified. A relationship for the modal phase shift is presented. This relation is applied to both single mode and two mode fibers in order to determine the sensitivity characteristics of fibers that are subjected to temperature changes.
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