A numerical model was developed to calculate the modal phase shift of a circular step index profile weakly guiding optical fiber under axial strain. Whenever an optical fiber is under stress, the optical path length, the index of refraction, and the propagation constants of each mode change. In consequence, the phase of each mode is also
modified. A relationship for the modal phase shift is presented. This relation is applied to bpth single-mode and two-mode fibers to determine the sensitivity characteristics of strained fibers. It was found that the phase shift is strongly dependent on the core refractive index nco. It was also found that it is possible to design fibers that are insensitive to axial strain. Practical applications of strain-insensitive fibers are discussed.