Anomalous effective strain-optic constants of nonparaxial optical fiber modes

We demonstrate that the experimental strain-optic coefficients for strong guided modes are not consistent with the accepted photoelastic theory. It is shown that for modes with significant nonparaxial components, such as modes guided by strong refractive index differences or in waveguides with dimensions that are much larger than the wavelengths used, the photoelastic theory should be modified to include the effect of the longitudinal components of the electromagnetic fields of the modes. Moreover, we highlight that the strain-optics coefficients depend on the state of polarization of the mode and provide a formula to calculate the necessary corrections.