Photoelastic Perturbations to the Dielectric Tensor Due to Guided Acoustic Modes

The photoelastic perturbations to the dielectric tensor in standard single-mode fibers, due to the oscillations of guided acoustic modes, are studied and formulated. Material displacement of acoustic modes is associated with local strain in every point within the fiber cross section. Strain, in turn, gives rise to dielectric perturbations, which scale with the magnitude of the acoustic modal displacement. The photoelastic perturbations propagate along the fiber axis with the frequency and wavenumber of the acoustic wave and may scatter and modulate optical fields. The effect of the photoelastic perturbations on guided light depends on the spatial overlap between their transverse profile and that of the optical mode. The position-averaged perturbations associated with radial modes are scalar, and their effect on guided light is independent of polarization. Torsional-radial acoustic modes, on the other hand, induce photoelastic birefringence with principal axes that follow those of acoustic displacement. Photoelastic perturbations are the most efficient for acoustic modes with cutoff frequencies between 300 and 500 MHz.