Forward Brillouin Scattering in Polarization-Maintaining Fibers

Polarization-maintaining fibers support guided acoustic modes that are more complex than those of single-mode fibers, due to the presence of strain rods. Forward Brillouin scattering interactions in those fibers can be intra-modal as well as inter-modal. Intra-modal interactions involve the stimulation of guided acoustic waves by a pair of optical fields that co-propagate in a common principal axis. The intra-modal processes can lead to phase modulation of co-polarized optical probe signals, similar to standard single-mode fibers. The forward Brillouin scattering spectra differ between the two axes. In addition, acoustic modes stimulated through an intra-modal process in one axis may also modulate a probe wave in the orthogonal axis. Such inter-polarization cross-phase modulation is analogous to the dynamic gratings of backward Brillouin scattering in polarization-maintaining fibers. In inter-modal forward Brillouin scattering, two optical fields of orthogonal polarizations stimulate the oscillations of a guided acoustic wave. The process can lead to the switching and coupling of probe waves of specific frequencies from one principal axis to the other. Such coupling is non-reciprocal: probe waves that counter-propagate with respect to the initiating optical fields undergo polarization switching, whereas co-propagating probe waves of the same frequencies are unaffected. The switching of counter-propagating probe waves is useful for the spatially distributed analysis of forward Brillouin scattering spectra.