Electrostrictive Stimulation of Guided Acoustic Modes in Standard Single-Mode Fibers

The stimulation of the guided acoustic modes of standard single-mode fibers by co-propagating optical field components is formulated. Effective stimulation requires that the electrostrictive force induced by optical waves and the displacement of the acoustic mode share the same temporal frequency and axial wavenumber. These conditions, in turn, are satisfied at acoustic frequencies that are close to the modal cutoff. The frequency dependence of modal stimulation follows a Lorentzian line shape. The strengths of the modal stimulation scale with the beating power between two optical field components. In addition, the stimulation efficiency scales with the overlap integral between the transverse profiles of force and acoustic displacement. The transverse symmetry of the electrostrictive force restricts the stimulation to guided acoustic modes that are either purely radial or torsional-radial with twofold azimuthal symmetry. The overlap between force and displacement is maximal for modes with cutoff frequencies in the range of 300–500 MHz. Electrostrictive stimulation is studied for pairs of optical tones that are co-polarized, of linear and orthogonal polarizations, and circular polarization. Stimulation by optical pulse envelopes of broad and continuous radio-frequency spectra is examined as well. Lastly, the linewidths of acoustic modal oscillations are related with the mechanical impedance of media outside the cladding boundary. The relation forms the basis of new optical fiber sensing concepts, based on guided acoustic waves.