Guided Acoustic Waves in Standard Single-Mode Fibers

This chapter provides the analysis of acoustic modes that are guided by the structure of standard single-mode optical fibers. The elastic wave equation is solved in cylindrical coordinates, and solutions are categorized according to their azimuthal symmetry. The boundary conditions of a bare fiber in air are formulated in terms of a matrice of coefficients, whose elements depend on the order of azimuthal symmetry. Discrete solutions of the boundary condition equations signify the cutoff frequencies of guided acoustic modes. Expressions are provided for the normalized transverse profiles of material displacement in modal solutions. Emphasis is given to radially symmetric modes and to torsional-radial modes with twofold azimuthal symmetry. The transverse profiles of these specific mode categories can match those of electrostrictive force terms that may be induced by guided light in the fiber.