Leaky Modes in Weakly Guiding Fiber Acoustic Waveguides

The analysis of leaky modes in a fiber acoustic waveguide with small differences between the acoustic parameters (Vr-, Vs, p) of its core and infinitely thick cladding is presented. These modes become ideally guided if both the shear-velocity and density differences vanish. The propagation constant of leaky modes is complex and has a small imaginary part that accounts for the attenuation of acoustic fields and power along the fiber. Aside from the attenuation coefficient, the trans- mission properties of leaky modes such as phase and group velocities, cutoff frequencies, and power flow are essentially the same as those of ideally guided modes that are independent of shear wave velocities. Furthermore, all modes are predominantly longitudinal if the differ- ence between the longitudinal velocities of the core and cladding is small. Their phase velocities lie between the two longitudinal velocities and they may not be of torsional or Stoneley wave types. The atten- uation coefficient, however, depends strongly on the shear velocity dif- ference t = lVSz - V~ssll/Vs, and is proven to be of U(t2) (subscripts 1 and 2 refer to the core and cladding regions, respectively). Waveguide materials and aspects of coupling techniques for longitudinal modes are also discussed.