A waveguide configuration comprising an optical core, an optical cladding, an acoustic core and an acoustic cladding. The acoustic core has two regions. The first region radial thickness is smaller than the optical core radial thickness and the sum of the first region radial thickness and the second region radial thickness is greater than the optical core radial thickness. The first region acoustic velocity is greater than the second region acoustic velocity and the acoustic cladding acoustic velocity is greater than the second region acoustic velocity. In one variation, the first region acoustic velocity is less than the second region acoustic velocity and the acoustic cladding acoustic velocity is less than the second region acoustic velocity.