  We investigate the radiating resonances for the Helmholtz equation in the two
dimensional space in the presence of an unbounded waveguide with a high
contrast index of refraction. Using a suitable asymptotic analysis of the
Green's function of the problem, we describe when these radiating resonances
appear, and we exploit these resonances to identify the thickness, location,
and index of refraction of the waveguide. We propose a suitable numerical
reconstruction algorithm that requires observations in a multifrequency range
containing the first radiating resonance.
