  At a transition in a wave-guiding structure, part of the incident energy is
transmitted and part of the energy is reflected. When the waveguide has
non-trivial topological properties, however, the transition may occur with no
backscattering, and with unusual modal coupling/transformations. Within this
context, we discuss the response of a nonreciprocal topological structure
composed of two nearby interfaces between oppositely-biased gyrotropic media
and an isotropic medium, which support unidirectional surface modes
(topological modes). We provide an exact Green's function analysis of this
structure, and we discuss how the topological surface modes are modified when
the two interfaces are brought closer and eventually merged. We show that the
resulting mode conversion is independent of the transition geometry.
