A method fabricates an interferometric tapered waveguide (ITWG) for a heat-assisted magnetic recording (HAMR) transducer. The ITWG is defined from at least one waveguide layer. The waveguide layer(s) include an energy sensitive core layer. The energy sensitive core layer has an index of refraction that varies in response to exposure to energy having a particular wavelength range. The step of defining the ITWG includes defining a plurality of arms for the ITWG. At least one phase difference between the arms is determined. At least one of the arms is exposed to the energy such that the index of refraction of the energy sensitive core layer in the arm(s) is changed and such that the phase difference(s) between the arms is changed.