A waveguide structure for a TAMR head is disclosed wherein at least one detection waveguide is formed parallel to a main waveguide and located a gap distance therefrom. A light source transmits light into the main waveguide and towards an ABS/medium interface. A plasmon generator converts light from the waveguide into plasmon waves that are directed onto a magnetic medium. Back reflected light is captured by the main waveguide, partially diverted into a detection waveguide, and transmitted to a photo detector that measures light intensity (IB) which correlates closely to the plasmon wave intensity at the ABS/medium interface. A controller linked to the photo detector is employed to calculate IB as a function of ABS/medium spacing in a non-write condition and this relationship can be used to control and maintain a constant plasmon wave intensity at the ABS during a series of TAMR write processes with a plurality of media.