An inherent problem to the study of waveguides with strong propagation losses by Scattering-type Scanning Near field Optical Microscopy is the coherent optical background field which disrupts strongly the weak detected near-field signal. We present a technique of heterodyne detection allowing us to overcome this difficulty while amplifying the near field signal. As illustrated in the case of a highly confined SOI structure, this technique, besides the amplitude, provides the local phase variation of the guided field. The knowledge of the complex field cartography leads to the modal analysis of the propagating radiation.