The simultaneous detection of TE- as well as TM-polarized light with a photon scanning tunneling microscope leads to a quasi- interference pattern of these mutually perpendicular polarized fields. This interference pattern has been observed in the optical field distribution as a function of both position and wavelength. Comparison of experimental data with simulations confirms the interference of mutually orthogonal fields. This quasi interference is caused by conversion of the linearly polarized light of both modes into elliptically polarized light by a fiber probe. (C) 2000 Optical Society of America.