We have investigated light propagation in optical devices by near-field scanning optical microscopy (NSOM) at the telecommunication wavelength of 1.55 &mgr;m. NSOM images obtained on the top of channel waveguides measure the mode profile perpendicular to the propagation direction and show a modulation of intensity along this direction. This modulation demonstrates the periodic variation of the mode size predicted for the propagation in small guides and marks the direction of propagation. We show that NSOM analysis can completely assess complex optical devices with subwavelength resolution: determination of the optical path, variation of the light intensity along this path, and measurement of local radiative losses. ©1998 American Institute of Physics.