We have used near-field scanning optical microscopy to measure the internal
optical modes inside glass waveguides and microring resonators, The period
of the observed standing modes provides a direct measure of the local effe
ctive index. The measured effective index and modal shape determines the va
lues of all components of the wave vector, The observed standing modes insi
de the ring resonator are unexpected, but are caused by the standing modes
in the coupling waveguide. Last, we describe a technique that can obtain de
tailed information about the locations of the dielectric interfaces.