Internal spatial modes in glass microring resonators

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 effective index. The measured effective index and modal shape determines the values of all components of the wave vector. The observed standing modes inside the ring resonator are unexpected, but are caused by the standing modes in the coupling waveguide. Last, we describe a technique that can obtain detailed information about the locations of the dielectric interfaces.