  We describe a method to map the standing-wave pattern inside a Fabry-Perot
optical cavity with sub-wavelength resolution by perturbing it with a
commercially available scanning near-field optical microscope (SNOM) tip. The
method is applied to a fiber Fabry-Perot microcavity. We demonstrate its use to
determine the relative position of the antinodes at two different wavelengths.
In addition, we use the SNOM tip as a point-like source allowing precise
positioning of a microscope objective with respect to the cavity mode.

Optics;Instrumentation and Detectors;Quantum Physics