Dispersion-free, multiple-beam interferometer.

A two-mirror Fabry-Perot interferometer is described haivng a dependence of transmission T˜ (ω) on frequency that is very different from the dependence T˜ (l) on the distance l between the mirrors. This feature is due to resonant dielectric mirrors in which the reflection phase and amplitude depend sharply on ω. The function T˜ (ω) can have several extrema ∂T˜ /∂ω = 0. At these points the interferometer becomes insensitive to a frequency change, whereas the dependence on l remains. Interferometer parameters are defined and some examples are considered. The dispersion-free interferometer can be used for measuring very small mechanical displacements with a light source with poor frequency stability. The applications to gravitational wave detectors and sensitive seismometers can be suggested if the small distance between the mirrors is acceptable.