Fabry-Perot Interferometer

This interferometer makes use of multiple reflections between two closely spaced partially silvered surfaces. Part of the light is transmitted each time the light reaches the second surface, resulting in multiple offset beams which can interfere with each other. The large number of interfering rays produces an interferometer with extremely high resolution, somewhat like the multiple slits of a diffraction grating increase its resolution.

Fabry-Perot Geometry

The Fabry-Perot Interferometer makes use of multiple reflections which follow the interference condition for thin films. The net phase change is zero for two adjacent rays, so the condition

represents an intensity maximum.

Fabry-Perot Resolution

A high-resolution interferometer, the Fabry-Perot Interferometer has a resolvance of

which means that the least separation of two spectral lines is given by

Resolvance examples

Calculation

This separation means that the two wavelengths satisfy the Rayleigh criterion. The interferometer can also be characterized by its free spectral range, the change in wavelength necessary to shift the fringe system by one fringe: