Planar Etalon Theory
The equation for the transmission of an ideal etalon, an Airy Function, is
If any phase change at the mirror surfaces is ignored then
The above figure plots the etalon spectral transmission. The distance between adjacent peaks is the Free Spectral Range (D) and the width (FWHM) of each peak is the resolution (d). The Free Spectral Range can be written three ways:
Another useful concept for etalons is the finesse (F). This dimensionless parameter is the ratio of the free spectral range to the peak width.
For an ideal etalon, only the mirror reflectivity determines the finesse.
Imperfections in the etalon such as not-perfect flatness and parallelism will degrade the finesse. TecOptics includes the various imperfections into our etalon design calculations so we guarantee all quoted specifications.