Optical resonators play an important role in amplifying laser beam. Fabry-Perot resonators ( etalons ) are optical cavities which are used for the purpose.
A simple Fabry-Perot etalon consists of two partially reflecting parallel mirrors with reflectances and separated by a length . For simplicity, consider an electromagnetic wave of amplitude incident normally on one of the faces of the etalon. The ray sufferes multiple reflection, bouncing to and fro between the mirrors. At each reflection, the field amplitude gets reduced by a factor or as the case may be.