Fabry–Pérot Etalon & Cavity Transmission Calculator

A Cavity Between Two Mirrors

A Fabry–Pérot etalon is a spacer layer trapped between two partial mirrors. Light transmits efficiently only at wavelengths where the round-trip phase is a multiple of 2π, producing a comb of sharp transmission peaks. Here two thin silver mirrors enclose a SiO₂ cavity.

Resonance, Free Spectral Range and Finesse

Constructive interference of the multiply-reflected beams gives transmission maxima whenever 2 n d cosθ equals an integer number of wavelengths. The spacing between adjacent peaks is the free spectral range; the sharpness of each peak relative to that spacing is the finesse, set by the mirror reflectance R.

Thicker cavities pack more peaks into the band (smaller FSR); higher mirror reflectance (thicker silver) raises the finesse and narrows each peak — at the cost of lower peak transmission from metal absorption.

Interactive design tool

This page includes a live transfer-matrix calculator for this design. Adjust the parameters to recompute spectra, layer stack, and performance figures in the browser, then open the design in the full calculator.