---
title: "Fabry–Perot Resonator calculator — AperiOptic"
description: "Free spectral range and finesse of an etalon. Free online optics calculator, with the formula and code snippets."
url: "https://aperioptic.com/calculators/fabry-perot-resonator"
---

# Fabry–Perot Resonator calculator

Free spectral range and finesse of an etalon.

Category: [Spectroscopy](https://aperioptic.com/calculators)

Formula: `FSR = c/(2·n·L) · ℱ = π/(2·asin((1−R)/(2√R))) ≈ π√R/(1−R) · δν = FSR/ℱ`

## Inputs

| Input | Parameter | Unit | Default |
| --- | --- | --- | --- |
| Length L | `L` | mm | 10 |
| Index n | `n` |  | 1 |
| Mirror R | `R` |  | 0.95 |

## Result with the default inputs

- FSR: 14.9896 GHz
- Finesse: 61.2342
- Linewidth: 244.7917 MHz

Open it with your own values: put each input in the address as a parameter, for example [`/calculators/fabry-perot-resonator?L=10`](https://aperioptic.com/calculators/fabry-perot-resonator?L=10). A unit other than the listed one goes in `<parameter>_unit`.

Read more: [Wikipedia: Fabry–Pérot interferometer](https://en.wikipedia.org/wiki/Fabry%E2%80%93P%C3%A9rot_interferometer)

[All optics calculators](https://aperioptic.com/calculators)

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The interactive page: https://aperioptic.com/calculators/fabry-perot-resonator (AperiOptic runs in the browser; free, no account). Every page of the site as Markdown: https://aperioptic.com/llms.txt