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Fabry-Pérot Interferometer

Multiple-beam interference and transmission resonances

Etalon Schematic & Beam Paths

Transmission Spectrum

Ring Pattern

Finesse (ℱ): -
FSR (Δλ): -
FWHM (δλ): -
Peak transmission: -
Resolving power: -

The Fabry-Pérot Interferometer

Invented by Charles Fabry and Alfred Pérot in 1899, this device consists of two parallel, partially-reflecting mirrors (an etalon). Light bouncing between the mirrors creates multiple-beam interference, producing sharp transmission peaks at resonant wavelengths. The finesse (sharpness of peaks) increases dramatically with mirror reflectivity.

Transmission: T = 1 / [1 + (2ℱ/π)² sin²(δ/2)]  where δ = 4πnd/λ

Finesse: ℱ = π√R/(1-R) determines peak sharpness. At R=0.9, ℱ≈30. At R=0.99, ℱ≈300. The Free Spectral Range (FSR) is the wavelength spacing between peaks: Δλ = λ²/(2nd). Fabry-Pérot interferometers are used in spectroscopy, lasers, and telecommunications for wavelength filtering and measurement.