FabryPérotInterferometer

The Fabry-Pérot interferometer is an optical instrument consisting of two highly reflective, partially transmitting mirrors facing each other to form an optical cavity. Light entering the cavity undergoes multiple reflections, producing interference that yields sharp transmission peaks at specific wavelengths. The device is used as a tunable, high-resolution spectral element in laboratory and astronomical settings.

Constructed as a pair of parallel plates or mirrors, the interferometer relies on constructive interference when the optical path between the mirrors satisfies a resonance condition. For light of wavelength λ, refractive index n in the cavity and incidence angle θ, constructive interference occurs when 2 n d cos θ = m λ, with m an integer. At normal incidence, this simplifies to 2 n d = m λ. The resulting transmitted spectrum consists of a series of closely spaced fringes whose spacing depends on the plate separation and refractive index.

Key metrics describe the performance of a Fabry-Pérot interferometer. The free spectral range, the wavelength separation between adjacent transmission peaks, is Δλ_FSR = λ^2 /(2 n d cos θ). The finesse, F, characterizes peak sharpness and is approximately F ≈ π sqrt(R)/(1−R) for mirror reflectivity R. The width of an individual transmission peak is roughly δλ ≈ Δλ_FSR / F. The resolving power, R_s ≈ m F, increases with higher finesse and larger order. The device can also function as a tunable narrowband filter by adjusting the cavity length.

Applications include high-resolution spectroscopy, astronomical observations, laser stabilization, metrology, and telecommunications as narrowband filters and wavelength references. The Fabry-Pérot interferometer was developed by Charles Fabry and Alfred Pérot in 1899 and remains a foundational tool in optical instrumentation.