interferometers

An interferometer is a device that splits a wavefront into two or more paths, then recombines them to produce interference. The resulting pattern depends on the relative phase of the waves, which in turn is determined by the optical path length difference, refractive index changes, or phase shifts along the paths. By analyzing the interference, one can infer very small changes in length, index, or phase with high precision.

Most interferometers split light with a beamsplitter, send the beams along separate arms, and then recombine them at a detector. In a two-beam setup, the output intensity I is I1 + I2 + 2√(I1I2) cos φ, where φ is the phase difference related to the path difference ΔL = nΔL and the wavelength λ. Coherence of the light source is important for stable interference.

Common configurations include the Michelson interferometer (two arms with a shared beamsplitter, widely used for precise length measurements), the Mach-Zehnder interferometer (two separate arms, often used in sensing and communications), Fabry-Pérot interferometers (a resonant cavity formed by two mirrors that enhances phase sensitivity), and the Sagnac interferometer (rotational sensitivity used in gyroscopes).

Applications span precision metrology, spectroscopy, vibration and displacement sensing, optical coherence tomography, and gravitational-wave detection. Notably, gravitational-wave observatories such as LIGO employ large, stabilized Michelson interferometers with resonant optical cavities to detect picometer-scale spacetime distortions over kilometers.

Interferometers require coherent light and are sensitive to environmental disturbances, including vibrations, temperature changes, and air currents, which are mitigated through isolation, stabilization, and careful calibration.