interferometer

An interferometer is a device that measures or exploits the interference of waves, typically light, to determine phase differences, path length differences, or small displacements with high precision. It relies on the coherence of the waves and stable optical paths to produce measurable interference fringes.

A common optical arrangement splits a source beam into two or more paths using a beam splitter, then routes and reflects the beams with mirrors before recombining them. The resulting intensity depends on the phase difference between the arms, which is proportional to the optical path length difference. Small changes in length or refractive index shift the fringes, enabling precise metrology.

Several well-known types exist. The Michelson interferometer uses two arms with end mirrors and a shared beam splitter, widely used in precision length measurements and in gravitational-wave detectors. The Mach–Zehnder interferometer splits and recombines paths without returning light to the source, useful for phase sensing and integration with samples. The Fabry–Pérot interferometer relies on multiple reflections between two parallel mirrors to create sharp spectral resonances. Sagnac interferometers, which send light around a loop in opposite directions, are sensitive to rotation. Variants and hybrids support spectroscopy, sensing, and imaging.

Applications span scientific and industrial fields. Optical interferometers enable high-precision length measurements, surface profiling, and Fourier transform infrared spectroscopy. In astronomy, optical and infrared interferometry improves angular resolution; radio interferometry in arrays like the VLA achieves high-resolution imaging by correlating signals from multiple antennas. Gravitational-wave observatories such as LIGO and Virgo employ long-baseline Michelson configurations with resonant cavities to detect minute spacetime distortions.

Interferometers require coherent light, careful alignment, and vibration isolation, but offer powerful sensitivity for measuring tiny optical and physical changes.