What Are Interferometers?

Interferometers are instruments that exploit the wave phenomenon of interference to make high-precision measurements of distance, displacement, wavelength, refractive index, and surface geometry. When two or more coherent waveforms are combined, their superposition produces a fringe pattern whose spacing and phase encode information about the optical path length difference between the interfering beams. By tracking changes in that fringe pattern, interferometers achieve length measurements with uncertainties at the nanometer level and below, making them among the most precise measurement instruments available in any scientific or engineering discipline.

The underlying physics applies to any wave: light, radio waves, microwaves, sound, neutrons, and matter waves from atomic sources have all been exploited in practical interferometer designs. The choice of wave type determines the measurable length scales, the achievable resolution, and the sensitivity to environmental disturbances such as vibration, temperature gradients, and atmospheric turbulence.

Optical Interferometers

Optical interferometers divide a coherent light beam along two or more paths and recombine the paths to produce interference fringes. The Michelson configuration, foundational to the 1887 Michelson-Morley experiment, uses a beamsplitter to route light along two perpendicular arms; path-length changes as small as a fraction of a wavelength shift the fringe pattern, enabling displacement measurements with sub-nanometer resolution. Fabry-Pérot interferometers bounce light between two parallel reflecting surfaces to generate multiple-beam interference, producing extremely narrow transmission peaks used in laser cavities, optical filters, and high-resolution spectroscopy. Mach-Zehnder designs, which route beams along physically separated paths before recombining them, are widely used in fluid-dynamics visualization, integrated-optic modulators, and fiber-optic sensors. The NIST Length Scale Interferometer, operational since 1965, calibrates graduated length scales to expanded uncertainties below one nanometer, demonstrating the extreme precision the technique supports in national measurement infrastructure.

Microwave and Radio Interferometers

At radio and microwave wavelengths, interferometry is the basis for aperture synthesis in radio astronomy. By correlating signals received simultaneously at widely separated antennas, a radio interferometer synthesizes an effective aperture comparable to the baseline distance between those antennas. Very Long Baseline Interferometry (VLBI) extends this principle to intercontinental baselines, producing angular resolutions that surpass any single optical telescope. The NIST neutron interferometry program applies similar quantum-coherence principles at the atomic scale, measuring fundamental neutron properties. In addition to astronomy, microwave interferometers are deployed as non-contact sensors for vibration measurement, structural health monitoring, and atmospheric path-length sensing in radar systems.

Atom and Quantum Interferometers

Matter-wave interferometers use the de Broglie wave nature of neutral atoms, electrons, or neutrons in place of photons. Cold-atom interferometers split an atomic wave packet along two spatially separated paths using laser pulses, accumulate a quantum phase difference due to acceleration or rotation, and then recombine the packets to produce interference fringes whose phase directly encodes inertial forces. This approach underlies atom-interferometry gravimeters, which measure the local gravitational acceleration to fractional precisions below 10 parts per billion, and atom gyroscopes, which compete with laser ring gyroscopes for navigation applications. Gravitational-wave detectors such as LIGO operate as kilometer-scale Michelson interferometers sensitive to mirror displacements smaller than 10 femtometers, a range documented extensively in the Physical Review Letters announcement of the first gravitational-wave detection.

Applications

Interferometers have applications in a wide range of disciplines, including:

• Precision dimensional metrology and semiconductor lithography alignment
• Gravitational-wave observatories and tests of fundamental physics
• Radio astronomy and VLBI geodesy for Earth orientation measurements
• Optical coherence tomography (OCT) in medical imaging
• Fiber-optic sensing for structural health monitoring, temperature, and strain
• Inertial navigation using atom or laser gyroscopes