Optical interferometry

Interference converts phase differences between coherent light waves into intensity variations. When two electric fields overlap, their relative phase determines whether they add constructively or destructively; changing the optical path by one wavelength returns the fringe pattern to the same phase. A Michelson interferometer splits light into two arms and recombines the returning beams, while a Mach–Zehnder arrangement keeps the beams in separate paths before recombination. A Fabry–Pérot interferometer instead uses multiple reflections between partially reflecting surfaces, producing sharply wavelength-selective transmission peaks.¹

The measured quantity is usually an optical path difference, the product of physical distance and refractive index along each path. A displacement of one mirror changes a round-trip Michelson path twice as much as the mirror motion, whereas a transparent specimen changes phase through its thickness, refractive index, and wavelength. Coherence length, polarization, beam quality, vibration, and air turbulence determine whether fringes remain visible and interpretable.

Interferometers measure length, displacement, angle, wavelength, refractive index, and surface shape with exceptional precision. In dimensional metrology, a stabilized laser and a calibrated interferometer track motion in coordinate-measuring machines, lithography stages, and precision machine tools; wavelength is the link between fringe count and displacement. Fringe analysis can also recover the shape of an optical surface, the thickness of a film, or the deformation of a component under load.

Phase-sensitive methods distinguish changes smaller than a single visible fringe by estimating the phase of the intensity signal, although absolute distance may require fringe counting, multiple wavelengths, or an additional coarse measurement. Refractive-index measurements are especially sensitive to temperature, pressure, humidity, and gas composition, so environmental compensation is essential. In astronomy, stellar interferometry combines light collected by separated telescopes to obtain angular information finer than a single telescope aperture could provide.

Optical interferometry underpins optical testing, spectroscopy, microscopy, and gravitational-wave detection. In surface metrology, interferometric microscopes map smooth surfaces and thin films without physical contact; white-light systems extend the method to rougher samples by using short coherence length to localize the fringe envelope. Fourier-transform infrared spectrometers use an interferometer to encode spectral information into an interferogram, which is mathematically transformed into a spectrum.²

The largest instruments use interference as a way to sense changes in spacetime rather than ordinary displacement. The Laser Interferometer Gravitational-Wave Observatory compares kilometer-scale optical paths and detects minute differential changes produced by passing gravitational waves. In astronomy, optical and infrared arrays such as the Very Large Telescope Interferometer combine separated apertures, but atmospheric turbulence and the need for precise delay compensation make observations technically demanding.

Interferometric measurements are often limited more by systematic error than by photon noise. Thermal expansion, laser frequency drift, imperfect alignment, polarization mixing, scattered light, and refractive-index fluctuations can all imitate a physical signal. Vacuum beam paths, environmental sensors, active stabilization, common-path designs, and differential measurements reduce these effects. The phase ambiguity of a single-wavelength measurement is another edge case: two surfaces separated by an integer number of wavelengths can produce the same phase, so synthetic wavelengths or low-coherence ranging may be needed.

Holographic and electronic-speckle interferometry extend the technique to vibrating, rough, or diffusely reflecting objects. They reveal displacement fields by comparing recorded wavefronts before and after deformation, making them useful in nondestructive testing and structural mechanics. Quantum-enhanced interferometry investigates whether nonclassical light can improve sensitivity, but classical laser interferometers remain dominant in practical metrology because they are stable, bright, and comparatively easy to control.³

Optical path difference

The difference in distance weighted by refractive index between two interfering optical paths.

Coherence length

The distance over which a light source maintains a predictable phase relationship.

Fringe

A bright or dark band produced by constructive or destructive interference.

Phase shifting

A method that changes the relative phase between beams to recover surface or displacement information.

Synthetic wavelength

An effective wavelength formed by combining measurements at two nearby optical wavelengths to extend unambiguous range.