Michelson Interferometer Coherence Calculator
Optical metrology & coherence optics: Calculate temporal coherence length (L_c), Optical Path Difference (OPD), interference fringe visibility contrast (V), and phase shift.
Light Source & Arm Displacements
Coherence & Interference Visibility
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Frequently Asked Questions
What is temporal coherence length in interferometry?
Coherence length (L_c = λ₀² / Δλ) is the propagation distance over which a light wave maintains a predictable phase relationship. In a Michelson interferometer, if the Optical Path Difference (OPD = 2|d₁ - d₂|) between the two arms exceeds L_c, waves arriving at the detector cannot interfere constructively or destructively, washing out all fringes.
Why can white light interfere in a Michelson interferometer only at zero path difference?
White light has an enormous spectral bandwidth (Δλ ≈ 300 nm), resulting in a microscopic coherence length of only 1 to 3 micrometers. Fringes only appear when the two interferometer arms are matched within a few microns, forming the basis of Optical Coherence Tomography (OCT) depth sectioning.
How does a Michelson interferometer achieve nanometer-scale displacement measurements?
Moving one of the mirrors by a distance of exactly half a wavelength (λ / 2) changes the round-trip Optical Path Difference by one full wavelength (λ), shifting the interference pattern by exactly one bright-to-dark-to-bright fringe cycle. Counting fringes provides direct sub-micron linear displacement calibration.