Coherence Length Calculator
Calculate coherence length and coherence time from source linewidth or wavelength bandwidth.
Temporal coherence length L_c = c / Δν is the path-length difference over which an interferometer can observe fringes with appreciable contrast. It is set by the source linewidth Δν (in frequency) or equivalently by the spectral bandwidth Δλ at a center wavelength. This tool accepts input in either form — frequency linewidth (Hz to THz) or wavelength bandwidth (pm or nm at a specified center wavelength) — and returns coherence length, coherence time, and the equivalent representation in the other domain. Presets cover the full practical range: incandescent (~300 THz bandwidth, µm-scale coherence), LEDs (~15 THz), sodium lamp (510 GHz), multimode HeNe (1.5 GHz, ~20 cm), single-mode HeNe (500 kHz, ~600 m), DFB lasers (1 MHz), and narrow-linewidth fiber lasers (5 kHz, ~60 km).