V-Number & Mode Calculator
Calculate V-number, number of guided modes, mode field diameter, and cutoff wavelength for step-index and graded-index optical fibers.
The V-number (normalized frequency) V = π d NA / λ is the fundamental dimensionless parameter governing how many modes a fiber guides. For V < 2.405 a step-index fiber supports only the fundamental HE₁₁ mode and is single-mode at that wavelength; above 2.405 it becomes multimode. This tool computes V from core diameter, numerical aperture, and wavelength — with presets for SMF-28 at 1310 nm and 1550 nm, and a 50/125 graded-index fiber at 850 nm. Outputs include single/multimode classification, mode count for multimode fibers (step-index: V²/2; graded-index: V²/4), mode field diameter via the Marcuse approximation (valid for single-mode fibers with 0.8 < V < 2.5), and cutoff wavelength — the wavelength above which the fiber transitions to single-mode operation.