2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/137336We address the long-standing unresolved problem concerning the V-parameter in a photonic crystal fiber (PCF). Formulate the parameter appropriate for a core-defect in a periodic structure we argue that the multi-mode cut-off occurs at a wavelength lambda* which satisfies V_PCF(lambda*)=pi. Comparing to numerics and recent cut-off calculations we confirm this result.3 pages including 2 figures. Accepted for Optics LettersOpticstext