Cut-off wavelength, λc, is defined in SMF as the shortest wavelength for which only the LP01 mode can propagate (single mode propagation). Light with the wavelength shorter than λc exhibits LP11 mode propagation (multi mode propagation). According to this equation, the cut-off wavelength for SMF is given by the following equation, that is, determined by the refractive index distribution and the core diameter.





The cut-off wavelength can be directly measured by bending method or multimode excitation method.