Theoretical cut-off wavelength is the shortest wavelength at which only the fundamental (LP01) mode can propagate in a single-mode fibre. At wavelengths below the theoretical cut-off wavelength, several modes propagate and the fibre is no longer single-mode, but multimode. This parameter can be computed from the refractive index profile of the fibre. In optical fibres, the change from multimode to single-mode behaviour does not occur at an isolated wavelength, but rather smoothly over a range of wavelengths. Consequently, for determining fibre performance in a telecommunication network, theoretical cut-off wavelength is less useful than the actual threshold wavelength for single-mode performance when the fibre is in operation. Thus, a more effective parameter, called cut-off wavelength, has been introduced for single-mode fibre specifications