Step-index fiber normalized frequency ($V=2\pi a\sqrt{n_1^2-n_2^2}/{\lambda }_0$)

The dimensionless waveguide parameter $V$ combines radius, vacuum wavelength and the numerical aperture factor. In the weak-guidance scalar model the first additional spatial family appears at $V=j_{0,1}=2.404825\dots$, the first positive zero of a Bessel function of the first kind. The fundamental family has no positive cutoff. Its two polarizations are not two distinct spatial profiles.