This CalcTown Calculator calculates the longitudinal propogation factor and phase velocity of an EM wave, which is propagating through an optical fibre.

*Please note that phase velocity can exceed the speed of light in vacuum, but it does not violate the special theory of relativity.

θ_{i} is the angle the ray makes with the fiber axis, before entering the fiber,

k_{0 }is the wavenumber of the EM wave in vacuum,

n is the refractive index along the central axis of the fiber,

v_{g }is the group velocity of the EM wave.

and frequency is related to k_{0 }by the equation c=2πf/k_{0}