The refractive index or phase index of a material defines the ratio of the speed of light in the material with respect to the speed of light in a vacuum, c. This speed is the phase velocity, vp, defined by the second equation below. It depends on both the waveguide material and the wavelength of the propagating light.
The first equation below shows how the refractive index of a material, n, relates the wavelength of light in the medium, λ, to the vacuum wavelength, λ0:
This means that the phase velocity, vp, of light in a medium is lower than the speed of light in a vacuum, c. For monochromatic sources, where dispersion is not significant, the velocity of light in the fiber can be considered to be related directly to the refractive index.
Related Terms: Phase Index