| the quantity which characterizes the material dispersion defined as:|
n is the refractive index,
N is the group index,
λ is the wavelength in vacuum, and
c0 is the speed of light in vacuum
NOTE 1 – For many optical fibre materials, M is zero at a specific wavelength λ0, usually around 1300 nm. The sign convention is such that M is positive for wavelengths shorter than λ0 and negative for wavelengths longer than λ0.
NOTE 2 – Pulse broadening caused by material dispersion in a unit length of optical fibre is give by M times spectral linewidth (Δλ), except at or near λ = λ0, where terms proportional to (Δλ)2 are important.