material dispersion coefficient [M()]
In an optical fiber, pulse broadening per unit length of fiber and unit spectral width, usually expressed in picoseconds per (nanometerkilometer).
Note 1: For many silica-based fibers, M(
) approaches zero at a specific wavelength 0 between
1. 3 
m and
1. 5 m. At wavelengths shorter than 0, M() is negative and increases with wavelength; at wavelengths longer than 0,, M() is positive and decreases with wavelength.
Note 2: Pulse broadening caused by material dispersion in a unit length of optical fiber is given by the product of M() and spectral width (
).
Note 3: The material dispersion coefficient M() is given by 
where 
is the refractive index of the material, N is the group index expressed as 
where is the wavelength of interest, and c is the velocity of light in vacuuo.