In an optical fiber, pulse broadening per unit length of fiber and unit spectral width, usually expressed in picoseconds per (nanometer•kilometer). 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.