In an optical fiber, pulse broadening per unit length of fiber and unit spectral width, usually expressed in picoseconds per (nanometerâ€¢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.