)] )])]: In an optical fiber, pulse broadening per unit length of fiber and per unit of spectral width, usually expressed in picoseconds per (nanometer•kilometer). Note 1: For many optical fiber materials, 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 (
), 
where n is the refractive index of the material, N is the group index expressed as
is the wavelength of interest, and c is the velocity of light in vacuum.