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.