material dispersion coefficient [M(λ)]

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(λ) approaches zero at a specific wavelength λ₀ between 1.3 m and 1.5 m. At wavelengths shorter than λ₀, M(λ) is negative and increases with wavelength; at wavelengths longer than λ_0,, M(λ) 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(λ) and spectral width (λ).

Note 3: The material dispersion coefficient M(λ) is given by

where η is the refractive index of the material, N is the group index expressed as

where λ is the wavelength of interest, and c is the velocity of light in vacuuo.