Material dispersion coefficient
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Material dispersion coefficient [ M ( λ )]: In an optical fiber, pulse broadening per unit length of fiber and per unit of spectral width, usually expressed in picoseconds per (nanometre·kilometre).
Note 1: For many optical fiber materials, M (λ) approaches zero at a specific wavelength λ0 between 1.3 μm and 1.5 μm. At wavelengths shorter than λ0, M (λ) is negative and increases with wavelength; at wavelengths longer than λ0,, 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 (Δλ),
where
n is the refractive index of the material
N is the group index expressed as λ is the wavelength of interest
c is the velocity of light in vacuum.
Source: from Federal Standard 1037C