Material dispersion coefficient

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 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 (Δλ), where n is the refractive index of the material, N is the group index expressed as λ is the wavelength of interest, and c is the velocity of light in vacuum.

Source: from Federal Standard 1037C