i need to know what r normal and anamolous dispersion?
Normal and Anomalous Dispersion
One distinguishes normal dispersion (for k'' > 0) and anomalous dispersion (for k'' < 0). Normal dispersion usually occurs for transparent media in the visible spectral region. Great care is recommended when the sign of dispersion is specified, because the ultrafast optics community identifies this with the sign of k'', while the opposite sign is usually used in fiber optics, where the dispersion is often specified with the parameter
dispersion of fibers
which is normally given in units of ps/(nm km) (picoseconds per nanometer and kilometer). The different signs result from using a frequency derivative in one case and a wavelength derivative in the other case. Note also that the conversion factor depends on the wavelength.
Between wavelength regions with normal and anomalous dispersion, there is a zero dispersion wavelength. The region around this wavelength can be special in some respects, not only concerning weak dispersive pulse broadening.
To summarise Steve's post, normal dispersion occurs where shorter wavelengths travel slower than longer wavelengths. Anomalous dispersion occurs when shorter wavelengths travel faster than longer wavelengths.
The zero dispersion point for optic fibres is around 1550 nm, which is why most communications systems use this wavelength.
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