Note that the spectral shape (i.e., the frequency dependence of the power spectral density of the optical field) is never changed by chromatic dispersion, since only phase changes are caused. Normal dispersion, for example, leads to a lower group velocity of higher-frequency components, and thus to a positive chirp ( instantaneous frequency rising over time), whereas anomalous dispersion creates negative chirps. Normal and Anomalous DispersionĬhromatic dispersion has an important impact on the propagation of light pulses, because a pulse always has a finite spectral width ( bandwidth), so that dispersion can cause frequency-dependent phase changes.Ĭonsequently, its frequency components propagate with different group velocities. The second-order dispersion, for example, is then given as the second derivative of β with respect to angular frequency. Tabulated index data are less suitable, since the numerical differentiation is sensitive to noise.įor light propagating in waveguides such as optical fibers, one considers the phase constant β instead of the wavenumber k. The dispersion of various orders for a medium can most conveniently be calculated if the refractive index is specified with a kind of Sellmeier formula. ![]() in numerical modeling to work directly with a table of frequency-dependent phase changes. It is therefore often more convenient e.g. Ultimately, the concept of Taylor expansion loses its value in this regime, where many dispersion orders have to be considered. When dealing with very broad optical spectra, one sometimes has to consider dispersion up to the fourth or even fifth and sixth order. Zero group delay dispersion is reached close to 1270 nm.ĭispersion of third and higher order is called higher-order dispersion. Mathematical Description of Chromatic DispersionĪs an example, the group delay dispersion of silica is +36 fs 2/mm at 800 nm, or −22 fs 2/mm at 1500 nm. The attribute “chromatic” is used to distinguish that type of dispersion from other types, which are relevant particularly for optical fibers: intermodal dispersion and polarization mode dispersion.Ĭhromatic dispersion can also occur from geometrical effects, for example see below the section on chromatic dispersion of optical components. That dependency results mostly from the interaction of light with electrons of the medium, and is related to absorption in some spectral regions see the article on Kramers–Kronig relations.Ī quantitative measure is the group velocity dispersion. The chromatic dispersion of an optical material is the phenomenon that the phase velocity and group velocity of light propagating in a transparent medium depend on the optical frequency. How to cite the article suggest additional literature More specific terms: normal and anomalous dispersion, material dispersion, waveguide dispersion, group delay dispersion, third-order dispersion, principal dispersion Using our ad package, you can display your logo and further below your product description.Īsk RP Photonics for any kind of calculations concerning chromatic dispersion and its effects in various situations.ĭefinition: the frequency dependence of the phase velocity in a transparent medium
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