Optical dispersion

In optics, dispersion (or optical dispersion) is a physical phenomenon that causes the separation of a wave into spectral components with different wavelengths, due to the dependence of the wave speed on the wavelength in the traversed medium. It is often described in light waves, but can occur in any type of wave that interacts with a medium or can be confined in a waveguide, such as sound waves. Dispersion is also called chromatic dispersion to emphasize its wavelength dependence. A medium that exhibits these characteristics with respect to the propagating wave is called dispersive.

There are generally two sources of dispersion: material dispersion, which results from the fact that the material’s response to the waves is frequency dependent, and waveguide dispersion, which occurs when the velocity of the wave in the waveguide depends on its frequency. The transverse modes of waves confined in a finite waveguide generally have different velocities (and field shapes), which depend on the frequency (i.e., the relative size of the wave, the wavelength, versus the size of the waveguide).

The dispersion in waveguides used for telecommunications results in signal degradation, since the different delay with which the different spectral components reach the receiver, “dirties” the signal over time or creates distortion. A similar phenomenon is the intermodal dispersion, caused by the presence of several modes in a guide at a given frequency, each of which has a different speed. A particular case is instead the polarization mode dispersion or PMD (polarization mode dispersion) that comes from the composition of two separate modes in polarization that travel at different speeds due to random imperfections that break the symmetry of the guide.

The dispersion of light in the glass of a prism is used to construct spectrometers and spectroradiometers. Holographic gratings are also used because they allow more accurate discrimination of wavelengths. Dispersion in lenses produces chromatic aberration, an undesirable effect that can distort images in microscopes, telescopes, and photographic objectives.

Given that substances transparent to light do not have the same refractive index for all monochromatic waves that make up a generic polychromatic incident beam, each light component is deflected differently from a prism made of the substance in question. The deflection undergone by radiation of a given wavelength in passing through a substance can be measured by the refractive index of that substance for that radiation.

The dispersion of a medium is measured by the Abbe number, also called the dispersive power, or constriction, or mean dispersion coefficient. Note, however, that sometimes dispersive power is defined as the inverse of Abbe’s number.

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