RP Photonics logo
RP Photonics
Technical consulting services on lasers, nonlinear optics, fiber optics etc.
Profit from the knowledge and experience of a top expert!
Powerful simulation and design software.
Make computer models in order to get a comprehensive understanding of your devices!
Success comes from understanding – be it in science or in industrial development.
The famous Encyclopedia of Laser Physics and Technology – available online for free!
The ideal place for finding suppliers for many photonics products.
Advertisers: Make sure to have your products displayed here!
… combined with a great Buyer's Guide!
VLib part of the

Group Index

<<<  |  >>>

Definition: the ratio of the vacuum velocity of light to the group velocity in a medium

German: Gruppenindex

Category: general optics

Formula symbol: ng

Units: (dimensionless)

How to cite the article; suggest additional literature

In analogy with the refractive index, the group index (or group refractive index) ng of a material can be defined as the ratio of the vacuum velocity of light to the group velocity in the medium:

group index

For calculating this, one obviously needs to know not only the refractive index at the wavelength of interest, but also its frequency dependence.

The group index is used, e.g., to calculate time delays for ultrashort pulses propagating in a medium, or the free spectral range of a resonator containing a dispersive medium.

For crystals or glasses, the group index in the visible or near-infrared spectral range is typically larger than the ordinary refractive index, which determines the phase velocity. This implies that the group velocity is often (but not always) lower than the phase velocity.

refractive index of silica

Figure 1: Refractive index (solid lines) and group index (dotted lines) of silica versus wavelength at temperatures of 0 °C (blue), 100 °C (black) and 200 °C (red).

Note that for optical fibers and other waveguides, one uses the so-called effective refractive index instead of the ordinary refractive index in order to calculate the group velocity, since waveguide dispersion has to be taken into account. Based on that, an effective group index of a fiber could be calculated.

See also: group velocity, refractive index
and other articles in the category general optics

If you like this article, share it with your friends and colleagues, e.g. via social media: