Definition: the ratio of the vacuum velocity of light to the group velocity in a medium
In analogy with the refractive index, the group index (or group refractive index) can be defined as the ratio of the vacuum velocity of light to the group velocity in the medium:
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.
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).
See also: group velocity, refractive index
Category: general optics
The Encyclopedia of Laser Physics and Technology is also available in the form of a two-volume book.
Maybe you would enjoy reading it also in that form!
The print version has a carefully designed layout and can be considered a must-have for any institute library, laser research group, or laser company.
You may order the print version via Wiley-VCH.
RP Photonics is pleased to grant open access to the online version of this encyclopedia as a service to the whole photonics community.
In-House Staff Training
e.g. on fiber lasers, diode-pumped lasers, etc.:
tailored courses will boost the effectiveness of your team!
Laser Design Services
Fast and efficient laser development is possible with the competent design services of RP Photonics, based on extensive experience, deep scientific knowledge and advanced software.
Your Advertisement at This Place
will be seen by many thousands of visitors per month.
These banners receive far over 100'000 page views per month.
Check the details.
