Encyclopedia … combined with a great Buyer's Guide!

Sponsors:     and others

Group Index

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

Alternative term: group refractive index

German: Gruppenindex

Category: general optics

Units: (dimensionless)

Formula symbol: <$n_\textrm{g}$>


How to cite the article; suggest additional literature

URL: https://www.rp-photonics.com/group_index.html

In analogy with the refractive index, the group index (or group refractive index) <$n_\textrm{g}$> of a material can be defined as the ratio of the vacuum velocity of light to the group velocity in the medium:

$${n_{\rm{g}}} = \frac{c}{{{\upsilon _{\rm{g}}}}} = c\frac{{\partial k}}{{\partial \omega }} = \frac{\partial }{{\partial \omega }}\left( {\omega \;n(\omega )} \right) = n(\omega ) + \omega \;\frac{{\partial n}}{{\partial \omega }}$$

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, for example, for calculating time delays for ultrashort pulses propagating in a medium, or the free spectral range of a resonator containing a dispersive medium.

For optical crystals or glasses, the group index in the visible or near-infrared spectral range is typically larger than the ordinary refractive index: the group velocity is somewhat smaller than the phase velocity. In certain special (artificial) situations, one obtains dramatically reduced group velocities (→ slow light), i.e., a very large group index.

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

Questions and Comments from Users


I wonder what is determining the wavelengths of cavity resonances, e.g. in a silicon ring resonator – is it group index or the effective refractive index?

The author's answer:

In short: a combination of both!

The mode spacing (the frequency spacing of the resonator modes) is determined by the group delay for one resonator round-trip. For a silicon ring resonator, containing a waveguide, the group delay is proportional to the geometrical round-trip length and to the effective group index. Here, “effective” means that we do not simply take a material property, but an effective value calculated for the waveguide structure. Further, “group index” means that we do not simply calculate the effective refractive index, which is relevant only for the phase delay, but the group index, which is relevant for the group delay. That calculation involves the use of frequency derivatives of propagation constants.


Which kind of index should be used to calculate refraction angles in the crystal?

The author's answer:

The refractive index, not the group index.


In an ideal Fabry–Perot resonator with R = 1, infinitely narrow linewidth and filled with a dispersive medium, is the free spectral range still determined by the group index?

The author's answer:

Sure, it is!


Can I calculate the group refractive index knowing the effective refractive index, for example in a step index optical fiber can we derive the group index? Will the group index be (ncore2)/neff?

The author's answer:

No, the group index is not determined by the effective refractive index. Additional information would be needed concerning frequency derivatives.


When determining the length difference of one arm of an MZI interferometer needed to get from one intensity minimum to the next, I have deltaL = lambda/(2*n) – or should I use the group index instead?

The author's answer:

The normal refractive index is what counts here; we are not considering frequency derivatives but simply how fast the optical phase delay changes spatially. Anyway, usually we vary path length differences in air, where both <$n$> and <$n_\textrm{g}$> are close to 1.

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here; we would otherwise delete it soon. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

Your question or comment:

Spam check:

  (Please enter the sum of thirteen and three in the form of digits!)

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.


Share this with your friends and colleagues, e.g. via social media:

These sharing buttons are implemented in a privacy-friendly way!

Code for Links on Other Websites

If you want to place a link to this article in some other resource (e.g. your website, social media, a discussion forum, Wikipedia), you can get the required code here.

HTML link on this article:

<a href="https://www.rp-photonics.com/group_index.html">
Article on Group index</a>
in the <a href="https://www.rp-photonics.com/encyclopedia.html">
RP Photonics Encyclopedia</a>

With preview image (see the box just above):

<a href="https://www.rp-photonics.com/group_index.html">
<img src="https://www.rp-photonics.com/previews/group_index.png"
alt="article" style="width:400px"></a>

For Wikipedia, e.g. in the section "==External links==":

* [https://www.rp-photonics.com/group_index.html
article on 'Group index' in the RP Photonics Encyclopedia]