Encyclopedia … combined with a great Buyer's Guide!

Sponsoring this encyclopedia:     and others

Coherence – a Black-or-White Issue?

Posted on 2009-06-23 as a part of the Photonics Spotlight (available as e-mail newsletter!)

Permanent link: https://www.rp-photonics.com/spotlight_2009_06_23.html

Author: , RP Photonics Consulting GmbH

Abstract: Optical coherence of lasers and other light sources is often treated as a black-or-white issue, although it actually is not. The article discusses some important considerations.

Ref.: encyclopedia article on coherence; The Photonics Spotlight 2006-09-22

Dr. Rüdiger Paschotta

Lasers are often said to be special light sources because they emit coherent light, whereas other light sources are incoherent. However, is this really a “black and white” issue?

Beginning with spatial coherence, there can really be gradual differences. Many lasers exhibit nearly the highest possible degree of spatial coherence, which would imply perfectly well-defined wavefronts of the output. Others exhibit a substantially reduced degree of spatial coherence, which is associated with a reduced beam quality. Compared with some really “incoherent” light source such as an incandescent lamp, the spatial coherence is still very high – sufficient to form a well directed laser beam, even if its focusability is not as high as for some other laser beams. That big difference in the degree of coherence between even poor lasers and other light sources justifies the practice of calling lasers coherent light sources, in contrast to incoherent sources like bulbs.

Concerning temporal coherence, the situation is somewhat more complicated. Some very carefully stabilized single-frequency lasers, used as optical frequency standards in frequency metrology, exhibit an extremely high temporal coherence, associated with a linewidth below 1 Hz (to be compared with the mean frequency of hundreds of terahertz). Most other lasers have a temporal coherence which is lower by many orders of magnitude. The largest optical bandwidth is reached by some few-cycle mode-locked titanium–sapphire lasers (see also: ultrafast lasers), emitting octave-spanning spectra. Looking at this huge bandwidth, one might believe that such lasers have an extremely low level of temporal coherence – comparable with that of a light bulb. This, however, is not true; the issue has been discussed in an earlier Spotlight article. Essentially, one has to consider that the spectrum (viewed with sufficiently high spectral resolution) forms a frequency comb, consisting of very narrow equidistant lines. So we see not only that lasers exhibit very different levels of temporal coherence, but also that one misses essential aspects when trying to quantify the temporal coherence with a single number such as a coherence length or optical bandwidth.

As a final remark, optical processes and certain techniques are called coherent or incoherent, depending on whether they are sensitive to (relative) optical phases. This is really a black-and-white issue. For example, coherent beam combining works only with interferometric stability of mutual phase relations, whereas spectral beam combining is not sensitive to such phases, as it exploits differences in optical frequency.

This article is a posting of the Photonics Spotlight, authored by Dr. Rüdiger Paschotta. You may link to this page and cite it, because its location is permanent. See also the RP Photonics Encyclopedia.

Note that you can also receive the articles in the form of a newsletter or with an RSS feed.


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

How do you rate this article?

Click here to send us your feedback!

Your general impression: don't know poor satisfactory good excellent
Technical quality: don't know poor satisfactory good excellent
Usefulness: don't know poor satisfactory good excellent
Readability: don't know poor satisfactory good excellent

Found any errors? Suggestions for improvements? Do you know a better web page on this topic?

Spam protection: (enter the value of 5 + 8 in this field!)

If you want a response, you may leave your e-mail address in the comments field, or directly send an e-mail.

If you enter any personal data, this implies that you agree with storing it; we will use it only for the purpose of improving our website and possibly giving you a response; see also our declaration of data privacy.

If you like our website, you may also want to get our newsletters!

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