### Question

What is the coherence length? What is the coherence time?

Asked by: Brian Wo

### Answer

Coherence length is the space over which a wave is 'nicely' sinusoidal. In other
words, it is the space over which we can predict the phase of a wave of beam (if we
are speaking of lasers). Coherence time is related to the classical uncertainty
principle and is thus a little more complex. It relates the bandwidth (spread in
frequency) of signal or wave to its temporal extent df = 1/dt. Thus phenomena with
long coherence times will be sharply peaking with respect to their spectrum, i.e.,
they will be composed of less frequencies. The limit of this is an infinite
coherence time, which would mean the signal is composed of a singular frequency.

Answered by: Seth Jonas, B.S., Physics Undergrad, UCF, Orlando FL

Coherence is the degree to which electromagnetic radiation maintains a
near-constant phase relationship, both temporally and spatially. The time over
which the phase relationship remains nearly constant is called the Coherence time.
Which is pretty logical if you think about it. The coherence time is approximately
equal to 1/delta v, where delta v, is the bandwidth of the source.

Therefore it follows that the path length corresponding to the coherence time is
called the coherence length.

Answered by: David Balson, Ph.D., String Theorist, PPARC, United Kingdom