Rami Arieli: "The
Laser Adventure" Chapter 10 Section 1, page 3
Coherence Length
Large temporal coherence means that when we split a wave
in two parts, and one part travel a different distance from the other part,
when the two parts recombine they produce large mutual interference.
Coherence Length
(Lc) is the maximum path difference
possible for a specific temporal coherence, which still shows interference.
This specific temporal coherence
is related to specific linewidth (delta n)
by the formula:
Lc = c/(Delta n)
Delta n = Frequency linewidth
of radiation, in units of Hertz [Hz].
c = Speed of light in vacuum, in units of [m/s].
Conclusion from this
formula:
Narrowing the linewidth
of radiation increase its coherence length.
This is the reason why it is recommended
to use a single (longitudinal) mode laser in applications related to interference
(such as holography).