Coherence length is important in holography and telecommunications engineering.
This article focuses on the coherence of classical electromagnetic fields.
In optical communications and optical coherence tomography (OCT), assuming that the source has a Gaussian emission spectrum, the roundtrip coherence length
If the source has a Gaussian spectrum with FWHM spectral width
It is important to note that this is a roundtrip coherence length — this definition is applied in applications like OCT where the light traverses the measured displacement twice (as in a Michelson interferometer).
In transmissive applications, such as with a Mach–Zehnder interferometer, the light traverses the displacement only once, and the coherence length is effectively doubled.
In long-distance transmission systems, the coherence length may be reduced by propagation factors such as dispersion, scattering, and diffraction.
Multimode helium–neon lasers have a typical coherence length on the order of centimeters, while the coherence length of longitudinally single-mode lasers can exceed 1 km.
Singlemode fiber lasers with linewidths of a few kHz can have coherence lengths exceeding 100 km.
Similar coherence lengths can be reached with optical frequency combs due to the narrow linewidth of each tooth.
[5] Cooling the low pressure sodium discharge to liquid nitrogen temperatures increases the individual D line coherence length by a factor of 6.
A very narrow-band interference filter would be required to isolate an individual D line.