Coherence length is the maximum distance a wave can travel while maintaining temporal coherence. It is influenced by factors such as wavelength and scattering. Applications include holography and telecommunications.
In optics, which is the branch of physics that deals with light and its properties, the coherence length (CL) is the maximum distance that a ray of light or other electromagnetic phenomenon can travel while maintaining a certain degree of temporal coherence . Time coherence refers to the sinusoidal shape of a traveling wave and the ability to predict where in its phase a wave will be at a specific moment in time. If light is coherent, it stays in phase with itself. Consequently, some texts also refer to the coherence time, which is the coherence length divided by the speed of light.
The coherence length is influenced by many factors: the purity and power of the light used, the specific wavelength, the presence of potential scattering and diffraction. Although the term “coherence length” is used primarily in optics, many of the concepts of optics have been generalized to any situation involving the propagation of waves, such as radio waves, sound waves, and compression waves. It is also used in discussions of superconductivity, possibly because electrons can also be viewed as waves under certain conditions.
A significant application of coherence length is holography, the recording and re-creation of three-dimensional images. Holography works by capturing the interaction between two laser beams: a reference beam and an object beam. The coherence length of the laser used is the maximum path difference that can be allowed between the beams, so it serves as a limit to the depth of the hologram that can be recorded. For a common five-milliwatt helium neon laser, this CL is limited to about 6-8 inches (15.2-20.3 cm).
Another application of coherence length is in telecommunications, the transmission of messages over an electromagnetic signal. Here the CL is the maximum distance a message can be sent without being retransmitted in some way. For radio waves, the length can be approximated by dividing the speed of light through that medium by the bandwidth of the signal. Interference, scattering and diffraction can reduce this range. For optical communications, CL is directly proportional to the square of the central wavelength of the source and inversely proportional to the refractive index of the medium used and the spectral amplitude of the signal.
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