Rayleigh length
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In optics, Rayleigh length is the distance along the propagation direction of the beam from the waist to the place where the area of the cross section is doubled.
For a Gaussian beam, the Rayleigh length is the distance from the beam waist, where the beam radius is increased by a factor 
. For a circular beam, this means that the mode area is doubled at this point.
A related parameter is the confocal parameter b, which is just two times the Rayleigh length.
