The distance along the axis over which a beam stays roughly collimated around its waist, ending where the radius has grown by the square root of 2. For a Gaussian beam it equals pi times the waist radius squared divided by wavelength, divided again by M-squared for real beams. Twice the Rayleigh range is the confocal parameter, the usable depth of focus in cutting, microscopy and nonlinear optics.
Tight focus gives us a 40 micron spot but only about a millimeter of Rayleigh range, so the height sensor has to be dead on.
Also heard as
- Rayleigh length
- zR
Related terms
Beam waist
The narrowest point of a focused or propagating laser beam, where its wavefront is flat.
Gaussian beam
A beam whose intensity falls off from the center as a bell curve, the shape of an ideal single-mode laser.
Divergence
How fast a laser beam spreads with distance far from its waist, given as an angle in milliradians.
M-squared
The beam quality factor: how many times worse a beam focuses and diverges than an ideal Gaussian of the same wavelength.