The far-field angle over which the beam expands, quoted as a half-angle or full angle in mrad. For a Gaussian beam the half-angle equals wavelength divided by pi times the waist radius, so smaller waists spread faster, and real beams spread M-squared times more. Expanding a beam before sending it a long distance lowers divergence, which is how beam expanders shrink far-field spots.
Divergence is 0.5 mrad full angle, so at the 20 meter target the spot's about a centimeter, too big for the detector.
Also heard as
- beam divergence
- far-field angle
Related terms
Beam waist
The narrowest point of a focused or propagating laser beam, where its wavefront is flat.
Rayleigh range
The distance from a beam's waist to where its cross-sectional area has doubled, a measure of depth of focus.
Beam expander
A pair of lenses arranged as a telescope that enlarges a collimated beam's diameter and cuts its divergence.
M-squared
The beam quality factor: how many times worse a beam focuses and diverges than an ideal Gaussian of the same wavelength.