Divergence is the angle the beam is expanding. It is usually in the units of milli-radians. (mrad)

Use this to determine the beam divergence given 2 beam diameter values at a given separation.

Calculate the focused spot size M = 1 for a perfect gaussian TEM00 beam

From...parallax-tech.com FAQ

What is the length of the beam in the area where the beam is less than 140% of the spot size.

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