Beam propagation (m(2)) measurement made as easy as it gets: the four-cuts method.
Tolerance analysis shows that an efficient M(2) measurement plan is a first cut (beam diameter measurement) at 0.5-2.0 Rayleigh ranges to one side of the waist, which is matched by interpolation between second and third cuts to the opposite side. The waist is measured by a fourth cut halfway between the matched diameters, yielding an easy two-parameter curve fit for M(2).
Johnston
T F
TF