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of high optical quality and known focal length. To properly obtain M², the following steps must be followed:
# Measure the D4? widths at 5 axial positions near the beam waist (the location where the beam is narrowest).
# Measure the D4? widths at 5 axial positions at least one Rayleigh length away from the waist.
# Fit the 10 measured data points to where is half of the beam width, and is distance in the direction of beam propagation,

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