Beamage-M² User Manual
Revision 6
15
4.1.2. Practical Measurement
In order to measure the M
2
factor, multiple slices of the beam within and beyond one Rayleigh Length
along the propagation axis must be considered (see
). For each one, the second order spatial
moment beam radius
𝑤(𝑧)
is measured. A hyperbola, which recalls the beam radius equation, is then
fitted with the results. The M
2
value is derived from that fit.
Since the distance range within which the measurements must be taken is too large (could be several
meters), the use of a focusing lens is mandatory. It is also mandatory to comply with ISO standard. It helps
to compress the slices of interest around the focal spot of the lens.
Please refer to ISO-11146 for more information.
Figure 4-2
: Positioning of the beam diameters when measuring a M
2
Left
: Five (5) beam diameters are taken in the focal region and three (3) beam diameters are taken
at the far field on each side of the caustic.
Right
: Five (5) beam diameters are taken in the focal
region and five (5) more beyond
2𝑧
𝑅
on one side only. The grey zone is the focal region, between
−𝑧
𝑅
and
+𝑧
𝑅
, the blue zones are for far field measurements and the white regions are the non-
essential measurement zones. Blue dots represent the position along the
Z
axis where beam
diameters are measured.
