THORLABS M2MS, Operating Manual

Page: 79 / 167

BP209 Series 7 Operation Instructions
Rev: 8.1, 14-Feb-2022 © 2022 Thorlabs Page 77
7.2 Beam Quality (M²) Measurement
The Beam Quality panel can be opened from the menu bar (Menu Windows ) or by clicking
the icon in the tool bar.
7.2.1 General
The M² value is an important measure of the beam quality. It is widely used in the laser industry
as a specification, and its method of measurement is defined in the ISO 11146 standard. It is
particularly useful to describe the degree of divergence and the focusability (minimum focus
diameter) of a real laser beam.
For more detailed information about Beam Quality, please see section M² Theory .
Beam Quality
Beam Propagation measurements according to the standard ISO 11146 describe the beam
quality using a single parameter, which is either the Times-Diffraction-Limit Factor M
2
or the
Beam quality K = 1/M
2
(also known as beam quality factor or beam propagation factor ).
Whereas the beam quality K is directly proportional to the quality level (K=1 optimal, decreasing
K stands for poorer quality), its reciprocal value M
2
(M
2
=1 optimal, increasing M for poorer
quality) is used more often.
Please do not confuse the beam quality and times-diffraction-limit factor (M
2
=1).
Diffraction Limit
Depending on the wavelength
l
and beam divergence angle
q,
the theoretical limit for the mini-
mum beam waist diameter d
0
is called the diffraction limit . The beam waist cannot be de-
creased beyond this value.
M
2
expresses how close the diffraction limit of the analyzed beam is to the diffraction limit of an
ideal Gaussian beam. The beam parameter product d
0
*
q
describes that mathematically. For
beams of less quality, the product d
0
*
q
is increased by the factor M
2
.
2
0 0
d θ M d θ ®
where d
0
is beam waist at the focus and q - the divergence angle.
M
2
is also known as
· the ratio of the waist diameter d
0
of the measured beam to that of an ideal Gaussian
beam (TEM
00
) at the same divergence angle q .
· the ratio of the divergence angle q of the measured beam to that of an ideal Gaussian
beam (TEM
00
) at the same waist diameter d
0
,
Lower beam quality results from laser imperfections like inhomogeneities which lead to appear-
ance of higher transverse modes.
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