THORLABS M2MS, Operating Manual

Page: 80 / 167

BP209 Series 7 Operation Instructions
Page 78
21424-D02
How to Measure Beam Quality
Ideal beam quality (K=1, M
2
=1) with a diffraction limited waist size is possible if only the funda-
mental mode TEM
00
(which has an ideal Gaussian shape) exists. The existence of higher
modes decrease beam quality which leads to larger waist diameters. Often, such distortions
can be easily discovered by looking at the non Gaussian beam profile.
But in many cases, several higher modes are distributed in a way that generate a nearly Gaus-
sian shape but the beam itself suffers from a bad beam quality.
The following example shows a beam with nearly perfect Gaussian shape but having a multi-
mode origin leading to bad quality M
2
= 1.79.
Note
A nearly Gaussian shape is not an indicator of high beam quality! Therefore, a single-shot
beam shape measurement from a Beam Profiler does not provide a correct information about
the beam quality.
Although a single Beam Profiler result is not a measure of beam quality, the Thorlabs Beam
Profiler Series can be used to accurately measure the beam quality. For this purpose, a beam
propagation measurement is carried out according to the ISO11146 standard. The key idea is
to measure the variation of beam diameter d(z) along the axis of beam propagation z.
When used with the M2MS measurement extension, the beam diameter and other parameters
are measured at several z-positions and stored by the software.
Besides learning the times-diffraction-limit factor M
2
, the Thorlabs Beam Propagation
measurement determines the following parameters of an optical beam:
·
beam waist width d
0x
, d
0y
·
beam waist z-position z
0x
, z
0y
·
Rayleigh range z
Rx
, z
R
y
·
divergence angle θ
x
, θ
y
·
waist asymmetry
·
divergence asymmetry