PRIMES
BeamMonitor BM+ with LDS 2.98
81
Revision 01/2018 EN
23.2
Calculation of beam data
For the calculation of the beam data not only the algorithms for the 2nd moment method are implemented as
demanded by the ISO standard 11145 but also the 86 % method which is widely-spread within the industry.
For the Gaussian TEM00-mode both methods offer similar results whereas in case of the majority of other
laser beams the 2
nd
moment method calculates bigger beam diameters than the 86 % method.
Laser radiation often is a mixture of different modes with different frequencies and coherent characteristics.
All known measuring procedures only provide little information on the beam. Therefore the calculated beam
parameters are always dependent on the measuring procedure. For the interpretation of the measuring
results it is important to be aware of this fact.
The calculation of the beam radius requires the following to preparatory steps:
1.
Measurement of the power density distribution
2.
Determination of the zero level
3.
Determination of the beam position
23.2.1
Determination of the zero level
The zero level can – for instance – be determined by means of a histogram by applying the frequency of the
measured power density values (please see Fig. 23.3).
•
Zero level of the signal
Frequenc
y (au)
Fig. 23.3: Schematic histogram of the scanned measuring points
The histogram shows how frequently a certain power density was measured. The maximum of this curve
indicates the power density of the zero level. The power density is deducted from all measured values of
the power density distribution.
It is important to measure the zero level accurately because even the slightest error would lead to a drastic
change as far as the volume is concerned. This in turn has a great impact on the measured beam radius.
