PRIMES
BeamMonitor BM+ with LDS 2.98
46
Revision 01/2018 EN
Focus radius
The focus radius is the smallest beam radius in the caustic. Generally, this value is similar to the smallest
value measured.
Due to different reasons it may occur that the adaption to the measurement values was not carried out.
This is the case if the compensation curve does not lie close to the measurement values. In this case the
parameters of the adapted compensation curve are to be discarded.
The review function (please see chapter page 36) provides more information on this topic.
Rayleigh length
The Rayleigh length is a derived parameter and describes the distance in z-direction with regard to which
the beam radius has increased by the factor
2
(=1.41) and concerning which the beam area has increased
by the factor 2. The Rayleigh-length increases with the beam propagation ratio and the focal width of the
focusing optic (please see chapter 21 on page 76). The doubled Rayleigh length is an approximate point
of reference, up to which material thickness (metal) a procession is possible with the optic employed.
In order to make sure that the adapted values have a high significance, the measurement is to be carried
out in a z-range of at least two Rayleigh-lengths. A range of four Rayleigh lengths – as demanded in the
ISO 11146 would be even better. 5 to 6 Rayleigh lengths would be ideal. However, this demand is often
confronted with the problem of quickly sinking power densities of the laser beam which is to be measured.
In case of a distance of two Rayleigh-lengths from the focus the power density has sunk to just a quarter.
In this case the caustic measurement consists of a compromise between the desired measurement range
in z-direction and the power density (signal-to-noise ratio) necessary for a perfect measurement.
Fig. 11.15: Result window
Caustic>>Advanced
For the examination of asymmetric beams the dimensions of the main axes of the beam can be determined.
On the basis of these values the program also calculates direction dependent beam propagation factors as
well as beam position values. The related curves are shown via the two check boxes radius x, y while the
numerical values are provided by the detail menu.
