PRIMES
BeamMonitor BM+ with LDS 2.98
79
Revision 01/2018 EN
23
Basis of beam diagnosis
23.1
Laser beam parameter
Raw beam diameter
Divergence angle
Focus diameter
Focal length f
z-axis
z-axis
Fig. 23.1: Sketch for the definition of beam parameters
23.1.1
Rotationally symmetric beams
According to ISO 11145 as well as ISO 11146 three beam parameters are necessary for the characterization
of a rotationally symmetric beam:
•
the z-position of the beam waist (focus) z
0
•
the diameter of the beam waist d
σ
F
•
the far field divergence angle
Θ
By means of these three values it is possible to determine the beam diameter at every spot along the propa-
gation direction. The following restriction is applicable: The divergence angle has to be smaller than 0.8 rad
and the focus diameter and the divergence angle were determined with the 2
nd
moment method.
Equation 1:
Furthermore, the beam propagation is described by means of the so called beam propagation ratio K.
Equation 2:
with:
K:
= beam propagation ratio
M
2
:
= beam propagation factor
λ
: =
wave length in a medium with the refractive index n
Θ
σ
: =
divergence angle
d
σ
0
: =
beam waist diameter
