© 2011 Thorlabs
145Application Note
6.7.1
Focal and stage length calculation
Focal length
The generated beam waist diameter d
0
must not decrease below the minimal
measurable beam diameter of the Beam Profiler.
The beam waist diameter d
0
is:
To fulfill this requirement for each wavelength and the highest focusability (M
2
=1) the
divergence angle must not exceed a maximum value
max
.
Depending on initial beam size d
init
, a minimal focal length f can be calculated.
Refer to diagram A for quick selection help.
Translation stage length
For optimal M
2
detection the translation range should be at least 5 times the
Rayleigh length of the focused beam to cover both the beam waist and the
neighboring divergent beam propagation.
Minimal translation stage length is:
where is:
L
min
minimum required stage length
z
R
Rayleigh length for a non-gaussian beam with fixed divergence angle
M
2
highest expected M
2
value
operating wavelength
d
0
beam waist diameter
Refer to diagram B for quick selection help.
Besides this minimum requirement, more flexibility is added to the setup by
selecting a longer stage.
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