© 2011 Thorlabs
87Operating the Beam Profiler
3.5.3
Selecting Focal and Stage Length
Since the BC1M2 Extension Set does not include focusing elements, it has to be
selected and ordered separately.
The following guideline should help selecting the optimal focal length of the lens as
well as the optimal length of the translation stage.
Selecting the focal length
The following diagram A helps selecting the optimal focal length f depending on the
operating wavelength and the unfocussed beam diameter. See Focal and stage
length calculation for details.
Diagram A
Select your operating wavelength at the x-axis of diagram A and go up to the curve
representing your initial (unfocussed) beam diameter. Then read out the minimal
required focal length at the vertical axis. Round it up to the next available focal length
(100 mm, 150 mm, 200 mm, ...). Longer focal lengths generate waist diameters
above 100 µm and relax the measurement requirements to the Beam Profiler.
Refer to the 'Accessories' tab for recommended focusing lenses. Be sure to order
the appropriate AR-coating for your wavelength range of interest.
Selecting the translation stage length
It is advisable but not necessary to choose the translation stage length at least as
long as the focal length of the focusing lens. For a correct M
2
measurement the
translation range has to be at least 5 times the Rayleigh length of the focused beam
to cover both the beam waist and the neighboring divergent beam propagation. The
Rayleigh length depends strongly on the generated waist diameter and also linearly
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