B-2
B Basics of the Measurement Procedure
Thus the resulting frequency at the detector f
mod
is given by
Equation B.6
Interferometers of this type which are directionally sensitive are described as
heterodyne.
B.2 Optical Configuration in the Scanning Head
In vibrometers from Polytec, velocity and displacement acquisition are carried
out using a modified Mach-Zehnder interferometer.
The optical configuration in the scanning head is shown schematically in the
following figure.
Figure B.1:
Optical configuration of the interferometer in the scanning head
The light source is a helium-neon laser which provides a linear polarized
beam. The BS1 beam splitter splits the beam into the object beam and the
reference beam.
The object beam passes through the polarizing BS2 beam splitter as well as a
λ
/ 4 plate, is then focused by the lens on the object and scattered back from
there. The polarizing BS2 beam splitter then functions as an optical
directional coupler together with the
λ
/ 4 plate, and deflects the object beam to
the BS3 beam splitter. The interference signal is created from the optical path
difference between reference and laser beam. The distance to the object goes
into the optical path difference with a factor of 2. As an option the objective
displays the object on the camera. If the spot diameter on the object is
minimal, the video image is sharply focused.
The Bragg cell in the reference arm of the interferometer generates the
additional frequency offset to determine the sign of the velocity.
The resulting interference signal of the object beam and the reference beam
is converted into an electrical signal in the photo detector and subsequently
decoded in the controller.
B
S
1
L
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B
S
2
R
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f
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Beam
Obj
e
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Beam
Sc
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m
irrors
Pris
m
B
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gg C
e
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B
S
3
Obj
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D
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L
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l/4
Summary of Contents for PSV-500
Page 10: ...1 6 1 Safety Information...
Page 14: ...2 4 2 Introduction...
Page 50: ...4 8 4 Making Measurements...
Page 104: ...C 2 C Declaration of Conformity...
