Polytec OFV-303 User Manual

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Optical interference can be observed when two coherent light beams are
made to coincide. The resulting intensity e.g. on a photo detector varies with
the phase difference ∆ϕ between the two beams according to the equation
Equation C.1
The phase difference ∆ϕ is a function of the path difference ∆ L between the
two beams according to
Equation C.2
where λ is the laser wavelength.
If one of the two beams is scattered back from a moving object (the object
beam), the path difference becomes a function of time ∆ L= ∆ L(t). The interfer-
ence fringe pattern moves on the detector and the displacement of the object
can be determined using directionally sensitive counting of the passing fringe
pattern.
On scattering from the object the object beam is subjected to a small fre-
quency shift which is called Doppler shift f
D
and is a function of the velocity
component in the direction of the object beam according to
Equation C.3
Superimposing object beam and internal reference beam i.e. two electromag-
netic waves with slightly different frequencies generates a beat frequency at
the detector which is equal to the Doppler shift. The ratio C.3 to determine the
velocity is, however, independent of its sign. The direction of the velocity can
be determined by introducing an additional fixed frequency shift f
B
in the inter-
ferometer to which the Doppler shift is added with the correct sign. Thus the
resulting frequency at the detector f
mod
is given by
Equation C.4
Interferometers of this type which are directionally sensitive are described as
heterodyne.
I ∆ϕ()
I
max
2
---------
1 ∆ϕcos
+
()⋅
=
∆ϕ 2π
∆L
λ
-------
⋅
=
f
D
2
v
λ
-----
⋅
=
f
mod
f
B
2
v
λ
---
⋅+=

Polytec OFV-303 Specifications

General

Brand	Polytec
Model	OFV-303
Category	Measuring Instruments
Language	English