Polytec PSV 300, Руководство по оборудованию

Страница: 85 / 105

   
                                                      
           
                  
Optical interference can be observed when two coherent light Optical interference can be observed when two coherent light beams are beams are
made to coincide. The resulting intensity e.g. on made to coincide. The resulting intensity e.g. on a photo detector varies with a photo detector varies with
the phase difference the phase difference
∆ϕ ∆ϕ
between the two beams according to the equation between the two beams according to the equation
Equation B.1 Equation B.1
The phase The phase difference difference
∆ϕ ∆ϕ
is a is a function of the path difference function of the path difference
∆ ∆
L between the L between the
two beams according to two beams according to
Equation B.2 Equation B.2
where where
λ λ
is the laser wavelength. is the laser wavelength.
If one of the two beams is scattered back from a moving object (the object If one of the two beams is scattered back from a moving object (the object
beam), the path difference becomes a function of time
beam), the path difference becomes a function of time
∆ ∆
L L = =
∆ ∆
L L (t). Th (t). The e
interference fringe pattern moves on the detector and interference fringe pattern moves on the detector and the displacement of the the displacement of the
object can be determined using directionally sensitive counting of the passing object can be determined using directionally sensitive counting of the passing
fringe pattern.
fringe pattern.
On scattering from the object the object On scattering from the object the object beam is subjected to a beam is subjected to a small small
frequency shift which is called Doppler shift f frequency shift which is called Doppler shift f
D D
and is a function of the velocity and is a function of the velocity
component in the direction of the object beam according to component in the direction of the object beam according to
Equation B.3 Equation B.3
Superimposin Superimposing object beam and g object beam and internal reference beam i.e. two internal reference beam i.e. two
electromagneti electromagnetic waves with c waves with slightly different frequencies generates a beat slightly different frequencies generates a beat
frequency at the detector which is equal to the Doppler shift. The ratio frequency at the detector which is equal to the Doppler shift. The ratio B.3 B.3 to to
determine the velocity is, however, independe determine the velocity is, however, independent of its sign. The nt of its sign. The direction of direction of
the velocity can be the velocity can be determined by introducing an additional fixed frequency determined by introducing an additional fixed frequency
shift f shift f
B B
in the in the interferomet interferometer to which the Doppler shift is added with the er to which the Doppler shift is added with the
correct sign. Thus the resulting frequency at the correct sign. Thus the resulting frequency at the detector f detector f
mod mod
is given by is given by
Equation B.4 Equation B.4
Interferomete Interferometers of this rs of this type which are directionally sensitive are described as type which are directionally sensitive are described as
heterodyne. heterodyne.
II
∆ϕ ∆ϕ ( ( ))
II
ma ma x x
2 2
--------- ---------
1 1
∆ ∆ϕ ϕ
cos cos
+ +
( ( )) ⋅⋅
= =
∆ϕ ∆ϕ
2 2
π π
L L
∆ ∆
λ λ
------ -------- ⋅⋅
= =
ff
D D
2 2
v v
λ λ
----- ----- ⋅⋅
= =
ff
mo mo d d
ff
B B
2 2
v v
λ λ
------ ⋅⋅
+ + = =