Polytec PSV 300

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              

Optical interference can be observed when two coherent light Optical interference can be observed when two coherent light beams arebeams are

made to coincide. The resulting intensity e.g. on made to coincide. The resulting intensity e.g. on a photo detector varies witha photo detector varies with

the phase differencethe phase difference

∆ϕ∆ϕ

between the two beams according to the equationbetween the two beams according to the equation

Equation B.1Equation B.1

The phase The phase differencedifference

∆ϕ∆ϕ

is a is a function of the path differencefunction of the path difference

∆∆

L between theL between the

two beams according totwo beams according to

Equation B.2Equation B.2

wherewhere

λλ

is the laser wavelength.is the laser wavelength.

If one of the two beams is scattered back from a moving object (the objectIf one of the two beams is scattered back from a moving object (the object

beam), the path difference becomes a function of timebeam), the path difference becomes a function of time

∆∆

LL==

∆∆

LL (t). Th(t). Thee

interference fringe pattern moves on the detector and interference fringe pattern moves on the detector and the displacement of thethe displacement of the

object can be determined using directionally sensitive counting of the passingobject 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 smallsmall

frequency shift which is called Doppler shift ffrequency shift which is called Doppler shift f

and is a function of the velocityand is a function of the velocity

component in the direction of the object beam according tocomponent in the direction of the object beam according to

Equation B.3Equation B.3

SuperimposinSuperimposing object beam and g object beam and internal reference beam i.e. twointernal reference beam i.e. two

electromagnetielectromagnetic waves with c waves with slightly different frequencies generates a beatslightly different frequencies generates a beat

frequency at the detector which is equal to the Doppler shift. The ratiofrequency at the detector which is equal to the Doppler shift. The ratio B.3

B.3toto

determine the velocity is, however, independedetermine the velocity is, however, independent of its sign. The nt of its sign. The direction ofdirection of

the velocity can be the velocity can be determined by introducing an additional fixed frequencydetermined by introducing an additional fixed frequency

shift fshift f

in the in the interferometinterferometer to which the Doppler shift is added with theer 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 fdetector f

modmod

is given byis given by

Equation B.4Equation B.4

InterferometeInterferometers of this rs of this type which are directionally sensitive are described astype which are directionally sensitive are described as