Examples of investigation and measurement

Contrast, interferogram
By displacing of one mirror by means of the linear displacement facility static and dynamic interference patterns can be generated and recorded. For dynamic measurements the screen is replaced by the photo detector which is directly connected to the oscilloscope. Advantageous is the use of a storage oscilloscope so that by moving the interferometer mirror, an interferogram can be produced as shown on the right side. From this curve, the contrast or visibility is derived. This type of detection allows also convenient readjustment of the components until an optimum contrast is reached.

Static interference pattern
From this type of interference pattern one gets information about the wave fronts. At this point, the discussion of Gaussian beams come into consideration. The influence of the radius of curvature of the interfering wave fronts becomes apparent. By adjustment of the telescope for beam broadening plane and curved wave fronts can be generated. The influence of the adjustment of the interferometer arms on the interference pattern can be demonstrated in a very impressive way.

Contrast function, coherence length
By measurement of the contrast as a function of the path difference of both interferometer arms, one gets the contrast function as an envelope of the interferogram. From this function the coherence length of the applied light source can be determined. The maximal coherence length is reached if the contrast becomes zero or reaches a technically realistic minimum. It particularly appears if the light source - here the HeNe-laser - has a two mode emission. In this case, the contrast function goes periodically through zero values as shown in the illustration on the right. The contrast function is recorded by measuring the contrast with the help of the photo detector and a storage oscilloscope for various path differences. The result shows that in general the measuring range of an interferometer is limited when using a two mode laser as light source. By measuring the spatial contrast function information on the coherence of the applied laser or light source can be determined.