Michelson Interferometer

Already in 1881, A. A. Michelson constructed an interferometer, which later also got his name, to prove successfully counter the theory of an universal ether assumed to exist at that time. Later on, he determined with this set-up the length of the basic meter in units of light wavelengths. Still, the promising use of interferometers in performing technical length measurements only reached significance after the discovery of the laser as a coherent light source.
Today, this contact less working high precision length measuring instruments have become an important tool for many areas in the machine building industry like adjustment, final control, incremental displacement measurement for CNC machines, the control of machine tools and for calibration procedures. With the newest laser, interferometers resolutions up to the nanometer range can be realised. The arrangement of the optical components has changed with regard to the original Michelson interferometer by the use of lasers as light sources. But with some exceptions, generally the two beam arrangements of Michelson is used. Within the frame of this experiment first the classical interferometer is set-up and the interference pattern are observed on a screen. To understand the observed interference pattern, the properties of Gaussian beams, wave fronts, radii of curvature and the superimposition of waves are discussed in the theoretical part of the manual.
Starting with a simple model of monochromatic radiation, the spectral bandwidth of a light source will be considered and the influence on the contrast of the interferometer discussed. The coherence length is introduced, defined and measured.
The applied HeNe-laser emits two orthogonally polarised modes with a coherence length of about 18 cm. In the second step, the Michelson set-up is upgraded to an technical interferometer.
 

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 this reason, 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 a comfortable readjustment of the components until an optimum contrast is reached.

Static interference pattern
From the type of interference pattern one gets information about the wave fronts. At this point, the discussion of Gaussian beams should 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 acquires periodically 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 is limited when using a two mode laser. On the other hand, information on the applied laser or light source can be found.