Already in 1881 A. A. Michelson constructed an interferometer, which later on also got his name, to counter prove successfully the theory of an universal ether assumed to be existing 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 high precision 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 of 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 nanometre 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 setup 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 a second step the Michelson setup is upgraded to an technical interferometer.
LM-0100 Michelson Laser Interferometer
The classical Michelson setup consists of the beam splitter, the mirror 1 and the mirror 2. The incident beam from a green laser is split into two beams at the beam splitter. The returning beams from mirror 1 and 2 are imaged by means of a diverging lens onto a translucent screen. Mirror 2 is mounted on a translation stage for precise change of the related optical path, particularly for white light interference. The beam expander provides an enlarged beam with plane wave fronts resulting in a fringe pattern with a parallel or circular pattern.