Sagnac interferometer

The presented Sagnac interferometer is used to make the Sagnac effect (named after Georges Sagnac) audible. To understand the Sagnac effect consider a circular interferometer with two counter-propagating laser beams (one clockwise the other one counter-clockwise) of the same frequency which will interfere at the detector. The amplitude of the resulting signal (the voltage of a photodiode) depends on the phase relation between the two laser beams. 

When the interferometer starts to rotate e.g. clockwise the clockwise propagating laser beam has to cover more distance because the detector is moving away. The counter-clockwise propagating beam has to cover less distance as the detector is moving towards it. Now the two laser beams have a different phase at the point where they interfere resulting in a different amplitude of the signal at the detector. 

 Sagnac Interferometer 

In this interferometer the laser beams are propagating through a silica fiber (the same that are used for telecommunication purposes) and the detector signal is used to adjust the frequency of an audio signal which is transmitted to the speakers.

Berhard Albrecht  
 (Gruppe Arno Rauschenbeutel, ATI, TU Wien)