Principle of Operation

In a ring laser consisting of three mirrors M1, M2, M3 and a HeNe-laser tube, two travelling modes are propagating inside the resonator. One travels clockwise and the other counter clockwise. At mirror M3 a small amount of the intensity of the individual modes is coupled out by means of a beam bender and beam splitter superimposed. If the resonator is not turned, both modes have the same frequency, f1 and f2. As soon as the resonator is turned around its axis with an angular speed corresponding to the angular frequency ω these frequencies are changed due to the Doppler effect. The difference in both frequencies are detected by the photo detectors PD1 and PD2 with subsequent signal processing. The so generated fringes will be summed up by means of a counter. The read out of the counter is directly proportional to the angle by which the ring laser has been turned. It is necessary that the ring laser oscillates in just one mode per direction. This requires a short HeNe-laser tube which is, however, commercially not available. Therefore, a single mode etalon is used to force the laser to the desired operation.
The experimental laser gyro (see the figure below) consists of a rugged turntable on which the ring laser is mounted. By means of a motorised rotational stage, the angular motion of the turntable is performed. The ring laser consists of three laser mirrors arranged at the corners of a 60° symmetric triangle. The point of rotation lies well within the centre of this triangle. At the mirror M3 a beam bending device is positioned in a way such that the clockwise and counter clockwise travelling modes are superimposed and their interference intensity is detected by means of two photo detectors. By counting the beat frequency and multiplying the result with the scale factor of the set-up, one gets the angle motion of the turntable.
For the prealignment of the ring laser, an external solid state laser emitting at a wavelength of 532 nm is provided.