Despite the existence of high precision satellite navigation (GPS) each transport vehicle which relies on navigation must have its own GPS independent navigation system to be prepared if the GPS may fail. Regardless of the manufacturer like Airbus or Boeing, air planes nowadays are equipped with laser gyros for navigation.
Shortly after the invention of the laser in 1960 the idea of Georges Sagnac from 1913 (France)was applied in conjunction with a HeNe ring laser. However the difference of such a ring laser gyroscope to the idea of Sagnac lies in the fact that within Sagnac’s set-up the light source is separate from the ring structure and the signal is as a phase shift between the counter propagating beams. In the laser gyroscope discussed and applied here, the light source is part of the ring laser and the output is a beat frequency between the counter propagating laser modes. This class of laser gyroscopes are termed as “active” and those of the Sagnac’ s type as “passive” laser gyroscopes. In general the active laser gyroscope provides a much higher precision and long term stability as against the passive ones. The precision of the laser gyro becomes more evident, when it is compared to other well known measuring devices for instance a micrometer screw with a resolution of 0.01 mm. It must have at least a length of at least 3 km (!) for having the same resolution.
Within this experimental system the basics of the laser gyro are explained and practically studied at the system, which allows full access to all components. The experimental laser gyroscope consists of a rugged turntable on which the ring laser is mounted. A rotational stage driven by a stepper motor which rotates the turntable. The angular speed and range can be set via the provided controller. The ring laser consists of three laser mirrors arranged at the corners of an equilateral triangle. The point of rotation lies well within the centre of this triangle. At one mirror a beam bending device is positioned in such a way that the clockwise and counter clockwise propagating modes are superimposed and their beat frequency is detected by means of two photo detectors. The signal of the photodetector has a phase shift of 90° to each other so that a subsequent direction discrimination is performed. The created TTL signal is fed to a frequency counter.
For the first alignment of the ring laser an adjustable green laser pointer is used. Once the system is aligned, the single mode etalon is inserted to obtain the required single mode operation. The beat frequency of the modes is measured as function of the angular speed. A special measurement is focused on the so called lock-in threshold, which is an unwanted effect of active laser gyroscopes.
At the mirror M3 a fraction of both the cw and ccw mode leaves the ring cavity formed by the three mirror M1, M2 and M3. Both modes are linearly polarised whereby the polarisation direction is defined by the Brewster windows attached to the laser tube. The cw beam enters and leaves directly the polarising beam splitter (PBS). The ccw beam is directed in such a way that its direction is perpendicular to the cw beam. The half wave plate (HWP) turns the polarisation direction of the ccw beam by 90° thus the beam is reflected at the PBS and travels from here onwards collinear to the cw beam which is necessary for the superimposition to obtain the beat frequency. After leaving the PBS both beams are travelling in one direction however, the polarisation state is orthogonal to each other. The quarter wave plate (QWP) converts both beams into circular polarisation, one right and the other left circularly oriented. If both modes having the same frequency the respective electrical field vector are rotating with the frequency ω the resulting vector is fixed in its orientation. As soon as there is a frequency shift between the two modes, the resulting vector starts to rotate with the beat frequency of ωb=ωcw - ωccw.
Vectorial presentation of the electrical field vector of the cw and ccw mode.
By placing photodetector behind a polarizer (P1) the rotating E vector is converted into a corresponding intensity variation. The neutral beam splitter (NBS) divides the two circular polarised beams into two channels each having a photodetector (PD1, PD2). The polariser P2 in front of the photodetector PD2 is tilted by 45° with respect to P1 resulting in a 90° phase shift between the signal of PD and PD2 which will be used in a phase discriminator to detect whether the gyroscope rotates cw or ccw.