3.1 - Traveling-wave laser gyrocompass

A direction-finding device has been designed making use of the properties of the ring laser. The order of magnitude precision for laser gyro is 0.1 degree per hour. The north determination requires long integration time when the plate is static. However, this may be vastly improved by a dynamic method. In this experiment, the ring laser is not rotated about its sensitive axis but about an axis parallel to its plane at constant rate. The beat frequency obtained is frequency modulated. The frequency deviation is proportional to the varying flux of the Earth's rotation through the cavity area. A discriminator detects this modulation. A reference light source which may be displaced about the rotation axis gives a synchropulse every time its beam is normal to the plate. This pulse is fed in a synchronous detector for detection of the output of the discriminator. A feedback control loop sets the reference so that the phase detector gives a null output, that is, when the synchropulse coincides with a maximum of the sine wave. The point source and the rotation axis then define the north direction. Results comparing static and dynamic measurements are given. Accuracy and integration time are discussed.