Abstract: An inertial sensor unit for determining north, has a housing and a positioning gimbal rotatably mounted in this housing about a substantially vertical axis. A releasable, cooperating detent mechanism defines three fixed angular detent positions of the positioning gimbal about the substantially vertical axis relative to the housing. The positioning gimbal is successively rotated into the three detent positions. A fiber optical gyro is mounted on the positioning gimbal. The input axis forms an acute angle with a substantially horizontal axis orthogonal to the above mentioned substantially vertical axis. A signal processing system is provided, to which the signals generated by the fiber optical gyro in the three detent positions are applied. The signal processing system provides, therefrom, the angle between a reference direction and north.

Abstract: A sensor which makes use of a non-reciprocal optical effect, such as the Sagnac effect has a passive ring resonator defining a closed light path. Light from a broadband light source is coupled to the passive ring resonator by 2.times.2 couplers in a first direction of propagation and a second direction of propagation opposite thereto. The 2.times.2 couplers also couple light out of the passive ring resonator. A 3.times.3 optical coupler is provided, by which light having circulated in said ring resonator in the first direction of propagation is superimposed to light having circulated in the ring resonator in the second direction of propagation to provide interference. The detector is exposed to this interfering light.

Abstract: A sensor is to measure a physical quantity which causes a non-reciprocal effect, such as the Sagnac-effect, in an optical ring resonator. This effect provides a difference of clockwise and counter-clockwise optical path lengths in the ring resonator. The difference is proportional to the physical quantity, such as rotation rate in the case of the Sagnac-effect. A semiconductor laser is coupled to the ring resonator and generates clockwise and counter-clockwise beams therein. The laser frequency is modulated by directly energizing the semiconductor laser such that sidebands of the frequency-modulated laser beams are in resonance with the ring resonator. A detector device responds to the difference of the clockwise and counter-clockwise light beams and generates a signal indicative of the measured physical quantity.