Abstract: The invention relates to an optical unit (17) for synchronizing clock signals. The unit (17) comprises at least two ring lasers (1', 1"), each ring laser (1', 1") generating a repetitive optical pattern at a different repetition frequency f.sub.i. The repetition frequency of at least one of the ring lasers (1', 1") is variable. The unit (17) comprises detection means (19) adapted to simultaneously receive the optical patterns from two ring lasers (1', 1") to be synchronized with each other and to compare these patterns. The ring laser, whose repetition frequency is variable, is controllable on the basis of a signal from the detection means, by which the optical path length of this laser is changed. The invention also relates to a high-frequency carrier transmission system comprising an optical unit as described hereinbefore.

Abstract: An optical unit for multiplying the frequency of a clock signal includes at least two series-arranged ring lasers. Each ring laser has a different resonance frequency f.sub.i. The unit has an input for receiving a signal, for example, a low-frequency electric signal for modulating one of the ring lasers. The repetition frequency of at least one of the ring lasers is variable by adapting the optical path length of the resonator.

Abstract: A transmission system having a polarization-sensitive receiver which includes polarization adaptor for changing the state of polarization of a received signal until it corresponds to an optimum polarization. For determining the direction in which the polarization should be adjusted, local signal generators produce polarization variations which influence the output signal of the polarization adaptor and the change in the signal is detected. In order to prevent slow variations in the received signal from affecting the polarization detection, a band-pass filter is included to suppress such variations.

Abstract: A multimode imaging component provided with a rectangular waveguide (14') having two ends (15, 19) is described. To avoid unwanted reflections of radiation in the waveguide, the surfaces (17, 21) between the windows (16, 18, 20, 22) at the ends are provided with means for reducing reflection, for example, anti-reflection coatings (24, 25). When such a multimode imaging component is used as a coupler in a ring laser, the radiation spectrum of the ring laser will not comprise unwanted spectral components.

Abstract: A ring laser is described, which has a ring (1) in which laser radiation is generated. A coupler (4) couples a part of the radiation in the form of two waves (7,8) to two waveguides (5,6). A combiner (11) combines the two waves to an output wave (12) in an output waveguide (13). By providing a reflector (16) in the output waveguide, a part of the output wave is reflected to the ring via the combiner, the two waveguides and the coupler. The radiation waves travelling in the ring couple to the reflected wave and thereby acquire a well-defined phase difference. The resultant phase equality of the two waves (7,8) at the input of the combiner ensures that the powers of the waves are added in phase so that the ring laser supplies the maximum possible power.