Abstract: A metallic ring is made of two metals, wherein one metal forms a major arcuate portion and the other a minor arcuate portion of the ring, thereby forming a thermocouple-type structure as a result of the two inter-metallic junctions. The metallic ring supports a surface plasmon whose energy is matched to the energy, i.e. wavelength, of an incident light beam so that the oscillating electromagnetic field of the light resonates with the plasmon. The resonating plasmon causes a temperature difference to arise between the two inter-metallic junctions in the ring. The different Seebeck coefficients of the two metals results in the temperature difference causing a net current to flow around the ring, which in turn generates a magnetic field. Such a thermoelectric metamaterial ring transforms high frequency optical energy into long duration magnetic radiation pulses in the terahertz range.

Abstract: In a telecommunication system, channels of information are modulated onto respective carriers that are swept repeatedly across a frequency range in a time-staggered fashion so that, at any one time each carrier is at a different frequency. The carriers are multiplexed onto a single communications link and separated, on reception, by filters swept in synchronism so as to de-multiplex the signals.

Abstract: A method of proving the integrity of optical paths in an optical communication system wherein the same diagnostic optical signal is transmitted along each path from one end, and each path is provided at its other end with a grating pattern, for example, for producing a pattern of time spaced echoes of the diagnostic signal unique to that path. In one particular embodiment the diagnostic signal comprises an amplitude modulated optical signal and the echoes are produced at a time spacing such that the modulation components of the echoes are in-phase at a different frequency for each path.

Abstract: An optical apparatus for use in association with an optical fibre having a core member and a cladding member which surrounds the core member and which has a refractive index less than the refractive index of the core member, the apparatus including optical means adapted to examine light radiated at an angle through the cladding member and thereby determine the characteristics of the optical fibre and/or facilitate the application of an illuminating beam of light at an angle through the cladding member and to thereby excite a single mode in the optical fibre; and means for maintaining the angle between the light and the fibre axis substantially constant.

Abstract: A fibre optic pressure sensor which includes at least two lengths of optical fibre each one of which is secured to a surface of a separate one of a number of spaced-apart member, at least one of which is resilient, the optical fibres being situated in the space between the said members and adapted to be brought into alignment when at least one of the said members is deflected by the application of external pressure to the sensor.

Abstract: In a disconnectable optical fibre butt connector, each of two fibres 1, 2 to be butted, is supported near an end by a bush 3 carried by a flanged carrier disc 4 in a seating of a flanged cap 8. An alignment unit comprises three alignment rods 12 located in a distortable elastic sleeve 13 which is enclosed in a rigid tube 15. To assemble the connector, the fibre ends 1, 2 are inserted between the alignment rods 12 from opposite extremities, and are pushed home till they butt each other. The caps 8 are then screwed onto opposite extremities of the tube 15, forcing the flanges 5 of the carrier discs 4 against the sleeve 13, which distorts, clamping the fibre ends 1,2 in alignment between the alignment rods 12. The alignment unit may form part of a pressure-tight bulkhead.

Abstract: In an optical fiber, light energy is transmitted in a number of modes having characteristic group velocities, and it is frequently necessary to limit transmission to modes whose group velocities are not excessively disparate. The invention avoids this limitation by providing a compensator (preferably) midway between two stations. The compensator pairs a fast mode in a fiber leading to one of the stations with a slow mode in a fiber leading to the other station, so that the group velocities for all pairs are approximately equal. The compensator comprises first, second and third focusing systems which are aligned with the fiber ends.The first system focuses energy received from one fiber into concentric rings whose radii are characteristic of the modes used in the fiber. The second system transposes, to a ring of maximum radius, energy focused at a ring of minimum radius, and vice versa.