Abstract: A communications cable based on optical fibres comprises a core 2 with external helical channels 13. The fibres 6 in the channels are accessible through removable parts 15 of an outer sheath 4.

Abstract: A pair of optical fibres which are to be joined are located with their end portions in coupling elements one of which is transparent to light. The coupling elements are located such that the ends of the fibres are in approximate alignment. Light is transmitted along one fibre so that it is received by the fibre in the transparent coupling element. Light which is not coupled into the core of the fibre is sensed by a sensor and the relative position of the fibres is adjusted until the level of sensed light is a minimum. The sensing of the light is carried out at positions spaced angularly about the waveguide so that an indication of the direction of any misalignment is obtained. The coupling elements are then secured in this position. The adjustment of the fibres can be carried out using a linear translation stage and a microprocessor which controls the stage in accordance with signals from the sensor.

Abstract: A method of growing crystalline semiconductors such as GaAs is disclosed. The method involves epitaxial deposition from the vapor phase and provides dopant material such as sulphur in the form of a molecular beam. The molecular beam is developed by effusion from a knudsen cell. The difficulties previously encountered using sulphur as such a cell are counteracted by use of an electrochemical cell as the sulphur source. The technique allows complicated doping profiles to be produced.

Abstract: A glass suitable for use as the core glass in a high numerical aperture optical fibre has a composition selected from the range of glasses which comprise(i) from 15 to 25 mole percent of soda,(ii) from 25 to 65 mole percent of silica and germania, taken together, the content of germania not exceeding 42 mole percent,(iii) up to 22.5 mole percent of boric oxide and barium oxide, taken together, the content of boric oxide not exceeding 20 mole percent, and the content of barium oxide not exceeding 12 mole percent, and(iv) from 0.01 to 1 weight percent of a redox buffering agent. The glass can be prepared by controlling the partial oxygen pressure of the melt so that it is approximately 10.sup.-5 atmospheres. An optical fibre can be produced, by using a soda-boro-silicate clad glass, and a core glass having the composition in the above range.

Abstract: A data transmission system to enable data to be transmitted along a transmission path at a rate C greater than the upper data rate limit A of the path. A data transmitter driven by an internal clock at C Hertz transmits data along the transmission path and at the same time also transmits a derived clock signal at a frequency C/N Hertz where (C/N)<A along the path. At the receiver the data is clocked into a series of data stores under the control of the transmitted clock signal and is thereafter recombined into a serial stream under the control of the clock signal driving the receiver.

Abstract: A digital switching device of the stored program type operating under the control of a central control unit has a first stage which multiplexes digital signals from a plurality of line units and separates data signals from signalling information. The data signals are fed to an input time switching stage. The input time switching stage operates under the control of the central control unit to transmit signals from an input port in a particular time slot on a highway to an output time switching stage. The output time switching stage transmits the data signals to the first stage which combines them with signalling information and after demultiplexing transmits them to appropriate ports.

Abstract: Gold is preferred as the conductor material in a metallization layer of a semiconductor device because of its high conductivity and freedom from electromigration effects but gold is inclined to diffuse into the semiconductor substrate typically silicon, so degrading the p-n junction characteristics within the semiconductor substrate and rendering the device inoperative. Previously this problem has been overcome by placing a protective barrier layer of titanium between the gold layer and the substrate. The gold/titanium interface is subject to corrosion and this corrosion adjacent the substrate containing the active areas of the device also leads to failure of the device. This is prevented by covering the gold metallization layer of the device on its top, bottom and side surfaces with titanium. This prevents the diffusion of the gold into any other layer of the semiconductor device above or below it and there is no gold/titanium interface exposed adjacent any active area of the device.

Abstract: In order to provide improved resistance to attack by atmospheric water etc. a clad glass optical fibre has a plastic coating which is in turn coated with a metal coating. The metal coating may in turn be coated with a second plastic coating. The metal coating is formed by drawing the plastic coated fibre through a globule of molten metal, the surface of which is exposed to an oxygen rich atmosphere.

Abstract: A wire wrapping tool comprises a head which is formed integrally with a shaft. The head includes a frusto-conical portion which has a chordally extending portion cut away to define a cutting edge. A blade portion is formed on the head, the blade having a slot through which a wire can pass. The head also has an axially extending bore into which a wire wrapping post can be inserted. In use the shaft is attached to a pistol gun to rotate the head. Wire from which the insulation is to be stripped is passed over the edge, through the slot and around a wrapping post. As the wire passes over the edge a longitudinal slit is formed in the insulation which is then stripped away from the conductive core as the wire passes through the slot.

Abstract: A data transmission of the videotex type in which coded information can be transmitted from a central information store to a plurality of terminals. The coded information includes groups of bits representing characters and groups of bits representing control codes. In the transmission technique used in the system a character attribute can be represented by a display control code which is transmitted in association with a cursor backspace code. By using this technique characters of different attributes can be displayed without a space being required between the characters on the display.

Abstract: A duct motor for carrying drawing ropes and the like through cable ducts consists of two inflatable gripper bags and a telescopic section. The duct motor is driven by compressed air, and operates by one gripper bag inflating, telescopic section extending, the other gripper bag inflating and the one gripper bag deflating, telescopic section contracting and the cycle being repeated. Compressed air is distributed within the duct motor by a distribution valve and a pair of inflation and deflation valves associated with each gripper bag. One distribution valve is not located between the gripper bags and this valve has a valve stem which is hollow, and is used for conveying compressed air from an input orifice to the remote end of the duct motor i.e. the other gripper bag.

Abstract: A supervisory channel is provided on a digital transmission system by frequency modulation of the data signal transmitted along a transmission link which includes repeaters. The frequency modulation can be carried out using an elastic store. Each repeater includes means for demodulating the frequency modulation so that each repeater can be addressed and interrogated. Each repeater can also phase modulate the data signals on the return link to a terminal station so that signals indicative of an interrogated condition can be transmitted to the terminal station.

Abstract: A pompon is provided that includes a plastic body member that has strands attached to an end thereof, and wherein a clip is provided for holding the strands in place on the member. One end of the holder or member is open so that a coupon or the like can be conveniently inserted within the member. Suitable advertisements, logos, and the like can be imprinted on the outer surface of the finger held pompom. The finger held pompon includes a receptacle that is constructed of plastic material of resilient characteristics whereby the user's finger can be conveniently inserted within the device to provide a convenient means of manipulating the pompon.

Abstract: An electroacoustic transducer for a microphone comprises an electret wherein an additional capacitance substantially greater than the capacitance normally inherent in the electret is connected in parallel with the electret. The additional capacitance may be of the order of 10 times greater than the capacitance normally inherent in the electret and may be produced inherently in the transducer or by a separate component connected to the transducer.

Abstract: A digital switching device of the stored program type operating under the control of a central control unit has an input time switching stage, an output time switching stage, and a space switching stage intermediate the time switching stages. Each time switching stage has a connection store for controlling reading of data from said stages, and the space switching stage includes comparators for comparing the contents of the connection stores. The space switch is controlled on the basis of the comparison.

Abstract: A graded index optical fiber is produced in a double crucible with an elongate heated exit nozzle. The core and cladding glasses of the fiber diffuse into one another in the nozzle.

Abstract: A double heterostructure semiconductor laser having the configuration of its optical cavity arranged to impose a constraint on the lasing filament as the spatial distribution of the lasing filament changes from that occuring at the start of the lasing regime, so that the optical flux emitted by the laser has self-saturating properties. This protects the laser from catastrophic facet erosion and enables a simpler control circuit to be used to drive this laser when it is used as a light source for an optical communications system.

Abstract: In previously proposed arrangements for charging an electret foil the electret charge implanted varies over the area of the foil. In order to promote more even charging of the foil, the method of the invention subjects the foil to a progressively increasing electric field. One form of apparatus used to carry out the method has rollers at progressively higher potentials and electric fields are applied across the foil between these rollers and a metallized layer on the rear face of the foil which is earthed by a roller.

Abstract: A method of proof testing an optical glass fibre by pulling it around a free roller whose radius is sufficiently small to impart the strain necessary to detect fibres whose surface or near surface flaws weaken the fibre below a required strength. Proof testers are disclosed incorporating, single rollers, a set of three parallel rollers and a jig holding four sets of three rollers. Proof testers incorporating the extra rollers whose axes are at different angles are shown to more thoroughly explore the surface of the fibre for cracks.

Abstract: A Group III element Q selected from gallium, indium and thallium containing a Group VI element X selected from oxygen, sulphur, selenium and tellurium as an impurity can be purified by adding, to molten Q, an element M capable of forming with X a stable solid compound substantially insoluble in liquid Q. The stable solid compound can be separated and the purified Q used in the epitaxial growth of a semiconductor material such as indium phosphide. Alternatively, in a process for the epitaxial growth of a semiconductor compound of Q in molten Q, X can be removed in situ by the addition of a suitable element of the M-type.