Abstract: Optical fibers are welded together by positioning the end parts in vacuum chucks 10 and 11. Piezo-electric elements 8 force the chucks towards one another. The piezo-electric elements are put into operation by the power supply of a welding arc operating between the chucks. The heated fiber end parts are thus moved together to ensure a good welded joint.

Abstract: The availability of service in a telephone exchange is evaluated by a technique which includes monitoring for each call type over a period of time the level of usage of the items of equipment required for that call type and evaluating using the monitored data the probability of each particular item not being able to provide a service when required. The probability figures of the call types can be combined to give an index figure indicative of the availability of service of the exchange.

Abstract: A filter network that exhibits a bandstop response comprises a network of basically low pass or high pass structure. The network is developed by choosing a bandstop transfer function, a fourth-orderelliptic is disclosed, which is converted to a low pass function. This function describes a passive low pass LC ladder network including a negative impedance element. A Bruton transformation is applied to the passive LC network to give an active version including an ungrounded negative resistance network in a series arm and frequency-dependent negative resistances in shunt arms without requiring inductance coils for its implementation.

Abstract: A waveguide cable comprises an axially extending polymer sleeve, or group of sleeves, the molecules in the sleeve(s) being longitudinally orientated. The or each sleeve contains one or more dielectric optical waveguides. A plurality of elongate filamentary strengthening members encased in polymer sleeves extend axially of the cable around the polymer sleeve or sleeves.

Abstract: This invention relates to the control of the wavelength of the emission from a semi-conductor laser 1. The laser driver 5 is connected in a feedback loop comprising a beam-splitter 6, a photodiode 8 and an amplifier 19, which loop controls the driver 5 to maintain the intensity of the laser emission constant. The emission is also sampled by a beam-splitter 7 and a photodiode 9, which is made frequency-dependent by a filter 10 interposed in the path of the beam. The outputs of the matched photodiodes 8 and 9 are fed via amplifiers 11 and 13 to the positive and negative inputs of a subtractor amplifier 12, the output of which is therefore dependent only upon the emission wavelength. This output is fed as negative feedback to an amplifier 15 for controlling an operating parameter (such as the temperature) of the laser 1 to maintain the wavelength constant.

Abstract: This invention relates to an adaptive demodulator for use in a phase shift keyed microwave communication system. The adaptive demodulator improves the system error rate at a particular carrier to noise ratio by monitoring the output of the demodulator and optimizing the phase offset of the carrier signal recovered in the demodulator to provide a high output. A particular example continuously varies the phase offset of the recovered carrier signal about the highest output value to track variations in the optimum phase offset.

Abstract: Semiconductor compounds which are alloys of group III-V compounds are grown by a liquid phase epitaxy method which includes heating growth apparatus in a reducing atmosphere while maintaining a solvent for the compound, a source of the group III-V compound and another element of the alloy separate from each other. After heating to reduce oxides, the element is added to the solvent, the source is brought into contact with the solvent and the resulting solution is brought into contact with a substrate to effect growth of the compound. Apparatus for carrying out the method is also described.

Abstract: A protection circuit for an Impatt amplifier includes a voltage comparator which is arranged to sense the terminal voltage of the or each Impatt diode. When the comparator senses that a diode terminal voltage has risen above a predetermined value a circuit is actuated which causes power to be disconnected from the amplifier. The present circuit does not require the use of expensive couplers.

Abstract: A digital switching device of the stored program type operating under the control of a central control unit has a multiplexing stage for multiplexing signals from a plurality of ports, an input time switching stage which receives multiplexed signals from the multiplexing stage and outputs them in appropriate time slots, an interface unit connecting the input time switching stage to an output time switching stage and a demultiplexing stage which demultiplexes signals from the output time switching stage for transmission to said ports. The interface unit is an interchangeable unit which either provides a direct connection between the input and output time switching stages or comprises a space switching stage.

Abstract: A digital switching arrangement for use in a telecommunications exchange handling channels of digital information in time division multiplex form. The digital switching arrangement being particularly suited for use in stored program processor controller environments. The arrangement comprises a digital switching network and a control equipment. The control equipment includes an input queue and an output queue each arranged to store processor input and output messages respectively. The control arrangement is arranged to asynchronously process each output message to process switching network path connections and to generate in the input queue an input message indicative of the actions performed. Each output message includes switching network identification information indicative only of the identities of the incoming and outgoing exchange highways and channels to be involved in the switching network path to be processed.

Abstract: The invention relates to the location of contact faults on electrically conductive cables such as telephone pairs. Contact fault location apparatus has a pair of resistors, a constant current source, and means such as a differential amplifier and meter for sensing the current flowing in the resistors. A pair of lines to be tested is looped at one end, the apparatus is connected across the other end and the current flowing in the resistors is measured. The ratio of the current is indicative of the distance to the fault.

Abstract: A submarine communications cable comprising a plurality of optical fibres extending longitudinally, an electrically conductive tube surrounding the plurality of fibres with radial clearance, the electrically conductive tube allowing the transmission of electrical power between repeaters and being rigid so that when incorporated in the cable it resists hydrostatic pressure at the sea bed.

Abstract: A range of alkali metal borosilicate glass compositions modified by the addition of alkaline earth metal oxides has been found to be especially suitable for the production of graded index optical fibre by thermal diffusion using the double crucible method. A core glass chosen from this range may be paired with a suitable cladding glass so that, in the nozzle of the double crucible, alkaline earth metal exchange occurs to give a composition gradient of alkaline earth metal oxide and hence a gradation of refractive index. Using these glasses, fibres have been produced having refractive index profiles that approximate closely to the ideal parabolic distribution, numerical apertures of up to 0.21, and best loss values as low as 6.4 dB/km at 850 nm.

Abstract: A control unit for establishing a four-wire connection between two terminals via a pair of telephone lines comprises switching means which are operable to provide a connection to a pair of telephone lines, signalling means connectible to said lines via said switching means to enable another control unit to be signalled via a line, and line send and receive circuitry connected to the switching means. The line send and receive circuitry has an input for connection to the send channel of a terminal and an output for connection to the receive channel of the terminal. A central control controls operation of the switching means and line send and receive circuitry to automatically establish a four-wire connection and adjust parameters of the line send and receive circuitry to compensate for loss on the telephone line.

Abstract: An alarm system for transmitting alarm information via telephone lines has a plurality of alarm transmitters which are located at subscribers' premises and are adapted to transmit signals indicative of the state of alarms associated with the transmitters along telephone lines to associated receivers. The receivers are located at one or more local exchanges and the receivers at a local exchange are all connected to a local processor. The or each local processor is connected to a central processor which can direct alarm information to one of a number of terminal stations. The local processors continuously scan the outputs of the receivers. When a local processor detects a signal indicative of an alarm condition it transmits a signal coded with the subscriber identity and destination for the alarm information to the central processor which routes the alarm message to the appropriate terminal station.

Abstract: An electret transducer backplate is made of a thermo-softening plastics material rendered electrically conducting by electrically conducting particles distributed therethrough. An electret foil diaphragm is mounted on the backplate and the relative flexural rigidities of the diaphragm and the backplate are such that, in use, the deflection of the diaphragm is at least twenty times as great as the deflection of the backplate. A method of making the transducer involves heating an electret foil, bonding the electret foil to a retaining ring while the foil is hot and securing together the retaining ring and the backplate.

Abstract: A converter for converting a delta sigma modulated signal into a pulse code modulated signal comprises an n bit counter, an (n-1) true/complement, zero one element which is controlled by the most significant bit of the counter and operates on (n-1) bits of the counter in accordance with the pulse density modulation input to produce weighted output samples. An m bit accumulator accumulates the weighted samples to produce pulse code modulated samples.

Abstract: The invention relates to a shrinkable sleeve device easily assembled with a body to be protected thereby. It comprises a dimensionally heat unstable sheet-like portion to embrace the body with unique means for securing edge portions thereof together. The device is particularly advantageous for application and use in restricted locations and for providing a protective sleeve by which cable ducts will not be unduly impeded.

Abstract: A method of enhancing the piezoelectric properties of a polymeric material exhibiting such properties includes the step of exposing the material to a quantity of .gamma. radiation lying within the range of from 1 Mrad to 200 Mrads. The material is subsequently poled. Best results are obtained when the material is exposed to between 1 Mrad and 100 Mrads of radiation. The material treated may be polyvinylidene fluoride.