Abstract: Optical four wave mixing is provided by interfering two waves on a liquid crystal layer (1) temperature stabilized just beneath its nematic/isotropic phase change transition temperature. The liquid crystal incorporates a guest dye to absorb the light producing a holographic thermal image which is accompanied by a holographic phase image. Once this phase image starts to build up it is illuminated with a `third wave` of light that is not absorbed so as to produce the required holographically diffracted `fourth wave`.

Abstract: It has been found that when a pulsed high-power laser beam is incident on the surfaces of a wide variety of materials, ripples are produced on such surface. This is due to interference between scattered waves and the incident beam producing intensity fringes, and hence localized heating. The dimensions of the ripples are dependent on the wavelngth of the incident light. This is used to produce permanent (after cooling) gratings on the outer surface of an optical waveguide, for example an optical fibre, on to which a laser beam is focussed by a lens system. Hence a grating whose length is a few hundreds of micrometers is produced. If the cladding is removed this effect of ripple generation is enhanced.

Abstract: A frequency selective optical polarizer is formed by pressing a length of single mode polarization maintaining fiber 12 against a ribbed plate 10 in order to generate a distributed strain pattern with a periodicity which causes for a particular optical frequency one of the guided modes to couple into an unguided mode while the other guided mode remains substantially uncoupled.

Abstract: A data transmission equipment, known as a primary digital multiplex (PDMX) consists of a bus structure formed by parallel lines on a printed circuit board, with the circuit units (1,2,3,4,5) on printed circuit cards set at right angles to the bus structure. The circuit units include a control unit (1,2) also called a core facilities unit, which acts as an interface to one or more 2Mb/sec. trunks, and so-called tributary units (3,4,5) which give access to subscriber and other outlets.Various networks using one or more such PDMX equipment are described.The signalling and control arrangements for the equipment use a signalling processing unit (2) and a tone processor (6). The signalling processing unit monitors the bus for signals and line conditions and issues signals appropriately. The tone processor functions in a generally similar manner.Note that PDMX is one of STC PLC Trade Marks.

Abstract: Novel derivatives of cyclic and acyclic terpenoid alcohols, having general formula (I) X--(CO).sub.n --O--T.sub.1, where n=O or l, T.sub.1 is a terpenoid group and X is a combination of cyclic groups, e.g. phenyl, cyclohexyl and pyrimidyl, X preferably contains up to three cyclic groups. Preferred derivatives are esters of cyclic terpenoid alcohols, e.g. menthol, borneol and isopineocampheol, e.g. formula (II) (-)-menthol. Ferroelectric smectic liquid crystal mixtures and devices containing these derivatives as chiral dopants in a tilted smectic host are also described. The properties of various other derivatives of terpenoid alcohols of formula (I) are also described, e.g. those of neomenthol, isolongifolol, fenchol, carveol, myrthenol, nopol, citronellol, perilla alcohol and dihydrocitronellol.

Abstract: A circularly polarized antenna including first and second multiple patch antenna structures dimensioned to operate at two distinct frequencies, each antenna structure consisting of four shorted patches, the patches of the first structure being spaced from a ground plane by dielectric material, the patches of the second structure being spaced from the patches of the first structure by dielectric material, the patches of the second structure each overlying a corresponding patch of the first structure and each having a dimension .lambda..sub.m.sup.(2) /4 which is less than the dimension .lambda..sub.m.sup.(1) /4 of the corresponding patch of the first structure, with feed means for each of the patches, the patches of both structures being disposed in the planes of the patches so that the radiating edges of the two patch structures form superimposed antenna structures.

Abstract: The method of measuring a resonator such as a quartz crystal is to calculate its admittance from the voltage transfer function of a suitable network incorporating the resonator. This admittance can be expressed in the form of a circle. Two initial measurements are taken from which an approximate circle is calculated. This circle is used to calculate the optimum measurement positions for the subsequent curve fitting routines, those positions being symmetrically placed around the series resonance. Each measurement is an average of a number of samples. The deviation from this average as well as the deviation from the circle defined by previous measurements is used to monitor the measurement. All the parameters are calculated by least square curve fitting of the first the admittance circle and then the variation of phase around the circle with frequency.

Abstract: Optical energy at a predetermined frequency or narrow band of frequencies is tapped from an optical fibre by causing spatially periodic deformation of the fibre such as to produce coupling of optical energy between modes of the fibre, for example between the core and cladding of a single mode fibre, at the predetermined frequency and removing the optical energy at the predetermined frequency, for example by means of a cladding mode tap for a single mode fibre. If the deformation means includes a mechanical grating then tuning of the coupling frequency can be achieved by changing the relative orientation (O) of the fibre and the grating.

Abstract: An arrangement for coupling an optical fiber (1) to a waveguide (8) which forms part of an integrated optical device (4) uses a block (3) with a narrow slot (2) to receive the bared fiber end. On top of this block is a retaining block (6), of transparent material such as glass. These are held together by epoxy, and the front face rendered smooth. A polishing block (10) is glued to the edge of the IO device (4), and the end of the block is polished. Then the fiber and the guide on the IO device are brought together in proper alignment and secured together.

Abstract: In order to prevent diffusion of silicon from under a titanium disilicide interconnect (1) and into an overlying aluminium layer (6), the disilicide is selectively nitrided by annealing in nitrogen at the points where interconnection between the disilicide and aluminium is required via holes (4) in a silicon dioxide layer (3). The titanium nitrode contacts (5) thus formed in a truly self-aligned manner provide a good barrier to silicon diffusion while having an acceptable low resistivity.

Abstract: In a data transmission system data is sent, after suitable conversion, as a ternary analog type signal. Using two-wire twisted pairs and hybrids, echo cancellation and feedback equalization are needed.Synchronization between the two ends, e.g. of a 144 Kb/sec. subscriber's loop is maintained by a low amplitude pilot tone sent with the data, and detects at the same time as the data is detected. Detection and elimination of this pilot tone use coefficient generation circuitry similar to those used in the equalizer and echo canceller.

Abstract: An electronic switching network for use as a transit exchange, interconnects PCM digital trunks, and handles large numbers, e.g. 1,000 inlets and 1,000 outlets of such trunks. The switching network is fully electronic, using integrated circuit units. To provide a self-checking function, the intelligence arriving via the trunks is converted (2) into a redundant line code, e.g. 3B-6B, which has spare code combinations which are used to provide an auxiliary data channel through the switch matrix (3). The converter (2) applies to that data channel the inlet port number for a connection via the switch matrix (3). At the outlet side another converter (4) converts back to the line code (e.g. HDB-3) used over the trunks, and also extracts the inlet port number. This is then compared with what it should be, and the result of this comparison indicates whether the switch is functioning correctly. Thus the checking occurs while the switch is handling traffic.

Abstract: A circularly symmetric reinstatement sheathed optical fibre splice butt joining the ends of two acrylate sheathed glass optical fibres is made by a method that includes the step of immersing the spliced fibres (14) in dichloromethane (64) to cause the acrylate to swell and longitudinally expand to close the gap (42) in the sheathing (41) left by the splicing process. The sheathing is clamped in this position while the solvent evaporates. The residual gap, which is comparable in length with the sheath diameter, is protected by the application of a drop of uncured resin which capillary forces constrain to assume a circularly symmetric form until the resin is cured.

Abstract: An optical crossbar switch employs the optical Kerr effect to cause switching of an input signal between two possible outputs. In the absence of a pump signal the output is on one fibre. In the presence of a pump signal the output is on the other fibre. The pump signal serves to produce rotation of the polarization of the input signal in a polarization maintaining coupler, the two possible polarization states being separated by a birefringent crystal or a polarization selective beam splitter.

Abstract: An injection laser array has 2.sup.n output elements (1) coupled by Y couplers (2, 4, 6) and (n-1) in sets of intermediate elements to a single element (7) in a tree configuration. The face end of the single element is provided with a high reflectivity coating (8) while those of the 2.sup.n output elements are provided with low a reflectivity coating (9).

Abstract: A solid electrolytic capacitor has an anode body (1) and an anode wire (2) and lead out connections (8, 3). In series with the connections and the body is a fusable link (6) formed of a composite of low melting point conductive plastics metal matrix. The fusable link (6) is in the form of a pad of this material. In a preferred form this material is made by compressing into sheets metal-coated polymer particles. The sheet is cut into pads and inserted into the capacitor assemblies to act as a combined thermal and electrical fuse. Preferably the pads are less than 1 mm thick and coated on both sides with solder and approximately 1 mm square. The pads can be reflow soldered between the anode and the lead frame or negative wire termination. Alternatively it can be reflow soldered between the anode wire and the positive wire termination. As the current reaches high level if a fault develops in the capacitor, the metal layer will melt and also melt the plastics.

Abstract: A design of plastics packaged glass optical fibre in which the plastics packaging consists of a urethane acrylate primary coating, a silicone rubber secondary coating and a nylon tertiary coating, the three coatings coacting to provide a pair of substantially decoupled mechanical filters.

Abstract: Polarization beam splitters/combiners or polarization insensitive beam splitters/combiners are manufactured from low or zero birefringence single mode optical fibre, rather than high birefringence fibre, by using a differential pulling technique to induce the appropriate amount of form birefringence, the fibres being stretched further than hitherto.

Abstract: Feedforward of the source signal into the compensation mode of an operational amplifier produces more than an order of magnitude improvement in small-signal capability at high frequencies. The technique is useful for active filters and wideband amplifiers. Further improvement will be available using amplifiers designed for this approach.