Abstract: An optical network comprises a central source providing light in a plurality of spaced wavelength bands and including variable-gain optical amplifiers enabling the relative intensity of light in respective wavelength bands to be varied. A wavelength-routed network (preferably but not necessarily passive) receives light in all the wavelength bands from the central source and routes each wavelength band to a respective terminals. The terminals are operable to modulate and return received light in any of the said wavelength bands, and may be simple and identical, requiring no precision light source, as would otherwise be needed to allow multiplexing to use optical fibre efficiently. The operator can adjust the semiconductor optical amplifiers to illuminate or disable individual terminals, as required, and can alter the intensity (and if required type) of light supplied to the terminals to maintain a satisfactory signal level, minimising need for engineer visits.

Abstract: An optical signal transmitter has an optical source for generating broad-band optical radiation, a wavelength division multiplexer optically linked to the optical source and operable to receive and slice spectrally the broad-band optical radiation, and at least one optical signal generator optically linked to the wavelength division multiplexer to receive a spectral slice of the broad-band optical radiation.

Abstract: An optical network having a spatially-multiplexed plurality N of optical waveguides is employed in conjunction with another, non-spatial signal multiplexing technique (e.g., frequency multiplexing) defining a plurality M of further optical signal channels thereby providing an enhanced number N.times.M of optical signal channels that can be selectively coupled to by optical signal processors.

Abstract: A monitor unit for detecting physical interference of a communications fibre in a transmission system includes a monitor circuit coupled to the communications fibre. A portion of a signal being transmitted along the communications fibre is coupled out of the fibre and into the monitor unit. A filter allows only one of two orthogonal polarization field components to pass through it to the monitor circuit. By detecting fluctuations in the polarization state of the filtered optical signal, the monitor circuit allows the detection of external influences to be determined.

Abstract: A neural network has inputs formed by square array of optical modulators M.sub.ij and outputs by optical detectors D.sub.ij coupled to threshold comparators. A holographic plate includes a spatial modulator whose elements are controlled by a controller to form an array of optical beams from a coherent optical source. Each optical beam optically interconnects a modulator M.sub.ij with a respective detector D.sub.ij. The weight values of the neural network are provided by the intensities of the optical beams. This obviates the need for an optical weighting mask between an array of light emitting diodes and a detector array allowing a higher density of lower power consumption components and reprogrammability of the network.

Abstract: An optical interconnection network includes an optical bus formed by a D-fibre embedded in a thermoplastic substrate with the flat of the D-fibre flush with the top surface of the substrate. A module similarly constructed is dimensioned to be a push fit in a wall structure formed on the substrate with the fibres and in a position to evanescently couple optical signals from one fibre to the other.

Abstract: A binary tree switching network in which the switching states are configured to switch an input signal from a selected input to an output by means of a control code set having a minimum Hamming distance (d) greater than one which eliminates cross-talk of order (d-1) and less. Such a network may be used for switching optical signals by means of optical beam deflector stages, each having a variable polarization rotator and a polarization sensitive deflector. Elimination of first order cross-talk is readily achieved by a modification stage arranged to receive a single input from the preceding stages and to pass it to the output.

Abstract: An optical communications network in which optical signals from a transmitter (T) are distributed to receivers (R.sub.i) via an optical waveguide (2) and couplers (C.sub.i) are amplified as they propagate along the waveguide (2). The amplifiers A.sub.i are all pumped by a single optical pump source (4) coupled to the optical fibre (2) by a dichroic coupler (6). By using the optical waveguide (2) to distribute the pump power to the amplifiers A.sub.i multiple optical pump sources are avoided.

Abstract: An optical distributor comprises an optical input (Il), and 2.sup.n optical outputs (01-08) , where n is an integer greater than 1, one or more plane mirror surfaces (8) and transflective means (2, 4, 6). The plane mirror surface or surfaces (8) and the transflective means (2, 4, 6) are arranged to provide one optical path between the input (Il) and each of the outputs (01-08), each path making n transitions only at the transflective means. In a preferred embodiment the transflective means comprise n separate beam splitter stages, the effective lengths of the n separate splitter stages (2, 4, 6) being substantially in the ratio 1.2 2.sup.n. The beam splitters may be formed by transflective coatings on glass substrates.

Abstract: Apparatus for generating of a comb having a large number optical teeth and comprises an optical loop (3) including an optical amplifier (5) a frequency shifter (4) and an optical filter (6). A signal from a laser (9) is partially coupled into the loop by a coupler (7) such that a part of it is output at output port (10). The signal in the loop (the feedback signal) is amplified by the amplifier (5) and frequency shifted by the frequency shifter (4) and is then partially coupled out of the loop by coupler (7). The part coupled out of the loop (the output signal) exits via output port (10). This process is repeated to produce the desired number of output signals exiting via output port (10). The signal from the laser (9) may be continuous or pulsed.

Abstract: An optical space switch comprising an optical output; three optical deflection stages each having a twisted-nematic liquid crystal polarization rotator responsive to a respective bi-state control signal and a calcite crystal deflection means for selectively deflecting optical signals according to their polarization. The deflection stages are arranged serially to define eight distinct source locations from where an optical signal is selectively deflectable successively by the deflection stages to the optical output. A distinct combination of states of the three control signals corresponds to each location. Each input is formed from an array of fibers so as to be capable of launching an optical signal which is spatially modulated transverse to the signal propagation direction. Each deflection stage preserves the spatial integrity of the deflected optical signals.

Abstract: A method of operating a multiple-access network. The network has a number of communication stations having receiving and transmitting portions (1, 2: 3, 4: 5, 6). The stations are physically coupled together by optical waveguides and arranged such that each transmitting portion (1, 3, 5) can transmit signals to one or more receiving portions (2, 4, 6) but cannot transmit signals to the receiving portion of the same station. One method comprises enabling two stations to communicate with each other by transmitting information simultaneously in the same channel. Another method comprises enabling two stations to communicate with each other by causing the transmitting portion of one station to transmit an interfering signal in the same channel as the information transmitted by the transmitting portion of the other station, the interfering signal being such that at other receiving portions the information and interfering signals are received together.

Abstract: An optical space switch is used in a centralized switching system for an optical network and connects light from one of a number of inputs (1) to a selected one of a number of outputs (6). The switch includes a number of multiplexed phase volume holograms (4), a spatial light modulator (3) for imposing a selected phase change on light passing through it, distribution means (2) for coupling light from the inputs (1) via the spatial light modulator (3) to the multiplexed phase volume holograms (4), and collection means (5) for collecting light diffracted by the multiplexed phase volume holograms (4). Each multiplexed phase volume hologram (4) diffracts light to a selected output (6) in dependence upon the phase change imposed by the spatial light modulator (3) on the light incident upon that hologram (4).

Abstract: A bidirectional optical space switch for selectively coupling an input signal to a selected output comprises two arrays of inputs (1, 8), two arrays of outputs (5, 9), polarizing beam splitters (2, 4), and a matrix of cells (3) each of which is selectively capable of varying the polarization state of light passing through it in response to an applied control signal. A first optical system (6) couples each of inputs (1) and a corresponding one of outputs (9) with a corresponding column of cells while a second optical system (7) couples each of inputs (8) and a corresponding one of outputs (5) with a corresponding row of cells. One of inputs (1) is switched to one of outputs (5) by activating the appropriate cell so that it rotates the polarization of light passing through it by 90.degree.. This also couples one of inputs (8) to one of outputs (9) thereby providing bidirectional switching.

Abstract: A node of an optical transmission network, for example a local area network, has a combined by-pass switch and optical modulator. The by-pass switch and modulator (45) comprises an optical switching matrix with two non-intersecting waveguides (W1, W2) connected by a further waveguide (W3). The junctions between the non-intersecting waveguides and the further waveguide perform both modulation of an optical carrier signal and re-routeing of the transmission path along the network.