Abstract: An add/drop multiplexer comprises passive optical components for wavelength division multiplexing. The multiplexer allows signals passing along fibers of a main trunk between terminal stations to be dropped to and added from a spur station, allowing a reduced number of spur fibers to be used since signals are routed between trunk fibers and spur fibers according to carrier wavelength.

Abstract: A number of add/drop multiplexers comprising passive optical components for wavelength division multiplexing are described. These add/drop multiplexers are particularly adapted for use in branching units (10) of scuh networks to allow signals passing along fibres (1, 2, 11, 12) of a main trunk between terminal stations (20, 30) to be dropped to and added from a spur station (40), the design of the add/drop multiplexers allowing a reduced number of spur fibres (3, 13) to be used as signals are routed between trunk fibers (1, 2, 11, 12) at spur fires (3, 13) according to carrier wavelength.

Abstract: An analogue telemetry system/method provides fault location in an optical transmission system including optical fibre amplifiers. Each amplifier includes an optical fibre amplifier with an AGC loop therearound, a respective optical pump source being included in the AGC loop. In the AGC loop the spontaneous emissions from the amplifier fibre is detected and employed to drive the pump source appropriately to maintain the amplifier output at a predetermined level. A supervisory tone is applied at the transmission end of the system. At each amplifier the presence of the supervisory tone or a distress tone from a previous amplifier is looked for. If either tone is detected the signal is amplified and transmitted to the next amplifier. If neither tone is detected a respective amplifier distress tone is applied to the respective optical pump source.

Abstract: An analogue telemetry system/method provides fault location in an optical transmission system including optical fibre amplifiers. Each amplifier has a respective optical pump source including an AGC loop around the optical amplifier's fibre. Spontaneous emission from the optical fibre amplifier is detected and employed to drive the respective pump source to maintain the amplifier output at a predetermined level. A supervisory tone is applied at the transmission end of the system and its presence/absence detected at each amplifier. In the event of "absence", a respective distress tone is applied at the relevant amplifier. At the receiver end of the system, the supervisory tone is sought and in the absence, the number of distress tones, if any present, is counted and the position of the fault is thus determined.

Abstract: A digital telemetry system/method for locating faults in an optical transmission system including optical amplifiers, in particular optical fiber amplifiers. A supervisory codeword is applied at the transmission end of the system. At each amplifier the presence of the supervisory codeword or a distress codeword from a previous amplifier is looked for. If either codeword is detected it is relayed to the next amplifier. If neither codeword is detected a respective distress codeword is applied. At the receiver end of the system the supervisory codeword is sought or, in its absence, a distress codeword or no codeword at all and from this information the position of a system fault determined.