Abstract: A polarization mode dispersion (PMD) compensation arrangement receives an optical input data signal which has been subjected to PMD. The arrangement comprises an adaptive chromatic dispersion compensator (24) and a first-order PMD compensator (20,22) in series, wherein the adaptive chromatic dispersion compensator is controlled to provide compensation for both chromatic dispersion and second order PMD. The compensation arrangement is used in a node of an optical network.

Abstract: An interferometer, such as a Mach-Zehnder type is fed with a pulse train. One arm is fed with a data stream, so as to modulate the pulse train according to the data stream. This enables the data stream to be regenerated, or sampled without converting from optical form into electrical form. Thus all optical regeneration, multiplexing, demultiplexing or retiming to remove jitter, can be achieved. An optical amplifier (20) in one arm of the interferometer enables the interference condition of the interferometer to be varied, to cause the pulse train to be modulated by the data.

Abstract: In an optical communications system including a transmitter (laser 1) and a receiver, with an optical amplifier (3) and an optical filter (4) therebetween, the position of the passband of the optical filter (4) is adjusted automatically in dependence on a received pilot carrier signal frequency transmitted with the data. This facilitates initial selection of transmitter laser (1) and avoids the need for transmitter laser frequency stabilisation. The optical filter acquires and tracks the optical carrier used by the transmitter.

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 diode-pumped fiber amplifier has its amplifying fiber divided into two parts so that light from both ends of a laser diode can be used for pumping. A second diode may be included for redundancy, its outputs being coupled via a pair of polarisation beam splitter/combiners.

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.