This invention relates to fibre break detection in optical signal transmission networks and more particularly to such networks employing optical amplifiers.
Optical fibre break detection is well known in which a signal is sent from one end of the fibre, is reflected from the break back to the sending end and used to determine the break position as a function of the time delay between sending and receiving back the signal. Such a system is known as Optical Time Domain Reflectometry (OTDR). In the case of long haul optical transmission networks it is necessary to employ one or more amplifier/repeater in order to compensate for signal degradation which occurs with increasing distance along the fibre. The amplifier/repeater in such a system are unidirectional and this prevents the use of conventional OTDR launching a probe pulse and detecting the backscattered signal on the same fibre.
Long haul amplified/repeated optical transmission systems normally employ pairs of fibres one for outbound signals and one for inbound signals each having unidirectional amplifier/repeaters and various optical coupler arrangements have been proposed which are intended to facilitate OTDR in such systems. Some arrangements known to us are described in:
(1) Novel Coherent Optical Time Domain Reflectometry For Fault Localisation of Optical Amplifier Submarine Cable Systems. Yukio Horiuchi et al.--KDD Laboratories IEEE Photonics Technology Letters, Vol.2, No. 4 April 1990. PA0 (2) Fault Location on Optical Amplifier Submarine Systems Masatoyo Sumida et al.--NTT Transmission Systems Laboratories IMTC, 10 May 1994, 0-7803-1880-3/94 IEEE.
Another fault detection arrangement is disclosed in European Patent Specification Number EP-A-0 111 582 in which an optical link having an outbound and inbound fibre comprises a concatenated series of optical couplers between the fibres each turned to a different test signal wavelength. The test signal employs several different wavelengths and each outgoing test signal wavelength on the outbound fibre is coupled back along the inbound fibre by a different one of the couplers. In this way the absence of a particular returned test signal wavelength is indicative of a fault in the fibre between a particular pair of couplers. This is an imprecise indication and does not permit the exact location of the break to be determined by time domain reflectometry.
Another fault detection arrangement is disclosed in British Patent Specification Number GB 2267 792A discloses a bidirectional optical transmission system in which outbound and inbound fibres are coupled by optical dividers to permit part of a reflected test signal to be coupled between the fibres to permit fault location by OTDR techniques. The coupling causes degradation of traffic signals due to the coupling and permits transfer of traffic signals as well as test signals between the two fibres. The coupling factor has to be sufficiently small to prevent significant degradation of the traffic signals and this results in low sensitivity of detection of OTDR due to low levels of test signal transferred.
There are disadvantages with all the previously mentioned known arrangements and the present invention seeks to provide an improvement over such arrangements.