Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1997-08-22
2000-06-13
Pascal, Leslie
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359161, H04B 1012
Patent
active
060756284
DESCRIPTION:
BRIEF SUMMARY
This invention relates to optical communications systems and in particular to the provision of time domain reflectometry to effect fault detection and location in such systems.
Optical time domain reflectometry (OTDR) is a well known technique used to determine the location of discontinuities in the impedance of a transmission medium (e.g. optical fibre wave guide, coaxial cable) by the measurement of the amplitude and delay of reflections of a travelling wave generated by the discontinuities. A description of OTDR is given in IEEE Journal of lightwave technology, Vol. 17 No. Aug. 8, 1989, pp1217 to 1224, and in specification No. WO-93/07687. This technique has been used to locate faults and breakages in the transmission medium. Conventionally, the travelling wave is generated by a short impulse, the amplitude, rise time and width of which combined with the bandwidth of the transmission medium and sensitivity of the reflection detection process determine the performance (resolution) of the method.
At present, time division reflectometry employs separate test equipment and requires a means of coupling this shared equipment into the access fibres. The use of the technique in the recently introduced passive optical network (PON) systems has been limited by considerations of cost and complexity. In particular, certain network operators require that reflectometry be performed at a wavelength different from the transmission wavelengths so as to ensure that the reflectometry testing does not interrupt normal data transmission within the network. This requires the installation of costly filter equipment and introduces consequent losses into the network.
The object of the invention is to minimise or to overcome this disadvantage.
According to one aspect of the invention there is provided an optical communications system including a data transmitter, a data receiver, and incorporating a time division reflectometry fault location arrangement, the system including means for transmitting a downstream data sequence having statistical properties equivalent to a continuous random binary sequence, means for generating an upstream data sequence orthogonal to the downstream data sequence, said upstream sequence having quiet periods containing no data, means for generating a time delayed copy of the downstream data sequence, means for correlating said time delayed copy during said quiet period with an echo of the downstream data sequence reflected from a fault, and means for adjusting said time delay whereby to achieve said correlation and to determine the location of said fault.
According another aspect of the invention there is provided a method of fault location in an optical communications system, the method including transmitting a downstream data sequence having statistical properties equivalent to a continuous random binary sequence, generating an upstream data sequence orthogonal to the downstream data sequence, said upstream sequence having quiet periods containing no data, generating a time delayed copy of the downstream data sequence, and correlating said time delayed copy during said quiet period with an echo of the downstream data sequence reflected from a fault.
In this technique, random or cyclic sequences are transmitted in place of single impulses, the reflections of the sequences being detected by a correlation process. The use of a pseudo random sequence enables reflections to be located up to the duration of the sequence length. This method has the benefit that the correlation between the launched and reflected signal may be performed over a greater proportion of the time than is generally possible for an impulse based system, and additionally, that the properties of the test signal may be designed to optimise the detector characteristics. This enables very high sensitivities to be achieved with relatively low cost optical laser and receiver components. It thus becomes practical to use a near conventional transmitter design and near conventional data receiver design such that the fault location system may
REFERENCES:
patent: 5177354 (1993-01-01), Tomita et al.
patent: 5187362 (1993-02-01), Keebie
patent: 5321541 (1994-06-01), Cohen
patent: 5491573 (1996-02-01), Shipley
patent: 5528404 (1996-06-01), MacKichan
patent: 5570217 (1996-10-01), Fleuen
patent: 5859716 (1999-01-01), O'Sullivan et al.
Fisher David Anthony
Roberts Kim Byron
Nortel Networks Corporation
Pascal Leslie
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