A method for the detection, localisation and quantization of faults in tree-structured passive optical networks is presented. This method is based on the analysis of OTDR (Optical Time Domain Reflectometry) traces. Several kinds of faults have been generated in a 1×8 passive optical network to illustrate the effectiveness of this technique. The use of OTDR has proven to be very efficient for the maintenance of optical links. The access to only one extremity of the link is necessary; the OTDR signal wavelength and the data signal wavelength can be different so that the monitoring of the link involves no interruptions of the communications. If a fault occurs in a point-to-point optical link, a simple comparison and analysis of OTDR traces leads to an accurate determination of its position, its kind and parameters such as its loss and return loss. The problem is more complex in the case of a tree-structured network, since the trace acquired at the top of the network results in the combination of the reflected and backscattered light signals coming from the different optical paths. If a fault occurs in a branch of a tree-structured network, it can be easily detected comparing the trace acquired at the head of the failed network with a reference trace. However the affected branch and the fault's parameters (kind, loss, return loss) are more difficult to determine. The method explained in this paper solves this problem. It is illustrated with the analysis of a 1×8 passive optical network. Several kinds of faults have been generated and the results obtained with a software implementing this method are presented.
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