Spread spectrum Bragg grating fibre fault finding and identification

This paper proposes a new technique for identifying faults within a passive optical network. A series of Bragg gratings are written on to each arm of the network such that the reflections from a single pulse will form a different spread spectrum code for each arm of the network. Since the code written on to each arm of the network is known, the reflections from that arm can be examined without interference from the reflections from the other arms in the network by correlating the returning signal with a spread spectrum code similar to that of the arm interrogated. If a fault occurs in the network somewhere before the Bragg gratings in that arm, the signal detected at the receiver from the Bragg gratings will be reduced. Conventional OTDR's quickly run out of dynamic range for Rayleigh backscatter when interrogating a network with many passive optical splits. Ifa fault occurs in a passive optical network, an OTDR would be able to detect it and determine a range to the fault, but would be unable to determine which arm of the network the fault was on. The novel technique described in this paper uses spread spectrum coding of Bragg gratings on each arm of the passive optical network to detect a fault, and the arm of the network that the fault is on can be identified by the coding. Exact positioning of the Bragg grating series on the arm with respect to the Bragg gratings on the other arms is not required due to the nature of the spread spectrum coding.