FAULT LOCATION IN OPTICAL COMMUNICATION SYSTEMS

Abstract: In an optical transmission system incorporating a passive optical network (PON), fault detection is effected by a time domain reflectometer technique in which the system transmitter generates a sequence having statistical properties equivalent to a continuous random binary sequence. A delayed copy of the sequence is correlated with an echo of the sequence reflected from a fault, the time delay required to effect correlation giving an indication of the fault location.

Description: FAULT LOCATION IN OPTICAL COMMUNICATION SYSTEMS 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. 8 August 89, 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 be combined with the normal data transmit and receive functions, with a time resolution of the order of a metre. In our arrangement and method, the transmitted data itself is used as test sequence. The receiver is provided with the ability to detect reflections using either (i) a correlation window noise tracking algorithm or (ii) by de-composition requiring the subtraction of an estimate of the transmitter primary interference path. In a second embodiment, a passive optical ranging method is employed to produce a system capable of detecting the loop delay of discontinuities in the fibre and hence determining cable breaks inferred by significant changes in the reflection of information. In this embodiment the pseudo random sequence may be launched at a level below that which would otherwise affect the error free detection of normal data transmission. Embodiments of the invention will now be described with reference to the accompanying drawings in which :- Figure 1 is a schematic diagram of an optical transmission system incorporating fault location equipment according to the invention; Figure 2 shows the time domain reflectometry or fault location in detail; Figure 3 shows a reflectometry arrangement coupled to a bidirectional amplifier station; and Figure 4 shows a further network arrangement. Referring first to figure 1 , the transmission system includes a data transmitter unit 1 containing a data sequence formatter 2, an optical transmitter 3 and an associated controller 4. The data transmitter unit is coupled via a fibre 5 to a passive optical splitter 6 and through a further fibre path to an optical receiver 7 and data receiver unit 8. The data receiving unit comprises an optical receiver 9 with its controller 10 and a data re-formatter 11 , providing data re-formatting as required for the next stage in the data path. The optical splitter allows optical signals to be combined and separated using optical wave guides, and has the additional property that the optical signal coupling is directional so that backward reflections due to the splitter itself, are typically one tenth or less in amplitude in comparison with the forward travelling light, this property applying to both transmission directions. On the upstream side of the system, a data receiver unit 13 is placed on a second fibre 18 connected to the left hand side of the splitter allowing it to see reflections from output ports of the splitter and fibre network to the right of the splitter (and any other upstream data source). This receiver signal processing unit contains an optical receiver 12, a controller 15, a signal processor block 17 and may also contain a data re-formatter block 16. In the arrangement of figure 1 , the data transmitter unit 1 sequence source formatter contains a data scrambler 19 which scrambles the data such that the downstream transmitted data sequence has statistical properties equivalent to those of a continuous random binary sequence. This allows the transmitted data itself to be used in the correlation process, this transmitted data being orthogonal to the upstream data which is also randomised. An upstream data formatter 21 which contains the same functions as the data transmitter unit 1 may transmit an upstream data sequence that leaves a segment, i.e. one or more time slots, of the upstream frame unused or reserved specifically to provide a quiet period in which the data receiver unit 13 can calculate the correlation coefficient between a delayed version of the transmitted data sequence from the transmitter unit 1 and potential reflections of it arising from discontinuities in the fibre at the splitter or to the downstream side of the splitter. ln its simplest form this original processing function, illustrated in figure 2, includes an optical to electrical conversion function 32 which converts the optical input signal into a voltage proportional to the optical power. After a low pass filter 33 to remove noise outside of the frequency band of the unwanted signals there is a DC balancing circuit comprising a comparator 34, a digital to analogue converter 35, a first up/down counter 36 and a D-type flip-flop 37. which adjusts the threshold level of the D/A comparator converter 35 to a value resulting in a substantially equal number of ones and zeros. This circuit is only active during that part of the frame in which there is no signal other than reflections from the fibre discontinuities, consequently the noise level of the receiver will in practice be of greater amplitude than the signals reflected. The product of the data sequence is formed by the digital multiplier 41 from the data sequence delayed by n clock cycles and the digitised data output from D-type flip-flop 37 and is integrated using an up down counter 39). By varying the value of the delay Z _n 40, the output of the up down counter 39 gives the correlation between the signal received as the upstream fibre and the downstream transmitted signal and hence directly allow measurement of reflections with a resolution approximately equal to one bit at the data clock rate. For accurate control of the decision threshold of the D-type flip flop 37 it is advantageous if the up-down counter used to drive the D to A converter has a step size that is a fraction of the D/A resolution (for example one sixteenth) in order that wander of the threshold value due to random noise does not dominate the decision process. In practice the noise level with no signal will be greater than the reflections due to cable discontinuities. Figure 1 shows how a receiver unit may be configured to provide the means for calculation of the amplitude of upstream reflections from the downstream signal path. In the case where there is not a transmitter on the downstream side of the splitter, this unit is then serving no purpose other than time domain reflectometry. Further receiver units may be added on the downstream side of the splitter to produce a simplex point to multipoint system and a separate fibre network provided for upstream. Alternatively the directional properties of the splitter may be sufficient to provide for duplex transmission in which case the receiver unit may be used both for time domain reflectometry and data reception and data transmitter units 21 employed. Referring now to figure 3, this shows a further reflectometry arrangement. As in the arrangement of figures 1 and 2, the transmitted data is used as the test signal. The reflectometer is coupled to an optical transmission path 41 on either side of a bidirectional amplifier 42 using the four port taps 43a, 43b that are provided for monitoring the amplifier performance. Typically the taps remove about 5% of the power being transmitted along te fibre path. In a bi-directional amplifier, signals are present, in different optical wavelength bands, in the forward and reverse directions. A four port tap is provided at both ends of the amplifier to monitor the input and output signals, and to perform analogue maintenance in both directions. The reflection measurement calculations can then be done in both directions. If the signals in the two directions are correlated, then interference can be produced in the reflection array. Known correlations, such as the SONET frame patterns, can be eliminated by zeroing those frequency components. Other signals may require more complex compensation using all four data arrays, or even the maintenance procedure of removing or changing one signal to prove that the result is not an artefact of signal correlation. Changing one signal e.g. to SONET AIS would eliminate its data content and break the uncompensated correlations. The output ports of the taps 43a, 43b are coupled each to a respective photodetector, e.g. a PIN diode 44a, 44b, 44c and 44d. The currents from the PIN photo diodes are amplified and band limited (~ 0 to 5 MHz) by respective transimpedance amplifiers 46a, 46b, 46c and 46d whose outputs are coupled each to a respective fast analogue to digital converter 47a-47d. The output signals from the respective analogue to digital converters are referred to below as the r1 , s1 , r2 and s2 signals. The fast, typically about 10 MHz, analogue to digital converters 47 simultaneously sample the levels of the outgoing and incoming optical signals and pass the resulting arrays of data to a signal processor 48 whereby the Fast Fourier transforms of the signals are computed, divided, and then Inverse Fast Fourier Transformed to produce a reflection array. The elements of this array r(n) correspond each to the reflection from a location at a distance n * Δl_ down the fibre, plus noise and interference. The largest reflections can be compared to thresholds, e.g. to generate alarms, and can be displayed to the user via the transmission system's software. Since the stimulus signal s(t) consists of all the low frequency optical signals present, the dominant noise term is the locally generated thermal noise. Improved sensitivity is achieved by lengthening the time span of the stimulus signal. The resolution of the distance is inversely proportional to the bandwidth of the stimulus signal. For a bandwidth of 5 MHz and assuming Nyquist sampling, a resolution of about 20 metres can be achieved. The spatial resolution can be improved by faster sampling or by phase correlation signal processing techniques to resolve better than one sampling interval. Note that this resolution depends on an acceptable autocorrelation function for the particular stimulus signal. This autocorrelation function can be computed as a check for the acceptability of the particular s(t) as a stimulus signal. One problem with using the on-line data signal as the stimulus is the possible correlation with the previous data signal giving rise to ghost reflections. Correlating the current stimulus with previous stimulus signals is one way to check for this phenomenon. If a significant correlation with a previous stimulus is detected, then the present stimulus is rejected. Note that lengthening the time span of a stimulus reduces the likelihood of this kind of correlation. The noise depends upon the amount of signal variation in the relevant frequency band (0-5 MHz) and the amount of averaging or filtering done to the results. The dither that is applied for analogue maintenance helps to ensure that there is low frequency content. The resolution of the distance depends upon the sampling frequency. For example, a sampling frequency of 10 MHz produces a ΔL of 100 ns divided by 5 ns/m, equivalent to a 20 m round trip, or 10 meters one way. The spatial resolution can be improved by faster sampling or by phase correlation signal processing techniques to resolve better than one sampling interval. By providing the bi-directional optical amplifier with isolators in the two different wavelength paths to restrict the direction of propagation of particular wavelengths, then the information from all four monitor ports can be used to compensate for signal correlations (where F represents the fast Fourier transform). If M(t) = F-1[ F{b}/F{a} ] and r2(t) = F"1[ F{d}/F{c} ] for the two directions, then the correlation of b and c can be subtracted from b to remove those elements in the frequency band travelling in the other direction, and similarly the correlation of d and a can be subtracted from d. r1 = F- 1 [ F{b}-F{c} * scale)/F{a} ] Where 'scale' is the correlation of b and c divided by the RMS value of c. Known reflections, such as those originating from within the amplifier can be calibrated in the factory, and then dynamically calculated as a function of the amplifier gain and subtracted off. Another interference compensation method is to examine the spectrums F{b} an F{c} , and zero or attenuate F{b} at those frequencies where F{c} is above a predetermined threshold relative to F{b}. This reduces the number of components that may interfere. A further development allowing simplex operation combined with time domain reflectometry is shown in figure 4. In this case two splitters (51 , 52) are employed, one for each transmission direction. A path 58 is provided for reflections from the downstream path to enter one leg of the upstream path. This will incur attenuation dependent on the upstream splitter loss and this could be increased as necessary to ensure that under no circumstances would a reflection from the downstream path compromise the performance of the upstream path. Similarly, reflections from the upstream path can enter the downstream path in the same way via path 59, but will be equally attenuated. Reflections in the upstream path to the left of the splitter in figure 4 may be determined using the same method as described employing one of the downstream optical receiver units and a downstream marshalling technique e.g.. as described in our specification No GB-A-2272608. In this case the data captured in this window would be correlated with the data received at the upstream receiver unit on the upstream side of the diagram to move all processing functions to the upstream side forming the head end of a passive optical network system. A second method is to introduce a path between the downstream and upstream splitter in which by virtue of the downstream splitter loss (typically 15 dB for a 32 way splitter), any attenuated remnant of the downstream data is transmitted towards the transmitter units on one downstream side of the diagram. In this case reflections from any discontinuity between the splitter and one downstream side optical transmitter units will pass back through the upstream splitter and arrive at the data receiver unit 55 (and the standby data receiver unit). Additional attenuation could be placed in the inter coupler path in order to eliminate the downstream signal on the upstream fibres compromising the performance of the transmitters (63, 64, 65) or the possibility of reflections from this being large enough to compromise the receiver performance (55, 56). Alternatively a dedicated test transmitter (59) (Fig. 3) could be used to provide a TDR mechanism in which case, in order to avoid interference with the upstream path, operation of this test transmitter in a very low power mode could be used. If there is no space in the upstream data format to allow for a correlation window, then adaptive coefficients can be used to eliminate the received signal components, thus providing effectively a decision directed equaliser where the residue is used as the correlator input to determine reflection coefficients, adaptation may employ the well known gradient estimation algorithm.

Claims: CLAIMS: 1. 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. 2. An optical communications system as claimed in claim 1 , wherein the correlation means comprises a digital multiplier, whose output is coupled to an up-down counter whereby to provide a count indicative of the correlation. 3. An optical communications system as claimed in claim 1 , wherein the system incorporates amplifier stations, and wherein the fault location arrangement is coupled to the system via one or more taps provided at a said amplifier station. 4. An optical communications system as claimed in claim 3, wherein wherein the correlation means includes means for determining the fast Fourier transforms of the upstream and downstream signals, means for determining the ratio of the transforms, and means for determining the inverese transform of said ratio whereby to generater a reflection array. 5. An optical communications system as claimed in any one of claims 1 to 4, and comprising a passive optical network. 6. 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. 7. A method as claimed in claim 6, wherein said correlation is effected by determining the fast Fourier transforms of the upstream and downstream signals, determining the ratio of the transforms, and determining the inverese transform of said ratio whereby to generater a reflection array.

Inventors: FISHER DAVID ANTHONY, ROBERTS KIM BYRON, FISHER, DAVID, ANTHONY, ROBERTS, KIM, BYRON
Applicants: FISHER DAVID ANTHONY, NORTHERN TELECOM LTD, ROBERTS KIM BYRON, FISHER, DAVID, ANTHONY, NORTHERN TELECOM LIMITED, ROBERTS, KIM, BYRON
Application year: 1995
Main classification: H04B10
IPC codes: G01M11, H04B10