Method with device for detecting reflections

Abstract: Method for determining fault locations in a local optical network. In the case of a fibre failure arising, a measuring signal is fed to the network, the reaction of the network to said measuring signal being analysed and converted into a location indication of said fibre failure. The output signal of the optical light source, in addition to the normal signal, is also modulated by the measuring signal. The measuring signal is preferably an OFDR signal, a signal having a frequency which varies according to a defined pattern. The signal reflected at the location of the fibre failure is converted into an electrical reflection signal. Said reflection signal is compared with the original measuring signal and, by means of frequency analysis of the two signals, the location of the fibre failure is calculated.

Description: A. BACKGROUND OF THE INVENTION The invention relates to a method for determining fault locations in an optical network for transmitting optical signals via optical fibres, such as, for example, telephone signals or TV signals between an exchange and subscribers, which optical signals are generated in a light source with a particular wavelength, of which the output signal, by means of an electro-optical modulator, is modulated by an electrical modulation signal fed to said modulator, an optical measuring signal being fed to the network at least in the case of a fibre failure arising, the reaction of the network to that measuring signal being analysed and converted into a location indication of said fibre failure. Such a method is disclosed by reference [1]. Therein it is proposed to use, for determining the location of a fibre failure, a pulse-shaped "OTDR" signal (OTDR = Optical Time Domain Reflectometer), via a separate "maintenance" wavelength. After having detected that somewhere in the network a fibre failure (for example, fracture) has arisen, it is first detected in which fibre or fibre "tree" the failure is situated and, thereafter, in order to determine at what distance from the signal source that failure is situated, a pulse signal is emitted via the fibre and, based on the reflection time thereof, the failure location is calculated. A drawback of the known method is that the entire network must be suitable for at least two different wavelengths. Emitting the pulse-shaped measuring signal -which must have a fairly large power - via the same wavelength as that via which the normal signal is transmitted, is undesirable, as the non-faulty subscribers would be inconvenienced by said pulse signal. Per se, another method for measuring reflection locations is disclosed by [2], namely the OFDR method (OFDR = Optical Frequency Domain Reflectometer). In this case, it is proposed to emit a measuring signal having a varying ("sweeping") frequency. The signal reflecting at the fault location is compared with the original measuring signal (actually with a replica thereof), and, by means of frequency analysis of the two signals, the reflection location can be calculated. A device using this method is known from [3]. B. SUMMARY OF THE INVENTION The object of the invention is to provide a method for determining the location of a fibre failure in an optical network, in which it is not necessary to employ a separate wavelength, while the connected subscribers are still not or virtually not inconvenienced by a measuring signal emitted on the network (while the normal signal transmission to or to/from the non-faulty subscribers thus continues to proceed). By means of the invention, provision is made for the output signal of the light source, in addition to said electrical modulation signal, to be furthermore modulated by said electrical measuring signal, resulting in a corresponding optical measuring signal. The measuring signal is thus transmitted through the fibres on the same light "carrier" as the normal signal. It is clear in this context that both the normal optical signal and the optical measuring signal will indeed reach the non-faulty subscribers, but will not reach the faulty subscribers. Both signals will be reflected at the location of the fibre failure. The reflected "normal" signal in this case is of no further significance; the parameters of the reflected measuring signal, however, such as, for example in the case of OTDR the precise moment of the pulse(s), or as in the case of OFDR the frequency spectrum of the measuring signal, are known. As a result it is possible to determine the failure location on the basis of the reflection signals. Since OTDR measuring pulses, if emitted via the same wavelength as the normal modulation signal - as proposed by the present invention, in contrast to the prior art - cause unacceptable inconvenience to the non-faulty subscribers, for that reason, in fact, the use of OFDR is preferred over OTDR. If, as in the known method, separate transmission wavelengths are employed for transmitting the normal signals and the measuring signal, OTDR pulses cause no inconvenience to non-faulty subscribers, since the signals, after all, are in fact transmitted entirely separately. The method according to the invention is thus preferably further characterised in that the electrical measuring signal is an OFDR signal having a frequency varying according to a defined pattern, reflections caused by the corresponding optical measuring signal at the location of the fibre failure resulting in an optical reflection signal which is converted by an opto-electrical converter into an electrical reflection signal, and said electrical reflection signal being compared with the electrical measuring signal, and the location of the fibre failure being calculated by means of frequency analysis of the two signals. If OFDR is used, the measuring signal "sweeps", as it were, through the normal signal, and as a result, non-faulty subscribers are in practice not inconvenienced or virtually not inconvenienced thereby. C. REFERENCES [1] Hoppitt, C.E.; Astbury, M.L.; Keeble, P.; Chapman, P.A.; Operations and maintenance experience of the Bishop's Stortford fibre trail; Third IEE Conf. Telecomm. Edinburgh, 17-20 March 1991 (Conf. Publ. No. 331), IEE, xiii+387 PP., PP.165-9 [2] MacDonald, R.I.; Frequency domain optical reflectometer; Appl. Opt. (USA), vol. 20, no. 10, PP. 1840-4, 15 May 1981 [3] Patent abstracts of JP-A-57 118 136, 22 July 1982. D. ILLUSTRATIVE EMBODIMENT Figure 1 shows an illustrative embodiment of the invention. What is shown is a passive optical subscriber network for distributing TV signals, incorporated in which is laser 1 of which the optical output signal is modulated by a (Mach-Zehnder) modulator 2. The modulated signal is transmitted, via a system of splitters 3 and fibres 4, to the subscriber side, where the signal is converted by an opto-electrical receiver into an electrical signal, which is presented to the subscriber apparatus. The Mach-Zehnder modulator 2 is driven by an electrical TV signal to be distributed to the subscribers. Said signal is AM/VSB modulated, for example, and can be demodulated on the subscriber side. The TV signal is presented via an electrical coupler 7, which has two inputs and one output. The output is connected to the electrical input of the modulator 2. The first input is presented with the TV signal, while the second input is connected to the output of a second coupler 8. As soon as a failure arises in the system, this is noticed virtually immediately by the system-monitoring hardware and software. If one (or more) fibres 4 prove to have broken, attempts will be made to detect the location thereof by emitting a measuring signal. The non-faulty subscribers, who continue to be supplied with signals in the normal way, should notice as little as possible of said measuring signal being emitted. According to the present illustrative embodiment, in the case of failure a measuring signal is presented to the input of coupler 8, which signal is generated by a "frequency sweep" generator (not drawn). The frequency sweep and the sweep frequency and possible further frequency data thereof are known. The measuring signal is presented to the modulator 2 via coupler 8 and coupler 7, and thus (co-)modulates the optical signal, as a result of which an optical measuring signal is emitted. The power of the measuring signal may remain limited, while it is still possible to carry out an effective measurement, an attractive characteristic of the OFDR method. At the place where the fibre has broken, the measuring signal is reflected. The reflected signal is picked up via the splitter 3 and is converted, via an optical (APD) receiver 9, into an electrical reflection signal. The original measuring signal and the reflected signal are both fed to a mixer 10 and via the latter to a frequency analyser 11, where the mixing product is subjected to frequency analysis. Based on that analysis, it is possible to determine the location of the failure.

Claims: Method for determining fault locations in an optical network for transmitting optical signals via optical fibres, such as, for example, telephone signals or TV signals between an exchange and subscribers, which optical signals are generated in a light source with a particular wavelength, of which the output signal, by means of an electro-optical modulator, is modulated by an electrical modulation signal fed to said modulator, an optical measuring signal being fed to the network at least in the case of a fibre failure arising, the reaction of the network to that measuring signal being analysed and converted into a location indication of said fibre failure, CHARACTERISED IN THAT the output signal of the light source, in addition to said electrical modulation signal, is furthermore modulated by said electrical measuring signal, resulting in a corresponding optical measuring signal, the reflected part of which is analyzed. Method according to Claim 1, CHARACTERISED IN THAT the electrical measuring signal is an OFDR signal having a frequency varying according to a defined pattern, reflections caused by the corresponding optical measuring signal at the location of the fibre failure resulting in an optical reflection signal which is converted by an opto-electrical converter into an electrical reflection signal, and said electrical reflection signal being compared with the electrical measuring signal, and the location of the fibre failure being calculated by means of frequency analysis of the two signals. Optical network, comprising a light source (1), a modulator (2), and optical fibres (4), all these for transmitting optical signals such as, for example, telephone signals or TV signals between an exchange and subscribers, light being generated with a defined wavelength in that light source, and the exiting light being modulated by the electro-optical modulator (2) under the control of an electrical modulation signal fed to said modulator, CHARACTERISED IN THAT said modulator (2) is connected to means (7) for feeding in said electrical modulation signal and, moreover, an electrical measuring signal, for detecting fault locations by reflectrometry. Optical network according to Claim 3, CHARACTERISED IN THAT the electrical measuring signal is an OFDR signal having a frequency varying according to a defined pattern.

Inventors: VERHOOF JEROEN WALTHERUS, VERHOOF, JEROEN WALTHERUS
Applicants: NEDERLAND PTT, KONINKLIJKE PTT NEDERLAND N.V.
Application year: 1993
Main classification: H04B10
IPC codes: H04B10, G01M11