BACKGROUND OF THE INVENTION 
     The present invention relates to the localization of faults in fiber optic systems. 
     Optical time domain reflectometers (OTDRs) are state of the art instruments to localize failures (e.g. breaks, bends, mechanical stress etc.) in fiber optic systems or networks comprising one or more fiber links e.g. for connecting one or more optical components. The reflectometric principle has evolved to a widely accepted technique for capturing backscatter signals because of its suitability for one-port measurements with distance ranges up to 200 km and even more. 
     An OTDR injects a pulsed signal into a fiber optic network comprising one or more optical components, e.g. fibers, filters, couplers, or the like. The pulsed signal typically is a laser pulse at a certain laser wavelength. A small amount of the pulsed signal is continuously scattered back in the opposite direction towards the OTDR, representing attenuation, loss, and reflectance in the optical network under test. By measuring the amount of backscattered and/or reflected signal versus time, the loss versus distance of the optical network is measured. The results of one or more individual measurements are usually combined and that combination represents an OTDR trace of the optical network. In a typical OTDR trace received from an OTDR measurement, the x-axis corresponds to the distance between the OTDR and a location in the optical network. The y-axis shows the power level of the reflected signal level revealing details about the optical link loss in the optical network. A detailed description of the current knowledge about OTDRs and analyses of OTDR traces is given in detail by the inventor in the book by Dennis Derickson: “Fiber optic test and measurement”, ISBN 0-13-534330-5, 1998 on pages 434 ff. 
     The OTDR allows the localization of faults in fiber optic systems by evaluating OTDR traces interpreting faults as specific discontinuities. This evaluation, however, finds a ‘natural’ limitation in particular for spatially extended fiber optic systems (e.g. long fibers) where the backscattered signal eventually becomes lower than the OTDR receiver&#39;s noise level. 
     A further limitation of the applicability of OTDR measurements for localization faults in fiber optic systems occurs when fiber optic links equipped with erbium doped amplifiers (EDFAs) need to be tested. In this environment, the standard OTDR faces some obstacles. Firstly, an EDFA produces considerable optical ASE noise (amplified spontaneous emission) which dazzles a sensitive instrument like an OTDR if no precautions were taken. This problem can be tackled e.g. with a narrow-band optical filter in the receiver path. Secondly, EDFAs commonly contain optical isolators that prevent scattered and reflected light from returning to the input port. Thirdly, the very high signal levels in an EDFA transmission link can show nonlinear interference with an OTDR&#39;s probe signal. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved possibility to localize faults in fiber optic systems. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims. 
     According to the invention, a spatially elongated optical element, such as an optical fiber, is provided with a plurality of reflecting events that are spatially allocated along the optical element. 
     Possible faults in the optical element are localized by emitting a signal into the optical element, measuring signals reflected from the reflecting events, and comparing the measured reflected signals with expected signals representing the optical element without faults. In case that there are no faults in the optical element, the measured reflected signals will substantially correspond to the expected signals. In case that there are one or more faults in the optical element, the measured reflected signals in a distance behind each one of the one or more faults will be attenuated to a certain degree, or, in other words, show at least a different (e.g. decreased) amplitude, or even disappear, with respect to the expected signals. 
     The invention can be executed by a system comprising means for emitting an optical signal into the optical element and means for measuring the reflected signals. This can be embodied e.g., by an OTDR as known in the art. The system further comprises means for comparing the measured reflected signals with expected signals representing the optical element without faults. This can be embodied and executed e.g. by a data processing unit which might be part of an OTDR or provides a separate device. 
     The expected signals can be received or determined, e.g. from a previous measurement, such as an acceptance measurement, or can be calculated or otherwise received for example from theoretical analysis (e.g. simulation or modeling) and/or from information about the fiber (such as physical properties). 
     This allows that a measurement of signals reflected at one or more of the plurality of the reflecting events provides information about possible faults in the optical element. 
     The reflecting events can be represented by any location where a change of the refractive index occurs and are preferably gratings such as fiber Bragg gratings. 
     The reflecting events can be embodied as individual devices, e.g. between successive parts of the optical element such as individual fiber parts, but are preferably incorporated within the optical element, e.g. as fiber Bragg gratings. 
     In a preferred embodiment, the reflectance value of each reflecting event is determined to depend on the distance to a measuring point at which the reflected signals are measured. Preferably, the reflectance values increase with increasing distance to the measuring point to compensate the attenuation of the element. This allows receiving reflected signals with substantially constant amplitudes independent of the distance of the reflecting event. This also allows reducing an influence of possible broadband noise sources, such as EDFAs, on the measurements. 
     The reflecting events are preferably selected to provide a defined reflectance characteristics versus wavelength. In a preferred embodiment, reflecting events, such as Bragg gratings, are used which provide a specific center wavelength at which a signal is reflected or partially reflected, while substantially all other wavelengths are not reflected. 
     In a preferred embodiment, the reflecting events are selected to provide different reflectance characteristics versus wavelength for different wavelengths. This allows employing measurements with different measuring wavelengths to localize individual reflecting events corresponding to the respective wavelength(s). This way the reflectance values of the reflecting events at a specific wavelength can be chosen independently and need not depend on the reflectivity and the associated insertion loss of the reflecting events at the other wavelengths. In that case, measurements for determining the time dependencies of the reflected signals are not required in order to clearly assign a specific reflected signal to a specific reflecting event. 
     The fault localizing according to the invention can be applied to single optical elements, e.g. for monitoring a specific fiber, and/or to an optical network comprising one or more individual elongated optical elements (e.g. fiber segments) that might be coupled between (optically non-elongated) optical components, such as optical filters, switches, or the like. In case that reflected signals from different reflecting events within the optical system to be measured/monitored exhibit the same characteristics, additional information has to be gathered in order to distinguish the different reflecting events. Such information can be time information e.g. the time between emitting a measuring signal and receiving the reflected signal as automatically determined by an OTDR. 
     Instead of one reflective event at a given location a pair or more than two reflective events can be grouped together to generate a specific pattern of reflections. Such a pattern can be detected easily and automatically directs to a specific location. 
     When optical amplifiers, such as EDFAs, are employed between successive optical elements, or parts thereof, each one of those optical elements is preferably provided with a sequence of reflecting elements with distance dependent reflectance values and/or with different reflectance characteristics versus wavelength for different wavelengths. This allows localizing faults in each one of the optical elements. In case that optical isolators are employed preventing scattered and/or reflected light from returning to the input measuring port, a loop back path (for back-travelling signals) or other adequate means might have to be provided to ensure that the reflected signals can return to the measuring point. 
     The measuring frequency is preferably selected to differ from possible or actual transmission frequencies applied on the optical element e.g. for communication purposes, so that the measurement can be executed independently of such ‘traffic’ on the optical element(s). 
     Other advantages versus the conventional OTDR measurements are: 
     strong return signals, since reflected signals are generally much stronger than backscattered signals, 
     high measurement speed, due to the normally stronger reflected signals (compared with the backscattered signals) which do not require excessive signal averaging over the time, 
     code correlation is possible to further enhance SNR, 
     live fiber monitoring is possible independent of fiber traffic, and 
     backscatter measurements are not necessary. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and many of the attendant advantages of the present invention will be readily appreciated and become better understood by reference to the following detailed description when considering in connection with the accompanied drawings in which: 
     FIGS. 1 a - 1   d  show an optical fiber  10  according to the invention and respective OTDR traces for examples according to the invention, and 
     FIGS. 2 a - 2   d  show an example of locating fiber breaks and losses in an EDFA link and examples for OTDR traces therefrom. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 a  shows an optical fiber  10  according to the invention. The fiber  10  comprises as a ‘fault coding’ a first reflective event RE 1  at a location x 1  (from a first end  20  of the fiber  10 ), a second reflective event RF 2  at a location x 2 , and a third reflective event RE 3  at a location x 3 . The reflective events REi can either be provided at equal distances or otherwise distributed over the length of the fiber  10 . In particular for monitoring faults in specific locations of the fiber  10 , one or more reflective events REi might be provided in a predetermined pattern within such specific locations. In the embodiment of FIG. 1 a,  the reflective events REi are provided by gratings, preferably Bragg gratings. 
     For localizing/monitoring faults in the fiber  10 , an OTDR  30  is coupled either directly or via a fiber network  40  to the fiber  10 . In operation, the OTDR  30  emits a measuring signal to the fiber  10  and measures signals reflected from the fiber  10 . For assessing whether a fault has occurred in the fiber  10 , the OTDR  30  compares the measured reflected signals with expected signals representing the fiber  10  without faults. The expected signals are preferably signals from a former measurement or from an initial measurement of the fiber  10 . 
     FIG. 1 b  shows an example of an OTDR trace  50  of the fiber  10  determined by the OTDR  30  for the fiber  10  of FIG. 1 a  in a fault-less condition. A second OTDR trace  60  of the same fiber  10  depicts a case wherein a fault occurs at a location f 1  between the reflective events RE 1  and RE 2 . In this case, the reflected signals from the reflective events RE 2  and RE 3  have been entirely disappeared because of a complete interrupt of the fiber  10  at f 1  e.g. due to a fiber break. 
     In the example of FIG. 1 b,  the reflective events are provided with substantially equal reflectance values, so that the heights H 1 , H 2 , and H 3  of the respective reflectance peaks, with respect to the last measured value before the respective reflective event RE i , are substantially equal. 
     FIG. 1 c  shows an OTDR trace  70  of another example, wherein the reflectance values of the reflective events RE i . reflective events are selected in a way that the absolute amplitude H, with respect to a defined absolute level, is substantially equal for reflective events REi. This is in particular advantageous in cases where the signal level of the OTDR trace  70  (at least partly) falls below a noise level N, so that the signal level of the measured signal (with exception of the reflectance peaks from the reflective events RE i ) is lower than the noise level N. This allows determining faults between successive reflective events RE i  over long distances even when a backscatter analyses is not possible anymore due to the noise level N. 
     In an embodiment for the example of FIG. 1 c,  wherein the intervening fiber segments between the reflective events RE i  and RE i+1  exhibit the attenuation A 1 , the reflectance value r i+1  , for a following reflective event RE i+1  is selected to compensate for two times the attenuation A i . 
     FIG. 1 d  depicts another example of the fiber  10 , wherein the reflectance values of the reflective events RE i  exhibit different wavelength dependencies. In FIG. 1 d,  the frequencies λ i  of the measuring signals from the OTDR  30  are selected in a way that the frequencies λ i  coincide with the center reflecting frequencies of the reflective events RE i.  For the sake of clarity, the three OTDR traces in FIG. 1 d  are depicted vertically spaced to each other. 
     The OTDR traces depicted in the FIGS. 1 exhibit a certain insertion loss for each reflective event RE i , indicated by vertical displacements of the reflected signal from the fiber  10  at the locations x i  (cf. also the aforementioned book by Dennis Derickson e.g. on page 457). It is clear, however, that the respective insertion loss value depends on the respective type and embodiment of the reflective event RE i . 
     It is apparent that one or more fibers  10 , e.g. as depicted in FIG. 1 a,  with identical or different fault coding can be applied for connecting other optical components. Although the OTDR traces will be more complex in that case, the principles of the invention are the same and can be applied thereto accordingly. 
     The invention shall now be explained for an example of locating fiber breaks and losses in EDFA links. However, it is clear that the invention can be used for any optical fiber arrangement with or without optical amplifiers. Further more, the example is based on measurements with different wavelengths and on reflecting events with different wavelength dependencies. However, it is clear that the principle of the invention does not require different wavelengths and that measurements with one wavelength and reflecting events with similar or identical wavelength dependencies can also be employed. In that case, the reflectance values are preferably adopted to depend on the distance from a measuring point, e.g. in a way that the reflectance increases with increasing distance, so that substantially constant amplitudes of the reflected signals can be received. 
     The invention avoids backscatter measurements in favor of improved immunity to high ASE signals, increased measurement speed, and enhanced measurement range. Instead of backscatter, the invention evaluates reflections on the link which were generated by reflecting events such as gratings e.g. worked into the fibers. Such gratings (e.g. fiber Bragg gratings) preferably act as partially blocking filters when exposed to light at selected wavelengths. 
     In a preferred embodiment as depicted in FIG. 2 a,  an optical fiber link  100  is furnished with gratings, preferably Bragg gratings, with different blocking wavelengths λ i  embedded at different locations into a fiber segment  10 A. The number of gratings, more precisely the distance between them, determines the spatial resolution at which a fault can be located. Probing the fiber  10 A under test with light pulses at a wavelength that hits the center frequency λ i  of such a grating causes a fraction of the pulses to reflect and travel back in the opposite direction. In case of a fault between successive gratings, the first grating (from the direction of the measuring point) still reflects the measuring signals while the fault fully or partly prevents the measuring signals to travel to the second grating, or at least attenuates them. In the arrangement of FIG. 2 a,  the fiber segment  10 A is coupled between optical amplifiers  105 A and  105 B. 
     In case isolators block the optical reverse path, a loop-back path  110  is preferably provided, which feeds the reflections into another fiber  10 B that carries the signals to the input port. The loop back path  110  can incorporate a forward path and a path for back-reflected light as well. In EDFA links (e.g. undersea links), this can be solved easily, because fibers are normally running in pairs. Such bi-directional links commonly are realized by two unidirectional fibers transmitting signals in opposite directions. An arrangement consisting of optical couplers that connects both unidirectional fibers can provide such a return path. 
     For a complete check of the fiber link  100  as depicted in FIG. 2 a,  a number of light pulses at different wavelengths λ i  with i=1,2,3,. . . is launched towards the fiber  10 A. The fiber  10 A responds to each individual probe signal with a returning pulse R i  that can be assigned to a specific location by the measured round trip times. A fiber break or additional link loss can be discovered and located by continuously comparing the height of the individual reflections R i  with expected values for a fiber without faults. In case of a fault, the heights of the individual reflections R i  after the fault will be different (here: decreased) compared to the expected values, or even disappear. 
     FIG. 2 b  shows a single pulse (or code) measurement at λ 3  which gives a return signal with a wavelength independent part R 0  (this is part of the forward travelling signal) and a wavelength dependent reflection R 3  at a position that corresponds to the location of fiber grating λ 3 . The fiber attenuation accounts for the smaller height of R 3 . 
     FIG. 2 c  shows the same measurement as in FIG. 2 c  at wavelength λ 1 . 
     FIG. 2 d  shows a measurement on the link  100  with adjusted reflectance values that result in equal reflection heights. In case of a fiber break between the locations of R 2  and R 3  the reflections R 3  and R 4  cannot be detected any more. 
     The FIGS. 2 b-d  show OTDR traces for ranges where the backscattered signals are below the noise level N. Thus, only the returning pulses R i  exceed the noise level N. 
     The measuring wavelengths are preferably selected to wavelengths at which the transmission link  100  does not show too much attenuation. The center wavelengths λ i  of the gratings should be chosen not to interfere with traffic signals on the link, i.e. other signals used e.g. for communication purposes. E.g. λ i  can either fit between an ITU (International Telecommunication Union) grid for DWDM (dense wavelength division multiplexing) systems, or lie outside the traffic band. 
     The reflectance values of the fiber gratings can be chosen to compensate for the fiber&#39;s attenuation. This way, the measured return signals can be adopted to provide substantially equal strength. If, for example, the link loss is approximately 10 dB, then the first grating should have a reflectance 20 dB lower than the last one on the link. This has the additional advantage that the reflected amount of ASE power is much smaller. 
     Since only the peak of reflections has to be measured, a code correlation scheme can be applied without the cumbersome problems with auto-correlation sidelobes superimposed to a backscatter signal.
An optical fiber is provided with a plurality of reflecting events spatially allocated along the optical fiber for localizing possible faults in the optical fiber. Possible faults are localized by emitting a signal into the optical fiber, measuring the reflected signals, and comparing the measured reflected signals with expected signals representing the optical fiber without faults. In case that there are one or more faults in the optical fiber, the measured reflected signals in a distance behind each one of the one or more faults will show at least a different amplitude, or even disappear, with respect to the expected signals. The expected signals can be received or determined, e.g., from a previous measurement, such as an acceptance measurement, or can be calculated or otherwise received from theoretical analysis (e.g. Simulation or modeling) and/or from information about the fiber (such as physical properties).