WO2003046504A1

WO2003046504A1 - System and method for monitoring optical fiber cables

Info

Publication number: WO2003046504A1
Application number: PCT/IT2001/000594
Authority: WO
Inventors: Enrico Domenico D'ascenzo
Original assignee: Sielte S.P.A.
Priority date: 2001-11-26
Filing date: 2001-11-26
Publication date: 2003-06-05
Also published as: AU2002222540A1

Abstract

System for monitoring optical fiber cables, comprising one or more emitting devices connected to a tramsmitting end of the cable to be monitored, one or more receiving devices connected to a receiving end of the cable to be monitored and a data processing unit apt to receive data from receiving devices and to carry out a method for processing data to obtain information on the condition of the cable being monitored.

Description

"SYSTEM AND METHOD FOR MONITORING OPTICAL FIBER CABLES"

DESCRIPTION

The present invention relates to a system and to a method for monitoring optical fiber cables, particularly effective in determining signal transients due to mechanical instability of the fibers and/or of components connected thereto.

To date, for monitoring and supervising an optical network at a physical level various methodologies are resorted to.

A first solution is to monitor by means of an optical source and of an optical power meter via a manual or automated measurement of the optical line attenuation, to be subsequently compared to a reference value. However, this solution is limited, in that it exclusively applies to unused (dark) optical fibers.

A second solution is based on the employ of an OTDR

(Optical Time Domain Reflectometer) , which enables to reconstruct from the backscattering diagram, cyclically acquired therefrom, the progress in time of the line attenuation.

This solution enables to locate the eventual failure and to single out the causes of degradation, be it of concentrated and/or of spread type. This methodology is intrinsically very onerous, requiring in the automated measurement processes the interfacing of the OTDR to an optical switch whose cost, increasing proportionally to the number of monitored fibers, might even exceed that of the same OTDR. However, this solution fails to provide a continuous monitoring of the fibers, due to its cyclic connection to the fibers of the monitored network.

Moreover, an OTDR-type monitoring system is unable to highlight transient attenuation phenomena. In fact, due to its operating principle in order to optimize the signal/noise ratio, the OTDR requires measuring times which increase proportionally to the length of the measured fiber. E.g., for particularly long optical fiber strands (>100 Km) , in order to provide a backscattering diagram offering a satisfactory compromise between measurable and dynamic resolution, time intervals of several minutes may be required.

Hence, it is apparent that transient phenomena of a duration shorter than the OTDR measuring time are in no way detectable.

Moreover, this solution, without the use of specific passive optical components like WDM (Wavelength Division Multiplexing) and filters, is exclusively applicable to dark (unused) optical fibers.

A further known solution, applicable also to optical fibers in use, is an extension of the former. The solution at issue, by means of passive optical componentry like WDM and band filters, is based on the overlapping of an OTDR-transmitted monitoring signal to the useful signal .

The two signals, of distinct wavelength (e.g., 1550 and 1625 nm) , are multiplexed onto the same fiber by means of a WDM. For an appropriate mutual insulation, the same signals require band filters which are also necessary to return the sole desirable signal .

To date, the latter solution is one of the most commonly adopted worldwide.

However, the OTDR mentioned above and optical componentry fail to implement an effective network monitoring. Suffice it to mention the occurrence of nonlinear effects (Raman scattering) , caused by the concomitant presence of OTDR of extremely high dynamics and of optical amplifiers exhibiting very high gains.

The nonlinear effects in turn adversely affect the transmission system in terms of BER (Bit Error Rate) degradation. Furthermore, another known solution is to near- continuously monitor a fraction of the optical power of the useful signal, via suitable devices connected upstream (transmitting side) and downstream (receiving side) of the optical line.

Each such device integrates the functions of passive sampling of a fraction of the useful signal and of measuring the power level thereof.

Therefore, a pair of these devices enable to monitor the line attenuation with respect to a predetermined reference value.

At times, such a solution, based on the monitoring of the transmitted useful signal, fails to discriminate the causes of the variation in the power thereof over time .

E.g., in the case of the actual multi-wavelength

DWDM (Dense Wavelength Division Multiplexing) systems, as it is known, Operative' variations of the transmitted optical power (e.g., due to the quenching of one or more laser carriers) may occur.

In this case, the use of a monitoring method destined to measure the attenuation suffered by the useful signal would not be overly significant. In fact, the method would not recognize the possible causes of signal variation over time, like: power variations in the signal envelope, degradation and/or failure of the interface optical components (straps and/or connectors) . Therefore, the margin of uncertainty would be too high with regard to the transmission capacity supported by a DWDM system.

An object of the present invention is to provide a method for monitoring an optical fiber cable apt to transport one or more operation signals at predetermined wavelengths, characterized in that it comprises the following steps: applying a reference sentinel signal at a transmitting end of the optical fiber cable; - extracting a resulting sentinel signal at a receiving end of said optical fiber cable via a sampling operation, obtaining a sampled signal; processing said sampled signal, obtaining information related to signal transients, wherein said reference sentinel signal is numerically synthesized and it comprises a plurality of sinusoidal components added among them, so as to obtain a reference signal apt to propagate along the optical fiber, said signal having a wavelength different from wavelengths typical of the operation signals, and having a power level sensibly lower with respect to said operation signals.

Moreover, a further object of the present invention is to provide a system for monitoring optical fiber cables, comprising: one or more emitting devices, each of said one or more emitting devices being apt to be connected to a transmitting end of an optical fiber; one or more receiving devices, each of said one or more receiving devices being apt to be connected to a receiving end of an optical fiber; - one or more local control unit, each of said one or more control units being apt to manage the operation of said one or more emitting devices and/or of said one or more receiving devices; a data transmission network; and - a central management unit, connected to said data transmission network, wherein each of said one or more control units is apt to transmit and to receive data from said central management unit, via said data transmission network. A first advantage of the system and of the method according to the present invention lies in the applicability thereof to optical fibers in use (with no degradation to the functionality thereof) as well as to dark optical fibers, allowing a continuous and real-time supervising of the operating conditions thereof.

A second advantage is entailed in that the method may also be used in the most advanced optical networks, which provide DWDM-type systems requiring sophisticated and reliable monitoring and supervising systems.

A third advantage of the system according to the present invention is that it enables to integrate the determination of the signal transients, e.g., due to the mechanical instability of the fibers and of each component connected thereto, to the standard functions of a monitoring system designed for present and future optical (transport and access) networks. A fourth advantage is provided by the fact that the system according to the present invention is able to control, from a monitoring center, the optical cables departing from the latter with a point-point, ring, single or composite star configuration. For each fiber of a cable trunk, functions of sole monitoring of the signal transients, of sole cyclic acquisition of the backscattering diagram, or integrated functions of transient monitoring and locating of the fiber failures and/or degradation may be provided. The cyclic acquisition of the backscattering diagram characterizing each fiber is systematically compared to the corresponding reference diagram.

The results of the comparison allow to timely highlight degradation phenomena of transmission qualities (increase of attenuation of an optical line and/or of patching junctions) as well as catastrophic failures

(fiber/cable breaking) , automatically locating the affected zone (optical line) or the spot whereat there be a possible singularity (junction or breaking) . A further advantage is provided by the fact that the system according to the present invention is based on a modular structure, having a correspondingly high functional flexibility, such as to allow a remarkable qualitative leap with respect to the actual methods for monitoring, alarm signaling and rationalizing of the maintenance and repair processes for multifiber optical cable networks. Further advantages, features and employ modes of the present invention will be made apparent in the following detailed description of preferred embodiments thereof, given by way of example and not for limitative purposes, making reference to the figures of the attached drawings, wherein: figure 1 is a flow chart of the method according to the present invention; figures 2 and 3 are graphs related to a reference auxiliary signal according to the present invention; figures 4 to 11 are graphs of signals used in the method according to the present invention; figure 12 is a graph corresponding to a received signal ; figure 13 is the Fourier transform of the signal of figure 12; figure 14 is a graph corresponding to the amplitude of the reference auxiliary signal according to the present invention; figure 15 is a graph corresponding to the noise introduced via the optical channel; figure 16 is a graph corresponding to the overlapping of the signals of figures 14 and 15; figure 17 is the Fourier transform of the signal of figure 16; figures 18 and 19 are signals used for monitoring, in absence and in presence of a transient, respectively; figure 20 is a block diagram of a system according to the present invention; figure 21 is a block diagram of a first embodiment of the system according to the present invention; figure 22 is a block diagram of a second embodiment of the system according to the present invention; figure 23 is a block diagram of a third embodiment of the system according to the present invention; figure 24 is a block diagram of a fourth embodiment of the system according to the present invention; and figures 25 to 29 are block diagrams of further embodiments of the system according to the present invention.

With reference to figure 1, a flow chart of the method according to the present invention is illustrated.

The method for monitoring is based on the processing of optical signals transmitted onto the fibers at issue, and it comprises a first step SI of applying a reference sentinel signal at a transmitting end of an optical fiber cable.

The reference sentinel signal has a shape particularly apt to cross long optical fiber strands and to be subsequently decoded by a receiver.

The reference sentinel signal is numerically synthesized and it has a waveform in the time domain such that the corresponding coherence function, computed in the frequency domain, have a particularly advantageous signal/noise ratio.

Next, figures 2 to 11 refer to the synthesis of the reference sentinel signal .

In particular, figure 2 is an example of reference sentinel signal, characterized by two signal sections identified by the time intervals TO and TI depicted in figure 3. A first signal section SSI, corresponding to the interval TO, consists of a monotonic signal of predetermined amplitude A0 and it is used to stabilize the emitting source, e.g., a laser diode, as a function of temperature. The amplitude A0 is computed as a function of the power delivered from the source and it is selected so as to enable a laser emission of a power of about 100 μW.

A second signal section SS2 , corresponding to the interval TI, consists instead of the sum of a plurality of sinusoidal components and of a continuous component.

Preferably, the interval TI has a duration of 10 ms and the frequency of each of the sinusoidal components is determined as a corresponding term of an arithmetic progression: fO, 2f0, 3f0, ..., nfO, wherein fO is preferably equal to 10 Hz. figure 4 is a graph related to the signal obtained as sum of the n sinusoidal components, whereas figure 5 is a diagram of the continuous component. figures 6 to 9 depict some of the sinusoidal components used.

Then, the strictly orthogonal sinusoidal components of the signal SS2 are subjected to a windowing operation with a Blackman window, whose graph is reported in figure

10, in order to prevent the maximum amplitude thereof from reaching levels apt to send the emitting source

(laser) out of the linearity zone. Then, adding all the 'windowed' sinusoidal components, the signal SS2 of figure 11 is obtained.

The signal SS2 is then added to a continuous component of an amplitude such that anyway the peak amplitude of the resulting signal be < A0 , hence such as to never exceed a 0.1 mW peak power value .

The sentinel signal obtained is that shown in figure 2.

A second step S2 of the method according to the present invention provides the extracting of a resulting sentinel signal at the receiving end of the optical fiber being tested.

In a subsequent step S3, the extracted signal is then processed and compared to the reference sentinel signal, in order to provide useful information about signal transients occurred during transmission in the optical cable.

Figure 12 is a diagram depicting a resulting sentinel signal, as received at the receiving end of the optical fiber being monitored. The step S2 of extracting the resulting sentinel signal is executed using known methodologies based on the Fourier transform. Figure 13 is the graph of the Fourier transform of the signal of figure 12.

Figure 14 is a graph showing the scanty amplitude used by the reference auxiliary signal. Figure 15 is a graph depicting the contribution noise introduced via the optical channel . In terms of amplitude the difference between the signals depicted in figures 14 and 15 is apparent.

Figure 16 is the graph corresponding to the overlapping of the signals depicted in figures 14 and 15.

Apparently, the reference sentinel signal is markedly contaminated by the noise introduced via the optical channel .

Figure 17 is the diagram of the Fourier transform of the signal of figure 16.

The sentinel signal, drowned in noise, was extracted from the combination of the signals depicted in the preceding figures 14 and 15.

These techniques are particularly convenient in this case, as it is desirable to maintain a bandwidth greater than 20 Hz in order to monitor the fiber with a sampling frequency of about 10 samples/sec.

Moreover, as the improvement in the signal/noise ratio due to the Fourier transform is of about the square root of N, where N is the number of sampled sites in the time interval at issue, selecting N=256, an improvement of about 24 dB is attained.

A further improvement of the signal/noise ratio may be attained carrying out an averaging operation on the modules of the Fourier transforms corresponding to the individual harmonic components forming the resulting sentinel signal.

Considering, e.g., a signal containing eight harmonic components, a further improvement of 9 dB in the signal/noise ratio is obtained.

Therefore, in the specific case, an 8 -channel receiving module is used. Each channel is sampled at a frequency of 2048 samples/sec, so as to obtain 256 samples in a 0.1 sec time interval.

The samples are inputted to a Goertzel algorithm which, as it is known, computes the module of the Fourier transform of the corresponding harmonic component.

The modules of the eight Fourier transforms corresponding to the eight channels of the receiving module are averaged to attain a further improvement of 9 dB in the signal/noise ratio.

Figures 18 and 19 are graphs related to the progress over time of this average value. In particular, figure 18 exemplifies a situation of appropriate operation and absence of failures along the optical fiber. In this case a function which is constant in time is obtained.

Figure 19 is instead an example related to a function obtained in case of failure onto the optical fiber, e.g., due to a mechanical stress, etc..

The function related to the resulting average value is compared to a corresponding value computed beforehand onto the reference signal or on a signal extracted under optimum conditions (e.g., at system installation).

If the result of the comparison deviates from one or more predetermined thresholds, a corresponding alarm routine is activated to communicate the recorded transient event to an operator or to a control center.

The decision-making mechanism over the transient event is based on the following parameters:

1) Transient entity. It is checked whether the transient amplitude is comprised within one of the value segments +/-sigma0, +/-sigmal, +/-sigma2, wherein sigmaO, sigmal and sigma2 are three alarm thresholds, each at an increasing level of deviation. This parameter indicates whether the load bearing structure of the optical cable be subjected to more or less intense mechanical stresses.

2) Transient recurrence in the time unit. This parameter informs as to whether the cause of the transient originates from a sporadic case, or, rather, to an anomaly due, e.g., to an undergoing soil cleavage.

3) Transient correlation to known phenomena (e.g., the transit of a train) . This parameter indicates a punctual stressing of the load bearing structure which, sooner or later, might degenerate.

This information is stored in a dynamic database, i.e., a database containing information about the events occurring on the plant over time. Therefore, the events are evaluated with respect to information stored in a static database, containing information on the plant consistency, its orography, the historical series of its failures, its traffic volume, etc., and finally, needwise, appropriate maintenance actions are undertaken.

Next, figure 20 is a block diagram of the system 1 according to the present invention, describing the main architecture thereof.

According to the layer model of the TMN (Telecommunications Management Network) , the main system architecture is structured according to three layers.

A first system layer 2, consisting of the NMC (Network Management Center) , of a set of terminals and of a general database of the monitored network. A second data communication layer 3 consisting of a (public and/or private) control network and by the units interfacing data to the system, like modem, router, etc.

A third testing layer 4, consisting of active and passive optical components (switches, couplers, filters, WDM, connecting straps) and by testing modules (OTDR, sources, photodetectors, etc.) slaved to a command module and to an optional local controller.

The local controller may e.g., be a PC or an Unix workstation, depending on the required reliability rate. The command module and the optional local controller manage the measuring and testing modules by means of a data bus based on the IEEE standard 488, and process the results from the (automatic and non-automatic) measuring comparing them to a local data base .

The flexible and modular architecture of the system enable to implement a plurality of configurations, anyhow referable to two classes of monitoring systems.

The monitoring systems lacking a failure locating function belong to a first class, whereas the monitoring systems providing a failure locating function belong to a second class. Next, figure 21 is a block diagram of a first embodiment of the system 1 according to the present invention, for monitoring dark optical fibers 11.

According to this first embodiment, the system 1 comprises a plurality of emitters 10, e.g., laser diodes emitting light at a predetermined wavelength, preferably selected within the following optical windows

(transmission bands) respectively centered in a neighborhood of the following values: 1310 nm, 1550 nm or

1625 nm. Each of such emitters 10 is connected to a transmitting end of one of the fibers of the optical cable 11 being monitored, via optical coupling means, e.g., optical connectors 12.

A first local control unit 100 comprises a first command module 13 managed, via a bus 14, by a first terminal 15, or, in absence of the latter, by a device having gateway function. The control unit 100 governs the ignition and/or the quenching of the laser diode emitters 10. At the receiving end of the optical cable 11 each fiber is connected, by means of a corresponding optical connector 12, to a receiving device 16, e.g., a photodiode .

A second local control unit 200 comprises a second command module 17, managed via a bus 18 by a second terminal 19 or, in the absence thereof, by a device having gateway function. The control unit 200 acquires the signals outputted by the receiving devices 16.

The two local control units 100 and 200 are connected to a central unit 20 (server) via a data transmission network 21. The central processor 20 supervises the operation of the entire system, acquires and processes the received data, and provides information on the condition of the monitored optical cable.

In the first control unit 100 there may be, as reported in the same Fig. 21, combinations of emitters 10 and of receiving devices 16. This configuration is possible when the attenuation of the optical strand being monitored (at the preselected wavelength) , enables a looping near to the remote terminal. In this case the entire system may be slaved to an individual control unit .

Next, figure 22 is a block diagram of a second embodiment of the system according to the present invention. Of this embodiment, exclusively the parts and the components different from the preceding ones will be described. For likewise components, the same numbers and names will be used.

The system of figure 22 is implemented for monitoring dark optical fiber cable, and it provides, with respect to the first embodiment disclosed above, the further function of locating eventual failures occurred along the fiber being monitored.

This function is implemented via an OTDR-type instrument 30, controlled via the terminal 15, or, in the absence thereof, by a device having gateway function.

A detailed description of the operation of this instrument will be omitted, as it is well-known to those skilled in the art. The OTDR 30 is activated when the system detects a degradation in one of the fibers being monitored, as disclosed above. In this case, the OTDR is connected to the 'defective' fiber, via an optical switch 31, to one of the optical couplers 32, having, e.g., a 50-50%-type partition ratio. Of course, this ratio could advantageously be modified as a function of the actual signal attenuation of the optical fiber being monitored.

This embodiment entails the advantage of determining the location of the failure along the fiber without requiring a continuous use of the OTDR which, as mentioned above, is disadvantageous and not always able to provide a desirable result.

As reported in the same figure 22, in the control unit installed at the OTDR side there may be combinations of emitters 10 and of receiving devices 16. This configuration is possible if the attenuation of the monitored optical strand (at the preselected wavelength) , enables looping near to the remote terminal . In this case, the entire system may be slaved to an individual control unit .

Next, figure 23 is a block diagram of a third embodiment of a system according to the present invention.

In this case the system is used for monitoring an optical fiber cable 40 in use.

The emitting devices 12 and the corresponding receiving devices 16 are connected to the optical fibers 40 via respective WDM-type 2 -wavelength optical couplers 33, in co-propagation or in counterpropagation, casewise. A fourth embodiment of a system according to the present invention, is depicted by the block diagram of the next figure 24.

As in the preceding case, the system is apt to monitor optical fibers 40 in use. However, in this case the system provides the additional failure location function.

This function is implemented via the use of an OTDR 30, according to modes analogous to the ones disclosed hereto.

The OTDR signal is 'injected' into the optical fiber via a WDM-type 3-wavelength optical coupler 34, or, e.g., via two cascade standard 2-wavelength WDMs .

Of course, further embodiments of the system, always falling within the scope of the present invention, may be provided.

By way of example, and with evident meaning of symbols and nomenclature, figures 25 to 29 are block diagrams of further embodiments of a system according to the present invention.

These further embodiments will not be detailed hereinafter, as easily derivable combining hereto disclosed embodiments.

In particular, figures 26 to 29 refer to systems apt to the concomitant monitoring of optical fibers unused and in use .

It is understood that the data processing step provided by the method for monitoring according to the present invention, could advantageously be implemented also in the realization of an instrument for monitoring OTDR-type optical fiber cables, comprising a data processing system based on the Gabor transform, overcoming several of the problems related to the limitations of these instruments.

The Gabor transform is a powerful mathematical tool developed by physicist Den Gabor, Nobel Prize in quantum mechanics, allowing to perform the spectrum analysis on a signal as a function of frequency as well as of time.

The Gabor transform may advantageously be applied in an instrument for monitoring to extract transients from the signal outputted by an OTDR-type device, performing a space-frequency analysis. This mathematical tool is of a general type. In the specific case, in order to suit it to the specific needs and to attain the desirable results, it has been used in conjunction with a specific function, denominated analysis function, specifically adapting the mathematical tool to the various types of signal used.

This analysis function, peculiar of the OTDR signals, optimizes the extraction of the transient signal .

In figures 30 to 34, by way of example, the experimental results of the application of the Gabor transform to a real signal, acquired by an OTDR in the course of a testing, are reported.

Hereinafter, the basic requirements of the testing are reported:

Total physical length of the optical section: 20.5 km; Physical length of the optical section interested by the acquisitions: 16 km; - OTDR pulse width: 1 μs; Pulse wavelength: 1550 nm;

Number of samples acquired in 16 km: 3902; Spatial resolution: 4 m; - Number of samples of final interest: 2048; Optical strand of interest: 8.58 km; Gabor transform Grid (space X frequency) : 64 X 32; Spatial resolution of the Gabor transform: 130 m; and Gabor Function used: exponential, of type: with Heaviside function.

Furthermore, a system according to the present invention, further entails an additional significant advantage with respect to the systems of the known art .

In fact, it is known that any system for supervising optical networks needs to be instructed in order to carry out the various measuring routines.

All extant systems are instructed according to rigid routines set up by the individual manufacturers.

Instead, the system subject matter of the present invention is fully programmable.

In fact, the system comprises a computing program, substantially based on a specifically designed and implemented programming language. This language consists of a series of primitives, each thereof executing an elementary measuring subroutine.

The sequential execution of a determined number of primitives constitutes a measuring routine.

The solution adopted enables to draw up enormously flexible measuring routines, such as to allow to ideally configurate the system both for the access and for the transport networks. Let us suppose, by way of example, that it be desirable to monitor the abscissa of the Km 12,450 of an optical fiber cable, as the former is drawn in correspondence of a railway bridge, and during the transit of a train remarkable cable vibrations, with the entailed transients onto the fibers could be induced.

Using this programming language, it is possible to set up a routine apt to monitor a sufficiently small neighborhood of the indicated abscissa, therefore enabling to detect small transients still at a latent stage.

In practice, the traditional systems do not allow this type of detection.

The primitives which generally constitute the programming language are the following: open_otdr () ; close_otdr () ; open_vxi ( ) ; close_vxi () ; open_switch ( ) ; close_switch () ; dspan ( ) ; mspan ( ) ; mpwidth () ; mmode ( ) ; mstartpos () ; mrefrindex () ; mwv ( ) ; run ( ) ; stop ( ) ; sleep () ; get_path() ; compare ( ) ; send_to_alarm() ; if 0; while () ; get_data () ; get_datum() ; get_alarm() ; sum ( ) ; mult () ; pulse () ; get_time () .

The present invention has hereto been disclosed according to preferred embodiments thereof, given by way of example and not for limitative purposes.

It is understood that other embodiments may be provided, all to be construed as falling within the protective scope of the invention, as set forth by the appended claims.

Claims

1. A method for monitoring an optical fiber cable apt to transport one or more operation signals at predetermined wavelengths, characterized in that it comprises the following steps: applying a reference sentinel signal at a transmitting end of the optical fiber cable; extracting a resulting sentinel signal at a receiving end of said optical fiber cable via a sampling operation, obtaining a sampled signal; processing said sampled signal, obtaining information related to signal transients, wherein said reference sentinel signal is numerically synthesized and it comprises a plurality of sinusoidal components added among them, so as to obtain a reference signal apt to propagate along the optical fiber, said signal having a wavelength different from wavelengths typical of the operation signals and having a power level sensibly lower with respect to said operation signals.

2. The method according to claim 1, wherein said reference sentinel signal is applied to an unused optical fiber.

3. The method according to claim 1, wherein said reference sentinel signal is applied to an optical fiber in use, by overlapping to said operation signals.

4. The method according to any one of the preceding claims, wherein said reference sentinel signal is emitted by a laser source.

5. The method according to any one of the preceding claims, wherein the wavelength of said reference sentinel signal is comprised in one of the following optical windows : 1310 nm; 1550 nm; - 1625 nm.

6. The method according to any one of the 'preceding claims, wherein said reference sentinel signal has a 100 μW peak power.

7. The method according to any one of the preceding claims, wherein said reference sentinel signal comprises a plurality of sinusoidal components, each of said sinusoidal components having a frequency determined by a corresponding term of an arithmetic progression.

8. The method according to claim 7, wherein said arithmetic progression is: fO, 2f0, 3f0, ..., nfo, wherein n is the number of said sinusoidal components and fO is a predetermined frequency.

9. The method according to claim 8, wherein n is equal to 8 and fO is equal to 10 Hz.

10. The method according to any one of the preceding claims, wherein said sinusoidal components are multiplied with a predetermined windowing signal .

11. The method according to claim 10, wherein the windowing signal is a Blackman window.

12. The method according to any one of the preceding claims, wherein said reference sentinel signal comprises a continuous component of a amplitude such that said reference sentinel signal have a 0.1 mW peak power value.

13. The method according to any one of the preceding claims, wherein said sampling operation is executed at a frequency of 2048 samples/sec.

14. The method according to any one of the preceding claims, wherein said step of processing said sampled signal comprises an operation of computing one Fourier transform per each of said sinusoidal components.

15. The method according to claim 14, wherein said step of processing said sampled signal comprises an operation of computing the modules of said Fourier transforms .

16. The method according to claim 15, wherein said modules are computed by means of a Goertzel algorithm.

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17. The method according to claim 15 o 16, wherein said are averaged among them, obtaining an average value.

18. The method according to claim 17, wherein said step of processing said sampled signal comprises an operation of comparing said average value to one or more predetermined thresholds.

19. A system (1) for monitoring optical fiber cables (11) , comprising: one or more emitting devices (10) , each of said one or more emitting devices (10) being apt to be connected to a transmitting end of an optical fiber (11) ; one or more receiving devices (16) , each of said one or more receiving devices (16) being apt to be connected to a receiving end of an optical fiber (11) ; - one or more local control units (100, 200) , each of said one or more control units being apt to manage the operation of said one or more emitting devices (10) and/or of said one or more receiving devices (16) ; a data transmission network (21) ; and - a central management unit (20) , connected to said data transmission network (21) , wherein each of said one or more control units is apt to transmit and to receive data from said central management unit, via said data transmission network.

20. The system according to claim 19, wherein at least one of said one or more emitting devices (10) is a laser diode apt to emit light at a predetermined wavelength.

21. The system according to claim 20, wherein said wavelength is comprised within one of the following optical windows : 1310 nm; 1550 nm; 1625 nm 22. The system according to any one of the claims 19 to 21, wherein at least one of said one or more receiving devices (16) is a photodiode .

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23. The system according to any one of the claims 19 a 22, further comprising coupling means (12, 32, 33, 34) of said emitting devices to said optical fibers to be monitored.

24. The system according to claim 23, wherein said coupling means comprises at least one optical connector (12) .

25. The system according to claim 23 or 24, wherein said coupling means comprises at least one optical coupler (32, 33, 34) .

26. The system according to claim 25, wherein said optical coupler (32) has a 50%-50%-type partition ratio.

27. The system according to any one of the claims 23 to 26, wherein said coupling means comprises at least one WDM-type 2-wavelength optical coupler (33) .

28. The system according to any one of the claims 23 to 27, wherein said coupling means comprises at least one WDM-type 3 -wavelength optical coupler (34) .

29. The system according to any one of the claims 19 to 28, wherein said coupling means comprises at least two cascade WDM-type 2-wavelength optical couplers.

30. The system according to any one of the claims 19 to 29, further comprising means (30, 31) for locating failures on said optical fibers. 31. The system according to claim 30, wherein said means (30,

31) for locating failures comprises an OTDR- type device (30) .

32. The system according to claim 30 or 31, wherein said means (30, 31) for locating failures comprises at least one optical switch (31) .

33. The system according to any one of the claims 19 a 32, wherein said central management unit (20) comprises means for storing data outputted by each of said one or more local control units (100, 200) .

34. The system according to claim 33, wherein said means for storing data comprises one or more databases.

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35. The system according to any one of the claims 19 to 34, wherein said central management unit (20) comprises means for processing data outputted by each of said one or more local control units (100, 200) .

36. The system according to claim 35, wherein said means for processing data comprises a computing program apt to analyze said data outputted by each of said one or more local control units and to provide indications related to eventual failures occurred on said optical fibers.

37. The system according to claim 36, wherein said computing program is apt to sequentially execute a plurality of elementary routines.

38. The system according to claim 37, wherein each of said elementary routines is selected from the following: open_otdr ( ) ; close_otdr () ; open_vxi ( ) ,- close_vxi () ; open_switch ( ) ,- close_switch() ; dspa O ; mspan ( ) ; mpwidth() ; mmode ( ) ; mstartpos () ; mrefrindexO ; mwv ( ) ; run ( ) ; stopO ; sleep () ; get_path() ; compare ( ) ; send_to_alarm() ; if 0; while () ;

RECTIFIED SHEET (RULE 91) ISA/EP get_data () ; get_datum() ; get_alarm() ; sum ( ) ; mult () ; pulse () ; and get_time () .

39. An instrument for detecting signal transients in a optical fiber cable comprising an OTDR-type device apt to transmit a first control signal along the cable and to receive a second reflected signal, characterized in that it comprises means for processing said second reflected signal, apt to provide indications about eventual signal transients occurred along said cable.

40. The instrument according to claim 39, wherein said means for processing said reflected signal comprises means for computing the Gabor transform of said second reflected signal.

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