Abstract:
The method for automatically testing optical fibres in multi-branch networks implements Optical Time Domain Reflectometry technology (OTDR) in which an optical signal pulse is pumped into a fibre and the respective backscattered signal is periodically analysed to obtain attenuation values according to distance. The method is used to identify the fibre and the position in which attenuation increase of the backscattered signal is found even when the signal is sent upstream of each branch point from which the individual fibres to be tested depart.

Description:
TECHNICAL FIELD  
       [0001]     The invention refers to optical telecommunications system testing tools and specifically refers to a method for automatically testing optical fibres in multi-branch networks.  
         [0002]     Optical fibre transmission systems are very widespread today and support very high speed audio and video transmissions. The need for reliable tools that are capable of detecting faults and deterioration of the physical carrier, i.e. of the fibre, is increasingly felt.  
       BACKGROUND ART  
       [0003]     Testing apparatuses normally test transmitted data to identify data transmission degradation. Unfortunately, considering that the systems are highly dynamic thus allowing error-free operation even in the presence of considerable line attenuation, damage beyond repair is already in progress when degradation is identified. The importance of testing systems, which are independent from the transmission apparatuses and capable of indicating not only extreme events (such as loss of optical fibre continuity due to breakage or opening of a connector), but also gradual deterioration in fibre efficiency, is evident.  
         [0004]     Systems with such characteristics implementing different technical solutions are currently marketed. The most common employ an optical reflectometer implementing OTDR (optical Time Domain Reflectometry) technology. The system pumps a light signal pulse at a different wavelength from that used for signal transmission so that it can be easily filtered before the reception apparatuses without interfering with transmission. The light pulse pumped by the reflectometer laser is backscattered on the fibre and returns to the instrument which uses it to trace the optical power of the line according to distance. The smallest line attenuation can be detected by periodically repeating the test on the fibre and comparing the current and the previously recorded or reference traces.  
         [0005]     Off-the-shelf systems of this kind are typically designed to work in long-haul networks and are used to test one optical fibre line at a time by means of one or more optical switching devices.  
         [0006]     The matter is more complicated when a multi-branch optical network (i.e. a network with several fibres formed through passive optical branch points from a primary line consisting of a single fibre) is to be tested. Analysing the optical reflectometer trace is more complex because the backscattered light from the various fibres is summed in the branch point before returning to the reflectometer. It is consequently difficult to identify the fibre in the network where the variation may have occurred.  
         [0007]     Testing each fibre would obviously increase costs both in terms of passive elements needed to pump optical signals at testing length into each optical fibre (WDM, optical filters, switch ports, etc.) and decreased analysis speed of the entire network.  
         [0008]     An OTDR trace analysis method is described in U.S. Pat. No. 6,028,661 dated 22 Feb. 2000. According to the described method, the trace acquired by the reflectometer is analysed by studying the correlation between adjacent points in the trace employing the solution of a system of equations based on the minimum square method. The solution of this system of equations (the number of which is equal to the number of branches forming the network), is used to estimate an attenuation coefficient for each branch. The variation of one of the coefficients indicates the presence of a variation in the corresponding fibre. This method is rather complicated and consequently slow to run and difficult to implement.  
       SUMMARY OF THE INVENTION  
       [0009]     The method for automatically testing optical fibres in multi-branch networks described in this invention, based on regular acquisition of OTDR traces of the concerned network, overcomes these shortcomings and solves the aforesaid technical problems. The method is particularly suitable for automatically testing branched networks, specifically several multi-branch networks and works upstream of the branch point in each network. The number of passive optical components needed to pump the test signal in the fibres and identify it at the end of the lines is decreased and the scanning frequency, i.e. the number of times a certain line is tested per unit of time, is increased.  
         [0010]     The method is capable of identifying the fibre and the section where the backscattered signal attenuation ratio is increased by very simply analysing the OTDR trace (consisting of the light backscattered by the various fibres of the multi-branch network); the analysis is consequently fast and easily implemented.  
         [0011]     The specific object of this invention is a method for automatically testing multi-branch network optical fibres as described in the preamble to claim  1 . 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]     These and other characteristics of the invention will now be described with reference to a preferred form of embodiment in the annexed drawings by way of example only, wherein:  
         [0013]      FIG. 1  is a sketch of the test bench and the network;  
         [0014]      FIG. 2  is an image on an optical reflectometer monitor in the event of a fault;  
         [0015]      FIG. 3  shows the possible images which may appear on the monitor of an optical reflectometer in the event of two different types of network faults. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     As mentioned above, the method implements OTDR technology according to which the traces referring to optical signals pumped on the network and backscattered are periodically analysed.  
         [0017]      FIG. 1  shows a test bench consisting of an off-the-shelf optical reflectometer ORM capable of working at a different wavelength from that normally used for telecommunications, e.g. 1625 nm. This wavelength is chosen so that the test can be conducted also on lines which are already engaged by normal traffic, providing that suitable branching devices, which are sensitive to the wavelength, and filters are installed.  
         [0018]     The bench also comprises an off-the-shelf optical switch module OSW which receives the pulse signal on fibre F 1  from ORM and sends it on three fibres F 2 , F 3  and F 4  in later instances to test several networks at the same time.  
         [0019]     The fibres are connected to the networks through shunt trips B 1 , B 2  and B 3 , needed to create a sufficient delay to make the signals backscattered by the optical fibres visible upstream of branch points D 1 , D 2  and D 3 , whose task is to suitably split the input electrical power and send the individual fractions into the various fibres, e.g. fibres F 8 , F 9 , F 10  and F 11  connected to D 1 .  
         [0020]     The optical test signals are pumped into the main fibres F 5 , F 6  and F 7  of the three networks through wave division multiplexers WDM 1 , WDM 2  and WDM 3 . Optical filters OF 1 , OF 2 , OF 3  and OF 4 , whose task is to permit the transit of optical signals used for transmitting data, and block those used for the test, are located on the end of the fibres.  
         [0021]     Obviously, the backscattered optical signals from the various fibres return to the reflectometer ORM to be analysed by crossing devices D 1 , D 2 , . . . , WDM 1 , WDM 2 , . . . , B 1 , B 2  . . . , and switching module OSW.  
         [0022]     Both the reflectometer ORM and the switching module OSW are controlled by a personal computer PC via an electrical connection RS. The personal computer can run the various steps of the method when equipped with suitable software.  
         [0023]     A typical image that may appear on the monitor of the optical reflectometer ORM is shown in  FIG. 2 . In this example, a reference trace TR (shown by the broken line) is shown along an alarm trace TA (shown by the solid line) In both cases, the ordinate shows the relative power of the backscattered signal and the abscissa shows the distance in kilometres from the reflectometer. The network under test consists of a main fibre which branches into four lower level fibres whose ends are approximately 5 kilometres, 7 kilometres, 10 kilometres and 14 kilometres away from the test bench. For the sake of simplicity, the network is considered to consist of the following five sections: 
        the first section consists of the single fibre which reaches the branch point from the reflectometer and is approximately 1.5 kilometres long;     the second section consists of the four fibres output from the branch point and goes from 1.5 kilometres to 5 kilometres;     the third section is reduced to three fibres and goes from 5 kilometres to 7 kilometres;     the fourth section is reduced to two fibres and goes from 7 kilometres to 10 kilometres;     the fifth section is reduced to one fibre only and goes from 10 kilometres to 14 kilometres.        
 
         [0029]     It is evident that each section starts and ends in correspondence with the end of a fibre and comprises all the fibres whose length are comprised within the boundaries of the section. The first segment of the traces visible on the monitor, referred to the first network section, is the sum of the backscattered powers of all fibres. As shown by the reference trace, the level decreases in correspondence with the end of each section—and consequently of each fibre—because the power contribution backscattered by the fibre ends at that point.  
         [0030]     To ensure easy application of the method, it is important to note that the lengths of the optical fibres in the network are all respectively different so that the end of fibre points of the various sections are different. This condition is easily obtained by including additional fibre sections along fibres with the same length.  
         [0031]     Attention must be given to attenuation points which may be present, such as couplings or connectors along the fibres, which may generate confusion. In this case, the approximate length of the fibres must be known.  
         [0032]     The trace analysis method, as mentioned, consists in comparing the periodically tested levels and the trace stored as a reference. Specifically, normalised power levels are considered with respect to the power level at network input so to free the test from inevitable variations in the output level of the reflectometer laser, which would change the vertical position of the traces.  
         [0033]     The alarm trace TA in  FIG. 2  shows that the backscattered signal level is lower in the forth section at a distance of approximately 8 kilometres from the reflectometer. The lowering is maintained until the end of the section, at a distance of 10 kilometres, after which the alarm trace and the reference trace overlap again.  
         [0034]     Having identified an alarm by detecting variations exceeding a predetermined threshold, the fibre in the section where the attenuation ratio has increased and the one involved in the fault must be univocally identified.  
         [0035]     This is because the fibre where the increase occurred is not immediately apparent because the fault may be located in any of the fibres forming that particular part of the trace.  FIG. 3 , for example, shows two traces TA 1  (solid line) and TA 2  (dotted line) which both present level decrease at 3 kilometres from the reflectometer, i.e. on the second section; in TA 1  the cause of attenuation is located on the 5 kilometre long fibre (i.e. the one that ends at the end of the second section) while in TA 2  the attenuation is located on the 14 kilometre long fibre (i.e. the one that ends at the end of the fifth section). The reference trace TR (dotted line) is the condition without increases in attenuation.  
         [0036]     The patterns of the two traces TA 1  and TA 2  near the attenuation increase point is identical. The difference appears at the end of each section: in the case of trace TA 1 , the increase in attenuation with respect to the reference trace TR remains only at the end of the second section, while in the case of trace TA 2 , the variation with respect to the reference trace affects the entire trace to the end of the fifth section.  
         [0037]     In general, the fibre on which the fault occurred is the one which ends in correspondence with the point which: 
        presents a reduced level of power with respect to that shown by the reference trace;     is the most distant from the test bench.        
 
         [0040]     The personal computer PC sets the tests and detects the results indicating alarms employing an application which implements the following method: 
        initialises test and analysis parameters according to the test bench operator;     generates a reference trace when it is sure the network is operating properly, measuring and storing the power levels of the backscattered optical signal according to distance, and identifying the ends of sections corresponding to ends of fibres according to power level attenuation;     periodically compares the levels at the end of section with the levels plotted when making the reference trace;     if a level attenuation exceeding a predetermined threshold is detected near the end of a section, passes into an alarm state and starts a detailed test cycle, by performing: 
            comparing the trace under test with the reference trace starting from the furthermost distance to identify both the first section where the attenuation increase is identified and the distance where the attenuation increase started.    
               
 
         [0046]     At this point, the fibre where the attenuation occurred is univocally identified being the one, as mentioned above, which ends in correspondence with the end of the identified section and the distance from the test bench where the fault occurred.  
         [0047]     The initialisation step of the method consists in defining the various test parameters for each of the networks under test, specifically: 
        number of networks to be tested;     number of fibres in each network;     alarm thresholds;     optical reflectometer test parameters, specifically: 
            wavelength;     pulse width;     maximum fibre length;     average result average time.    
               
 
         [0056]     It is noted that the description herein is provided by the way of an example only. Variants and changes are possible without departing from the scope of the present invention.