Abstract:
A method of testing optical fibers in which a reference trace is generated and the power levels of backscattered optical signals are measured are stored according to distance, thereby identifying fiber section ends according to the attenuation in power levels. If a power level attenuation exceeds a predetermined threshold near the end of a section is detected a series of detailed checks is run comparing the trace tested with the reference trace starting with the furthest distance and identifying the first end where there is an increase in attenuation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a national stage of PCT/EP 03/01360 filed 12 Feb. 2003 and based upon the Italian application TO 2002 A 000168 filed 28 Feb. 2002. 
   FIELD OF THE INVENTION 
   The invention relates to optical telecommunications system testing tools and specifically relates to a method for automatically testing optical fibers in multi-branch networks. 
   BACKGROUND OF THE INVENTION 
   optical fiber 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 fiber, is increasingly felt. 
   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 fiber continuity due to breakage or opening of a connector), but also gradual deterioration in fiber efficiency, is evident. 
   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 out ahead of the reception apparatuses without interfering with transmission. The light pulse pumped by the reflectometer laser is backscattered on the fiber 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 fiber and comparing the current and the previously recorded or reference traces. 
   Off-the-shelf systems of this kind are typically designed to work in long-haul networks and are used to test one optical fiber line at a time by means of one or more optical switching devices. 
   The matter is more complicated when a multi-branch optical network (i.e. a network with several fibers formed through passive optical branch points from a primary line consisting of a single fiber) is to be tested. Analyzing the optical reflectometer trace is more complex because the backscattered light from the various fibers is summed in the branch point before returning to the reflectometer. It is consequently difficult to identify the fiber in the network where the variation may have occurred. 
   Testing each fiber would obviously increase costs both in terms of passive elements needed to pump optical signals at testing length into each optical fiber (WDM, optical filters, switch ports, etc.) and decreased analysis speed of the entire network. 
   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 analyzed 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 fiber. This method is rather complicated and consequently slow to run and difficult to implement. 
   SUMMARY OF THE INVENTION 
   The method for automatically testing optical fibers 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 into the fibers 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. 
   The method is capable of identifying the fiber and the section where the backscattered signal attenuation ratio is increased by very simply analyzing the OTDR trace (consisting of the light backscattered by the various fibers of the multi-branch network); the analysis is consequently fast and easily implemented. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     These and other characteristics of the invention will now be described with reference to a preferred embodiment in the annexed drawing by way of example only, wherein: 
       FIG. 1  is a sketch of the test bench and the network; 
       FIG. 2  is an image on an optical reflectometer monitor in the event of a fault; and 
       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 
   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 analyzed.  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. 
   The bench also comprises an off-the-shelf optical switch module OSW which receives the pulse signal on fiber F 1  from the optical reflectometer ORM and sends it on three fibers F 2 , F 3  and F 4  in later instances to test several networks at the same time. 
   The fibers 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 fibers 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 fibers, e.g. fibers F 8 , F 9 , F 10  and F 11  connected to branch point D 1 . 
   The optical test signals are pumped into the main fibers 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 ends of the fibers. 
   Obviously, the backscattered optical signals from the various fibers return to the reflectometer ORM to be analyzed by crossing devices D 1 , WDM 1 , B 1 , B 2  . . . , and switching module OSW. 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. 
   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 kilometers from the reflectometer. The network under test consists of a main fiber which branches into four lower level fibers whose ends are approximately 5 kilometers, 7 kilometers, 10 kilometers and 14 kilometers 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 fiber which reaches the branch point from the reflectometer and is approximately 1.5 kilometers long; 
   the second section consists of the four fibers output from the branch point and goes from 1.5 kilometers to 5 kilometers; 
   the third section is reduced to three fibers and goes from 5 kilometers to 7 kilometers; 
   the fourth section is reduced to two fibers and goes from 7 kilometers to 10 kilometers; the fifth section is reduced to one fiber only and goes from 10 kilometers to 14 kilometers. 
   It is evident that each section starts and ends in correspondence with the end of a fiber and comprises all the fibers 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 fibers. As shown by the reference trace, the level decreases in correspondence with the end of each section—and consequently of each fiber—because the power contribution backscattered by the fiber ends at that point. 
   To ensure easy application of the method, it is important to note that the lengths of the optical fibers in the network are all respectively different so that the end-of-fiber points of the various sections are different. This condition is easily obtained by including additional fiber sections along fibers with the same length. 
   Attention must be given to attenuation points which may be present, such as couplings or connectors along the fibers, which may generate confusion. In this case, the approximate length of the fibers must be known. 
   The trace analysis method, as mentioned, consists in comparing the periodically tested levels and the trace stored as a reference. Specifically, normalized 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. 
   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 kilometers from the reflectometer. The lowering is maintained until the end of the section, at a distance of 10 kilometers, after which the alarm trace and the reference trace overlap again. 
   Having identified an alarm by detecting variations exceeding a predetermined threshold, the fiber in the section where the attenuation ratio has increased and the one involved in the fault must be univocally identified. 
   This is because the fiber where the increase occurred is not immediately apparent because the fault may be located in any of the fibers 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 kilometers from the reflectometer, i.e. on the second section; in TA 1  the cause of attenuation is located on the 5 kilometer long fiber (i.e. the one that ends at the end of the second section) while in TA 2  the attenuation is located on the 14 kilometer long fiber (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. 
   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. 
   In general, the fiber 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. 
   The personal computer PC sets the tests and detects the results indicating alarms employing an application which implements the following method: 
   initializes 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 fibers 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. 
   At this point, the fiber 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. 
   The initialization 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 fibers in each network;   alarm thresholds;   optical reflectometer test parameters, specifically:   wavelength;   pulse width;   maximum fiber length;   average result average time.       

   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.