Abstract:
The present invention concerns a system  10  using optical time-domain reflectometry (OTDR) to test a plurality of optic fiber lines  13  in a telecommunications network, more particularly suited to tree topology networks of PON type (Passive Optical Network). Said system comprises a plurality of fiber optic lines  13 , a coupler  7  having one input and a plurality of outputs, each of said outputs being connected to one line of said plurality of lines  13 , said system  10  being characterized in that it comprises means  14  for separating each of said lines  13  into two channels: a first channel  18  to receive a first test impulse corresponds to a first test and a second channel  19  to receive a second test impulse corresponding to a second test, the length of said second channel being greater than the length of said first channel by a predetermined overlength  15 , each of said overlengths  15  being different for each of said lines  13.

Description:
RELATED APPLICATIONS 
   This application is related to and claims the benefit of priority from French Patent Application No. 02 10946, filed Aug. 30, 2002. 
   FIELD OF THE INVENTION 
   The present invention concerns a system using optical time-domain reflectometry (OTDR) to test a plurality of fibre optic lines in a telecommunications network, more particularly suited to tree topology networks of Passive Optical Network (PON) type. 
   BACKGROUND OF THE INVENTION 
   The flow rate of data transmitted by telecommunications networks is continually increasing. Therefore, increasingly more optic fires are used in these networks to meet these high speed requirements. 
   For every optic fibre installed, a verification of its characteristics must be made to ensure that they meet specifications and have no breaks or major attenuation. 
   The most frequently used devices to conduct these verification operations are devices called Optical Time-Domain Reflectometers (OTDRs). Optical reflectometry is also used by the operator to detect the position of a fault for faster, more efficient network repairs. 
   The principle of the OTDR technique is the detection and analysis, as a function of time, of the light backscattered by small imperfections and impurities present in the fibre (phenomenon known as Rayleigh back-scattering) and of the light reflected within the fibre (reflection on connectors, splices . . . ). The method consists of sending out a short impulse, from one end of the fibre, which propagates along the fibre and of measuring the quantity of light, as a function of time, which is backscattered towards a detector. On account of small imperfections and impurities in the fibre, part of the light is scattered in all directions. An ultrasensitive detector measures the quantity of backscattered light, i.e. which moves in the opposite direction to the direction of the incident impulse. With knowledge of the quantity of light that is at all times backscattered towards the detector, it is possible to determine the distribution of losses in the optic fibre. Therefore, a loss or fault at a determined point of the fibre will give rise to transitory discontinuity in the backscattered Optical Power tracing. 
   In tree topology networks, as in point-to-point networks, the OTDR system must be able to precisely locate faults occurring on the line. But this operation is made difficult on a tree-type network since all backscattered signals of all the lines are added together. Even if it is always easy to measure the distance between the fault and the OTDR, it is much more difficult to determine on which lines the faults have occurred. 
   One solution consists of placing a selective mirror at the end of the line of each subscriber, said mirror reflecting a predetermined wavelength, 1625 nm for example. Each subscriber is at a different distance from the detector of the OTDR system. The presence of the mirror leads to the presence of a reflective peak on the backscattered Optical Power tracing. The presence of a fault translates into transitory discontinuity in the monotonicity of the backscattered Optical Power tracing, indicating the distance between the OTDR and the fault. But with this indication alone, the fault may be located on any of the fibres of the tree network. It is the reflective peak which enables determination of the fibre on which the fault is located. Said reflective peak, highly attenuated or inexistent, indicates the presence of a fault on the line with which the mirror is associated. This line can be identified since each mirror is at a different distance from the OTDR. 
   However, the use of this kind of solution raises certain difficulties. 
   Firstly one must make sure that each mirror is at a different distance, otherwise the reflective peaks will be confused and it will no longer be possible to make a distinction between two branches. This is not an easy condition to meet, since it is difficult to know the exact length of the fibre on account of all the overlengths stored in the bays or boxes. 
   Also, when a fault occurs on two different lines, the faults are observed on the OTDR display. Two transitory discontinuities in the monotonicity of the Optical Power tracing are visible. These two faults will also lead to the presence of two attenuated peaks derived from the reflection of the two mirrors associated with each of the two faulty lines. The presence of the two peaks enables identification of the two faulty lines. However, it is difficult to allocate each of the two transitory discontinuities in the monotonicity of the tracing to the faulty line associated with it, and hence to determine the location of the fault on the faulty line. This difficulty in locating the fault in the event of several faulty lines is all the more critical since the losses on each of the two lines are substantially the same. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   The present invention sets out to provide a system for determining and locating faults using an optical time-domain reflectometer (OTDR) in a network comprising a plurality of fibre optic lines, said system, in the event of a plurality of faults on a plurality of faulty lines, making it possible to determine the faulty lines and to locate the faults associated with each of the faulty lines even if the fibre length of the different lines is the same. 
   For this purpose the present invention puts forward a system for determining and locating faults using optical time-domain reflectometry in a network comprising a plurality of optic fibres to be tested, said system comprising a coupler having an input and a plurality of outputs, each of said outputs being connected to one line of said plurality of lines to be tested, said system being characterized in that it comprises means for separating each of said lines to be tested into two channels:
         a first channel to receive a first test impulse corresponds to a first test,   a second channel to receive a second test impulse corresponding to a second test,
 
the length of said second channel being greater than the length of said first channel by a predetermined overlength, each of said overlengths being different for each of said lines.
       

   With the invention, a first impulse sent into the first channels of each line can detect whether there are any faults. The presence of a fault will give rise to transitory discontinuity in the monotonicity of the backscattered Optical Power tracing, this discontinuity being positioned at the location of the fault. In the event of a fault, a second impulse is sent into the second channel of each line. Each second channel has an overlength of optic fibre relative to the first channel. Also, this overlength is different on each line. Consequently, transitory discontinuity in the monotonicity of the backscattered Optical Power tracing, detected during the first measurement and associated with the fault, will move along the tracing during the second measurement of a length, equal to the overlength along which the light will have travelled. Therefore, measurement of the movement of the transitory discontinuity in the monotonicity of the tracing will give the value of the travelled overlength. As each line has a different overlength, it is very simple to associate a fault with a line irrespective of the fact that there may be several simultaneous faults on several lines. 
   Since the first measurement gives the distance between the OTDR and the fault, it becomes possible to geographically locate the fault and a technician can be sent to that location for its repair. 
   There are several ways of generating the first and second impulse. 
   One first solution consists of sending an impulse at a certain test wavelength which divides into two impulses, one first impulse entering the first channel and a second impulse entering the second channel. In this case the presence of a fault will give rise to a triple transitory discontinuity in the monotonicity of the backscattered Optical Power tracing, the first discontinuity being positioned at a point corresponding to the distance of the fault, the second being positioned at a point shifted by a length equal to the overlength travelled by the light, and the third discontinuity being positioned at a point shifted by a length equal to twice the overlength. With this solution, it is possible to detect the fault with one single test impulse divided into two impulses. 
   A second solution consists of using an impulse at one first wavelength which solely enters the first channel and a second impulse at a second wavelength, different from the first, which solely enters into the second channel. In this case, it is necessary to use a wavelength demultiplexer or adequate filters on each of the channels. 
   Finally, the system of the invention is a purely passive system. 
   Advantageously said overlength is an overlength of optic fibre. 
   According to one first embodiment, said means for separating each of said lines into two channels are a demultiplexer having at least two outputs. 
   Advantageously, each of said two channels are regrouped on said lines from which they are derived via a multiplexer having at least two inputs. 
   According to a second particular embodiment, one of said two outputs is connected to a saturable absorber device. 
   According to a third embodiment, said means for separating each of said lines into two channels are formed of a switch. 
   According to a fourth embodiment, said means for separating each of said lines into two channels are a coupler having at least two outputs. 
   Advantageously, each of said channels are regrouped on said lines from which they are derived by a coupler having at least two inputs. 
   A further subject of the invention is a method for determining and locating faults using an optical time-domain reflectometer (OTDR) in a network comprising a plurality of fibre optic lines using the system of the invention, characterized in that it comprises the following steps:
         sending an impulse at a first wavelength into said first channel   if a fault is detected, sending an impulse at a second wavelength into said second channel.       

   Advantageously, said first wavelength is different from said second wavelength. 
   A further subject of the invention is a method for determining and locating faults by optical time-domain reflectometry in a network comprising a plurality of optic fibre lines using a system according to the invention, characterized in that said method comprises the following steps:
         sending a first impulse at a predetermined wavelength, the power of said first impulse being such that said saturable absorber device is in a blocked state,   if a fault is detected, sending a second impulse at said predetermined wavelength, the power of said second impulse being such that said saturable absorber device is in a pass state.       

   The present invention also concerns a method for determining and locating faults by optical time-domain reflectometry in a network comprising a plurality of optic fibre lines using a system according to the invention, characterized in that said method comprises a step in which an impulse is sent at a predetermined wavelength, said impulse separating into a first impulse at said predetermined wavelength in said first channel and a second impulse at said predetermined wavelength in said second channel. 
   A further subject of the invention is an optical network characterized in that it comprises:
         an optical time-domain reflectometer device for sending and analysing test impulses,   a coupler having at least one input and at least two outlets, the input of said coupler being adapted to receive an impulse from said optical reflectometer device,   at least two systems according to the invention, each of said two systems having its respective input connected to one output of said coupler,   a plurality of couplers, called subscriber couplers, each of the outputs of said two systems being connected to an input of one of said subscriber couplers, and each output of said subscriber couplers being connected to a wavelength filtering device, the non-filtered wavelength being different for each output of one same subscriber coupler.       

   Advantageously, said coupler has one input and two outputs, each of said two systems according to the invention has one input and four outputs, and each of said subscriber couplers has one input and four outputs. 
   Finally, a further subject of the invention is a method for determining and locating faults in a network by optical time-domain reflectometry according to the invention, characterized in that it comprises the successive sending of four impulses and four different lengths, said four wavelengths respectively corresponding to the wavelengths that are not filtered by said wavelength filtering devices. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the present invention will become apparent in the following description of embodiments of the invention given for illustrative purposes and which are in no way restrictive. 
     In the following figures: 
       FIG. 1  is a diagram of the architecture of a tree topology network of PON type incorporating a system of the invention, 
       FIG. 2  is a diagram of a system for determining and locating faults using an optical time-domain reflectometer (OTDR) system, according to a first embodiment of the invention, 
       FIG. 3  is a diagram of a system for determining and locating faults using an optical time-domain reflectometer (OTDR) system, according to a second embodiment of the invention, 
       FIG. 4  schematically shows the architecture of a point-to-point type network incorporating a system of the invention, 
       FIG. 5  schematically shows a system for determining and locating faults using an optical time-domain reflectometer (OTDR) system, according to a third embodiment of the invention, 
       FIG. 6  is a diagram of a system for determining and locating faults using an optical time-domain reflectometer (OTDR) system, according to a fourth embodiment of the invention, 
       FIG. 7  schematically shows the architecture of a tree and branch network of PON type, incorporating a system according to a fourth embodiment of the invention, 
       FIG. 8  shows a subscriber Final Drop Point (FDP) device such as used in the architecture of  FIG. 7 . 
   

   In all the figures, common items carry the same reference numbers. 
   DETAILED DESCRIPTION 
     FIG. 1  schematically shows the architecture of a tree network  1  of PON type (Passive Optical Network) comprising two systems  10  and  10 ′ according to the invention. 
   Network  1  comprises:
         a central office  2     an optical time-domain reflectometry system  3     a multiplexer/demultiplexer,  4     a first coupler  5 ,   first means  6  for length adaptation   a second coupler  7     second means  8  for length adaptation   subscriber terminals  9         

   Tree network  1  of PON type is a point-to-multipoint system enabling bi-directional data exchange between a central office  2  and subscriber terminals  9  via optic fibres  11  over a distance of the order of twenty kilometres. The wavelengths used for the optic signals during such exchange are generally 1310, 1490 and 1550 nm. Most often, data is transmitted from the central office  2  to the terminals  9  at 1550 nm via Time Division Multiplexing (TDM); data is transmitted from terminals  9  to the central office  2  at 1310 nm via Time Division Multiple Access (TDMA). 
   The optical time-domain reflectometry system  3  makes it possible to send test impulses at wavelengths of 1625 and 1650 nm. 
   On its two inputs, multiplexer/demultiplexer  4  receives data derived from central office  2  and impulses derived from the OTDR system. Output fibre  11  of the multiplexer/demultiplexer  4  is divided into eight optic fibre lines  12  by the first coupler  5 ; the eight lines  12  enter into the first length adaptation means  6 . 
   Each of the eight lines  12  is then divided into four optic fibre lines  12  by the second coupler  7 ; the four lines  13  enter into the second length adaptation means  8  and are then connected to subscriber terminal  9 . 
   System  10  according to the invention comprises the second coupler  7  and length adaptation means  8  and will be described more precisely with reference to  FIGS. 2 and 3  which form two embodiments of the invention. System  10 ′ of the invention comprises the first coupler  5  and length adaptation means  6  and is made in identical fashion to system  10 . 
     FIG. 2  is a diagram of system  10  for determining and locating faults using an optical time-domain reflectometer (OTDR) system, according to a first embodiment of the invention. 
   System  10  of the invention comprises:
         a coupler  7     length adaptation means  8         

   Length adaptation means  8  comprise four length adaptation modules  17 . 
   Each of said modules  17  comprises:
         a demultiplexer  14  having one input and two outputs   two optic channels  18  and  19 , channel  19  having an optic fibre overlength  15 ,   a multiplexer  16  having two inputs and one output.       

   Optic fibre overlength  15  is different for each of modules  17 : for example 10 m, 15 m, 20 m and 25 m of overlength can be taken for each of modules  17 . 
   Each of the four outputs of coupler  7  is respectively connected to the input of demultiplexers  14  belonging to one of modules  17 . 
   The two outputs of demultiplexer  14  are respectively connected to channel  18  and channel  19 . 
   The two channels  18  and  19  are respectively connected to the two inputs of mutliplexer  16 . 
   Channel  18  is used to transfer optic signals at 1625, 1550, 1490 and 1310 nm. 
   Channel  19  is used to transfer optic signals at 1650 nm. 
   Therefore, during formal data exchange operation, optic data circulates solely on channel  18  which allows wavelengths of 1550, 1490 and 1310 nm. 
   If a first test laser impulse is send at a wavelength of 1625 nm, this impulse is also sent into channel  18  as far as the subscriber. 
   If a fault is detected by the OTDR system such as shown in  FIG. 1 , a second laser impulse is sent at a wavelength of 1650 nm. This second impulse is guided into channel  19  via demultiplexer  14 . Therefore the laser impulse at 1650 nm propagates along a length of fibre greater than the length on which the first impulse propagated at 1625 nm. 
   For example, a fault detected during the first measurement gives rise to transitory discontinuity in the monotonicity of the backscattered Optical Power tracing, this discontinuity being positioned at a length of 500 m. This first measurement indicates the position of the fault on the faulty line. 
   During the second measurement using a wavelength of 1650 nm, this discontinuity in monotonicity moves by a length of 15 m and is therefore positioned at 515 m. This length of 15 m corresponds to an overlength associated with a particular, fully identified line. 
   Therefore, each of overlengths  15  being different for each of modules  17 , measurement of the movement of the discontinuity associated with the presence of a fault makes it possible to determine line  13  on which the fault is located. 
     FIG. 3  is a diagram of system  10  for determining and locating faults using an optical time-domain reflectometer (OTDR) system, according to a second embodiment of the invention. 
   System  10  is identical to the one shown in  FIG. 2 , with the difference that channel  19  is used to transfer optic signals at 1625 nm and comprises a saturable absorber device  20  in series which passes signals beyond a certain received signal power. 
   Therefore during normal data exchange operation, optic data circulates solely on channel  18  which allows wavelengths of 1550, 1490 and 1310 nm. 
   If a first test laser impulse of low power is sent at a wavelength of 1625 nm, this impulse is sent into channel  18  as far as the subscriber. This low power impulse is blocked on channel  19  by saturable absorber  20 . 
   If a fault is detected by the OTDR system such as shown in  FIG. 1 , a second laser impulse of greater power is sent at a wavelength of 1625 nm. This second, stronger powered, impulse propagates partly along channel  19  so that saturable absorber  20  changes to a passing state, and partly on channel  18 . Therefore the second laser impulse propagates along a length of fibre that is greater than the length on which the first impulse was propagated. 
   Consequently the discontinuity in the monotonicity of the backscattered Optical Power tracing associated with a fault detected during the first measurement moves during the second measurement by means of overlength  15 . 
   Each of overlengths  15  being different for each of modules  17 , by measuring the movement of the transitory discontinuity in monotonicity, it becomes possible to determine the line  13  on which the fault is located. 
   The embodiments just described concern the architecture of a point-to-multipoint network, but an architecture  1 ′ can also be considered, as shown in  FIG. 4 , of a point-to-point architecture comprising a system  10  of the invention. 
   Network  1 ′ comprises:
         a central office  2 ,   an optical time-domain reflectometry system  3 ,   data lines  24 ,   an optic fibre  27 ,   test lines  25 ,   multiplexers  21 ,   a coupler  7 ,   length adaptation means  8 ,   subscriber lines  26 ,   subscriber terminals  9 .       

   This type of network  1 ′ allows data exchange between central office  2  and subscriber terminals  9 , central office  2  having as many input nodes as there as subscriber terminals  9 , each input node being connected to a terminal  9  via a data line  24  multiplexed by one of multiplexers  21  on whose output is a subscriber line  26 . Data lines  24  operate at wavelengths of 1550, 1490 and 1310 nm. 
   Network  1 ′ also allows testing of subscriber lines  26  via an optical time-domain reflectometry system  3  connected by optic fibre  27 , operating at 1625 nm, to coupler  7 . Fibre  27  is divided into four test lines  25  by coupler  7 ; the four test lines  25  enter into length adaptation means  8 . 
   Each of the four lines  25  is then multiplexed on one of multiplexers  21 . 
   System  10  of the invention will be described with more precision below with reference to  FIG. 5 . 
     FIG. 5  schematically shows a system  10  for determining and locating faults using an optical time-domain reflectometer (OTDR) system according to a third embodiment of the invention, adapted to a point-to-point network. 
   System  10  such as shown in  FIG. 5  is identical to the one shown in  FIG. 2  with the difference that it comprises a switch  22  switching an input on two outputs instead of a demultiplexer, and a switch  23  switching two inputs on an output instead of a multiplexer. 
   A first test laser impulse is first sent at a wavelength of 1625 nm into channel  18  as far as the subscriber by means of switches  22  and  23 . 
   If a fault is detected by the OTDR system such as shown in  FIG. 4 , a second laser impulse is sent at the same wavelength of 1625 nm along channel  19  by switching the two switches  22  and  23 . Therefore the second laser impulse propagates along a length of fibre greater than the length along which the first impulse was propagated. 
   Consequently the discontinuity in monotonicity of the backscattered Optical Power tracing associated with the fault detected during the first measurement will be moved during the second measurement by means of overlength  15 . 
   Each of overlengths  15  being different for each of modules  17 , measurement of the movement of the absorption peak makes it possible to determine the line  13  on which the fault is located. 
     FIG. 6  is a diagram of a system  100  for determining and locating faults using an optical time-domain reflectometer (OTDR) system, according to a fourth embodiment of the invention. 
   System  100  such as shown in  FIG. 6  is identical to system  10  shown in  FIG. 2  with the difference that each of the four length adaptation modules comprises a coupler  28  dividing an input on two outputs instead of a demultiplexer, and a coupler  29  regrouping two inputs on one output instead of a multiplexer. 
   Use of system  100  shown in  FIG. 6  consists of sending an impulse at a certain test wavelength λ TEST  which separates into two impulses reach representing approximately one half of the test impulse. There is therefore a first impulse which passes into the first channel at λ TEST  and a second impulse entering the second channel at λ TEST . 
   In this case, the presence of fault gives rise to a triple transitory discontinuity in the monotonicity of the backscattered Optical Power tracing. The first discontinuity is positioned at a point corresponding to the distance of the fault, the second is positioned at a point shifted by a length equal to the overlength travelled by the light and the third is positioned at a point shifted by a length equal to twice the overlength. With this type of solution it is therefore possible to detect the fault with a single test impulse separated into two impulses. 
   System  100  in  FIG. 6  is advantageously used in a tree network of PON type such as shown in  FIG. 7 . 
     FIG. 7  is a diagram of the architecture of a tree network  200  of PON type (Passive Optical Network) comprising two systems  100  such as shown in  FIG. 6 . 
   Network  200  comprises:
         a central office  202 ,   an optical time  6  domain reflectometer (OTDR) system,   a multiplexer/demultiplexer  204  with at least two inputs and at least one output,   a coupler  205  with at least one input and at least two outputs,   two systems  100  such as shown in  FIG. 6  corresponding to primary subscriber connections of PFP type (Primary Flexibility Point)   eight final subscriber connection devices  206  of Final Drop Point (FDP) type with one input and four outputs which will be described more in detail with reference to  FIG. 8     32 subscriber terminals  209         

   Tree network  200  of PON type is a point-to-multipoint system enabling bi-directional data exchange between the central office  202  and subscriber terminals  209  via optic fibres  211  over a distance in the order of twenty kilometres. Data is transmitted from the central office  202  to the terminals  209  at a wavelength λ W  via Time Division Multiplexing (TDM); data is transmitted from terminals  209  to the central office  202  at another wavelength via Time Division Multiple Access (TDMA). 
   With the time-domain reflectometer system  203 , it is possible to send test impulses at four test wavelengths λ TEST1 , λ TEST2 , λ TEST3 , λ TEST4 . 
   Multiplexer/demultiplexer  204 , on its two inputs, receives data derived from central office  202  and the impulses derived from OTDR system  203 . Outlet fibre  211  of multiplexer/demultiplexer  204  is divided into two optic fibre lines  212  by the first coupler  205 ; the two lines  212  enter into the two systems  100 . 
   Each of the eight output lines  231  of systems  100  then enter into the eight devices  206  of final drop point type (FDP) 
   Each of the 32 outputs of the 8 FDP devices  206  is connected to a subscriber terminal  209 . 
     FIG. 8  shows a device of Final Drop Point (FDP) type with one input and four outputs. 
   Device  206  comprises:
         a subscriber coupler  215  with one input and four outputs so that input line  213  is separated into four lines  216 ,   four filters  214  respectively allowing the passage of wavelengths λ TEST1 , λ TEST2 , λ TEST3  and λ TEST4 .       

   Therefore, each line  216  only allows one test wavelength to pass. 
   Evidently, each filter also allows data transmission wavelengths λ W  to pass. 
   Network  200  such as shown in  FIG. 7  enables easy and efficient fault determination and location. 
   Four test impulses corresponding to the four test wavelengths λ TEST1 , λ TEST2 , λ TEST3  and λ TEST4  are sent in succession. 
   If a fault is located between the two systems  100  and FDP devices  206 , each impulse is sufficient to determine and locate the fault and will give rise to triple discontinuity; the first discontinuity being positioned at a point corresponding to the distance of the fault, the second being positioned at a point shiftted from the first by a distance equal to the overlength travelled by the light and the third being positioned at a point shifted by a length equal to twice the overlength. 
   If the fault is found between FDP devices  206  and subscribers  209 , only one of the four test impulses will give rise to a triple discontinuity, the first discontinuity being positioned at a point corresponding to the distance of the fault, the second being positioned at a point shifted by a length equal to the overlength travelled by the light, and the third being positioned at a point shifted by a length equal to twice the overlength. Knowledge of this wavelength is sufficient to determine the subscriber line on which the fault is located. 
   Evidently the invention is not limited to the embodiments just described. 
   While remaining within the scope of the invention, it is possible in particular to modify the wavelengths used and the number of coupler inputs used. 
   Also, the second embodiment was described with a saturable absorber which may be replaced by any optical component having open or closed mode operation such as a bi-stable component. 
   In addition the coupler and length adaptation means may be made in one same integrated module, a semiconductor for example, on which the optic fibre overlengths are connected. 
   Finally the overlengths may or may not be integrated in the module and are not necessarily optic fibres.