Patent Publication Number: US-9419708-B2

Title: Live monitoring of raman and fiber degradation in DWDM networks using in-service OTDR

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is the first application filed in respect of the present invention. 
     TECHNICAL FIELD 
     The present invention relates generally to fibre-optic communication systems, and in particular to techniques for live monitoring of Raman and fiber degradation in Dense Wavelength Division Multiplexing (DWDM) networks using in-service Optical Time Domain Reflectometry (OTDR). 
     BACKGROUND 
     For the purposes of the present disclosure, the term “in-service” shall be understood to refer to an optical link in which at least one channel is carrying (or is available to carry) data traffic. Similarly, “in-service OTDR”, and “in-service Raman” and the like shall be understood to refer to OTDR measurements and Raman amplification operation pertaining to an “in-service” optical fiber link. 
       FIGS. 1A-1D  illustrate representative Wavelength Division Multiplexed (WDM) or Dense Wavelength Division Multiplexed (DWDM) optical systems  2  known in the art. In all four examples, the system  2  comprises an optical fiber link  3  having a known length which extends between a set of transmitters  6  at a transmitting end of the system  2 , and a corresponding set of receivers  8  at a receiving end of the system. Typically, the optical link  3  will include two or more optical fiber spans  4 , interconnected by discrete optical devices  5  such as, for example, Erbium Doped Fiber Amplifiers EDFAs and optical switches. In the illustrated arrangement, optical MUX and DEMUX devices,  10 ,  12  are provided to combine optical channel signals from each transmitter into a WDM signal for transmission through the optical fiber span, and for supplying each wavelength channel from the WDM signal to a respective receiver. However, it will be appreciated that other suitable MUX/DEMUX arrangements may be used if desired. For the purposes of the present invention, each fiber span  4  can be any suitable type and length; each transmitter may be tuned to any suitable wavelength channel and may use any suitable modulation technique to transmit data through the fiber span; and each receiver may use any suitable detection (i.e. direct or coherent detection) and decoding techniques. 
     Optical Time Domain Reflectometry (OTDR) is a well-known technique which can be used to obtain an impulse response of an optical fiber span  4 , and extract useful information regarding optical properties of the fiber link  3 . For example, OTDR has been successfully used to obtain information about fiber attenuation profile (i.e. loss vs. distance), and point losses and/or reflections due to physical problems with the fiber (such as a “pinched” fiber). Degradation or impairment of fiber properties over time may be monitored by saving a reference OTDR measurement, and comparing it with a new OTDR measurement made at a later time. 
     In the examples of  FIGS. 1A-1D , OTDR is implemented by means of an OTDR subsystem  14  connected to the optical fiber link  3  at a suitable location. In the illustrated examples, only one OTDR subsystem  14  is shown. However, in practice, any suitable number of OTDR subsystems  14  may be employed, and these may be connected to the optical fiber link  3  at any suitable location(s). In the examples of  FIGS. 1A and 1B , the OTDR sub-system  14  is connected to the fiber link  3  such that OTDR pulses are launched into the fiber span  4  and propagate in the same direction as traffic launched from a transmitter  6 . This is referred to as co-propagating OTDR. In the examples of  FIGS. 1C and 1D , the OTDR sub-system  14  is connected to the fiber link  3  such that OTDR pulses are launched into the fiber span  4  and propagate in the opposite direction as traffic launched from a transmitter  6 . This is referred to as counter-propagating OTDR. 
     An advantage of OTDR is that it permits measurement of the optical characteristics of an optical fiber link installed in a network. Typically, OTDR is used to evaluate the performance of unused (i.e. “dark”) fibers, which have been installed, in the network but which are not carrying any optical channel signals. This provides an effective means of determining whether or not an installed optical fiber link is ready to support optical channel traffic when needed. However, in some cases it is also possible to perform OTDR measurements of “in-service” fibers by selecting a wavelength for the OTDR measurements that will not interfere with optical channel signals within the fiber. 
     Meanwhile, the increasing demand for bandwidth and signal reach is driving the development of technologies capable of transmitting more and more bits per second in the DWDM (dense wavelength division multiplexing) spectrum. Supporting such demands requires better Optical Signal-to-Noise ratio (OSNR) for each traffic signal. A commonly used technique to improve OSNR is to deploy Raman amplifiers employing co- or counter propagating pump lasers. Raman amplifiers work by taking the advantage of nonlinear Stimulated Raman Scattering (SRS) phenomena in the fiber, and the interaction between Raman pump laser light and the optical channel signals propagating in the fiber. 
     Typically, Raman amplification is implemented by means of a Raman module  16  or card, that includes a pump laser  18  and a coupler  20  for optically coupling pump light from the pump laser  18  into an optical fiber span  4  at a desired location. As the pump light propagates through the link  3 , energy is coupled from the pump light into any optical signals having a wavelength lying within a Raman gain region, thereby amplifying any such optical signals. In the examples of  FIGS. 1A and 1D , the Raman module  16  is connected to the fiber link  3  such that the pump light propagate in the same direction as traffic launched from a transmitter  6 . This is referred to as co-propagating Raman. In the examples of  FIGS. 1B and 1C , the Raman module  16  is connected to the fiber link  3  such that the pump light propagates in the opposite direction as traffic launched from a transmitter  6 . This is referred to as counter-propagating Raman. 
     In practical optical transmission systems, any suitable number of Raman modules  14  may be provided, and may be connected to the link  3  at any suitable location. For example, Raman modules may be provided at both the transmitter and receiver ends of the link  3 . If desired, one or more Raman modules may be provided at a discrete optical device  5  so as to inject pump light into a desired fiber span  4 . In systems having more than one Raman module  14 , the respective Raman pump lasers  18  may be tuned to a common wavelength, or to respective different wavelengths, as desired. In some cases, a Raman module may include more than one pump laser  18 , and these lasers may be tuned to the same, or different wavelengths, as desired. 
     In  FIGS. 1A-1D , the length of a fiber span  4  (L) is measured from the point at which the OTDR sub-system  14  is connected to the fiber. Thus, in  FIGS. 1A and 1B , the origin (z=0) is located at the transmitter end of a span  4 , and distances are measured toward the receiver end of the link  3 . Conversely, in  FIGS. 1C and 1D , the origin (z=0) is located at the discrete optical device  5 , and distances are measured toward the transmitter end of the link  3 . This nomenclature is relevant to the present technique because OTDR measures distance from the point of injection of the OTDR pulses. 
     Because Raman amplification utilizes the fiber itself as the gain media, the optical characteristics of the installed fiber link must be well defined before turning the pump laser(s) on. This is very important both in terms of safety and performance, as pump laser power levels can be high enough to cause damage due to point losses such as poor splices or dirty connectors. Conventional OTDR methods can be used to accomplish the required characterization of the installed optical fiber link. Typically, a so-called “short trace” is used to characterise portions of a fiber link where Raman gain is expected to be high, which corresponds to the portion of the fiber nearest to the Raman module. If the short trace identifies a problem with the fiber, such as a high loss, the fiber link is disqualified and a service technician or management system may be notified. A long trace is typically used to detect faults along the entire length of the fiber link, and so may be used to locate a pinch or cut in the fiber span. The short and long OTDR traces are commonly used together to obtain a “snapshot” of the optical link characteristics. 
     However, when the Raman pump lasers are turned ON, OTDR traces are adversely affected by nonlinear Raman scattering from the pump light. Because the intensities of both the Raman scattering and the OTDR return signal are affected by the properties of the optical fiber, there is no reliable method of analysing OTDR trace information to distinguish degradations in Raman gain from degradations in fiber performance. As a result, the conventional use of OTDR for analysing optical fiber characteristics is limited to situations in which the Raman pump lasers are turned OFF, which implies that the transmitters  6  are not transmitting optical signal traffic through the fiber link  3 . 
     As noted above, conventional OTDR trace information is obtained when Raman amplification is OFF, i.e., no Raman gain and therefore no traffic (which implies that the optical link  3  is out of service). The two conventional applications of OTDR comprise: short trace, to investigate if the fiber qualities for Raman amplification with regard to safety and performance concerns; and long trace, in fault scenarios such as fiber cut when an automatic fiber cut location analysis is performed. In both of these applications, OTDR trace information is obtained only when the optical link is out of-service. 
     Consequently, network elements typically use a telemetry channel to estimate the overall gain experienced by wavelength channels/signals when the link is in-service. This technique provides a blind estimation of the gain due to the presence of Raman pump light. As a result any degradation in provided Raman gain cannot be simply analyzed to be due to the real degradation in Raman gain or due to non-Raman related issues. 
     It would be desirable to provide techniques for analysing in-service fiber link characteristics using real-time OTDR trace information in the presence of Raman amplification. In this respect, “real-time” should be understood to refer to information that is obtained from measurements with minimum processing delay, so that the measurements and the information obtained therefrom, provide a high-fidelity representation of the current state and/or performance of the optical fiber link. 
     SUMMARY 
     Aspects of the present invention provide methods and systems to analyse fiber link performance using OTDR trace information in the presence of Raman amplification. 
     According to one aspect, the present invention provides a method of analysing performance of an optical fiber link. As a preliminary step, a reference trace indicative of a distributed optical performance of the optical fiber link is derived. During in-service operation of the optical fiber link, an Optical Time Domain Reflectometry (OTDR) sub-system measures an OTDR trace with Raman amplification ON, and a real-time cumulative Raman Gain profile of the optical fiber link is calculated based on the reference trace and the measured OTDR trace. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Representative embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: 
         FIGS. 1A-1D  are block diagrams schematically illustrating elements of a conventional optical link including OTDR measurement and Raman amplification; 
         FIG. 2  is a flow chart illustrating principal steps of a process in accordance with a representative embodiment of the present invention; 
         FIGS. 3A-3D  are charts illustrating representative gain profiles in the links of  FIGS. 1A and 2B ; 
         FIG. 4  is a flow chart illustrating principal steps of a process in accordance with a representative embodiment of the present invention; and 
         FIG. 5  is a flow chart illustrating principal steps of a process for obtaining real-time OTDR trace information in accordance with a representative embodiment of the present invention 
     
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DETAILED DESCRIPTION 
     The present invention provides a technique for analysing performance of an optical fiber link by extracting real-time OTDR trace information from the in-service link, in the presence of Raman amplification. 
     In order to monitor the performance of an optical link, it would be desirable to obtain the following information separately: 
     a) The distributed Raman gain profile G(z) which defines the Raman gain as a junction of distance. The distributed Raman gain profile is useful in that it can be monitored over time and Raman-related performance degradation may be detected and localized. 
     b) A real-time OTDR-like trace obtained “in-service”, with Raman amplification ON. Conventional OTDR trace information (i.e. when Raman amplification is OFF) enables the fiber quality to be monitored and defects can be detected and localized. Providing equivalent information for in-service fiber links would enable monitoring of fiber link performance over time, to detect degradation or change not necessarily affecting the Raman gain but the overall performance. 
     For the purposes of the present description, an embodiment is described in which a counter propagating Raman light arrangement is used (See  FIG. 1B ), but it will be appreciated that the same techniques can be used to analyse embodiments using co-propagating Raman pump light ( FIG. 1A ). In fact, it will be appreciated that the present techniques may be used to analyse the optical characteristics of an optical link  3  having any desired combination of co-propagating and counter propagating pump lights launched from any desired number of Raman modules  16 , each of which may be located at any suitable location within the link  3 . In some embodiments, OTDR sub-systems  14  and Raman modules  16  may be integrated together into a common module, but this is not essential. 
     Interestingly the real-time OTDR trace information generated using the present techniques is also a function of distance (because the OTDR trace profile is a function of distance). Therefore by understanding the physics behind these measurements we can extract the information lacking in conventional techniques. 
     In principle the main difference between conventional OTDR trace information and OTDR trace information with Raman amplification ON is due to the contribution of the nonlinear Raman scattering between the Raman pump light and the OTDR pulses. This nonlinear Raman scattering contribution is directly related to the Raman gain. Accordingly, it is possible to estimate the Raman gain profile G(z), by subtracting the OTDR trace with Raman amplification OFF from the OTDR trace with Raman amplification ON. Thus:
 
 P ( z ) ON   −P ( z ) OFF   =G ( z )[dB]  (1)
 
     Where P(z) ON  and P(z) OFF  represent the OTDR power measured when Raman amplification is ON and OFF, respectively. G(z) represents the cumulative Raman gain profile (as a function of distance z). Assuming that Raman amplification is implemented using N pump lasers  18  (which may be provisioned in one or more Raman modules  16 ), the cumulative gain G(z) can be expressed as: 
     
       
         
           
             
               
                 
                   
                     
                       G 
                       ⁡ 
                       
                         ( 
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                     ⁡ 
                     
                       [ 
                       dB 
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                   = 
                   
                     
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                         n 
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     Where: C R (n) is the Raman cross gain between the OTDR wavelength and the n th  pump wavelength; P n  is the output power of the n th  pump laser; and α n  is the fiber attenuation loss for the n th  pump laser wavelength. 
     The right side of equation (2) can be approximated by one term representing an averaged single pump. With this simplification, equation (2) can be rewritten as: 
     
       
         
           
             
               
                 
                   
                     
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                       ⁡ 
                       
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     Where the subscript “0” identifies an equivalent pump with an average attenuation loss α 0 , and C R0 P 0 =Σ n=1   N C R (n)P n . Note that equation (3) has no approximation for a Raman amplification arrangement based on a single pump laser. Recall that the measurement at z=L (distance from Raman connection) shows the total gain experienced due to the presence of Raman. As illustrated in the flow chart of  FIG. 4  (described in greater detail below), this is used in conjunction with the measured telemetry signal gain for narrowing down the troubleshooting if necessary. 
     Taking the derivative of equation (3), and the natural logarithm of the derivative yields the following: 
     
       
         
           
             
               
                 
                   
                     
                       ∂ 
                       G 
                     
                     
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                     ln 
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                       z 
                     
                     + 
                     cte 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Where cte   ln(P 0 C R0 ) and may be treated as a constant value. Equations (4) and (5) describe the physics in measured gain. Equation (4) defines a distributed incremental Raman gain profile which can be used effectively (as well as equation (2)) to analyse the real time Raman gain (as a function of distance z) within the optical fiber link. 
     Similarly equation (5) defines the attenuation/loss profile of the fiber link for the averaged pump wavelength. This profile is analogous to an OTDR trace obtained at the averaged pump wavelength, and can be used in a manner closely similar to a conventional OTDR trace. 
       FIG. 2  illustrates a representative method for obtaining a real-time Raman Gain profile an in-service optical link. In some embodiments, this process may be executed by a processor of an integrated module comprising a Raman module  16  and an integrated OTDR sub-system  14  of the type described above. In other embodiments, this algorithm may be implemented by a management system in communication with a Raman module  16  and an OTDR sub-system  14  which may or may not be integrated with the Raman module  16 . In some cases, the algorithm may be embodied as machine-readable instruction code stored on a non-transitory machine-readable storage medium for controlling a suitable processor to execute the algorithm.  FIGS. 3A-3D  illustrate representative trace information obtained using the method of  FIG. 2  for an optical fiber span  4  having a length L=120 km. The example of  FIGS. 3A and 3B  illustrate trace information for a case in which the OTDR pulses and Raman pump lasers are co-propagating with respect to each other. Conversely, the example of  FIGS. 3C and 3D  illustrate trace information for a case in which the OTDR pulses and Raman pump lasers are counter-propagating with respect to each other. 
     Referring to  FIG. 2 , at a first step (step S 2 ), a reference trace is derived, which is indicative of a distributed optical performance of the optical fiber link with no Raman amplification. In some embodiments, the reference trace may be derived by calculating an estimate of the fiber loss using data of the known optical performance of each of the components of the optical fiber link  3 . In other embodiments, the reference trace may be derived by performing a conventional OTDR trace measurement with Raman Amplification OFF, to yield the reference OTDR trace  22  shown in  FIGS. 3A and 3C . For this reference OTDR trace measurement, the OTDR pulse wavelength may be selected to lie within the Raman gain region. In some embodiments, the OTDR pulse wavelength may also be selected to lie outside the EDFA gain region. In some embodiments, two or more OTDR pulse wavelengths may be used, in which case each OTDR wavelength may be respectively selected to lie inside or outside of the Raman and EDFA gain regions, as desired. In some embodiments, at least one OTDR wavelength may be selected to lie within a spectral region known to be sensitive to fiber faults such as, for example bending, pinching, or water saturation of the cable containing the fiber. Subsequently, an in-service OTDR trace measurement is performed, using the same OTDR pulse wavelength(s) with Raman Amplification ON. This operation yields an in-service OTDR trace  24  ( FIGS. 3A and 3C ). Using the reference and in-service OTDR traces  22  and  24 , the cumulative Raman gain profile  26  ( FIGS. 3B and 3D ) can be calculated (at S 6 ) following equation 1 above. 
     As may be appreciated, the process described above will yield a cumulative Raman gain profile at the (or each) OTDR wavelength selected for the OTDR trace measurements. If desired, this process may be executed using an OTDR pulse wavelength corresponding to a wavelength channel of the WDM system, so as to develop a cumulative Raman gain profile  26  pertaining to that wavelength channel. If desired, this process may be repeated for two or more different wavelength channels of the WDM system, so as to develop a family of Raman gain profiles pertaining to some or all of the channels of the WDM system. If desired, this family of Raman gain profiles may be analysed to derive further information regarding the optical characteristics of the optical fiber link. For example, variations in Raman gain as a function of wavelength, as well as estimates of Group delay and dispersion can be readily obtained from the respective Raman gain profiles obtained at different wavelengths. 
     In some embodiments, the process described above with reference to  FIGS. 2 and 3  is performed during initial deployment of an optical link. In this case, the cumulative Raman Gain profile may be used “as-is”, or further processed (for example using equation 4) to obtain the incremental Raman gain profile and/or the attenuation/loss profile (for example using equation 5). These results may be used alone, or in combination to validate whether or not the optical link is performing properly before starting to carry traffic. These profiles may also enable point faults to be located within the link, so that, for example, service personnel can investigate and take corrective action. 
       FIG. 4  illustrates another representative algorithm for extracting real-time OTDR trace information from an in-service optical fiber link. In some embodiments, this algorithm may be executed by a processor of an integrated module comprising a Raman module  16  and an integrated OTDR sub-system  14  of the type described above. In other embodiments, this algorithm may be implemented by a management system in communication with a Raman module  16  and an OTDR sub-system  14  which may or may not be integrated with the Raman module  16 . In some cases, the algorithm may be embodied as machine-readable instruction code stored on a non-transitory machine-readable storage medium for controlling a suitable processor to execute the algorithm. 
     In a preliminary step (S 8 ), the process described above with reference to  FIGS. 2 and 3  are performed to generate a reference Raman gain profile G ref (z). In addition, a reference Telemetry Gain can be measured in a conventional manner. The reference Raman gain profile G ref (z), Telemetry Gain profile, and the OTDR trace profile measured with Raman Amplification OFF (P OTDR   OFF ) may then be saved for later use. These operations may be performed prior to the optical link  3  entering service, for example during System Layout and Testing (SLAT) or as part of a failure recovery. 
     In some embodiments, the OTDR wavelength is selected such that it is in the Raman Gain range, but outside of the gain range for Erbium Doped Fiber Amplification (EDFA). In embodiments in which both Raman and EDFA are used in the link, this selection of OTDR frequency enables in-service monitoring of Raman Gain and Fiber performance without interference from EDFA modules that may also be operating on the link. 
     Once the optical link has entered service, real-time OTDR data can be obtained by first measuring (at S 10 ) the OTDR power level P OTDR   ON  as a function of distance (z). Known techniques may then be used (at S 12 ) to process the measured OTDR power level data so as to filter measurement noise. For example, a smoothing or low-pass filtering operation may be performed to reduce scatter in the measured OTDR power level data. The filtered data can then be used, in conjunction with the reference OTDR power level P OTDR   OFF  and equation 1 to calculate the real-time Raman gain profile G(z) (at S 14 ). 
     As noted above, the Raman Gain value at z=L (i.e. G(L)) shows the total gain experienced due to the presence of Raman amplification. Accordingly, if the difference between the reference and real-time Raman gain values at z=L is less than a predetermined threshold (at S 16 ) then the measured telemetry gain can be treated as a reasonable estimate of the Raman gain (S 18 ). In some embodiments, the threshold value may be provisionable by the user. On the other hand, if the difference between the reference and real-time Raman gain values at z=L is equal to or greater than the threshold value, then the Raman pump laser(s) is(are) checked (at S 20 ) to determine whether or not they are operating at their target output power level. If the Raman pump laser(s) is(are) operating at their target output power level, then the real-time OTDR trace information can be calculated from the real-time Raman gain profile G(z) (at S 22 ) as described in greater detail below. On the other hand, if the Raman pump laser(s) is(are) not operating at their target output power level, then the real-time Raman gain profile G(z) can be re-calculated (at S 24 ) based on the actual Raman pump laser output power, before evaluating the real-time OTDR trace information. 
     In some embodiments, the reference and/or real-time Raman gain profiles G Ref (z) and G(z) may be used to optimize an optical performance of the optical link  3 . For example, consider an embodiment in which Raman modules are provided with multiple pump lasers tuned to different frequencies. In such a case, a family of real-time Raman gain profiles G(z) may be derived, as described above, using a range of different OTDR wavelengths. This information may be used to adjust the respective power level of each Raman pump laser so as to at least partially compensate wavelength dependence of the Raman gain. In a further example, the real-time Raman gain profile G(z) may be used in conjunction with the known location and launch direction of each Raman pump laser to maximize Raman gain in portions of an optical fiber link  3 , while at the same time avoiding damage to sensitive optical devices in the link. Other possible uses of the real-time Raman gain profile G(z) will be, or become, apparent to those of ordinary skill in the art, based upon the present description. 
       FIG. 5  is a flow chart showing principal steps in a process for evaluating the real-time OTDR trace information based on the reference and real-time Raman gain profiles G Ref (z) and G(z), respectively. At a first step (S 26 ), the reference and real-time Raman gain profiles G Ref (z) and G(z), respectively, are compared. This comparison may be used to identify changes in the optical characteristics of the optical link  3  since the reference Raman gain profile G Ref (z) was generated. For example, the appearance of a point loss in the real-time Raman gain profile G(z) that is not present in the reference Raman gain profile G Ref (z) may indicate a problem that requires intervention by a service technician. Then, the incremental Raman gain profile 
               ∂   G       ∂   z           
is calculated (at S 28 ) by taking the derivative with respect to distance (z) of the real-time Raman gain profile G(z). The loss/attenuation profile of the link can then be estimated (at S 30 ) by taking the natural logarithm of the incremental Raman gain profile
 
                 ∂   G       ∂   z       .         
As described above, this estimate can be used (at S 32 ) in a manner directly analogous to conventional OTDR trace information, to examine the link performance in the presence of Raman amplification.
 
     The above-described embodiments of the invention are intended to be examples only. Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.