Abstract:
The present invention relates to method for interpreting data obtained by measuring a length of optical fiber using an optical time domain reflectometer (OTDR), and comparing that measurement to a reference measurement. The technique uses statistical inference to determine a likely cause for a length measurement to be shorter than a reference length. One technique uses a chi-squared best fit of an array reflectance spike occurrences along the fiber to a historical reference array. In that way, it can be determined whether the missing portion of the tested fiber is at an end or between the ends, providing evidence that the short length measurement results from a fiber break or from the intentional removal of a reserve loop.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of application Ser. No. 10/897,929, entitled “Application of Statistical Inference to Optical Time Domain Reflectometer Data,” filed on Jul. 23, 2004 now U.S. Pat. No. 6,989,893. 

   FIELD OF THE INVENTION 
   The present invention relates generally to optical network troubleshooting. Specifically, the invention provides a method for interpreting an optical fiber measurement trace taken by an optical time domain reflectometer (OTDR) using statistical inferences. 
   BACKGROUND OF THE INVENTION 
   Telecommunications network operators rely heavily on the integrity of their fiber cable networks in competing for customers on the basis of quality of service. Carriers can no longer tolerate service outages on cables, or even single fibers, designed to transport numerous gigabit-per-second optical channels. 
   Once an optical cable has been installed, network providers must be certain that each separate fiber span matches or exceeds the carrier&#39;s specifications. Testing and troubleshooting of in situ optical fiber cable is frequently carried out using optical time domain reflectometer (OTDR) instruments, such as the Series FTB-7000B ODTR sold by EXFO Electro-Optical Engineering Inc. of Vanier (Quebec) Canada. 
   The OTDR characterizes fibers at a high level of detail, generating distance versus attenuation data, as well as insertion loss measurements for all splices, defects, kinks, and breaks. An OTDR functions by injecting a short, intense laser pulse into the optical fiber and measuring the backscatter and reflection of light as a function of time. A simplified, schematic representation of an OTDR  110  is shown in  FIG. 1 . The OTDR includes a laser source  116  and a detector  117 , each connected to a subject fiber  100  via a coupler  118 . Laser energy is injected into the fiber  100  in the form of hundreds of pulses per second. A portion of the laser energy travels to the fiber termination, but splices, breaks, bends and any other anomalies in the fiber reflect some portion of the laser energy. Those characteristics can be located along the fiber by observing the round trip transmission time of the reflected laser energy. The characteristics are measured thousands of times and averaged to increase accuracy. 
   The reflected light characteristics are analyzed to determine the location of any fiber optic breaks or splice losses.  FIG. 2  shows a sample output trace of a typical OTDR. Distance along the fiber cable is represented in kilometers on the x-axis  210 , and attenuation  220  of the Laser signal is represented in decibels on the y-axis  212 . 
   Many of the characteristics seen on an OTDR trace are the result of Fresnel reflectance caused by abrupt changes in the index of refraction (ex: glass/air). For example, the mechanical splice connector  221  and span end  223  shown in the trace of  FIG. 2  are the result of glass/air interfaces. Using the location of the span end  223  on the x-axis, a length of the fiber span may be accurately calculated. A fusion splice  222  eliminates a glass/air interface, but it nevertheless generates a substantial level of reflected signal power as compared to the backscatter level, and is detectable on the trace. Floor noise  224  may be seen at distances beyond the termination  120  ( FIG. 1 ) of the fiber. 
   The proper interpretation of OTDR data—whether generated manually or automatically—frequently depends upon its comparison to one or more reference traces. A reference trace, however, is a static snapshot of a fiber that quickly becomes out of date, both from aging characteristics of the optical material, and as a result of network maintenance. 
   For example, a test OTDR trace may be compared to a reference trace, and the test trace may reveal a shorter overall fiber length than that shown by the reference trace. That situation raises the question whether the shorter fiber length is due to an accidental cable cut, or is due to a cable loop being removed in the course of normal network operations. Similarly, if a test trace displays a higher loss reading, the question arises whether this an equipment fault, or is simply aging of the fiber. 
   A wide body of tools exists in the industry to analyze a test trace, and to compare that trace to an assumed good reference. However, working in the reverse direction—comparing the reference to the newer test trace—gives a significant advantage to the process. 
   There is presently a need for a set of statistical tools to increase confidence in the comparison and analysis of OTDR data. Those tools should permit the automatic detection of certain conditions such as: 
   Reference Trace is invalid and must be updated; and 
   Reference Trace is valid, but test trace is suspect. 
   Even if both traces are valid, the toolset should allow additional information to be extracted from the test trace, such as a possible future failure type or failure point. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the needs described above with a method for analyzing a measurement of reflective characteristics in an optical fiber. Parameters are received from a test OTDR trace T test  of the optical fiber, the parameters including a gross distance measurement D test  and at least one array parameter P             test . Parameters from a reference OTDR trace T ref  are retrieved from a memory, the parameters including a gross distance measurement D ref  and at least one array parameter P           ref . D ref  and D test  are compared, and if D ref  is greater than D test , then correlating fiber characteristics represented by P           ref  with fiber characteristics represented by P           test  to determine a location of a portion of fiber represented by P           ref  that is not represented by P           test .
   If D ref  is less than D test , then D ref  may be replaced in the memory with D test . The step of correlating fiber characteristics may use an inferential statistical method. The inferential statistical method may include performing a chi-squared best fit of the parameter P             test  against the parameter P           ref .
   The method may also comprise the step of selecting a significance level α for the correlation of the fiber characteristics. 
   The parameter P             test  may be an array R          of reflectance spikes detected along the fiber. If D ref  is not statistically significantly greater than or less than D test , and a reflectance array R           test  from the test trace T test  varies statistically significantly from a reference reflectance array R           ref , then D ref  may be replaced in the memory with D test .
   The parameter P             test  may be an array IL          of instantaneous loss values measured along the fiber, or may alternatively be an array GL          of gross loss values for points along the fiber.
   The step of comparing D ref  and D test  may comprise the steps of assigning an experimental error σ associated with the measurement of D ref  and D test ; if D test +2σ&gt;D ref  then conclude that D test  is less than D ref ; if D test &gt;D ref +2σ then conclude that D test  is greater than D ref ; and if D test &lt;D ref &lt;D test +2σ then conclude that D test  may be equal to D ref . 
   If D ref  is less than D test , then D ref  may be replaced in the memory with D test . 
   In another embodiment of the invention, a machine readable medium is provided containing configuration instructions for performing the above method. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an OTDR system connected to a fiber cable. 
       FIG. 2  is a sample graphical output from an OTDR system. 
       FIG. 3  is a schematic diagram showing a system utilizing one embodiment of the invention. 
       FIG. 4  is a flow chart depicting a method according to an embodiment of the invention. 
   

   DESCRIPTION OF THE INVENTION 
   The present invention provides a process for using a test OTDR trace to either validate or refute an assumed good reference trace. The process also allows additional determinations to be made about the validity of the test trace, and to refine error analysis in an alarm situation. 
   A block diagram of an exemplary system  300  in accordance with one embodiment of the invention is shown in  FIG. 3 . Raw measurement data is transmitted from the laser detector  310  to an OTDR processor  320 . In one typical OTDR instrument, the processor is integral with the laser detector, the laser and the coupler ( FIG. 1 ) and is programmed to determine fiber characteristics based on the test trace. The OTDR processor  320  may have integral memory (not shown) for storing past measurement results. 
   Test trace characteristics are transmitted from the OTDR processor  320  to a statistical post-processor  330 . The term “statistical post-processor” as used herein does not preclude statistical processing from being performed in the OTDR processor as well. Indeed, the measurement averaging and other statistical functions used in calculating the trace characteristics are typically carried out in the OTDR. 
   The statistical post-processor  330  may be part of the OTDR processor or may be a separate processor residing, for example, in a desk-top computer. The statistical post-processor  330  has access to a machine readable medium  340  on which are stored instructions that, when executed by the processor, perform the method of the invention. The machine readable medium may be removable media such as an optical disk or magnetic disk, or may be fixed magnetic disk. The medium may alternatively be internal volatile or non-volatile memory. 
   In executing the process of the invention, the statistical post-processor accesses memory  350  to recall reference trace data or reference test trace characteristics stored on that memory. The memory  350  may reside within the statistical post-processor  330  or within the ATDR processor  320 . The reference trace characteristics are used for comparison to the test trace characteristics to detect changes in the fiber. 
   Certain statistical characteristics of each of the trace data sets are initially computed for the Test trace (T test ). If those characteristics are not already available for the Reference Trace (T ref ), then they are computed for that trace as well. For example, the following values may be computed: 
   D Gross Distance (overall length of the fiber tested) 
   R            array of Reflectance spikes (inflection points) found along the fiber
   GL            array of Gross Loss values for all points along the fiber
   IL            array of Instantaneous Loss values (rate of loss) for points along the fiber
   DV            array Divergence Values between the two traces (IL           ref −IL           test )
   The gross distance or length values are computed by determining the location along the fiber of the span end spike. For example, in the trace shown in  FIG. 1 , the fiber has a length value of approximately 10 km to the span end  223 . The spike at the span end is often fairly well defined in the trace because it appears as a Fresnel reflection of the glass/air interface of a polished fiber end. 
   The array parameters R           , GL         , IL          and DV          are arrays containing values for a given characteristic at a plurality of points along the fiber optic cable.
   In a method of the invention  400 , shown in  FIG. 4 , the gross distance, or length, calculated from the test trace is compared (step  405 ) to the gross distance calculated from the reference trace. 
   In comparing values such as fiber length for equality or inequality, an experimental error (σ) may be taken into account. A reading of D implies an actual value of |D+/−σ|. So, to compare D 1  and D 2 , the following multi-valued logic must be used: 
   D 1 +2σ&lt;D 2 : D 1  is definitely less than D 2    
   D 1 &gt;D 2 +2σ: D 1  is definitely greater than D 2    
   D 1 &lt;D 2 &lt;D 1 +2σ: D 1  and D 2  may or may not be equal 
   Determinations of equality and relative magnitude made in the presently described method are preferably made using a similar multi-valued logic. A value of the experimental error σ may be calculated from the measurement data, or may be estimated based on past performance. 
   Based on the relative calculated values for the gross distance D test  of the test trace T test  and the gross distance D ref  of the reference trace T ref , three possible scenarios exist; each of those scenarios is examined separately in a preferred method according to the invention. 
   The case (decision block  420 ) in which D ref &lt;D test  is the most straightforward case. If the test trace identifies a longer fiber length, then the test trace is adjudged to be superior to the existing reference trace, the assumption being that the only possible explanation is the (intentional) physical installation of additional cable. In that case, the reference trace T ref  may be replaced (step  421 ) with the test trace T test , which is determined to be more current. 
   Still, a χ 2  (chi-square) standard deviation test (not shown) on T ref  versus the initial length-matched portion of T trace  can be performed. That additional test gives a confidence factor in the test trace; a failure indicates the trace is likely invalid. 
   Another possible case is the case (decision block  410 ) where D ref &gt;D test . That is a more complex case. While the underlying network has undisputedly changed, it is necessary to determine whether the new (shorter) cable more or less functional. The cable may be optically shorter due to an accidental cut or partial failure (negative impact). Conversely, a loop or splice section may have been removed intentionally during maintenance (neutral/positive impact). Essentially, the technique attempts to discover which is a more accurate picture of the desired network—T ref  or T test . 
   To establish which of the above alternatives is most likely, the following assumption is made: if the section missing from the newer (test) trace lies between both endpoints, then the change is assumed intentional. Stated conversely: an accidental cut will cause a discontinuity between the endpoints of a cable, whereas maintenance operations preserve end-to-end continuity. 
   Given that assumption, it is possible to determine the cause of the length discrepancy by establishing a correlation (step  411 ) between the fiber characteristics of various sections of T ref  and T test . An attempt is made to make both a positive correlation (the missing section is definitely at the far end, indicating a likely fiber cut) and a negative correlation (the missing fiber is definitely not an endpoint, indicating a maintenance-related intentional shortening). 
   A positive correlation can be tested (step  412 ) by one of several inferential statistical methods. Essentially, a two-tailed hypothesis test is desired, with null hypothesis T test =(segment of) T ref . The significance level (α) of the match can be tuned by application and situation. 
   For instance, a chi-squared best fit can be performed against R             test  and the initial length-matched portion of R           ref . A successful test indicates T ref  as valid; in practice, such a result would indicate a likely fiber break (step  414 ). A failure leads us to reject the null hypothesis and assume T test  to be superior (step  413 ).
   In the absence of data for R            (i.e. a fiber section either without splices, or with a very small sample set of splices), the same correlations can be performed on GL          and IL         , though a correspondingly higher confidence factor must be chosen.
   If the relative magnitude of D ref  and D test  cannot be resolved within statistical bounds, and the optical path length remains statistically unchanged, there are still opportunities for extracting data on the relevance of T ref . Any statistically significant variance in R            yields the conclusion that the reference trace is dated. Variances in GL          or IL         are less meaningful, but may still trigger an exception if a preset value is exceeded.
   If, through any of the above statistical determinations, the test trace is adjudged superior, various possibilities exist for remedial action. An alarm condition can be suppressed, the existing reference can be automatically replaced with the new trace, or the operator(s) can be alerted to take manual action. The actual action taken by the system is application- and condition-dependent. 
   In situations where the reference trace is superior, there still exists the opportunity to extract meaningful data from a statistical comparison of the traces. In an alarm condition, a one-sample z-test on IL             test  will determine whether the alarm is most likely due to a point fault (line cut or equipment failure) or a cumulative fault (degraded signal at multiple points). A chi-squared standard deviation computation between T ref  and T test  will give a confidence factor for the alarm itself, with the opportunity to suppress possibly spurious alarms.
   The foregoing Detailed Description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the Detailed Description, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. For example, while the processor performing the statistical calculations included in the method of the invention is disclosed to be a separate unit form the OTDR unit, those units may be integrated. Further, the specifically-described statistical techniques may be replaced with other known techniques. For instance, chi-squared best-fit technique may be replaced by least square or a regression fitting technique. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.