Patent Publication Number: US-2023152183-A1

Title: Fiber Span Characterization Utilizing Paired Optical Time Domain Reflectometers

Description:
TECHNICAL FIELD 
     The inventive concepts described in detail below are related to the characterization of the transmission properties of optical fiber as provided through the use of optical time domain reflectometers (OTDRs). 
     BACKGROUND OF THE INVENTION 
     OTDRs have been used extensively to determine optical fiber characteristics such as attenuation, reflections, and the like, in order to optimize the working levels of associated transmitter and receiver equipment. An OTDR module typically includes an optical source used to generate optical pulses that are injected into the fiber being analyzed, and an optical receiver for detecting light from the optical source that is back-reflected by the fiber. An associated processing module utilizes the timing information of the input pulse train and the optical power in the return back-reflected light to create an output (typically referred to as an OTDR trace) that defines the overall loss along the fiber span, as well as an identification of physical changes/reflection points (e.g., connectors, splices, and the like) along the measured span. 
     While extremely useful in both installation and monitoring of optical fiber links between network nodes, the operational range of OTDRs is limited by the amount of optical power that may be launched into a given optical fiber, as well as the length of the fiber span itself. 
     SUMMARY OF THE INVENTION 
     The advanced fiber characterization capabilities provided by the system of the present invention are based upon the utilization of a pair of OTDRs, disposed at opposite terminations of a given fiber span, to address the operational range limitations of the prior art, while also supplying measurements of additional fiber characteristics beyond those associated with a conventional OTDR trace output. 
     In accordance with the principles of the present invention, a separate OTDR module is located at either termination of a defined optical fiber link (also referred to at times as a “fiber span”, or simply a “link” or “span”). Each OTDR performs standard reflectometry measurements and transmits the results to monitoring equipment in a typical manner. The pair of OTDR standard traces may then be combined in a particular manner (“stitched together”) to create an OTDR trace of the entire fiber span (essentially doubling the operational range of prior art OTDR measurement capabilities). In particular and as discussed in detail below, the traces may be combined in either the time domain or the loss domain to create a composite trace that provides an end-to-end characterization of the complete fiber span. 
     Inasmuch as each OTDR includes both a transmit component and a receive component, a communication channel may be created along the fiber span between a transmit component in a first OTDR and a receive component in a second OTDR. Test signals transmitted along this communication channel may be used to determine the optical link length, as well as the optical signal loss across the link. The paired OTDR apparatus of the present invention may also be used to provide wavelength-dependent characteristics of the fiber span (e.g., chromatic dispersion, Raman gain) by the inclusion of a multi-wavelength transmitter in one of the OTDR modules. Measurements of polarization-dependent loss may also be provided by configuring at least one OTDR module to include a polarization controller with the transmit component. Using a combination of both a polarization controller and a multi-wavelength light source of optical probes provides all of these measurement capabilities, as well as the ability to determine the differential group delay (DGD) and polarization mode dispersion (PMD) of the fiber span being evaluated. 
     An exemplary embodiment of the present invention may take the form of a system for characterizing an optical fiber span extending between a first optical node at a near-end termination of the optical fiber span and a second optical node at a far-end termination of the optical fiber span, based upon a near-end optical time domain reflectometer (OTDR) coupled to a near-end termination of the optical fiber span, a far-end OTDR coupled to the far-end termination of the optical fiber span, and a “characterization element” in communication with both OTDRs. The near-end and far-end OTDRs each including a light source for injecting an optical probe signal (for example, an optical pulse train) into the optical fiber span, a receive component for measuring back-reflected light from the optical probe, and a processing module for generating an OTDR trace in a known manner. The characterization element estimates an optical path length (or optical signal loss) based on measurements performed by both OTDRs and combining the near-end OTDR trace with the far-end OTDR trace in a trace-stitching procedure (based upon the estimated length or loss) to create as an output an end-to-end OTDR trace that characterizes an extensive portion of the optical fiber span. 
     An exemplary method of creating an end-to-end OTDR trace of a given optical fiber span, in accordance with the principles of the present invention, may include the steps of: (1) performing a first OTDR procedure from a first, near-end termination of the optical fiber span, using a near-end OTDR module and generating therefrom a near-end OTDR trace; (2) performing a second OTDR procedure from a second, far-end termination of the optical fiber span, using a far-end OTDR module and generating therefrom a far-end OTDR trace; (3) estimating an optical path length (or optical signal loss) of the optical fiber span; (4) defining an OTDR trace stitching point as a mid-point of the estimated optical length (loss); (5) starting at the stitching point of the far-end OTDR trace, identify loss values for a set of individual span locations from the stitching point to the far-end termination of the optical fiber span; (6) adding an inverse of each identified loss value to the near-end OTDR trace to form a combined OTDR trace of the optical fiber span; and (7) inserting anomalies present in the far-end OTDR trace at defined locations into the combined OTDR trace, forming as an output the end-to-end OTDR trace of the optical fiber span. 
     Other and further aspects and embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings, where like numerals represent like components in several views: 
         FIG.  1    illustrates an exemplary advanced OTDR system in accordance with the principles of the present invention, utilizing a paired arrangement of separate OTDR modules; 
         FIG.  2    shows a typical OTDR trace as created by a first OTDR of the paired arrangement of separate OTDR modules, providing fiber characterization from a first end (e.g., “near-end”) of a given optical fiber span; 
         FIG.  3    shows a typical OTDR trace as created by a second OTDR of the paired arrangement of separate OTDR modules, providing fiber characterization from a second end (e.g., “far-end”) of a given optical fiber span; 
         FIG.  4    demonstrates an initial step in stitching together the traces of  FIGS.  2  and  3   , forming a “reversed” version of one of the traces and using an estimated optical path length to properly position the start location of the reversed trace; 
         FIG.  5    shows a next step for the stitching process as shown in  FIG.  4   , illustrating the relocation of the starting point of the reversed trace in  FIG.  3    and the identification of the “stitching point” based upon the relocation of the reversed trace; 
         FIG.  6    is an end-to-end OTDR trace formed by reversing the magnitude of the second trace to form the final result, describing the complete fiber span; 
         FIG.  7    demonstrates an initial step in alternative method of stitching together the traces of  FIGS.  2  and  3   , in this case based upon the estimated optical signal loss; 
         FIG.  8    shows a following step in the process as associated with  FIG.  7   , where the reversed trace is moved to position its start location to coincide with the optical signal loss, and again defining a “stitching point” where the two traces meet; 
         FIG.  9    is an end-to-end OTDR trace formed by reversing the magnitude of the trace values beyond the stitching point; 
         FIG.  10    illustrates an alternative embodiment of the present invention, in this case incorporating a multi-wavelength light source (here, a tunable wavelength source) within at least one OTDR module of the paired arrangement of OTDR modules, utilized for providing wavelength-dependent measures of optical path length (related to the fiber&#39;s chromatic dispersion characteristic) and optical path loss (e.g., a Raman gain profile); 
         FIG.  11    shows a variant of the embodiment of  FIG.  10   , in this case using a tunable filter in combination with the receiver to provide wavelength-dependent power measurements 
         FIG.  12    shows a different type of multi-wavelength OTDR module that may be used in the embodiment of  FIG.  10   , in this case using a plurality of single-wavelength lasers for providing optical probe signals at different wavelengths; 
         FIG.  13    illustrates another embodiment of the present invention, in this case including an adjustable polarization state controller that is used in combination with the optical probe signal to perform measurements of polarization-dependent loss (PDL) across the fiber span; and 
         FIG.  14    depicts yet another embodiment of the present invention, incorporating the wavelength-dependent measurement capabilities of the embodiment of  FIG.  10    with the polarization-dependent measurement capabilities of the embodiment in  FIG.  13    where the ability to utilize both wavelength- and polarization-dependent measurements allows for the characterization of complex fiber qualities (such as, for example, differential group delay and polarization mode dispersion). 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    illustrates an exemplary advanced OTDR system  100  in accordance with the principles of the present invention, utilizing a pair of OTDRs  10 A,  10 B to perform extensive testing and monitoring of a fiber link  20  disposed between OTDR  10 A and OTDR  10 B. While not required for practicing the inventive techniques, OTDRs  10 A,  10 B are typically located in a pair of optical nodes  1 A,  1 B (respectively) that include conventional optical communication equipment represented by elements  2 A,  2 B. A pair of optical switches, couplers, or wavelength-dependent multiplexers  3 A,  3 B are included in each node  1 A,  1 B and may be used to couple an associated OTDR to fiber link  20  in order to perform fiber characterization measurements. 
     Each OTDR  10  includes a light source  12  for providing the optical probe signal that is coupled into fiber link  20  and used in a manner well-understood in the art to create back-reflected light in the return direction that is used to generate an OTDR trace as the output from OTDR  10 . In many cases, the optical probe signal takes the form of an optical pulse train, but other types of optical probe signals (e.g., continuous-wave signal, a digital signal having a particular coding scheme, etc.). 
     An optical receiver  14  (e.g., photodetector) is also located in each OTDR  10  and used to measure the back-reflected light created by the optical probe as it propagates along fiber link  20 . In this particular configuration, an optical circulator (or coupler)  16  is used to control/direct the signal flows between transmitter light source  12 , receiver  14 , and fiber link  20 . Various other arrangements may be used to control the directions of the propagating signals without affecting the inventive techniques as described below. Each OTDR  10  also includes a processing module  18  that functions in a well-known manner to develop the OTDR trace output based upon the reflection measurements performed by receiver  14 . At times, OTDR  10 A may be referred to as the “near-end” OTDR and OTDR  10 B may be referred to as the “far-end” OTDR. 
     In accordance with the principles of the present invention, the operation of OTDRs  10 A,  10 B is controlled such that only one module is performing OTDR measurements at any given point in time. Otherwise, having an optical probe signal propagate from both ends of fiber link  20  would result in creating interference between the two probe signals and prevent the collection of useful data for characterizing the fiber. In many cases, processing modules  18 A,  18 B may communicate with each other over an established communication channel (as discussed below) to share scheduling/monitoring information (for example) and avoid the possibility of both OTDRs attempting to obtain reflectometry measurements at the same time. Alternatively, an internal controller timing element (not shown) may be included in each OTDR and used to schedule the functionality of the components in accordance with the principles of the present invention. 
     In yet other embodiments, a separate (perhaps network-based) OTDR controller  30  may be included within advanced OTDR system  100  and used to control the operation of the OTDRs, where for example controller  30  may be used to transmit “start” and “stop” commands to each OTDR  10 A,  10 B to prevent the possibility of simultaneous OTDR measurement sequences. Controller  30  may be used to activate opposing pairs of “optical probe”/receiver to perform the time-of-flight measurement (signal loss calculation, for example) to determine the optical path length. Additionally, controller  30  may include the processing capability discussed below to combine the OTDR traces generated by processing modules  18  and provide as an output a detailed characterization of fiber link  20 . OTDR controller  30  may be operated by a network management system, using well-known techniques. 
     For the purposes of explanation, it is presumed that an OTDR measurement process is initiated by controller  30  sending a command signal to near-end OTDR  10 A to “start” the procedure. In this particular configuration, a control signal path (which may be an electronic signal path, a wireless RF path, or the like) is shown between controller  30  and processing module  18 A, since module  18 A is generally used to control the operation of light source  12 A. Upon receipt of the “start” command, light source  12 A directs an optical probe signal along fiber like  20 . Light source  12 A is configured to generate the optical probe at a pre-defined optical wavelength, and if taking the form of a train of optical pulses light source  12 A is further configured to produce the pulse train at a predetermined pulse rate. Reflections from the optical probe are measured by receiver  14 A and sent to processing module  18 A. The propagating optical probe signal will continue to be transmitted and the reflections measured until near-end OTDR  10 A receives a command from controller  30  (and/or processing module  18 A) to “stop” the measurement process. 
       FIG.  2    shows a typical OTDR trace that is created by processing module  18 A upon compiling the reflection measurements from receiver  14 A in a known manner. The trace (referred to at times hereafter as OTDR trace A) illustrates optical power loss (in dB) as a function of distance along fiber link  20 . Here, the span is measured in kilometers (km), with the origin defined as a first termination  20 A of fiber link  20  (that is, the near-end location where the optical probe is coupled into fiber link  20 ). A spike X1 (i.e., a reflection point) is shown to have been identified at a location at a first distance from origin  20 A, with a transition X2 (attributed to a bulk optical loss) shown at a position further along the fiber span. Reviewing the results shown in  FIG.  2   , it is clear that the operational range of OTDR  10 A is about 110 km, since any measurements made beyond this point are consumed within the system noise floor. The operational range of an OTDR is known to be limited by factors such as, but not limited to, the optical power of the transmitted optical probe signal, as well as the amount of receiver noise (the combination of these factors determining the SNR of the back-reflected light). 
     The results shown in  FIG.  2    are thus typical of conventional OTDR systems, providing necessary information on the properties of a portion of a fiber span. If the span is relatively short (say, 100 km or less), it is possible that the entire span may be characterized by this single OTDR procedure. However, without an a priori knowledge of the span length or signal loss, it is not possible to determine how much of the complete fiber link has been characterized using the single OTDR arrangement of the prior art. 
     The impediments to fiber span characterization based upon the limited operational range of a conventional OTDR module is addressed by the inventive advanced OTDR system, which utilizes a paired configuration of near-end and far-end OTDRs in a manner that allows for the complete span to be characterized. In accordance with the principles of the present invention a second, separate OTDR measurement is performed, this time using a far-end OTDR  10 B located at the opposite end of fiber link  20 . That is, OTDR  10 B is used to create a trace that characterizes fiber link  20  starting from opposing endpoint (i.e., “far-end”) termination  20 B, looking “backwards” toward near-end termination  20 A. Again, OTDR controller  30  may be used to send “start” and “stop” commands to OTDR  10 B, along a signal path to processing module  18 B, to control the transmission of the optical probe from light source  12 B along fiber link  20  and the measurement of back-reflected light by receiver  14 B. Alternatively, as discussed above, an established communication channel between processing modules  18 A,  18 B (or embedded OTDR controllers, not shown) may work together to control the activation of OTDR  10 B once OTDR  10 A is no longer sending an optical probe along fiber link  20 . 
       FIG.  3    illustrates an OTDR trace (denoted for the purposes of the present invention as OTDR trace B) that may be generated by far-end processing module  18 B at the completion of the measurement process. The starting point for OTDR trace B is shown as far-end termination  20 B of fiber link  20  and progresses along fiber link  20  toward near-end termination  20 A. OTDR trace B shows the presence of a spike X3 (due to a reflection) at a location immediately beyond starting far-end termination  20 B. Again, the operational range of the measurement capabilities of OTDR  10 B is clearly represented by the degradation in useful measurements upon reaching the noise floor level. Presuming that both OTDRs exhibit similar functionality, the operational range of each module will be essentially the same. 
     In order to properly combine the data presented in these OTDR traces, either the optical span length L or optical signal loss P of fiber link  20  must be known (or estimated as best as possible). Advantageously, the paired OTDR modules of the present invention may be used to determine both the optical length, as well as optical loss, of fiber link  20  and thereafter use this information to create the end-to-end OTDR result. That is, by knowing one of these span-based values (length or loss), one trace may be reversed with respect to the other and positioned to coincide with an endpoint defined by the span length or loss values. 
     Turning to a description of a methodology for determining the span-based length or loss values (and as briefly mentioned above), a communication channel may be established between the paired OTDRs that allows for an exchange of information with respect to, for example, signal timing, measured power, and the like. The shared information may then be used to derive either optical span length L or optical signal loss P. For example, accurate timing/synchronization of the paired OTDRs may be provided upon establishment of the communication channel, and may then be used to estimate the optical span length L and/or optical signal loss P. 
     One technique for determining the optical length of the span is to measure the total time for light to traverse fiber link  20 , denoted Δt span  and referred to at times as a “time of flight” measurement. If both OTDRs  10 A,  10 B operate using the same system clock (e.g., upon establishment of the communication channel between the paired devices), it is possible to control near-end light source  12 A and far-end receiver  14 B (or the opposite pairing of far-end light source  12 B and near-end receiver  14 A) to measure the propagation interval of the transmitted optical probe. For example, if there is a 1 ms time interval measured from “transmit” to “receive”, it can be presumed that the link has an optical length L of about 200 km. This 200 km span length may be used below at times as an exemplary span length when discussing various features of the inventive OTDR system, with the understanding that this example value is in no way limiting. 
     A determination of the optical signal loss P may be created by measuring both the transmit power at light source  12 A and the received power at receiver  14 B. Calculating the difference between these two power levels defines the power loss PAB, which may thereafter be used in combination a power loss measured in the opposite direction PBA (i.e. from light source  12 B to near-end receiver  14 A) to determine an optical signal loss P of fiber link  20 . 
     Once either the optical span length L or optical signal loss P is known, it is possible to “stitch together” the information in OTDR traces A, B to form an end-to-end trace that fully characterizes the entire span. Accomplishing this stitching proceeds by “reversing” the information presented in one of the OTDR traces with respect to the other, and then shifting the location of the reversed trace until the pair of traces display either the estimated values of L or P.  FIGS.  4 - 9    illustrate the details of this approach. 
     In particular,  FIGS.  4 - 6    illustrate a process of stitching together OTDR traces A, B based upon knowing the optical link length L.  FIG.  4    illustrates a first step in the process of combining the OTDR traces, which includes “reversing” one trace relative to the other. In this example, OTFR trace B is reversed. For the purposes of the present invention, reversal involves changing the sign of the measured losses (starting at end point termination  20 B) from negative to positive values, resulting in the upward trend of the trace moving from end point termination  20 B toward endpoint termination  20 A). 
     Also shown in  FIG.  4    is the marked location of the derived optical span length L. With OTDR trace B reversed and the derived optical link length L noted, OTDR trace B is re-positioned until its starting point (i.e., far-end termination  20 B) lines up with span length L, as shown in  FIG.  5   . This is possible since it is known a priori that data collection for OTDR trace B is initiated at this defined endpoint termination  20 B. The result as shown in  FIG.  5    defines a length-based “stitching point” SPL intermediate of the two endpoints, where the traces meet. With stitching point SPL determined, the final step in stitching-together process of creating an end-to-end OTDR is to reverse the loss magnitude of OTDR trace B, resulting in the end-to-end OTDR trace as shown in  FIG.  6   , which properly characterizes the complete fiber span. Obviously, trace B may also be used as the primary trace, with trace A reversed and incorporated into trace B using this same approach. It is to be understood that if a given fiber span is extremely long (e.g., many hundreds of kms), the doubling of the operational range may be insufficient to fully characterize a middle portion of the span that is beyond the range of the OTDR measurements as performed from each endpoint. 
     As mentioned above, the calculated link loss P may be used instead of the determined fiber span length to stitch together the pair of OTDR traces.  FIGS.  7 - 9    illustrate a process of stitching together OTDR traces A, B based upon knowing the optical signal loss P across the span. Similar to the plots shown in  FIG.  4   ,  FIG.  7    illustrates both OTDR traces A, B from  FIGS.  2  and  3   , with the values for OTDR trace B reversed in the same manner. In this case, the derived value of optical signal loss P is marked along the y-axis and reversed trace B is shifted in position until its starting point is aligned with this known loss value.  FIG.  8    illustrates the result of this movement of reversed trace B, denoting a loss-based stitching point SP P  where the two traces meet along the y-axis direction. In order to form the end-to-end trace, the next step is similar to that described above with the length-based approach; namely, the data points along the trace B section are reversed in value, arriving at the desired end-to-end OTDR result shown in  FIG.  9   . While there may be slight differences between the end-to-end OTDR traces shown in  FIG.  6    and  FIG.  9   , they are considered minimal and due to variations involved in determining the total optical path length or total optical signal loss. 
     In further accordance with the principles of the present invention, a paired arrangement of OTDRs may be used to provide additional characterizations of a fiber span by using a combination of a transmitter light source in a first OTDR with a receiver component in a second OTDR (as used in the manner described above to determine the optical path length and/or optical signal loss of the fiber span) to form a communication channel. For example, the use of a transmitter/receiver combination in a paired OTDR configuration may be used to provide wavelength-dependent and/or polarization-dependent characteristics of the fiber span. 
       FIG.  10    shows an alternative advanced OTDR system  160  formed in accordance with the present invention to determine wavelength-dependent characteristics of fiber link  20 . In particular, advanced OTDR system  160  is able to perform measurements of wavelength-dependent changes in both the optical path length and optical path loss of fiber link  20 , based upon using wavelength-dependent transmissions from a near-end OTDR module to a far-end OTDR module (or vice versa). It is to be understood that these properties of fiber link  20  are measured by advance OTDR system  160  in addition to generating the end-to-end OTDR trace created in the manner discussed. Similar to the embodiment as described, each OTDR module  60  includes a light source  62  for producing the optical probe, a receiver  64 , and processing module  68  (with perhaps a circulator/coupler  66  controlling signal path directions). An associated fiber characterization element  70  may be used in a manner similar to OTDR controller  30  discussed above to control the operation of OTDRs  60 A,  60 B to generate OTDR traces and produce therefrom a “end-to-end” OTDR trace that characterizes fiber span  20 . Otherwise, processing modules  68  (or embedded controllers) may be particularly configured to control the multi-wavelength testing of fiber link  20 . 
     In the particular arrangement as shown in  FIG.  10   , near-end light source  62 A is configured as a multi-wavelength laser source that is capable of providing an optical probe at selected wavelengths, under the control of processing module  68 A. In order to create wavelength-based measurements, far-end receiver  64 B is used to measure the optical power of the optical probe as a function of wavelength. While in most cases light source  62 B is similarly configured as a multi-wavelength source, it is not required for collecting the wavelength-dependent information. However, inasmuch as near-end light source  62 A will be communicating with far-end receive component  64 B to perform the wavelength-dependent testing of fiber link  20 , receive component  64 B may be configured to exhibit a sufficiently broad response (in terms of bandwidth) so that an accurate wavelength-dependent response is observed at the far end of fiber link  20 . 
     The pairing of near-end light source  62 A with far-end receive component  64 B is thus used in accordance with this embodiment of the present invention to perform measurements of wavelength-dependent optical path length (i.e., chromatic dispersion properties) and also, if required, wavelength-dependent measurements of optical path loss (associated with creating a Raman gain profile for fiber link  20 ). Chromatic dispersion occurs since optical signals operating at different wavelengths propagate at different speeds along a section of optical fiber. Therefore, using a similar methodology as discussed above to determine the optical length of fiber link  20  (for defining the “stitching point”), receive component  64 B may be used in combination with processing module  68 B to ascertain an arrival time for an optical probe operating at a given wavelength. Thus, after transmitting optical probes at a set of different wavelengths within the wavelength range of light source  62 A, a set of arrival times (associated with each individual optical probe wavelength) is collected. These values may be used by processing module  68 B (or sent to fiber characterization component  70 ) to determine the chromatic dispersion of fiber link  20 , which is a measure of time arrival delay as a function of wavelength (typically measured as ps/nm). By also knowing the length of the span (as determined by time-of-flight, loss measurements, or the like, as discussed above), the chromatic dispersion coefficient, which is a measure of chromatic dispersion per km of fiber (ps/nm-km), may also be determined. The chromatic dispersion values may be plotted as a function of wavelength, with the derivative of this plot being an alternative way to describe and define the chromatic dispersion coefficient associated with the fiber span under evaluation. 
     In applications where fiber link  20  is used as a distributed Raman amplifier, it is useful to know its gain profile as a function of wavelength. Additionally, accurate monitoring of the net gain on a recurring basis is important to ensure and maintain an acceptable level of OSNR for all wavelength channels. The paired combination of multi-wavelength light source  62 A and broadband receiver  64 B may be used to perform this measurement. In some cases, a “pilot tone” from light source  62 A may be injected into fiber link  20  along with the actual optical signal traffic. Alternatively, if the propagating WDM signals are affected by pilot tones, coded signals (such as CDMA, for example) may be used to reduce the signal power while ensuring sufficient received power to properly characterized the gain profile. Additionally, the pilot tone may be injected into fiber link  20  from light source  62 B, thus propagating in the opposite direction with respect to the optical signal traffic. 
     Moreover, the ability of broadband receiver  64 B to measure the optical power in each different wavelength optical probe may be used in combination with the known transmitted power of each optical probe to create accumulated Raman gain of fiber link  20 . This is an option when the arrangement is provided as a distributed Raman amplifier, with the ability to test the span under different wavelength conditions providing the ability to see variations in gain as a function of wavelength. The distributed gain along the length of the span (at a given wavelength) will be evident from the complete span OTDR trace that is prepared by the paired combination of OTDR  60 A,  60 B. 
       FIG.  11    illustrates a variation of the advanced OTDR system of  FIG.  10   , where in this case an advanced OTDR system  160 A utilizes a tunable optical bandpass filter in combination of the far-end receiver so as to ensure that only a relatively narrow band surrounding each transmitted optical probe wavelength is measured at receiver  64 B. Referring to the particulars of  FIG.  11   , a tunable wavelength filter  67  is used in combination with receiver  64 B, where the tunable center wavelength of filter  67  is controlled in synchronization with the tuning of light source  62 A so that receiver  64 B is operating at the same wavelength as multi-wavelength light source  62 A (and thus stepping through the same set of wavelength settings in synchronization with light source  62 A). In particular, OTDR controller  70  may be used to provide wavelength control signals to both multi-wavelength light source  63 A and receiver bandpass filter  67  such that both elements operate at the same wavelength at the same time. 
     Various arrangements may be used to form multi-wavelength light source  62 A.  FIG.  10    illustrates the use of a “tunable” laser source, which consists of a single laser source that may be controlled by an external signal (here, supplied by controller  70  via processing module  68 A) to operate at a desired wavelength.  FIG.  12    shows an alternative OTDR module, here noted as  60 A. 1 , that utilizes an array of individual single-wavelength laser sources  62 A- 1 ,  62 A- 2 , . . . ,  6 A 2 -N to provide a set of input optical probes at fixed wavelengths. As with the arrangement of  FIG.  10   , signals from controller  70  and/or processing module  68 A may be used to control the wavelength sequence of the optical probes that are transmitted along the associated fiber span (not shown in  FIG.  12   ). It is to be understood that this multi-source embodiment may also be used in combination with the tunable-wavelength receiver filter as shown in  FIG.  11   . For this combination, the command signals used to select the specific laser source is synchronized with the tuning of the filter&#39;s center wavelength. 
     The performance of optical communication systems is increasingly influenced by the polarization of the optical signals passing through the network. Additionally, the increasing length of fiber links within a network has brought attention to factors such as polarization-dependent loss (PDL), which is a known signal distortion that accumulates over distance and may have a deleterious impact on the transmitted optical signals. 
       FIG.  13    illustrates an embodiment of the advanced OTDR system of the present invention, here denoted as advanced OTDR system  180 , that is particularly configured to provide polarization-dependent loss measurements. As with the embodiments described above, advanced OTDR system  180  comprises a paired arrangement of OTDR modules  80 A,  80 B, with each module including a light source  82 , receiver  84 , and processing module  88  for use in generating conventional OTDR traces. Again, these traces may be stitched together in the manner outlined above to form an end-to-end OTDR trace of the complete span. 
     In this case of also providing polarization-dependent measurements, OTDR module  80 A is shown as further including a polarization controller  83  that is positioned at the output of light source  82 A (which in this instance is a “fixed wavelength” source, such as light source  12 A of advanced OTDR system  100 ). An external fiber characterization module  90  is used in this embodiment to control polarization controller  83  so that the polarization state of the output optical probe from light source  82 A steps through a complete sequence of polarizations. The control signals from module  90  may be directly received by polarization controller  83 , or pass through processing module  88 A as an intermediary component. Alternatively, processing module  88 A itself may be configured to provide for a continuous movement/change in the polarization state of polarization controller  83 . 
     Polarization-dependent loss, therefore, may be characterized by using receiver  84 B to measure the received optical probe power. For the purposes of the present invention, polarization-dependent loss is defined as a measure of the peak-to-peak difference in transmission of an optical signal along the fiber span as the polarization is cycled through all possible polarization states. In most cases, it is defined as the ratio of the maximum and minimum signal powers, where 
     
       
         
           
             
               P 
               ⁢ 
               D 
               ⁢ 
               
                 L 
                 
                   d 
                   ⁢ 
                   B 
                 
               
             
             = 
             
               1 
               ⁢ 
               0 
               * 
               log 
               ⁢ 
                  
               
                 
                   ( 
                   
                     
                       P 
                       Max 
                     
                     
                       P 
                       Min 
                     
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
     It is also possible to combine the multi-wavelength attributes of advanced OTDR system  160  with the polarization-dependent aspects of system  180  to create an advanced OTDR system  200 , shown in  FIG.  14    as including both a tunable wavelength light source  92  and a tunable polarizer  94 . Beyond providing the results mentioned above, the collection of both wavelength-dependent dispersion and loss measurements in combination with polarization-dependent loss information over the tunable wavelength range allows for an associated fiber characterization module  210  to also develop differential group delay and polarization mode dispersion information for fiber link  20 . This collection of information is extremely robust and well beyond the capabilities of a conventional OTDR system. 
     It is to be understood that the present invention may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately-detailed optical communication system. While the invention has been described in connection with several preferred embodiments, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.