Abstract:
An accurate method for characterizing optical fiber using a modest enhancement to equipment that is already in place for other purposes within an optical transmission system is thereby disclosed. In particular, optical amplifier light sources already embedded in transmission equipment are used to provide accurate characterization information in addition to their conventional purpose. Therefore, the invention permits fiber characterization at little additional cost over the optical amplifier light sources already present in a system. Furthermore, the need of going out to the field, disconnecting the transmission equipment and performing the field measurements with separate test equipment is eliminated.

Description:
FIELD OF THE INVENTION 
     This invention relates to methods and apparatus for testing optical fiber communication links in optical communication systems and, more particularly, to methods and apparatus for characterizing an optical fiber using optical amplifier light sources. 
     BACKGROUND OF THE INVENTION 
     Optical transmission systems are quickly evolving to multi-Terabit per second (Tb/s) capacity. Successful deployment of these systems requires accurate characterization of the fibers that connect all the transmission equipment together. Such characterization information includes fiber attenuation, chromatic dispersion, polarization mode dispersion (PMD) and back-reflection among others. This information allows the network provider to deploy the appropriate compensation techniques to mitigate deficiencies in the fibers, and also allows the network provider to understand the ultimate transmission distance limitations within the system. 
     In the field, fiber characterization information must be evaluated in conjunction with the installation of new optical transmission systems or the upgrade of existing routes to higher bit rates. For example, control of the total chromatic dispersion of transmission paths is critical to the design and construction of long-haul, high-speed, high-capacity telecommunications systems. Similarly, PMD of older installed fibers is typically much higher than recently manufactured fibers, and system integrators often need to measure these installed fibers as they plan to upgrade their systems to higher bit rates. 
     Fiber characterization is of particular importance to high-capacity fiber optic systems such as Dense Wavelength Division Multiplexed (DWDM) systems with many high bit-rate (e.g. 10 Gb/s and above) channels in place. In many cases, it is not possible to install the system at all without this information. There are currently at least ten different optical fiber types examples of which include Non-Dispersion Shifted Fiber, Dispersion Shifted Fiber, True Wave Classic®, True Wave Plus®, True Wave Minus®, True Wave RS®, All Wave®, LEAF®, E-LEAF®, LS®, etc. Each of these fiber types have different characteristics yielding varying levels of performance. Identification of the fiber type is, therefore, essential in providing accurate characterization information as system upgrade costs differ radically based on the type of fiber installation. 
     Where the fiber characterization information may be estimated from knowing the fiber length and fiber type, it may be possible to deploy a system for applications that do not have extended reach or capacity. However, suppliers in this case would be unable to permit customers to deploy these systems in aggressive applications. For example, such a case may occur when a customer wants extended reach or capacity and is unwilling to characterize the fiber in full. In addition to the above, some customers do not have accurate records regarding the type of fiber they have installed in the field. This information is essential to allow them to estimate the fiber characteristics from the length. 
     Generally, the customer may estimate fiber characteristics from length and type information. However, this does not supply them with enough information for suppliers to specify the most aggressive system. Subsequently, the customer may have to take measurement equipment out to the field to collect accurate fiber characterization information. This requires a coordinated effort (potentially at both ends of the fiber) to identify the fiber strand in question, disconnect the transmission equipment, make the measurement, record the results, and communicate these results to the equipment deployment team. 
     Several measurement techniques presently exist for characterizing optical fibers, the most popular of which is Optical Time Domain Reflectometry (OTDR). Optical time domain reflectometers (OTDRs) are frequently used to measure a variety of optical fiber properties. OTDRs operate by sending a short pulse of laser light down an optical waveguide fiber and observing the small fraction of light that is scattered back towards the source. This small fraction of light represents attenuation and reflectance in the optical fiber under test. By measuring the amount of backscattered and/or reflected signal versus time, the loss versus distance of the optical fiber is measured. Furthermore, it is known that an OTDR can be employed in combination with a variable wavelength laser source in order to display the effect of wavelength dependent fiber attenuation and chromatic dispersion of an optical fiber path. 
     However, OTDR systems requiring a wavelength-tunable pulsed light source and/or multiple light sources for the test signals impose a high degree of complexity not to mention cost. Furthermore, the use of separate OTDR systems to collect reflectance data necessitates the dispatching of experienced craftspeople to the field with a high level of expertise to perform the measurements. An inexpensive technique for testing optical fibers without the use of a tunable OTDR pulsed light source would, therefore, be highly beneficial. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a method for characterizing an optical transmission fiber using a modest enhancement to equipment that is ordinarily embedded within an optical transmission system. The apparatus of the invention is embedded in the transmission equipment itself, thereby eliminating the need to dispatch a craftsperson for performing field measurements. The method and apparatus of the present invention, therefore, eliminates the need for having to take the optical transmission system out of operation in order to isolate the optical transmission fiber to be characterized. Although it is already known to embed separate test equipment into a system application using established methods (e.g. OTDR), this is necessarily more expensive than the method and apparatus of the present invention since it requires the use of additional test laser sources. 
     According to a first broad aspect of the invention, there is provided a method for characterizing an optical transmission fiber comprising operating an optical light source so as to launch an optical signal into a first end of the optical transmission fiber, detecting at a second end of the optical transmission fiber a residual optical signal resulting from the optical signal launched into the first end of the optical fiber and characterizing the optical transmission fiber on the basis of the detected residual optical signal. 
     According to another broad aspect of the invention, a Raman pump laser is used as the light source for characterizing the optical transmission fiber. 
     According to another broad aspect of the invention, an optical service channel (OSC) laser source is used as the light source for characterizing the optical transmission fiber. 
     According to another broad aspect of the invention, a plurality of optical light sources, each operating at a particular wavelength, are used for characterizing the optical transmission fiber in a wavelength band defined by the plurality of optical light sources. 
     According to another broad aspect of the invention there is provided a method for characterizing an optical transmission fiber comprising operating an OSC laser source so as to launch an optical signal into a first end of the optical transmission fiber, detecting a reflected optical signal at the first end of the optical transmission fiber resulting from the optical signal launched into the first end of the optical transmission fiber and characterizing the optical transmission fiber on the basis of the reflected Raman pump light detected at the first end of the optical transmission fiber. 
     According to another broad aspect of the invention, there is provided an optical transmission system for transmitting multi-channel optical signals over an optical transmission path, the optical transmission path including an optical transmission fiber interposed between a first node and a second node wherein the multi-channel optical signals are transmitted over the optical transmission fiber from the first node to the second node, the optical transmission system comprising at least one optical light source provided at the second node operable to inject an optical signal into the optical transmission fiber for transmission to the first node, at least one photodetector at the first node for detecting a residual optical signal resulting from the optical signal injected into the optical transmission fiber at the second node and a processing agent for characterizing the optical transmission fiber on the basis of the optical signal injected at the second node and the residual optical signal detected at the first node. 
     According to another broad aspect of the invention there is provided apparatus for characterizing an optical transmission fiber comprising at least one optical light source operable to inject an optical signal into a first end of the optical transmission fiber, at least one photodetector at the first end of the optical transmission fiber for detecting a reflected optical signal resulting from the optical signal injected into the first end of the optical transmission fiber and a processing agent for characterizing the optical transmission fiber on the basis of the optical signal injected into the first end of the optical transmission fiber and the reflected optical signal detected at the first end of the optical transmission fiber. 
    
    
     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 drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an example portion of a typical optical fiber communication link employing Raman amplification and rare-earth doped fiber amplification. 
     FIG. 2 provides an example implementation used to characterize an optical fiber communication link according to a first embodiment of the invention. 
     FIG. 3 is an example of an implementation used to characterize an optical fiber communication link according to a second embodiment of the invention. 
     FIG. 4 is an example of an implementation used to characterize an optical fiber communication link according to a third embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Long distance lightwave communication systems require optical amplifiers for boosting optical signal levels sufficiently to compensate for losses experienced along the optical fiber transmission medium. Optical amplifiers may generally be classified into two categories: lumped amplifiers and distributed amplifiers. An exemplary lumped amplifier is the rare earth doped fiber amplifier, which offers substantial benefits because of its simplicity, low cost, and connective compatibility with existing optical fibers. These amplifiers increase the optical signal power of a supplied input signal via stimulated emission of fiber dopants such as erbium that is subject to an optical pump source. A series of fiber amplifiers, therefore, compensates for attenuation in the system via a gain process. 
     Distributed amplifiers include Raman amplifiers in which amplification is based on stimulated Raman scattering (SRS). Raman gain is generated by direct optical pumping of the transmission fiber and hence such amplifiers provide distributed amplification over an extended portion of the transmission path, often on a substantially uniform basis. 
     As a result, Raman amplification may also be employed as a gain mechanism for effectively reducing the impact of transmission path loss. Distributed Raman amplification increases the distance between the optical amplifiers (EDFAs) used to boost data signals thereby cutting costs and allowing higher data capacity by spacing wavelength channels closer together. Raman amplification is, therefore, often used in conjunction with doped fiber amplification. 
     As an example, consider FIG. 1 which depicts an example of a typical arrangement provided in an optical fiber communication link employing both Raman amplification and rare-earth doped fiber amplification. A section of optical transmission fiber  10 , used to carry a series of optical traffic wavelengths  12  in one direction is connected to a wavelength division multiplexer (WDM) filter  16 . In general, a plurality of Raman pump lasers  17  feed into the WDM filter  16  which couples a corresponding set of Raman pump wavelengths into the optical transmission fiber  10 . The WDM filter  16  is further connected to an erbium-doped fiber amplifier (EDFA)  18  whose amplified output is coupled into a subsequent section of optical transmission fiber  11 . 
     The WDM filter  16  performs the function of coupling the Raman pump laser wavelengths into the optical transmission fiber  10 . The Raman pump lasers  17  are used to supply energy in the form of Raman pump wavelengths  14  to the optical transmission fiber  10 . As indicated, these lasers often transmit power in the opposite direction to regular traffic, represented in FIG. 1 by the traffic wavelengths  12 . As is well known, these pump sources may alternatively be arranged to transmit power in the same direction as regular traffic but this is generally avoided since it leads to an associated performance penalty. The fiber medium acts to transfer energy in the Raman pump wavelengths  14  into energy for the traffic wavelengths  12 , which results in a gain process. This gain process works against the attenuation in the fiber plant. An assortment of laser wavelengths is often used to make the gain as broadband (in wavelength terms) as possible. 
     In most current systems, such as that depicted in FIG. 1, the EDFAs are enhanced by Raman amplification in which high-power laser light is sent in the direction opposite that traveled by the data signals (traffic), thereby transforming part of the optical transmission fiber, itself, into an amplifier of the signals passing through it. 
     In view of the above, the technique of the present invention proposes to set up a receiver at the far (transmit) end of the optical transmission fiber to receive residual Raman pump light. For example, FIG. 2 provides an arrangement that may be used to characterize an optical transmission fiber  20  according to a first embodiment of the invention. A span of optical transmission fiber  20  is connected between a WDM splitter  23  and a WDM coupler  26 . Preceding the WDM splitter  23  is an EDFA  25 , the input of which is coupled to a length of optical transmission fiber  21 . Following the WDM coupler  26  is a second EDFA  28  whose amplified output is coupled into a subsequent length of optical transmission fiber  22 . A plurality of Raman pump lasers  27  feed into the WDM coupler  26  which couples a corresponding set of Raman pump wavelengths into the optical transmission fiber  20 . These Raman pump wavelengths will be counter-propagating to the normal direction of traffic flow which is from EDFA  25  to EDFA  28 . The WDM splitter  23  is responsible for providing the counter propagating Raman wavelengths to a plurality of photodetectors  29 . Each Raman wavelength is detected independently by a separate photodetector. Alternatively, the counter propagating Raman wavelengths may be detected together and then separated by a suitable signal processing technique on the basis of their modulation. The photodetectors  29  are further coupled to a respective processing agent  24 . 
     Other well-known parts which may comprise an optical communication system such as the drive electronics, splices, attenuators, couplers, etc., are considered to be conventional and known in the art and have thus been omitted from all the figures. 
     The WDM coupler  26  and WDM splitter  23  are well known components in the art and are simply filters used to combine or separate optical signals, respectively, according to their wavelength. In the arrangement of FIG. 2, the Raman pump lasers  27  are used to inject Raman light into the optical transmission fiber  20  via the WDM coupler  26 . The photodetectors  29  may then receive and measure the power in the residual counter propagating Raman light via the WDM splitter  23 . 
     By performing such power measurements, two important fiber parameters may be determined i.e. the attenuation of the fiber as it applies to the Raman pump wavelength band and the chromatic dispersion of the fiber as it applies to the Raman pump wavelength band. These measurements may then be converted to respective estimates for fiber attenuation and chromatic dispersion in the traffic wavelength band. 
     Generally speaking, no traffic wavelengths should be present during the fiber characterization process according to the present invention. This condition can obviously be met before the system is actually put into service, as will most often be the case when fiber characterization takes place. However, it should be noted that the characterization process of the present invention may indeed occur with live traffic present, provided that the equipment vendor has qualified potential traffic performance impacts with the customer who has accordingly accepted such implications. 
     The first fiber parameter that may be determined using the arrangement of FIG. 2 is the attenuation of the fiber as it applies to the Raman pump wavelength band. For each wavelength, this will be the difference between the launch power of the respective Raman pump laser  27  and the received power at the corresponding photodetector  29 , taking into account fixed path losses which occur within the amplifier blocks (i.e. WDM splitter  23  and WDM coupler  26 ) and depletion due to any existing traffic wavelengths if at all present. The fixed path losses associated with the WDM splitter  23  and WDM coupler  26  may be calibrated in the factory at ‘start of life’ as is well known in the art. 
     The arrangement presented in FIG. 2 may further be used to determine the chromatic dispersion of the optical transmission fiber  20  as it applies to the Raman pump wavelength band. To do so, known driving signals (in frequency, amplitude and phase) are applied to the Raman pump lasers  27  and phase information is then collected at the receiving photodetectors  29 . Then, the relative difference in phase between the source driving signals at the Raman pump lasers  27  and the received signals at the photodetectors  29  for each pair of adjacent Raman wavelengths corresponds to the different propagation time of the wavelengths, which can then be used to derive the chromatic dispersion for the particular wavelength interval. 
     Both of the measurements described above will obviously require some form of communication path to be established between source and receiver i.e. between the Raman pump lasers  27  and the photodetectors  29 . Advantageously, most current optical transmission systems already typically incorporate some form of communication path between optical components such as the amplifier sites (i.e. EDFA  25  and EDFA  28 ) depicted in FIG.  2 . In such instances, the required communication infrastructure may be realized using the well known Optical Service Channel (OSC) or any other equivalent. 
     The chromatic dispersion and attenuation profiles derived for the optical transmission fiber  20  in the Raman pump wavelength band may then be converted to corresponding estimates for the traffic wavelength band. Specifically, the chromatic dispersion and attenuation information, as determined above for the Raman pump wavelength band, may be compared against similar known characteristics (or signatures) in the Raman pump wavelength band for the different fiber types (e.g. Non-Dispersion Shifted Fiber, Non-Zero Dispersion Shifted Fiber, TrueWave, LEAF, etc.). In this way, the type of fiber or at least the family of fibers to which the fiber belongs may be identified. 
     In FIG. 2, the processing agent  24  manages the entire measurement and characterization process. The processing agent  24  may be implemented by some form of control software with the necessary processor complex (i.e. cpu, memory, non-volatile memory, communications ports and other necessary hardware). Those skilled in the art will appreciate that such a processing agent may be resident on one or more amplifier circuit packs and their associated processor complex or on an external computing platform. 
     The known characteristics (e.g. attenuation and chromatic dispersion) in the Raman pump wavelength band for each fiber type may subsequently be organized in database form and stored in, for example, the random access memory (RAN) of the processing agent  24 . Such a database, for example, may comprise a series of records each of which consists of a known attenuation and chromatic dispersion profile in the Raman pump wavelength band for each fiber type. A successful match between the measured characteristics and a particular record in the stored database will then yield the fiber type. 
     It should be noted that the determination of the precise fiber type is not critical in a given fiber characterization process. The key parameters of interest are the attenuation and chromatic dispersion in the traffic wavelength band. As demonstrated, the technique of the present invention may be used to determine these parameters with respect to the Raman pump wavelength band. Based on this information, the attenuation and chromatic dispersion for the traffic wavelength band may be estimated using a suitable extrapolation algorithm. 
     In some specific situations, the network provider may desire to provision the appropriate fiber type i.e. stipulate what fiber is really there instead of looking for a match. Typically, this may over-ride the fiber type determined by the above characterization process. If a significant mismatch between the detected fiber type and the provisioned fiber type is observed, an alarming mechanism may be triggered to alert the user in order that appropriate courses of action be taken. For example, after clearing the alarm, the fiber type may be re-provisioned to match the detected fiber type. 
     Finally, the chromatic dispersion and attenuation characteristics of the transmission fiber for normal traffic flow may be estimated, using known relationships between these parameters in the Raman pump wavelength band and the traffic wavelength band for the fiber type in question. In other words, by allowing determination of the type of fiber, the present invention further provides for the determination of fiber attenuation and chromatic dispersion in the traffic wavelength band. 
     Advantageously, the arrangement of FIG. 2 is very well suited to measure the polarization mode dispersion (PMD) of the length of transmission fiber in question. Polarization mode dispersion is a form of pulse dispersion caused by the varying polarization properties of optical fiber. If undetected, it can result in the receiver being unable to interpret the signal correctly and cause high bit error rates. 
     In addition to the inherent, varying polarization properties of fiber, stress and other environmental influences (such as changes in temperature and humidity) will directly affect the severity of the impact of PMD, causing it to change over time. In fact, the initial PMD measurement taken when the fiber is first pulled is likely to be different from the PMD reading after cabling or installation. 
     MD can dramatically decrease a fiber optic network&#39;s performance, particularly those networks operating at high data rates such as OC48 or OC192. Therefore, it&#39;s critical to test installed fiber before upgrading to higher data rates or installing DWDM systems. In this way, the link performance may be qualified and, if necessary, the use of some form of active PMD compensation may be planned for. 
     To measure the PMD of a length of optical transmission fiber, two Raman pump lasers may be used to realize a pulse-delay type of setup similar to FIG.  2 . In this case, however, each Raman pump laser  27  will feed into a polarization rotator (not shown) prior to the WDM coupler  26  in order that orthogonal polarization states may be launched into the optical transmission fiber  20 . Then, the resulting difference in propagation time between the polarization modes as measured at the far-end of the fiber defines the differential group delay and allows for direct observation of the impact of PMD. 
     In another embodiment of the invention, characterization of an optical transmission fiber may be conducted using an OTDR type arrangement such as that depicted in FIG.  3 . In this case, a length of optical transmission fiber  30  is coupled to a WDM splitter/coupler  36  which precedes an EDFA  38 . The amplified output of the EDFA  38  is coupled into a subsequent length of optical transmission fiber  31 . A plurality of Raman pump lasers  37  feed into an optical circulator  34 , and a corresponding set of Raman wavelengths  32  are then provided to the WDM splitter/coupler  36  to facilitate Raman amplification within the preceding length of optical transmission fiber  30  under normal system operation. 
     However, in this particular embodiment, at least one or more Raman pump lasers may further serve as the optical source(s) for an OTDR type function. Accordingly, the WDM splitter/coupler  36  is also connected via the optical circulator  34  to a power monitor  39  which measures any back-reflections  35  propagating in the optical transmission fiber  30 . A processing agent  33  is further coupled to the power monitor  30 . As in the embodiment of FIG. 2, the role of the processing agent  33  includes managing all aspects of the fiber characterization process. 
     Characterization of optical transmission fiber in accordance with the embodiment of FIG. 3, therefore, does not require any far-end detectors as depicted in FIG.  2 . Instead, the one or more Raman pump lasers  37  are used to provide a type of Optical Time Domain Reflectometer (OTDR) application. For example, a Raman pump laser  37  may be used to supply a short pulse of light  32  which is launched into the optical transmission fiber  30  via the WDM coupler/splitter  36 . The Raman pump light  32  is transmitted down the transmission fiber  30 , and backscattered light in the form of a series of back-reflections  35  is then monitored at the source end by the power monitor  39 . Just like any well known commercially available OTDR, this technique may be used to supply a profile of fiber reflection and attenuation versus distance. 
     At present, OTDR employs separate test equipment and requires a means of coupling this equipment into the access fibers. This usually requires the installation of costly filter equipment and introduces consequent losses into the system. However, the inventive technique provides an OTDR function at little cost over the Raman pump lasers. The collected reflection data will be directly applicable to the Traffic Wavelength Band provided there are no wavelength sensitive devices in the fiber path. The reflected light may be monitored for abrupt changes indicative of a loss or fault in the optical transmission fiber. As per conventional OTDR techniques, the distance of the loss or fault from the launch end of the fiber can be determined from the time interval between the launch and the return of the reflected peak. Note that the reflection locations will generally be wavelength independent. 
     Therefore, in addition to their primary role in providing Raman amplification for ordinary optical transmission systems, the Raman pump lasers may serve a dual function in accordance with the embodiments of FIGS. 2 and 3 in that they may also act as optical sources for fiber characterization measurements. 
     Those skilled in the art will appreciate that optically amplified systems such as the one depicted in FIG. 2 ordinarily have one or more Optical Service Channels (OSCS) that create a messaging link between respective amplifier sites. In such arrangements, each amplifier site is usually equipped with both an OSC laser source and an OSC receiver. All measurements that require only one wavelength can, therefore, be made with this laser source instead of the Raman pump laser. OSC laser sources are, however, generally configured to propagate light in the same direction as the traffic wavelengths; otherwise their application in the above context is identical to that of the Raman pump laser embodiment described in accordance with FIG.  2 . 
     For example, FIG. 4 depicts a further embodiment of the present invention which utilizes an OSC laser source, typically found in optically amplified systems, for performing optical fiber characterization. In this particular arrangement, a span of optical transmission fiber  40  is connected between a first WDM filter  43  and a second WDM filter  46 . Preceding the first WDM filter  43  is a first EDFA  45 , the input of which is coupled to a preceding length of optical transmission fiber  41 . Following the second WDM filter  46  is a second EDFA  48  whose amplified output is coupled into a subsequent length of optical transmission fiber  42 . 
     In the optically amplified system of FIG. 4, each amplifier site is equipped with an OSC source and OSC receiver as is convention. Specifically, an OSC laser source  44  and an OSC receiver  47  are coupled to the WDM filter  43  at the first EDFA  45 . Similarly, an OSC laser source  50  and an OSC receiver  51  are coupled to the WDM filter  46  at the second EDFA  48 . Sample photodetectors  49  and  52  are further coupled to WDM filters  43  and  46 , respectively. The sample photodetectors  49  and  52  are coupled to processing agents  54  and  55 , respectively, which are responsible for managing all aspects of the fiber characterization process. 
     Consequently, fiber characterization measurements requiring only one wavelength (e.g. attenuation, OTDR applications) may alternatively be made using the OSC laser source typically found in such systems. For example, the OSC laser source  50  coupled to the WDM filter  46  may be used to launch an optical pulse into the length of optical transmission fiber  40 . The sample photodetector  52  may then be used to monitor the back-scattered signal which, of course, will be of the same wavelength as that produced by the OSC source  50 . This technique may, therefore, provide for an OTDR-type measurement similar to the one described in accordance with FIG.  3 . Obviously, the use of two light sources (Optical Service Channel or otherwise) may further yield some chromatic dispersion information. 
     In another embodiment, the OSC laser source  50  may be used to determine, for example, the fiber attenuation in an application similar to the embodiment presented in FIG.  2 . For example, the OSC laser source  50  may be operated so as to launch an optical pulse into the length of optical transmission fiber  40 . The sample photodetector  49  may then be used to detect and measure the power in the residual pulse arriving at WDM filter  43 . In this way, the attenuation of the optical transmission fiber  40  as it applies to the particular operating wavelength of the OSC laser source  50  may be determined. 
     Advantageously, then, most of the components required to perform the characterization technique of the present invention are already embedded in existing optical transmission equipment, thereby eliminating the effort currently required to test optical fibers. Although it is well understood to embed test equipment into a system application (e.g. conventional OTDR), such methods are necessarily more expensive than the present invention since they require the use of additional laser sources and costly filters. In the technique of the present invention, fiber characterization information is provided at little additional cost over the Raman pump lasers which are already embedded in most transmission equipment to facilitate Raman amplification within the fiber medium. Subsequently, only low-cost detectors are needed to detect and measure the residual Raman light or backscatter reflections. 
     In the future, all transmission system vendors will require extremely accurate fiber characterization data from their customers in order to specify aggressive (i.e. high capacity and long reach) fiber optic transmission systems. The high capacity optical transport business is growing at an astonishing rate each year due to the explosive growth in Internet applications. In order to stay competitive, transmission system vendors must meet and exceed their competition&#39;s ability to specify the highest capacity, longest reach systems. In addition, the commercial use of Raman amplification to extend the distance between amplifier sites is becoming more and more popular thereby providing a readily available and cost-effective platform for implementation of the inventive technique. 
     While preferred embodiments of the invention have been described and illustrated, it will be apparent to those skilled in the art that numerous modifications, variations and adaptations may be made without departing form the scope of the invention as defined in the claims appended hereto.