Abstract:
A passive optical element defining an optical propagation path is tested by coupling a buffer fiber between an input of the propagation path and an optical time domain reflectometer. The OTDR launches optical radiation into the buffer fiber via one end thereof, measures power level of return light received at the OTDR via the buffer fiber, and creates an OTDR signature representing power level of return light as a function of distance from the end of the buffer fiber. The OTDR selects a first marker point by applying data reduction to a portion of a segment of the OTDR signature corresponding to the buffer fiber, selects a second marker point downstream of the input of the optical propagation path, and calculates a first power difference value as difference between a power level at the first marker point and a power level at the second marker point.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims benefit of U.S. Provisional Application No. 61/242,892 filed Sep. 19, 2009, the entire disclosure of which is hereby incorporated herein by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to a method of testing a passive optical element, such as an optical fiber or an optical coupler. 
     Operators of data communication networks employing optical fiber cables require that the optical fibers forming the cables meet very strict requirements regarding change in optical power loss through the fibers under various mechanical stresses. The mechanical stresses may be applied by direct mechanical action, such as bending, twisting and crushing, or result from environmental effects, such as change in temperature. Testing to determine change in optical power loss of an optical fiber under change in mechanical stress due to environmental effects is referred to herein as enviro-mechanical testing whereas testing to determine change in optical power loss of an optical fiber under change in mechanical stress due to direct mechanical action is referred to herein as direct mechanical testing. 
     The current procedure for enviro-mechanical testing of optical fiber involves monitoring loss at one or more critical transmission wavelengths using a light source and power meter, applying a stress, and measuring the change in optical power loss resulting from the applied stress against relevant industry and/or end-user specifications.  FIG. 1  shows one typical arrangement for performing a test to measure change in power loss. An optical fiber under test (FUT)  8  is placed in a suitable test fixture  10 , which comprises an environmental stress chamber in which a controlled environmental stress may be applied to the FUT. The FUT has fiber pigtails that extend to the exterior of the stress chamber and are connected by fusion splices  12 , jumpers  14 , and optical connectors  16  to, respectively, a light source  18  and a power meter  20 , which also are located outside the stress chamber. 
     The light source/power meter (LSPM) approach to measuring changes in loss is subject to disadvantage. For example, the real changes in loss of the FUT might be obscured by drift in the power output of the light source or drift in the response of the power meter over the duration of the test, and there may also be drift in loss of the optical connectors  16 . 
     The test arrangement shown in  FIG. 1  may be adapted to test several fibers substantially concurrently, such as a test sample of fibers in a multi-fiber cable, each connected to its own pair of jumpers  14 , by providing optical switches  22  (shown in dashed lines) between the jumpers  14  and, respectively, the light source and power meter and controlling the switches  22  to select the fibers in turn. However, optical switches introduce another source of drift in power loss in the test channel (the optical path between the light source and the power meter). 
     Although drift in the source power and detector response can be monitored using a reference channel, the reference channel and the FUT cannot be monitored simultaneously. Further, in order to utilize a reference channel it would generally be necessary to interpose an optical switch between each jumper  14  and the adjacent optical connector  16  and drift in loss in the optical switches may impair the accuracy with which the change in power loss of the FUT can be measured. 
     Another arrangement that may be used for enviro-mechanical testing of an optical fiber is shown in  FIG. 2 . The fiber under test (FUT) has an upstream end, which is outside the stress chamber and is connected by a fusion splice  12  to the downstream end of a buffer fiber  26 . The upstream end of the buffer fiber  26  is connected through a optical connector  30  to the port of an optical time domain reflectometer (OTDR)  34 . The terms “upstream” and “downstream” are used herein relative to the direction of propagation of light from the port of the OTDR towards the FUT. The downstream end of the FUT is connected by a fusion splice  12  to the upstream end of a second buffer fiber  36 . 
     In operation, the OTDR  34  acquires a data set that can be represented graphically as a trace showing power loss through the test channel (the optical path into which light is launched by the OTDR, and from which return light is received by the OTDR) as a function of distance. This trace, commonly referred to as a signature, may have the appearance shown in  FIG. 3 . Each segment of the signature corresponds to a segment of the test channel. In  FIG. 3 , the peak  40  originates from reflection in the optical connector  30 , the peak  42  originates from a reflection at the glass-air interface at the far end of the buffer fiber  36 , the substantially linear segments  44 ,  46  represent the power of return light received from the buffer fibers, and the steeper substantially linear segments  48 ,  50  represent power of return light received from the fusion splices. The operator places markers  52 ,  54  on the FUT and the OTDR measures the power loss in the segment between the markers. The measurements are not affected by loss drift in the elements, such as connectors and switches, outside the segment of the test channel that is between the markers. However, noise on the OTDR signature makes a two-point loss measurement of this type inherently noisy, injecting uncertainty into the measurement and possibly obscuring real changes in power loss. 
     Direct mechanical testing may be performed using an equipment arrangement that is schematically similar to that shown in  FIG. 2 . In the case of direct mechanical testing, however, the test fixture  10  applies stress to the FUT by direct mechanical action. Generally, the power loss is measured before the stress is applied and after the stress has been removed for a sufficient time to allow the FUT to recover. 
     Current standards for power loss change require a maximum loss change for 90% of the fibers in a fiber optic cable when placed under a specified stress of no greater than 0.05 dB. Current methods for measuring loss change have a precision no better than +/−0.05 dB. It is desirable that the minimum measurement error should be substantially less than the maximum permitted loss. 
     SUMMARY OF THE INVENTION 
     In accordance with the subject matter claimed herein there is provided a method of testing a passive optical element defining an optical propagation path, comprising coupling a first end of a first buffer fiber to an input of the optical propagation path and coupling a second end of the buffer fiber to an optical time domain reflectometer (OTDR), employing the OTDR to launch optical radiation into the first buffer fiber via the second end thereof, measure power level of return light received at the OTDR via the second end of the first buffer fiber, and create a first OTDR signature representing power level of return light as a function of distance from the second end of the first buffer fiber, selecting a first marker point by applying data reduction to at least a portion of a segment of the first OTDR signature corresponding to the first buffer fiber, selecting a second marker point downstream of the input of the optical propagation path, and calculating a first power difference value as difference between a power level at the first marker point and a power level at the second marker point. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG. 1  illustrates schematically a first equipment arrangement for measuring optical power loss in an optical fiber, 
         FIG. 2  illustrates schematically a second equipment arrangement for enviro-mechanical testing of an optical fiber, 
         FIG. 3  illustrates a graph that is useful for explaining one method for using the equipment arrangement shown in  FIG. 2 , 
         FIG. 4  illustrates a second graph that is useful for explaining a second method for using the equipment arrangement shown in  FIG. 2 , 
         FIG. 5  is an enlarged view of a portion of the graph shown in  FIG. 4 , and 
         FIG. 6  illustrates schematically a modification of the equipment arrangement shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     The arrangement shown in  FIG. 2  may be used to perform an alternative method for measuring power loss with a substantially higher precision than the method described with reference to the signature shown in  FIG. 3 . 
     Referring to  FIG. 4 , the OTDR is able to use conventional signal processing techniques to distinguish the segments of the signature corresponding to the two buffer fibers  26 ,  36  from other portions of the signature and to determine a power level associated with each buffer fiber by applying data reduction to a sufficiently complete amount of the information contained in the segments of the signature corresponding to the two buffer fibers. In a practical implementation of the alternative method, the data reduction is achieved by fitting two straight lines  26 A,  36 A to the buffer fiber segments respectively using a least squares approximation algorithm. Referring to  FIG. 5 , the OTDR then selects a first data point  26 B on the fit line  26 A corresponding to a point on the buffer fiber  26  that is close to the fusion splice  12 . For example, a data point that corresponds to a point on the buffer fiber that is one meter closer to the distal end of the buffer fiber than a point on the signature where the power differs by a predetermined amount from the power of the fit line (indicating that the latter point corresponds to a point on the fusion splice), may be selected as the first data point. The OTDR selects a second data point  36 B on the fit line  36 A in similar fashion and measures the power loss between the two selected data points. 
     The test fixture  10  is then used to apply a stress to the FUT and the OTDR acquires a second signature while the FUT is under stress (in the case of an enviro-mechanical test) or after the stress has been removed (in the case of a direct mechanical test). 
     The OTDR repeats the analysis and fits straight lines to the two buffer fiber segments of the second signature. The OTDR verifies that the two fit lines of the second signature have the same respective slopes as the corresponding fit lines of the first signature. If the fit lines do not have the same slope, it implies a change in one or both of the buffer fibers, which would invalidate the test. Assuming that the fit lines have the proper slopes, the OTDR selects two data points on the second pair of fit lines corresponding to points on the test channel at the same respective distances from the OTDR as the points that correspond to the selected data points of the first signature and measures power loss between the two data points of the second signature. 
     By using points on the fit lines to define the power levels that are measured, the effect on the power loss measurement of noise present on the OTDR signature is substantially reduced. Measurements performed using the method described with reference to  FIGS. 4 and 5  show variations in measurement precision of less than 0.003 dB over several weeks of observation. 
     The power loss measurements includes power loss in the fusion splices but since the fusion splices are outside the test fixture, the losses in the fusion splices are not affected by conditions inside the test fixture and any change in loss is caused by the applied stress. As in the case of the method described with reference to the signature shown in  FIG. 3 , the OTDR calculates the change in power loss due to the stress by subtracting the first power loss measurement from the second power loss measurement. 
     In order to test adequately a fiber optic cable containing multiple optical fibers, it may be necessary to test all of the fibers or a representative sample of the fibers, depending on the number of fibers in the cable. In a development of the method described with reference to  FIGS. 4 and 5 , it is possible to test multiple fibers substantially concurrently using the apparatus shown in  FIG. 6 . In this case, a medial length segment of the fiber optic cable  60  is placed in the test fixture  10  while the two opposite end segments of the cable remain outside the test fixture, so that each fiber to be tested has two pigtails outside the test fixture. Each fiber to be tested is connected by its upstream and downstream pigtails to its own upstream and downstream buffer fibers (for clarity, only one FUT and one pair of upstream and downstream buffers are shown in  FIG. 6 ) by fusion splices. The distal end of each upstream buffer fiber  26  is connected by an optical connector to a downstream port of an optical switch  62 . The optical switch  62  has one upstream port and multiple downstream ports and is operative to connect the upstream port to the downstream ports selectively. The port of the OTDR is connected to the upstream port of the optical switch  62  by a fusion splice. 
     Using the apparatus shown in  FIG. 6 , as described thus far, the OTDR first tests all the fibers with the cable in an unstressed condition, by acquiring and analyzing the signature of each test channel in turn, by selecting the test channels sequentially using the optical switch. The test fixture is then used to apply a stress to the cable and the OTDR repeats the operation, either while the cable is under stress (in the case of an enviro-mechanical test) or after the stress has been removed (in the case of a direct mechanical test). The OTDR is therefore able to measure the change in loss of each fiber to be tested. 
       FIG. 6  also illustrates that a two-port OTDR may be used to test the fibers from each end. In this case, a second optical switch  64  is needed in order to allow the OTDR to launch optical radiation into the fibers under test in turn via the respective buffer fibers  36 . 
     In accordance with another modification of the method described with reference to  FIGS. 4 and 5 , the downstream buffer is omitted and the downstream data point is a point on the FUT. In this case the downstream data point may be selected by applying data reduction to a segment of the FUT although it would be possible to select the downstream data point based only on distance from an end of the FUT. Generally, it will be desirable for the downstream data point to be distant from the upstream data point, but it is not necessary that the downstream data point be outside the test fixture  10 . It will be understood that measurement precision is reduced relative to the method described with reference to  FIGS. 4 and 5 , but the setup time and measurement time may be reduced, and the equipment cost may be reduced. 
     The method described with reference to  FIGS. 4 and 5  is applicable to testing not only optical fibers but also other passive optical elements such as couplers, wavelength division multiplexers and connectors. However, when testing an optical element having an optical path less than about 70 m in length (depending on the particular OTDR that is used to acquire the data set) it will not normally be satisfactory to omit the downstream buffer, as discussed above in the case of testing a fiber. 
     The operating wavelength of a laser diode used in an OTDR depends on temperature, and power loss in an optical fiber depends on wavelength. Therefore, it is desirable to avoid changes in temperature during the test. 
     It may take several hours to complete a variety of tests on a cable containing multiple fibers and during that time the temperature in a typical test laboratory may change by an amount such that the operating wavelength of many OTDRs may change significantly. It is therefore desirable to employ an OTDR that is stable under change in ambient temperature, since this may be more reliable and less expensive than controlling the ambient temperature in the test laboratory. The Photon Kinetics 8000 OTDR has been found particularly suitable. 
     Generally, the length of the (or each) buffer fiber should be at least 100 m in order to provide a sufficient number of data points for fitting a line to the signature segment corresponding to the buffer fiber, where the line will not be influenced to an undesirable degree by noise on the signature. 
     As suggested above, fitting a straight line to the signature segment corresponding to a buffer fiber is not the only method by which data reduction may be applied to determine a power level associated with the buffer fiber. Other possible data reduction techniques include simply averaging over the segment corresponding to the buffer fiber (or over a portion of the segment corresponding to the buffer fiber). Also, in the event that the data reduction involves fitting a line to the signature segments, other approaches than least squares may be employed. 
     It will be appreciated that the invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims, as interpreted in accordance with principles of prevailing law, including the doctrine of equivalents or any other principle that enlarges the enforceable scope of a claim beyond its literal scope. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated. The word “comprise” or a derivative thereof, when used in a claim, is used in a nonexclusive sense that is not intended to exclude the presence of other elements or steps in a claimed structure or method.