Simultaneous bi-directional optical time domain reflectometry method
1. Technical Field 
     The present invention relates to optical time domain reflectometry (OTDR) testing of optical fibers, and more particularly to an optical time domain reflectometry testing method which provides for accurately performing simultaneous bi-directional optical time domain reflectometry testing measurements. 
     2. Background Art 
     Optical time domain reflectometry (OTDR) testing is a proven non-destructive technique for determining the existence and location of fiber breaks and for measuring both splice and transmission loss in an installed fiber system. OTDR is an optical pulse echo system which utilizes an optical probe pulse from a laser which is introduced into the optical fiber to be tested. The probe pulse undergoes scattering continuously along the length of the fiber so as to produce a low-level reflected signal. The degree of scattering is dependent on the amount of transmission loss in the fiber and is primarily caused by Rayleigh scattering from inhomogeneities in the glass. The reflected signals propagate back to the input end of the fiber where they are detected by a photodetector, displayed on a sampling oscilloscope, and analyzed by a computer. 
     In the past, it was necessary to take a measurement in each direction through a fiber and then average the measurements for accurate analysis since varying fiber parameters result in bi-directionality of the trace in OTDR testing. Although the use of a mirrored surface on one end of a test fiber in order to allow simultaneous two-way OTDR testing of an optical fiber has been previously proposed, there has to date been no practical and technically viable system to accurately perform simultaneous bi-directional OTDR measurements. This is apparently due to the fact that a &#34;noise&#34; contribution due to previously unrecognized reflectivity at the interface of the launch fiber and the OTDR testing apparatus can significantly affect the reverse trace signal and resulting fiber measurements. The present invention has recognized this previously unappreciated phenomenon and provides for a practical and accurate method for performing simultaneous bi-directional optical time domain reflectometry (OTDR) measurements which includes a correction algorithm for removing the superfluous reflectivity signal from the reverse trace of the laser pulse signal. 
     DISCLOSURE OF THE INVENTION 
     In accordance with the present invention, applicant provides an improved optical time domain reflectometry (OTDR) method for testing optical fibers which provides for accurate simultaneous bi-directional measurement utilizing a correction algorithm for the reverse trace signal which removes unwanted signal contributions resulting from reflectivity at the interface of the OTDR testing apparatus and the launch fiber. 
     The simultaneous bi-directional OTDR testing method for measuring certain characteristics of single mode and multi-mode optical fibers includes splicing a test fiber between a launch fiber which is connected to an OTDR testing apparatus and a termination fiber which has a highly reflective coating at its terminal end. A light pulse is then introduced by a laser into the launch fiber by means of conventional OTDR testing apparatus. An optical detector is utilized to detect the forward trace signal (T1) and the reverse trace signal (T2) generated by the light pulse. In order to facilitate accurate testing measurement of the optical fiber, the reverse trace signal (T2) from the light pulse is corrected with an algorithm in order to remove reflected signals originating at the interface of the OTDR testing apparatus and the launch fiber connected thereto, and the forward trace signal (T1) and the corrected reverse trace signal (T2) are then analyzed to accurately measure certain desired characteristics of the test fiber. 
     Accordingly, it is a general object of the present invention to provide an improved method of OTDR testing. 
     More specifically, it is an object of the present invention to provide a practical and accurate simultaneous bi-directional OTDR testing method. 
     Still another object of the present invention is to provide a method for correcting the reverse trace signal T2 in simultaneous bi-directional OTDR testing in order to provide for accurate optical fiber measurements. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Some of the objects of the invention having been stated, other objects will become evident as the description proceeds, when taken in connection with the accompanying drawings in which: 
     FIGS. 1 and 1A are a schematic view of an OTDR testing apparatus in accordance with the present invention; 
     FIG. 2 is a schematic representation of the signal elements present in the forward trace (T1) and the reverse trace (T2) resulting from introduction of a light pulse into the launch fiber of the system; 
     FIG. 3 illustrates a sample trace (T2) before correction; 
     FIG. 4 illustrates the center of the trace (T2) both before, and after application of the correction; and 
     FIG. 5 illustrates the agreement achievable between standard FFP measurement methodology and the simultaneous bi-directional OTDR method of the invention. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     With particular reference to FIG. 1 of the drawings, a system for performing the simultaneous bidirectional OTDR measurement is shown. A test fiber 12 is spliced between a launch fiber 14 and a termination fiber 16. Termination fiber 16 is connected to a connectored fiber having a thin layer of gold sputtered onto the end thereof. The reflectivity (Re) from the gold sputtered end of termination fiber 16 is greater than 90% with only some degradation due to a splice between the connectored fiber and termination fiber 16. Since test fiber 12 is spliced between launch fiber 14 and termination fiber 16, termination fiber 16 and the connectored fiber are a permanent part of the entire measurement system and the reflectivity Re is constant for all measurements. 
     The lengths of the launch fiber 14 and the termination fiber 16 are chosen to sufficiently suppress the effects of input and reflected pulse saturation on the test link and to provide a sufficient number of data for accurate linear regression on these portions of the pulse trace, and are each most suitably greater than 200 meters in length. Although there is no absolute need to match the performance characteristics of launch fiber 14 and termination fiber 16 when the ratio of reflectivity at input (Ri) and reflectivity at the reflecting end of the termination fiber (Re) is small, careful selection of the launch and termination fibers (as described hereinafter) minimizes the error in the correction of the reverse trace as provided by the method of the present invention. For measuring the back-scattered light or signal from the forward and reverse traces, a commercially available OTDR testing apparatus 18 is provided which utilizes a single-mode dual wave length source module with a masking function to further minimize effects of detector saturation from the reflected pulse. 
     THEORY OF THE OTDR METHOD 
     The theory of optical fiber back-scatter as measured by the method of the invention will now be set forth with particular reference to FIG. 2 of the drawings. The signal P(t) detected by OTDR testing apparatus 18 is dependent on the proportion of total attenuation scattered and confined in the counter propagating mode, as shown in Equation 1 below: 
     
         P (t)=EO*Vg*e.sup.-2α.sub.t.sup.z *B(z) (Equation 1) 
    
     In the foregoing Equation 1, EO is input power, Vg is group velocity, .sup.α t  is total contribution to attenuation, variable z is the distance to the scattering point measured from the signal input end of the link arriving at time t=2*z/Vg. B(z), the source of OTDR bidirectionality, is the local scattering factor dependent on the Rayleigh scattering portion of attenuation and a fiber parameter dependent containment factor. 
     With simultaneous bi-directional OTDR methodology, Equation 1 is valid only for the initial forward trace (T1 of FIG. 2) represented by the equation t&lt;=2*L/Vg, where L is the total fiber length. For t22 2*L/Vg, the trace is a superposition of all independent signals which arrive back at the optical detector of OTDR testing apparatus 18 at the same time t. FIG. 2 illustrates the possible signal paths for the initial forward trace (T1) and the next trace or reverse trace (T2) where 2*L/Vg&lt;t&lt;4*L/Vg. The primary signal B of reverse trace T2 is due to scattering at a point Z&#39; as measured from the remote end of termination fiber 16. Also, at the same time t, two equivalent signals A and C are returned from point z wherein the sum of the two signals is an attenuated reproduction of forward trace T1 arising from the non-zero reflectivity Ri at the input of launch fiber 14. The reflectivity Ri is due to reflectivity between the interface of launch fiber 14 and OTDR testing apparatus 18 and may also include reflected signals from internal optics of OTDR testing apparatus 18. In order to accurately measure reverse trace T2, the contaminating signal due to reflectivity Ri at the input must be subtracted from measured trace T2. 
     The desired reverse trace T2 is obtained by first transforming the logarithmic data (see Equations below), then subtracting the appropriate fraction of the forward trace from the acquired trace of t22 2*L/Vg. FIG. 3 shows a sample trace through t=6*L/Vg, before correction. The correction algorithm requires comparing the return losses D1 and D2 as shown. FIG. 4 shows the center of the trace T2 before and after application of the correction routine. 
     Averaged bi-directional traces can be used to accurately calculate attenuation and splice loss in an optical fiber. 
     Test Results 
     To verify the accuracy of the simultaneous bidirectional OTDR method, 12 cabled depressed cladding single-mode fibers were measured using the described system. The splices to launch fiber 14 and termination fiber 16, also depressed cladding fibers, were mechanical V-groove splices with index matching liquid having varying splice quality. Attenuation measurements from the average of the bi-directional traces were compared with standard cut-back method attenuations at 1.3um and 1.5um both before and after correction according to the present method. The results are shown in Table 1 below and clearly demonstrate that recognition of the input reflectivity Ri allows for significant improvement of attenuation measurements. 
     
                       TABLE 1                                                     
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            1.3 um      1.55 um                                           
            -x    σ   -x     σ                                
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Uncorrected trace                                                         
               .036   .032      .031 .036                                 
Corrected trace                                                           
              -.008   .015      .011 .017                                 
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 Attenuation error: OTDR  cutback (dB/km)                                 
 
    
     Also of interest, to test the feasibility of using the method of the present invention to estimate mode field diameters (MFD) of single mode fibers, the splice losses due to splicing of the test fiber to the launch and termination fiber were measured using the simultaneous bi-directional OTDR method in the forward and reverse directions. With the MFD of the two splices measured by the far-field pattern (FFP) method, the MFD at each end of the test fibers were calculated from the splice losses. FIG. 5 illustrates the excellent agreement achievable between standard FFP measurement technique and the simultaneous bi-directional OTDR measurement method of the invention at 1300nm and 1550nm. The repeatability of the MFD measurement was measured to have one standard deviation of σ=0.lum or less at both wavelengths. 
     Equations For Removing Reflectivity From Reverse Trace T2 
     The logarithmic equations for correcting reverse trace T2 are complex from a mathematical standpoint, but they are set forth below in a step-by-step fashion to provide a more complete understanding of the novel method of the invention. I. The signal is received by the detector at time t: 
     
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Case 1  O &lt; t &lt; 2*L/Vg   (Forward trace T1 only)                          
Equation 1:                                                               
P(t) = E.sub.o Vg e.sup.-2α.sbsp.T.sup.z B(z)                       
Case 2  2*L/Vg &lt; t &lt; 4*L/Vg                                               
                         (T2 contaminated                                 
                         reverse signal)                                  
Equation 2:                                                               
P(t) = E.sub.o Vg e.sup.-2α.sbsp.T.sup.L [e.sup.-2α.sbsp.T.sup
.Z&#39;  Re.sup.2 B(Z&#39;) + 2e.sup.-2α.sbsp.T.sup.Z ReRi B(Z)]            
 ##STR1##                                                                 
 ##STR2##                                                                 
Case 3  4*L/Vg &lt; t &lt; 6*L/Vg                                               
                         (T2 contaminated                                 
                         reverse signal)                                  
Equation 3:                                                               
P(t) = E.sub.o Vg e.sup.-4α.sbsp.T.sup.L [e.sup.-2α           
.sbsp.T.sup.Z&#39;  2Re.sup.3 Ri B(Z&#39;) +                                      
3e.sup.-2α.sbsp.T.sup.Z Re.sup.2 Ri.sup.2 B(Z)]                     
 ##STR3##                                                                 
 ##STR4##                                                                 
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     The desired reverse trace is found as the primary signal of Case 2: 
     
         P(t) .sub.reverse =E.sub.o Vg [.sup.-2α.sub.T.sup.L Re.sup.2 ]e.sup.-2α.sub.t Z&#39;B(Z&#39;) Equation 4 
    
     Therefore, we need a method to determine the signal: 
     
         E.sub.o Vg [2e.sup.-2α.sbsp.T.sup.L ReRi]e.sup.-2α.sbsp.T.sup.Z B(Z) 
    
     (Note that this is the forward trace T1 (Case 1) attenuated by a factor 2e -2 α.sbsp.T L  ReRi.) 
     II. Determination of attenuation factor 2e -2 α.sbsp.T L  ReRi. 
     (a) Let DO be the signal drop from the beginning of the OTDR trace (t=0, Z=0) to the beginning of the first reflected trace (t=2*L/Vg+Δt, Z&#39;≃L, Z≃O) 
     (b) Let D1 be the signal drop between the end of the forward OTDR trace (t=2*L/Vg-Δt, Z=L) and the beginning of the first reflected trace (t=2*L/Vg +Δt, Z&#39;≃L, Z≃0) 
     (c) Let D2 be the signal drop between the end of the first reflected trace (t=4*L/Vg-ΔT, Z&#39;≃,O,Z≃L) and the beginning of the first reflected trace (t=4*L/Vg+Δt, Z&#39;≃L,Z≃O) . ##EQU1## D0, D1, D2 are measured directly from the OTDR trace, in dB. Then: 
     
         -[D0+(D2-D1)]=-2α.sub.T(dB)+ 10Log2ReRi+E 
    
     Note: Recognize that 10Log[e -2 α.sbsp.T L  ReRi]=[-2α t  +10Log2ReRi]. This allows for subtraction of the attenuated forward trace from the measured reflected trace. E represents a small error as follows: ##EQU2## 
     III. To minimize errror E: 
     (a) Set B(L)=B(O) in the system. Then: ##EQU3## 
     (b) Since B(Z)≃K W (Z) -2 , therefore set ##EQU4## then ##EQU5## is known and E≃O for single mode fibers. 
     (c) Since B(Z)=K&#39; NA (Z) 2 , set ##EQU6## then ##EQU7## is known and E≃O, for multi-mode fibers. 
     The legend for the variable designations set forth above is as follows: 
     L: Length of fiber from OTDR input to mirror 
     Vg: Group velocity (Constant for equations set forth above) 
     α T  : Total local loss component 
     α T  (dB): Total loss of span in logarithmic units 
     Z: Distance measured from input end 
     Z&#39;: Distance measured from mirror 
     B(Z): Scattering fraction and containment factor (fiber location dependent) 
     Re: Mirror reflectivity 
     Ri: Reflectivity at OTDR/fiber interface 
     W: Mode field radius (single mode fiber characteristic) 
     NA: Numerical aperture (multi-mode fiber characteristics) 
     IV. Measurement in case of very long test fibers: 
     Use of the correction method described above requires the ability to measure D2. Depending on length L and the OTDR apparatus used, this may not always be possible to do (e.g., we may not be able to measure and correct in the same measurement). 
     To solve this problem, we can determine 10Log2ReRi in advance by measuring a link with only launch and termination fibers first, calculating 10Log2ReRi and using this information to correct the longer span after measurement. This may be important, especially for field applications. 
     It will be understood that various details of the invention may be changed without departing from the scope of the invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation-the invention being defined by the claims.