Abstract:
An Optical Time Domain Reflectometer (OTDR) tests an optical fiber by generating, transmitting, and receiving light signals from an optical fiber. The OTDR generates light signals having different characteristics and stitches these light signals into an OTDR trace. Backscatter and properties such as dynamic range effect the quality of the OTDR trace.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from U.S. Provisional Application No. 62/074,883 filed Nov. 4, 2014, the disclosure of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    Exemplary embodiments relate to an Optical Time Domain Reflectometer (OTDR), and more particularly to merging OTDR traces generated by an OTDR using different settings respectively. 
       BACKGROUND ART 
       [0003]    An OTDR is an instrument used to detect and characterize features in an optical fiber. These features may include spontaneous increase of signal level due to reflection at a small air gap between parts of a connector, or a drop of signal level caused by a splice loss or a power splitter. In some situations, reflectance or optical loss could represent a defective fiber. 
         [0004]    An OTDR analyzes optical signals through an optical fiber and may output such signals as OTDR traces. OTDR traces are generally captured in the presence of noise making it difficult to detect or reliably characterize signal characteristics having a magnitude less than that of the noise. 
         [0005]    In addition to noise, dynamic range and dead-zone of OTDR traces are relevant to OTDR instrumentation. Dynamic range is a backscatter level at the entrance of a fiber compared to noise level such that a better dynamic range indicates a smaller noise level. Dead-zone is a section of an OTDR trace immediately following a spontaneous increase of signal level due to reflection, such that a trace may appear functionally saturated, before the power of received light falls within some level, such as 1.5 dB for reflective event and 0.5 dB for non-reflective event. 
         [0006]    Present techniques for overcoming such noise and inability to interpret signal characteristics, such as within a dead-zone of an OTDR trace, have disadvantageous. Increasing the power of a light pulse may increase the complexity and price of an instrument, as well as may cause safety issues. Averaging repeated measurement results may increase test time and thereby decrease productivity. Increasing a duration of a light pulse may cause an OTDR trace to exhibit at least longer dead-zones thereby preventing any measurement during a greater portion of the trace. Applying a filter to an OTDR trace may have similar effect as increasing a duration of a light pulse, and the effect of the filter may diminish as length of the filter increases. Accordingly, a technique for OTDR traces is needed which may overcome these disadvantages. 
       SOLUTION TO PROBLEM 
       [0007]    In accordance with an aspect of the present invention(s), there are several modifications to OTDR systems that can provide improved measurement accuracy. This includes capturing a series of OTDR traces, that use different light pulse-widths optimized to capture different characteristics of the same test network. These traces are then stitched together to produce a composite or merged trace that uses the optimum elements from each individual trace. This composite trace provides shorter dead-zones with higher noise immunity than a single trace using a single pulse-width. 
         [0008]    It is an aspect of the exemplary embodiments to provide an OTDR capable of dealing with events and short pulse-width signals without sufficient dynamic range. For example, for a fast falling of a short pulse-width trace, an OTDR may be configured to determine a point where a shorter pulse-width trace and a longer pulse-width trace intercept. If an OTDR trace distance to the point of intersection of the two traces exceeds a distance of a dead-zone of the longer pulse-width trace and there is no significant signal level rise within this distance, then the OTDR will add data from at least the shorter pulse-width trace, or its filtered version, to an OTDR trace, even when the shorter pulse-width changes rapidly. The OTDR trace may continue to add data from the shorter pulse-width trace until the two traces reach the same level, at which point the longer pulse-width trace and shorter traces may be stitched together, or other optical characteristics are detected. 
       ADVANTAGEOUS EFFECTS OF INVENTION 
       [0009]    Advantages and benefits of the above-described exemplary embodiments include, but are not limited to merged traces having simultaneously lower backscatter noise levels and a shorter dead-zone, overall, as compared to a trace captured with a pulse-width longer than the shortest pulse-width and shorter than the longest pulse-width used before a merge or stitching process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  illustrates an OTDR system including input and output sections according to exemplary embodiments; 
           [0011]      FIG. 2  illustrates a selector section according to exemplary embodiments; 
           [0012]      FIGS. 3A and 3B  illustrates OTDR traces according to exemplary embodiments; 
           [0013]      FIG. 4  illustrates a flowchart according to exemplary embodiments. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0014]    Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. However, known functions associated with the exemplary embodiments or detailed descriptions on the configuration and other matters which would unnecessarily obscure the present disclosure will be omitted. 
         [0015]      FIG. 1  is a view of an OTDR system  1000  including a light generation section  101 , a light I/O  102 , an exit path  103 , a return path  104 , a detection section  105 , a selector  106 , a processor  107 , a display  108  and a memory or storage  109 . 
         [0016]    The memory or storage  109  may store executable instructions that, when executed by the processor  107 , cause the processor to perform algorithms according to exemplary embodiments. 
         [0017]    According to exemplary embodiments, the processor  107  of the OTDR system  1000  algorithms control the light generation section  101  to generate light pulses having different durations, pulse-width, to be sent to a fiber through exit path  103  of the light I/O  102  and to be reflected from the fiber through the return path  104  of the light I/O  102 . The processor  107  may also be configured to control the light generation section  101  to generate light pulses not only having different pulse-width respectively, but also having different intensity and repetitions of previously transmitted light signals according to algorithms. 
         [0018]    The detection section  105  receives the light signals, having at least respectively different pulse-width, from the return path  104  of the light I/O  102  and converts the light signals into electrical signals, such as by a photodetector according to exemplary embodiments. 
         [0019]    The optical signals, having at least different pulse-width, may be further processed with respective gains, such as a low-level gain to accommodate high-amplitude signal at a near end, close to the OTDR, and a high-level gain to accommodate a low-amplitude signal for a far end, away from the OTDR, along a fiber. As discussed further below, the optical signals may be stitched into a single OTDR trace in a horizontal direction of the trace, where the horizontal direction of the trace may represent distance from the OTDR. 
         [0020]    Various methods for processing light signals according to exemplary embodiments include, increasing power of light pulse, repeating and averaging measurements, increasing duration of a light pulse and filtering captured traces, at least to reduce noise levels. 
         [0021]    The selector  106  receives the electrical signals corresponding to the light signals, having at least respectively different pulse width, and may be controlled by the processor  107  to select various ones of the electrical signals and various portions of the electrical signals for being output to an OTDR trace by the display  108 , as further discussed with respect to the flowcharts of  FIGS. 4 and 5 . 
         [0022]    The processor  107  further stores such data in a memory  109  and processes the stored data to provide outputs to at least the display  108 . 
         [0023]      FIG. 2  is a view of a selector section  2000  including the selector  106  having an input  210  and an output  220 . The input  210  and the output  220  may each represent more than one signal path according to exemplary embodiments. A selection unit  205  of the selector  106  operates in conjunction with the processor  107  or may autonomously operate to determine signals and portions of signals to be displayed by the display  108 . The selector  106  further includes receivers  201 - 205  each configured to receive and buffer signals corresponding to at least respective pulse-width denoted by “L”. 
         [0024]      FIG. 3A  illustrates a graph  3000   a  including various OTDR traces  301   a - 303   a  each representing light signals having different pulse-width. For example, trace  301   a  may represent a light signal having a 5 ns pulse-width; trace  302   a  may represent a light signal having a 30 ns pulse-width, and trace  303   a  may represent a light signal having a 300 ns pulse-width. 
         [0025]    To reduce scaling errors of the traces, backscatter levels of at least two traces obtained by using different pulse-width may be normalized by multiplying each trace by a product of pulse-width, number of averages and gain of another trace, according to exemplary embodiments. These coefficients could be divided by a maximum common denominator before the normalization to avoid processing overflow. Although at least two traces may be normalized as described above, more than two traces may also be normalized similarly by multiplying each trace by a product of pulse-width, number of averages and gain of another trace, according to exemplary embodiments. 
         [0026]    The trace  302   a  ends its respective dead-zone and intersects the trace  301   a  at stitching point  310 . The trace  303   a  ends its respective dead-zone and intersects the trace  302   a  at stitching point  311 . 
         [0027]    As illustrated in  FIG. 3A , the trace  301   a  experiences the shortest dead-zone, and the trace  303   a  experiences the longest dead-zone. Further, the trace  301   a  experiences the greatest amount of noise after its respective dead-zone, and the trace  303   a  experiences the least amount of noise after its respective dead-zone. The shorter pulse-width trace  301   a,  although noisier than the longer pulse width trace  303   a,  exhibits optical characteristics at a shorter fiber distance than the longer pulse-width trace  303   a  at least because of the shorter dead-zone of the shorter pulse-width trace  301   a;  further, the characteristics at the shorter fiber distance exhibited by the shorter pulse-width trace  301   a  are not exhibited by the longer pulse-width trace  303   a  because of its respective dead-zone. 
         [0028]      FIG. 3B  illustrates a graph  3000   b  having trace portions  301   b - 303   b  stitched together from the traces  301   a - 303   a  of graph  3000   a.  For example, as trace  301   a  experiences a shortest dead-zone by representing at least a shortest pulse-width, the portion  301   b  of the trace  301   a  is selected as a portion to be displayed by the graph  3000   b.    
         [0029]    The trace  302   b  experiences less noise than the trace  301   a;  however, the trace  302   b  has a longer dead-zone. The dead-zone of the trace  302   b  ends at stitching point  310  and therefore, the trace  302   b  may be prioritized over the noisier trace  301   a.  A trace portion trace  302   b  of the trace  302   a  is stitched to the noisier trace portion  301   b.    
         [0030]    Although trace portion trace  301   b  is noisier than trace portion  302   b,  the trace portion  301   b  provides non-dead-zone data at a shorter distance than could be reliably represented the longer pulse represented by the trace  301   b.    
         [0031]    Further, the stitching point  311  represents a point where the dead-zone of the trace  303   a  has ended and is prioritized over the noisier trace  302   a,  and therefore, the trace portion  303   b  may be stitched at stitching point  311  thereby providing a less noisy signal at greater distances from the OTDR. 
         [0032]    The stitching may be progressive rather than sudden. For example, areas around the stitching point  310  and stitching point  311  may be ratios of the signals about the respective points. The ratio may be different at each stitching point. 
         [0033]    Further, the traces  301   a - 303   a  also have different gains applied thereto respectively in addition to representing different pulse-width signals and therefore may reduce scaling of an OTDR trace. 
         [0034]    Meanwhile, if the data of the stitched OTDR trace comes from either the shorter pulse-width signal or the longer pulse-width signal, there may be one unused trace. According to exemplary embodiments, the longer pulse-width signal may be replaced by a combined trace from the two traces according to the following formula: 
         [0000]        L ′( n )= a*L ( n )+ b*S′ ( n )   (1)
 
         [0035]    L(n) denotes a longer pulse-width signal, S′(n) denotes a moving average filtered version of a shorter pulse-width signal S(n); “a” and “b” are two parameters to be chosen such that a ratio of total noise of L′ (n) to combined minimizes a backscatter noise level. It is noted that the pulse-width of S(n) is Ps, the shorter pulse-width, and in order to obtain S′(n) in view of the longer pulse-width P 1  as L(n), the optimum moving average filter length to compute S′(n) from S(n) should be P 1 -Ps. 
         [0036]      FIG. 4  illustrates flowchart  4000  of an algorithm performed according to exemplary embodiments. At S 400  an OTDR receives signals representing at least different pulse-width. 
         [0037]    At S 401 , the OTDR determines that the signals rise spontaneously and amplitude of a short pulse-width trace is greater than amplitude of a longer pulse-width trace. 
         [0038]    At S 402 , the OTDR adds data corresponding to the short pulse-width signal even though this signal may be noisy. A filtered version of the short pulse-width signal may be added. 
         [0039]    At S 403 , the OTDR compares the amplitude of the short pulse-width signal to that of a longer pulse-width signal. 
         [0040]    At S 404 , the OTDR has determined that the amplitude of the short pulse-width signal is greater than that of the longer pulse-width signal, and therefore, the longer pulse-width signal remains in a dead-zone. Processing returns to S 403 . 
         [0041]    This process may continue until the short pulse-width trace has a number of points having negative value exceeding a defined threshold such that the short pulse trace no longer has a sufficient dynamic range. From this point, the short pulse-width trace may only be selected in case of a large spontaneous rise of signal level, according to exemplary embodiments. 
         [0042]    However, at S 405 , the OTDR has determined that the amplitude of the short pulse-width signal is less than or equal to that of the longer pulse-width signal, and therefore, the longer pulse-width signal is outside of its dead-zone and the OTDR stitches the longer-pulse-width signal or a filtered version of the longer pulse-width signal onto an OTDR trace. The above formula may represent multiple ratios for any of a first light signal and subsequent light signals having respectively different pulse-width. 
         [0043]    Although exemplary embodiments of the disclosure have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these exemplary embodiments without departing from the principles and spirit of the exemplary embodiments, the scope of which is defined in the claims and their equivalents. 
       REFERENCE SIGNS LIST 
       [0044]    OTDR  100   
         [0045]    light generation section  101   
         [0046]    light I/O  102   
         [0047]    exit path  103   
         [0048]    return path  104   
         [0049]    detection section  105   
         [0050]    selector  106   
         [0051]    processor  107   
         [0052]    display  108   
         [0053]    memory  109   
         [0054]    receivers  301 - 305   
         [0055]    selection unit  205   
         [0056]    input  210   
         [0057]    output  220   
         [0058]    traces  301   a - 303   
         [0059]    traces  301   b - 303   b    
         [0060]    stitching point  310   
         [0061]    stitching point  311   
         [0062]    OTDR system  1000   
         [0063]    selector section  2000   
         [0064]    graph  3000   a    
         [0065]    graph  3000   b    
         [0066]    flowchart  4000