In measuring the attenuation of multimode graded index fiber, differences have been noted between the results of the cut-back method and the optical time domain reflectometry (OTDR) method. Under certain conditions, the OTDR results can be higher by 0.1 to 0.3 dB/km than the cut-back results. In attempting to understand the origin of these differences, we have characterized the attenuation using several different OTDRs and measured the differential mode attenuation (DMA) and intensity profiles of the fibers. Using this information and backscatter intensity, we created a model to simulate both OTDR and cut-back measurements. The model showed that the DMA results could account qualitatively for the large difference between OTDR and cut-back methods, and also for larger differences when the OTDR had a smaller launch spot size. When the spot size is small, a larger fraction of the light is confined to the center of the core. If the fiber under test has a defect at the core center, the OTDR will measure a larger attenuation than the cut-back method.
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机译:在测量多模渐变折射率光纤的衰减时,已注意到切回法和光时域反射法(OTDR)方法的结果之间存在差异。在某些条件下,OTDR结果可能比缩减结果高出0.1至0.3 dB / km。为了理解这些差异的起因,我们使用几种不同的OTDR对衰减进行了表征,并测量了光纤的差模衰减(DMA)和强度分布。利用这些信息和反向散射强度,我们创建了一个模型来模拟OTDR和缩减测量。该模型表明,DMA结果可以定性地解释OTDR和削减方法之间的巨大差异,并且当OTDR的发射点尺寸较小时,也可以解释较大的差异。当光斑尺寸较小时,较大部分的光将被限制在纤芯的中心。如果被测光纤的纤芯中心存在缺陷,则OTDR的衰减将比缩减方法大。
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