Understanding differences between OTDR and cut-back attenuation measurements in multimode graded index fiber

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.