Simultaneous Measurement of Differential Modal Group Delay in Few-Mode Fibers on the Basis of Optical Time Domain Reflectometer with Delay Lines

Mode-division multiplexing (MDM) transmission using few-mode fibers (FMFs) has been studied as a promising approach to expand transmission capacity per fiber. In MDM transmission system, the computational load of the system increases with the differential modal group delay (DMD) between the propagation modes. Therefore, evaluating the DMD of FMF is necessary to implement MDM transmission systems. In this paper, we propose a method to simultaneously measure the DMDs for all linearly polarized (LP) modes in FMFs. The method involves using a commercially available optical time domain reflectometer (OTDR). It measures the round-trip impulse responses for all LP modes simultaneously using delay lines of different lengths and a mode-selective coupler. The DMDs for all LP modes are obtained from these round-trip impulse responses. By using commercially available OTDR and delay lines, the proposed method can measure the DMD easily and reasonably compared with the conventional method. We use delay lines for each mode to distinguish the signal of each mode and crosstalk component. We also clarify the conditions for the length of delay lines given to each mode. We measured the DMD for two types of FMFs to verify the applicability of the proposed method.