Measuring modal delays of few-mode fibers using frequency-domain method

A simple and robust frequency-domain method for measuring modal delays of few-mode fibers is presented. The method utilizes the inverse Fourier transform of a measured complex transfer function to determine modal delays between different modes. Through proper treatments of the data, we can de-alias the signal and obtain the accurate values of the time of flights and the modal delays in the right time sequence. The method has been validated through comparing with direct time-domain measurements. A few-mode fiber supporting four modes has been studied in detail with wavelength-resolved time of flights and modal delays obtained. We also illustrate how the new measurement method can provide a way to measure the crosstalk for mode division multiplexing system. The method can be readily used for FMF measurements without obvious limitation on the number of modes involved or measurement wavelength.