Differential mode delay diagnostics for LP modes traversing a few-mode fiber.

We provide differential mode delay (DMD) diagnostics for linearly polarized (LP) modes traversing a few-mode fiber (FMF). We employ a modal interferometer method with a butt coupling mechanism to measure the DMD between any two modes guided in FMFs with step-index (SI) and depressed-cladding profiles. The measurement principle is based on investigating a transmitted spectrum through temporal decomposition by means of a Fourier transform. At least four different temporal waveforms are needed for an FMF supporting four LP modes to estimate the DMD between two different LP modes. A butt coupling mechanism with an air gap between an FMF and a dispersion-shifted fiber is employed for simultaneous DMD measurement. The present diagnostics requires some knowledge of the bending loss characteristics of the FMFs and the butt coupling properties of the circular symmetry modes or circular asymmetry modes in their electric fields. The DMD diagnostics are realized taking the bending loss and butt coupling characteristics into consideration. We present experimental results for the DMD and DMD slope as a function of the wavelength in a 1450-1625 nm telecommunication band.