Low-DMD few-mode fiber with distributed long-period grating.

We propose a distributed grating-assisted few-mode fiber (DG-FMF) to reduce the differential mode delay (DMD). By introducing asymmetric long-period fiber gratings with random exposure directions in a step-index few-mode fiber during fiber manufacture process, the strong random mode coupling is generated along the entire length of the fiber. Simulation results show that mean DMD in the DG-FMF can be reduced to less than 12 ns at a distance of 100 km with a period of 514 μm and grating strength of 1×10(-6). The DMD reduction in our DG-FMF is guaranteed within the entire C-band, and in a wide temperature range from -20°C to +60°C.