Study On Bend-insensitive Few-mode Fibers

With the rapid increasing of the capacity of fiber optical transmission systems, Mode Division Multiplexing (MDM) in fibers appears as a promising technological path to increase the capacity of optical networks through an increase in the capacity per fiber strand. Recently, there has been considerable interest in the study of few-mode fibers for MDM applications. However, few researches on the bend performance of high order modes in few-mode fiber are involved so far.In this paper, we systematically investigated the bending losses of higher order modes of few-mode fiber. The theoretical formulas of bending loss for high-order mode are derived by using perturbation method, and these formulas are suitable for investigating the bending performance of few-mode fibers with complex configuration.The bend insensitive trench-assisted step-index fiber and parabolic-index fiber are investigated in terms of bending losses and the mode field diameter (MFD) for LP01, LP11LP21and LP02mode. The results show that the distance between the core and the trench layer, the width of the trench layer, the refractive index of the trench layer (or the coating layer), the normalized frequency, and the core radius (numerical aperture) are all the influencing factors for the bending loss and MFD of each mode. With the same bend radius, the bending loss increases with increasing order of mode. There is an optimization point of bending loss existing for the core to trench distance, and for higher mode, this optimization point is closer to core layer than that of the fundamental mode. The coating layer could give rise to resonance of the bending loss curve, and the amplitude and the frequency of the resonant curve for high-order modes are higher than that of the fundamental mode. The MFD of each mode could be inhibited by the trench layer, especially for high order modes. Therefore, the MFD could be increased by increasing the core to trench distance. In comparison with trench-assisted step-index fiber, the trench-assisted parabolic-index fiber provides a better bending performance with the same number of propagation mode. The conclusions are of significance for the design and manufacture of bend-insensitive few-mode fibers.