An efficient scheme of intermodal distributed Raman amplification using tailored doping profiles in spatial-division multiplexed coherent fiber-optic transmission systems

For next-generation long-haul communication systems, space-division multiplexing (SDM) is suggested as a promising technique for providing orders of magnitude increase in transmission capacity. Optical amplifiers for multimode fibers are the crucial components to realize SDM systems, while few-mode fiber (FMF) has the advantage of strong intensity overlap between different modes that allows sharing of single pump across multiple signal modes. FMF-based erbiumdoped fiber amplifiers (FM-EDFA) have been intensively studied. Since SDM is exceedingly reliant on DSP technique and thus has more stringent requirement on noise performance, FMF-based distributed Raman amplifier (FM-DRA) benefits from relatively lower noise figure (NF) compared with FM-EDFA. Yet much less work has been done in this area. To implement FM-DRAs in SDM systems, their performance should be carefully optimized. In this paper, modal gain and saturation characteristics of intermodal Raman amplification in FMFs have been fully investigated by tailoring the refractive-index profiles and doping levels of different FMF designs. For 50-km-long FMF with -3 dBm signal at 1550 nm and a 20-25 dBm pump at 1455 nm, the optimized modal-equalization of Raman gain and NF are provided depending on various fiber cross-section and pumping configuration with respect to modes/wavelengths. The pumpsignal modal-overlap integrals for each of the four mode-groups with normalized frequency V=5 have been exploited, resulting in a mean gain of 10 dB within 1 dB of equalization for 16 gain coefficients. Our results show it should be possible to design FMF with larger intermodal nonlinearity and better dispersion characteristics to achieve desirable Raman gain.