In this study, we have developed a 6×6 mode-division-multiplexing communication experimental system using a photon lantern as the mode multiplexer/demultiplexer. Further, we can achieve a signal transmission rate of 6×8.5 Gbit/s in a 10-km few-mode fiber using a phase modulation and coherent detection system via offline digital signal processing. The results denote that the bit error rate of each signal can be less than 10 -3 when the received signal powers of LP01, LP11a, LP11b, LP21a, LP21b, and LP02 are -37.84 dBm, -36.47 dBm, -36.20 dBm, -35.27 dBm, -35.37 dBm, and -35.79 dBm, respectively.