为了有效降低光接入网系统的成本，提出了一种利用低模式串扰复用器、解复用器和少模光纤实现的双向波长重用的模式复用接入网结构。在下行传输中，少模光纤中的一个模式用于传输光载波，其他模式用于传输下行信号。在上行传输中，光载波被分束到所有的光网络单元，用于上行信号的重新调制。通过利用低串扰的模式复用/解复用器和少模光纤，载波和各个信道可以有效地分离。进一步地，为了减少双向瑞利反向散射噪声，提出上下行传输不同的边带信号。利用该方案，实现了双向对称的基于强度调制和直接检测的2×12.5 Gbit/s正交相移键控正交频分复用信号在4模式11.5 km少模光纤中的传输。

To effectively reduce the cost of optical access network, a wavelength reused Mode-Division-Multiplexing (MDM) scheme for bidirectional optical access network with low mode-crosstalk Multiplexer/Demultiplexer (MUX/DEMUX) and Few Mode Fiber (FMF) is proposed. In downstream (DS) transmission, one of the spatial modes in FMF is used to transmit optical carrier while the others are used as DS signal channels. In upstream (US) transmission, the carrier is split to all the ONUs for US re-modulation. By utilizing mode MUX/DEMUX with low mode crosstalk and FMF, the carrier and each signal channel can be effectively separated. Moreover, to reduce the bidirectional Rayleigh backscattering noise, different optical sidebands for DS and US transmission are proposed. With the proposed scheme, symmetrical bidirectional 2×12.5 Gbit/s Quadrature Phase Shift Keying (QPSK) Orthogonal Frequency Division Multiplexing (OFDM) Intensity Modulation and Direct Detection (IM-DD) transmission over 11.5 km 4-mode FMF is experimentally demonstrated.