Three-dimensional photonic devices can process multiple signals simultaneously in terms of multi-dimensional space division multiplexing

which increase the transmission capacity and improve the integrated density.In this paper

based on selfimage principle of 3D Multimode Interference（MMI）coupler

a 3D Beam Splitter（BS）with mode conversion is designed

which can be easily integrated.The 3D-BS with mode conversion can convert the input beam at the fundamental mode into three output beams at the fundamental mode and three output beams at the first-order mode.The length

width and height of the MMI are designed with polymer materials and optimized to be 3 398.17

50.00 and 29.37μm respectively.The simulation results show that the total transmission efficiency of the BS is 85.5%

and the transmission efficiency of the output fundamental mode is 28.9% with the maximum imbalance of 0.010 dB.The conversion efficiency of the output first-order mode is 56.6% and its maximum imbalance is 0.004 dB.