To further improve the communication capacity

a wavelength-mode division multiplexer/demultiplexer based on photonic crystals is proposed.Consisting of four combination resonator

four wavelength selective reflection cavities

and a pair of asymmetric parallel waveguides for mode conversion

the wavelength-mode division multiplexer/demultiplexer is realized by adjusting the radius and refractive index of the cavity as well as the width and the coupling length of the asymmetric parallel waveguide.Finite Difference Time Domain（FDTD）methods are used to simulating the performance of mode division multiplexer/demultiplexer.Numerical simulations show that the designed device has the potential for high-capacity Wavelength Division Multiplexing-Mode Division Multiplexer（WDM-MDM）optical communication systems with a low insertion loss<0.11 dB and a low mode crosstalk <-16 dB

which can meet the requirements of today’s large capacity optical communication system.