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An eight-channel 400 GHz-spacing planar waveguide multi/demultiplexer employing etched diffraction grating is designed and fabricated on a silicon-on-insulator platform with a 220 nm thick top layer.The design parameters are optimized by scalar diffraction simulation to obtain optimal performance with a small footprint of only 0.75 mm2.A bi-level adiabatic taper is used to connect the input/output waveguide and the slab waveguide so as to reduce the insertion loss and the broadening of the diffraction angle of the propagating light.The device is fabricated in a two-step process with E-beam lithography and dry etching,and the alignment accuracy is 1Onm or even better.Measurements show that the insertion loss is 7.35 dB and the crosstalk between adjacent channels is about-15 dB.Ways to improve the performance are also investigated in detail.