Four kinds of wavelength division multiplexer in silicon photonics are introduced, the silicon nanowire arrayed waveguide grating and the etched diffraction grating can increase output channels easily, which fits to high count dense wavelength division multiplexer application; multi-count Mach-Zehnder inteferometer (MZI) and micro-ring wavelength division multiplexer need to be cascaded, but they can not control their wavelength and channel space easily, which fits to low count application. Meanwhile, the silicon nanowire arrayed waveguide grating and the etched diffraction grating are designed and fabricated. The crosstalk of the arrayed waveguide grating by widening the arrayed waveguide width is less than -15 dB, and the insertion loss is reduced by 3 dB using a 2D photonic crystal mirror. Besides, the related monolithic silicon photonic chips are also shown.