Design Of Multi-Channel Mode (De) Multiplexer Based On Micro-Waveguides

Wavelength division multiplexing technology is developed to improve the transmission capacity and bandwidth of optical communication systems and meet the demand of exponential network traffic growth.However,the capacity of the traditional wavelength division multiplexing system has almost reached the limit under the Shannon limit of nonlinear optical fiber transmission.It is necessary to further improve the data transmission capacity of the system and the capacity of a single wavelength channel.Researchers have proposed new multiplexing technologies:polarization division multiplexing and mode division multiplexing.Polarization division multiplexing and mode division multiplexing technologies use dual-polarization and multi-mode technology to realize parallel channel data transmission at multiple wavelengths.These technologies can not only save physical space but also reduce system costs.In particular,the mode division multiplexing technology has a higher information transmission density and is considered to be the most promising technology.Based on the mode division multiplexing system,this paper proposes and designs some high-performance and powerful silicon photonic integrated mode（de）multiplexers by properly mixing wavelength division multiplexing and polarization division multiplexing technologies.The main research work is as follows:（1）Optical waveguide is a dielectric device that guides the propagation of light waves.It is an important part of photonic integrated devices.Based on the study and analysis of optical waveguides,this paper discusses the mode field distribution,coupled-mode theory,and numerical calculation method for designing devices in silicon-based optical waveguides.（2）A design method of an asymmetric directional coupler is proposed.The directional coupler is cascaded on the SOI platform to realize a dual-polarization 10-channel mode（de）multiplexer.In the mode division multiplexing system,the application of hybrid polarization division multiplexing technology is realized.The designed（de）multiplexer consists of three cascaded coupling sections and a polarization beam splitter.Each coupling section includes two or four parallel cascaded asymmetric directional couplers for（de）multiplexing TE₁～TE₅and TM₁～TM₃high-order modes.And the polarization beam splitter is used to combine/separate TE₀and TM₀fundamental modes.In order to meet the phase matching conditions and efficiently perform mode coupling conversion,the sizes of the drop waveguides and bus waveguides are optimized according to the mode dispersion curve of the silicon waveguide.To effectively reduce the footprint,the directional couplers in this device are cascaded in parallel so that the total coupling length is only 59.3μm.The simulation results show that the device has large working bandwidth（～80 nm）and good performance.It has low insertion loss（>-0.99 dB）and low mode crosstalk（<-15.6 dB）in the wavelength range of 35 nm.（3）A tapered adiabatic coupler is designed.A 6-channel mode（de）multiplexer is realized by cascading four tapered adiabatic couplers and a directional coupler.Four tapered adiabatic couplers are optimized for（de）multiplexing TE₁～TE₄mode,TE₀mode is input/output along the bus waveguide,and the directional coupler is used for（de）multiplexing TE₅mode.The experimental results show that the mode（de）multiplexer has low insertion loss（>-0.99 dB）and mode crosstalk（<-21 dB）in the operating wavelength of 1500～1600nm.In addition,within the waveguide fabrication tolerance range（±40 nm）,the insertion loss of the mode is greater than-2.4 dB and the crosstalk is less than-15.2 dB.（4）A microring resonator with wavelength selectivity is designed.A 4-channel mode multiplexer based on cascaded microring resonators is implemented on the SOI platform.By optimizing the structural parameters of the microring resonator and controlling its working state,the coupling conversion between the fundamental mode and any mode is realized.The simulation results show that within the working bandwidth of 1520～1580 nm,the device can realize four modes of TE₀～TE₃multiplexing and simultaneously support multiple wavelength channel multiplexing.It realizes the function of mode division multiplexing is compatible with wavelength division multiplexing.The experiments also show that it has low mode crosstalk（>-3.1 dB）and insertion loss（<-15.8 dB）at 1552 nm.