On-chip Mode Splitter Based On Planar Optical Waveguides

Mode splitter is a kind of passive photonic devices which is used to split beams of different modes from a flow of light signals and lead them to other waveguides.In recent years,with the development of Mode-Division-Multiplexing,mode splitter has gradually come under the spotlight of tremendous researchers,and it also has been widely used in areas such as optical communication system,photonic device fabrication and optical interconnect.In this thesis,mode theory was exploited to change the structures of silicon waveguides.And then,a mode splitter without changing the mode order and a polarization beam splitter which is able to split all the TM modes are designed and simulated via numerical calculation methods.First,we investigated the development of mode splitter both in domestic and abroad,noting that the current polarization beam splitter can only work at fundamental modes and the mode order changes inevitably when a specific mode is split.And then,we researched four kinds of numerical calculation methods and compared the difference of Beam Propagation Method and Finite-Difference Time-Domain method.The structure model of planar waveguides and mode coupling theory were introduced in the following part,in which the significance of disturbance was highlighted.In the fourth part,after researching the characteristics of modes in strip and slot waveguides,we found that when an air slot was introduced to a strip waveguide,the distribution and effective index of TE₁ mode changes little,but that of the TE₀mode alters significantly.Based on this discovery,we designed a mode splitter without changing the mode order.The simulation via FDTD method suggested that high coupling efficiency?>-1 dB?,low mode crosstalk?<-16 dB?,wide bandwidth?¹00 nm?and large fabrication tolerance?±10 nm?can be obtained.After the structure was improved,BPM method was used to verify the rationality of the proposed device.It's found that the device can not only split TE₁ mode without changing the mode order,but also split modes of higher orders without changing the mode order.At last,we designed a polarization beam splitter which is able to split all the TM modes.The simulation via FDTD method indicated that the extinction ratio was higher than 10 dB over a bandwidth of 60nm while all the TE modes propagated along the input waveguide with low loss of less than-0.5dB.With a variation of10nm in the width,the device could also work well.