Research On Polymer Optical Waveguide Devices For On-board Optical Interconnects

With the increasing requirements of high-performance computers in terms of high-speed,large-bandwidth,low-loss,and large-capacity transmission,the traditional printed circuit board（PCB）based on the electrical interconnection has been increasingly difficult to meet the demands of the high-performance transmission due to their limitations in electromagnetic interference,bandwidth,and power consumption.In contrast,the optical interconnect has the advantages of anti-electromagnetic interference,high-speed data transmission,large bandwidth,and low power consumption,and has become an effective solution to PCB based on electrical interconnects.The polymer optical waveguide has the advantages of good flexibility,low cost,high stability,simple preparation process,etc.And its preparation process is compatible with PCB technology,which becomes a pivotal transmission medium for the board-level optical interconnects.It is very significant to research high-performance passive optical waveguide devices for module connection,optical splitting,filtering,multiplexing,and other functions to realize the board-level optical interconnects with high flexibility,high density,and complex functions.In this paper,we carried out the research on polymer passive optical waveguide devices for board-level optical interconnects,including the polymer S-shaped waveguide for connecting modules and mode（de）multiplexers for increasing transmission capacity.The finite element method（FEM）and beam propagation method（BPM）are used to design structures and analyze their transmission performance.The main research contents and results are as follows:1.Structural design and transmission performance analysis of the polymer S-shaped waveguides for board-level optical interconnects.A novel S-shaped waveguide with offsets and trenches is proposed to reduce the large bending loss of the low-index-contrast S-shaped polymer waveguide.Firstly,we used the FEM in the COMSOL software to determine the single-mode transmission condition,and we established the S-shaped waveguide with offsets and trenches（SW-OT）model in RSoft software.Then,we analyzed the influences of parameters on the bending loss of the S-shaped waveguide by the BPM,and the parameters include the offset,trench width,distance between trenches and the waveguide,and the refractive index of the medium filling in trenches.Finally,we compared the bending loss of the standard S-shaped waveguide without offset and trenches,the S-shaped waveguide with only offsets,the S-shaped waveguide with only trenches,and the SW-OT structure.The results show that the bending loss of the SW-OT is only 0.11d B when the bending radius is1000μm,which is 89.70%lower than that of the S-shaped waveguide with only offsets,and when the bending radius is greater than or equal to 2000μm,the bending loss is less than 0.1d B.Therefore,using both the offsets and the trenches can effectively reduce the bending loss of the low-index-contrast S-shaped polymer waveguide,and realize the low-loss transmission of the S-shaped polymer waveguide with a small bending radius.2.Structural design and transmission performance analysis of the polymer mode（de）multiplexer for board-level optical interconnects.A polymer mode（de）multiplexer based on cascaded tapered couplers is proposed aiming at the demand for large-capacity information transmission in the board-level optical interconnects.Firstly,we analyzed the modes of the rectangular waveguide in COMSOL software and calculated the effective refractive index of the supported modes by rectangular waveguides with different widths to determine the waveguide size.A mode（de）multiplexer structure model based on cascaded tapered couplers was established in RSoft software.Then,we analyzed the influence of parameters on the mode coupling by the BPM.These parameters include the width of the tapered waveguide,the height of the tapered waveguide,the spacing of the tapered waveguide,and the uniformity of the waveguide surface.The performance indicators of this device were calculated for TE and TM polarization within the C+L band,respectively.The performance indicators include the mode coupling ratio,the extinction ratio,and the excess loss.The results show that,for the multiplexing and demultiplexing process of the device,the coupling ratio of the E₁₁,E₂₁,and E₃₁ modes is greater than 0.97.The excess loss is less than 0.15d B,and the extinction ratio is greater than 20d B.Besides,this device has friendly fabrication tolerance and weak polarization sensitivity,which is expected to be applied to the mode-division multiplexing system on large-bandwidth board-level optical interconnects.