Polymer Waveguides And Devices For Board-level Optical Interconnects

The demands of short-reach optical interconnects for high-speed and high-density data transmission increase dramatically due to the rapid development of large data centers and high-performance computers,which gives an ever increased burden on the copper-based onboard optical interconnects.On the other hand,optical interconnects have drawn significant attentions thanks to their advantages in bandwidth,integration density,power consumption and electromagnetic compatibility.Optical interconnects are now developing from long-haul communications to board-level interconnects.Among the transmission mediums for boardlevel optical interconnects,polymer waveguide is a power-efficient and cost-effective solution due to its low processing and material cost,high integration density,as well as the excellent compatibility with printed circuit boards(PCBs).Multimode polymer waveguides combined with the on-board optical transceivers at 850 nm becomes one of the most competitive solutions for board-level optical interconnects.Additionally,on-chip optical interconnects is considered the most effective solution for next-generation high-bandwidth and low-power chip.Siliconbased optoelectronic integration operating at long wavelengths is the most promising solution for on-chip optical interconnects.Hence,single-mode polymer waveguides operating at 1310 nm and 1550 nm are considered to be the key transmission medium for high-bandwidth and highdensity optical interconnects from chip to chip.Besides the polymer waveguides,waveguide devices are essential for board-level optical interconnects with specific function and topology.Moreover,three-dimensional(3D)waveguide devices need to be developed urgently for highdensity integration.This dissertation focuses on the high-bandwidth and high-density optical interconnects.The research contents that include polymer waveguides and devices are organized as follows:1.Design and fabrication of multimode polymer waveguides with high performance and wide bandwidthLow-loss multimode polymer waveguides combined with the low-cost and high-density optical transceivers at 850 nm is a main solution for board-level optical interconnects.However,the multimode dispersion limits their transmission bandwidth and distance.Moreover,for the industrial application with 1-m-long backplane,the practical performances of the multimode waveguides need to be evaluated comprehensively.We aim at improving the comprehensive performance of the multimode waveguides by choosing the suitable materials and optimizing the fabrication process.Research contents about the multimode polymer waveguides are as follows.1)Design,fabrication and performance evaluation of meter-level multimode polymer waveguidesBy choosing suitable sets of materials and optimizing the fabrication process,meter-level polymer waveguides with low loss(0.046 dB/cm)and low crosstalk(<-56 dB)are fabricated successfully.PAM4 data transmission up to 56 Gb/s is realized over the meter-level multimode waveguide.Moreover,the waveguides show good environmental stability after the aging test and five solder reow cycles.The experimental results demonstrate that the fabricated multimode polymer waveguides are good candidates for high-speed and meter-scale on-board optical interconnects.2)Design and fabrication of new-type polymer waveguides based on 3D direct writing techniqueMultimode polymer waveguides with circular cores are fabricated successfully by the 3D direct writing technique.The fabricated waveguide exhibits a low loss(<0.044 dB/cm),a low crosstalk(<-58 dB)and negligible coupling loss with multimode fibers(MMFs).750-mm-long waveguide in spiral design is fabricated successfully,and error-free NRZ signal transmission at 25 Gb/s is realized.Moreover,4×25 Gb/s SWDM transmission is realized on a straight polymer waveguides for the first time.The 3D direct writing technique shows great potential in fabricating polymer waveguides for high-bandwidth and high-density optical interconnects.2.Design and fabrication of single-mode polymer waveguides at 1550 nmSingle-mode polymer waveguides operating at 1310 nm and 1550 nm are considered to be the key transmission medium for board-level high-speed optical interconnects while the siliconbased on-chip optical interconnects is realized.Compared with the multimode waveguide,the small core size of the single-mode waveguide makes it more stricter to the fabrication process and more sensitive to the roughness of the sidewall.We demonstrate the design and fabrication of circular-core 1550 nm single-mode polymer waveguides with graded-index profiles.The waveguides are fabricated by commercially available UV-curable epoxies using 3D direct writing method.Due to the immunity to the sidewall roughness,the crosstalk of waveguides fabricated by the 3D direct writing technique is 20 dB lower than that of the waveguides fabricated by the lithograph method.The coupling loss with single-mode fibers is as low as 0.78 dB.Meanwhile,error-free NRZ signal transmission at 25Gb/s is realized.The experimental results imply that the fabricated waveguides are promising for single-mode board-level optical interconnects with high speed,high density and low crosstalk.3.Design and fabrication of key passive devicesIn order to realize complicated functions of board-level optical interconnects,polymer waveguide devices are investigated as follows:1)Design and fabrication of splitter/combiner1 × 32 planar Y-splitter/ combiner is fabricated using the lithography method,which shows good uniformity among all output ports,and the maximum imbalance is 1.39 dB.1 × 4 3D Y-splitter/combiner is designed and fabricated by the 3D direct writing technique for highdensity 3D integration.The fabricated device realize the communication between waveguides in multilayers.The insertion loss of the 1 × 4 splitter is 1.1 dB,and the maximum imbalance is1.24 dB.When working as a 4 × 1 combiner,the average insertion loss is as low as 1.73 dB.2)Design and fabrication of the 3D directional coupler3D directional coupler is fabricated by the 3D direct writing technique for 3D integration in single-mode board-level optical interconnects.Coupling ratio of 58 : 23 : 19 at 1550 nm is realized by optimizing the design of the device to reduce the turbulence of the monomers.3)Design and fabrication of the mode(de)multiplexerIn order to increase the transmission capacity further,we apply the mode division multiplexing(MDM)technique to board-level optical interconnects.We fabricated the mode(de)multiplexer based on the tapered mode-selective directional coupler using the 3D direct writing technique.The fabricated mode(de)multiplexer has a coupling ratio of higher than 0.93 and an extinction ratio of larger than 23 dB within a broadband of 100 nm around 1550 nm.This dissertation demonstrates the great potential of polymer waveguides and devices for high-bandwidth and high-density board-level optical interconnects.Moreover,multiple polymer waveguides and devices have been realized by using the 3D direct writing technique,which implies that the 3D direct writing technique is promising for high-bandwidth,especially 3D high-density board-level optical interconnects.