Research On Silicon-based Integrated Subwavelength Mode Division Multiplexing Photonic Devices And Their Inverse Design

With the rapidly development of 5G communication,clouding computation,data center,on-chip optical interconnect on SOI platform,as a more promising and attractive technology,has attracted tremendous attention to alleviate the communication bottleneck and satisfy the rapidly increasing demands for high transmission capacity,high density integration and low power consumption.Mode division multiplexing technique enables us to use multiple spatial modes as independent single carriers to transfer data information,providing a new and efficient solution to enhance the transmission capacity in a single wavelength carrier.Conventional waveguide-based silicon multimode devices suffer from large footprint,which limit the development of monolithic high densely integrated mode division multiplexing systems.On the other hand,Recently,enabling one to engineer the refractive index distribution flexibly and manipulate light field at the nanoscale,inverse design method have drawn tremendous attention to realize ultra-compact and highly functional on-chip devices simultaneously.However,there are two major challenges in theoretical research and practical application for the inverse designed nanophotonic devices.Firstly,in terms of theoretical research and optimization methods,the current inverse design method lacks of a clear theoretical model.More specially,for a given a functionality,design footprint,device materials and ultimate device performance are not clear and lack of theoretical prediction.The optimization process is extremely sensitive to the initial random pattern,which suffer from the low design efficiency.Secondly,the etching feature size of inverse-designed nanophotonic device is usually random and irregular,which severely suffer from lag effect and restrict the widespread use of inverse-design methods in practical applications.In this thesis,we foucs on these two common scientific problems of inverse design method and silicon photonic integrated subwavelength multimode devices and inverse design method,make extensive research,and also make great progress.The main research contents are as follows:(1)In this thesis,we propose and establish a set of physical model,efficient optimization method and fabrication procedure for the inverse-designed silicon photonic integrated devices.First,we have established physical model of subwavelength photonic devices and proposed the novel theory-assisted inverse design method.According to the optical physical theory model,we can pre-set high performance device initial pattern and minimum topology structure,and then achieve the novel theory-assisted numerical optimization inverse design method.By greatly overcoming randomness of conventional inverse design optimization process,we achieve the high optimization efficiency of inverse design method for ultracompact and high perform integrated silicon photonic devices.Second,we propose the Ph C-like subwavelength structure and make full use of its uniform feature size to relax the fabrication errors of lag effect,which is utilized to greatly improve the robustness and manufacturability of the device and pave the way for the large-scale application of inverse design.In this way,we designed and experimentally demonstrated a variety of multimode waveguide devices with high performance ultracompact footprint and fabrication robustness.(2)We proposed and experimentally demonstrated an ultracompact silicon photonic subwavelength multimode multiplexer based on the subwavelength structure asymmetric Y-junction.According to equivalent theoretical model,a theory-assisted inverse design method is proposed and mannually-preset initial pattern are introduced to reduce the randomness of inverse design,to improve the design efficiency and device performance.By engineering material index and manipulating phase profiles of light at the nanoscale,we realize SW structure asymmetric Y-junctions with large divergence angles to reduce the device footprint.The footprint of the fabricated two-mode MUX and three-mode MUX was2.4×3?m~2,3.6×4.8?m~2,respectively.The footprint of the devices is almost two orders of magnitude smaller than that of conventional one,which offer an effective approach to promote practical applications of densely integrated photonic MDM systems for on-chip optical interconnection.(3)We proposed and experimentally demonstrated two ultracompact multimode waveguide crossings.First one is a silicon integrated inverse-designed dual-mode waveguide crossing based on subwavelength MMI coupler.Firstly,the dual-mode waveguide crossing physical model based on digital silicon subwavelength structure MMI is established.We also use theory-assisted inverse design method and pre-set device footprint,feature size,initial pattern of inverse design region according to the subwavelength MMI equivalent model to improve the design efficiency and device performance.The fabricated device exhibited high performance with insertion loss less than0.6 d B,crosstalk less than-30 d B and a compactfootprint of 4.8×4.8?m~2,which is two orders of magnitude smaller than that of conventional ones.(4)Three typies of subwavelength structure multimode photonic devices in optical transmission link were proposed and experimentally demonstrated including polarization rotator,dual mode power splitter and multimode bent waveguide.An ultracompact silicon integrated inverse-designed polarization rotator based on subwavelength structure asymmetric waveguide with the high perforamcne of high extinction ratio was proposed and experimental demonstrated in single step etching,which occupied the footprint of only4.8×4.8?m~2.The fabricated device also exhibited high performance with insertion loss less than 0.7 d B and polarization extinction ratio more than 19 d B.A wideband dual-mode3 d B power divider based on axisymmetric three-branch waveguide was proposed and experimentally fabricated,which occupied the footprint of only 2.88×2.88?m~2.Similarly,the inverse-designed multimode bent waveguide based on sub-wavelength asymmetric Y-junction was proposed and experimentally demostrated.The two-mode and three-mode multimode bent waveguide were fabricated in the ultracompact footprint of 2.4×3?m~2,3.6×4.8?m~2,respectively.The presented inverse design method enable subwavelength devices to reduce the footprint two orders of magnitude smaller than that of conventional ones,which will show great application prospects in designing the next generation of integrated photonic devices.