Research On Silicon On-chip Multiplexing Devices Using Subwavelength Structure

With the emergences of new Internet services such as the telecommute,cloud computing,5G,and autonomous driving,mankind has entered the era with an explosive data growth.The signal transmission and exchange capacity in broadband networks and data centers has increased by more than 10 folds in the past 4 years,which is beyond the traditional Moore's Law.It is highly desired to establish a broad-bandwidth,low-latency,high-speed,and low-cost data communication network to promote the development of the communication industry.Currently,the optical communication technology is advancing towards a large-capacity optical communication system with a high modulation rate,high spectrum efficiency,low power consumption and low cost.Multiple physical dimensions of the optical carrier such as wavelength,polarization,mode,etc.,can be employed to further scale the transmission capacity of the optical fiber or waveguide.Among them,wavelength-division multiplexing(WDM)technology,which utilizes multiple wavelength channels to carry different modulated signals,can effectively scale the transmission capacity and is among the most widely used multiplexing technologies.However,this technique requires multiple lasers,which greatly increases the system cost,and the limited optical communication bandwidth also restricts the WDM technology towards more signal channels.Recently,several novel on-chip multiplexing tecnologies have been proposed as alternative methods to scale the signal channels,such as the space-division multiplexing(SDM),the mode-division multiplexing(MDM),and the polarization-division multiplexing(PDM).Among them,the MDM and PDM technologies are attractive since they can effectively improve the transmission capacity of silicon chip while maintaining a compact footprint.However,it is challenging to use only one physical dimension of an optical carrier to realize an ultra-high-capacity transmission system.A hybrid multiplexing technology which combines multiple physical attibutes is highly desired.In this paper,we start from the Maxwell's equations and the mode coupling theory and explore the on-chip multiplexing technologies.We demonstrated multiple mode-,polarization-,wavelength-multiplexing devices by using subwavelength-scale devices.The main research achievements of this thesis can be summarized as follows:1.On-chip mode multiplexing devices based on subwavelength grating structure.Mode(de)multiplexers are important optical components in future large-capacity on-chip optical communication system.They can effectively scale the signal channels by using the mode dimensions in the silicon waveguide to achieve a high-capacity optical switching and transmission.Currently,most mode multiplexers suffer from large crosstalk between the mode channels,and are sensitive to waveguide dimension variations.To solove these issues we designed two MDM devices based on subwavelength grating(SWG)structures:1)An 11-channel mode multiplexer based on subwavelength grating.We propose and experimentally demonstrate a mode multiplexing device based on SWG directional couplers,which is capable of combining or separating 11 channels on TE₀?TE₁₀ modes.It is the highest-order multi-channel mode multiplexing device ever reported to the best of our knowledge.The device shows good insertion losses and acceptable crosstalk.Thanks to the flexibility of the SWG structure in effective refractive index engineering,the proposed mode(de)multiplexer has a good fabrication tolerance compared with the traditional MDM devices.2)Mode filter based on subwavelength grating contra-directional coupler.A mode blocking filter based on the SWG structure is proposed and experimentally demonstrated.The device can be used to filter a selected mode in the multimode waveguide without affecting the normal transmission of other modes.This device can be used in MDM systems to block a selected mode channel,in analogy to the wavelength blocker in an optical add-drop multiplexing(OADM)system,to reduce the crosstalk and improve the data transmission quality of the MDM system.As a proof-of-concept experiment,we demonstrated two mode filters which can block the TE₀ mode and the TE₁ mode in the bus waveguide,respectively.The proposed device has the potential to be scaled to an arbitrary mode blocking filter by designing the structural parameters,which can be used in MDM systems to suppress inter-modal crosstalk and improve the system performances.2.Polarization handling devices based on subwavelength structureThe high refractive index contrast between the silicon waveguide and the Si O₂cladding makes SOI platform highly polarization dependent and results in severe polarization-related dispersion and losses.Polarization diversity scheme is a promising solution to eliminate the polarization sensitivity of silicon photonic circuits.Various polarization handling devices,such as polarization beam splitters(PBSs)and polarization rotators(PRs)have been reported.In this work,we introduce subwavelength structure to the polarization handling devices to optimize the device fabrication tolerance and polarization extinction ratio(PER).Here we demonstrated two polarization handling devices:1)Polarization splitter rotator based on subwavelength grating structure.We experimentally demonstrated a compact polarization splitter rotator(PSR)by using a SWG-based directional coupler.Low insertion losses and high PER values are achieved in a 40-nm bandwidth for the two polarizations.Compared with the conventional PSRs,our device shows larger tolerance(±40 nm)to waveguide-width variations thanks to the flexibilities of the SWG waveguide.2)On-chip polarizer based on the nanohole array structure.An ultra-compact and broadband transverse magnetic(TM)-pass polarizer is experimentally demonstrated using a photonic crystal(Ph C)nanohole structure.By engineering the period of the circular holes,the fundamental transverse electric(TE₀)mode is suppressed due to the bandgap of the nanohole array,while the TM₀ mode propagates with a negligible insertion loss.Simulation results predict that the bandwidth of the device can reach 245 nm with a 33-d B polarization extinction ratio(PER).The fabricated device is very compact with a total length of 7.21?m and compatible with the CMOS fabrication process.3.Multi-dimensional multiplexing devices based on the subwavelength structureLeveraging the physical dimensions of an optical carrier(e.g.,wavelength,mode or polarization)allows significant scaling of the transmission capacity for optical communications.Here we propose a scheme for implementing on-chip silicon(de)multiplexers with simultaneous wavelength-,mode-and polarization-division(de)multiplexing capability.The device is constructed by using cascaded subwavelength grating(SWG)-based contra-directional couplers.To verify the feasibility of the proposed structure,we perform a proof-of-concept experiment of an 8-channel(de)multiplexer with2 wavelengths,2 modes and 2 polarizations.The insertion losses are lower than 6.6 d B and the crosstalk values are below-18.7 d B at around 1540 nm and 1550 nm for all the 8channels.