On-chip Silicon Photonic Devices For Mode-Division Multiplexing

Today,all kinds of smart mobile devices and internet services have reshaped the way we live,leading to an exponential increase in the data traffic sent over global communication networks.Thus higher-density and higher-speed data transfer is of necessity to serve applications in telecom networks,data centers,and high-performance computers,etc.This has pushed traditional electronics links on back foot and has driven researchers to look in optical communication circuits on a complete system level.Among a variety of material platforms,silicon photonics is well-known to be the mainstream and this matter of fact comes from its potential merits of CMOS compatibility,low cost,low loss,high yield and compact integration.In this context,several multiplexing techniques have been proposed and developed as a tool to further push the boundary of photonic capacity.On top of the wavelength-division multiplexing(WDM),mode-division multiplexing(MDM)technology,one of the most important space-division multiplexing(SDM)technologies,has also been brought into the vision.Yet,there are still some important issues to be addressed.Basically,considering the high refractive index contrast of silicon platform,modes in silicon core are sensitive to the dimension variation,thus it is not always easy in developing robust designs of mode multiplexers with high performance,multi channels and wide bandwidth.Also,the demonstration of hybrid multiplexing system utilizing multiple mode and wavelength channels is still absent.Meanwhile,in order to realize effective and flexible routing of mode channels,multimode circuits with energy-efficient and reconfigurable configuration is required.Under this research frame,this thesis aims to explore the potentials toward robust,large-scale,and reconfigurable MDM-based photonic devices and systems,which is organized as below:As an initial step,we set up general framework to embed signal channels for both modes and wavelength as a system.This is followed by a specific demonstration containing 10 mode channels and 8 wavelength channels,which is fabricated and characterized with acceptable performance.In this system,80 signal channels supported in one physical tube can be demultiplexed with low excess losses(1.8dB?5dB)and low crosstalks(-12dB?21dB),which shows the most channels than ever reported.Then it comes to the scheme of mode routing,where nonvolatile optical phase change material is utilized.An add/drop mode-selective multiplexer for reconfigurable devices is designed with structure simplicity and compact footprint.It's shown to have low excess losses(<1 dB)and low intermodal crosstalk(<-20 dB)for all mode channels over an 80nm wavelength range.The effective design is enabled to add/drop certain mode-channel in-situ to/from the bus waveguide in the ON state,potentially paving a way for complicated and flexible on-chip multimode data routing.Finally,the MDM devices tend to suffer from the fabrication inaccuracy,turning into my further tolerance study.Detailed investigation has been carried out from every kind of dimension deviation for two typical design,clarifying the specific drawbacks,respectively.This turns into the efforts in conceiving another devices structures.One is an improved device taking advantages of subwavelength structures,which is known for dispersion and index engineering.The coupling losses for the first three TE modes are improved by almost 10 times upon a 20nm variation on width.Another one is designed employing adiabatic/straight-waveguide DC for TE-/TM-mode conversion in one mode multiplexer.This strategy enable simultaneously the act to relieve width sensitivity and polarization control in vertically asymmetric waveguide.These discussions and approaches are useful for the community of multimode photonics and the future exploration of mode multiplexing.