Silicon Photonics Devices Based On Gratings

As the "Moore's law" is slowing down,the concept of "More than Moore" is starting to get more and more attention from both the academic and industrial world.Silicon photonics is a promising candidate of "More than Moore" not only to boost the computing capacity of the existing multicore and multi-CPU systems,but also to expand the functionality of semiconductor devices.Grating,a widely-used structure in traditional optics,is also playing an important role in the area of silicon photonics.Due to the characteristics of easy fabrication and complementary metal-oxide semiconductor(CMOS)technology compatibility,grating has been implemented in various silicon photonics devices such as hybrid integrated lasers,couplers,reflectors and biological sensors.The various implemented forms and flexible designability makes the grating structure still an attracting subject to further miniaturize,integrate,and enrich the functionality of the silicon photonics.This thesis focuses on grating application of distributed feedback(DFB)resonators,waveguide-to-fiber couplers and contra-directional couplers.The research accomplishments and innovations are as follows:(1)In order to explore the possibility of increasing luminescesnce power by including more Ge quantum dots in a large-mode-volume DFB cavity,design strategy of longitudinal mode stretching is proposed to increase the mode volume of DFB cavity without increasing the resonating linewidth.A 1/4 shifted DFB cavity with a large-mode-volume of 75.39?m3 and small resonant linewidth of 69pm is demonstrated experimentally on 220nm-thick silicon-on-insulator(SOI).The extinction ratio of the resonant peak is 15dB.Comparisons with other cavity designs are made to verify the design strategy of longitudinal mode stretching to enlarge the mode volume of the cavity.(2)A DFB cavity with side-current-injected structure is designed and demonstrated experimentally in order to explore the luminescence properties of Ge quantum dots in large mode volume cavity.The device is based on rib waveguide on 790nm-thick SOI with 30 layers of Ge quantum dots grown by molecule beam epitaxy.A wide electroluminescence spectrum is observed with injected current of 40mA from grating couplers at the cavity ends,indicating spontaneous emission of the Ge quantum dots in the cavity.(3)Compact grating couplers based on waveguides on multimode slab are designed and fabricated.On 700nm-thick silicon nitride-on-insulator(SNOI),a grating coupler with footprint of 70.2?m ×19.7?m,designed for strip waveguide,achieves peak coupling efficiency of-3.dB and ldB bandwidth of 54nm by experiment.On 790nm-thick SOI,a compact grating coupler designed for rib waveguide is also demonstrated experimentally with peak coupling efficiency of-6.18dB and 3dB bandwidth of 45.8nm.(4)A novel two-mode grating assisted contra-directional coupler(TGACC)which can filter two mode channels with two superposed grating components is demonstrated.Simulations shows that by apodization of the superposed grating components,both the sidelobes,crosstalk,self-reflection and reflection coupling can be suppressed simultaneously.A mode-channel switchable TGACC for 2.54nm-wide wavelength band centered at 1548.Onm by 50K thermal-optic tuning is demonstrated experimentally to show that the proposed TGACC can have various applications.(5)A novel multi-waveguide DFB cavity is proposed and analyzed,which has a single resonant mode oscillating in multiple waveguides.Calculations shows the multi-waveguide cavity can have a mode volume several times larger than a single-waveguide cavity,with a smaller threshold gain density.Cavity with 5 waveguides is verified by finite-difference time-domain simulation.Staged experimental result for device fabrication is obtained.