Microring Resonators Based On Strongly-confined Optical Waveguides And Their Thermo-optical Performances

Miniaturization is one of the research directions for integrated opto-electronic devices.A very effective method to develop compact or ultracompact devices is using strongly-confined optical waveguides.Microring resonators(MRRs)become indispensable elements for photonic integrated circuits because of their strong flexibility and functionality.Therefore, researches on MRRs are very important.In this thesis,two kinds of strongly-confined optical waveguides are involved,namely,SOI(Silicon-on-insulator)nanowire waveguides and SU-8 ridge waveguides.Compact MRRs based on these strongly-confined optical waveguides and their thermal performances are studied theoretically and experimentally.Optical lightwave theory and numerical methods build up the basis of integrated optics. Here,various numerical methods,including finite-difference method(FDM),beam propagation method(BPM),finite-difference time domain(FDTD)method,are introduced and applied to analyze and design various photonic devices.In this thesis,two-step FDTD method is utilized to analyze the butt-coupling between tapered lens fibers(TLFs)and a SOI rib waveguide.Two types of TLFs are considered,namely,a tapered-cladding TLF and a tapered-core TLF.The BOR(or radial)FDTD is first used to simulate light focusing for two types of TLFs.It is shown that the tapered-core TLF has a smaller focus spot size and a larger transmission loss than the tapered-cladding TLF.The butt-coupling between a TLF of either type and a SOI rib waveguide is then simulated by a 3D FDTD method.The tapered-core TLF has a smaller coupling loss to the SOI rib waveguide but a similar total loss(the sum of the coupling loss and the transmission loss),as compared with the tapered-cladding TLF.According to the heat conduction theory,thermal modeling of optical waveguides is presented.Based on this thermal modeling,a photonic SOI ridge wire with a submicron metal heater is proposed.Its thermal analysis is presented and compared with the conventional buried optical waveguide.The simulation indicates that the power consumption of the proposed SOI waveguide structure is almost less than 1/10 of the latter,and the response time is about half of that of the latter.This proposed structure also helps reduce the manufacture cost,because only one photolithography process is needed.Furthermore,optical performances of this structure are optimized,and an ultracompact widely tunable thermooptical(TO)MRR filter is designed.The calculation shows that the designed MRR has a wide tuning range of about 20nm with a low heating power of 5mW.The fabrication processes for SOI based ultracompact optical waveguides and devices are summarized.Here also present the manufacturing processes for the thermooptical devices based on the photonic SOI ridge wire with a submicron metal heater.We conduct researches on fabrication arts of SU-8 ridge waveguides and devices.Two kinds of technologies are compared,i.e.,nanoimprint lithography and direct ultraviolet(UV)photolithography.The latter is easier and is still able to provide enough fabrication tolerance for our designed SU-8 optical waveguides(several microns).Therefore,in this thesis,SU-8 based photonic devices are all developed by direct UV photolithography technology,which are proved effective by low loss optical waveguides and devices,e.g.,small multimode interference(MMI)coupler, MRR,etc.A2×2 tapered MMI coupler is designed and fabricated by using air-cladded SU-8 ridge waveguides,which are effective in reducing the device size and improving the self-imaging quality.The parabolically tapered MMI section is used to reduce further the size of the MMI coupler.The measurement results show that the fabricated 2×2 tapered MMI coupler has relatively small excess loss and nonuniformity for both polarizations in a broad wavelength range.Introducing this 2×2 tapered MMI coupler into MRR coupling region,we develop a compact MRR device.Its optical performances are characterized.TO effect of such MMI-based MRR are studied experimentally and the measurement results indicate that the resonant wavelength will shift over 10nm when the temperature is increased by 100℃.This is useful for developing TO devices with a wide tuning range.Considering the strong light confinement of SU-8 based air cladded ridge waveguides and the resonance enhancement of MRR structure,the last experiment presented in the thesis is carried out to explore the thermal nonlinearity of SU-8 polymer.The measurement results are analyzed and the infrared light absorption coefficient of SU-8(about 0.179cm^-1)is obtained,which establishes the basis for the future work involved in all-optical communication.