Study On Ultra-Thin Silicon-Based Integrated Waveguide Bragg Gratings

With the urgent demand on capacity,speed and cost of information transmission and processing,silicon photonics has become a hot research topic and has been regarded as one of the most promising plarforms for intergrated photonic devices due to their small footptint,low cost and compatibility with complementary metal-oxide-semiconductor(CMOS)fabrication process.Bragg gratings have been widely used as key functional components in optical communications and optical signal processing.Hence,silicon-based integrated waveguide Bragg gratings(IWBGs)have important academic value and extensive applications in integrated optoelectronics.It has become one of the research hotspots in recent years.Compared to fiber Bragg gratings,IWBGs are more compact and possess many different properties and applicaitons due to the high index contrast between the silicon and silicon dioxide.Silicon-based IWBGs have been widely studied and applied in various devices,e.g.optical filters,optical delay lines,wavelength division multiplexers,polarization beam splitters and sensors.However,there still exist many challegings in implementation of silicon-based IWBGs with high quality.Moreover,more functions based on WBGs need to be developed to satisfiy the versitle application demands.Hence,it is still a challenging and important subject to study on the silicon-based IWBGs.This dissertation will present partial solutions to solve the existing issues in inplemetation of silicon-based IWBGs.In particular,this dissertation will present a novel silicon-based spiral IWBG and a channel-spacing tunable comb filter based on two linearly chirped WBGs.The major research work and the novelties are summarized as follows:First of all,the working principle,key parameters and simulation methods of silicon-based IWBGs are elaborated.We utilize the coupled-mode theory to describe the working principle of the silicon-based IWBGs.We deduce the analytical expression for the key performance metrics of the uniform Bragg grating,including Bragg wavelength,stop bandwidth,grating coupling coefficient,transmittance and reflectivity.Three types of numerical simulation methods are introduced and compared.The discussions in this part provide a theoretical basis to the design of the silicon-based IWBGs.To make use of the reflection of the silicon-based IWBGs without a circulator,we propose to design two identical IWBGs in the two arms of a Michelson interferometer.The working principle of this structure is first elaborated.We present the advantages of the proposed structure over the exisiting solutions.Then we design and optimize a chirped IWBG.In order to achieve a small couping coefficient and reduce the width-induced phase noise,the chirped IWBG is implemented in a 220-nm-thick multimode waveguide.Single-mode waveguides with adiabatic couplers are placed before and after the multimode waveguide in order to predominantly excite the fundamental mode of the multimode waveguide.The period of chirped IWBG is designed to satisfy the second-order Bragg condition due to the limited resolution of the 180-nm fabrication process.The Michelson interferometer comprising two identical optimized chirped IWBGs is experimentally demonstrated.The passband width is0.4 nm and the extinction ratio is 15 dB.The group delay variation range is 60 ps.After that,we propose a novel spacing-tunable comb filter using a Michelson interferometer comprising two identical linearly chirped Bragg gratings(LC-BGs).A theoretical model using the transfer-matrix method is proposed to study the transmission characteristics of the proposed comb filter.In order to achieve active tuning of the comb filter using the thermo-optic effect,we propose a novel waveguide-based resistive heater following the previous work in our group.Its thermal tuning efficiency and free-carrier absorption induced excess loss are studied by numerical simulations.Based on these simulation results,the active tuning property of the proposed comb filter is studied using the proposed theoretical model.Our numerical example shows that the channel spacing can be continuously tuned form 8.21 nm to 0.19 nm(number of comb passbands increasing from 1 to 65)while maintaining an out-of-band rejection ratio of greater than 30 dB.The influence of apodization of the Bragg gratings on the device performance is explored.This is the first proposed channel-spacing tunable comb filter on SOI(silicon on insulator).Though small grating coupling coefficient can be obtained and width-induced phase noise can be reduced by using a wide multimode waveguide,this solution is not compatible with many applications because the use of multimode waveguides will casue cross coupling between many modes and strongly degrade the device performance,especially in grating-assisted couplers and FSR-free Bragg grating filters.Moreover,only the second-order diffraction in Bragg gratings can be used due to the limited resolution of 180-nm fabrication process.The coupling-coefficient of the second-order Bragg grating differs from that of the first-order Bragg gratings.However,nowadays most grating-based devices work in the first-order Bragg reflection.This dissertation proposes a novel 60-nm-thick waveguide based IWBG.The characteristics of the IWBG based on the 60-nm-thick waveguide are analyzed.Simulation results indicate that the single-mode ultra-thin strip waveguide processes a fundamental mode with much lower confinement in the waveguide core compared to the typical 500 nm×220 nm single-mode strip waveguide.A small coupling coefficient thus can be achieved using relatively large corrugations on the single-mode strip waveguide sidewalls.As the mode of the ultra-thin waveguide interacts weakly with the sidewalls,the waveguide is less sensitive to the sidewall roughness.Meanwhile,the IWBGs in ultra-thin waveguides can be patterned by 248-nm DUV photolithography.After that,we design and experimentally demonstrate a series of basic structures based on ultra-thin waveguides,including grating couplers,micro-ring resonators,1×2 MMI couplers,2×2 MMI couplers,and Mach-Zehnder interferometers.The average propagation loss of the ultra-thin waveguides is 0.61 dB/cm.This waveguide loss is almost 5 times smaller than that of a regular 500 nm×220 nm waveguide fabricated using the same fabrication process.Then,we design and fabricate IWBGs in single-mode ultra-thin waveguides.The maximum extinction ratio among the fabricated IWBGs is 50 dB and the narrowest bandstop width is 0.67 nm.Integrated resistive heaters are used to actively tune the IWBGs.This dissertation experimentally investigates the effect of grating length,coupling coefficient and apodization function on the transmission spectra of IWBGs.The delay characteristics of IWBGs are also investigated using a modulation phase-shift technique.A tunable delay of 49 ps is obtained in an apodizd IWBG.As the IWBGs based on single-mode ultra-thin waveguides are still sensitive to the height variation,we design spiral-shape IWBGs to reduce the height variation.By wrapping a long grating into a spiral,the straight-line distance between any two points on the grating is minimized and thereby minimizes the waveguide thickness variation in the grating.We theoretically analyze and simulate the effect of key design parameters of the spiral waveguide on the transmission performance of the spiral IWBGs,especially the bending radius.Then the thermal tuning structure is optimized by numerical simulation.After that,the experimental transmission spectra of spiral IWBGs are measured.The spiral IWBG exhibits a single narrow transparent peak with a Q-factor of~1×10~5 in a broad stopband,induced by the phase shift of the S-junction at the spiral center.We propose a theoretical model based on the transfer matrix method to fit the experimental results.The thermal active tuning of the spiral IWBGs is also investigated.Experiments show the transparent peak can periodically shift in the stopband upon heating the S-junction.We experimentally explore the effect of bending radius on the device performance,especially on the Q-factor of the transparency peak.We also investigate the dependence of the peak transmittance and Q-factor on the reflectivity of the spiral IWBG.At last,we summarize all the research work in this dissertation.The outlook of the silicon-based ultra-thin waveguides and IWBGs is presented.