Research On Optical Control And Modulation In Silicon Microring Resonator

As a fundamental component in very-large-scale integration circuits, the silicon chiphas been the mainstay of the electronics industry. With the requirement ofhigh-performance computing and the rising of interconnect density between data centers,the limitations of copper as an interconnect medium in terms of its loss, bandwidth andcrosstalk are becoming increasingly obvious. To break the limitations, optical interconnectis considered as a promising solution. Silicon-based photonic integrated devices havebroad prospects for development because they could be cheaply made using standardsemiconductor fabrication techniques and integrated with microelectronic chips.Recently, researches on both the active and passive devices with silicon-on-insulator (SOI) material were underway, and became a hot-point in opto-electronics.Silicon photonic devices not only provide low-cost opto-electronic solutions for opticalcommunication systems, but also gain wide-spread applications in energy, environment,medicine, biology, sensor and etc. Microring resonator (MRR) is one of the mostimportant devices in integrated optoelectronic technologies. Thanks to forming of itsresonance does not need cleaving facet, microring is born with the advantage ofintegration and its micro/nano size supports large-scale optoelectronic integration on chip.This dissertation will first introduce the basic theoretical knowledge and characterizationof the MRR. After that, theoretical analysis and modeling of silicon microring resonatorswith mode splitting is carried out. Based on the silicon microring resonators with modesplitting, tunable optical delay lines are experimentally demonstrated and an electro-opticmodulator is theoretically designed. The major research achievements and contributions ofthis dissertation are summarized as follows:1. Modeling of the mode-splitting in silicon microring resonators1）Analysis of the mode-splitting in silicon microring resonator based on coupled modetheory (CMT). At first, we discuss the generation of mutual coupling betweenclockwise and anti-clockwise modes in silicon microring. The influence of the mutualcoupling on the mode-splitting is investigated by introducing the coupling coefficient μ With the increase ofμmutual coupling becomes stronger and mode-splittingphenomenon is more obvious.2）Modeling of the mode-splitting in silicon rings by considering the sidewall corrugationas a group of quasi-Bragg-gratings. The gratings are with the height of30-50nm andthe period of~100nm. Mode-spilitting is investigated by introducing the reflectionproperty of the quasi-Bragg-gratings to the spectral response of ring resonators. Theaccuracy of the model is verified by fitting with the experimental results.2. Experimental demonstration of optical tunable delay lines in siliconmicroring resonators1）Experimental demonstration of tunable slow-light in single-waveguide-coupled siliconring resonators with mutual mode coupling. In the through port, negative groupvelocity dispersion (GVD) is obtained when the center wavelength of the optical pulsematches the split resonances. In the reflection port, positive GVD is achieved,resulting in tunable slow light around the resonances. Pulse delay is for the first timeexperimentally demonstrated in the reflection port of single waveguide coupled ringresonator and the delay time achieves25ps.2）Experimenal demonstration of continuously-tunable fast-to slow light based ondouble-waveguide-coupled ring resonators with mutual mode coupling. In the dropport, positive group delay is obtained when the central wavelength of the optical pulsematches the split resonance, while negative group delay is obtained with the centralwavelength of optical pulse locates between the split resonances. Based on the abovefeatures, continuous tuning of fast-to slow light is for the first time experimentallydemonstrated using thermal effect. Pulses are tuned from6-ps advancement to29-psdelay.3）Proof-of-concept demonstration of a microring resonator structure to achieve enhancedfast light. By connecting an under-coupled microring resonator to a Sagnac loopreflector, input light passes through the ring resonator twice. As a result, the absolutevalue of group delay is doubled and fast light enhancement is realized. In theexperiment, microfiber ring resonator is utilized to demonstrate the fast light effect inprinciple. Advancement of25ps is obtained for a return-to-zero (RZ) pulse train at the data rate of5-Gb/s.3. Design of a silicon electro-optic modulator based on microringresonators with tunable mutual mode coupling1）Realization of tunable mutual mode coupling through a tunable Bragg grating, whichis formed by etching periodic holes on part of the racetrack ring resonator. Mutualcoupling occurs between back-reflection mode and forward travelling mode with thetunable grating. By modulating the effective index of the grating based on free carrierplasma effect, reflection spectrum blue-shift, leading to the variation of the gratingreflectivity at the resonance. With the increase of the grating reflectivity, the mutualcoupling becomes stronger.2）Design of an optical intensity mosulator based on the tuning of mutual coupling. Withthe change of the reflectivity, mutual coupling becomes stronger resulting inmode-splitting in the drop port. The spectral response at the resonances transits frompeak to dip，the output power becomes lower, and thus optical modulation is realized.Simulations show that the proposed modulator can achieve a modulation depth of~13dB with a3-dB bandwidth of10GHz. Meanwhile, a moderate insertion loss of1.9dBand low energy consumption of122.3fJ/bit are obtained.