| Silicon not only plays an irreplaceable role in the microelectronics industry, butalso becomes the first choice of the photonic integration and the optoelectronic hybridintegration. In1980s, people proposed the silicon-based photonic technology. Sincethen, many kinds of silicon-based photonic devices have been successfully developed,Raman silicon lasers, silicon-on-insulator waveguides, silicon modulators and so on.However, silicon is a kind of centrosymmetric material and it does not have the linearelectro-optic effect in the bulk, which limits the development of silicon-basedphotonics greatly. Although silicon has the Kerr effect and the Franz-Keldysh effect,these effects are usually very weak, which cannot satisfy the needs of practicalapplication. At present, most high performance silicon-based electro-opticalmodulators are based on the plasma dispersion effect. However, the inversionsymmetry of silicon can be destroyed by applying the electric field on thenear-intrinsic silicon, as a result, linear electro-optic effect will happen in silicon,which is called electric-field-induced linear electro-optic effect. This mechanism canbe used for manufacturing silicon-based electro-optic modulators, and the modulationbandwidth of this modulator can be up to100GHz.First of all, the fundamental principles of the electric-field-induced linearelectro-optic effect and the plasma dispersion effect are analyzed in this paper, and atransverse electro-optic modulation system is designed to measure theelectric-field-induced linear electro-optic effect, the Kerr effect and the plasmadispersion effect simultaneously. According to the truth that these effects havedifferent polarization dependences and frequency responses, these effects aresuccessfully distinguished and compared. The experimental results show that theelectro-optic signal from the electric-field-induced linear electro-optic effect is muchlarger than that from the plasma dispersion effect, which indicates that it is feasible tofabricate the silicon-based electro-optic modulator based on the electric-field-induced linear electro-optic effect.Moreover, a silicon electro-optic modulator based on the electric-field-inducedlinear electro-optic effect is designed, which possesses Mach-Zender interference(MZI) structure, and the principle of this structure is interpreted. According to thesingle-mode transmission conditions and the effective index method, the ridgewaveguide structure of the silicon modulator is analyzed and designed to meet thesingle-mode transmission conditions. The external height of the ridge waveguide is0.5μm, the height of the core of the ridge waveguide is4μm, and the waveguidewidths are3μm、4μm、5μm and6μm respectively. Based on the beam propagationmethod, the transmission characteristics of the MZI structure is simulated by using theOptiBPM software, and the effects of the splitting angle of Y-branch waveguide, andthe waveguide width on the insert loss of the modulator are considered especially. Thetheoretical results indicate that the insert loss is about5.4dB when the waveguidewidth is6μm and the angle of the Y branch is3.8°, and the insert loss decrease to3.5dB when the angle of the Y branch is2.4°. In addition, we also try to replace theY-branch structure by the multi-mode interference (MMI) structure to realized thecoupling between the waveguides, and analyze and simulate the MMI structure, andthe results show that the insert loss can be decreased to0.1dB by using MMIstructure.According to the design and simulation results, we put forward a fabricationproject of the silicon-based electro-optic modulator and designed a set ofphotolithography mask of the whole device by using the L-Edit software. Furthermore,the prototype device of the silicon-based electro-optic modulator on thesilicon-on-insulator (SOI) substrate is manufactured by using conventionalsemiconductor planar technology. The total length of the modulator is7.2mm, thelengths of the input and output straight waveguide are1mm, the angle of the Y branchis3.82°, the distance and length of the parallel straight waveguides are40μm and4mm respectively. The PIN electrical structure and the traveling wave modulationstructure are applied, and the core of the ridge waveguide is near-intrinsic, whoseresistivity is about10000Ω·cm. Finally, the performance parameters of the modulator are elementarily measured. The insertion loss is about16.6dB, the modulation depthis82.2%, the extinction is about7.5dB, the half wave voltage is2.5V, and themodulation bandwidth is more than10MHz. Although the performance of thefabricated modulator needs to be improved further, it is a novel attempt to apply theelectric-field-induced linear electro-optic effect to the silicon modulator, andtheoretically, the performance of this kind of modulator based on theelectric-field-induced linear electro-optic effect can be improved obviously. Forinstance, the free-carrier absorption should be induced effectively because the core ofthe ridge waveguide is the near-intrinsic silicon, which can decrease the insertion lossof the modulator. In addition, the electric-field-induced linear electro-optic effect is apolarization effect, so the response time is very short (about several femtoseconds),thus, the modulation speed of the modulator on this effect will be very fast. In a word,the modulator based on the electric-field-induced linear electro-optic effect is verypotential. The researches in this thesis will promote the development of siliconphotonics. |
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