Study On Strained Silicon And All-Optical Signal Processing Techniques For Optical Communication

With the emergence of intelligent terminals and rapid development of new services, network traffic has been in explosive growth. As the physical carrier layer of mobile networks, the Internet and the Internet of things,optical communication and network technologies have been constantly evolving and developing in order to meet the growing demand for bandwidth. The future optical transmission link will be developing toward ultra-high speed, ultra-large capacity, low power consumption and ultra-long haul. In addition, to accommodate the intelligent, efficient, green,flexible and reconfigurable developing trends of future optical communication and network, the future optical backbone and access network will be evolving to SDN.All-optical signal processing technology has been an attractive field of optical communications, it has advantages of processing optical signal with high speed, flexibility and efficiency, and it is the key technology to achieve ultra-high speed optical transmission and future software-defined optical backbone and access network. Silicon is the ideal medium for all-optical signal processing, since it is CMOS compatible and it has advantages of low price, transparency in the communication band, high refractive index, small size, low power consumption and easily integrated with existing electronic and photonic devices in a single chip. However,silicon has a symmetric crystal structure, which prohibit the presence of second order nonlinear susceptibility, which is the key to achieve Pockels electro-optic modulation and other second order optical effects. The symmetric structure of silicon can be broken using strain, and the second order nonlinear susceptibility can be induced in silicon. The silicon with broken symmetric structure is called strained silicon. The second order nonlinear susceptibility induced in the strained silicon can be used to realize silicon-based electro-optic modulator and other optical signal processing techniques, which has an important impact on the realization of photonic-electronic integration and development of optical communication.In addition, as an active device, semiconductor optical amplifier (SOA) has also been widely used for all optical signal processing over the past decades,due to its advantages of high nonlinearity, low power consumption and easily integration.This dissertation focus on the research of strained silicon related technology and SOA based optical signal processing. The innovations and achievements are as following:Firstly, a new method called SIMOX 3D sculpting is presented to realize strained silicon. This method involves formation of SiO2 stripe mask, implantation of oxygen ions and high temperature annealing (HTA).Where the oxide mask is present, the oxide mask decelerates implanted oxygen ions, after HTA a buried rib silicon waveguide can be formed underneath the oxide mask. Oxidation of the SIMOX layer is accompanied by large volumetric expansion, and the volumetric expansion will induce stress in buried silicon waveguides. Therefore, buried and strained silicon optical waveguide is realized. The top silicon layer can be utilized forfabrication of other optical and/or electronic devices, therefore vertical integration(3D integration) of strained silicon with other optical and/or electronic devices on the same substrate can be realized. A buried silicon waveguide using SIMOX 3D sculpting was fabricated, and the stress in waveguide was measured via Raman spectroscopy, the stress value is -163MPa. Therefore, the concept of using SIMOX 3D sculpting to realized strained silicon is proven.Secondly, a mechanics model of SIMOX 3D sculpting was developed.The model is consist of stress formation and stress relaxation. The stress distribution in buried silicon waveguides was calculated using this model.The modeled stress from the stress model shows agreement with the measured stress provided via Raman spectroscopy. The stress model is also capable of analyzing the strain gradient in buried silicon waveguides,which is crucial to study the strain induced second order nonlinear susceptibility in strained silicon.Thirdly, the nanoscale stress in the strained silicon waveguide was measured using tip enhanced Raman spectroscopy. The TERS system is realized by optically coupling of a Raman spectrometer and an atomic force microscopy (AFM) operated in the non-contact mode with side illumination. Based on the Raman scattering theory, a model, which relates the observed Raman peak shifts to the localized stresses for our TERS experiments, was presented. The tip-induced electric-field enhancements,tip-induced depolarization of incident light and oblique incidence geometry of the TERS system were included in the model. Based on the observed Raman peak shifts, a two-dimensional stress map inside the silicon waveguide with spatial resolution of about 20nm was obtained. The strain-induced second order optical nonlinearity inside the SIMOX 3-D sculpted buried silicon waveguides is analyzed, and the second order optical nonlinearity in the center of the silicon waveguide is about 0.9pm/V,which suggests that the SIMOX 3-D sculpted strained silicon might be a potential metamaterial for electro-optical modulation and optical signal processing.Fourthly, a novel all-optical quantization and coding method exploiting polarization switches (PSWs) based on nonlinear polarization rotation (NPR) in semiconductor optical amplifiers (SOAs) is proposed.This method is realized using the NPR-based quantizer-coder (NPR-QC)array. Each NPR-QC is composed of two procedures: pre-quantization-coding and dynamic gain compensation, and both procedures are realized using a PSW. A theoretical model for the PSW is presented based on the propagation equation of the input electric field and the rate-equations of the SOA. The transfer functions (output light power versus pump light power) of independent PSW with respect to injection current, polarization angles of pump light and probe light, and different SOAs are simulated and analyzed. The parameters of different PSW are optimized to realize multi-period transfer functions, and half-, single-, and two-period transfer functions for 3-bit long all optical quantization and coding is realized numerically for the first time. The effective number of bits (ENOB), the limitation of bandwidth and conversion speed and the scalability of this quantization and coding method are also investigated. The proposed all-optical quantization and coding scheme, combined with existing all-optical sampling techniques, will enable ultrafast A/D conversion at high operating speed and with high resolution, and allows low optical power requirements, photonic integration, and easy scalability.