Research On Novel Mechanism Of On-chip Optical Mode Conversion Based On The Brillouin Scattering

In the optical communicate system, all of the dimensions of the optical wave(the amplitude, frequency, phase and the polarization) have been utilized to carry signal for improving the capacity of the optical communication to deal with our increasingly information-driven society. For further increasing the communicate capacity, the new dimension of the optical wave is required. In the few-mode fiber, the orthogonality of the space modes can be used as a new dimension to realize the Mode-division multiplexing(MDM), which is similar to Polarization Division Multiplexing(PDM). And this is considered as a promising technology for the next multiplicative capacity growth for optical communication by exploring new degrees of freedom.MDM has an important theoretical and practical signification in meeting the requirements of rapidly increased communication services. Mode conversion is a key technique to build up MDM transmission system with the property of flexible and compatible to the existed optical communication networks. In addition, in the recent years, the optical devices have a rapid development in on-chip integration. Thus, an integration, large bandwidth and tunable optical mode convertor will be the direction of future research and development. Supported by the National Basic Research Program of China, this thesis will focus on the theories of the optical mode conversion based on Forward Stimulated Brillouin Scattering(FSBS) and acousto-optics effect, design and fabrication of the phonon-photonic hybrid crystal waveguide. The major research achievements of this dissertation are summarized as follows:Firstly, we deeply study the physical mechanism of Stimulated Brillouin Scattering(SBS), which is established by taking into account the radiation pressure and the electrostriction force simultaneously. Based on this, the acousto-optics coupled-wave equations are derived in the SBS process. And then, it is theoretically shown that the Forward-SBS can be used to realize the optical mode conversion. In addition, we obtain the propagation direction and existential mode of acoustic wave from the phase-matched condition which is required in the Forward-SBS process.Secondly, based on the analysis of the acousto-optic interaction and the coupled-wave equations in Forward-SBS process, we obtain the basic requirements of the nanoscale waveguide for realizing the SBS. The first is that both optical and acoustic waves can be confined and guided with low-loss in the same waveguide. And then, the photonic and phononic dispersions should be independently controlled by different structural parameter of the waveguide. Based on this, we proposed a hybrid phonon-photonic crystal waveguide which is suspended in air over the silica substrate and contains a rectangular optical waveguide and the phonon crystal.Thirdly, through the analysis of the material characteristics, we choose the silicon(Si) and silicon nitride(Si3N4) to serve as the material of the optical waveguide and phonon crystal, respectively. To confine the optical and acoustic waves effectively, a hybrid waveguide is designed by embedding the Si line defect in the Si3N4 phononic crystal slab. Influences of waveguide structural parameters on the properties of acoustic waves are analyzed and discussed by taking into account three different kinds of lattices. Through the simulation, the appropriate structural parameters are obtained to enhance the acousto-optic interaction. It is shown that the hybrid waveguide with the honeycomb lattice is the most beneficial among the three different kinds of lattices.Fourthly, we propose a scheme for on-chip all optical mode conversion based on FSBS in a hybrid phononic-photonic waveguide with the appropriate structural parameters. The numerical simulation is carried out for the mode conversion from the fundamental mode 11 xE to the higher-order mode 21 xE. The results indicate that the mode conversion efficiency is affected by the waveguide length and the input pump light power, and the highest efficiency can reach upto 88% by considering the influence of optical and acoustic absorption losses in the hybrid waveguide. Additionally, the large conversion bandwidth can be achieved in 1550 nm communication band.Fifthly, a hybrid photonic-phononic waveguide is proposed to realize the reversible and tunable optical mode conversion based on the dynamic grating. The appropriate structural parameters of the hybrid waveguide, which can realize three different kinds of mode conversion, are obtained by considering the optical and acoustic wave's dispersion characteristics. The theoretical model and simulation of the acousto-optic effect are presented to illustrate the mode conversion over a large bandwidth in the hybrid waveguide. The theoretical analysis shows that the acoustic wave amplitude determines the mode conversion efficiency in a fixed-length hybrid waveguide. Furthermore, we obtain three different mode conversions by choosing the different acoustic wave frequency. It is predicted that the more kinds of mode conversions can be obtained by designing a suitable hybrid waveguide based on the acousto-optic effect. A chip-based tunable mode converter scheme with a large operation bandwidth will play an important role in modern integrated photonics.Sixthly, the acousto-optic hybrid waveguide is created using the optoelectronic micronanoelectronic manufacturing process platform. By testing on the hybrid waveguide, it is demonstrated that the hybrid wavegudie can realize the Forward-SBS.