Research On The Enhancement Of Forward Stimulated Brillouin Scattering In Silicon-Based Integrated Optical Waveguides

Silicon photonics is to research the integrated optical system which uses silicon as the optical medium,offering the advantages of CMOS-compatibility,low cost,and the ability to mass produce.There are numerous unique optical nonlinear effects in silicon materials,including the Brillouin scattering effect.The Brillouin scattering effect is a nonlinear effect of acousto-optical interactions that bridges the gap between the optical field,the mechanical acoustic field,and the microwave signal.However,the optical waveguide in silicon photonics has many shortcomings for realizing the Brillouin effect.First,the Brillouin scattering effect requires a very large device size to achieve strong acousto-optical interactions.Second,the Brillouin scattering effect requires a sufficiently large pump power,which is limited by the on-chip optical power injected by the fiber.And the silicon-based optical waveguides suffer from the nonlinear loss of the silicon material.In order to further enhance the FSBS effect in the silicon-based optical waveguides and thus realize devices and applications based on the FSBS effect.Based on the theoretical principle of on-chip FSBS,the thesis uses new device structures,new materials and external sound field injection to design and fabricate some devices to enhance the FSBS effect.The main research results of this thesis are summarized as follows.（1）The scalar model for calculating the SBS gain coefficient in bulk materials and the vector model for calculating the SBS gain coefficient in integrated optical waveguides are proposed.Based on the phase-matching conditions and the acousto-optical coupling wave equations,the properties of optical and acoustic fields and the dynamics of three SBS processes in integrated optical waveguides are analyzed.The experimental principle of the heterodyne FSBS-enhanced four-wave mixing is presented for guiding the design,fabrication and measurement of FSBS-based devices.（2）A Bragg grating-based FP resonant cavity is designed and fabricated for implementing on-chip FSBS effect enhancement.Firstly,suspended straight waveguides with different widths are fabricated,which can realize tunable Brillouin center frequency of the FSBS.By adding Bragg gratings at both ends of the suspended straight waveguide,a FP resonant cavity with FSBS effect is formed.The pump light and Stokes light resonate in the FP resonant cavity at the same time,thus enhancing the FSBS interaction.The Brillouin gain coefficient is 1016.4 m^-1W^-1,the Brillouin frequency is 5.4 GHz,and the Brillouin gain linewidth is 16.8 MHz.The Stokes signal intensity in the FP resonant cavity is improved by6.2 d B compared with the suspended straight waveguide when the pump light power is 35m W,and is improved by 5.8 d B compared with the suspended straight waveguide when the detected optical power is 35 m W.（3）Cascaded racetrack microrings are designed and fabricated to enhance the on-chip FSBS effect.Through the vernier effect,the cascaded racetrack microring is able to generate split resonant peaks to resonantly enhance the pump light and Stokes and anti-Stokes.Compared with single racetrack microring,cascaded racetrack microrings can greatly reduce the device size and thus improve the device integration.The anti-Stokes and Stokes amplification of 3.0 d B and 2.4 d B is also achieved.Meanwhile,for both single racetrack microring and cascaded racetrack microrings,the Stokes signal is improved by 23.3 d B and18.1 d B compared with the suspended straight waveguide at a pumping power of 35 m W,and by 25.9 d B and 21.8 d B compared with the suspended straight waveguide at a detection power of 35 m W.At the same time,a single sideband modulation with 18 d B of the rejection ratio is achieved in the cascaded racetrack microrings.（4）A germanium-silicon alloy waveguide structure is proposed for achieving FSBS effect enhancement.The material properties of the germanium-silicon alloy,including optical and mechanical parameters,are presented.Then the optical force distribution and the two main acoustic field modes in the germanium-silicon alloy are simulated,and the variations of Brillouin frequency shift and Brillouin gain are calculated for different device sizes and components of the germanium-silicon alloy.Finally,the variation of the Stokes net gain is calculated for different components with different waveguide lengths and pumping powers.The results show that the germanium-silicon alloy withSi₀.₂Ge_0.8component is able to achieve a maximum Stokes net gain of 10.32 d B at an injected optical power of 100 m W and a waveguide length of 1.23 cm.（5）A hybrid waveguide structure of AlN and SOI is proposed to realize FSBS effect enhancement.The piezo-opto-mechanical hybrid waveguide has both piezoelectric and Brillouin-opto-mechanical effects,thus converting the externally injected microwave signal into a mechanical acoustic field and thus realizing the energy conversion from pumped to scattered light.A novel electrode structure is designed to enable intra-mode FSBS and inter-mode FSBS in the hybrid waveguide.The piezoelectric-mechanical and Brillouin-opto-mechanical coupling coefficients are numerically simulated.The results show that when the injected microwave signal is 100 m W,the optical phase modulation up to the fifth-order sideband can be generated.By optimizing the waveguide structure parameters,a microwave-optical wave coupling efficiency of 80%can be achieved.