Research Of Forward Stimulated Brillouin Scattering Based On The Silicon Integrated Waveguide

With the development of big data,cloud computing,internet of things and other information technology,the demand for transmission,processing and storage of high-speed network data is becoming higher and higher.Optical interconnect is an emerging solution of data interconnect and great potential to have the advantages of great bandwidth,high transmission speed and low power consumption.Those overcome the limited bandwidth,large time delay,large power consumption and large crosstalk of the traditional electric interconnect.Silicon-based photonic integrated devices with CMOS-compatible processing are the key devices to realize optical interconnection technology and have great wide applications in optical communications on-chip,high-speed optical interconnects,optical biosensing,microwave photonics and other fields due to their advantages of high integration,low cost and low power consumption.Silicon-based photonic integrated devices exhibit the ability of quasi-lossless signal transmission at the optical communication.The high-index-contrast can dramatically shrink the component size.This not only generates the strong optical confinement but also strengthens the optical energy density within the waveguide which can stimulate many nonlinear optical effects and be served as various functional silicon photonic integrated devices.Stimulated Brillouin scattering?SBS?,as one of the most common nonlinear optical effects,is widely applied in optical sensing,microwave photonics and all-optical signal processing.Therefore,an integrated on-chip SBS device will play an important role in future information systems.Supported by the National Basic Research Program of China,this thesis will focus on optimized designs of silicon-based photonic integrated waveguides,the theoretical analyses,simulations and experiments for forward stimulated Brillouin scattering?FSBS?.The main research achievements of these studies are described as follows:?1?We study the fundamental principle of FSBS,the coupled wave equations in the process of acousto-optic interaction and the difference between forward and backward SBS phase-matching conditions.Then,through the electrostrictively stress and Maxwell stress,the physical mechanisms of electrostriction force and radiation pressure are established.By accurately incorporating the effects of two kinds of forces,the full-vectorial theoretical formulation of FSBS gain coefficient is analyzed.The formulation is applicable to the calculation of FSBS gain coefficient for microscale and nanoscale optical waveguides and has an extensive applicability.?2?A honeycomb phoxonic crystal waveguide is proposed to realize slow light enhanced FSBS.The waveguide,which is formed by periodically arranging the medium with different optical and acoustic properties,simultaneously supports the photon and phonon band gaps.By introducing the line defect into the phoxonic crystal waveguide,not only the slow light enhancement effect is generated,but also the corresponding Brillouin optical and acoustic modes are confined in the line defect.Therefore,the acousto-optic coupling is effectively enhanced which is beneficial to excite the intramode and intermode FSBS.By the coupled-mode equations,we explore the evolution of optical powers in FSBS process,Stokes amplification and optimized waveguide length taking into account the slow light enhancement effect and nonlinear optical loss in the phoxonic crystal waveguide.The simulation results indicate that large Stokes amplification of 18 dB is obtained with 100 mW pump power in a short waveguide length of 200?m.?3?We present the generation of FSBS in hybrid phononic-photonic waveguides.To confine the optical and acoustic waves simultaneously,a hybrid waveguide is designed by embedding the silicon line defect in the silicon nitride phononic crystal slab.By comparing the influences of waveguide structural parameters of three kinds of lattices on the properties of acoustic waves,the appropriate lattice and structural parameters are obtained to enhance the acousto-optic interaction.The influences of pump power,acoustic loss,nonlinear optical loss and lattice constant on the acousto-optic interaction in FSBS are analyzed and discussed.In the experiment,we fabricate the honeycomb hybrid waveguide with a CMOS compatible technology.The forward Brillouin frequency shift is measured up to 2.425 GHz and the acoustic Q-factor of the corresponding acoustic mode is 1100.The characteristics of acoustic and optical fields in hybrid phononic-photonic waveguides can be independently controlled by respectively changing the structures of phononic crystal waveguide and Si waveguide,which is potential to realize a variety of integrated acousto-optic devices?4?A partly suspended silicon nanowire racetrack microring is proposed for resonance-enhanced FSBS.The resonator is fabricated on a standard SOI wafer with a 220nm top Si layer.The large refractive index difference between Si waveguide and SiO₂substrate produces the strong optical confinement.An etch for the SiO₂ substrate is carried out to fabricate the partly suspended structures which is beneficial to confine the acoustic field in the racetrack microring to achieve enhanced acousto-optic interaction.In the experiment,the Brillouin amplification of 2.25 dB is achieved under a low-power pump laser of 8 mW.The influences of the waveguide width on the Brillouin frequency shift are presented and analyzed.The results indicate that the Brillouin frequency shift is conveniently manipulated by the changes in waveguide widths.The partly suspended microring waveguide device has advantages of low Brillouin threshold,high integration,small size and easy fabrication.?5?We demonstrate the forward cascaded Brillouin lasing exploiting a silicon-based rectangular spiral microring resonator.To realize the enhanced Brillouin nonlinearity,the optical and acoustic fields are effectively confined by partly suspending the spiral resonator.The free spectral range is precisely designed to match the half of the Brillouin frequency shift to guarantee Brillouin laser oscillation.The spiral structure allows a 0.6368 cm long resonator waveguide to be enclosed in a footprint of 250×330?m².In the experiment,the fabricated spiral resonator is incorporated in a fiber loop to serve as not only a resonance-enhanced element to generate the internal pump lasing for Brillouin scattering but also a Brillouin gain medium to excite Brillouin lasing.Finally,four anti-Stokes and three Stokes lasing lines are obtained with the Brillouin frequency shift of about 12.0463 GHz.The proposed approach provides a potential way to implement CBL on a silicon-based chip.