Study On Output Optical Coupling Structure Of The Integrated Silicon-based Ⅲ-Ⅴ Lasers And Relative Fabrication Process

Silicon photonics(SiPh)is the study of photonics on silicon-based material platforms,which is closely related to the design methods and manufacturing processes of silicon-based integrated circuits.The research topics of SiPh include but are not limited to photon generation,optical transmission,optical modulation,and light detection on silicon.SiPh is a promising solution to achieve large-scale optoelectronic monolithic integration.It has the potential to overcome the integration problems of discrete optical components such as high cost,large footprint,and high insertion loss.SiPh offers industry-level solutions of optical waveguides,optical modulators,and photodetectors.Silicon-based photonic-integrated circuits(PICs)hold promise in an increasing number of applications including telecommunications,integrated neurophotonics for optogenetics and brain imaging,integrated ion traps for quantum information processing,and miniaturized display systems.However,the efficient coupling between the monolithic Ⅲ/Ⅴ laser and a silicon waveguide on an SOI(Silicon On Insulator)wafer still presents challenges due to poor mode matching and a refractive index difference between the light source and passive waveguides.The monolithically integrated Ⅲ/Ⅴ laser usually has thick(about 2-3 μm)buffer layers to block the defects from threading dislocation.This makes the coupling between the laser and a standard Si EC(edge coupler)difficult because the SiPh devices fabricated by CMOS foundries usually use a 220-nm-thick Si waveguiding layer on top of the 2-μm buried oxide layer.Therefore,it is significant to study how to efficiently couple light between a monolithic III/V laser and a standard silicon photonic waveguide in a CMOS compatible platform.This work is supported in part by the National Key Research and Development Program of China under Grant 2018 YFB2200104 and in part by the Program of Beijing Municipal Science and Technology Commission under Grant Z191100004819012.The foundation of the work is the bilayer SiN interlayer photonic coupling structure fabricated in our group.In my work,the type of the photonic material is extended and the coupling efficiency is greatly improved.This work introduces the silicon waveguiding layer in the coupling system.And a bi-layer 5-tip SiN EC and a bi-layer SiN-Si transition were realized on the SOI wafer in this work.These two devices were integrated together on a chip to make the total output optical coupling structure of the integrated silicon-based Ⅲ-Ⅴ laser.This coupling structure has high coupling efficiency and broad optical bandwidth.And the output leading waveguide of the coupling structure is a standard SiPh single-mode silicon waveguide.Apart from the coupling structure,a reconfigurable SiPh logic device was measured in this work.The electronic-controlled device indicates promising application scenarios of the silicon-based optoelectronic devices integrated on an SOI wafer.The main research achievements and innovations of this work are as follows:1.A SiN-on-Si 3-D photonic integration structure was designed and fabricated in this work.A bi-layer SiN EC and a SiN-Si interlayer transition coupler were fabricated in the 3-D photonic integration structure.These couplers have high coupling efficiency and broad optical working bandwidth.The silicon waveguiding layer introduced enables the future integration of advanced active SiPh devices such as silicon modulators and germanium photodetectors,which cannot be made by the material of SiN.2.A bi-layer 5-tip SiN EC based on the SiN-on-Si photonic integration structure was proposed in this work.It was designed to integrate a Ⅲ/Ⅴ-on-Si lasers on a SiPh PIC.Although the laser emission position is 1 μm higher than Si waveguide plane,this EC can align well with the laser emission position both in the vertical and the horizontal direction.Lasing power was efficiently coupled from the laser output mode into a singlemode SiN waveguide through the SiN EC with a coupling efficiency of 92.8%at the wavelength of 1550 nm,a 1-dB-drop bandwidth of more than 362 nm,and taking only a small footprint with 38.2 μm coupling length.The SiN EC also had good alignment tolerances,with the vertical/horizontal 1-dB-loss misalignment tolerance of 0.5 μm and 0.8 μm at the wavelength of 1550 nm.3.The determining factors of the coupling efficiency of the SiN-Si interlayer transition were studied in this work.The optimized transition structure was obtained.The input of the transition structure is a SiN singlemode waveguide and the output is a Si single-mode waveguide.The coupling length was 85 μm and the interlayer spacer thickness was 250 nm.The simulated coupling efficiency from the lower SiN waveguide to the bottom Si waveguide was 95.6%at TE polarization at 1550 nm.The coupling efficiency was higher than 94.0%from the wavelength of 1490.5 nm to 1797.2 nm.The peak efficiency was 96.3%at 1588.9 nm.The simulated total coupling efficiency between the silicon-based Ⅲ-Ⅴ laser and the total optical output coupling structure(one bi-layer 5-tip SiN EC and one SiN-Si interlayer transition coupler)was 88.7%at 1550nm.This is also the laser-to-Si waveguide coupling efficiency.4.The fabricated SiN sing-mode waveguides,Si single-mode waveguides,bi-layer 5-tip SiN EC,SiN-Si transition,and the total SiN-onSi output optical coupling structure were measured by an accurate coupling system designed for nanophotonic devices.The measured Si waveguide loss was 4.1 dB/cm at 1550 nm,and the SiN single-mode waveguide loss was 6.3 dB/cm.The measured coupling efficiency of the SiN-Si interlayer transition coupler is 91.5%,which was slightly lower than the simulation results.The measured coupling efficiency of the bi-layer 5-tip SiN EC was 72.5%,and the measured overall coupling efficiency of the SiN-on-Si output optical coupling structure was 78.4%.Besides,the reconfigurable SiPh PIC logic devices were also measured in this work.The thermo-optic effect was successfully used to change the transfer function of the PIC logic device.By applying different electrical bias signals,one PIC device realized different optical logic operations including "AND","OR","NOT",and "XOR".