| Silicon-based optoelectronic chips become the core technology in the post-Moore era because of its advantages in high-speed,high-density,high-bandwidth,multi-function,small-size,low-power consumption,low-cost and compatibility with the complementary metal-oxide-semiconductor(CMOS)technology.C urrently,silicon-based optoelectronic chip technology is reaching maturity for a majority of photonic devices such as passive waveguides,modulators and photodetectors,while directly epitaxial Ⅲ-Ⅴ lasers on silicon are still in the exploratory stage.As an indirect bandgap semiconductor material,silicon is unsuitable for the realization of light sources,so how to integrate lasers with other optical components is a key factor that limits the development of silicon-based optoelectronic chips.Instead,Ⅲ-Ⅴ semiconductors such as GaAs and InP are direct bandgap materials with high luminescence efficiencies,making them more suitable for producing efficient and reliable lasers.Therefore,it has been proposed to integrate high-quality Ⅲ-Ⅴ materials onto the existing Si photonic platform to realize on-chip light sources.At present,bonding technology and directly epitaxial growth are two main types of approaches to integrate light sources on the Si platform.The integration method using bonding technology is currently reaching maturity.However,the price and size of Ⅲ-Ⅴ substrates and the complex bonding processes result in a high cost,limiting the economies of scale.Yet,directly epitaxial Ⅲ-Ⅴlasers on silicon can integrate the process to the greatest extent,and are expected to achieve large-scale manufacturing of wafers with high yield,so it is considered to be one of the most promising solutions for realizing light sources on silicon substrates.In recent years,it has made great progress on directly epitaxial Ⅲ-Ⅴ quantum dot lasers on silicon,but a thick buffer layer and defect filter layers increase the difficulty of light coupling from the laser to the on-chip Si waveguide greatly,so the research results are limited to the performance improvement of a single device.Therefore,it is an urgent problem to couple the output light from electrically pumped Ⅲ-Ⅴ lasers epitaxially grown on silicon into on-chip Si waveguides.Based on the above background,we research the monolithically integrated structure of the directly epitaxial Ⅲ-Ⅴ quantum dot laser on silicon and the silicon waveguide which revolves around the question of how to couple the output light of the Ⅲ-Ⅴ laser epitaxially grown on silicon into the on-chip silicon waveguide.The specific work and research results are as follows:(1)We proposed a design scheme to enable the monolithic integration between silicon waveguides and 1.3μm wavelength band Ⅲ-Ⅴquantum dot lasers,which are epitaxially grown on silicon with an asymmetric structure.The deep trenches are etched down to the Si substrate and Ⅴ-grooves are prepared further in the deep trenches,then use selective area epitaxy to grow Ⅲ-Ⅴ lasers,which can reduce the number and thickness of Ⅲ-Ⅴ epilayers,so that the difficulty of non-planar growth can be reduced.And a GaAs coupling layer is inserted into the lower cladding layer of the laser,which can make the optical field distribution of the laser shift down,so that the coupling difficulty between laser and silicon waveguide is reduced.Besides,a mode-size converter with a three-segment tapered structure is designed to couple the output laser into the standard single-mode silicon waveguide.For the laser,the composition and the thickness of AlGaAs cladding layers and AlGaAs transition layer,and the thickness of GaAs coupling layer are optimized based on the optical waveguide theory.When the thickness of GaAs coupling layer is 320nm,the upper cladding layer is 0.6μm Al0.7Ga0.3As,the lower cladding layer is 1.2μm Al0.25Ga0.75As,and the transition layer is 20nm Al0.45Ga0.55As,the optical confinement factors of the active region and the coupling layer are 45.34%and 40.69%,respectively.Then the length of the mode-size converter with a three-segment tapered structure is further optimized by the mode-matching method.When the lengths of the three tapered structures of the mode-size converter are 50μm,53μm and 10μm respectively,a coupling efficiency of 65%can be obtained between the laser and the Si waveguide.This scheme provides a feasible way for the on-chip integration of lasers epitaxially grown on silicon.(2)We propose a scheme to couple the output light of 1.3μm microdisk quantum dot lasers epitaxially grown on silicon into standard single-mode silicon waveguides.For microdisk lasers,a double-ended radial gap coupled waveguide structure is used to achieve single-mode operation and directional output.The gradient index lens mode-size converter is used to realize the conversion of the light spot in the thickness direction,and the disk mode converter is designed to convert the high-order transverse mode into the fundamental transverse mode,so the output laser can be converted into a mode suitable for transmission in a standard single-mode silicon waveguide.First,we theoretically analyzed the mode characteristics of the microdisk laser,and optimized the output waveguide width and coupling gap using the three-dimensional finite-difference method to achieve stable single-mode lasing of the microdisk laser in the 1.3μm wavelength band.The simulation results show that the introduction of the double-ended waveguide structure is helpful to realize the single-mode operation of the microdisk laser.When the output waveguide width is 0.75μm and the coupling gap is 0.09μm,the laser can achieve stable single-mode resonance,the main lasing mode is TE38,4,the mode wavelength is 1.269μm,the mode quality factor is 6409,and the radial waveguide coupling efficiency 66%.Then the length of the mode-size converter is optimized,and when the length is 15.5μm,the coupling efficiency between the mode-size converter and the silicon waveguide is 68%.Finally,the disk mode converter is optimized.When the radius is 1.2μm and the output waveguide width is 0.35μm,the conversion efficiency between the higher-order mode and the fundamental mode is 99%. |
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