Study Of On-chip Output Optical Coupling Structure For Ⅲ-Ⅴ Semiconductor Laser On Si-based Monolithic Photonic Integrated Chip

With the rapid development of digitalization,the demands for high-speed data transmission and mass big data storage are becoming urgent.Silicon photonics applies CMOS compatible processes to enable large-scale,energy-efficient photonic integrated circuits(PICs)with high yield and low cost.This is a promising solution to overcome the bottleneck of computing performance and data transmission bandwidth.Recent progress shows the long-wavelength Ⅲ/Ⅴ-on-Si quantum dot lasers with a good lifetime that meets the commercial standard.At the same time,over-30 GHz-bandwidth silicon photonic platforms have been realized in commercial foundries that include low-loss passive components,high-speed modulators,and Ge-on-Si photodetectors.Therefore,the monolithic integration of Si-based PICs with Ⅲ/Ⅴ-on-Si lasers using CMOS-compatible processes is becoming a significant agenda in the field of silicon photonics.However,due to the mode mismatch and position misalignment between the Si-based edge-emitting lasers(EELs)and the standard silicon photonic waveguide,it is challenging to realize high efficient optical coupling between the active and the passive devices.This work is supported by the National Key Research and Development Program of China and the Program of Beijing Municipal Science and Technology Commission.In order to couple the emission from the monolithic laser to the waveguides in the silicon photonic chips,this work designs integrated photonic coupling structures on a silicon nitride-on-silicon(SiN-on-Si)platform for the Ⅲ/Ⅴ-on-Si lasers.The performance of the SiN-on-Si photonic devices is also verified after device fabrication.The major results and innovations are as follows.(1)Design a novel on-chip CMOS-compatible output optical coupling structure for Ⅲ/Ⅴ semiconductor laser on Si-based monolithic photonic integrated chip,including SiN-based edge coupler and a meta-structure-based bi-layer SiN interlayer transition.In the coupling structure,the vertical coupling distance is 1.4 μm and the total coupling length from the input port of the SiN edge coupler to the output port of the SiN interlayer transition is 32.72 μm.Simulation shows that the total coupling efficiency from a Si-based 1550 nm EEL to the lower SiN waveguide by these integrated SiN-based devices is-8.92 dB(where edge coupler:-3dB/each,interlayer transition:-5.92 dB/each).The 1-dB optical working bandwidth of the coupling structure is about 131 nm across C-band.The major advantage of the coupling structure is that it can have large vertical coupling distance,very small footprint,and very broad optical working bandwidth simultaneously with decent coupling efficiency for monolithic laser integration scenarios.(2)Fabricate and measure the coupling structure for the Si laser integration.Measurement results of the coupling structure based on external tunable laser source show that the total coupling efficiency from an EEL to the lower SiN waveguide by these integrated SiN-based devices is-9.5 dB(where edge coupler:-3.4 dB/each,interlayer transition:-6.1 dB/each).The 1-dB optical working bandwidth is more than 58.1 nm across C-band.The measurement results are satisfy the simulation results well except the bandwidth.The poor coincidence of the bandwidth is due to the tooth width error in electron beam lithography process.When using the on-chip multi-mode FP laser for characterization,the measured coupling efficiency of SiN edge coupler is-4.1 dB at 1545.34 nm(FP laser fundamental mode wavelength).The measurement results become a little worse than the measurement using the external laser source.The possible reason is that the on-chip laser works in multi-longitudinal mode,and its wave of the fundamental mode deviates from the design value.In addition,the transverse mode size of on-chip laser does not match that of SiN edge coupler,and the parameters of the gap filling material between the on-chip laser and the SiN edge coupler are not consistent with the design parameters.The experiment results demonstrate the feasibility of integration of a Si-based laser with the coupling structure.The differences between the measurement conditions and the parameters of the simulation model need to be solved in the follow-up study.Further analysis also shows that the performance of the coupling structure still has a greater improvement.