Research On Novel Vertical Grating Coupler For VCSEL-based On-chip Optical Interconnect

With the further development of the information society and the arrival of the era of big data,the communication rate is increasing day by day,and electrical interconnection faces the defects of large transmission delay and susceptibility to electromagnetic interference in high-speed data transmission.Therefore,using photons as information carriers to replace electrons,it is expected that optical interconnects with faster transmission rates and stronger electromagnetic interference resistance can be used to replace electrical interconnects to complete the transmission of high-density information to solve the bottleneck of electrical interconnection technology.Silicon-based photonic integrated circuits have great advantages of low cost,high integration density,and compatibility with mature CMOS processes,and are considered to be a promising on-chip optical interconnect solution.After decades of research,silicon-based photonic integration has developed rapidly,but the problem of the light source on the silicon substrate has not been solved.Hybrid integrated technology is an alternative solution that can provide integrated light sources for silicon-based photonic integrated circuits,but how to achieve efficient optical coupling of external light sources and waveguides on silicon substrates remains a challenge.In addition,the vertical cavity surface emitting laser(VCSEL)has the characteristics of low threshold,low power consumption and small size,and is suitable for silicon-based hybrid integrated systems.However,due to the polarization switching of VCSEL,in addition to the standard silicon photonic waveguide has a strong polarization selectivity,these effects will reduce the coupling efficiency between VCSEL and silicon waveguide.If it is not resolved,the bit error rate of the on-chip optical communication/interconnection system based on this scheme will rise,resulting in a decline in communication quality.The main research results and innovations are as follows:1.The principle of VCSEL polarization control is analyzed,and the theoretical models of VCSEL polarization control are studied,such as the four-level VCSEL polarization dynamics model and the external cavity optical feedback polarization control model.The analysis and design methods of the grating coupler are studied,such as rigorous coupled wave analysis and finite-difference time-domain.2.A SiN-on-Si bi-layer vertical grating coupler(VGC)is designed and proposed.The GC can control the polarization state of the light output and at the same time realize the efficient coupling between the VCSEL and waveguide.The grating coupler is used for hybrid integration with VCSEL,and the integrated structure is used as a silicon-based hybrid integrated laser.Simulation analysis shows that the coupling efficiency reaches-3.6 dB at 1550 nm TE mode.When the external cavity length is 1 μm and the bias current is less than 2.6 mA,the output of VCSEL can be stabilized in single TE mode.3.A single-layer VGC based on SOI is proposed.The GC can control the polarization state of the VCSEL to stabilize it in the TE mode while maintaining efficient coupling of the VCSEL to the silicon waveguide.Simulations show that the coupling efficiency reaches-6.5 dB at 1550 nm TE mode.When the external cavity length is 0.6 μm and the bias current is less than 3.9 mA,the output of VCSEL can be stabilized in the single TE mode.4.The layout of the single-layer SOI VGC is drawn.The fabrication of the reflective sub-wavelength focusing grating with focal length of 400μm is accomplished.According to the experimental data,the reflective index of reflective sub-wavelength focusing grating is 80.5%,the FWHM is 168 nm,and optical power at the focal point reaches 414 μw.