A Study On Low-loss Optical Connection Devices On Chip

With the development of information technology,communication systems and data centers have more requirement on transmission speed and bandwidth.Optical interconnection with high speed and large bandwidth is a promising solution.Large-scale photonic integrated circuits(PICs)have attracted extensive attention since silicon-photonic devices are easy to miniaturize and integrate and are compatible with CMOS processes.To achieve large-scale and high-density PICs,the use of optical connection devices,such as waveguide crossings and coupling structures is inevitable.However,directly crossing two waveguides would lead to significant light scattering and crosstalk.Meanwhile,there is mode field mismatch in the process of optical transmission from single-mode fiber to waveguide which will lead to a lot of coupling loss.Therefore,it is necessary to design low loss on-chip optical connection devices.A variety of planar waveguide crossings were designed,including silicon waveguide crossings based on multimode interference couplers,inverse design and Gaussian beam synthesis method and SiN planar waveguide crossings based on multilayer platform.The above devices were fabricated by CMOS compatible processes.The waveguide crossing based on multimode interference couplers had a loss of 0.035 d B and had a crosstalk lower than-31 d B.The waveguide crossing based on inverse design had a loss of 0.09 dB due to the low fabrication tolerance.Based on the method of Gaussian beam synthesis,the waveguide crossing was demonstrated by introducing inverse tapers to lower the loss.It had a crosstalk lower than-40 d B and a loss of 8 md B,which is,to our knowledge,the lowest loss of planar silicon waveguide crossing working at O-band.Besides,the SiN planar waveguide crossing with a loss of0.012 d B and a crosstalk of lower than-60 d B was achieved.Based on the SiN-SiN-Si multilayer platform,low-loss 3D waveguide crossings and interlayer coupling structure were designed and fabricated by CMOS process.3D waveguide crossings with a loss of 1.4 md B were achieved by selecting appropriate waveguide parameters and SiO₂ layer thickness.By introducing the elliptic parabolic taper,the interlayer coupling structure with a short coupling length of 140?m was achieved and the coupling loss was 0.14 d B.In addition,based on the mode expansion method,a rib-waveguide to strip-waveguide transition structure was designed.By introducing tapers and multi-mode waveguides,the device has a simulated loss of 0.042d B.Two kinds of backside coupling structures were designed based on the grating coupler.The first kind of structure required substrate etch.The optical fiber was inserted vertically into the bottom of the chip.The cladding ramp reflection could improve the coupling efficiency of grating and the simulation loss was 1.44 d B.In the second structure,the lens was etched at the bottom of chip without wafer-through etch,and the apodized grating coupler was introduced to enhance the coupling efficiency and the simulation loss was 1.68 dB.