Effect Of Fabrication Variation On The Performance Of Silicon Thermo-optic Switches

With the development of optical communications and interconnections,silicon photonics has received more and more attention due to many advantages of silicon,such as low cost and compatibility with existing fabrication platforms.High-speed,high-integration density,low-power,low-loss optoelectronic devices are the research focus of silicon photonics.Optical switches,an important type of silicon photonic devices,also have received significant attention due to its applications in optical interconnections and communications,and in reconfigurable quantum photonic circuits.Therefore,their performance closely affects the working state of the entire optical transmission circuits.Fabrication variation is inevitable in the chip manufacturing process.The manufacturing process flow of silicon-based passive and active devices is described in detail,and the possible Fabrication variations and their effects on the performance of passive devices in the manufacturing process are pointed out.For example,for the two parallel waveguides of a directional coupler(DC),three possible variations are analyzed:synchronous variation in width or height,differential variation in width with identical hight,and differential variation in both width and height.The results show that even a small differential variation has a much greater effect on the splitting wavelength than a large synchronous variation.The 1×2 and 2×2 Mach-Zehnder thermo-optic switches based on SOI wafers are designed and fabricated by a CMOS foundry.After testing,the 1×2 switch insertion loss is less than 1.5dB,the extinction ratio can reach 20dB,the switching power is 16.5mW,the rise time(10%?90%)and the fall time are 3.6us and 19us,respectively;The 2×2 switch insertion loss is below 1 dB,the extinction ratio of all four paths can reach 30 dB(@1628 nm),the switching power is 33.7 mW,the rise time and fall time are 10.26 us and 19.2 us,respectively.The requirements of optical interconnects are basically realized.Here,we focus on the impact of fabrication variation on 2×2 thermo-optic switches.This paper illustrates the complexity of fabrication variation in active silicon photonic devices,and an approach to unraveling the complexity through separating the device level and structure level.It is found that the extinction ratio of the bar port and the cross port of the 2×2 switch has a difference of nearly 20 dB at some wavelengths,and the wavelength corresponding to the maximum extinction ratio is shifted from the designed 1550 nm to 1628 nm.Device physics analysis shows that this can be explained by synchronous variation of the splitting ratios of the two directional couplers in a switch.The calculations show that 2?3nm differential variation in width and height can shift the 3dB splitting wavelength by 70nm.Besides,a slight asymmetry between two arms of the MZI may also appear.The above phenomenon can be perfectly explained by these two possible reasons.For the single-waveguide crossing in the switch array,a crossing structure based on MMI is designed.The measured insertion loss is-0.14dB and the crosstalk is-40dB.In addition,a MMI-type double-crossing structure is also designed and optimized.The simulation results show that the insertion loss is less than 0.3dB,and the crosstalk value is below-25dB.