Linearity Study Of Silicon Microring-Assisted MZI Modulator

Silicon photonics has been rapidly developed for its advantages of high integration density,low power consumption,low cost,and compatibility with CMOS processes.Microwave photonics takes the advantages of photonic devices to implement microwave functions.It has the advantages of large bandwidth and immunity to interference.In recent years,it has received extensive attention and has been widely studied.The integration of microwave photonic subsystems by using silicon photonics can combine the advantages of photonic and microwave technologies,which has become an important direction for the development of microwave photonics in the future.In both digital optical communication links and microwave photonics links,electro-optic modulators are indispensible to convert electrical signals into optical ones.In microwave photonic systems,the non-linearity of the modulator will generate high-order harmonics and intermodulation distortion,while in digital optical communication systems,the non-linearity of the modulator degrades the signal quality during high-level modulation and long-distance transmission.This thesis focuses on the linearity study of a silicon microring-assisted Mach-Zehnder modulator.The sources of nonlinear distortion in the silicon modulator are analyzed first.Then,a theortical model used for linearity optimization of the modulator is established.After analyzing the modulation process,the coupling coefficient of the microring resoantor and the phase difference between the two arms of Mach-Zehnder interferometer(MZI)are appropriately set to get anti-Fano resonances.The non-linearity caused by the sinusoidal transfer function of the MZI structure and the superlinear phase modulation of the microring resonator can compensate each other,leading to a linear transfer function.Meanwhile,by considering the non-linearity caused by the PN junction and selecting the appropriate operating point in the transmission spectrum,the linearity of modulator can be further improved.This is also the traditional method to optimize the linearity of the modulator by using a microring-assisted MZI.We find that when the device operates in the Fano resonance wavelength,although the transfer function does not provide an ideal linear modulation range,the modulation efficiency is higher.As a result,a higher microwave link gain is obtained,leading to a larger spurious-free dynamic range(SFDR).The optical and electrical characteristics of the microring-assisted Mach-Zehnder modulator are simulated and analyzed.COMSOL and Lumerical simulation softwares are used to study the electrical and optical properties of the device and optimize its performance.Finally,the linearity of the device is simulated.The thesis presents experimental results of the silicon microring-assisted Mach-Zehnder modulator.The on-chip insertion loss of the device is 5 dB.The modulation efficiency is 0.96 V·cm under a reverse bias voltage of 3 V.High modulation linearity can be obtained at both the Fano resonant and the anti-Fano resonant wavelengths,of which the latter has a higher carrier-to-distortion ratio.However,as the Fano resonance has a higher modulation efficiency due to its sharp slope of the transfer function,a larger SFDR is obtained.The experimental results agree well with the theoretical calculations.Experimental results indicate that the best SFDR of the third-order intermodulation distortion is 111.3 dB·Hz^2/3measured at the 1 GHz frequency,comparable to the commercial lithium niobate modulators.