Enhanced Linearity of MOSCAP-Driven Silicon Microring Modulator

Optical microring modulator plays a critical role in optical communication and computation due to its ultra-compact footprint and low-driving voltage; however, the linearity of microring resonator is constrained by its intrinsic Lorentzian transfer curve. In this article, we analyze and characterize the linearity of a metal-oxide-semiconductor capacitor (MOSCAP) silicon microring modulator (Si-MRM) driven by transparent conductive oxide (TCO) gate. Our theoretical analysis shows that due to the varying quality factor (Q-factor) with respect to the gate voltage, the linearity of MOSCAP Si-MRM can be improved when the input wavelength is set on the left side of the transmission dip. By fitting a linear line to the transfer curve with driving voltage from 0 to -4V, our experimental results prove significant linearity improvement from left-sided modulation compared to right-sided modulation: the coefficient of determination ($\text{R}^{2}$) increases from 0.9806 to 0.9861 and the Root Mean Square Error (RMSE) decreases from 0.0321 to 0.0215. Spurious-Free Dynamic Range (SFDR) measurement also achieves convincing results of 106 dB·$\text{Hz}^{2/ 3}$ from left-sided modulation as compared to 98.5 dB·$\text{Hz}^{2/ 3}$ from right-sided modulation and 104.3 dB·$\text{Hz}^{2/ 3}$ of reversed-PN junction Si-MRM that is the best result from existing literature. The enhanced linearity due to the varying Q-factor from MOSCAP Si-MRM provides a new mechanism to reduce signal distortion and improve dynamic range, which are essential to RF-photonic systems and high-precision optical computing.