  In order to overcome the challenge of obtaining high modulation depth due to
weak graphene-light interaction, a graphene-on-silicon nitride (SiNx) microring
resonator based on graphene's gate-tunable optical conductivity is proposed and
studied. Geometrical parameters of graphene-on-SiNx waveguide are
systematically analyzed and optimized, yielding a loss tunability of 0.04
dB/{\mu}m and an effective index variation of 0.0022. We explicitly study the
interaction between graphene and a 40-{\mu}m-radius microring resonator, where
electro-absorptive and electro-refractive modulation are both taken into
account. By choosing appropriate graphene coverage and coupling coefficient, a
high modulation depth of over 40 dB with large fabrication tolerance is
obtained.
