This paper presents a theoretical analysis of a silicon all-optical modulator based on free-carrier injection by two-photon absorption (TPA) in a highly light-confining structure. In spite of the weak optoelectronic properties of silicon, strong light confinement allows high modulation depths in very compact devices requiring low-energy pump pulses. This analysis is applied to 1-5 µm radius silicon ring resonators with the pump pulse coupled on-chip and including in the model the scattering loss due to sidewall roughness originating from the fabrication process. The calculations show that using this scheme, modulation depths greater than 80% can be achieved, with no more than 3 pJ of pump pulse energy, at speeds on the order of 10 GHz.
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