Nonreciprocal light propagation in a cascaded all-silicon microring modulator

Optical isolators and circulators are critical building blocks for large-scale photonic integrated circuits. Among the several methods proposed to realize such nonreciprocal devices, including heterogeneous integration with garnet-based materials or using nonlinearities, dynamic modulation of the waveguide properties is a potentially practical and easily accessible method. However, most proposals relying on this method rely on modulators with a very large footprint, limiting their practical applicability. This paper overcomes this issue by presenting a method to achieve nonreciprocal optical transmission taking advantage of compact ring modulators. We use a cascaded system of microring modulators with a footprint as small as 15 mu m x 220 mu m and propose that, by tuning the relative time delay between the RF driving signals and the optical delay between the modulators, nonreciprocal transmission can be achieved. We present a detailed theoretical analysis of our design and investigate the origin of the asymmetric transmission. The modulators were designed and fabricated on IMEC's Silicon-on-Insulator platform iSiPP50G. We achieve a 16 dB difference between forward and backward optical signals at a driving voltage (V-pp) of 8 V at 6 GHz. Moreover, we analyze the impact of fabrication imperfections on the device performance. Our work leads to a significant reduction in device footprint compared to formerly explored solutions using dynamic modulation and is well suited for monolithic integration with photonic integrated circuits