Mario C. M. M. Souza

Embedded coupled microrings with high-finesse and close-spaced resonances for optical signal processing

Single microring resonators have been used in applications such as wavelength multicasting and microwave photonics, but the dependence of the free spectral range with ring radius imposes a trade-off between the required GHz optical channel spacing, footprint and power consumption. We demonstrate four-channel all-optical wavelength multicasting using only 1 mW of control power, with converted channel spacing of 40-60 GHz. Our device is based on a compact embedded microring design fabricated on a scalable SOI platform. The coexistence of close resonance spacing and high finesse (205) in a compact footprint is possible due to enhanced quality factors (30,000) resulting from the embedded configuration and the coupling-strength dependence of resonance spacing, instead of ring size. In addition, we discuss the possibility of achieving continuously mode splitting from a single-notch resonance up to 40 GHz.

Modeling quasi-dark states with temporal coupled-mode theory.

Coupled resonators are commonly used to achieve tailored spectral responses and allow novel functionalities in a broad range of applications. The Temporal Coupled-Mode Theory (TCMT) provides a simple and general tool that is widely used to model these devices. Relying on TCMT to model coupled resonators might however be misleading in some circumstances due to the lumped-element nature of the model. In this article, we report an important limitation of TCMT related to the prediction of dark states. Studying a coupled system composed of three microring resonators, we demonstrate that TCMT predicts the existence of a dark state that is in disagreement with experimental observations and with the more general results obtained with the Transfer Matrix Method (TMM) and the Finite-Difference Time-Domain (FDTD) simulations. We identify the limitation in the TCMT model to be related to the mechanism of excitation/decay of the supermodes and we propose a correction that effectively reconciles the model with expected results. Our discussion based on coupled microring resonators can be useful for other electromagnetic resonant systems due to the generality and far-reach of the TCMT formalism.

Hybrid multimode resonators based on grating-assisted counter-directional couplers

Research thrusts in silicon photonics are developing control operations using higher order waveguide modes for next generation high-bandwidth communication systems. In this context, devices allowing optical processing of multiple waveguide modes can reduce architecture complexity and enable flexible on-chip networks. We propose and demonstrate a hybrid resonator dually resonant at the 1st and 2nd order modes of a silicon waveguide. We observe 8 dB extinction ratio and modal conversion range of 20 nm for the 1st order quasi-TE mode input.

Photonic molecules for application in silicon-on-insulator optical sensors

Optical sensors based on integrated photonics have experienced impressive advancements in the past few decades and represent one of the main sensing solutions in many areas including environmental sensing and medical diagnostics. In this context, optical microcavities are extensively employed as refractive index (RI) sensors, providing sharp optical resonances that allow the detection of very small variations in the surrounding RI. With increased sensitivity, however, the device is subjected to environmental perturbations that can also change the RI, such as temperature variations, and therefore compromise their reliability. In this work, we present the concept and experimental realization of a photonic sensor based on coupled microcavities or Photonic Molecules (PM) in which only one cavity is exposed to the sensing solution, allowing a differential measurement of the RI change. The device consists of an exposed 5-μm radius microdisk resonator coupled to an external clad microring resonator fabricated on silicon-on-insulator (SOI) platform. This design allows good sensitivity (26 nm/RIU) for transverse electrical mode (TE-mode) in a compact footprint (40 × 40 μm2), representing a good solution for real-life applications in which measurement conditions are not easily controllable.

Low-power four-channel wavelength multicasting in embedded microring resonators

We demonstrate four-channel all-optical wavelength multicasting using only 1 mW of pump power and channel spacing of 40-60 GHz. Our device is based on a compact embedded microring design fabricated on a scalable SOI platform.