Design of a vanadium dioxide-based dual-polarization optical PAM4 modulator

Abstract

CMOS compatibility and large variations in optical properties of vanadium dioxide (VO2) during its insulator-metal phase transitions have made this material an interesting choice for realization of optical modulators in the silicon-on-insulator platform. However, despite its promising properties, the phase transition time of electrically actuated VO2 is on the order of nanoseconds, which limits the maximum achievable bit rate of VO2-based optical modulators. To overcome this limitation and increase the maximum bit rate of VO2-based optical modulators, the implementation of dual-polarization four-level pulse amplitude (PAM4) modulation is proposed here as a viable solution. In the design process of the dual-polarization PAM4 modulator, a VO2-based double-slot waveguide is proposed, which, by adjusting its geometrical parameters, can support single transverse electric (TE) or single transverse magnetic (TM) mode. For each polarization, the optical mode of the designed waveguide can provide four different propagation loss levels based on the conduction phase of its VO2 layers. Designed TE and TM waveguides are then coupled to a silicon access waveguide in a directional coupler structure to form the dual-polarization optical PAM4 modulator at the telecommunication wavelength of 1.55 μm. The geometrical parameters of TE and TM waveguides and those of the coupling region are optimized so that equally spaced output power levels can be achieved in four operating states of the designed modulator in both TE and TM polarizations. The overall footprint of the proposed modulator along with the required electrical contacts is estimated to be below 100 μm2. Numerical simulations based on three-dimensional finite-difference-time-domain method show energy consumption of 2.5 pJ/bit and 3.9 pJ/bit for TM and TE polarizations, respectively, while implementation of the dual-polarization PAM4 modulation scheme increases the modulation bit rate of the proposed modulator by a factor of four compared to previously reported VO2-based optical modulators.