Proposal and Analysis of a Silicon MMI Coupler-Based Electronically Controllable Photonic Switch

Abstract

In this paper, we propose and analyze an electronically controllable $2 \times 2$ multimode interference (MMI) coupler-based photonic switch (PS) in form of a lateral p-i-n diode, which is realized on the silicon-on-insulator platform. The switching has been achieved by applying external bias voltage, which induces free carrier dispersion effect-based refractive index change. Theoretical analysis of the proposed PS based on perturbation theory is presented, and the results are verified by numerical simulations and full vectorial analysis. Analysis and simulation show that the proposed multimode interference photonic switch (MMIPS) can offer maximum 22.7 dB extinction ratio (ER) and $\sim$ 1.5 dB of maximum excess loss (EL). The maximum switching voltage is found to be $\sim$ 2.25 V. Optimization in terms of device length, doping concentrations, width of the intrinsic region, and position of the access waveguides for achieving desired values of ER and EL has been presented. Analysis also indicates strong polarization dependence of the proposed switch. Steady-state analysis of the proposed MMIPS confirms its operation over entire C-band (1530–1565 nm) with maximum 0.25 and 0.35 dB optical power variation in bar and cross port, respectively. It is found that the MMIPS can easily withstand a temperature variation of 20 K. Moreover, we show that change in switching performance due to temperature variation can also be compensated by adjusting external bias voltage. This paper also investigates the fabrication tolerances of MMIPS in terms of imperfections in device length, width, and doping concentrations. Study shows that $\pm \text{100}$ nm variation in device length and $\pm \text{50}$ nm variation in device width introduce maximum optical power variations of 0.3 and 0.1 dB in cross port (and 0.2 and 0.07 dB in bar port), respectively. The doping concentration variation, as permissible in state-of-the-art fabrication process, is found to introduce negligible penalty on the switching performance. Transient analysis and equivalent circuit model of the proposed switch confirms more than 1 GHz of switching speed. Small-signal equivalent circuit of the proposed MMIPS predicts power dissipation in the orders of tens of milliwatts.