Silicon Photonics Foundry Fabricated, Slow-Light Enhanced, Low Power Thermal Phase Shifter

In this research, we developed a low-power silicon photonics foundry-fabricated slow-light thermal phase shifter (SLTPS) where the slow-light (SL) effect is achieved using an integrated Bragg grating (BG) waveguide. Heating the grating induces a red shift in the transmission spectrum, leading to an increased group index $n_g$ during operation, which facilitates a further reduction in the voltage needed for a $\pi$ phase shift, i.e. $V_{\pi}$. Additionally, we investigated a compact Mach-Zehnder Interferometer (MZI) that incorporates the SLTPS in both arms with a phase shifter length of 50 $\mu$m. A detailed theoretical analysis was conducted to address the non-idealities of the SL-MZI due to uneven optical power splitting and unbalanced loss in the two MZI arms. The $V_{\pi}$ and power consumption for a $\pi$ phase shift $(P_{\pi})$ of the SL-MZI were quantified for operation in the slow light regime, demonstrating a $V_{\pi}$ of 1.1 V and a $P_{\pi}$ of 3.63 mW at an operational wavelength near the photonic band edge. The figure of merit (FOM) $P_{\pi} \times \tau$ is commonly used to assess the performance of thermal optical switches. The SL-MZI in this work has achieved a low $P_{\pi} \times \tau$ of 5.1 mW $\mu$s. Insertion loss of the SL-MZI ranges from 1.3 dB to 4.4 dB depending on the operation wavelength, indicating a trade-off with the $V_\pi$ reduction.