Novel Folded Structure Recycling Phase Modulator for Ultralow Vπ and Enhanced Low-Vπ Bandwidth

Electro-optic phase modulators (PMs) with an ultralow half-wave voltage $(V_{\pi})$ are fundamental devices for optical communication and microwave photonics systems. Here, a folded recycling PM is developed to achieve both an ultralow $V_{\pi}$ and a large $\text{low}-V_{\pi}$ bandwidth via a thin-film lithium niobate (TFLN) platform. This novel design enables a high modulation efficiency of the recycling modulation structure and a large low-$V_{\pi}$ bandwidth by reducing the time delay of the optical loop-back waveguide. The fabricated PM is characterized by an ultralow $V_{\pi}$ of 2.84 V@48.1 GHz, and particularly a large low-$V_{\pi}$ bandwidth of 2.6 GHz in each modulation period from 3.5 to 50 GHz.