Compact Lithium Niobate Michelson Interferometer Modulators Based on Spiral Waveguides

We demonstrate ultra-compact and highly efficient electro-optic Michelson interferometer modulators on thin film lithium niobate based on spiral-shaped waveguides. The modulator utilizes the in-plane isotropy of the Z-cut lithium niobate refractive index to achieve space-efficient spiral waveguides that are modulated using bottom and top electrodes. Monolithic optical rib waveguides are achieved using dry etching of lithium niobate with bottom and top cladding layers made of silicon dioxide and SU-8 polymer, respectively. The proposed modulator requires a total area of 175×175 μm2 to accommodate a 9-mm long waveguide, owing to the optimized design of the spiral inner radius and the gap between adjacent turns. The vertical distance between electrodes is engineered to achieve a half-wave-voltage-length product Vπl less than 2.02 V.cm with low optical propagation loss of 1.3 dB/cm. The 3-dB electro-optic bandwidth of the fabricated modulators varied between 4.2 GHz and 17.8 GHz for total spiral lengths of 9 mm and 1.2 mm, respectively. The compact modulator architecture fulfills the pressing demand for high-density photonic integrated circuits in modern data centers and telecommunication networks.