High-Efficiency and Flat-Top Electro-Optic Frequency Comb Using a Single Modulator and Drive on Thin-Film Lithium Niobate

Optical frequency comb sources are essential components for applications in high-capacity optical communication, large-scale optical computing systems, and high-precision laser-based light detection and ranging. These applications require high-performance, flexible, and easy-to-operate chip-scale integrated frequency comb generators featuring good spectral flatness and high efficiency. Here, we demonstrate a new design strategy to achieve flat-top electro-optic frequency combs on the thin-film lithium niobate platform using a hybrid-mode Mach-Zehnder modulator structure. The proposed device adopts only a single modulator and a single microwave source, greatly reducing the overhead on the microwave components as well as the operation of the device. The fabricated device shows a low optical loss of similar to 0.2 dB, a tunable central wavelength up to a range of similar to 80 nm, a flexible frequency spacing from 18 to 38 GHz, and a wide 5 dB flat-top optical bandwidth of similar to 6.38 nm. A double-pass configuration with a compact footprint of 0.745 x 0.07 cm is also introduced to fully exploit the coplanar microwave waveguide on the chip. A similar comb performance is obtained using this device, showing a doubled efficiency for comb generations.