Ultra-compact thin-film-lithium-niobate photonic chip for dispersion compensation

Thin-film-lithium-niobate (TFLN) photonics has attracted intensive attention and become very popular in recent years. Here, an ultra-compact TFLN on-chip dispersion compensator is proposed and realized to provide a promising solution for dispersion control. The proposed dispersion compensator is composed of chirped multimode waveguide gratings (CMWGs) arranged in zigzag-cascade, enabling high footprint compactness and scalability. Particularly, these CMWGs are circulator-free and very convenient for cascading, owing to the TE0–TE1 mode conversion and the assistance of the TE0–TE1 mode (de)multiplexer. The present configuration with CMWGs in zigzag-cascade also overcomes the drawback of being unable to use waveguide spirals for large-range time delay and dispersion control due to the TFLN’s anisotropy. In addition, positive/negative dispersion control is realized by appropriately choosing the input port of the CMWGs. In the experiment, 2-mm-long CMWGs are used to provide a dispersion value of about +1.5 ps/nm and −1.2 ps/nm over a 21-nm-wide bandwidth, and there are up to 32 CMWGs in cascade demonstrated experimentally, showing a maximal dispersion of 49.2 ps/nm and −39.3 ps/nm. The corresponding average propagation loss is as low as 0.47 dB/cm, and the fabricated chip with 32 CMWGs in zigzag-cascade has a footprint as compact as 0.16 × 4.65 mm2. Finally, the present on-chip dispersion compensator is used successfully to compensate for the dispersion originating from a 5-km-long singlemode fiber (SMF) and high-quality eye-diagrams are achieved for the recovered 40 Gbps OOK signals, showing great potential for optical systems such as high-speed interconnects in datacenters.