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

Abstract 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 TE 0 –TE 1 mode conversion and the assistance of the TE 0 –TE 1 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 mm 2 . 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.