Four-Channel Broadband Mode (De)multiplexer Based on Thin-Film Lithium Niobate Platform

The lithium-niobate-on-insulator (LNOI) platform has recently emerged as a promising candidate for advanced photonic functions due to its excellent electro-optic coefficient. However, there remain some challenges associated with the etching of LNOI, which typically results in a decreased performance of the fabricated devices. For instance, fabrication errors may reduce the bandwidth of the mode multiplexer and demultiplexer (MMUX/DEMMUX), thereby limiting the capacity of the communication transmission. In this study, a four-channel broadband MMUX/DEMMUX based on an LNOI strip waveguide is experimentally demonstrated by employing an asymmetrical directional coupler structure. To avoid phase mismatch caused by etching depth error, an LNOI strip waveguide was introduced instead of a ridge waveguide. Additionally, an auxiliary spiral loss line was introduced to consume the residual energy of incomplete coupling due to fabrication error and ensure the low crosstalk of the device. Experimental results show that the device achieves a bandwidth exceeding 130 nm with a crosstalk of less than -10.6 dB, making it achieve the largest multiplexing bandwidth reported for LNOI-based platforms. Furthermore, a clear eye diagram at 64 Gbps demonstrates the capability for the high-speed communication offered by the fabricated device.