Reflective-path-enabled folded AWG on thin-film lithium niobate with 20 GHz fine channel spacing

Thin-film lithium niobate (TFLN) is a promising platform for high-speed optical interconnects, yet implementing arrayed waveguide gratings (AWGs) on X-cut TFLN remains challenging due to phase errors induced by the intrinsic refractive index anisotropy, which has prevented the realization of narrow-channel-spacing WDM devices on this platform. Here, we propose and demonstrate, for the first time, a folded AWG architecture with reflective optical paths on X-cut TFLN that mitigates anisotropy-induced phase distortions. As a proof-of-concept, the device delivers a 20 GHz channel spacing with 10 effective channels, an adjacent-channel crosstalk of -14.4 dB, and an insertion loss of 11.9 dB. To the best of our knowledge, this constitutes the first sub-50 GHz wavelength-division multiplexer on the X-cut TFLN platform, establishing an architectural pathway toward on-chip high-precision spectral sensing and signal detection.