Coherent demodulation using silicon-nitride integrated photonics for visible and short near-infrared optical communications

Photonic integrated circuits (PICs) are gaining significant attention in the visible and short near-infrared spectral regions for diverse applications such as spectroscopy, optical atomic clocks, quantum optics, and optical communications. Among various material platforms, silicon nitride (SiN) is particularly noteworthy due to its wide bandgap, low propagation losses, broad transparency, and well-established fabrication processes. Furthermore, thin-film lithium niobate (LN) has emerged as a promising platform for high-speed, power-efficient modulation in these spectral regimes, complementing the capabilities of SiN. Consequently, the integration of SiN and LN is well-suited for optical communication transmitters. However, on-chip receivers for such systems remain unexplored. We present the first demonstration of an optical coherent communication system at a short near-infrared window, utilizing a SiN-based integrated 90° optical hybrid. Our demonstration operates over a 13.9-THz bandwidth with 10-Gbaud binary phase-shift-keyed signals, which is comparable to or even more than the bandwidth of the conventional fiber-optic systems. Furthermore, we demonstrate a 2.2-m free-space optical transmission using the SiN chip as an application of visible and short near-infrared coherent communication. This demonstration highlights the potential of fully integrated high-performance coherent transceivers for new wavelength windows.