Broadband wavelength-flattened directional coupler on silicon nitride for quantum photonic integrated circuits

Integrated silicon nitride photonics has emerged as a key enabling technology for quantum technologies. Among the essential devices, beam splitters in the form of integrated directional couplers are widely used in circuits such as Hong-Ou-Mandel interferometry. However, the wavelength-dependent nature of these couplers limits their use in experiments involving broadband photons. In this work, we experimentally demonstrate a wavelength-flattened directional coupler on the silicon nitride platform, exhibiting a nearly wavelength-flattened splitting ratio over the measured telecom wavelength range. This integrated photonic circuit is compatible with broadband quantum photonic circuits, including Hong-Ou-Mandel interferometry, and supports the development of integrated single- and photon-pair sources.