On-chip optical frequency combs could revolutionize a vast range of applications such as optical atomic clocks [1], astronomy [2,3], coherent optical communication [4,5], ultrafast distance measurements by means of LIDAR [6–9], spectroscopy [10], and integrated microwave photonics [11,12]. It could also be applied in optical neural computing [13,14] and quantum optics [15,16]. Silicon nitride (SiNx) has been the main platform for optical comb generation by way of Kerr nonlinearity as the material combines the lowest linear loss with a relatively high refractive index and strong Kerr effect among many material platforms [17]. SiNx is widely used in microelectronic and photonic fabrication flows, making it a readily acceptable candidate for monolithic integration with active photonic components and high-frequency electronics, which would be key to the practical applications of optical comb sources.