Frequency comb generation in the green using silicon nitride microresonators
Optical frequency combs enable precision measurements in fundamental physics and have been applied to a growing number of applications, such as molecular spectroscopy, LIDAR and atmospheric trace-gas sensing. In recent years, the generation of frequency combs has been demonstrated in integrated microresonators. Extending their spectral range to the visible is generally hindered by strong normal material dispersion and scattering losses. In this paper, we report the first realization of a green-light frequency comb in integrated high-Q silicon nitride (SiN) ring microresonators. Third-order optical non-linearities are utilized to convert a near-infrared Kerr frequency comb to a broadband green light comb. The 1-THz frequency spacing infrared comb covers up to 2/3 of an octave, from 144 to 226 THz (or 1327-2082 nm), and the simultaneously generated green-light comb is centered around 570-580 THz (or 517-526 nm), with comb lines emitted down to 517 THz (or 580 nm) and up to 597 THz (or 502 nm). The green comb power is estimated to be as high as -9.1 dBm in the bus waveguide, with an on-chip conversion efficiency of -34 dB. The proposed approach substantiates the feasibility of on-chip optical frequency comb generation expanding to the green spectral region or even shorter wavelengths.