Octave-spanning dissipative Kerr soliton frequency combs in Si₃N₄ microresonators

Octave-spanning, self-referenced frequency combs are applied in diverse fields ranging from precision metrology to astrophysical spectrometer calibration. The octave-spanning optical bandwidth is typically generated through nonlinear spectral broadening of femtosecond pulsed lasers. In the past decade, Kerr frequency comb generators emerged as a novel scheme offering chip-scale integration, high repetition rate, and bandwidths that are only limited by group velocity dispersion. The recent observation of the dissipative Kerr soliton (DKS) operation regime, along with dispersive wave formation, has provided the means for fully coherent, broadband Kerr frequency comb generation with an engineered spectral envelope. Here, by carefully optimizing the photonic Damascene fabrication process, and dispersion engineering of Si₃N₄ microresonators with a free spectral range of 1 THz, we achieve bandwidths exceeding one octave at low powers (~100  mW) for pump lasers residing in the telecom C band (1.55 μm) as well as in the O band (1.3 μm). Precise dispersion engineering enables emission of two dispersive waves, increasing the power in the spectral ends of the comb, down to a wavelength as short as 850 nm. Investigating the coherence of the generated Kerr comb states, we unambiguously identify DKS states using a response measurement. This allows demonstrating octave-spanning DKS comb states at both pump laser wavelengths of 1.3 μm and 1.55 μm, including the broadest DKS state generated to date, spanning more than 200 THz of optical bandwidth. Octave-spanning DKS frequency combs can be applied in metrology or spectroscopy, and their operation at 1.3 μm enables applications in biological and medical imaging such as Kerr-comb-based optical coherence tomography or dual-comb coherent anti-Stokes Raman scattering.