Octave-spanning dissipative Kerr soliton frequency combs in Si_3N_4 microresonators

Abstract

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 have emerged as novel scheme offering chip-scale integration, high repetition rate and bandwidths that are only limited by group velocity dispersion. The recent observation of Kerr frequency combs operating in the dissipative Kerr soliton (DKS) regime, along with dispersive wave formation, has provided the means for fully coherent, broadband Kerr frequency comb generation with engineered spectral envelope. Here, by carefully optimizing the photonic Damascene fabrication process, and dispersion engineering of Si3N4 microresonators with 1 THz free spectral range, we achieve bandwidths exceeding one octave at low powers (O(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. Equally important, we find that for THz repetition rate comb states, conventional criteria applied to identify DKS comb states fail. Investigating the coherence of generated, octave-spanning Kerr comb states we unambiguously identify DKS states using a response measurement. This allows to demonstrate 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 form essential building blocks for 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.

Emerging material systems for integrated optical Kerr frequency combs

“…Additionally, due to the increase in refractive index, the optical mode area is smaller and more tightly confined within the device. Therefore, the dispersion of the cavity can be engineered by changing the geometrical parameters of the waveguide cross section [351] [335] [352]. However, these types of designs are only possible when a device geometry can be tightly controlled.…”

Kerr-Microresonator Soliton Frequency Combs at Cryogenic Temperatures

“…The results of this study open up new directions in advancing chip-scale frequency comb optical clocks and metrology at cryogenic temperatures.Temporal dissipative Kerr soliton pulses generated in nonlinear microresonators are a promising technology for metrological applications of frequency combs [1]. Their ability to cover a spectral region over an octave [2,3] while operating in a low-noise, phase-stable configuration enables new classes of chip-scale photonics components such as optical frequency synthesizers [4,5], optical clocks [6,7], and microwave generators [8,9]. However, reaching soliton states in practice can be challenging.…”

Chip-Based Frequency Combs for High-Resolution Optical Coherence Tomography

“…Si3N4 frequency combs have recently been recently generated using a reflective semiconductor amplifier as a pump source enabling a millimeter sized electrically pumped source 22 In order to enable a large imaging range using the optical comb as an OCT source of at least 2 mm (comparable with commercial OCT imaging range), we design the combs with a small spectral line spacing of 0.21 nm (corresponding to 38 GHz) using a large microresonator with a perimeter of 1.9 mm (see the Supplementary Materials). This perimeter is at least an order of magnitude larger than traditional high confinement micro-resonators 14,24,25 . In order to achieve sufficient optical power build up and enable comb generation in such a large cavity 8,26 , we rely on the extremely low loss platform recently demonstrated 27 .…”