Octave-spanning dissipative Kerr soliton frequency combs in $Si_3N_4$ microresonators (1701.08594v2)

Abstract: Octave-spanning, self-referenced frequency combs are applied in diverse fields ranging from precision metrology to astrophysical spectrometer calibration. In the past decade, Kerr frequency comb generators have emerged as alternative 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 $\mathrm{Si_{3}N_{4}}$ microresonators with $1\,\mathrm{THz}$ free spectral range, we achieve bandwidths exceeding one octave at low powers ($\mathcal{O}(100\,\mathrm{mW})$) for pump lasers residing in the telecom C-band ($1.55\,\mathrm{\mu m}$), as well as for the first time in the O-band ($1.3\,\mathrm{\mu m}$). 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 $1.3\mathrm{\,\mu m}$ and $1.55\,\mathrm{\mu m}$ including the broadest DKS state generated to date, spanning more than $200\,\mathrm{THz}$ of optical bandwidth. Octave spanning DKS frequency combs can form essential building blocks for metrology or spectroscopy, and their operation at $1.3\mathrm{\,\mu m}$ enables applications in life sciences such as Kerr comb based optical coherence tomography or dual comb coherent antistokes Raman scattering.

• The paper demonstrates that Si3N4 microresonators can generate octave-spanning dissipative Kerr soliton combs with a 1 THz free spectral range and pump power around 100 mW.
• The paper employs advanced dispersion engineering via the photonic Damascene process to achieve precise control over microresonator dimensions, enabling dual dispersive wave emission.
• The paper confirms coherence of DKS states using response measurements, paving the way for on-chip integrated frequency combs in precision metrology and spectroscopy.

Octave-Spanning Dissipative Kerr Soliton Frequency Combs in Si$_3$N$_4$ Microresonators

The paper in discussion presents a significant advancement in the generation of octave-spanning frequency combs using dissipative Kerr soliton (DKS) states within silicon nitride (Si$_3$N$_4$) microresonators. These achievements are made possible through careful dispersion engineering and an optimized photonic Damascene fabrication process, which collectively enhance the performance and applicability of Kerr frequency comb generators in precision metrology, spectroscopy, and other domains.

Technical Achievements and Numerical Results

Kerr frequency combs have gained attention for their potential in chip-scale integration and high repetition rates. However, achieving octave-spanning bandwidth with low power consumption has been a challenge. This paper accomplishes such a feat with Si$_3$N$_4$ microresonators featuring a free spectral range (FSR) of 1 THz. The resulting comb bandwidths exceed one octave with only $\mathcal{O}(100\,\mathrm{mW})$ of pump power, showcasing efficient energy use.

Using pump lasers in both the telecom C-band (1.55 µm) and the O-band (1.3 µm), the authors realized octave-spanning DKS combs, achieving unprecedented performance. Specifically, a 200 THz optical bandwidth DKS state was demonstrated, notably being the broadest reported at the time. The researchers used a response measurement technique to confirm coherence and specifically identify DKS states, addressing challenges with conventional state identification methods that may fail at these bespoke conditions.

Dispersion Engineering and Fabrication Techniques

Central to their success is the precision dispersion engineering made possible by the photonic Damascene approach. This method allows for high-fidelity control over microresonator waveguide dimensions, reducing fabrication variability to the extent necessary for controlled dual dispersive wave emission from the microresonator combs. The paper notes that altering waveguide dimensions by as little as 10 nm can significantly influence dispersive wave positions, underscoring the criticality of precise fabrication processes.

Despite inherent challenges such as wafer bowing and local non-uniformity during chemical mechanical planarization, the authors optimized their process to produce microresonators with consistent dimension control necessary for the desired optical performance.

Implications and Future Directions

The results suggest that Si$_3$N$_4$ microresonators, when fabricated with the techniques detailed, present a viable path forward for practical implementations in optical systems requiring broad bandwidths and high coherence. This has implications for the future of integrated frequency comb systems, potentially enabling fully chip-scale, self-referenced frequency combs and enhanced dual comb setups for applications like coherent anti-Stokes Raman scattering.

Further research could focus on extending these techniques across different material platforms and refining the integration of these microresonators with other optical and electrical functionalities on-chip. Additionally, exploring the utility of such systems in biological and medical imaging could offer novel insights and advancements, particularly given the spectral flexibility facilitated by these advances.

In summary, this paper outlines substantial innovations in the field of integrated photonics and ultrafast optics, grounded in meticulous procedural advancements and addressing both theoretical and practical demands for modern optical applications.