Observation of Interband Kelly Sidebands in Coupled-Ring Soliton Microcombs

Optics Physics
soliton microcombs Kelly sidebands coupled-ring optics nonlinear optics

Observation of Inter-Band Kelly Sidebands in Coupled Ring Comb Microcombs

Background Introduction

Kelly Sidebands (KSS) are special dispersive waves that appear in mode-locked systems and have been extensively studied in fiber lasers. However, due to the shorter path lengths of integrated photonics microresonators, their generation in integrated microcombs is relatively challenging. Despite these difficulties, Kelly Sidebands have been recently observed in pulse-pumped soliton microcombs with broken symmetry. In this research context, the authors explore the generation of Kelly Sidebands through continuous wave excitation in partially coupled racetrack resonator microcombs.

Paper Source

This paper is co-authored by Maodong Gao, Zhiquan Yuan, Yan Yu, Warren Jin, Qing-Xin Ji, Jinhao Ge, Avi Feshali, Mario Paniccia, John E. Bowers, and Kerry J. Vahala, who are affiliated with the California Institute of Technology, the University of California, Santa Barbara, and Anello Photonics. The paper was published in July 2024, in Volume 11, Issue 7 of the journal “Optica.”

Research Process

Research Object and Methods

  1. Experimental Setup and Detection Methods:

    • Using a coupled ring device prepared using CMOS-compatible technology.
    • Employing continuous wave laser pumping near the region of anomalous dispersion in the upper frequency band to generate bright soliton pulse pairs.

  2. Experimental Steps:

    1. Prepare Si3N4 microresonator racetracks.
    2. Measure the integrated dispersion of the two mixed mode families.
    3. Employ continuous wave laser pumping near the anomalous dispersion band to generate soliton pulse pairs.
    4. Observe the spectral positions of the soliton frequency comb and KSS.

Key Experiments and Parameters

  • During the experiment, partially coupled racetrack resonators were used. This structure generates two optical bands, and the partial coupling of the racetrack resonator results in broken system symmetry, allowing the soliton and Kelly Sidebands to couple between different frequency bands.
  • Studies were conducted on the tuning of Kelly Sidebands positions under both continuous wave pumping and pulse pumping conditions. Comparison of simulation and experimental results analyzed the differences in Kelly Sideband generation in partially coupled and fully coupled devices.

Experimental Results

Results Under Continuous Wave Excitation

  • In the partially coupled racetrack resonator, bright soliton pulse pairs were generated by continuous wave laser pumping near the anomalous dispersion window. The spectrum showed dispersive waves and Kelly Sidebands present in two different frequency bands.
  • Specific results showed dispersive waves observed at two points on the integrated dispersion curve, while Kelly Sidebands were located in a different frequency band. These characteristics underscored the necessity of symmetry-breaking in the formation of Kelly Sidebands.

Results Under Pulse Pumping

  • Using pulse-pumped electro-optic comb and pulse compression methods, soliton pulse pairs demonstrated similar spectral results.
  • Experimental results also showcased the single-pulse pumping repetition frequency’s effect on the spectral positions of dispersive waves and Kelly Sidebands, which were observed by tuning the pump pulse’s repetition frequency.

Numerical Simulation Analysis

  • Using the coupled mode Lugiato–Lefever Equation (LLE), the generation of Kelly Sidebands in partially coupled and fully coupled devices was simulated.
  • Simulation results showed that in the partially coupled condition, both dispersive waves and Kelly Sidebands appeared in the spectrum, whereas, in the fully coupled condition, only dispersive waves appeared, unable to generate Kelly Sidebands.
  • Additionally, the simulation results reflected the interrelationship between Kelly Sideband formation and broken symmetry.

Conclusions and Significance

  • This study demonstrated the method of generating Kelly Sidebands through soliton microcombs in partially coupled racetrack resonators. By breaking system symmetry, this method reduced the power requirement for generating Kelly Sidebands and allowed for continuous wave excitation to generate Kelly Sidebands.
  • The experimental and simulation results confirmed the potential of this multi-band system in broadening the microcomb spectrum, providing a new pathway for engineered Kelly Sidebands.
  • Considering the potential applications of Kelly Sidebands in optical frequency division (such as in optical frequency combs), this research outcome holds significant scientific and practical value.

Research Highlights

  • The generation of Kelly Sidebands via partially coupled resonators provides an innovative method to expand the microcomb spectrum.
  • Observations from experiments and simulations demonstrated Kelly Sideband generation’s dependence on system symmetry breaking, offering new insights for further microcomb system design.
  • Numerical simulations precisely compared the differences in Kelly Sideband generation under varying coupling conditions, validating the experimental results’ effectiveness.

Additionally, based on high-precision spectrometer measurements, this paper further confirmed that Kelly Sidebands share the same carrier-envelope offset frequency with the main frequency comb grid. The research results in this paper have significant implications for the engineered application of optical frequency combs and provide new directions for future related research.