Raman induced soliton self-frequency shift in microresonator Kerr frequency combs (1506.08767v1)

Abstract: The formation of temporal dissipative solitons in continuous wave laser driven microresonators enables the generation of coherent, broadband and spectrally smooth optical frequency combs as well as femtosecond pulses with compact form factor. Here we report for the first time on the observation of a Raman-induced soliton self-frequency shift for a microresonator soliton. The Raman effect manifests itself in amorphous SiN microresonator based single soliton states by a spectrum that is hyperbolic secant in shape, but whose center is spectrally red-shifted (i.e. offset) from the continuous wave pump laser. The Raman induced spectral red-shift is found to be tunable via the pump laser detuning and grows linearly with peak power. The shift is theoretically described by the first order shock term of the material's Raman response, and we infer a Raman shock time of 20 fs for amorphous SiN. Moreover, we observe that the Raman induced frequency shift can lead to a cancellation or overcompensation of the soliton recoil caused by the formation of a (coherent) dispersive wave. The observations are in excellent agreement with numerical simulations based on the Lugiato-Lefever equation (LLE) with a Raman shock term. Our results contribute to the understanding of Kerr frequency combs in the soliton regime, enable to substantially improve the accuracy of modeling and are relevant to the fundamental timing jitter of microresonator solitons.

Raman Induced Soliton Self-Frequency Shift in Microresonator Kerr Frequency Combs

The paper "Raman Induced Soliton Self-Frequency Shift in Microresonator Kerr Frequency Combs," introduces a novel observation regarding the role of the Raman effect on soliton dynamics within microresonators. This research delineates the intricate dynamics of Kerr frequency combs in a microresonator environment shaped by the temporal dissipative soliton regime. The authors investigate the Raman-induced soliton self-frequency shift in silicon nitride (SiN) microresonators, providing a significant contribution to the understanding of soliton formation, their frequency behavior, and modeling.

The Raman effect, well-documented in fiber optics, has been observed for the first time in microresonators made from amorphous SiN. The paper finds that the Raman effect causes a red-shift in the soliton spectrum—observable as a shifting of the center frequency of solitons away from the continuous wave pump laser frequency. Numerically, this phenomenon aligns well with simulations based on the Lugiato-Lefever equation (LLE) augmented with a Raman shock term. The soliton frequency shift is characterized using a Raman shock time of 20 fs for SiN, established through experimental validation and simulation alignment.

Significant experimental observations are reported, where soliton-based frequency combs exhibit a notable spectral red-shift originating from the laser detuning. The shift changes continuously with the detuning parameter and is reversible within defined soliton existence ranges. The ability to modulate the soliton frequency shift offers insights into potential applications in tuning and optimizing Kerr combs for various optical applications, including coherent communications and spectroscopy.

Interestingly, the self-frequency shift is shown to potentially counteract the soliton recoil effect caused by coherent dispersive waves (DWs). This counterbalance can lead to a reduction or elimination of spectral recoil, conjecturing combinations of higher-order dispersion and Raman effects in microresonator platforms. Such effects have implications for designing microresonators to suppress DW interactions, which can be particularly advantageous for applications requiring stable multi-channel comb generation.

The research presents several implications and possibilities for future exploration in photonic technologies and related fields:

1. Enhanced Modeling Techniques: The integration of Raman effects into theoretical models provides a more accurate framework for simulating microresonator dynamics, facilitating the design of advanced photonic devices with predictable behavior.
2. Noise and Jitter Analysis: Understanding the intrinsic noise and jitter performance of soliton combs will be crucial for applications where timing precision is paramount, such as optical clocks and precision navigation systems.
3. Broadening of Raman Effect Applications: By extending these insights beyond SiN, the research encourages investigation into other amorphous materials like aluminum nitride, potentially leading to new avenues in nonlinear optics with broader material choices.

In conclusion, this paper significantly advances the detailed understanding of nonlinear optical effects in microresonators, emphasizing Raman interactions. It not only sets the stage for more comprehensive simulations by incorporating previously unmodeled physical phenomena but also expands the practical utility of Kerr combs in diverse technological applications. Such findings hold the potential for impacting a wide array of optical systems, demanding further research into the fundamental and applied aspects of microresonator-based frequency combs.