A new publication from Opto-Electronic Advances; DOI 10.29026/oea.2025.240257 , discusses generation of avoided-mode-crossing soliton microcombs.
Optical frequency combs refer to spectra composed of a series of frequency components that are uniformly spaced, resembling the teeth of a comb, thus the term "optical frequency comb." In recent years, optical frequency combs generated in microresonators (known as microcombs) have attracted significant attention due to their high repetition rates, broad bandwidths, and compactness. These advantages have shown immense potential in applications such as coherent communication, precise ranging, high-resolution spectroscopy, and quantum optics. To date, numerous materials, including silica (SiO₂), silicon nitride (Si₃N₄), and lithium niobate (LiNbO₃), have been explored for the research of optical resonators. Among these, silicon oxynitride (SiON) has gained widespread use in the manufacturing of various high-quality photonic devices due to its mature processing technology and relatively low nonlinear losses. However, a critical challenge arises in the process of soliton microcombs generation: thermal effects. When light propagates within the microresonator, interactions between the light and the material generate heat, leading to temperature fluctuations inside the cavity. These fluctuations, in turn, cause a deviation of the pump frequency from the resonant frequency of the microcavity, significantly increasing the difficulty of capturing stable solitons. To address this issue, researchers have proposed various techniques, such as rapid frequency tuning, forward and backward frequency-scanning, Euler-bent microcavities, and auxiliary laser heating balance. However, these methods still face limitations in terms of operational complexity and the flexibility of temperature control. Therefore, achieving soliton microcombs in a simpler and more efficient manner has become a key focus in the development of optical frequency comb technology. Additionally, due to the influences of higher-order nonlinearities and mode crossings, previously studied soliton microcombs exhibit phenomena such as soliton oscillation tails and abrupt spectral envelope variations in both the time and frequency domains. These phenomena introduce additional noise in practical applications, thereby limiting the precision of soliton microcombs. Although researchers have generated single solitons and soliton crystals with smooth spectral envelopes through methods like phase modulation, the degree of smoothness still requires further improvement.
To address the aforementioned challenges, a collaborative team from Northwest A&F University, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Nanyang Technological University, and the Xi'an Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, has proposed a novel auxiliary laser thermal balancing method for exciting avoided-mode-crossing soliton microcombs. This approach successfully generates soliton microcombs with ultra-smooth spectral envelopes by precise thermal regulation in a SiON microcavity with smooth sidewall and uniform thickness, thereby avoiding issues such as mode crossings and soliton oscillation tails. Figure 1(c) presents the experimental setup for generating the soliton microcomb, where the key component is a microcavity made of SiON. Figures 1(a) and (b) show the physical image of the microcavity within a butterfly-shaped package, as well as a SEM image of the microresonator. Figure 1(d) presents the spectrum of a single soliton (blue curve), with the red dashed line representing the fit of the spectral envelope to a sech² function. Figure 1(e) illustrates the equidistant distribution of the spectral envelope of the single soliton, indicating that the soliton microcomb is generated in a low-noise state.
Figures 2(a-c) show the experimental results for the generated single soliton, dual soliton, and quadruple soliton states, with the spectrum of the auxiliary laser being filtered out. The experimental results indicate that the generated solitons avoid interactions between different modes, thus possessing extremely smooth spectral envelopes. Importantly, this phenomenon is independent of the pump itself. Figures 2(d-e) present the theoretical simulation results based on the Lugiato-Lefever equation. A comparison with Figures 2(a-c) reveals that the spectra of the generated solitons closely match the theoretical simulations. This smooth spectral envelope with low redundant noise and minimal inter-line power variations is crucial for applications in precise ranging, spectroscopy, and astronomy. Furthermore, our experimental study also demonstrates the presence of Raman-induced self-frequency shift in the spectrum, which manifests as a redshift of 4.4 nm at the spectral center of the pump wavelength, as shown in Figure 2(a). From this, the Raman shock time can be deduced to be 2.7 fs.
Interestingly, further research demonstrates that the proposed soliton excitation scheme allows for the flexible generation of different dual-soliton states by controlling the temperature of the microcavity as well as the wavelength and power of the auxiliary light. Figure 3 shows the smooth spectral envelopes of four distinct dual-soliton states obtained experimentally. These different dual-soliton states exhibit significantly different spectra, which result from the interference between the Fourier components of each individual soliton.
This study applies an auxiliary laser thermal balancing scheme, which enables flexible thermal regulation, to a silicon oxynitride microresonator with smooth sidewalls and uniform thickness. This approach successfully addresses the thermal effects encountered during the microcomb generation process and generates soliton states with ultra-smooth spectral envelopes. This advancement opens new avenues for the practical application of micro-combs in fields such as precision measurement, spectroscopic analysis, and astronomical observation. Moreover, the simplicity and efficiency of the auxiliary laser thermal balancing scheme enhance its feasibility for practical applications, potentially accelerating the commercialization of optical frequency comb technology. Moving forward, the research team will continue to explore the excitation mechanisms of soliton microcombs on other material platforms, further advancing the development of optical frequency comb technologies.
Keywords: perfect solitons / thermal dynamics / auxiliary laser heat balance / Raman effect
