A self-starting bi-chromatic LiNbO3 soliton microcomb

Abstract: For its many useful properties, including second and third-order optical nonlinearity as well as electro-optic control, lithium niobate is considered an important potential microcomb material. Here, a soliton microcomb is demonstrated in a monolithic high-Q lithium niobate resonator. Besides the demonstration of soliton mode locking, the photorefractive effect enables mode locking to self-start and soliton switching to occur bi-directionally. Second-harmonic generation of the soliton spectrum is also observed, an essential step for comb self-referencing. The Raman shock time constant of lithium niobate is also determined by measurement of soliton self-frequency-shift. Besides the considerable technical simplification provided by a self-starting soliton system, these demonstrations, together with the electro-optic and piezoelectric properties of lithium niobate, open the door to a multi-functional microcomb providing f-2f generation and fast electrical control of optical frequency and repetition rate, all of which are critical in applications including time keeping, frequency synthesis/division, spectroscopy and signal generation.

• The paper demonstrates a self-starting soliton microcomb using LiNbO₃’s photorefractive effect to achieve bistable, self-triggered soliton mode-locking.
• It reports Kerr solitons with an optical Q factor of 2.2×10⁶ and nearly 199.7 GHz FSR, enabling precise frequency stabilization and robust soliton control.
• The study achieves concurrent second-harmonic generation and quantifies a 6.3 fs Raman shock time, paving the way for integrated solutions in optical metrology and communications.

Microhistorical Advancements in Self-Starting LiNbO₃ Soliton Microcombs

The documented research paper proposes a technical innovation in the generation of soliton microcombs within high-Q lithium niobate (LiNbO₃) resonators, marking a noteworthy advancement in the domains of optics and photonics. The material, renowned for its second and third-order nonlinearities, exhibits versatility that opens up avenues for a plethora of applications, especially in optical communication and metrology.

The core achievement of this study is the demonstration of a self-starting soliton microcomb that leverages the intrinsic properties of LiNbO₃, in particular, its photorefractive effect. This effect significantly contributes to the stabilization of comb generation, allowing self-starting soliton mode-locking without the need for external triggering. Notably, the paper presents the formation of a soliton state from frequency sweeps initiated in the red-detuned state, highlighting the unique bistability introduced by the photorefractive effect as opposed to the thermo-optic effect usually limiting other microresonator platforms.

In the study, the generation of Kerr solitons in LiNbO₃ microresonators was successfully demonstrated, reaching optical Q factors of $2.2 \times 10^6$ with an FSR of nearly 199.7 GHz. The elegance of using lithium niobate lies not only in its nonlinear optical properties but also in its photorefractive response, which introduces a significant advantage by enabling bi-directional control over soliton states across the instability regime. This control is crucial for applications requiring precise modulation and stability in optical comb generation. The paper further elucidates how the photorefractive effect enhances soliton number switching, introducing both the potential for soliton count modulation and frequency stabilization within the cavity.

Another pivotal aspect of the research is the demonstration of concurrent second-harmonic generation (SHG) within the soliton microcomb, facilitated by LiNbO₃'s quadratic nonlinearity. This functionality accentuates the potential for developing compact, efficient f-to-2f self-referencing systems critical for coherent comb application in optical clocks and frequency synthesis. Specifically, the paper illustrates the presence of a distinct second-harmonic spectrum corresponding to the fundamental soliton spectrum, reinforcing the potential of lithium niobate devices to generate bi-chromatic frequency combs which could accelerate advancements in integrated photonics.

Moreover, the paper provides quantitative analysis on the self-frequency shift of solitons, attributed to the material's Raman effect. The empirical data yield a Raman shock time constant of 6.3 fs for LiNbO₃, a contribution previously unquantified, allowing predictive modeling for soliton management in nonlinear fiber systems.

The implications of these results are profound, as they not only enhance the functional capability of soliton microcombs but also present opportunities for multifaceted integration in photonics technology. Future explorations could intensely benefit from this dual nonlinear capability, leveraging both Kerr and non-Kerr mechanisms, potentially transforming our approach to integrated photonic systems. The intrinsic electro-optic and piezoelectric properties of LiNbO₃, coupled with its photoconductivity and temperature sensitivity, position it as a promising medium for future innovations in optical processing, sensor networks, and on-chip photonic systems, thereby broadening the horizon for practical deployments in frequency metrology and optical communications.