Researchers Achieve Soliton Microcombs in Lithium Niobate Chips

Recent advancements in integrated photonics have led to the successful demonstration of soliton microcombs in high-Q microresonators using X-cut thin-film lithium niobate (TFLN) chips. A team of researchers, led by Professor Fang Bo from Nankai University and Professor Qi-Fan Yang from Peking University, published their findings in the journal eLight. This breakthrough has the potential to enhance various optical applications, including communication and timing technologies.

Thin-film lithium niobate has gained attention due to its ultralow optical losses and impressive electro-optic efficiency. These properties have enabled the development of high-speed modulators and efficient frequency doublers, which are essential components for creating chip-based optical frequency combs, or microcombs. Microcombs play a crucial role in integrating microwave and atomic systems, facilitating applications such as optical frequency synthesis and advanced computational tasks.

Despite the promise of TFLN, previous attempts to achieve soliton formation were hindered by a strong Raman response associated with extraordinary-polarized light. Instead, researchers observed Raman lasing. The new study addresses this challenge by precisely orienting the racetrack microresonator relative to the optical axis, effectively mitigating the Raman nonlinearity. This innovative approach allows for soliton formation when pumping with continuous-wave lasers.

Key Findings and Experimental Setup

The research team conducted an extensive characterization of the polarization-dependent Raman response of the X-cut TFLN chip using Raman spectroscopy. They noted that the Raman intensities decreased as the pump polarization shifted from parallel to perpendicular to the optical axis.

Two racetrack microresonators with different orientations were tested. In the first device, the waveguides were perpendicular to the optical axis, resulting in a strong overall Raman response. Conversely, in the second device, the waveguides were aligned parallel to the optical axis, leading to a weaker Raman response and enabling the generation of soliton microcombs.

The experiments demonstrated soliton microcombs utilizing synchronized pulsed lasers, yielding higher optical-to-optical conversion efficiencies and a broader spectral range. The characteristic step-like comb power was observed during frequency scanning, indicating robust soliton formation across a tuning range of approximately 340 kHz for the electro-optic comb repetition frequency.

The optical spectrum of the soliton state spanned from 1400 nm to 1750 nm, showcasing a sech²-shaped spectral envelope. This comprehensive analysis of soliton microcombs on X-cut TFLN chips signifies a significant advancement toward fully integrated on-chip comb functionality.

Implications for Future Technologies

The ability to generate soliton microcombs on X-cut TFLN chips presents a clear pathway for advancements in photonics. Unlike silicon nitride microcombs, the X-cut lithium niobate platform allows for monolithic integration with electrodes designed for high-speed modulation. This integration provides additional control over both the repetition frequency and carrier-envelope offset frequency of soliton microcombs.

Moreover, the combination of X-cut TFLN with periodically poled lithium niobate (PPLN) waveguides enhances the prospects for on-chip self-reference capabilities. These developments are set to lay the groundwork for the realization of chip-based optical clocks, building on recent advancements in visible laser technologies and photonic-integrated atomic systems.

This research was supported by the Beijing Natural Science Foundation and the National Natural Science Foundation of China. The full findings can be accessed through the publication in eLight.

As integrated photonics continues to evolve, the implications of these findings may significantly impact a range of fields, from telecommunications to precision measurement, offering new avenues for innovation and technological advancement.