Advancements in integrated photonics have reached a significant milestone with the successful demonstration of soliton microcombs in high-Q microresonators on X-cut thin-film lithium niobate (TFLN) chips. This achievement, led by a research team from Nankai University and Peking University, marks a crucial step in enhancing the capabilities of photonic devices for various applications.
The integration of multifunctional material platforms has become essential for supporting diverse on-chip optical operations. TFLN has emerged as a leading candidate due to its ultralow optical losses, strong second-order nonlinear response, and high electro-optic efficiency. These attributes have enabled the development of high-speed modulators and efficient frequency doublers, essential components for creating chip-based optical frequency combs.
Soliton microcombs play a vital role in the co-integration of microwave and atomic systems, which are crucial for applications such as optical frequency synthesis and timekeeping. Achieving complete comb functionalities on photonic chips necessitates the integration of high-speed modulators and efficient frequency doublers, both of which are available in a monolithic form on X-cut TFLN.
Despite these advancements, the strong Raman response associated with extraordinary-polarized light previously hindered soliton formation, favoring Raman lasing instead. The research, detailed in a recent article published in eLight, addresses this issue by demonstrating soliton microcombs through a precise orientation of the racetrack microresonator concerning the optical axis. This innovative approach mitigates Raman nonlinearity, thus facilitating soliton formation under continuous-wave laser pumping.
The team successfully extended the soliton microcomb spectra to 350 nm with pulsed laser pumping, showcasing the enhanced capabilities of TFLN photonics. This progress paves the way for the monolithic integration of fast-tunable, self-referenced microcombs, which hold promise for significant advancements in optical communication, computation, timing, and spectroscopy.
To explore the polarization-dependence of the Raman response, the researchers employed Raman spectroscopy. They observed that Raman intensities decreased as the pump polarization transitioned from parallel (extraordinary light) to perpendicular (ordinary light) to the optical axis. Two racetrack microresonators with distinct orientations on X-cut TFLN-on-insulator chips were tested. In the first device, the straight waveguides were oriented perpendicular to the optical axis, resulting in a robust overall Raman response. In contrast, the second device, with waveguides parallel to the optical axis, exhibited a weaker Raman response, allowing for successful soliton microcomb generation.
The experimental setup further revealed that soliton microcombs could also be generated using synchronized pulsed lasers, which resulted in higher optical-to-optical conversion efficiencies and a broader spectral range. Throughout the experiments, the researchers observed the characteristic step-like comb power during frequency scanning, demonstrating soliton formation across a tuning range of approximately 340 kHz for the electro-optic comb repetition frequency.
The optical spectrum of the generated single soliton state spanned from 1400 nm to 1750 nm, characterized by a sech²-shaped spectral envelope. The phase noise spectra of the soliton microcomb and driven microwave further highlighted the performance of the system.
The successful demonstration of soliton microcombs on X-cut TFLN chips offers a clear path toward fully integrated on-chip comb functionality. Unlike traditional silicon nitride microcombs, this platform allows for monolithic integration with electrodes, facilitating high-speed modulation and providing greater flexibility for feedback control of both repetition frequency and carrier-envelope offset frequency.
Moreover, the integration with periodically poled lithium niobate (PPLN) waveguides enables on-chip self-reference, laying the groundwork for the development of chip-based optical clocks. This work builds on recent advancements in visible laser technologies and photonic-integrated atomic systems, marking an important step forward in the field of integrated photonics.
This research was supported by the Beijing Natural Science Foundation, the National Natural Science Foundation of China, and the high-performance computing platform of Peking University, among others. The findings are anticipated to significantly influence the future of photonic technology and its applications across various fields.
