Recent research has achieved a significant milestone in integrated photonics with the successful demonstration of soliton microcombs on X-cut thin-film lithium niobate (TFLN) microresonators. A team of scientists, led by Professor Fang Bo from Nankai University and Professor Qi-Fan Yang from Peking University, published their findings in the journal eLight. This advancement opens new avenues for on-chip optical operations, crucial for applications like optical frequency synthesis and high-precision timekeeping.
The evolution of integrated photonics has created a pressing demand for materials that can support a variety of optical functions. Thin-film lithium niobate has emerged as a leading candidate due to its ultra-low optical losses and high electro-optic efficiency. These properties have already facilitated the development of cutting-edge high-speed modulators and effective frequency doublers. Microcombs, which are chip-based optical frequency combs, have reshaped the landscape of integrated photonics, allowing for the co-integration of microwave and atomic systems.
While the remarkable properties of TFLN chips have attracted attention, the strong Raman response associated with extraordinary-polarized light has hindered the formation of solitons, instead promoting Raman lasing. The recent work from the research team has successfully addressed this challenge. By carefully orienting the racetrack microresonator in relation to the optical axis, they mitigated Raman nonlinearity, facilitating soliton formation under continuous-wave laser pumping. This breakthrough allows for soliton microcomb spectra to extend up to 350 nm with pulsed laser pumping, enhancing TFLN photonics capabilities.
Key Findings and Methodology
The researchers conducted a thorough analysis of the polarization dependence of the Raman response in their X-cut TFLN chips using Raman spectroscopy. They discovered that Raman intensities decreased as the pump polarization shifted from parallel to the optical axis towards perpendicular alignment. The team tested two racetrack microresonators with different orientations on TFLN-on-insulator chips.
In the first device, where the straight waveguides were perpendicular to the optical axis, the fundamental transverse electric (TE) mode polarized along the optical axis resulted in a stronger Raman response. The corresponding Raman-Kerr comb spectrum reflected this increased response. Conversely, in the second device, where the straight waveguides were parallel to the optical axis, the weaker overall Raman response led to successful soliton microcomb generation.
The experiments also highlighted the potential for generating soliton microcombs using synchronized pulsed lasers. Such configurations promise higher optical-to-optical conversion efficiencies and a broader spectral range. Notably, the optical spectrum of the single soliton state spanned from 1400 nm to 1750 nm, following a sech²-shaped spectral envelope.
Implications for Future Research
The successful demonstration of soliton microcombs using X-cut TFLN chips represents a significant stride toward fully integrated on-chip comb functionality. Unlike silicon nitride microcombs, the TFLN platform offers the advantage of monolithic integration with electrodes for high-speed modulation. This integration enables fast feedback control of both the repetition frequency and the carrier-envelope offset frequency of soliton microcombs.
Furthermore, the ability to incorporate periodically poled lithium niobate (PPLN) waveguides allows for on-chip self-referencing. These developments 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, among others. As researchers continue to explore the potential of TFLN and soliton microcombs, the implications for optical communication, computation, timing, and spectroscopy remain promising and significant.