Reviewed by Lexie CornerSep 23 2024
A group of scientists at Harvard University has developed design guidelines that effectively suppress Raman lasing in favor of soliton frequency comb generation. The study was published in Light: Science & Applications.
Thin-film lithium niobate microresonator Raman lasing suppression methods and octave-spanning frequency combs. a, Thin-film lithium niobate microresonator Raman lasing suppression methods, including free spectral range (FSR) engineering and dissipation spectrum engineering (top and bottom panels, respectively). In the former, the microresonator FSR is ideally larger than the Raman gain bandwidth, and the Raman gain peak placed in between Raman modes, such that the effective Raman gain coefficient is reduced for Raman modes. In the latter, the microresonator is lossy at Raman mode frequencies, due to excess coupling between the microresonator and bus waveguide. Both methods maintain a critically coupled pump mode and increase the Raman lasing threshold, such that the Kerr parametric oscillation threshold may be lower. b, Entirely connected, octave-spanning dissipative Kerr soliton (DKS) frequency comb spectra from dispersion-engineered microresonators, utilizing the FSR engineering method (blue) and the dissipation engineering method (green). Image Credit: Yunxiang Song, Yaowen Hu, Xinrui Zhu, Kiyoul Yang, Marko Lončar.
Professors Marko Lončar and Kiyoul Yang from the John A. Paulson School of Engineering and Applied Sciences were involved in the study.
The development of optical frequency combs completely transformed timekeeping and frequency metrology.
To further extend these functionalities, miniaturizing such combs onto photonic chips primarily uses microresonator Kerr soliton frequency combs. This enables portable solutions for stable optical, millimeter wave, and microwave frequency generation, as well as ultrafast spectroscopy and laser frequency synchronization.
Low-noise soliton frequency combs are crucial for many applications, but they typically require complex off-chip devices like frequency doubling and electro-optic division to stabilize the soliton repetition rate and carrier-envelope offset. The ultralow-loss thin-film lithium niobate photonic platform, with its strong electro-optic effect and efficient second-harmonic generation, shows promise as a host for fully-stabilized, integrated frequency combs made on-chip.
However, it has not been demonstrated that octave-spanning solitons with fully connected spectra are suitable for stabilization using periodically-poled waveguides and integrated electro-optic modulators. One of the challenges is that low-threshold Raman lasing in lithium niobate microresonators, a nonlinear process, competes with the Kerr effect, preventing parametric oscillation and mode-locked soliton states.
Therefore, strict guidelines are needed to suppress Raman lasing in lithium niobate microresonators. This would enable the creation of octave-spanning soliton sources and facilitate their reliable integration into large-scale photonic systems and stabilization at the chip level.
Thin-film lithium niobate has proven to be a very powerful material for photonics. So, our goal was to develop a comprehensive understanding for soliton microcomb generation on this platform, so they could be used in conjunction with its well-established active modulation and frequency conversion capabilities, ultimately for building better integrated comb sources.
Yunxiang Song, Ph.D. Student, Quantum Science and Engineering, Harvard University
The scientists showcased Kerr soliton frequency combs with octave spans on the thin-film lithium niobate photonic platform. They developed design guidelines to consistently suppress Raman lasing in favor of soliton frequency comb generation. These guidelines focus on engineering the microresonator mode spacing and dissipation profile.
The researchers demonstrated an octave-spanning soliton from 131 to 263 THz by increasing the microresonator mode spacing beyond the Raman gain bandwidth.
They achieved over 88 % success in fabricating soliton-supporting microresonators across various designs, including different dispersions and resonator sizes. They also demonstrated an octave-spanning soliton from 126 to 252 THz with no spectral gaps by increasing dissipation near Raman lasing modes using a pulley-type coupling.
This Raman suppression technique could simplify nonlinear frequency comb generation in other electro-optic and crystalline materials, like thin-film lithium tantalate, where Raman lasing interference is likely. It also paves the way for further development of soliton frequency combs on thin-film lithium niobate. Furthermore, the fully coupled, octave-spanning soliton states could enable system-level applications like comb-referenced laser spectroscopy and optical frequency synthesis.
The team believes that “although still in its early stages, the reliable fabrication of octave-spanning soliton frequency combs shows potential for developing monolithic and compact comb-driven photonic systems based on thin-film lithium niobate.”
Journal Reference:
Song, Y., et al. (2024) Octave-spanning Kerr soliton frequency combs in dispersion- and dissipation-engineered lithium niobate microresonators. Light: Science & Applications. doi.org/10.1038/s41377-024-01546-7