image: Figure | Hybrid Kerr-electro-optic frequency comb on thin-film lithium niobate. a, 3-D illustration of the hybrid Kerr-EO frequency comb using both Z-cut and X-cut TFLN, consisting of a DKS microresonator chip and an EO phase modulator chip, respectively. b, Schematic of the hybrid Kerr-EO frequency comb generation process. A DKS frequency comb (blue) serves as the source, and comb line generates EO sidebands (red) around it at multiples of the microwave modulation frequency. The final output of the hybrid comb generator consists of both blue and red lines. c, Electro-optically modulated single DKS spectrum, featuring 2,589 comb lines spaced by 29.308 GHz and spanning 75.9 THz.
Credit: Song, Y., Hu, Y., Lončar, M. et al.
Optical frequency combs have revolutionized technologies such as atomic clocks, spectroscopy, and ultra-fast lasers by acting as precise bridges between optical and microwave frequencies. As the demand for high-performance and miniaturized photonic systems increases, the challenge lies in delivering frequency combs that are both broadband and microwave spaced, all within a compact, chip-scale platform. Traditional approaches based on single optical nonlinearities have either produced wide comb spans with coarse line spacings or narrowly spaced lines with limited spectral coverage.
In a new paper published in Light: Science & Applications, a team of researchers led by Professors Kiyoul Yang and Marko Lončar at the John A. Paulson School of Engineering and Applied Sciences, Harvard University introduces a hybrid Kerr-electro-optic frequency comb that resolves this limitation. Using thin-film lithium niobate, the team combines a dissipative Kerr soliton (DKS) and electro-optic (EO) modulation in a cascaded architecture. This resulted in a frequency comb containing 2,589 evenly spaced comb lines at 29.308 GHz spacing, spanning a total bandwidth of 75.9 THz, all while combining only two photonic chips.
This hybrid Kerr-EO comb exploits the unique nonlinear properties of lithium niobate by employing separate crystal cuts: Z-cut for broadband and stable DKS formation, and X-cut for efficient high-speed EO modulation. The DKS is coherently densified using EO modulation, yielding an optical comb structure that can be directly interfaced with conventional microwave electronic equipment Notably, the comb exhibits excellent long-term stability and can be electronically tuned and phase-locked to a microwave reference, offering precise control over its frequency output.
“By leveraging the complementary strengths of Kerr and electro-optic effects on lithium niobate, we’ve designed a comb source that achieves both large spectral coverage and dense line spacing, capabilities that have been difficult to realize simultaneously,” the researchers explain.
The demonstrated device not only pushes the limits of integrated photonic performance but also opens new possibilities for compact, low-power systems in applications such as frequency metrology, optical communications and computing. The hybrid Kerr-EO comb could serve as the foundation for next-generation chip-scale optical frequency references, as well as microwave and terahertz frequency synthesizers.
Journal
Light Science & Applications
Article Title
Hybrid Kerr-electro-optic frequency combs on thin-film lithium niobate