Reviewed by Lexie CornerJan 24 2025
In a study published in Nature, researchers from the China Academy of Science, the Colorado School of Mines, and EPFL developed an electro-optic frequency comb generator with over 2000 comb lines, achieving a spectral coverage of 450 nm.
The hybrid-integrated electro-optic frequency comb generator. More than 2000 comb lines covering 450 nm spectrum can be generated within a footprint smaller than 1 cm2. Image Credit: Junyin Zhang (EPFL).
In modern optics, frequency combs are essential tools for applications in telecommunications, environmental monitoring, and astrophysics. Despite their utility, designing compact and efficient frequency combs has been challenging.
Electro-optic frequency combs, introduced in 1993, produce optical combs via cascaded phase modulation. However, their high power requirements and limited bandwidth hindered further development, allowing femtosecond lasers and Kerr soliton microcombs to dominate the field. These alternatives, while effective, require sophisticated tuning and significant power, limiting their practical deployment.
Recent advances in thin-film electro-optic integrated photonics have renewed interest in materials such as lithium niobate. Yet, achieving broader bandwidths with reduced power consumption remains difficult, and lithium niobate's intrinsic birefringence further constrains performance.
Researchers have addressed these limitations by integrating microwave and optical circuit designs on a lithium tantalate platform. Lithium tantalate offers 17 times lower birefringence than lithium niobate, enabling improved bandwidth and reduced microwave power consumption—cutting power requirements nearly 20-fold compared to previous designs.
The team introduced an "integrated triply resonant" design, where two optical fields and one microwave field resonate simultaneously. This was achieved through a co-designed system combining monolithic microwave circuits with photonic components. A distributed coplanar waveguide resonator was integrated into lithium tantalate photonic circuits, enhancing microwave confinement and energy efficiency.
The device’s small size, with a footprint of 1×1 cm², minimizes interference between light waves, enabling smooth and consistent frequency comb generation. The system operates using a simple, free-running distributed feedback laser diode, eliminating the complex tuning processes required by Kerr soliton-based combs.
The comb generator achieves ultra-broadband coverage of 450 nm and stable operation over 90 % of the free spectral range. Its stability and simplicity make it suitable for field-deployable applications.
The device's robust design and compact size have potential applications in fields such as precision gas sensing for environmental monitoring and laser ranging for robotics. The co-design approach, combining photonic and microwave engineering, highlights the potential for creating next-generation photonic devices.
All samples were fabricated at the EPFL Center of MicroNanoTechnology (CMi) and the Institute of Physics (IPHYS) cleanrooms. The lithium tantalate-on-insulator (LTOI) wafers were provided by Shanghai Novel Si Integration Technology (NSIT) and the SIMIT-CAS.
Journal Reference:
Zhang, J., et al. (2025) Ultrabroadband integrated electro-optic frequency comb in lithium tantalite. Nature. doi.org/10.1038/s41586-024-08354-4