Ultra-Broadband Electro-Optic Comb Packs 450 nm of Precision

Researchers from the Colorado School of Mines, the China Academy of Science, and EPFL have developed a new ultra-broadband electro-optic comb that can precisely measure light at 450 nm. The compact device, fitting on a coin-sized chip, advances the development of precise and efficient photonic technologies. The findings were published in the journal Nature.

Frequency combs are essential tools in modern optics. They act as precise rulers for measuring light, enabling progress in areas like astrophysics, environmental monitoring, and telecommunications. Despite their importance, designing compact and efficient frequency combs has remained a significant challenge.

Electro-optic frequency combs were introduced in 1993 and showed early promise by generating optical combs through cascaded phase modulation. However, their development slowed due to high power requirements and limited bandwidth. Over time, femtosecond lasers and Kerr soliton microcombs took precedence, but their high energy demands and complex tuning processes have limited their practical applications.

Recent advancements in thin-film electro-optic integrated photonic circuits have revived interest in these combs, particularly with materials like lithium niobate. While lithium niobate offers useful properties, its intrinsic birefringence, which splits light beams, has restricted bandwidth and made it difficult to achieve greater efficiency with lower power consumption.

Researchers have addressed this issue by integrating microwave and optical circuit designs into a lithium tantalate platform, a material with 17 times lower birefringence than lithium niobate. Led by Professor Tobias J. Kippenberg, the team developed an electro-optic frequency comb generator that achieves a 450 nm spectral range with more than 2000 comb lines. This design improves bandwidth and significantly reduces power requirements compared to earlier models.

The device utilizes an "integrated triply resonant" architecture, where two optical fields and one microwave field resonate simultaneously. This approach combines photonic elements with monolithic microwave circuits. By incorporating a distributed coplanar waveguide resonator on lithium tantalate photonics integrated circuits, the team enhanced microwave confinement and energy efficiency.

Lithium tantalate’s lower birefringence made it possible for the comb generator to be designed in a compact 1 × 1 cm² size. This reduced interference between light waves and ensured stable, reliable frequency comb generation. Unlike Kerr soliton combs, the device operates with a simple, free-running distributed feedback laser diode, making it easier to use.

With its broad spectral range and stable performance across most of the free spectral range, the comb generator provides practical solutions for field applications without requiring intricate tuning. Its robust design and compact form make it suitable for areas such as environmental monitoring, where precise gas sensing is vital, and robotics, which relies on accurate laser-based measurements.

The study also highlights the potential of combining photonic and microwave engineering to develop next-generation devices. The research was supported by the Swiss National Science Foundation (SNSF).

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