A new integrated design for electro-optic (EO) frequency combs expands the bandwidth of the comb and significantly reduces its microwave power requirements, compared to previous designs. The integrated EO frequency comb could benefit robotics, environmental sensing, spectroscopy, astronomy, and other fields that require precise, efficient measurement of light. An international team comprising researchers from École Polytechnique Fédérale de Lausanne (EPFL), the Colorado School of Mines, and the Chinese Academy of Sciences, led by professor Tobias J. Kippenberg, created the EO comb generator using an integrated triply resonant architecture. This architecture features three interacting fields — two optical and one microwave — that resonate in harmony. The researchers combined monolithic microwave integrated circuits with photonic integrated circuits (PICs) on a platform of thin-film lithium tantalate (LiTaO3), a low-birefringence material. By embedding a distributed, coplanar waveguide resonator on LiTaO3-based PICs, the team significantly improved microwave confinement and energy efficiency, reducing its microwave power requirements almost 20-fold. Traditionally, lithium niobate (LiNbO3) is used to enable EO frequency combs. While this material can be used to make combs that are precise and simple to operate, the combs made with LiNbO3 have limited spectral coverage. This is due to the large amount of microwave power required to drive the nonresonant capacitive electrodes and the strong intrinsic birefringence of LiNbO3. The birefringence of LiNbO3 places an upper limit on the comb’s achievable bandwidth. LiTaO3 has 17 x lower intrinsic birefringence than LiNbO3. Using LiTaO3 in an integrated triply resonant approach, the team designed a compact EO comb with broad bandwidth and low power requirements. With its resonantly enhanced EO interaction and the reduced birefringence in LiTaO3, the new comb design demonstrated a 4-fold comb span extension and a 16-fold power reduction compared to the conventional, nonresonant microwave design. 2. Courtesy of Junyin Zhang/EPFL." style="width: 400px; height: 224px; float: left; margin-top: 7px; margin-right: 10px; margin-bottom: 7px;" /> The hybrid-integrated, electro-optic frequency comb generator. More than 2000 comb lines covering a 450 nm spectrum can be generated within a footprint smaller than 1 cm2. Courtesy of Junyin Zhang/EPFL. Driven by a hybrid integrated laser diode, the spectral coverage of the new comb design spans over 450 nm (more than 60 terahertz), with more than 2000 comb lines, exceeding the limits of current EO frequency comb technologies. The comb achieves stable operation across 90% of the free spectral range, eliminating the need for complex tuning mechanisms. The stability and simplicity of the new comb design could advance the use of EO combs for field-deployable applications. The comb fits within a 1 square centimeter (1 x 1 cm2) footprint. Its compact size is made possible by leveraging the low birefringence properties of LiTaO3, which minimize interference between light waves, enabling smooth, consistent frequency comb generation. The device is operated using a simple, free-running distributed feedback laser diode, making it much easier to use than Kerr soliton combs. Additionally, the researchers found that strong EO coupling led to an increased comb existence range, approaching the full free spectral range of an optical microresonator. Until now, building compact and efficient EO frequency combs has been a challenge. Although they were introduced in 1993, their development has been hampered by their need for large amounts of power and their bandwidth restrictions. Femtosecond lasers and Kerr soliton microcombs also have been used to measure light. While effective, these technologies require complex tuning and high amounts of power, limiting their field-ready use. The ultrabroadband, integrated, EO frequency comb using LiTaO3 could transform the role of optical frequency combs in applications where precise laser ranging is required, like robotics, and in fields where accurate sensing is essential, like environmental monitoring. The ultrabroadband comb generator, combined with detuning-agnostic operation, could advance chip-scale spectrometry and ultra-low-noise millimeter wave synthesis. The methodology used by the team to co-design microwave and photonics could be extended to a range of integrated EO applications. The successful use of this methodology to create EO combs highlights the potential to integrate microwave and photonic engineering for next-generation devices. The research was published in Nature (www.doi.org/10.1038/s41586-024-08354-4).