Optical frequency combs (OFCs), as high-precision light sources, play crucial roles in optical communications, spectroscopy, and precision ranging. Traditional OFCs primarily rely on mode-locked lasers or microcavity nonlinear effects. While offering excellent performance, these systems suffer from complexity, large size, and high power consumption. Lithium niobate (LN), with its outstanding electro-optic (EO) coefficient and broad spectral transparency, provides an ideal platform for miniaturizing electro-optic frequency combs (EOFCs) while enabling low power consumption and fast tuning. Recent advances in thin-film lithium niobate (TFLN) platforms have further reduced on-chip EO frequency comb devices to millimeter-scale dimensions, achieving comb bandwidths of hundreds of nanometers with ultrahigh repetition frequency stability. However, conventional EOFCs, despite their stability and tunable bandwidth, face challenges such as high driving power, significant RF reflection losses due to lumped capacitive electrode designs, and the need for expensive external isolators, severely limiting their chip-level integration. Addressing these issues—achieving low power consumption, high compatibility, and isolator-free on-chip EOFCs—has become an urgent challenge.

To overcome these limitations, the research team proposed a novel coplanar waveguide microwave resonator electrode design based on the TFLN platform, optimizing electric field distribution and impedance matching through a quarter-wavelength resonant structure. This design enhances the electric field intensity by 3.6 times, significantly improving EO modulation efficiency, while reducing reflected coefficient to -46 dB, enabling direct driving without isolators. In experiments, the team achieved an 85-nm-wide frequency comb with 430 spectral lines at a repetition rate of 25.6 GHz using a moderate electrical power of 740 mW (28.7 dBm). Compared to traditional lumped-capacitor designs, the new approach doubles the comb bandwidth (2.2× improvement) under the same power and operates with a lower loaded optical cavity quality factor(Q_L=8.5×10⁵), demonstrating its superior efficiency. These findings are published in Photonics Research (Issue 2, 2025) under the title "Microwave-resonator-enabled broadband on-chip electro-optic frequency comb generation" and featured as the cover article.

The proposed quarter-wavelength resonator structure (Figure 1) forms standing waves through a short-circuit termination, significantly enhancing the electric field. Simultaneously, phase-matching ensures that optical pulses circulating in the resonator cavity constructively interfere with the microwave field, enabling efficient modulation. The high Q factor of the optical resonator extends photon lifetime, exponentially boosting sideband generation efficiency. Furthermore, the team developed a theoretical model for the microwave resonator's electrical response, which aligns closely with experimental results, providing critical guidance for extending operational frequencies in practical applications. The optimized microwave resonator achieves impedance matching with driving circuits, suppressing reflection coefficients to -46 dB and effectively resolving power reflection issues in conventional designs. The final design reduces RF reflectivity to below 1% within a 1 GHz range around the target frequency of 25 GHz, marking the first isolator-free monolithically integrated solution.

Figure 1. Working principles of microwave resonator-enabled broadband EOFC generation. (a) Comparison between lumped capacitor and (b) microwave resonator schemes. (c) Schematic of electric field distribution and phase-matching conditions in the microwave resonator-based comb generator.

In experimental validation, the team successfully integrated optical racetrack resonators and coplanar microwave resonator electrodes on a 4-inch TFLN wafer using ultraviolet lithography and metal deposition. Under the same driving power of 740 mW (Figure 2), the microwave resonator generated an 85-nm comb bandwidth, far exceeding the 38-nm bandwidth of traditional lumped-capacitor designs, all without external isolators. Further tests showed that even with deviations in the optical free spectral range (25.67 GHz), the comb bandwidth remained above 75 nm (Figure 3), demonstrating strong tolerance to fabrication variations. Additionally, by adjusting the microwave resonator and interdigitated electrode lengths, the team achieved resonance in the 10 GHz band (Figure 4), validating the design's scalability across multiple frequencies.

Figure 2. (a) Micrograph of a traditional lumped-capacitor design and (b) the proposed coplanar microwave resonator-based EOFC generator. (b) SEM images of the microwave resonator's shorted end and coupling capacitor. (c) Comparison of comb spectra under 740 mW driving power: (i) lumped capacitor and (ii) microwave resonator designs. (iii) Numerical simulations of experimental results (i, ii) with potential higher optical Q and input power.

Figure 3. EOFC generation under frequency detuning. Black curves show the normalized electric field of the microwave resonator (left axis, bottom scale); blue dots represent measured comb spectra (right axis, top scale).

Figure 4. Microwave resonators for different target frequencies. (a) Micrograph of 10 GHz microwave resonators. Measured resonance frequencies: (b) 9.82 GHz and (c) 9.67 GHz.

This work demonstrates three key innovations: First, through electric field enhancement and impedance matching, the driving power is reduced to 740 mW, achieving a power efficiency metric of 116 nm·W^-1/2·MQ^-1, the best in its class. Second, it pioneers isolator-free on-chip EO comb generation, greatly simplifying system architecture. Third, the design employs CMOS-compatible TFLN fabrication, with a compact footprint of 5 mm×1 mm, enabling wafer-scale production with high process compatibility and cost-effectiveness. These breakthroughs unlock broad prospects for practical applications of optical frequency combs.

Prof. Cheng Wang commented: "The innovation of lithium niobate EO combs lies not only in technical breakthroughs but also in their vast application potential. In optical communications, they enable ultra-dense wavelength-division multiplexing with low-noise signal generation. In precision measurement, they provide stable light sources for lidar and atomic clock calibration. In the field of perception and detection, they provide wide-spectrum and high-precision light source solutions for high-sensitivity gas sensing, distributed fiber monitoring, and biomedical imaging. Recently, by optimizing waveguide structures and modulation schemes, the team expanded comb teeth to the hundred-nanometer scale using wafer-scale TFLN processes—a critical step toward industrialization."

Currently, the team is pursuing further optimizations, including: (1) developing microstructured electrodes to reduce microwave losses and enhance modulation efficiency; (2) improving optical designs and fabrication to boost cavity Q beyond 2×10⁶, targeting comb span over 150 nm; (3) exploring millimeter-wave and terahertz resonator designs for 6G communications; and (4) advancing system-level integration by monolithically combining lasers, modulators, and detectors into fully functional photonic chips. These efforts aim to transition EOFC technology from labs to industrial applications.

In summary, this study addresses the challenges of reflection and power consumption in on-chip EO combs through innovative microwave resonator designs, establishing a new paradigm for integrated, low-cost frequency comb systems. Combining theoretical depth with practical feasibility, the technology holds promising potential in communications, sensing, and quantum technologies.