Chongfan Technology
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08
2025
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04
Nature Communications | Single-channel 1.79 GHz absolute distance measurement
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Introduction
With the rapid development of artificial intelligence and autonomous driving technology, achieving high-speed and high-precision 3D environmental perception has become a core challenge. Current intelligent driving systems employ environmental perception schemes mainly including pure vision (cameras), millimeter-wave radar, and lidar. Among them, lidar (LiDAR), as a key sensor, has advantages such as high resolution and strong resistance to active interference, but sampling rate and unambiguous range The trade-off problem has long restricted the performance and cost of lidar hardware.
The principles and methods of lidar ranging widely used today mainly include the time-of-flight (ToF) method and the frequency-modulated continuous wave (FMCW) method. Taking the time-of-flight (ToF) method, which has achieved large-scale commercial application, as an example, although this method can achieve a megahertz sampling rate, it is limited by the pulse emission interval and it is difficult to achieve long-distance absolute distance measurement. In terms of maximum detection distance, it is not advantageous compared with the lower-cost millimeter-wave radar, and it also faces problems such as poor anti-interference ability and poor eye safety. The frequency-modulated continuous wave (FMCW) method can achieve a large absolute distance measurement range with lower optical power, but the time required for single-distance measurement is limited by the principle and algorithm.
Tsinghua University Department of Precision Instruments and School of Electronics and Information Engineering, Sun Yat-sen University have collaborated to achieve technological breakthroughs from both methodological and device perspectives, proposing a repetition-rate modulated frequency comb based on integrated lithium niobate electro-optic frequency comb coherent ranging method (Repetition Rate Modulated Frequency Comb, RRMFC), successfully achieving 1.79 GHz single-channel acquisition rate absolute distance measurement, which is an order of magnitude higher than existing technologies, and is expected to simplify the hundreds of parallel detection units in existing lidar systems to one, providing a new solution for real-time high-precision perception in fields such as autonomous driving and drones.
Design ideas and working principles
The principle of the distance measurement system designed by the team based on the repetition-rate tunable integrated lithium niobate electro-optic frequency comb is shown in Figure 1 As shown. This design uses an electrically controlled repetition-rate integrated electro-optic frequency comb as the light source, and linearly scans the repetition rate of the electro-optic frequency comb in the GHz range. According to the tuning characteristics of the electro-optic frequency comb, the frequency scanning range of the nth comb tooth is n times the electro-tuning range, and the frequency scanning process of each comb tooth is synchronized. The electro-optic frequency comb signal containing all comb teeth is reflected by the target object in the ranging Michelson interferometer, and coherently detected with the local oscillator signal. The beat frequency signals of the positive and negative symmetrical comb teeth after coherent detection are shown in the upper right of Figure 1 as shown, their beat frequencies are multiples and can reach their maximum simultaneously. The actual detected beat frequency signal is the sum of all the above beat frequency signals. Just as the spectral changes from sinusoidal signals to periodic pulse signals in signal and system, the actual detected beat frequency signal, compared with the sinusoidal signal (usually obtained by the FMCW system), has the same period and significant peaks ( lower right of Figure 1 ). By directly detecting the time difference between every two adjacent peaks in the time domain, the absolute distance of the target to be measured can be calculated.
Figure 1: RRMFC distance measurement principle diagram
Source: Nature Communications
RRMFC distance measurement has the following significant advantages:
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High ranging refresh rate. Because each period has sufficiently significant time-domain characteristics ( Figure 2a ), every two time-domain peaks can be used to complete a ranging measurement, so the refresh rate of the distance data is much higher than the frequency corresponding to the time of a single frequency sweep, and can reach the fundamental frequency of the beat frequency signal at most.
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Unambiguous range not limited by the method. Compared with ToF systems, the principle of RRMFC is based on frequency sweep coherent detection, so the absolute distance measurement range is not limited by the method itself to a considerable extent ( Figure 2b ).
Figure 2: Advantages of the RRMFC distance measurement method
Source: Nature Communications
Electro-optic frequency comb device design and characterization
To fully utilize the performance of RRMFC, the electro-optic frequency comb needs to scan several GHz of repetition frequency within a few microseconds. Existing electro-optic frequency comb structures struggle to meet this stringent performance requirement. Based on its research background, the research team proposed an integrated lithium niobate straight waveguide folded optical path phase modulation electro-optic frequency comb structure ( Figure 3 ). This structure consists of an integrated lithium niobate electro-optic phase modulator with a length of 5 centimeters. It can achieve a unique, unrestricted repetition frequency adjustment capability while maximizing the phase modulation depth (increasing the number of electro-optic frequency comb teeth). To achieve a modulation length of 5 centimeters and minimize microwave loss, the research team proposed a folded optical path structure and applied microwave transmission line technologies such as chamfered electrodes and air bridges.
Thanks to its simple and effective structural design, this electro-optic frequency comb not only achieves an unrestricted adjustment range during repetition frequency tuning but also maintains a stable and smooth change in the number of teeth during the adjustment process. The device fabrication was mainly completed on the research team's processing platform. Figure 3b shows the packaged phase modulator electro-optic frequency comb. This device can generate 59 teeth at 6 GHz and smoothly transition to 53 teeth at 10 GHz ( Figure 3c ).
Verification of high-speed distance measurement
The research team used the fabricated phase modulation electro-optic frequency comb to conduct experimental verification work on distance measurement. The experiment designed verifications for different target distances, using different frequency sweep ranges and frequencies. In the verification for a target distance of 62 m, the highest ranging refresh rate reached 1.79 GHz ( Figure 4a ). The ranging refresh rate increases with the increase in the distance to be measured ( Figure 4b ), the range of frequency sweeping, and the speed of sweeping. Figure 4c shows the experimental results of the ranging Allan variance. For each distance, frequency sweep range, and rate, the minimum relative Allan variance can reach the order of 10−5; in 3 m distance measurement, the minimum Allan variance reached 13.8 micrometers.
Figure 4: RRMFC ranging results
Source: Nature Communications
The research team also conducted experiments for three-dimensional environmental perception. In the experiment, the signal light path was emitted to various directions in space using a two-dimensional galvanometer. An unpolished aluminum plate engraved with the word “THU” was placed in the ranging light path ( Figure 5a ). In the experiment, a single-channel RRMFC distance measurement system was used to carry out three-dimensional distance measurement, and the measurement results are shown in Figure 5b . Thanks to the high-speed measurement capability of RRMFC, the experiment achieved a single-channel point frequency as high as 3 MP/s. This point frequency data has already surpassed the total point frequency of current multi-channel lidar systems and still has considerable room for improvement compared to the theoretical acquisition rate of RRMFC. Compared with traditional lidar, this method improves the distance measurement accuracy by more than an order of magnitude at the same refresh rate.
Figure 5: 3D perception experiment based on RRMFC
Source: Nature Communications
Summary and Outlook
In this work, the research team proposed an absolute distance measurement scheme based on the integrated lithium niobate straight waveguide folded optical path phase modulation electro-optic frequency comb structure, achieving an ultra-high refresh rate absolute distance measurement of 1.79 GHz in a single channel. This system has advantages such as high ranging refresh rate and unrestricted unambiguous range, and can achieve single-channel three-dimensional spatial perception. This system can further improve the point frequency and image pixel number of automotive lidar, enabling the perception of distant small objects (such as suddenly appearing pedestrians or bicycles), and further improving the safety of intelligent driving systems.
In future research directions, the realization of a fully solid-state lidar based on RRMFC will be a challenging and meaningful task. Currently, integrated lithium niobate on-chip lasers and amplifiers already have technical routes and solutions. Using solid-state components (such as phased arrays) to emit signal light into a sufficiently large three-dimensional field of view will provide practical help for the implementation of RRMFC technology. In addition, RRMFC can also be combined with dual-comb ranging methods to further improve performance and achieve ultra-long-range, ultra-high-precision, and ultra-fast absolute distance measurement.
Paper Information
Qi, Y., Jia, X., Wang, J. et al. 1.79-GHz acquisition rate absolute distance measurement with lithium niobate electro-optic comb. Nat Commun 16, 2889 (2025).
https://doi.org/10.1038/s41467-025-58018-8
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