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Optical frequency combs, as a time and frequency "ruler," have important applications in precision ranging. Conventional dual-comb ranging schemes utilize the optical Vernier effect to achieve long-distance measurements, and they typically require asynchronously secondary sampling, either after changing the repetition rates or swapping dual-comb roles.

These approaches have a commonly overlooked issue: When considering real-time distance variations induced by target motion or atmospheric turbulence in practical measurement scenarios, the asynchronously secondary sampling will introduce substantial absolute distance measurement error, namely asynchronous measurement error (AME).

In a study in Science Advances, Prof. Zhang Wenfu's team from the Xi'an Institute of Optics and Precision Mechanics (XIOPM) of the Chinese Academy of Sciences proposed an on-chip cross dual-microcomb (CDMC) ranging method based on dispersion interferometry. This method resolves the AME issue by eliminating secondary measurements through one-shot spectral sampling of cross dual-microcomb carrying distance information in the frequency domain.

Researchers used the laser-assisted intracavity thermal balance scheme to generate two single soliton microcombs. Through thermal tuning, they optimized the frequency interval between the two pump lasers to about half the repetition rate, yielding the CDMC. The comb-teeth of CDMC exhibited cross-distribution and a uniform pattern, enabling precise one-shot spectral sampling using optical spectrum analyzer or photodetector arrays.

A series of ranging experiments were conducted using CDMC. The standard deviation of the stepwise measurement was 3.72 μm @ 7.14 m. The standard deviation of the fixed-point measurement was 56 nm @ 0.3 m, and the corresponding minimum Allan deviation was 5.63 nm @ 56 s. Comparative experiments demonstrated significant AME under dynamic conditions (60 mm @ 0.3 m), whereas synchronous measurements completely eliminated the AME.

Detailed analysis revealed that the repetition rate jitter limits the extended nonambiguity range (NAR). By implementing the microwave injection locking technique, the repetition rate jitter was reduced from 1 kHz to 2 Hz, thereby extending the NAR from 3 mm to 339 m.

"The CDMC ranging method has the potential for full-chip integration and demonstrates significant advantages in long-range, dynamic, and absolute distance measurement applications," said Prof. Zhang Wenfu from XIOPM.