Significance Distance measurement is a common basic technology in the field of geometric measurement and has broad applications in scientific research and industry. Currently, high-precision distance measurement is normally achieved using the interferometric method, and the distance results can be directly traced to the optical wavelength. However, the phase ambiguity hinders the application of the traditional interferometric method in long-distance absolute positioning, such as space missions, including tight formation-flying satellites, antenna measurement, spacecraft rendezvous and docking, as well as precision manufacturing and assembly, including aircraft manufacturing, satellite equipment manufacturing, and synthetic aperture optical system assembly. Fortunately, invention of the optical frequency comb (OFC) provides great opportunities for geometric measurements.

In recent years, several OFC-based methods have been proposed for the measurement of distances, e.g., the intermode beat, dispersive interferometry, pulse alignment, and dual-comb methods. Compared with conventional methods, OFC-based methods are capable of resolving the problem of phase ambiguity and measuring the absolute distance. Among them, the dual-comb ranging method makes full use of the characteristics of OFC in the time and frequency domains and exhibits reasonable dynamics, precision, and unambiguity range. The dual-comb method opens up a new direction for distance measurement and is expected to bring great benefits to optical metrology. Since 2009, many advances have been achieved in dual-comb ranging techniques. However, there are still several challenges involving principle research and industrial applications. Hence, it is necessary to summarize progress of the dual-comb ranging technique to guide future development in this field more rationally.

Progress The progress of the dual-comb ranging method is illustrated in Fig. 1. The concept of dual-comb was first proposed in 2002 by Schiller, and was first applied to absolute distance measurement in 2009 by Coddington et al from NIST. Recently, the proposed dual-comb ranging method primarily uses fiber combs as the light source and achieves absolute distance measurement based on two principles, namely, time-of-flight-based and phase-based ranging. Optimizing the parameter model and suppressing the frequency noise of a dual-comb system are two key technologies to achieve high-precision distance measurement. Here, the time-of-flight ranging method mainly focuses on the former, while achieving the phase-ranging method is based on the latter. Since 2013, domestic and foreign researchers have explored the performance improvement, function expansion, and applications of the dual-comb ranging method, including the dead-zone elimination of dual-comb ranging, nonambiguity range extension, improvement of ranging precision and response speed, dual-comb multi-degree-of-freedom sensing, and instrumentation and applications of the dual-comb ranging technique. Since 2018, research on the microresonator-based dual-comb ranging method has been reported. The microcomb has the characteristics of miniaturization and easy on-chip integration. Compared with the fiber comb, the microcomb offers a better application prospect in the field of ranging, and provides new opportunities for further developments of dual-comb ranging.

Conclusions and Prospects In summary, the dual-comb ranging technique provides an efficient tool for absolute distance measurement with a large unambiguity range, high precision, and high speed, and it has also become a hot spot in the field of ranging. Such an overall performance brings great benefits to various tasks in optical metrology. With the continuous in-depth and detailed explorations of dual-comb ranging, it is expected to become a portable instrument product widely used in scientific research and industry.