In this paper, we propose a method aiming to measure the absolute distance via the slope of the inter-mode beat phase by sweeping the repetition frequency of the frequency comb. The presented approach breaks the inertial thinking of the extremely stable comb spacing, and the bulky phase-locking circuit of the repetition frequency is not required. In particular, the non-ambiguity range can be expanded to be infinite. To verify the performance of presented method, a series of distance experiments have been devised in different scenarios. Compared with the reference values, the experimental results show the differences within 25 µm at 65 m range in the laboratory, and within 100 µm at 219 m range out of the lab.

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Absolute distance measurement based on the asynchronous optical sampling with using a dual-comb system has the characteristics of large range, fast measurement speed, and high accuracy, which has wide application prospects in the field of precision measurement of geometric quantities, such as the space technology, equipment manufacturing, etc. Recently, the invention of the femtosecond frequency comb is a milestone in the field of precision length measurement. Many approaches to the absolute distance measurement have been proposed. Among them, the dual-comb system with asynchronous optical sampling can realize a length measurement with fast speed, high accuracy, and long range. Especially, the temporal method combining the asynchronous optical sampling with nonlinear intensity cross-correlation can effectively avoid influencing of the carrier-envelope offset frequency on the ranging accuracy in the measurement process. The time-of-flight information can be obtained by the time interval between the reference pattern and the measurement pattern. Even so, the selection of the repetition rate and the difference of repetition rates will strongly influence the temporal sampling interval of the measurements. Therefore, the theoretical model and key parameters for the ranging are numerically studied for the non-linear asynchronous optical sampling by using a dual-comb system of absolute distance measurement. After analysis, the effects of source parameters (repetition frequency and repetition frequency difference), fine fitting of second harmonic signal, and timing jitter on ranging accuracy are studied respectively. The numerical analysis results show that the method of choosing a reasonable repetition frequency and repetition frequency difference is beneficial to the improvement of the ranging accuracy. When the sampling interval of the dual-comb system is a constant, the time value between the reference and measurement patterns can be obtained by the interpolation method of fine curve fitting, and it will further improve the ranging accuracy. In addition, the time jitter of the femtosecond pulses is also an important factor that can affect the ranging accuracy. By changing the difference in the repetition rate, the measurement speed can also be improved. After that, the cumulative ranging error caused by time jitter can be reduced. Therefore, the appropriate increasing of measurement speed can effectively reduce the influence of timing jitter on ranging.

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