Measurement of propagation delay in fiber with dual microcombs at the sub-femtosecond level

In recent years, the propagation delay in optical transmission systems has received increasing attention. Including precise time synchronization protocol, optically controlled distributed coherent radar and other technical fields. The traditional optical time domain reflectometer (OTDR) measures the propagation delay of optical fibers from a single end, using several nanosecond pulses with measurement accuracy of the same magnitude. Meanwhile, as an ultrafast pulse light source with multiple equidistant optical modes, optical frequency combs have shown significant advantages in many precision measurement scenarios such as long-distance ranging, spectroscopy and frequency modulation radar due to their excellent time-frequency characteristics. In this article, a measurement method of propagation delay in fiber based on dual microcombs at the sub-femtosecond is proposed. Based on the principle of linear optical sampling(LOS), the interval of the signal pulses can be stretched in the time domain, and high-precision measurement is carried out. In order to improve the coherence of the dual-comb in the system, we use the same continuous-wave laser source to pump the dissipative Kerr soliton (DKS) comb in two microcavities with slightly different free spectral ranges of~25GHz. In the end, we achieved a femtosecond level accuracy of 8.04fs at single LOS measurement. Moreover, due to a high update rate exceeding 1MHz, we obtained a sub femtosecond level measurement accuracy of 0.744fs with an average processing time of 0.1 milliseconds. This method has unique advantages of fast measurement speed and high accuracy, and has broad application potential in various fields such as optical communication and optical distributed sensing.