The synchronisation of time and frequency between remote locations is crucial for many important applications. Conventional time and frequency dissemination often makes use of satellite links. Recently, the communication fibre network has become an attractive option for long-distance time and frequency dissemination. Here, we demonstrate accurate frequency transfer and time synchronisation via an 80 km fibre link between Tsinghua University (THU) and the National Institute of Metrology of China (NIM). Using a 9.1 GHz microwave modulation and a timing signal carried by two continuous-wave lasers and transferred across the same 80 km urban fibre link, frequency transfer stability at the level of 5×10−19/day was achieved. Time synchronisation at the 50 ps precision level was also demonstrated. The system is reliable and has operated continuously for several months. We further discuss the feasibility of using such frequency and time transfer over 1000 km and its applications to long-baseline radio astronomy.
We experimentally demonstrate a robust scheme to select a single high-order harmonic among the harmonic comb by using a driving laser field with subcycle waveform control, which is synthesized by the fundamental 800 nm laser pulse and two controlling laser pulses at 400 and 267 nm with perpendicular polarizations. By controlling the relative phase among the pulses of different colors, a single high-order harmonic is selectively enhanced while the adjacent harmonics are greatly suppressed with the intensity contrast increased by more than 1 order of magnitude and the peak intensity enhanced simultaneously by more than 2 orders of magnitude compared to the case by using only the fundamental 800 nm laser pulse. Such phenomena can be mainly attributed to the intra-atomic phase matching realized with the sub-cycle waveform controlled field.
We demonstrate a fiber-based multiple-access ultrastable frequency dissemination scheme over an 83 km fiber link. As a performance test, we reproduce the disseminated 9.1 GHz radio-frequency modulation signal at an arbitrary point in the dissemination channel. Relative frequency stability of 7×10(-14)/s and 5×10(-18)/day is obtained. Highly synchronized frequency signals can be regenerated along the entire fiber pathway and its applications are discussed.
We demonstrate a fiber-based, multiple-access optical frequency dissemination scheme. Without using any additional laser sources, we reproduce the stable disseminated frequency at an arbitrary point along the fiber link. Relative frequency stabilities of 3×10(-16)/s and 4×10(-18)/10(4) s are obtained. A branching fiber network for high-precision synchronization of optical frequency is made possible by this method, and its applications are discussed.
We experimentally demonstrate the macroscopic evolution of quantum-path distributions in harmonic emission with spatial and spectral resolution from an argon gas jet, and obviously observe that the spatial profiles of harmonics are gradually split into two components (the red and blue shifts) when the driving laser intensity is increased. Moreover, the red and blue shifts in quantum-path distributions are experimentally traced and clarified in the spatial and spectral domain by choosing the focal position. These results give a more comprehensive understanding and therefore a better control of harmonic emission.
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