The generation and transfer of time and frequency standards is essential to the national economy, national defense and our daily life.From a scientific point of view, frequency standards with high accuracy and stability can help scientists solve many fundamental scientific questions, such as changes in fundamental physical constants, dark matter detection, solid tides etc. From the first atomic clock invented in 1955, the frequency stability based on atomic frequency standards continues to increase. From microwave to optical frequency atomic clock atomic (optical clock) is the current trend and the major developed countries are committed to the development of optical clocks. In 2014, the uncertainty of the optical clock has reached the order of 10-18[1]. In 2015, the frequency shift from uncertainty of strontium atomic clock light system in China Institute of Metrology has reached 2.3 × 10-16 [2].
However, the traditional method of transmitting microwave frequencies based on free space does not meet t such a high-precision frequency standard. In the GPS system, for example, takes a long time (average time: day) to average out variations in the environmental conditions of the transmission path, it cannot be provided for a short time with high accuracy synchronization signal stabilization time necessary. Therefore, GPS-based timing system does not have the ability to deliver a new generation of highly stable optical clocks. The present study show that the use of fiber-optic networks isolated from the environment for precision frequency transfer, and to take the initiative after the compensatory measures, the frequency transfer instability can achieve stable 10-19 days. In June 2015, Europe began to implement a plan by more than four units of optical fiber light clock. These units include the British National Institute of Physics NPL, the Paris Observatory, the German Technical Physics Institute PTB and the Italian National Institute of Metrology (INRIM) [3].
Currently, precise frequency transfer and time synchronization based on the fiber link has become an important means of time-frequency transmission. In the specific implementation of the program, we use electronically and optically combined system design.Meanwhile, detect the phase vibration due to temperature, mechanical vibration during the optical fiber transmission and use active compensation to improve its stability. Frequency transfer through the fiber, we need to transmit the atomic clock / optical precision frequency reference from one end to another end without loss. The use of optical carrier radio frequency transmission technology program is divided into the following three methods: RF modulation transfer, optical frequency transfer and optical frequency comb transfer. The latest research progress is Droste S. [4], who completed from the German Max Planck Institute Quantum Optics (MPQ) - German Federal Institute of Technology Experimental Physics (PTB) -MPQ constructed 1840 km of fiber optical frequency transfer ,the frequency transfer instability can reach 10-15 / 100 s.
Peking University has completed the experiment at 120 km carrier-grade optical fiber, optical frequency comb as a carrier burst and using feedforward compensation techniques before digital technology and precise frequency and wavelength division multiplexing optical fiber transmission and two-way synchronization with high accuracy time synchronization. Frequency transfer instability reached 6.18 × 10-20 / 2000 s, the time synchronization accuracy of <40 ps. Functional block diagram shown in Figure 1.
On the basis of 120 km fiber-precision frequency transfer and time synchronization, we completed a pair of four-star network points.Star networking principle block diagram shown in Figure 2.
Frequency precision and time synchronization technology is an important basis for research in the field of quantum information, which belongs to the high-tech frontier. Frequency and time transfer and synchronization between the optical fiber communication technology research network based on the physical media sites, development of the same frequency source and one-to-many broadcast of high-precision technology, and to improve inter-site time synchronization accuracy on this basis, can provide core technical support for critical applications navigation, electricity, communications and so on.
References:
[1] Bloom, B. J., Nicholson, T. L., Williams, J. R., et al.. An optical lattice clock with accuracy and stability at the 10-18 level. Nature, 2014, 506:71-75.
[2] Lin YiGe, Wang Qiang, Li Ye, et al.. First evaluation and frequency measurement of the strontium optical lattice clock at NIM. Chin. Phys. Lett., 2015, 32(9):090601.
[3] E. Gibney. Atomic clocks face off: next generation of hyper-precise timekeepers can only be tested against each other. Nature, 2015, 522:16-17.
[4] Droste, S., Ozimek, F., Udem, T., et al.. Optical-frequency transfer over a single-span 1840 km fiber link. Phys. Rev. Lett., 2013, 111(11):110801.