Study On The Key Techniques For Precision Transfer Of Optical Frequency Standard Via Fiber Links

Optical atomic clock has the properties of high accuracy and stability. It has potential to be the new definition of "second", which could be used in physical constant measurement, deep space navigation, geodesy, timing synchronization and so on. Most of these applications based on optical clock require precise frequency transfer of clock signal between distant laboratories. Therefore, it is of great importance to study on the optical frequency standard transfer. There are two key techniques in the study. For one thing, it is necessary to obtain a light source which has a linewidth less than 1 Hz and frequency instability better than 10"15 to act as a clock laser. For another, as the frequency instability of optical clock has reach the level of 10’18, it is indispensable to design a transfer scheme with higher stability to meet the demand for precise frequency transfer and keep the original coherence of the optical clock.This paper mainly introduces the realization of long-distance coherent optical frequency transfer based on the two techniques:1. Two sets of 1.5 μm ultrastable laser systems with sub-Hz linewidth are developed. They are realized by using PDH technique to lock the frequency of the laser to the resonance frequency of FP cavity with ultrahigh finesse. Thus, the property of the laser light is largely dependent on the stability of cavity length and the precision of servo system. The stability of cavity length is affected by the ambient vibration and temperature fluctuation. We design a supporting structure to make the cavity insensitive to vibration and vacuum chamber to diminish the influence of temperature fluctuation on the cavity length. The evaluation of the 1.5 μm ultrastable laser system shows that each laser has the most probable linewidth of 0.26 Hz and frequency instability of 8.6×10-16(1s), approaching to the thermal noise limitation of the cavity. It can meet the requirement for study of coherent optical frequency transfer. As far as we know, the laser has the narrowest linewidth among all the lasers have been reported that are stabilized to a cavity at normal temperature.2. An optical frequency standard transfer system based on telecommunication fiber is developed. The random phase noise induced by the fiber length fluctuation is extracted by using double-pass method and compensated by adjusting the driving frequency of an AOM. In the paper, we firstly realize the optical frequency standard transfer in a 50-km-long spooled fiber. The instability of transfer reaches 2×10-17(1s), and the relative broadening linewidth is suppressed to 1 mHz. Furthermore, it is also realized in a 32-km-long telecommunication fiber. The instability of transfer reaches 3.5×10-17 (1s), and the linewidth of the transferred light remains at 0.26 Hz, as same as that of the light source. Finally, we establish a repeater amplification system to extend the fiber transfer length to 82 km. The instability of transfer reaches 4×10-17 (1 s), and the relative broadening linewidth is also suppressed to 1 mHz. All the results shown above indicate that the optical frequency standard transfer system can meet the demand for transfer of the best optical clock in the world. And it is also proved that the transfer technique based on repeater amplification system is competent for long-distance coherence transfer in fiber link.