Experimental Study On Optical Frequency Transfer Via Optical Fibers

The atomic clocks are important tools for time and frequency standards. In the recent years, along with the rapid development of the technologies including laser cooling and optical frequency comb, both the instability and accuracy of the optical clocks from different countries have been improved to 10-18 level in several groups. Nowadays the uncertainty of the best optical clocks surpasses the primary frequency standard(cesium fountain clocks) by two orders of magnitude. Therefore, optical clocks are considered as promising candidates for the redefinition of the second in the International System of Units(SI). Nevertheless the well-established satellite-based microwave frequency transfer techniques like TWSTFT, which reach a relative transfer instability of 10-16 after 1 day, are not adequate for transferring the optical frequency of the state-of-the-art optical clocks. Fortunately various methods have been developed to transfer time and frequency signal via optical fibers with ultrahigh precision. Particularly the relative instability of optical frequency transfer via fibers has reached to 10-20 level after averaging for 1 day. As the instability is two orders of magnitude higher than that of the optical clocks, the fiber-based optical frequency transfer can be applied to distribute the optical frequency and remotely compare the optical clocks from different laboratories.This thesis introduces the research on optical frequency transfer via fiber links in NTSC. The content of this thesis can be summarized as the following sections:1. The theoretical study on fiber-based optical frequency transfer. The principles of fiber-induced noise and fiber noise compensation scheme using the so-called “Doppler cancellation technique” are investigated. To disscuss the fundamental limitations, a transfer model is built and the transfer function of phase noise compensation is derived. The results show the relative instability of optical frequency transfer is mostly limited by the fiber delay and the free-running phase noise of the fiber link.2. The experiments on the direct distribution of optical frequency via fiber are demonstrated. And the optical frequency transfer via a 100km-level urban fiber link shows a relative instability of a few part of 10-20. Here the direct distribution means point-to-point frequency transfer without any optical amplification. Firstly optical carrier transfer is demonstrated over a 1km fiber spool. Then using the improved experimental device for optical frequency transfer, we distribute a coherent optical carrier with 1k Hz linewidth over a 70 km fiber spool in the lab, which shows a relative instability of 10-15 at 1s. Ultimately a 1.9Hz-linewidth laser is applied as the optical carrier and transferred over a 112 km urban fiber link with a relative instability of 2.5×10-16/1s and down to 10-20 level after 104 s averaging time.3. Study on long-distance transfer of optical frequency is described. We compared two different methods to increase the distance of optical frequency transfer, optical power amplification and cascaded transfer. A new scheme for the optical frequency signal amplification in the cascaded repeater station is presented, in which the optical signal can be amplified by an EDFA, while the phase noise induced by optical power amplification can be detected and compensated by an AOM. The new scheme is proved to be suitable for the repeater station with more compact structure and lower cost.4. Prospective investigation is peformed on optical frequency transfer networks with multiple users. A new method of remotely compensating the fiber-induced noise for optical frequency transfer is reported, which can be used for establishing optical frequency transfer networks. The three basic network structures of optical frequency transfer networks are analyzed, including the ring structure, the star structure and the tree structure. The new remotely-compensating scheme is quite suitable for branch distribution of optical frequency in the tree structure. Test of optical frequency transfer with remotely-compensating scheme over 50 km fiber spool shows a relative instability of 6.9×10-17 /1s and 3.0×10-18 /104 s, which is close to the former locally-compensating method. Therefore, the remotely-compensating method can be applied for establishing the three structures of optical frequency transfer networks, with less complexity and more independence.