| Nowadays,both the frequency instability and uncertainty of the best optical atomic clock have reached 10-18.It has great potential to be the next generation of time and frequency standard.With such high accuracy frequency standards,new physical phenomena and laws will be discovered.Once optical clocks at different places are connected,it will play an important role in many fields such as geodesy,high-accuracy metrology,and tests of basic physics.Whereas the transfer instability of frequency based on satellites in one day is only 10-16,which is unable to meet the requirements for transferring the signal from optical clock.Fortunately,the transfer instability and uncertainty of optical frequency via optical fibers can be as low as 10-19,which satisfies the requirements of frequency dissemination and comparison of optical clocks.Therefore,how to link the signal from optical clocks to the telecom-band lasers and how to cancel the fiber-induced phase noise are the key to realize high precision frequency dissemination via fiber.In this paper,two narrow-linewidth ultrastable lasers systems at 1557 nm are firstly established by frequency stabilizing the laser frequency to 10-cm-long FP cavities with the Pound-Drever-Hall technique.By frequency comparing between two similar systems,the most probable linewidth of each laser is 0.26 Hz,and the frequency instability of each laser at 1-30s averaging time approaches to the cavity-thermal-noise-limit length stability of 8×10-16.Based on the narrow-linewidth ultrastable lasers at 1557 nm,the frequency link between optical clocks and telecom-band lasers could be realized with a femtosecond optical frequency comb.In order to cancel the environmental noise induced light phase noise via optical fiber,two techniques have been developed.For one,the fiber noise is cancelled at the local site while for the other the fiber noise is cancelled by the remote users.We realize the optical frequency coherence transfer via 75-km-long fiber link with FNC system at local site.By comparing the light before and after the fiber,the frequency instability induced by the fiber link is 2.6×10-17 at 1 s averaging time,scales down to5.1×10-19 at 5000 s averaging time.The frequency uncertainty induced by the fiber link is 2.2×10-19.Then we established 50 km+50 km transfer system with a repeater station,the transfer instability is 5.5×10-17 at 1 s averaging time and 5.5×10-19 at10000 s averaging time.The frequency uncertainty induced by the fiber link is3.1×10-19.In order to realize the optical frequency transfer from one point to multiple points,optical frequency transfer system via fiber with FNC system at remote site is developed.At the remote site,the frequency noise induced by the time base at the remote site is eliminated.In this way,we realize optical frequency transfer via25-km-long fiber link and 50-km-long fiber link,respectively.The frequency instability induced by the fiber link is?2-4?×10-17 at 1 s averaging time and 1×10-19 at1000 s averaging time.The frequency uncertainty induced by the fiber link is better than 3.0×10-19.For this scheme,the users at remote site could independently compensate the fiber noise.However,the servo band of this scheme is narrower than that of system with FNC system at local site.The results indicate that both systems with FNC system at local site and remote site satisfy the requirements of frequency dissemination from the best optical clocks in the world without degrading their performance. |
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