Further, since the pump laser frequency will be locked at the signal laser frequency to achieve a narrow gain bandwidth (i.e., quasi-unidirectional amplification), Rayleigh scattering and point reflections have negligible effects during transmission of the optical frequency 22, 23. Fiber Brillouin amplification (FBA) is an alternative technique that can provide an amplification gain of 50 dB with approximately 30 mW pump power only. However, the bidirectional nature of the frequency transfer system (Doppler phase noise cancellation technology) limits the gain of the system and deteriorates the stability of transmission this is because the point reflections and Rayleigh scattering in the fiber link may send feedback into the EDFAs, which may trigger the stimulated effect 20, 21.
In order to transmit an optical signal across a distance up to thousands of kilometers, the noise introduced by the optical fiber link should be eliminated and the power loss caused by long-distance transmission should be compensated in order to significantly improve the signal-to-noise ratio of the optical signal detected at both the local and remote end.Īn erbium-doped fiber amplifier (EDFA) is the most commonly used amplifier for amplification in an optical fiber link. Therefore, optical fiber links have been widely investigated as possible media for transferring today’s most stable existing frequencies to remote locations 10, 11, 12, 13, 14, 15, 16, 17, 18, 19. Traditional satellite-based frequency transfer is far from meeting the requirements as its stability is only 10 −16 over an entire day. However, studies in fundamental physics, navigation, time keeping, or geodetic applications 7, 8, 9 require that the clock signal be transferred to remote sites. They serve as frequency references for testing basic theories such as quantum electrodynamics, general relativity, and their foundations, for example, to verify the constancy of the fundamental constants and other experiments that can be performed locally 5, 6. State-of-the-art optical frequency standards are superior to the current cesium-based atomic clocks, in which a fractional systematic uncertainty 1, 2 can reach as low as 10 −18, and the fractional instabilities 3, 4 in the same regime are below 10 −17.
Immense progress in the development of frequency standards has facilitated high performance in various research fields.
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