This works demonstrates an improved local two-way optical phase comparison method. Based on this scheme, we implement a real-time frequency comparison over a bi-directional 200-km fiber spool link with a Bi-EDFA. The achieved relative frequency stability is 9.7E-18 at 1 s integration time and reaches 1.4E-21 at 10,000 s.

Two-way Optical Phase Comparison at 10 -21 Level over 200-km fiber link

In this paper, we present a high-precision optical frequency noise detection and comparison technique using a two-way transfer method over a 260 km field fiber link. This method allows for the comparison of optical frequencies between remote optical references without the need for data transfer through communication. We extend a previously established two-way comparison technique to obtain all data at the local site. Two optical carrier signals are injected into the bidirectional fiber from both ends, and one carrier is reflected back from the remote end. This enables the phase comparison of the two carrier signals at a single site without the need to transmit experimental data. The common-mode frequency noise induced by the bidirectional fiber link is detected and effectively suppressed without the need for sophisticated active fiber noise control. Our demonstration system, which uses a 260 km field fiber link and a common laser source, achieves a fractional instability of 2.5×10−17 at 1 s averaging time and scales down to 3.5×10−21 at 8000 s. This scheme offers the distinct advantage of completing the comparison at a single site, eliminating the need for remote data transfer via communication. This method is expected to enhance reliability for high-precision frequency comparisons between remote optical clocks and advanced atomic clocks. 

High-Precision Fiber Noise Detection and Comparison over a 260 km Field Fiber Link

This works demonstrates an improved local two-way optical phase comparison method without the requirement of synchronization. Based on this scheme, we implement a real-time frequency comparison over a bi-directional 50-km fiber spool. The achieved relative frequency stability is 1.2E-16 at 1 s integration time and reaches 1.3E-21 at 40,000 s.

Two-way optical phase comparison at 10 −21 level

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Two-way optical phase comparison at 10 −21 level

Two-way Optical Phase Comparison at 10 -21 Level over 200-km fiber link