A multiple-access underwater frequency transfer scheme using terminal phase compensation is demonstrated. With this scheme, a highly stable 100 MHz frequency signal was disseminated over a 3 m underwater link for 5000 s. The timing fluctuation and fractional frequency instability were both measured and analyzed. The experimental results show that with the phase compensation technique, the total root-mean-square (RMS) timing fluctuation is about 3 ps, and the fractional frequency instabilities are on the order of 5.9 × 1 0 − 13 at 1 s and 5.3 × 1 0 − 15 at 1000 s. The experiment results indicate that the proposed frequency transfer technique has a potential application of disseminating an atomic clock to multiple terminals.
A multiple-access underwater frequency transfer scheme using terminal phase compensation is demonstrated. With this scheme, a highly stable 100 MHz frequency signal was disseminated over a 3 m underwater link for 5000 s. The timing fluctuation and fractional frequency instability were both measured and analyzed. The experimental results show that with the phase compensation technique, the total root-mean-square (RMS) timing fluctuation is about 3 ps, and the fractional frequency instabilities are on the order of 5.9 × 1 0 − 13 at 1 s and 5.3 × 1 0 − 15 at 1000 s. The experiment results indicate that the proposed frequency transfer technique has a potential application of disseminating an atomic clock to multiple terminals.
We demonstrated an underwater frequency transfer technique with a green diode laser. The characteristic of the timing fluctuation and instability for the transfer technique was analyzed and simulated. With this technique, we had transferred a highly stable 100-MHz frequency signal over an underwater link with distances of 3 m, 6 m, and 9 m for 5000 s, respectively. The experimental results involving the underwater transfer of the 100-MHz radio-frequency signal shows that the rms timing fluctuations are 5.9 ps (3-m link), 6.4 ps (6-m link), and 8.4 ps (9-m link). The calculations also show that the relative Allan deviations for the 3-m, 6-m, and 9-m transmission links are 5.6 × 10−13 at 1 s and 5.3 × 10−15 at 1000 s, 5.8 × 10−13 at 1 s and 1.1 × 10−14 at 1000, and 6.8 × 10−13 at 1 s and 1.1 × 10−14 at 1000 s, respectively. The measured instabilities were lower than Rb and Cs atomic clocks, implying that the proposed frequency transfer scheme can potentially be used to transfer the signals of these atomic clocks over underwater links.
We demonstrated an atmospheric radio-frequency transfer over a 50 m outdoor free-space link using a compact diode laser with an electronic phase compensation technique. With transferring a 1 GHz microwave signal in 5000 seconds, the total root-mean-square (RMS) timing fluctuation was measured to be about 560 fs, with fractional frequency instability on the order of 4 × 10-13at 1 s, and order of 4 × 10-16at 1000 s. This atmospheric frequency transfer scheme can be used to disseminate a commercial Rb or Cs clock via a free-space transmission link.
High-precision time–frequency is widely used in time measurement, satellite navigation, scientific research, and other fields. With the rapid development of optical clock technology, the fractional frequency instability and uncertainty of optical clock have reached 10−18 orders of magnitude, which is expected to contribute to generating the International Atomic Time and may even be used to redefine the “second” in the future. Therefore, the long-distance transfer of time–frequency signals between optical atomic clocks is of great significance. Free-space optical frequency transfer technology is one of the important technologies for solving the space-based optical clock comparison because of its high transfer precision and easy networking characteristics. In order to solve the long-distance space-based optical clock comparison, this paper investigates a free-space active phase noise compensation method using an Acousto-Optic Modulator (AOM), based on the traditional optical fiber phase noise compensation scheme. This new method is more flexible and scalable than the optical fiber time–frequency transfer technology. The optical frequency transfer over a 64 m free-space link is demonstrated. The fractional frequency transfer instability during free running is 9.50 × 10−16 at 1 s, and 4.44 × 10−16 at 2000 s, and the fractional frequency instability after compensation is 7.10 × 10−17 at 1 s, 3.07 × 10−19 at 2000 s, which is about 1–3 orders of magnitude better than that in free running, and provides a feasible scheme for space-based optical clock comparison.
We demonstrate a new ultra-stable frequency dissemination scheme on L band over 100m atmosphere. By compensating phase noise actively and a series of frequency conversions, the phase fluctuation is well compensated. Two links disseminate different frequency signals phase locked to a common reference simultaneously, and recover the disseminated frequency respectively at remote site. The stability of dissemination with phase compensation in free space is 3×10-13/s and 4×10-17/day. The relationship between dissemination stability and propagating distance is also discussed.
In this Letter, we propose a multiple access fiber-optic radio-frequency transfer, time, and phase synchronization scheme using passive optical coupling and symmetric carrier configuration. In the proposed scheme, the RF signal is distributed to distant nodes via a symmetric carrier configuration, which inherently suppresses link length-dependent phase difference. Access nodes are connected to the fiber link via passive optical coupling. Time signals are generated at each node by leveraging transferred frequency signals and synchronized by time-division multiplexing transmission and active delay compensation. Experimentally, a 2 GHz RF signal is transferred to access and terminal nodes over 20-km and 60-km fiber links, respectively, and compensated separately. The measured frequency stability is better than 4×10 −14 and 2×10 −16 at averaging time of 1 s and 10 4 s, respectively. For the synchronized link, the measured time stability is better than 7 ps at 1 s and 4 ps at 10 4 s. The phase consistency of RF signals is better than 10 ps (2 % of the entire signal cycle) when subjected to changes in link length and system restart.
We demonstrated a portable atmospheric radio-frequency transfer over a 120 m outdoor free-space link using a compact diode laser as the clock carrier. Timing fluctuation and Allan Deviation are both measured to characterize the instability of transferred frequency incurred during the transfer process. In this experimental setup, all components were powered by a full-charged battery, and the experiment was conducted in an open-air environment, which is far from the laboratory-based room. With transferring a 100 MHz microwave signal in 4500 seconds, the total root-mean-square (RMS) timing fluctuation was measured to be about 22 ps, with fractional frequency instability on the order of 3 × 10-12at 1 s, and order of 4 × 10-14at 1000 s. We found that the majority of this excess timing fluctuation in our experiment is attributed to the air turbulence which could contribute a jitter at tens picosecond level in a normal windy night. This portable atmospheric frequency transfer scheme can be used to fast build an atomic clock-based frequency free-space transmission link since its stability is superior to a commercial Rb or Cs clock.