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Review of Scientific Instruments

Field trial of stable radio frequency transfer system in 200 km metropolitan optical fiber link

Chenxia Liu, Hanyue Wang, Ge Song, Hao Gao, Jiahui Cheng, Tao Liu, Song Yu

Published Oct 1, 2024• 1 citation

Abstract

In this paper, we demonstrate stable radio frequency (RF) transfer via metropolitan optical fiber link in the Beijing area. The phase variation of the RF signal is compensated by a phase conjugation method incorporating two high-performance phase-locked loops. The wavelength conversion module extends the transmission length to 200 km with only two parallel 50 km dark optical fibers available. We optimize the configuration of dispersion compensation and optical amplification due to the high loss (0.31 dB/km) of the optical fiber link. At the same time, comparative experiments verify the short-term instability limitation that arises from the group velocity dispersion of the optical fiber link. The measured standard Allan deviation of the 2.4 GHz RF transmission system with dispersion compensation is 4.5 × 10-14/1 and 2.6 × 10-17/20 000 s, which is superior to that of the reference rubidium clock. The short-term instability of the system is deteriorated to 2.5 × 10-13/1 s without dispersion compensation.

DOI: 10.1063/5.0222748

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We have demonstrated accurate frequency transfer via an ultra long span optical-fiber link by passive phase conjugation, which is based on frequency mixing. Combing with the phase-lock-loop, our compensation technique can eliminate the effect of phase noise that introduced during the optical-fiber transmission, so the phase of remote signal is synchronized to the phase of local reference signal. The transfer stability of our experiment with 2.4 GHz frequency signal through a 1007 km fiber link is 6.9_10-14/s and 4.1_10-16/104s.

This paper presents the system for stable radio frequency (RF) transfer with phase modulation. The phase modulator is not biased, which eliminates the bias drifting problem existing in intensity-modulation-based RF transfer systems. The phase modulation to intensity modulation (PM-to-IM) conversion is realized by using dispersive fiber. Phase variation of the transmitted RF signal is compensated by utilizing passive compensation method. A 2.4 GHz RF signal is transmitted via 125 km single mode fiber (SMF) link. The measured standard Allan deviation (ADEV) of the stable RF transmission system is 3.66×10−13@1 s and 2.26×10−16@10000 s, which is superior to that of the reference cesium clock.

We demonstrate a photonic radio-frequency transmission system via optical fiber. Optical radio-frequency signal is generated utilizing a Mach-Zehnder modulator based on double-side-band with carrier suppression modulation scheme. The phase error induced by optical fiber transmission is transferred to an intermediate frequency signal by the dual-heterodyne phase error transfer scheme, and then canceled by a phase locked loop. With precise phase compensation, a radio frequency with high-phase stability can be obtained at the remote end. We performed 20.07-GHz radio-frequency transfer over 100-km optical fiber, and achieved residual phase noise of -65 dBc/Hz at 1-Hz offset frequency, and the RMS timing jitter in the frequency range from 0.01 Hz to 1 MHz reaches 110 fs. The long-term frequency stability also achieves 8×10(-17) at 10,000 s averaging time.

In this paper, we propose a phase-conjugation-based fast radio frequency (RF) phase auto stabilization technique for long-distance fiber delivery. By phase conjugation at the center site, the proposed scheme pre-phase-promotes the RF signal with the shift which is acquired by round-trip transferring another RF whose frequency is half of the one to be sent. Such phase pre-promotion is then used to counteract exactly the following retard induced by one-way delivery. Different from the previous phase-locking-loop-based schemes, the proposed open-loop design avoids the use of any tunable parts and dynamic phase tracking, enabling a fast phase stabilization at the remote site. An end-less compensation capacity can also be achieved. Our design is analyzed by theory. Experimentally, the new scheme is verified by transferring a frequency of 2.42 GHz through a 30-km optical fiber link. Significant phase drift compression is observed. The rapid phase stabilization is verified by introducing sudden time delay change into the link. The recovery time equals to the round-trip time of the link plus the transitional duration of the delay change, which is much shorter than the traditional trial-and-error phase locking loop. Important issues of the system design are discussed.

The phase instability induced during the transfer of radio frequency and optical clock signals through the turbulent atmosphere was measured in a rooftop experiment. Radio frequency intensity modulation of a laser to transmit signals over 100 m results in an Allan deviation of 1.31x10(-10) at 1 s. Optical transfer is more accurate at 1.68x10(-13) at 1 s. As a consequence, fiber links are more suitable for the transfer of optical frequencies over very long distances while free space transmission might find applications in short distances of less than 1 km.

An rf dissemination system using an optical fiber link has been developed. The phase noise induced during optical fiber transmission has been successfully cancelled using what we believe to be a novel fiber-noise compensation system with a combination of electrical and optical compensations. We have performed rf transfer in a 114 km urban telecom fiber link in Tokyo with a transfer stability of 10(-18) level at an averaging time of 1 day. Additionally, a high degree of continuous operation robustness has been confirmed.