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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Stable 2.4 GHz Radio Frequency Transmission Based on Phase Modulation

Chenxia Liu, Tao Liu, Tianwei Jiang, Song Yu

Published Apr 24, 2022• 4 citations
Publisher

Abstract

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.

DOI
10.1109/eftf/ifcs54560.2022.9850722

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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.
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.
We propose a passive compensation fiber-optic radio frequency (RF) transfer scheme with a nonsynchronized RF stable source during a round-trip time, which can avoid high-precision phase-locking and efficiently suppress the effect of backscattering only using two wavelengths at the same time. A stable frequency signal is directly reproduced by frequency mixing at the remote site. The proposed scheme is validated by the experiment over a 40 km single mode fiber spool using nonsynchronized common commercial RF sources. The influence of the stability of nonsynchronized RF sources on the frequency transfer is investigated over different length fiber links.
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.
We propose a novel scheme that uses only a single passive phase compensation device to achieve stable optical and radio frequency joint transfer. The phase noises of optical and radio frequency can be simultaneously compensated by passively embedding their phase information on the two optical carrier sidebands generated by an electro-optical modulator without using the phase discrimination and active servo controller. As a result, this scheme has many advantages, such as high spectral purity, short settling time and infinite compensation accuracy. We experimentally demonstrate the joint transfer of optical and 1 GHz RF over 120 km fiber spools. The optical frequency stability achieves 6.9 × 10 −17 at 1 s and 7.03 × 10 −19 at 10000 s, while the 1 GHz RF is 6.47 × 10 −13 at 1 s and 3.96 × 10 −16 at 10000 s.
In this Letter, we propose a fiber-based stable radio frequency transfer system by a hybrid frequency modulation scheme. Creatively, two radio frequency signals are combined and simultaneously transferred by only one laser diode. One frequency component is used to detect the phase fluctuation, and the other one is the derivative compensated signal providing a stable frequency for the remote end. A proper ratio of the frequencies of the components is well maintained by parameter m to avoid interference between them. Experimentally, a stable 200 MHz signal is transferred over 100 km optical fiber with the help of a 1 GHz detecting signal, and fractional instability of 2×10(-17) at 10(5) s is achieved.
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.