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A point to point stable radio frequency (RF) signal transfer scheme is proposed and experimentally demonstrated via 100 km noisy urban optical fiber link in Beijing area. We utilize passive frequency mixing method to compensate the variations of the group delay in the optical fiber link. Undistinguished backscattering noise is effectively suppressed by using only two sets of optical transceivers operating on different wavelengths. At the same time, an auxiliary RF signal is employed to accomplish the phase conjugation and effectively eliminate the influence of intrinsic nonlinearity in the mixers. The configuration of dispersion compensation and optical amplification is optimized due to the high loss (31 dB) of this fiber link. To carry out the field trial, two prototype modules are fabricated consisting of devices of the transmitting site and receiving site. The measured fractional frequency instability (Allan deviation) of the 2.4 GHz RF transmission system is 6.3×10 −14 /1 s and 5.0×10 −17 /10 4 s, which is superior to that of the reference rubidium clock. The experimental result proves that our frequency transfer system is capable for operating on the underground fiber link and valuable for further applications, such as remote atomic clock comparison.INDEX TERMS Radio frequency, optical fiber communication, optical fiber networks, radio astronomy.

Herein, we verify that a Raman/EDFA hybrid amplifier can improve the stability of fiber-optic time and frequency synchronization systems compared to the Er3+-doped fiber amplifier (EDFA), owing to its higher gain and lower noise figure (NF) performance. We studied the variation law of Raman gain efficiency for a fiber Raman amplifier (FRA) as a function of pump power and input signal power, designed a bidirectional Raman/EDFA hybrid amplifier, and proved that equivalent NF below 0 dB can be obtained. Finally, hybrid amplifiers were compared to EDFAs in a free-running frequency synchronization system. The transfer stabilities reached 1.9678 × 10−13/1 s and 2.0248 × 10−13/1 s when FRA + EDFA and EDFA + FRA configurations were used, respectively, both exhibiting better performance than the stability of 3.0905 × 10−13/1 s obtained by EDFA.

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To meet the demand of flexible access for high-precision synchronization frequency, we demonstrate multi-node stable radio frequency (RF) dissemination over a long-distance optical fiber. Stable radio frequency signals can be extracted at any node along the optical fiber, not just at the endpoint. The differential mixing structure (DMS) is employed to avoid the frequency harmonic leakage and enhance the precision. The phase-locked loop (PLL) provides frequency reference for the DMS while improving the signal to noise ratio (SNR) of dissemination signal. We measure the frequency instability of multi-node stable frequency dissemination system (MFDS) at different locations along the 2,000 km optical fiber. The measured short-term instability with average time of 1 s are 1.90 × 10−14 @ 500 km, 2.81 × 10−14 @ 1,000 km, 3.46 × 10−14 @ 1,500 km, and 3.84 × 10−14 @ 2,000 km respectively. The long-term instability with average time of 10,000 s are basically the same at any position of the optical fiber, which is about (6.24 ± 0.05) × 10−17. The resulting instability is sufficient for the propagation of precision active hydrogen masers.