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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 $6.3\times 10^{-14}/1$ s and $5.0\times 10^{-17}/10^{4}$ 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.

Stabilized Radio Frequency Transfer via 100 km Urban Optical Fiber Link Using Passive Compensation Method