Abstract

The stable long-distance transmission of radio-frequency (RF) signals holds significant importance from various aspects, including the comparison of optical frequency standards, remote monitoring and control, scientific research and experiments, and RF spectrum management. We demonstrate a scheme where an ultrastable frequency signal was transmitted over a 50 km coiled fiber. The optical RF signal is generated using a two-section distributed feedback (DFB) laser for direct modulation based on the reconstruction equivalent chirp (REC) technique. The 3-dB modulation bandwidth of the two-section DFB laser is 18 GHz and the residual phase noise of −122.87 dBc/Hz is achieved at 10-Hz offset frequency. We report a short-term stability of 1.62×10⁻¹⁴ at an average time of 1 s and a long-term stability of 6.55×10⁻¹⁸ at the measurement time of 62,000 s when applying current to the front section of the DFB laser. By applying power to both sections, the stability of the system improves to 4.42×10⁻¹⁸ within a testing period of 56,737 s. Despite applying temperature variations to the transmission link, long-term stability of 8.63×10⁻¹⁸ at 23.9 h can still be achieved.

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