Phase demodulation method for stable long-haul frequency transmission based on the Michelson interferometer

In this Letter, we present a fiber-optic radio frequency (RF) transmission scheme based on phase modulation with an interferometric detection structure. A self-developed Michelson interferometer (MI) is used to demodulate the frequency signal via an electrically controlled optical shifter. The two complementary outputs from the interferometer are detected using a balanced detector, which suppresses the common-mode noise of the fiber link. The structure is tested in the laboratory using a frequency transfer system over a 560 km fiber link. Experimental results show that a stable 2.4 GHz frequency transmission with a fractional frequency instability of 3.9 × 10^-14 at 1 s and 6.2 × 10^-17 at 10,000 s is achieved. Compared with the frequency transmission system based on intensity modulation and direct detection, the frequency instability is improved from 7.3 × 10^-14 to 3.9 × 10^-14 at 1 s. We believe that the proposed method will be useful for the construction of time-frequency synchronous fiber networks.