Bidirectional Optical Carrier Frequency Commonality Method for Optical Fiber Delay Measurement

The transmission delay in an accurate time-transfer system using an optical fiber is calculated by accurately measuring the roundtrip delay of the fiber. The delays in the forward and backward paths of the transmission line must be identical to achieve a highly accurate transmission delay compensation. The effect of chromatic dispersion, which occurs when the optical wavelengths of the forward and backward signals are different, and the effect of backscattering, which occurs in the fiber, are crucial and must be considered for higher accuracy. In this study, we present a fiber transmission delay measurement method that uses optical signals of the same wavelength for the forward and backward paths to overcome these problems. Signal separation can be achieved between the forward and backward signals and between the transmission and backscattered signals by shifting the modulation frequency of a sub-carrier of the optical signal at the remote station. The experimental results demonstrate that a highly accurate time-transfer of more than 1 ns can be expected over a 250 km fiber transmission.