Single-way Fiber-based Time Transfer with Active Detection of Time Transfer Variations

With an increasing demand for distributing accurate time, preferably accessible in real time, better techniques for time transfer is needed. The inevitable variations in the time transfer delay must be detected and compensated for, if an absolute precision better than μs-level is needed, for transmission distance exceeding a few km. This is usually solved through two-way transfer, where the transmitter can estimate and forward the information on the actual transfer time at every instant, and this can be performed both over aerial transmission and fiber. If the number of users is high, there will be a complex and large network of two-way time signal transmissions. In comparison a single, one-way broadcasting is more desirable, and would be enabled if the variations in transmission time could be estimated from the received data at the far end. Some preliminary results of this time transfer technique based on transmission of a repetitive signal, modulated on two lasers at different wavelengths and transmitted through an optical fiber, has been presented previously. These data showed a strong correlation between a change in transfer time at one wavelength, and the transfer time difference for the signals at the two wavelengths. In this paper, the setup and the measurement results have been improved and new data are gathered, which shows improvement in the reliability and quality of this technique. There is to our knowledge no other technique for time transfer over wire-line solutions, where the variations in transfer time is measured and compensated for, that only needs transmission in one direction.