Picoseconds-accurate fiber-optic time transfer with relative stabilization of lasers wavelengths

In this paper we focus on analyzing the accuracy of time transfer in bidirectional fiber optic links. It has been pointed out that one of the main uncertainty contributions in such links is related to insufficient stabilization of the lasers’ wavelengths and to the asymmetry of propagation caused by the fiber's chromatic dispersion. To reduce these contributions, we propose a scheme where the difference (offset) of the wavelengths of forward and backward lasers is stabilized and where optical interleavers are used as diplexers. This allows implementing a λ-swapping technique to assess dispersion-related link asymmetry during the link calibration. For the investigation of these approaches and the measurement of the associated stability we utilize a frequency-synchronized offset laser module (FSOLM) to realize an offset of 25 GHz. In this paper, we show that with the proposed λ-swapping technique the uncertainty associated with the propagation asymmetry due to fiber chromatic dispersion can be reduced to values below 2.5 ps for links up to 1000 km long, making it insignificant compared to other uncertainty contributions. A laboratory proof-of-concept experiment, in which distances up to 540 km were tested, showed good agreement between the measured and the anticipated propagation delay of investigated links.