Abstract
Accurate time transfer is routinely performed using GPS, however an order of magnitude better accuracy can be achieved when signal transfer over optical fibres is used (e.g., in [1], fibre transfer over 73 km with <100 ps precision was achieved as compared to <700 ps for the GPS-based system). Unfortunately, the propagation delay through an optical fibre changes due to temperature variation. This is commonly compensated for by transferring the time information bi-directionally over a single optical fibre with subsequent cancellation of the propagation delay variations [1]. However, to avoid any signal degradation due to Rayleigh back-scattering and reflections at fibre connectors, it would be advantageous to transmit the information in both directions using different wavelength, each of them in a unidirectional sense [2]. However, this requires two transmitters operating at different wavelengths. Due to the limited stability of (low-cost) telecom-grade transmitters (generally ±0.1nm [3]), the propagation delay in both directions can change in an uncorrelated manner and thus would not be cancelled out, degrading the precision of the time transfer.
© 2015 IEEE
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