Low-cost monitoring of the wavelength difference of two transmitters for two-way time transfer over optical fibre

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 could not be compensated by a known stable difference, degrading the precision of the time transfer.

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ORCID for Radan Slavík: orcid.org/0000-0002-9336-4262

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