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Detecting underwater earthquakes is crucial for understanding the interior of the Earth and its dynamic behaviour. However, only a limited number of permanent ocean bottom seismometers exist today because of their prohibitive installation cost. Here we show that, using state-of-the-art frequency metrology techniques, standard telecommunication optical fibre links can be used as earthquake detectors. With this technique, the optical fibre cables already installed on the sea floor could be used to implement a global network for underwater earthquake detection. In our experiments we have detected earthquakes on terrestrial and submarine optical fibre links located in the UK, Italy and Malta and of length up to 535 km. Local and tele-seismic events were detected, with epicentre distance ranging from 25 to 18,500 km. These include earthquakes in New Zealand, Japan, Iraq and Mexico. We propagate laser light into the optical fibre and we measure the perturbations induced by the seismic waves to the received optical phase. In order to achieve the ultra-high sensitivity required to detect seismic noise we use state-of-the-art optical frequency references based on telecommunications lasers frequency locked to ultra-low expansion glass Fabry-Pérot cavities. In contrast with existing distributed acoustic sensing techniques, which measure backscattered light in the fibre and are limited to a few tens of kilometres, our technique measures the transmitted light (rather than reflected) and can be used over thousands of kilometres. Our technique requires only a single telecommunication channel on ordinary data-carrying optical links. Experimental results will be presented at the conference followed by discussions on using the proposed technique for the implementation of a global network for the detection of submarine earthquakes based on the existing telecommunications underwater infrastructure., peer-reviewed

Very Long Baseline Interferometry experiments require an extremely precise synchronization between the atomic clocks keeping the time and frequency standards at radiotelescope observatories. Recently the availability of fiber optic links from a few radio observatories and their national metrological institutes has made the streaming of extremely stable frequency standards via optical atomic clocks possible (even two orders of magnitudes better than Rubidium or Hydrogen maser standards). Firstly, we present the infrastructure of the Italian Link for Frequency and Time (LIFT) and results of the MetGeSp project aimed at finally creating a common clock between two of the antennas of the VLBI Italian Network. Secondly, the results are shown from VLBI experiments in which the rms phase noise was used to accurately compare the synchronicity of atomic clocks located at a few European stations (Medicina, Noto, Yebes, Torun, and Matera). VLBI clock timing proves a valid alternative to satellite-based techniques such as the Global Navigation Satellite System or the Two-Way Satellite Time and Frequency Transfer.

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We present the advantages of dual-polarization coherent receivers and digital signal processing in metrology applications for high accuracy frequency distribution over long-distance fiber links. We experimentally demonstrate optical frequency distribution at 10−16 accuracy on 300 km installed fiber.