Frequency dissemination through optical fibers

Atomic frequency standards are not transportable, with few exceptions. Hence, specific techniques based on satellites have been developed for the remote comparison and the dissemination of frequency standards. Nevertheless, more than 20 days of measurement are needed to compare state-of-the-art Cs-fountains, and years would be required for the comparison of optical clocks. If optical fibers are used instead of satellites the measurement can be 10’000 times faster.

Coherent optical links are based on the transfer of a frequency-ultrastable laser along standard telecom fibers. The lenght variations of theĀ  fibers due to temperature changes or acoustic noise are compensated by the Doppler-cancellation technique; this allows the delivery of the same optical frequency at places located several hundreds of km apart.

Since 2005, an optical fiber network is being developed in Europe, connecting research facilities with high-level metrological needs. INRIM is involved in several projects funded by EU (NEAT-FT, OFTEN, CLONETS) targeting at the development of a European fiber Network of Metrology Insitutes and has established a national backbone for frequency dissemination to several laboratories of the Country (LIFT project) at an improved level.

Up to now, we disseminate optical and microwave frequency references with uncertainty <1E-18 at the Institute of Radioastronomy in Medicina (BO) and at the European Laboratory for Non Linear Spectroscopy-LENS in Sesto Fiorentino (FI), accross a 642-km long optical fiber. Present applications go beyond metrology, and aim at improving the resolution in atom and molecular spectroscopy as well as in Very Long Baseline Interferometry.

We are extending the backbone southward, to provide the Galileo Precise Time Facility in Fucino with UTC timing signals. We are also working to establish frequency dissemination to the Institute of Geodesy in Matera. This will pave the way for new experiments in relativistic geodesy.

We have developed an optical link to the Laboratorie Souterraine de Modane on the French border. This span will allow a connection to the European fiber network through the French one and has been used for a proof-of-principle geodesy experiment with optical clocks (ITOC project).

In recent years we have investigated alternative techniques for optical amplification based on distributed Raman scattering; Raman amplification is bidirectional and allows high and distributed gain, hence offering some advantages over custom-developed bidirectional Erbium-Doped Fiber Amplifiers especially for ultra-long hauls bridging.

We have studied alternative techniques for the remote comparisons of atomic clocks based on two-way schemes, and investigated the ultimate limits in the frequency transfer resolution.

We explored the possibility of using optical fibers for the realization of giant optical fiber gyroscopes, with applications in geophysics and rotational ground motion.

Further readings

Theses and positions available!

  • Development of multi-point dissemination techniques on stabilized optical fibers
  • Development of remotely-referenced optical sources
  • Remote comparison of atomic clocks
  • Low-noise digital electronics & phase-measurements

Contact us: c.clivati [at] inrim.it, d.calonico [at] inrim.it

 

Funded by:

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