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The standard metre from a hole in the wall

A new method for comparing optical clocks in different places, connected by an optical fibre, has been developed at PTB and tested over a long-distance fibre link. Using this method, an optical frequency derived from an optical frequency standard (atomic clock) was transmitted over a 211-km-long fibre link with an accuracy of one part in 1017.

An interferometer with fibre-coupled, acousto-optic modulators detects variations of the optical path length in the transmission fibre.

The best atomic clocks and optical frequency standards, designed to achieve an accuracy of better than one part in 1016, can be found in different laboratories, and frequently in different countries. One method for accurately comparing these frequency standards and for disseminating high-precision frequencies to industrial and research facilities is the transmission of highly stable optical frequencies via optical fibres as they are used for telecommunications. Buried underground, optical fibres are well-protected and have a low attenuation of approximately 0.25 dB per kilometre for radiation in the near infrared.

In the concept developed at PTB, the frequency of an optical standard is phase-coherently transferred to the near infrared range using a femtosecond optical comb generator. As part of an international cooperation, an optical frequency synthesized in this way has now been fed into an optical fibre link spanning 86 km, thus connecting two different research institutes in Paris. Using additional fibre spools, the fibre link was extended to 211 km to test the technique over longer fibres.

Due to mechanical, acoustic and thermal influences on the optical fibre, the optical path length varies. At the end of the transmission path, this variation manifests itself as fluctuations in the transmitted carrier frequency. These fluctuations were detected by an interferometric method and suppressed by about three orders of magnitude using feedback control. In this way, the optical frequency was transmitted with a relative uncertainty of about one part in 1017.

With the transfer (to infrared) and fibre optic transmission of an optical carrier frequency, a unique method is available to PTB which allows us to compare optical clocks separated by very long distances. This method also enables the distribution of reference frequencies to research laboratories and enables their use in addressing fundamental questions in physics.

Contact at PTB:

Phone: +49-531-592-0