Transfer of stable optical frequency for sensory networks via 306 km optical fiber link

Optical fiber links for distribution of highly-stable optical frequencies were experimentally tested by many metrology laboratories in the past fifteen years. But recent development of new optical sensors for industrial application puts demands on a technology transfer of this high-end technology from laboratory experiments to the real industry. The remote calibration of interrogators of Fiber Bragg Grating strain sensory networks is one of important examples. We present a 306 km long optical fiber link established in the Czech Republic where a coherent transfer of stable optical frequency has been firstly demonstrated. The link between ISI CAS Brno and CESNET Prague uses an internet communication fiber where a window 1540 to 1546 nm in DWDM grid is dedicated for the coherent transfer and 1 PPS signal. The optical frequency standard at 1540.5 nm is used for the coherent transfer where compensation of the Doppler shift induced by the optical fiber is done by an acousto-optic modulator driven by a servo loop. The closed loop action is continuously recorded. This enables to compute changes in the transport delay introduced by external influences on the optical line. A comparison with a different measuring method based on analyzing the transport delay of a 1 PPS signal transmitted via the same DWDM window is done. This comparison is a subject of results of the paper.