Ultrastable microwave transfer of cesium frequency standard over 20 km of optical fiber

Precise absolute frequency measurements of transitions in atomic, ionic, and molecular systems provide an excellent means for stringent tests of fundamental physics. In many cases, however, the accuracy of an optical frequency measurement is limited by the accuracy of the local, commercial frequency reference used in the experiment. To address this limitation, we have established a dark optical fiber link between Colorado State University (CSU) and the National Institute of Standards and Technology (NIST) radio station, WWV, to transfer the microwave signal generated by an ensemble of cesium beam atomic clocks located at the radio station. This frequency transfer scheme allows the timescale at WWV, which is referenced to UTC(NIST), to act as the primary frequency reference for measurements taking place at CSU. The link was established using pre-existing, commercially available telecommunications fibers. We have implemented an active pathlength stabilization system to eliminate frequency drifts due to fluctuations in the optical path length of the fiber. We have also compared stabilization schemes which involve fiber noise cancellation using a single fiber and a pair of fibers in the same bundle. The transferred cesium signal is measured against a local rubidium reference and a frequency comb locked to a ULE cavity as tests of the stability of the link.