Tomoya Akatsuka, Takashi Goh, Hiromitsu Imai, Katsuya Oguri, Atsushi Ishizawa, Ichiro Ushijima, Noriaki Ohmae, Masao Takamoto, Hidetoshi Katori, Toshikazu Hashimoto, Hideki Gotoh, and Tetsuomi Sogawa
We report a cascaded optical fiber link which connects laboratories in RIKEN, the University of Tokyo, and NTT within a 100-km region using a transfer light at 1397 nm, a subharmonic of the Sr clock frequency. The multiple cascaded link employing several laser repeater stations benefits from a wide feedback bandwidth for fiber noise compensation, which allows constructing optical lattice clock networks based on the master-slave configuration. We developed the laser repeater stations based on planar lightwave circuits to significantly reduce the interferometer noise for improved link stability. We implemented a 240-km-long cascaded link in a UTokyo–NTT–UTokyo loop using light sent from RIKEN via a 30-km-long link. In environments with large fiber noise, the link instability is 3 × 10−16 at an averaging time of 1 s and reaches 1 × 10−18 at 2,600 s.