Coherent Optical Frequency Transfer Via 110-Km Urban Fiber Network

Optical frequency at 1542 nm was coherently transferred over a 110-km optical fiber using an urban fiber network (JGN2) between Tsukuba and Kashiwa cities. The phase fluctuation induced by the fiber length fluctuation was actively reduced by stabilizing the fiber length using a fiber stretcher and an acousto-optic frequency shifter. Residual frequency instability of the transmitted light at the remote end was reduced down to 1.5×10-15 at an averaging time of 1 s.