Phase coherent transfer and retrieval of terahertz frequency standard via optical fiber with 10−18-level accuracy and stability

We demonstrate a THz frequency reference transfer over a 20 km optical fiber with accuracy and stability at the level. Phase information of the THz frequency standard is coherently duplicated onto an optical carrier as an intermediary for exploiting low-loss optical-fiber technology. The transferred information on the optical carrier is retrieved into the THz domain again without phase decoherence. This THz reference transfer can be applied for the remote frequency calibration of diverse instruments working in the THz region.