Phase-coherent transfer and retrieval of terahertz frequency standard over 20 km optical fiber with 4 × 10−18 accuracy

Abstract

We demonstrate a terahertz (THz) frequency reference transfer with high accuracy and stability. 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 without phase decoherence. The THz reference transfer system, which comprises frequency-comb-based THz-to-optical and optical-to-THz synthesizers connected by a 20 km phase-noise-compensated fiber, is operated with 4 × 10−18 fractional frequency accuracy at 0.3 THz. This THz reference transfer is available for the remote frequency calibration of diverse instruments working in the THz region.