Possibilities and limitations of the offline scheme frequency transfer method

For a high-frequency radio interferometry system such as ngVLA (next generation Very Large Array, receiving frequency is up to 116 GHz), the distribution of a high-frequency reference signal can be advantageous, in that the local oscillator (LO) phase drift associated with frequency multiplication at each remote antenna is minimized. The reference frequency signal is transmitted from a reference station to each antenna as a beat signal of dual-wavelength lightwave signal generated using a high-extinction-ratio Mach-Zehnder modulator. Radio interferometers require correlation processing and signal integration to observe extremely weak radio sources. Atmospheric scintillation correction is also performed, particularly in high-frequency observations. We propose phase compensation for the frequency transmission by offline processing as the same with atmospheric phase scintillation correction. Off-line compensation has advantages with respect to the cost-effectiveness of phase compensation and feedback bandwidth concerns that depend on the delay of the phase-locked loop, including transmission lines. In our fiber transmission scheme, the method measures the double difference phases between the phases before and after the roundtrip of the two lightwaves. Particularly characteristic is that the roundtrip phase of each lightwave of the dual-wavelength signal is measured independently, which provides accurate compensation for chromatic dispersion. Experiments were conducted of 100-GHz reference frequency transmission over optical fiber spools up to 400 km. Additionally, we made frequency transfer experiments using the installed fiber link and the fiber installed in the driving antenna. Based on the results, we will report on the application possibilities and limitations of the proposed dual-wavelength frequency transfer scheme.