Parametric sensitivity characterization of two-way time transfer over hollow-core fiber links up to 91 km

Hollow-core fiber (HCF) offers a promising route to precision time transfer by reducing light–silica interaction. We experimentally compare two-way time transfer over HCF and single-mode fiber (SMF) links of 54, 68, and 91 km using wavelength scans, controlled bidirectional attenuation, and long-term measurements. At 91 km, HCF reduces the accumulated dispersion from 1195 to 324 ps/nm and the wavelength-to-delay sensitivity by approximately a factor of 3.6. It maintains a continuous lock at 0, 5, and 10 dB added attenuation, whereas the SMF link exhibits intermittent loss of lock at 5 and 10 dB. In a 24-h common-clock comparison over 68 km, the mean output offsets are approximately 78 ps for HCF and 411 ps for SMF, and the out-of-loop time deviations at 1000 s are 0.35 and 0.54 ps, respectively. A separate simultaneous 24-h measurement gives mean one-way link-delay excursions of 0.77 and 3.17 ns. These results demonstrate the system-level benefits of HCF for precision two-way time transfer, combining reduced wavelength sensitivity with improved tolerance to additional link loss and better long-term timing performance in the tested systems.