Thermal delay response and high-stability RF transfer in hollow-core fibers

The coherent operation of distributed radar systems requires stable radio frequency (RF) reference distribution among spatially separated nodes. In conventional single mode fiber (SMF) links, thermally induced delay variations often require active compensation, thereby increasing system complexity and limiting rapid deployment. We compared the thermal delay response and 12 GHz RF transfer stability of nested antiresonant nodeless fiber (NANF), hollow-core photonic bandgap fiber (HC-PBG), and SMF links. The NANF exhibited a thermal coefficient of delay (TCD) of 6.5 ps/(km·K) and a normalized delay variation of 195 ps/km under ±30 °C cycles, which were 6.0 and 7.7 times lower than those of SMF, respectively. Without active control, both tested hollow-core fiber (HCF) links outperformed SMF. The NANF ADEV reached 1.01 × 10^-16 @10000s, while the SMF value was 50.2 times higher. With active control, the NANF ADEV approached the noise floor (1.28 × 10^-18@10000s) with 2.28 × 10^-18 @10000s, outperforming SMF (3.95 × 10^-18@10000s). The tested HC-PBG link outperformed SMF only without active control. These results demonstrate the potential of the tested HCF links, particularly NANF, for stable unidirectional frequency transfer without active control and simplified RF reference distribution in mobile distributed radar systems.