Application of phase-stabilized optical fiber in transmission of reference and IF signals in VLBI observation

A phase-stabilized optical fiber (PSOF) cable, which is 880 m long, was successfully used to transmit a 100-520 MHz IF signal and 10 MHz reference frequency signal in Mark III very long baseline interferometry (VLBI) observations. Phase stability of the 1760-m round-trip PSOF cable was measured by a dual mixer time difference system. Results of the measurements show that the stability of the PSOF cable system is about 50 times better than that of an ordinary coaxial cable system. The measured phase fluctuations are smaller than the tolerance limit required for a coherence loss of less than 20% in VLBI observations with an integration time of several hundred seconds. Internal accuracies of the international geodetic VLBI observations, in which the PSOF cable system is used at a station, are proven to be in the level of the present-day standard. The results show that the performance of the cable is at least comparable with, and probably better than, that of coaxial cables of much shorter length widely used in VLBI stations