A Novel High Resolution Optical Time Delay Cable Measurement System Utilized in Fiber Verification in Radio Interferometry

A long distance optical fiber cable measurement is described that delivers ~0.2 picosecond (ps) delay resolution with capability for high accuracy relative distance extraction over 10's of microseconds of delay. This is necessary in radio interferometry, for verification, but has general applicability to many situations found in optical fiber transport. For radio interferometry one can use direct RF over fiber to transport the hydrogen maser clock source, and measurement of fiber to certain points in the cable system is very important. The long distance measurements are used to show that the thermal stability of the buried optical fiber cable keeps the propagation delay changes on the optical fiber link stable to less than 9.5 ps over 20 minutes, meeting a key requirement for phase margins for interferometry. The measurement method shows great promise in general optical fiber measurement for optical cables over long distances, with high resolution and dynamic range, enabling capabilities better than those offered by normal OTDR (Optical Time Domain Reflectometer) or OFDR (Optical Frequency Domain Reflectometer) methods.