Long range time transfer using optical fiber links and cross comparison with satellite based methods

In this thesis our objective is to develop a scalable network time and frequency transfer approach, providing multi-user dissemination, compatible with large telecommunication net- works and competitive with GNSS-based time distribution. Therefore we are concerned with methods for use in packet- based networks, like the Network Time Protocol (NTP) and Precision Timing Protocol (PTP). We also concentrate on “unidirectional” links, where the forward and backward sig- nals between network nodes propagate over separate fibers, not within the same fiber (“bidirectional” links). In particular we use a method called White Rabbit PTP (WR). This is a novel technology developed at CERN, based on PTP while using Synchronous Ethernet and other techniques to achieve high performance. It demonstrates sub-nanosecond time sta- bility and synchronization of arrays of instruments over 10 km scale networks. We are particularly interested in extending this method for large scale distribution of references at re- gional or national level, over links of up to 1000 km. We first study extensively the default performances and limitations of White Rabbit network equipment, in particular the White Rab- bit switch. We make various improvements to its operation: on the locking of the grandmaster switch to the external ref- erence, thus improving its short-term stability by more than an order of magnitude; optimizing the locking bandwidth of the slave switch; and increasing the PTP messaging rate be- tween master and slave switches. We then study medium and long-distance WR links. We construct a 100 km, uni- directional link using fiber spools in the laboratory. We dis- cover that the short-term performance is limited by chromatic dispersion in the fiber, while the long-term performance is degraded by the influence of temperature variations on the fiber. To limit the effect of chromatic dispersion for long-haul links, we propose the use of a cascaded approach. We re- alise a national scale, cascaded, 500 km link, again utilizing fiber spools. We use Dense Wavelength Division Multiplexing methods to construct this link by mutliple passages through shorter spools. We achieve a frequency transfer stability of 2 ×10−12 at one second of integration time and 4 ×10−15 at one day, limited by thermal noise in the long term. We achieve a time stability of 5 ps at one second of integration time, decreasing to a minimum of 1.2 ps at 20 seconds and remaining below one nanosecond for longer averaging times. These performances are similar in the short term, and two or- ders of magnitude better in the long term, than good quality GPS receivers. We expect thermal fluctuations and there- fore the effect of fiber thermal noise to be suppressed by a factor of approximately five for installations in the field. Fi- nally we make preliminary investigations of time calibration of WR links. The main challenge here is to measure the opti- cal length asymmetry between the two fibers used for signal transfer in the forward and backward directions. We demon- strate a fiber swapping technique, using a mid range, subur- ban White Rabbit link over dark fiber. We then describe and test a new variational method for calibration, involving a dif- ferential measurement method based on operating two WR links at different wavelengths over the same optical fiber link.