Precise Time and Frequency Transfer over Phase Compensated Optical Fibre Links Established through White Rabbit-Technology

Precise time and frequency signal transfer is important for various daily life applications such as telecommunications, navigation, meteorology, finance, and smart power grid management, as well as in scientific experiments. In addition, time-frequency signal transfer and atomic clock comparison is an essential activity for timekeeping. With the development of modern advanced atomic clocks (e.g., caesium fountain, optical clock), traditional satellite-based time and frequency transfer methods are insufficient to compare the performance of these ultra-stable clocks. On the other hand, optical fibres-based time-frequency transfer methods have emerged as a promising solution for transferring reference signals over long distances and ultra-stable clock comparison as they offer low loss, high bandwidth and immunity to electromagnetic interference. However, fluctuation in propagation delays within an optical fibre link due to environmental conditions, such as temperature variations and mechanical stress, affects the stability and accuracy of time-frequency transfer over optical fibre links. This thesis aims to identify the factors that deteriorate the stability and to develop solution to mitigate their effect on time transfer accuracy over White Rabbit (WR)-technology based optical fibre links. Five research questions (RQ) are formulated to achieve the thesis aims, and related study findings are discussed in chapters 3 to 7 of the thesis, respectively. In the first part of the research (RQ1, chapter 3), we present performance evaluation of White Rabbit-Network (WRN) based optical fibre links for transferring precise time and frequency signals to distant locations as well as comparing the performance of atomic clocks. We also investigate the temperature sensitivity of WR components (WR nodes and optical fibres). The second part of the research (RQ2, chapter 4) demonstrates feedback loop-based dynamic phase variations compensation within the WRN. In this study, we find that introducing dynamic phase corrections successfully compensates the time delays/phase variations that arise due to varying temperature conditions and improves the accuracy and stability of WRN-based time transfer links. In the third part (RQ3, Chapter 5), we examine the performance of a WR-based time transfer link using underground telecommunication fibres and prove its suitability for real field applications. In the fourth part (RQ4, chapter 6), we present the development of a compact and cost-effective testbed to introduce automated dynamic phase corrections and mitigate the effect of temperature variations around the optical fibres on time transfer accuracy. The fifth part (RQ5, Chapter 7) focuses on analyzing and modelling phase noise within the WRN. We present the modelling of the phase noise based on experimentally recorded measurements. Furthermore, we present verification of developed phase noise models by generating simulated phase noise using these models. A close match of the PDF of simulated phase noise data with the PDF of experimentally recorded phase noise verifies that the developed models are correct and can be referenced for further phase noise studies. A key contribution of this research work is that we establish phase-stabilized WRN-based optical fibre links to transfer time over long fibre links within 100 ps uncertainty. Moreover, we utilize the phase-stabilized WRN-based optical fibre link as a time traceability link and compare the performance of ultra-precise timescales. In addition, we develop a compact and cost-effective testbed for introducing automated dynamic phase corrections. Lastly, we develop WRN phase noise models based on experimental time delay measurements. Knowledge of the stability and accuracy of WR-technology based optical fibre links, the temperature sensitivity of WR components, and phase noise models would be very useful to the time and frequency metrology community for incorporating further modifications and improving its performance. Such modifications will ultimately lead to employing these links to mirror or replicate reference atomic clock signals at distant locations with high accuracy and fulfil the requirement of several critical and unique applications.