Frequency Transfer Techniques and Applications in Fiber Optic Communication Systems

Doctoral thesis, 2013

Modern society is dependent on communications and the developments of these increases constantly through a seemingly endless demand for communication services and thereby synchronization and time. This is confirmed by a vast range of research on communications, irrespective of technology and protocol. Historically, the national metrology institutes are the distributors of stable accurate time and frequency through national timescales, but that situation has changed with the arrival of Global Navigation Satellite Systems (GNSS) such as GPS (Global Positioning System). The introduction of GNSS-based solutions has resulted in improvement for system users and owners in need of time and frequency. When using a GNSS receiver, sufficient accuracy and precision is often achieved. However, a disadvantage of this development is that GNSS-solutions are based on weak radio signals that can be interfered with. The main objective of the research that forms the groundwork for this thesis is the development of new fiber based methods for time and frequency. The aim is to complement GNSS-based methods for redundancy, with the intention of strengthening the robustness of the Swedish infrastructure. The research has resulted in two unique and innovative transmission technologies, one of which has been patented (two-color, one-way). The first method is based on a non-insertion technology that utilizes passive listening to existing data frames in a fiber optical network and does not require any particular bandwidth. This technology only uses a fraction of the optical signal for time and frequency measurement from an indirect connection to the network. This method has resulted in a precision relative to the GPS carrier phase of less than 1 ns root mean square for distances exceeding 1,100 km. This precision has been achieved for all of the included experiments, conducted within the framework of the thesis, regardless of configuration. The other fiber based technology is a one-way method that uses two wavelengths (colors) for the realization of a correction algorithm and signals thereto. It was developed because the symmetry required for performing two-way time and frequency transfer is rarely precise enough. This optical fiber technique was evaluated with respect to a GPS precise point positioning technique in an urban fiber optical network. The difference in frequency stability between the two systems has been shown to be about 3 × 10-15 over an averaging interval of 10,000 s for a distance of 3 km. The method has also been evaluated in several laboratory experiments with fiber distances up to 160 km. The best performing result is presented as time resolved transit time variations compared with arrival time difference. The standard deviation of the difference between the reference measurement and the one-way, two-color technique result is 3.12 ns and the data showed temperature dependence in transit time of 6 ns / °C.