Sub-Picosecond Active Timing Control over Fiber Optic Cable

Two-way time transfer over fiber optic cable [l] has been demonstrated over long baselines [2] and has been used recently to provide coherence to enable L-Band interferometry. In order to support coherent interferometry at higher frequencies and enable other high precision applications over long baselines, an active control concept was developed that enables sub-picosecond control of the propagation delay of an optical fiber. In previous implementations, time synchronization over fiber optic channels was achieved by steering time at a remote node to time at a master node by changing the frequency of a direct digital synthesizer (DDS) based on processed two-way time and two-way phase data [3]. In this paper, we introduce a different approach to active synchronization over fiber optic. This approach eliminates the dominant noise source from previous implementations (the DDS) by using an all-optical scheme to actively control the propagation delay of a long-haul fiber. Two loops using different mechanisms enable the control of the time delay of a fiber to the sub-picosecond level.