KEYWORDS: Clocks, Receivers, Transmitters, Free space optics, Channel projecting optics, Time metrology, Signal detection, Global Positioning System, Avalanche photodetectors
This paper introduces a method and experiment using free-space, pulsed, optical time transfer, through turbid water to synchronize spatially distant chip-scale atomic clocks. This experiment uses one-way time transfer. Clock discrepancy is computed by comparing the measured departure time from Module A to the measured arrival time at the Module B, after accounting for the estimated time of flight. The effect of the turbidity is calculated and compared through clock discrepancy and time transfer stability for an empty tank, still water, turbid water, and fog.
We present and analyze an experiment to characterize the effect of turbidity to pulsed optical time transfer through water. During optical time transfer through water, two modules are each equipped with an atomic clock and timing electronics to keep and compare time independently. One module emits a laser pulse to a remote clock. The departure of the laser pulse from is time stamped with respect to a reference clock. The arrival of the pulse at the remote unit is time stamped with respect to its clock. The discrepancy between clocks is computed by comparing the measured departure time from plus the estimated time of flight of the pulse to the measured time of arrival at the remote unit. The estimated time of flight and the resulting time transfer performance are affected by the pulse propagation channel conditions. To analyze the effects of various channel conditions, we present the clock discrepancy, the measurement noise of an array of time stamps, and the Allan deviation. Allan deviation is a common metric in the time and frequency community to evaluate the stability of a series of events, where lower values are correlated with greater stability. This is an especially effective tool for studying nonstationary processes, where mean and deviation are a function of time, rather than constant. Additionally, insight into the dominant noise contributions of the frequency instability can be drawn by plotting the Allan deviation versus averaging time. In order to evaluate the free-space optical time transfer, Allan Deviation plots are generated for the empty, still water, and turbid water filled tank. Optical time transfer Allan deviation is compared to Allan Deviation generated from simultaneous frequency counter measurements through coaxial cables to differentiate between clock and channel stability.
Free-space, optical time-transfer can synchronize two spatially distant clocks. Currently, Global Positioning System (GPS)-based clock synchronization is a common technique to adjust clocks around the globe to a time standard. GPS-based clock measurements utilize radio signals that travel from GPS satellites to receivers. Electromagnetic signals, like radio waves, that travel through the ionosphere interact with the free electrons that causes signal propagation delay proportional to 1=f2 and the total electron content, where f is the signal frequency. Uncertainty in the in propagation delay correlates to synchronization uncertainty. Radio frequency (1 GHz) clock synchronizations use atmospheric models and long integration times to correct for propagation delay uncertainty, however it is challenging to model the ionosphere. By utilizing optical frequencies (100 THz) for clock synchronization, the uncertainty in the propagation delay is reduced by a factor of 1010 enabling high-performing synchronization without ionosphere corrections. This paper introduces and evaluates the performance of free-space, pulsed laser clock synchronization in a laboratory setting. The pulsed laser synchronization technique is directly compared to a cable and frequency counter-based clock discrepancy measurement. This presentation evaluates the short and long term stability of time transfer between chip scale atomic clocks (CSAC) and compares them to a more stable oscillator.
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