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.
This effort designed and tested new algorithms and deployable scintillometer hardware for ocean optical turbulence characterization. Novel features include a hand-deployable design, a non-laser optical source, a rapidly adjustable propagation length, and a collocated multi-instrument environmental sensor package. Undersea testing was contingent on several accomplishments, including developing robust algorithms and data logging methods, integrating compact optics and electronics, and engineering handheld-sized pressure vessels suitable for field experimentation. The test assembly was deployed in 428-m Pacific Ocean water from a small boat. Direct measurements revealed the ocean’s refractive-index structure parameter (Cn2 from 1.9×10−11 m−2/3 to 2.3×10−10 m−2/3) and the inner scale of optical turbulence (l0 from 0.5 mm to 1.5 mm). Onboard temperature, depth, beam attenuation, and backscattering sensors corroborated key regions of interest, namely the thermocline. By integrating turbulence, temperature, depth, attenuation, and backscattering measurements within a single hand-portable assembly, we increased our understanding of ocean optical dynamics while demonstrating the practicality of a low size, weight, and power scintillometer.
The inherent and apparent optical properties (IOPs and AOPs) of seawater limit the performance of free-space optical (FSO), underwater wireless optical communication (UWOC), and imaging systems. Absorption, scattering, and downwelling irradiance are three such properties that influence system performance and often evolve independently. In situ measurements of multiple IOPs and AOPs would provide environmental sensing for fielded optical systems, but such comprehensive measurements are typically expensive or impractical. This effort analyzed existing oceanographic data sets to uncover wavelength-dependent correlations between IOPs, AOPs, test depths, and ocean depths. We then employed machine learning (ML) methods to predict the optical properties of diffuse attenuation (Kd) and backscatter (bb) using beam attenuation (c) and compared these results to ground-truth values. Predicted values of Kd and bb were well matched to their ground-truth data. Furthermore, we demonstrate ML-based Jerlov optical water type classification using beam attenuation as the optical data input. With our methods validated, we collected new optical data sets and processed them using our ML models. Results are promising and indicate future in situ classification and prediction capability. To highlight one practical application, we present a preliminary ML-enabled performance estimator for a generic FSO or UWOC system.
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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