The Fundamental Limit of Fiber-Optic Sensors in the Infrasonic Region

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Abstract Title: The Fundamental Limit of Fiber-Optic Sensors in the Infrasonic RegionNontechnical: Fiber-optic sensors have been widely used in industry and research. However, one of their fundamental properties, the intrinsic limit of sensitivity, has not been fully understood. Specifically, at infrasonic frequencies (below 20 Hz), there has been no direct observation of the inherent noise in fiber-optic sensors, and the available theories remain inconclusive. The proposed project aims to address this problem by devising a set of experiments to probe the sensitivity-limiting noise in optical fibers and supporting the experimental study with advanced theoretical modeling. By uncovering the physics underlying the ultimate limit of sensor performance at low frequencies, the research will substantially deepen the understanding of infrasonic fiber-optic sensing, allowing future sensor designers to exploit the full potential of fiber-optic sensors at an unprecedented level of sensitivity. Moreover, the novel sensor designs used in the experiments will serve as blueprints for future ultra-sensitive distributed infrasound sensors, which are critical for monitoring mass-destruction weapons, earthquakes, volcanic eruptions, glacial motions, etc. The project will directly fund multiple students at both undergraduate and graduate levels and will generate capstone and summer research opportunities for college and high school students. It will also help create a new research thrust, precision fiber-optic sensing, at the University of Alabama in Huntsville, and improve the presence of NSF in the state of Alabama.Technical: The overarching goal of the planned research is to understand the physics that sets the ultimate limit of fiber sensor sensitivity at low frequencies. The investigation will primarily focus on direct measurement of the spontaneous thermal noise generated by optical fibers in the infrasonic region. A parallel effort will also be dedicated to the development of a three-dimensional visco-elastic model with concentric structures to describe the thermomechanical noise in optical fibers. To address the challenges facing the measurement of the minuscule thermal noise at infrasonic frequencies, a new sensor design based on a Mach-Zehnder-Fabry-Perot hybrid interferometer will be employed. Preliminary theoretical analysis has shown that such a scheme is able to raise the sensor sensitivity by a factor of 104, hence extending the thermal noise-dominated spectral region to well below 1 Hz. The scientific merit of the proposed research rests upon its primary goal toward uncovering the fundamental physical law of fiber thermal noise. The mystery surrounding the 1/f behavior of fiber thermal noise has puzzled researchers for two decades. There is an urgent need within the fiber-optic sensor community for a thorough investigation specifically targeting the low-frequency characteristics of thermal noise. By leveraging new sensing concepts such as hybrid interferometers, the proposed work will completely transform optical sensing for low-frequency signals and open up a new paradigm of infrasonic technologies.