Towards a neutrino trap

Design, optimization, realization and characterization of a fiber laser hydrophone

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A neutrino is a intriguing subatomic particle. Although the neutrino is the second most abundant known particle in the universe and billions of neutrinos pass through our own body each second, it is not easy to detect them and they remain poorly understood. The detection of neutrinos can help us understand various events happening in space and answer fundamental questions about the universe. One possibility to detect neutrinos is acoustic detection in the ocean.

The goal of this thesis is to design, optimize, manufacture and test a hydrophone that can be used to detect ultra-high energy neutrinos, when implemented in a large-scale telescope. The hydrophone designed in this research is a transducer which converts acoustic pressure to strain in an optical fiber, this strain can be measured precisely. To be implemented in a telescope for the detection of ultra-high energy neutrinos the fiber optic hydrophone has to meet a number of design requirements. The most vital requirements concern the sensitivity and frequency range of the hydrophone. The transducer should have a frequency range from 1 to 50 kHz and should be able to measure the softest noise in the ocean, hence there is a minimum required strain in the fiber per Pascal pressure. Within the frequency range mentioned, it is preferable that the transducer behaves linearly. Consequently, the first eigenfrequency of the sensor should be above 50 kHz. Such a high eigenfrequency means that the sensor must be stiff and as this conflicts with the objective to maximize strain, a trade-off had to be made.

Different design methods, such as topology optimization and parametric optimization, were investigated. Four hydrophones were manufactured and tested. The measurements were corroborated by numerical simulations using finite element method.