The average worldwide internet traffic demand in 2022 is projected to be over 1200 Terabits per second. A fifth of this data would be transmitted using mobile networks. One of the technologies used for this is radio frequency (RF) telecommunication, but this technology is reaching its limits. Despite ongoing development, typical data rates are still in the order of Gigabits per second per link.

TNO is working on a telecommunication link (called TOmCAT) that can reach data transfer rates of a Terabit per second. The high data rate is achieved using a very promising alternative to RF telecommunication: optical telecommunication, which is also known as laser communication.

In order to reach the intended data rates, the data needs to be spread over multiple optical frequencies. These signals need to be combined into one transmitted beam using a free-space optical bulk multiplexer.

The laser beams that are transmitted by their collimators need to be aligned with respect to each other in order to reach the satellite as one beam. The footprint available for the required alignment mechanisms is very limited. Furthermore, the system needs to achieve a good thermal and mechanical stability in order to meet the strict specifications.

The aim of this thesis is two-fold: to show the need for achieving state-of-the-art alignment specifications with strict footprint constraints, and to defend the steps taken to achieve these requirements. The research spans the entire design process of the alignment assembly: from higher/system level trade-offs and calculations, to the derivation of the design specifications, to the conceptual and detailed design, and concluding with the manufacturing and testing of the first prototype.

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.