Adventures in Gravitational Wave Detector Control: A Round-Trip Journey
Authors:   , Maggiore Riccardo 
Published: March 2024

With significantly improved sensitivity, the Einstein Telescope (ET), along with other upcoming gravitational wave detectors, will mark the beginning of precision gravitational wave astronomy.

For ET to operate, we must transcend the current state of the art. For controls, the only way to do this involves examining existing systems to identify and understand the limiting constraints. This information will lay the foundation for refined strategies for future detectors.

My work focused on the controls of Virgo with the aim of transferring the acquired knowledge for the ET design. This endeavour not only supports the planning for ET but also supports and enhances Virgo, ultimately leading to a better understanding of the detector.

To surpass the performance of current detectors, we may need to consider strategies that differ from the current methods in use. From an optical perspective, this indicates the necessity to explore a wider variety of optical configurations beyond the ones presently employed. This is the purpose of Chapter 2. Here, we present a novel technique to control strongly coupled cavities. Compared to what we are used to seeing in GW detectors, the coupled cavity behaves in a substantially different manner, requiring the development of specialised techniques, which is the main outcome of the chapter. This marks the first successful design of a sensing scheme for this type of cavity.

Sensing strategies, such as the one shown in Chapter 2, rely on light modulation-demodulation techniques to generate error signals. These sig- nals are normally used as part of a feedback control system to keep the optical system operational. The control system acts by zeroing the error signals. If the error signals present an offset, the operating point, which is the point where the error signals are zero, no longer coincides with the desired working point. Not being precisely at the working point can lead to unwanted radiation-pressure effects.
In Chapter 3, we explain how optical imperfections can result in the cre- ation of offsets in these type of error signals. Furthermore, we will show how, for marginally stable optical resonators, the creation of offset is more pronounced.

ET is expected to outperform current detectors, especially in the low- frequency range, with an anticipated sensitivity improvement of approx- imately one million times compared to existing detectors in that range. However, a key limitation observed in this frequency region, as seen in LIGO detectors, is the presence of angular control noise. It is reasonable to assume that such constraints could obstruct the ET’s objectives unless the design intelligently avoids these issues. Thus, evaluating the noise performance of the ET is crucial and, to do this effectively, developing an accurate model is essential. Chapter 4 focuses on a detailed model of Advanced Virgo, which, for the first time, closely matches the measured data. This enables us to make strong predictions about ET’s performance.

Chapter 5 focusses primarily on a series of simulations for the lock acquisition of the central part of the Virgo interferometer.