Development of a novel alignment sensing and control technique for the output mode cleaner at GEO600

Abstract

We are in the era of multi-messenger astronomy, with gravitational wave detectors playing a pivotal role. All current long-baseline detectors are advanced versions of the basic Michelson interferometer. One of them is GEO600, located in Hannover, Germany, where this thesis is based. Once a detector is commissioned, it has to be maintained at a favourable operating state. This is mostly done with automatic control loops. Two of the most important kinds of control are length control and angular control. This thesis is about the development of a high bandwidth autoalignment control scheme for the detector's output beam to the output mode cleaner located just before the main photodiode. The scheme is called modulated differential wavefront sensing (MDWS) and is based on the well-known differential wavefront sensing alignment technique into the regime of beams with strong higher-order mode content. The MDWS control surpasses the pendulum resonances without additional auxiliary modulations. Due to these advantages, it is foreseen that it can replace the current autoalignment scheme called the beacon dither scheme that has a low bandwidth of less than 20\,mHz. In chapter 2, the implementation of the MDWS scheme for a stable-low-bandwidth state is shown where a bandwidth of up to 2 Hz could be achieved. This is already 100 times more than the beacon scheme's bandwidth. During the course of this work, challenges were encountered that prevented the full time implementation in a high-bandwidth state. To better understand the observed behaviour, the system was modelled in Simulink (described in chapter 3) along with a combination of modelling tools like Mathematica and Finesse and a noise budget was made with the help of the SimulinkNb tool. To make the model realistic, measurements were performed that gave a deeper understanding of several subsystems and these results have been used as model input. The knowledge gained about the system through the modelling work will form the basis of further development of the MDWS scheme and eventually in its full-time implementation.