Ways to stop mirrors from moving unnecessarily : design of advanced gravitational wave detectors

Abstract

Now that we have entered an era of gravitational wave astronomy, gravitational wave detectors are expected to detect several events a month (or more) in the near future. The current ground based detectors are undergoing upgrades, and future gravitational wave detectors, using exotic techniques and aiming at better sensitivity with lower noise contributions, are under consideration. To maintain strain sensitivity, precise control of various interferometric degrees of freedom in the detectors is required. The core work of this thesis involves the analysis of a sensing and control scheme for a third generation gravitational wave observatory, the Einstein Telescope (ET). A novel technique that will allow us to sense all the longitudinal degrees of freedom of the interferometer independently has been proposed in this thesis. Assuming conservative servo designs, a shot noise limited displacement noise budget for the low frequency part of ET (ET-LF) has been simulated using the simulation tools Finesse and SimulinkNb. It is thus shown that the control scheme does not inject additional sensing noise from auxiliary degrees of freedom into the measured gravitational wave strain channel. The simulated demonstration of the controllability of ET-LF with the new GHz control scheme is an important finding as current detectors are limited by control noise in the low frequency (< 10 Hz) regime. The new technology should be tested thoroughly to ensure that it can be incorporated into detectors. The new sensing scheme proposed for ET-LF can be tested and tweaked for low noise operation at the AEI 10m prototype before transferring the technology to full-scale detectors. In this thesis, a control scheme was developed to independently sense and control the three longitudinal degrees of freedom of the AEI 10m prototype using a GHz sub-carrier. The sum of the control noise from all the loops was shown to be below the quantum noise limit predicted for the AEI 10m prototype. The sensing scheme presented for ET-LF could also be tested in GEO600 since this detector presently suffers from signal recycling cavity length (SRCL) control noise. Different fundamental questions may be addressed by deploying the new sensing scheme at AEI 10m prototype versus GEO600. While the scheme implemented through the input port of the interferometer at the AEI 10m prototype would allow us to understand the influence of cavity stability parameters on control signals, the scheme implemented through the dark port of GEO600 (as proposed for ET-LF) provides answers for the mitigation of the SRCL control noise in the interferometer.