@@ -168,8 +168,12 @@ UoB = University of Birmingham\\
\include{CPSdiff}
\include{laserstab}
\chapter*{Conclusions}
\chapter*{Summary}
This thesis is intended to provide a contribution to the improvement of gravitational-wave interferometers in the lower frequency bandwidth (below 30 Hz). My PhD project then focussed on developing new devices and improving already existing control structures for this aim, through three different experimental works: the study of optical levers for sensing and reducing tilt motion, the modification of the control system of LIGO in order to improve the control of seismic motion on the platforms and the frequency stabilization of the laser surce for the 6D isolation system device.\\
The optical lever can be in principle a good device to sense tilt motion over long lever arms. However, the noise budget indicated a small frequency window of good operation, while below 0.1 Hz the levers are limited by the ground motion along the z axis, but it opened the way to further tests to improve the technology: with a good sensing system of tilt motion, the addition of an actuation system able to reduce this noise will be crucially helpful to stabilize the suspension points of the optical chains and then of the whole cavity.\\
The study on the CPS and LSC offloading is promising to provide a significant contribution to the improvement of LIGO LSC signals and the detector stability when it is running in observing mode. The tests at LHO demonstrated that the experiment succeeded in lowering the seismic motion of the platforms by a factor of 3 at low frequencies and that also the DARM signal benefited from it. The simulations have shown that it is possible to reduce the differential motion of the chambers by a factor of 3 in order of magnitude below 0.1 Hz. The test on the Power Recycling Cavity Length highlighted that the signal can be controlled by the ISI according with the software simulations.\\
The results of the laser stabilization experiment showed that it is possible to stabilize the frequency of the laser source of the 6D device using the technology presented: a compact, easy to handle setup which makes use of small interferometers of the same type that are used inside the 6D sensor. With this technology, we managed to reach a frequency stabilization of 3.6 $\times$ 10$^3$ Hz/$\sqrt{Hz}$ at 1 Hz, without the need of installing the prototype in vacuum. This is already a promising result, but not yet sufficient for the requirements of 6D, especially below 1 Hz. This experiment requires further tests in vacuum in order to isolate the HoQIs and improve the performance.\\
In conclusion, all the three experiments proposed in this thesis can provide an important contribution for the low frequency noise reduction and are worthy of further tests and developments, as demonstrated by the experimental results and the simulations.
