The amplitude of a gravitational wave is typically very small and corresponds to a variation of the arm length of the order of $\Delta L \sim10^{-18}$ m. This means that, if we want to measure a considerable phase shift, the sensitivity of the instrument depends on the length of the arms.
\paragraph{Fabry-Perot cavities}
A useful way to increase the length of the arms is to make the laser beam travel back and forth inside an optical cavity delimited by two mirrors, called a \textit{Fabry-Perot cavity}: here, thanks to the multiple reflections, the optical path length will be longer. This process returns in a longer optical arm length, proportional to the quality factor of the cavity, which depends on the reflection coefficients of the two mirrors, named \textit{Finesse} (F):
A useful way to increase the length of the arms is to make the laser beam travel back and forth inside an optical cavity delimited by two mirrors, called a \textit{Fabry-Perot cavity}: here, thanks to the multiple reflections, the optical path length will be longer. This process returns a longer optical arm length, proportional to the quality factor of the cavity, which depends on the reflection coefficients of the two mirrors, named \textit{Finesse} (F):
\begin{equation}
\centering
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@@ -117,7 +117,7 @@ Noises can be of fundamental, technical and environmental origin. Fundamental no
Technical noises arise from electronics, control loops, charging noise and other effects; environmental noises include seismic motion, acoustic and magnetic noises: these noises can be reduced once identified and carefully studied.\\
\noindent
This thesis focuses on the improvement of the seismic isolation system. Seismic motion is measured using inertial sensors which are placed on the suspension benches. The residual motion affects the stability of the resonant cavities and limits the sensitivity of the detector in the low frequency band. The goal is to provide solutions to reduce seismic motion and improve the detector sensitivity.
This thesis focuses on the improvement of the seismic isolation system. Seismic motion is measured using inertial sensors which are placed on the suspension benches. The residual motion affects the stability of the resonant cavities and limits the sensitivity of the detector in the low frequency band. The goal is to provide solutions to reduce the coupling of seismic motion to the interferometer and improve the detector sensitivity.
\section{LIGO seismic isolation system}
\label{ligosei}
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@@ -150,7 +150,7 @@ The BSCs have a similar design as the HAMs, but they have two stages of ISI to s
\end{figure}
\paragraph{The sensors on the chambers}
The devices dedicated to monitoring the seismic motion are inertial and displacement sensors, which are horizontal and vertical, according to the different motion they need to sense. Currently, no sensors for tilt motion are installed on the platforms. Actuators are paired to each sensor, for active isolation of the sensed noise. The vertical displacement sensors are called Capacitive Position Sensors and are placed between every stage of every chamber: they measure the relative motion between the platforms. These are the sensors we will use in Chapter \ref{CPSdiff}. The vertical and horizontal inertial sensors with the dedicated actuators are placed on the platforms, underneath the optical tables, measuring the seismic motion in the horizontal and vertical directions. The position and the use of these sensors are different for HAM and BSC chambers, depending on the number of stages and the presence of the suspensions. The calibration and the specific role of each sensor into the seismic isolation system can be found in \cite{kisselthesis}, with references to the covered range of frequencies in \cite{kisseltalk3}.\\
The devices dedicated to monitoring and providing feedback the seismic motion are inertial and displacement sensors, which are horizontal and vertical, according to the different motion they need to sense. Currently, no sensors for tilt motion are installed on the platforms. Actuators are paired to each sensor, for active isolation of the sensed noise. The vertical displacement sensors are called Capacitive Position Sensors and are placed between every stage of every chamber: they measure the relative motion between the platforms. These are the sensors we will use in Chapter \ref{CPSdiff}. The vertical and horizontal inertial sensors with the dedicated actuators are placed on the platforms, underneath the optical tables, measuring the seismic motion in the horizontal and vertical directions. The position and the use of these sensors are different for HAM and BSC chambers, depending on the number of stages and the presence of the suspensions. The calibration and the specific role of each sensor into the seismic isolation system can be found in \cite{kisselthesis}, with references to the covered range of frequencies in \cite{kisseltalk3}.\\
\begin{figure}[h!]
\centering
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@@ -165,7 +165,7 @@ Active isolation implies a sensing system of the noise to reduce and a control s
\noindent
The control loop of a generic ISI stage on the X degree of freedom is simplified in the block diagram in Fig. \ref{control}. The platform motion is the sum of the input disturbance and the contribution from the control signal and it is measured by relative position and inertial sensors. This motion is then low- and high-passed via filters suitably built to fit the requirements and tuned to obtain the best performance combining the best results of both filters. This technique is called \textit{blending}, and the frequency where the relative and the inertial sensors contribute at their best is called the \textit{blend frequency}. The result of this blend is called the \textit{super sensor}. The output of the super sensor feeds the feedback loop, where the actuators close the loop \footnote{A general overview of control loops theory is given in Appendix B}.\\
The sensor correction loop takes the ground motion signal from an inertial instrument, filtering it before adding it to the relative sensor signal. This filter is needed because the sum of the motions from the ground inertial and the relative sensors can in principle provide a measurement of the absolute motion of the platform. However, the ground sensors are affected by low frequency noise and need to be suitably filtered.
The \textit{sensor correction loop} takes the ground motion signal from an inertial instrument, filtering it before adding it to the relative sensor signal. This filter is needed because the sum of the motions from the ground inertial and the relative sensors can in principle provide a measurement of the absolute motion of the platform. However, the ground sensors are affected by low frequency noise and need to be suitably filtered.
\begin{figure}[H]
\centering
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@@ -215,8 +215,8 @@ In particular, DARM is exactly the gravitational wave signal and thus the most i
\end{figure}
\noindent
During the time at LIGO Hanford, some of the work has been devoted on the optimization of the time spent by cavities in resonance, using a new concept based on the communication between the optics and the platforms where they are placed.\\
During the time at LIGO Hanford, some of the work has been devoted to the optimization of the time spent by cavities in resonance (i.e. the duty cycle), using a new concept based on the communication between the optics and the platforms where they are placed.\\
\noindent
As we will see, time in stable mode is crucial to assure higher chances of detection of gravitational-wave candidates. Small disturbances during the operational mode can compromise the detector while observing, losing stabilization (locking). This means that operators need to spend time to lock the instrument again and reset it in observing mode, time that is precious and that could instead be spent detecting events.\\
As we will see, time in stable mode is crucial to assure higher chances of detection of gravitational-wave candidates. Small disturbances during the operational mode can compromise the detector while observing, losing stabilization (lock). This means that operators need to spend time to lock the instrument again and reset it in observing mode, time that is precious and that could instead be spent detecting events.\\
This work in particular intends to give a contribution to the improvement of the sensitivity and stabilization of LIGO at low frequencies.
