@@ -12,7 +12,7 @@ Gravitational waves are an astrophysical event that takes place when massive obj
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@@ -12,7 +12,7 @@ Gravitational waves are an astrophysical event that takes place when massive obj
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\noindent
The effect of the gravitational waves when they pass through an object is to produce a deformation on the physical lengths (L). This effect is very small ($\Delta$L/L $\sim$ 10$^{-21}$): masses able to deform the fabric of the spacetime and generate gravitational waves are of the order of more than the solar mass $M_{\odot}$, so they need to be looked for in the Universe.\\
The effect of the gravitational waves when they pass through an object is to produce a deformation on the physical lengths (L). This effect is very small ($\Delta$L/L $\sim$ 10$^{-21}$): masses able to deform the fabric of the spacetime and generate gravitational waves are of the order of more than the solar mass $M_{\odot}$, so they need to be looked for in the Universe.\\
\subsection{A difficult detection}
\subsection{A challenging detection}
Detecting gravitational waves is particularly hard, because the effect is very small, and the sensitivity required for an instrument to see it must be suitable.\\
Detecting gravitational waves is particularly hard, because the effect is very small, and the sensitivity required for an instrument to see it must be suitable.\\
The challenging goal of detecting gravitational waves opened a research field dedicated to the development of new technologies, that could help to obtain the sensitivity necessary for the detection to happen.\\
The challenging goal of detecting gravitational waves opened a research field dedicated to the development of new technologies, that could help to obtain the sensitivity necessary for the detection to happen.\\
This research is important, because detecting gravitational waves means looking at the sources which produced them. There is still a gap in the knowledge of many astrophysical objects, such as Black Holes (BH), Neutron Stars (NS), Supernova events: this new-born branch of astrophysics will help to fill the gap and increase our knowledge of the Universe.\\
This research is important, because detecting gravitational waves means looking at the sources which produced them. There is still a gap in the knowledge of many astrophysical objects, such as Black Holes (BH), Neutron Stars (NS), Supernova events: this new-born branch of astrophysics will help to fill the gap and increase our knowledge of the Universe.\\
@@ -90,6 +90,6 @@ We will see in the next chapters that one of the most important noise sources, a
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@@ -90,6 +90,6 @@ We will see in the next chapters that one of the most important noise sources, a
The importance of the opening of the lower frequency window has been widely outlined and highlighted \cite{lantztalk}: the final goal is to reduce the noise coupling into the gravitational-wave signal, and an important contribution could be provided by the efforts of the people working on the seismic noise suppression. \\
The importance of the opening of the lower frequency window has been widely outlined and highlighted \cite{lantztalk}: the final goal is to reduce the noise coupling into the gravitational-wave signal, and an important contribution could be provided by the efforts of the people working on the seismic noise suppression. \\
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It is in this frame that the work exposed in this thesis finds place. The experiments carried on cover both the studies for noise suppression of seismic platforms on gravitational-wave detectors, and the development of new devices for sensing and reduce seismic motion.
It is in this frame that the work exposed in this thesis finds place. The experiments carried on cover both the studies for noise suppression of seismic platforms on gravitational-wave detectors, and the development of new devices for sensing and reducing seismic motion.
\chapter{Laser stabilization for 6D isolation system device}
\chapter{Laser stabilization for 6D isolation system device}
In this chapter I will introduce the 6D device, a new technology for inertial isolation. This project has been presented to the scientific community at the 10th ET Symposium in 2019 \cite{poster}. My contribution to the development of this technique focussed on the sensing side: a laser will be injected into the device and will need to be stabilized in frequency. To do it, we propose a new technique based on compact interferometry.\\
In this chapter I will introduce the 6D device, a new technology for inertial isolation. This project has been presented to the scientific community at the 10th ET Symposium in 2019 \cite{poster}. My contribution to the development of this technique focussed on the sensing side: a laser will be injected into the device and will need to be stabilized in frequency for a low-noise readout of the sensing system at lower frequencies. To do it, we propose a new technique based on compact interferometry.\\
This work has been done entirely at UoB: the design of the project has been conducted in 2020, while the experiment has been built and tested from September 2020, when the University accorded me the permission to return to the lab, to July 2021.
This work has been done entirely at UoB: the design of the project has been conducted in 2020, while the experiment has been built and tested from September 2020, when the University allowed the return to the laboratory, to July 2021.
\section{6D inertial isolation system overview}
\section{6D inertial isolation system overview}
The 6D inertial isolation system is a device based on a new technology under development at University of Birmingham and at Vrije Univestiteit in Amsterdam, which could enable detection of gravitational waves at 10 Hz and below \cite{6d}. We have already seen the importance for this frequency window to be opened (chap 2): this facility can be installed on Earth-based interferometers of every type, on or under ground, allowing the different instruments to easily use the same device.\\
The 6D inertial isolation system is a device based on a new technology under development at University of Birmingham and at Vrije Univestiteit in Amsterdam, which could enable detection of gravitational waves at 10 Hz and below \cite{6d}. We have already seen the importance for this frequency window to be opened (chap 2): this facility can be installed on Earth-based interferometers of every type, on or under ground, allowing the different instruments to easily use the same device.\\
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@@ -214,7 +214,7 @@ The performances of the setup depend strongly on the HoQIs because they are the
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@@ -214,7 +214,7 @@ The performances of the setup depend strongly on the HoQIs because they are the
\end{figure}
\end{figure}
\subsection{Tested noise sources}
\subsection{Tested noise sources}
There are several noise sources to take into account: air currents and vibrations from electronics and cables have been reduced placing the optical setup into a foam box and moving the electronic devices suitably. Cables have been isolated from the table and the breadboard by rubber feet.\\
There are several noise sources to take into account and that we tested and minimized: air currents and vibrations from electronics and cables have been reduced placing the optical setup into a foam box and moving the electronic devices suitably. The lasers have been left outside the box to avoid overheating inside, due to their heat dissipation. Cables have been isolated from the table and the breadboard by rubber feet.\\
\paragraph*{Acoustic noise}
\paragraph*{Acoustic noise}
The test in Fig. \ref{sound} shows that the setup is sensitive to acoustic noise: we injected a sound at 75 Hz and both HoQIs clearly detected it. Moreover, we found out that HoQI1 is detecting some noise around 22 Hz that HoQI2 is not able to sense: the two peaks in the figure are present in every condition of the laboratory and time of the day. The source of this noise is still under investigation: it could be a permanent sound in the lab non audible by humans. The fact that only HoQI1 can detect it could be due to its position with respect to the noise source: it might be closer to it than HoQI2. Imperfections in the optics and general setup of the HoQIs are also taken into account.\\
The test in Fig. \ref{sound} shows that the setup is sensitive to acoustic noise: we injected a sound at 75 Hz and both HoQIs clearly detected it. Moreover, we found out that HoQI1 is detecting some noise around 22 Hz that HoQI2 is not able to sense: the two peaks in the figure are present in every condition of the laboratory and time of the day. The source of this noise is still under investigation: it could be a permanent sound in the lab non audible by humans. The fact that only HoQI1 can detect it could be due to its position with respect to the noise source: it might be closer to it than HoQI2. Imperfections in the optics and general setup of the HoQIs are also taken into account.\\
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@@ -226,9 +226,16 @@ The test in Fig. \ref{sound} shows that the setup is sensitive to acoustic noise
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@@ -226,9 +226,16 @@ The test in Fig. \ref{sound} shows that the setup is sensitive to acoustic noise
\end{figure}
\end{figure}
\paragraph*{The role of the temperature}
\paragraph*{The role of the temperature}
Temperature changes affected dramatically the measurements: the two lasers can be driven also via temperature modulation. This method has been used to move the beat-note peak along the frequencies and set it around 60 Hz, being this the setpoint we decided for it. However, both laser modules are sensitive to changes of the room temperature, which make the peak move out from the setpoint on large time scales (~hours): this affects long time measurements. The stabilization of the room temperature requires the use of the air conditioning, which in turn creates air currents visible by the setup below 1 Hz.\\
Temperature changes affected dramatically the measurements. The two lasers can be driven also via temperature modulation: this method has been used to move the beat-note peak along the frequencies and set it around 60 Hz, being this the setpoint we decided for it. However, both laser modules are sensitive to changes of the room temperature, which make the peak move out from the setpoint on large time scales (~hours): this affects long time measurements. The stabilization of the room temperature requires the use of the air conditioning, which in turn creates air currents visible by the setup below 10 Hz (Fig. \ref{ACtest} shows the difference between two tests taken with and without AC).\\
Temperature changes are also responsible for deformations of metals; this induces noises into the HoQI platforms because of the different materials they are built of: platform, screws and post holders expand in different ways with temperature changes, and this produces deformations and friction between the metals, which translate into displacement noise visible by the HoQIs. This issue has been reduced by inserting rubber rings between the junctions where different metals are mounted.\\
Temperature changes are also responsible for deformations of metals; this induces noises into the HoQI platforms because of the different materials they are built of: platform, screws and post holders expand in different ways with temperature changes, and this produces deformations and friction between the metals, which translate into displacement noise visible by the HoQIs. This issue has been reduced by inserting rubber rings between the junctions where different metals are mounted.\\
\begin{figure}[h!]
\centering
\includegraphics[scale=0.3]{images/AConoff.png}
\caption[Test of the impact of AC on the frequency stability]{Test of the impact of AC on the frequency stability. From this plot the free running frequency measured by the beat-note is compared to the frequency measured when the setup is in loop in different AC conditions: the red trace shows a measurement taken when the AC was on, during the night: the air currents are affecting the setup below 10 Hz; the black trace shows the same test with no AC: below 10 Hz the trace is much quieter. The higher noise above 10 Hz is due to the fact that this test has been taken in daylight time, and HoQIs suffered the vibrations of the building. After this test we reduced the free space OPL between the optics were possible, filled the empty spaces of the box and reduced the free space between the last optic and the beat-note photoreceiver, to reduce air flows. All the following tests have been taken with AC off.}
\label{ACtest}
\end{figure}
\paragraph*{HoQIs performances}
\paragraph*{HoQIs performances}
When monitoring the output of the HoQIs, we noticed that HoQI2 is much noisier than HoQI1: Fig. \ref{hoqi2} shows an out of loop measurement of the output of both HoQIs. This discrepancy has been investigated: possible reasons for that could arise from the laser source of HoQI2, alignment and clipping on the optics, fringe visibility, spurious light, mechanical defects in HoQI2 setup. The laser source has been changed to be the same as HoQI1 and further tests showed that HoQI2 is performing the same way. This relieved the laser of any responsibility, since now the same source is feeding the two HoQIs in the same way. The alignment and the clipping on the optics have been carefully checked and possible sources of stray lights have been meticulously covered. The fringe visibility has been double-checked: the test is still showing more noise from HoQI2 output. What remains to inspect is the possibility of mechanical defects in the optics or in the setup of HoQI2, the latter being a concrete possibility due to errors in manufacturing.\\
When monitoring the output of the HoQIs, we noticed that HoQI2 is much noisier than HoQI1: Fig. \ref{hoqi2} shows an out of loop measurement of the output of both HoQIs. This discrepancy has been investigated: possible reasons for that could arise from the laser source of HoQI2, alignment and clipping on the optics, fringe visibility, spurious light, mechanical defects in HoQI2 setup. The laser source has been changed to be the same as HoQI1 and further tests showed that HoQI2 is performing the same way. This relieved the laser of any responsibility, since now the same source is feeding the two HoQIs in the same way. The alignment and the clipping on the optics have been carefully checked and possible sources of stray lights have been meticulously covered. The fringe visibility has been double-checked: the test is still showing more noise from HoQI2 output. What remains to inspect is the possibility of mechanical defects in the optics or in the setup of HoQI2, the latter being a concrete possibility due to errors in manufacturing.\\
Another reason of concern about HoQI2 behaviour is that it is not consistent with tests in loop (see Fig. \ref{looptest} later): this might be due to intensity noise coupling, which effect might be more evident than in HoQI1 due to internal defects, giving a noisier output when the setup is in loop.
Another reason of concern about HoQI2 behaviour is that it is not consistent with tests in loop (see Fig. \ref{looptest} later): this might be due to intensity noise coupling, which effect might be more evident than in HoQI1 due to internal defects, giving a noisier output when the setup is in loop.
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@@ -246,7 +253,7 @@ We also monitored the power of the lasers
\subsection{Loop performances}
\subsection{Loop performances}
The behaviour of the two HoQIs has been tested in loop and out of loop, to check if they are detecting and responding correctly to the injection of the controller filters through the input modulation of the laser modules. The expectation is that the HoQIs output in out-of-loop mode should show the injection of the gain: Fig. \ref{looptest} shows that the expectations are satisfied.\\
The behaviour of the two HoQIs has been tested in loop and out of loop, to check if they are detecting and responding correctly to the injection of the controller filters through the input modulation of the laser modules. The expectation is that the HoQIs output in out-of-loop mode should show the injection of the gain: Fig. \ref{looptest} shows that the expectations are satisfied.\\
This test shows that HoQI2 is in general noisier than HoQI1, especially above 1 Hz: this affects laser stabilization measurement and loop stability, thus it has been deeply investigated. The higher intensity fluctuations of laser2 can partially explain the reason of HoQI2 noise.
This test confirms that HoQI2 is in general noisier than HoQI1, especially above 1 Hz: this affects laser stabilization measurement and loop stability, thus it has been deeply investigated. The higher intensity fluctuations of laser2 can partially explain the reason of HoQI2 noise.
\subject{A Thesis submitted for the degree of PHILOSOPHIAE DOCTOR}
\title{Innovative perspectives for seismic isolation of gravitational-wave detectors}
\title{Innovative perspectives for seismic isolation of gravitational-wave detectors}
\author{Chiara Di Fronzo}
\author{Chiara Di Fronzo}
\publishers{\raggedleft{{\normalsize Institute of Gravitational Wave Astronomy\\ School of Physics and Astronomy\\ University of Birmingham\\ December 2021}}}
\date{}
\date{}
\titlehead{A Thesis submitted for the degree of Philosophiae Doctor}
\publishers{School of Physics and Astronomy\\Gravitational Waves Department\\ University of Birmingham}
\begin{document}
\begin{document}
\maketitle
\maketitle
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\newpage
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\frontmatter
\frontmatter
\chapter{Statement of originality}
\chapter{Statement of originality}
I confirm that the work presented in this thesis is original and has been entirely carried out by the author, started and completed at the University of Birmingham. The work done in collaboration with other scientific groups and/or abroad has been suitably highlighted throughout the thesis.\\
I confirm that the work presented in this thesis is original and has been entirely carried out by the author, started and completed at the University of Birmingham. The work done in collaboration with other scientific groups and/or abroad has been suitably highlighted throughout the thesis.\\
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%I am particularly grateful to Dr. Conor Mow-Lowry and the University of Birmingham, for giving me the opportunity and the funding to join the Gravitational waves group and contribute to the development of exciting science. This was also possible thanks to the support of the Royal Astronomical Society and the Institute of Physics, which allowed me to take part to conferences and workshops abroad.\\
%I am particularly grateful to Dr. Conor Mow-Lowry and the University of Birmingham, for giving me the opportunity and the funding to join the Gravitational waves group and contribute to the development of exciting science. This was also possible thanks to the support of the Royal Astronomical Society and the Institute of Physics, which allowed me to take part to conferences and workshops abroad.\\
%During my stay at LIGO Hanford site, I need to warmly thank Caltech for providing me accommodation and travel: this experience was very important for my studies.\\
%During my stay at LIGO Hanford site, I need to warmly thank Caltech for providing me accommodation and travel: this experience was very important for my studies.\\
%The completion of the work presented in this thesis would not have been possible without the action of the UoB, which accepted my application for an extension of my studies: the lockdown in 2020 stopped my lab work and the support of the UoB has been crucial to accomplish my project in the best way.\\
%The completion of the work presented in this thesis would not have been possible without the action of the UoB, which accepted my application for an extension of my studies: the lockdown in 2020 stopped my lab work and the support of the UoB has been crucial to accomplish my project in the best way.\\
%
%\noindent
%\chapter{Grazie!}
%On a personal note, \textit{grazie} Conor for always being supportive, it was important in the hardest times! Grazie, with great respect.\\
%On a personal note, \textit{grazie} Conor for always being supportive, it was important in the hardest times! Grazie, with great respect.\\
%
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%\textit{Grazie} to all my PhD colleagues who shared the stress and the fun of this experience with me. Riccardo, thanks for pushing me to go further, do better, stay stronger. George, thank you for the chess \& Czech classes, you opened my mind. Amit, your positivity is always contagious, thank you for sharing it!\\
%\textit{Grazie} to all my PhD colleagues who shared the stress and the fun of this experience with me. Riccardo, thanks for pushing me to go further, do better, stay stronger. George, thank you for the chess \& Czech classes, you opened my mind. Amit, your positivity is always contagious, thank you for sharing it!\\
%
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%I really need to thank all the people on LIGO Hanford, who made me feel included and part of a great family. \textit{Grazie} Jenny and Jim for your infinite patience, and I was very lucky to meet Dripta, an amazing housemate and friend, thank you for all the time you chose to share with me.\\
%I really need to thank all the people on LIGO Hanford, who made me feel included and part of a great family. \textit{Grazie} Jenny, Jim, Rahul and Jeff B. for your infinite patience and for teaching me so much of LIGO; and I was very lucky to meet Dripta, an amazing housemate and friend, thank you for all the time you chose to share with me.\\
%\textit{Grazie} to my family in Seattle, zia Claudia, zio Carlo, Rossana \& Jim, Carla \& Wayne, who helped me to feel at home during my stay in the USA.
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%To my friends in Italy: Vero \& Ludo, Alice \& Anna Paola, Martina, Francesca, Edo \& Nani, \textit{grazie} for your incredible will to stay close to me despite the 2000 km-distance.\\
%To my friends in Italy: Vero \& Ludo, Alice \& Anna Paola, Martina, Francesca, Edo \& Nani, \textit{grazie} for your incredible will to stay close to me despite the 2000 km-distance.\\
