\chapter{Control of seismic platforms motion and LSC offloading}
\label{CPSdiff}
During 2019, I spent some months working at the LIGO Hanford site (Washington, USA). This experience allowed me to be critically involved in the complicated life of a gravitational-wave interferometer. In particular, I was given the opportunity to study how to improve LIGO performances at low-frequency, focussing on the reduction of seismic motion of the platforms where the optics are located.\\
During 2019, I spent some months working at the LIGO Hanford site (Washington, USA). This experience allowed me to be critically involved in the complicated life of a gravitational-wave interferometer. In particular, I was given the opportunity to study how to improve LIGO performance at low-frequency, focussing on the reduction of seismic motion of the platforms where the optics are located.\\
In this chapter I will demonstrate how we can modify seismic control configuration of LIGO: in particular, this study should help reducing the differential motion between the chambers, making them move in sync, and help reducing and stabilizing the rms motion of the auxiliary sensors, through an LSC offload. The final goal is to obtain different and possibly better performance for seismic motion stabilization, faster and longer locking mode and, ultimately, more gravitational waves detections. The detailed computations included in this chapter are original and partially presented to the LIGO community and stored in LIGO DCC \cite{proposal}\cite{technote1} .\\
This work has been developed in collaboration with LIGO Hanford and LIGO Livingston laboratories, Stanford University, MIT and UoB and completed at UoB during 2020.\\
This chapter is partially including some technical notes I shared with LIGO collaboration and the contents of this study have been presented at conferences and workshops \cite{chiatalk}.\\
which is what we expect to be the signal of the differential motion sensed by the CPSs. In order to see this signal, we need to implement the modifications of the filters involved in the loop, as shown in the following section.
\section{Analysis of feasibility}
The next step is to study how to modify the low and high pass filters in order to obtain the best performances from each one in the new configuration of the chambers \cite{technote3}. To do this, we are going to change the blending filters, i.e. those filters whose combination gives the best performance of the set low+high pass filters.\\
The next step is to study how to modify the low and high pass filters in order to obtain the best performance from each one in the new configuration of the chambers \cite{technote3}. To do this, we are going to change the blending filters, i.e. those filters whose combination gives the best performance of the set low+high pass filters.\\
If by definition we have L+H=1\footnote{This definition arises from the need to accounting for unconditional loop stability and noise contributions. For details about blending filters, refer to \cite{kisselthesis}.}, we can write it as:
\begin{equation}
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@@ -497,7 +497,7 @@ Through CPSs locking, we reduced the differential motion of HAM2 and HAM3 chambe
\label{chamb}
\end{figure}
\noindent
The same work is foreseen to be done for the other cavities: the very short period of time available during the commissioning break allowed us to modify only the control loop for PRCL. Moreover, during the commissioning break, time is also used to work on the chambers, profiting of the out-of-lock mode. This means that, for every attempt of software modification, a locking trial was needed, to see if the new configuration of the instrument was giving better performances and, also, if it was affecting negatively other sides of the instrument. To try to lock LIGO, we needed people not to work besides the chambers. This was a huge and collaborative work, which involved many people on site, and their time. Despite these challenges, the results obtained are encouraging and validated the analysis of feasibility exposed.
The same work is foreseen to be done for the other cavities: the very short period of time available during the commissioning break allowed us to modify only the control loop for PRCL. Moreover, during the commissioning break, time is also used to work on the chambers, profiting of the out-of-lock mode. This means that, for every attempt of software modification, a locking trial was needed, to see if the new configuration of the instrument was giving better performance and, also, if it was affecting negatively other sides of the instrument. To try to lock LIGO, we needed people not to work besides the chambers. This was a huge and collaborative work, which involved many people on site, and their time. Despite these challenges, the results obtained are encouraging and validated the analysis of feasibility exposed.
\paragraph{The Power Recycling Cavity Length (PRCL)}
We need to connect the ISI to the cavity and to do it we need to know how the PR cavity is going to communicate with the ISI (refer to Chapter \ref{LIGO} for details on the PR cavity). The block diagram in Fig. \ref{prcl} illustrates the simplified concept of the PR cavity connected to the ISIs of the block of HAM2 and HAM3 chambers \footnote{Some insights about the shape of the transfer function of the suspensions are in Appendix C.}.\\
@@ -161,11 +161,11 @@ The devices dedicated to monitoring the seismic motion are inertial and displace
\end{figure}
\paragraph{Stabilizing the ISI}
Part of the work presented in this thesis focussed on the enhancement of the performances of the active isolation system of the ISIs for both BSC and HAM chambers.\\
Part of the work presented in this thesis focussed on the enhancement of the performance of the active isolation system of the ISIs for both BSC and HAM chambers.\\
Active isolation implies a sensing system of the noise to reduce and a control system to compensate the disturbance. Each platform includes relative position sensors, inertial sensors and actuators, working in all degrees of freedom.\\
\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 performances 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 exposed in Appendix B}.\\
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 exposed 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.
@@ -64,9 +64,9 @@ The optical levers can in principle reduce tilt motion below 1 Hz; the use of ca
\clearpage
\chapter{Acknowledgements}
%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.\\
%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.\\
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. Thanks to the Albert Einstein Institute (Hannover) for providing their facilities for my tests.\\
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.\\
@@ -142,7 +142,7 @@ The device described in this chapter should involve sensing and actuation for th
The purpose when thinking of interferometers is to help reducing the RX motion on the HAM chambers that propagates into the suspensions.
\section{Experiment design}
In order to understand the feasibility of the project in terms of performances, we have to estimate the noise budget and the sensitivity of the system.\\
In order to understand the feasibility of the project in terms of performance, we have to estimate the noise budget and the sensitivity of the system.\\
\noindent
Let's start from the block diagram of the system, in Fig. \ref{BD}.
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@@ -155,7 +155,7 @@ Let's start from the block diagram of the system, in Fig. \ref{BD}.
\noindent
In the block diagram all the noises we have to deal with are described: the most relevant in terms of contributions are the shot and the thermal noises; then there are all the noises related to the electronics, like dark current, flicker and op-amp noises, usually given in the datasheet of the devices.\\
Beyond them, we have to consider the relative intensity noise (RIN), due to instabilities in the laser intensity: this kind of noises reduces the signal-to-noise ratio, limiting the performances of the electronic transmission. This may be reduced by making the signal positions independent of illumination intensity.\\
Beyond them, we have to consider the relative intensity noise (RIN), due to instabilities in the laser intensity: this kind of noises reduces the signal-to-noise ratio, limiting the performance of the electronic transmission. This may be reduced by making the signal positions independent of illumination intensity.\\
The translation coupling noise due to the motion of the platform where sensors are set is also considered: this gives a contribution in the measurement in terms of linear displacement, while we are measuring the angular motion of the platforms.
\subsection{Quadrant Position Devices}
...
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@@ -290,7 +290,7 @@ The same computation gives the result for the coordinate y:
\end{equation}
\noindent
In order to estimate the resolution of the device and provide an estimate of its performances, we need to account for the noises coming from the QPD and external sources.
In order to estimate the resolution of the device and provide an estimate of its performance, we need to account for the noises coming from the QPD and external sources.
\subsection{Photon shot noise}
\label{sn}
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@@ -569,7 +569,7 @@ The pressure has been set at 5 $\times$ 10$^{-3}$ mbar. What we expect is to fin
In this conditions, also the signals from the L4C seismometers and accelerometers (Watt's Leakage) placed on the Central bench have been measured (Fig. \ref{central}). The plots with the UoB electronics show that there is some leakage below 10 Hz, probably due to saturation, in the measurement of the accelerometers.\\
\noindent
QPD performances are shown in the plots \ref{qpd_fin}. With AEI boxes we had expected results: no variations in the power fluctuation peaks and expected behaviour of pitch and yaw.\\
QPD performance is shown in the plots \ref{qpd_fin}. With AEI boxes we had expected results: no variations in the power fluctuation peaks and expected behaviour of pitch and yaw.\\
However, with UoB pre-amp the measurements do not seem consistent with what we expected: we think that some non-linearities in UoB pre-amp could be the cause of the problem. This is still under investigation at UoB.
\begin{figure}[h!]
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@@ -627,5 +627,5 @@ Noise measurements of CDS with unplugged electronics have been taken, to check i
\addcontentsline{toc}{section}{Conclusions}
The analysis of feasibility of this experiment showed that 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. It is anyway a good device to be tested.\\
During the test of the prototype, the measurements have shown that we had issues when calibrating the device due to problems highly related to electronics from UoB, since the tests with the AEI electronics showed that the optical setup was well built and aligned. The very short time of the visit did not allow to take more in-depth tests.\\
Other possible reasons to investigate for better performances might lie in the structure of the prototype: further tests might be useful to understand if the device can be improved by changing the position of the lens with respect to the QPD, and let the diode sit at the focus on the lens. This solution will concentrate the power and decrease the size of the beam.\\
Other possible reasons to investigate for better performance might lie in the structure of the prototype: further tests might be useful to understand if the device can be improved by changing the position of the lens with respect to the QPD, and let the diode sit at the focus on the lens. This solution will concentrate the power and decrease the size of the beam.\\
The device is currently not suitable for the purposes we tested for, but it opened the way to further tests to improve the technology: since the pitch and yaw tests have shown that the optical lever might be sensitive to the vertical motion of the bench, a reduction of this motion might be of great impact to improve the sensitivity of the levers \cite{luise}. With a good sensing system of tilt motion, the addition of an actuation system able to reduce this motion will be crucially helpful to stabilize the suspension points of the optical chains and then of the whole cavity.
