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%%\title{CH.5 LSC signal optimization on LIGO}
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\chapter{LSC signals optimization on LIGO}
\label{LSCdiff}

\noindent
In the previous chapter I collaborated to enhance the performances of LIGO at low frequencies, modifying the software set up of the ISIs to reduce the motion of the chambers. As I anticipated, a consequence of this work could be the improvement of the LSC signals from LIGO cavities. Among them, DARM is particularly important, because it represents the gravitational wave signal. In this chapter I am going to show you how we tried to apply the new CPS configuration in order to obtain improvements in LSC signals at LIGO Hanford. This work has been performed during the 2019 commissioning break, in collaboration with LIGO Livingston Observatory.

\section{LSC signals in LIGO cavities}
Length to Signal Control is a crucial part of LIGO: as we know, gravitational waves can be seen by the interferometer if its sensitivity is high and stable enough. Optical signals in the cavities are very important in this frame: 

\section{Optimization: LSC offloading}
We saw that the cavities (and the optical signals) in LIGO are affected by the ISI motion, simply because they lie on them. Given the work done with the CPSs to suppress the ISI motion (described in the previous chapter), we should see an improvement on LSC signals. This is not immediate, though, nor trivial, because the optics are just set on the optical bench, without any communication with the ISI. The motion of the optics on the chambers due to other factors than seismic is not seen by the platforms: if we could connect this motion to the platform via software, this would make the optics and the platform more dependent on each other. This means that we can control the stabilization of the cavity lengths also with the ISIs.\\
What we expect is a faster reach of locking and a longer state of lock of the interferometer during observing runs.

\noindent
This work has been performed on LIGO during the commissioning break between O3a and O3b observing runs, in October 2019. The reason of this choice is that we needed the interferometer to \textit{not} be observing, since we were going to modify some software structure of the instrument.\\

\noindent
During the commissioning break, we performed the experiment: through CPSs locking, we reduced the differential motion of HAM2 and HAM3 chambers and made them to move in sync. So they can be considered as a whole block. The IMC is entirely lying on HAM2 and HAM3, and it is straightforward to use it as a witness: to make this real, we need to feed the HAM2-HAM3 block with IMCL. This will lock the cavity signal to the HAM2-HAM3 block. The same feeding will be performed with PRCL, SRCL, DARM and MICH cavities, which optics lie on the other chambers, in and out the corner station. Fig. \ref{chamb} illustrates the chambers and the locations of the cavities.

\begin{figure}[h!]
\centering
\includegraphics[scale=0.5]{images/chambs.jpg}
\caption{Sketch of the blocks and the locations of the cavities.}
\label{chamb}
\end{figure}

\noindent
To lock the LSC signals to ISIs, we need to do something similar to what we did in the previous chapter: we need to connect via software two different set ups which do not talk to each other. We decided to start from the Power Recycling Cavity Lecgth (PRCL) because we locked HAM2 and HAM3 chambers, so it was natural to start to lock the cavities on the x axis. 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 software for PRCL, since the job involved the request of permissions to modify the structure of the interferometer and the synchronization with the job of other people working on different parts of LIGO. Moreover, during 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.

\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 works.

\begin{figure}[h!]
\centering
\includegraphics[scale=0.7]{images/PRCLfeed.png}
\caption{Block diagram of PRCL locked to ISI.}
\label{prcl}
\end{figure}

\section{Beyond}

%\begin{thebibliography}{9}
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%\bibitem{phd} D. Tuyenbayev, \textit{Extending the scientfic reach of Advanced LIGO by compansating for temporal variations in the calibration of the detector.}, PhD thesis, University of Texas, 2017
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