\chapter{Reducing differential motion of aLIGO seismic platforms}
\noindent
During 2019, I spent some months working on 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 live.\\
In this chapter and in the following I am going to show you in detail how we can modify the software set up of LIGO in order to obtain a different and possibly better performance. This work has been developed in collaboration with LIGO Hanford and LIGO Livingston laboratories, Stanford University, MIT and UoB, and this chapter is partially including some technical notes I shared with LIGO collaboration.
\section{Motivation: Duty cycle on LIGO}
Lock loss events are the main sources of preventing continuous observations for long periods of time: when light loses resonance in the cavities, a lock loss happens and the control system of the optical cavities are under effort to restore stabilization. This means that during lock loss the interferometer is no longer able to be stable and the observing time is interrupted.\\
Duty cycle is one of the main topic where commissioners focus on before starting an observing run. It is needed not only to observe more gravitational waves, but also to identify noise sources and improve sensitivity.\\
Since the number of detected events over a time period N(t) is proportional to the volume of Universe under observation V, the observing time t and the rate R of astrophysical sources that can occur in a certain volume:\\
Since the number of detected events over a time period N(t) is proportional to the volume of Universe under observation V, the observing time t and the rate R of astrophysical sources that can occur in a certain volume:
\begin{equation}
\centering
...
...
@@ -75,21 +73,20 @@ We started our design on chambers of x arm. Along this direction, the Input Mode
\noindent
In the next section we will demonstrate that CPS are good witnesses to sense differential motion and they also can be used to lock the chambers with each other.
\section{CPS Differential motion and locking}
\subsection{Sensing differential motion via CPS}
Capacitive Position Sensors (CPS) measure the relative motion between two stages of the isolation system. On HAM chambers, they are are set between HEPI and ground, and between Stage 1 and HEPI. On BSC chambers, they also measure the relative motion between Stage 1 and Stage 2. Plots is Fig. \ref{diff} show the differential motion seen by the CPS between BSC and HAM chambers: the sensors are reliable for this measurement, and they put in evidence that the HAM chambers have a more synchronized motion with respect to the motion between HAM and BSC and BSCs only. This means that the block of HAM chambers on x arm is more relatively stable and can be used as driver for the other chambers, with the mode cleaner acting as witness.
\section{Sensing differential motion via CPS}
Capacitive Position Sensors (CPS) measure the relative motion between two stages of the isolation system. On HAM chambers, they are are set between HEPI and ground, and between Stage 1 and HEPI. On BSC chambers, they also measure the relative motion between Stage 1 and Stage 2. Plots is Fig. \ref{diff} show the differential motion seen by the CPS between BSC and HAM chambers: the sensors are reliable for this measurement, and they put in evidence that the HAM chambers have a more synchronized motion with respect to the motion between HAM and BSC and BSCs only. This means that the block of HAM chambers on x arm is more relatively stable and can be used as driver for the other chambers, with the mode cleaner acting as witness.\\
\caption[CPS differential motion]{CPS differential motion between the HAM and BSC chambers along x axis. ISIs move in common, particularly in the same building. This can be confirmed by noting that the difference between two chambers is much lower than individual chambers.}
\label{diff}
\end{figure}
\newpage
\noindent
We projected the CPS of the x axis chambers to the suspension point in order to obtain PRCL and IMCL traces sensed by the CPS. For BSCs, we decided to sum the contributions of the CPSs on stage 1 and stage 2 and to project this sum.\\
One of the main differences between the behaviour of CPS IMCL and CPS PRCL, is that the former is obviously involving only the HAM chambers. Since HAM2 and HAM3 have a very good common motion, IMCL can be considered more stable with respect to PRCL, which instead involves also BSCs. Indeed, CPS PRCL is following the only BSCs at frequencies below 0.02 Hz.\\
...
...
@@ -97,13 +94,13 @@ Fig. \ref{sus} shows plots of PRCL and ICML by CPS projection to suspoint. These
\caption[CPS suspoint projections]{CPS suspoint projections: IMCL and PRCL. The calibrated PRCL trace used as comparison has been de-whitened.}
\label{sus}
\end{figure}
\subsection{Locking chambers via CPS}
\section{Locking chambers via CPS}
In the previous section we demonstrated that CPSs are good sensors for differential motion and can be used to monitor chamber motions at lower frequencies. Given that and remembering the aim of stabilize the motion of the chambers making them moving in sync, it is possible to use the CPSs to lock HAM2 and HAM3 together, HAM4 and HAM5 together, BSCs in the Corner Station together and BSCs hosting the ETMs together. This will stabilize the ISI differential motion with respect to a driving chamber.\\
Since we saw that HAM2 and HAM3 show a very good common motion and that we can use the IMC as a witness of it, our first step has been to lock the HAM2 and HAM3 chambers together by feeding HAM3 a calculated differential CPS signal. This is done in practice with an additive offset to the setpoint of the HAM3 isolation control loop.\\
The block diagram in Fig. \ref{ham2b}, shows the structure of HAM2, where the signals from $d_{2}$ and $i_{2}$ represent the offsets given by CPS and inertial sensors.\\
...
...
@@ -490,3 +487,6 @@ We need to connect the ISI to the cavity and to do it we need to know how the PR
@@ -248,8 +248,8 @@ Several tests have been taken in different conditions for noise hunting along th
\begin{figure}[h!]
\centering
\includegraphics[scale=0.3]{images/test2807.png}
\caption{Last laser stabilization test}
\includegraphics[scale=0.3]{images/result.png}
\caption{Results of frequency stabilization: the in-loop red trace shows the frequency stabilization process as detected by the frequency counter monitoring the beat-note between the two lasers; the black trace is the expected gain activated by the controllers, which is set to maximise the stabilization below 1 Hz.}
\label{test}
\end{figure}
...
...
@@ -265,7 +265,8 @@ This test highlights also that HoQI2 is still noisier than HoQI1, despite the us
@@ -117,6 +110,26 @@ SRCL = Signal Recycling Cavity Length\\
TEC = Thermo-Electric Controller\\
UoB = University of Birmingham\\
\chapter{Structure of this thesis}
An introduction to frame the work and structure of the thesis go here.\\
STRUCTURE OF THESIS [DRAFT]\\
PART I: Gravitational astrophysics\\
Chapter 1: Gravitational waves and sources\\
Chapter 2: low frequency window and multimessenger astronomy\\
PART II: Detectors and seismic isolation\\
Chapter 3: Interferometry and Advanced LIGO\\
Chapter 4: Inertial sensors and optical levers\\
PART III: Lowering seismic noise\\
Chapter 5: Seismic isolation at LHO\\
Chapter 6: Laser stabilization for 6D seismic isolation\\
Appendix A: first GW detection\\
Appendix B: control loops
\mainmatter
\part{Gravitational-wave frontiers}
...
...
@@ -127,20 +140,19 @@ UoB = University of Birmingham\\
\part{Lowering seismic motion}
\include{oplevs}
\include{CPSdiff}
%\include{LSCdiff}
\include{laserstab}
\appendix
\include{A}
\include{B}
%\appendix C on my work in LIGO labs
\backmatter
\listoffigures
\listoftables
\begin{thebibliography}{}
%chapt 1
\bibitem{wei} S. Weinberg \textit{Gravitation and Cosmology: principles and applications of the General Theory of Relativity}, John Wiley \& Sons, Inc., 1972
\bibitem{nar} J. V. Narlikar \textit{An introduction to Relativity}, Cambridge University Press, 2011
...
...
@@ -149,6 +161,8 @@ UoB = University of Birmingham\\
\bibitem{first} B. P. Abbott, \textit{Observation of Gravitational Waves from a Binary Black Hole Merger}, Phys. Rev. Lett. 116, 061102, 2016
%chapt3
\bibitem{ligo} Advanced LIGO Systems Group, \textit{Advanced LIGO Systems Design}, DCC document T010075-v3, 2017
\bibitem{abb} B. P. Abbott et al, \textit{GW150914: The Advanced LIGO Detectors in the Era of First Discoveries}, Phys. Rev. Lett. 116, 131102, 2016
...
...
@@ -160,7 +174,41 @@ Beginning of Gravitational Wave Astronomy}
\bibitem{lsc} K. Izumi, D. Sigg, \textit{Advanced LIGO: length sensing and control in a dual recycled interferometric gravitational wave antenna}, 2017 Class. Quantum Grav. 34 015001
\bibitem{phd} D. Tuyenbayev, \textit{Extending the scientific reach of Advanced LIGO by compensating for temporal variations in the calibration of the detector.}, PhD thesis, University of Texas, 2017
\bibitem{intro} B. Lantz et al., \textit{Estimates of HAM-ISI motion for A+}, T1800066-v2, March 2018, https://dcc.ligo.org/LIGO-T1800066
\bibitem{intro2} S. Cooper et al., \textit{Ham ISI model}, Technical note, University of Birmingham, March 2018, https://dcc.ligo.org/LIGO-T1800092
\bibitem{ven} Venkateswara et al., \textit{Subtracting tilt from a horizontal-seismometer using a ground-rotation-sensor}, Bulletin of the Seismological Society of America (2017) 107 (2): 709-717
\bibitem{mca} C. Di Fronzo \textit{Optical sensors for improving low-frequency performance in GW detectors}, Mid-Course Assessment, University of Birmingham, 2018
\bibitem{poster2} C. Di Fronzo et al., \textit{Optical Lever for interferometric inertial isolation}, poster, LVC meeting, Maastricht 2018, https://dcc.ligo.org/LIGO-G1801693
\bibitem{sina} S. M. Köhlenbeck, \textit{Towards the SQL Interferometer Length Stabilization at the AEI 10 m-Prototype}, PhD thesis, Gottfried Wilhelm Leibniz Universität Hannover, 2018
\bibitem{J} R.V. Jones et al., \textit{Some developments and applications of optical levers}, 1961 J. Sci. Instrum. 38 37
\bibitem{kaz} K. Agatsuma, \textit{Optical lever for KAGRA}, GW monthly seminar at Tokyo, 2014
\bibitem{lan} B. Lantz et al. \textit{Review: Requirements for a Ground Rotation Sensor to Improve Advanced LIGO}, Bulletin of the Seismological Society of America, Vol. 99, No. 2B, 2009
\bibitem{coll} C. Collette et al. \textit{Review: Inertial Sensors for Low-Frequency Seismic Vibration Measurement}, Bulletin of the Seismological Society of America, Vol. 102, No. 4, 2012
\bibitem{dan} D. E. Clark, \textit{Control of Differential Motion Between Adjacent Avanced LIGO Seismic Isolation Platforms}, PhD thesis, Stanford University, 2013
\bibitem{ast} S. M. Aston at al., \textit{Update on quadruple suspension design for Advanced LIGO}, Class. Quantum Grav., 29 (2012) 235004 (25pp)
\bibitem{image} B. Shapiro, \textit{Brief Introduction to Advanced LIGO Suspensions}, LAAC Talk, August 2014, https://dcc.ligo.org/LIGO-G1400964
%cpsdiff
%6D
\bibitem{poster} C. Di Fronzo et al, \textit{A 6D inertial isolation system}, ET Symposium, 2019, poster, https://dcc.ligo.org/LIGO-G1900741
\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
\bibitem{intro} B. Lantz et al., \textit{Estimates of HAM-ISI motion for A+}, T1800066-v2, March 2018, https://dcc.ligo.org/LIGO-T1800066
\bibitem{intro2} S. Cooper et al., \textit{Ham ISI model}, Technical note, University of Birmingham, March 2018, https://dcc.ligo.org/LIGO-T1800092
\bibitem{ven} Venkateswara et al., \textit{Subtracting tilt from a horizontal-seismometer using a ground-rotation-sensor}, Bulletin of the Seismological Society of America (2017) 107 (2): 709-717
\bibitem{mca} C. Di Fronzo \textit{Optical sensors for improving low-frequency performance in GW detectors}, Mid-Course Assessment, University of Birmingham, 2018
\bibitem{poster} C. Di Fronzo et al., \textit{Optical Lever for interferometric inertial isolation}, poster, LVC meeting, Maastricht 2018, https://dcc.ligo.org/LIGO-G1801693
\chapter{Optical Levers for tilt motion reduction}
\label{oplevs}
In this chapter I will introduce the sensors dedicated to measure the seismic motion. They need to account for horizontal, vertical and tilt displacements in all degrees of freedom in order to be efficient and the technology for their improvement is currently pushing and competing on sensing as low as possible seismic motion. As we know from the previous sections, on an interferometric detector seismic motion affects the stabilization of the supports where the optics lie. This produces unwanted noise at low frequencies (< 30 Hz), which reduces the sensitivity of the detector.\\
During the first year of my PhD studies, I investigated the use of optical levers to tilt motion: a device has been built at UoB and tested at the Albert Einstein Institute (AEI) in Hannover in June 2019.\\
In this chapter I will introduce the sensors dedicated to measure the seismic motion. They need to account for horizontal, vertical and tilt displacements in all degrees of freedom in order to be efficient and the technology for their improvement is currently pushing and competing on sensing as low as possible seismic motion. On an interferometric detector, seismic motion affects the stabilization of the supports where the optics lie. This produces unwanted noise at low frequencies (< 30 Hz), which reduces the sensitivity of the detector.\\
During the first year of my PhD studies, I investigated the use of optical levers to reduce tilt motion: a device has been built at UoB, and tested at the Albert Einstein Institute (AEI) in Hannover in June 2019.\\
The content of this chapter has been re-adapted from my MCA report \cite{mca}. A poster about this project has been presented at the LVK meeting in Maastricht (September 2018) \cite{poster}.
\section{Inertial sensors affected by tilt-coupling}
There are many contributions affecting aLIGO sensitivity at low frequency. One of the most investigated is the tilt of HAM vacuum chamber of ISI benches, which dominates above 1 Hz \cite{intro}.\\
For the rotational degrees of freedom, getting a good estimate of ground motion is not trivial because no rotational sensors capable of measuring the ground motion in rotation at low frequencies have been installed yet on aLIGO \cite{intro2}.\\
However, there could a possible way to measure angular displacements of the benches very precisely (10$^{-12}$ rad/$\sqrt{Hz}$) and to actively control them. This could be done by optical levers.\\
However, there could be a possible way to measure angular displacements of the benches very precisely (10$^{-12}$ rad/$\sqrt{Hz}$) and to actively control them. This could be done by optical levers.
\begin{figure}[h!]
\centering
\includegraphics[scale=0.85]{images/HAMoplev.PNG}
\caption{Plot of the contributions to the Suspension point L motion at LHO HAM5. The pitch (RX) contribution dominates above 1\,Hz (Figure taken from \cite{intro}).}
\includegraphics[scale=0.8]{images/HAMoplev.PNG}
\caption[Example of tilt-coupling contributions at LHO]{Plot of the contributions to the Suspension point L motion at LHO HAM5. The pitch (RX) contribution dominates above 1\,Hz (Figure taken from \cite{intro}).}
\end{figure}
\newpage
\paragraph*{Horizontal sensors}
The most important problem, in order to achieve good isolation, is the sensitivity of the horizontal sensors to rotation (Fig. \ref{a}). If we could independently measure the rotation, we could calculate the true translation motion.\\
...
...
@@ -31,6 +32,7 @@ When a rotation around the center of mass occurs, an additional term F$_{tilt}$
\centering
F_{tilt} = mg\sin\theta
\end{equation}
\noindent
where m is the mass and g = 9.8 m/s$^2$ is the gravitational acceleration.\\
\noindent
...
...
@@ -61,6 +63,7 @@ m(W+Y)s^2 = -bWs -kW + mg\theta
\centering
W = \frac{ms^2}{ms^2 + bs +k}\left(-y + \frac{g}{s^2}\theta\right)
\end{equation}
\noindent
Remembering that s = i$\omega$ in a steady-state situation, we have:
...
...
@@ -95,13 +98,10 @@ If $\theta \ll$ 1, $\cos \theta \rightarrow$ 1: this means that the vertical sen
\begin{figure}[h!]
\centering
\includegraphics[scale=0.8]{images/vert.PNG}
\caption{Tilting of vertical sensor (Figure taken from \cite{ven}).}
\caption[Tilting of vertical sensor]{Tilting of vertical sensor (Figure taken from \cite{ven}).}
\label{v}
\end{figure}
%\section{LIGO inertial sensors}
%UNA CARRELLATA VELOCE DI COME VIENE ISOLATO LIGO
\section{Optical levers}
In general, an optical lever is a convenient device that makes use of a beam light and a position sensor to measure a small displacement and thus to make possible an accurate measurement of angles. This method is a very useful approach in sensitive non-contacting measurements. There is a light source, typically a laser, impinging on an optic reflecting the beam on a position device, which records any displacement of the beam, i.e. of the optic.\\
\noindent
...
...
@@ -127,8 +127,10 @@ The device described in this chapter should involve sensing and actuation for th
\caption{Basic principle of the optical lever used for sensing and actuation for seismic isolation.}
\label{z}
\end{figure}
\noindent
The purpose when thinking of interferometers is to help reducing the Rx motion on the HAM chambers that propagates into the suspensions.
\subsection*{Noise budget}
\section{Noise budget}
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.\\
\noindent
Let's start from the block diagram of the system, in Fig. \ref{BD}.
...
...
@@ -144,14 +146,14 @@ In the block diagram all the noises we have to deal with are described: the most
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.\\
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.
\subsubsection{Quadrant Position Devices}
\subsection{Quadrant Position Devices}
The Quadrant Position Devices (QPD) are the position devices usually involved with optical levers. They consist of four distinct and identical quadrant-shaped photodiodes that are separated by a small gap (typically, $\sim$0.1 mm) and together form a circular detection area capable of providing a 2D measurement of the position of an incident beam.\\
When light is incident on the sensor, a photocurrent I is detected by each section in Fig. \ref{j}.\\
When light is incident on the sensor, a photocurrent I is detected by each quadrant Q in Fig. \ref{j}.\\
\begin{figure}[h!]
\centering
\includegraphics[scale=0.8]{images/quad.PNG}
\caption{Basic scketch of the segmented photodiode.}
\includegraphics[scale=0.6]{images/quad.PNG}
\caption[QPD segmented details for beam position detection]{View of the segmented photodiode. Each quadrant Q receives a photocurrent which is the signal responsible for any displacement detection: depending on which quadrant is receiving more or less photocurrent, it is possible to derive the position of the beam onto the active area.}
\label{j}
\end{figure}
\noindent
...
...
@@ -182,7 +184,7 @@ If a symmetrical beam is centred on the sensor, four equal photocurrents will be
%In this example, after pre-amplification, each adjacent pair of quadrant signals is fed to a differential amplifier. These signals then give partial information about motion in the x or y axis. The signals from each axis are then summed by final stage of amplification, giving the x and y position signals.\\
%Note that also, it may be useful to generate a total intensity signal by summing all of the quadrants; this may be used to normalise the position signals to make them independent of illumination intensity (\cite{qpd}).
\subsubsection{Spot position and displacement}
\subsection{Spot position and displacement}
At the light of what we have seen about QPDs, we have to compute where the beam is on the photodiode: the coordinates of the beam depend on the photocurrents. If we are dealing with a Gaussian beam, they are proportional to the Gaussian intensity:\\
\begin{equation}
...
...
@@ -267,7 +269,7 @@ The same computation gives the result for the coordinate y:
Because of the fact that the working principle of the QPD is based on tracking the motion of the centroid of power density, it is useful to compute the contribution of the shot noise.\\
The shot noise is the fluctuation of the photon counting on the photodetector. This fluctuation obeys the Poisson statistics, but for a large mean number of photons ($<N> \gg1$), it approaches the Gaussian one, with standard deviation $\sigma$ = $\sqrt{<N> }$.\\
...
...
@@ -323,7 +325,7 @@ If we have a laser wavelength $\lambda$ = 1064 nm and an input power P$_0$ = 1 m
%APPROFONDISCI FACCENDA DEL QUADRANTE DIVISO!\\
%Note that the QPD is composed by 4 photodiodes, so we should consider for each section 1/4 of the shot noise previously computed. However, we can consider the device as a unique device because...
\subsubsection{Thermal noise}
\subsection{Thermal noise}
\label{tn}
The other, important noise affecting the measurements is the thermal noise due to the resistor of the photodiode R. It is given by:
...
...
@@ -348,7 +350,7 @@ So, considering T=300 K at room temperature, we have:
T_{h}=1.47 \times 10^{-12}\frac{W}{\sqrt{Hz}}.
\end{equation}
\subsection*{Resolution}
\subsection{Resolution}
Now that we have extracted the noise budget of our system, we can determine the sensitivity $\alpha$ of the sensor. This means that we want to know the efficiency of our system in measuring angles (in rad/$\sqrt{Hz}$).\\
So, according to the block diagram in Fig. \ref{BD}, to obtain the angle measurement we have that:
...
...
@@ -371,6 +373,50 @@ So, according to the block diagram in Fig. \ref{BD}, to obtain the angle measure
\centering
\alpha = 3 \times 10^{-12}\frac{rad}{\sqrt{Hz}}.
\end{equation}
\noindent
This value is of the order of magnitude of the sensitivity of optical levers anticipated earlier.
\subsection{Estimated sum of contributions}
In order to obtain a plot of the noise budget for the optical lever prototype, we need to take into account some more elements to add to the ones just computed:
\begin{itemize}
\item The motion along z of the platforms is used as noise: however, at low frequency, sensors are not sensitive to this motion so what we need is a differential motion between HAM chambers (say HAM4 and HAM5 for this derivation); the best estimation we have is the platform z motion measured by GS13s. This motion is given by channels of LIGO Livingston data;
\item The best performance of current tested optical levers is the one tested at the AEI and shown in Fig. 4.4 of reference \cite{sina};
\item The ground z motion of the chambers is given by the Beam Rotation Sensors (BRS) and used as noise source. This motion is taken from channels of LIGO Livingston data;
\item The Rx motion is given by the CPS on HAM4 and HAM5 and it is used for comparison with the optical lever performance. This motion is taken from channels of LIGO Livingston data.
\end{itemize}
\noindent
The optical lever performance reported in \cite{sina} takes into account the motion along x axis, the spot displacement of the beam on the photodiode and the displacement of the photodiode itself.\\
The differential Z motion is given by the difference between the z motion measured by the GS13 sensors on HAM4 and HAM5:
\begin{equation}
\centering
\Delta Z = (GS13^{HAM5}_z - GS13^{HAM4}_z).
\end{equation}
\noindent
GS13 motion of both chambers needs to be re-calibrated, because it does not take into account the low frequency range.\\
\noindent
All the noise sources are divided by the lever arm, in order to obtain an estimation in radians.\\
A low pass filter (LP) at 1 Hz is applied to the BRS motion and a high pass filter (HP) is applied to the $\Delta Z$ motion at 0.1 Hz.\\
Summing all the noise elements in quadrature, we have the total noise performance of the optical lever, which is shown in the plot in Fig. \ref{oplevnoise}:
\caption[Optical lever noise budget]{Optical lever total noise budget.}
\label{oplevnoise}
\end{figure}
\section{Design of the prototype}
The optical design has been simulated, taking into account some general constraints of the sensor: generally, the QPD diameter is around 10 mm, so the beam size should not exceed 1-3 mm; gaps in quadrant photodiodes are of the order of tens $\mu$m. Moreover, it is ideal for the setup to be compact.\\
...
...
@@ -391,15 +437,14 @@ The prototype and its own pre-amplifying electronics has been built at UoB (Fig.
\begin{figure}
\centering
\includegraphics[scale=0.5]{images/OpLev20.jpg}
\caption[Photo of the optical lever prototype]{Photo of the optical lever prototype.}
\caption[Photo of the optical lever prototype]{Photo of the optical lever prototype as built at UoB.}
\label{oplev20}
\end{figure}
\section{Test at the AEI}
The aim of the visit was to test the optical lever prototype in vacuum. We used the South bench of the 10 m prototype at AEI in HAnnover.\\
The device and part of its electronics have been adjusted in order to match the requirements for a measurements using the CDS and facilities at AEI. The main modifications are the following:
\paragraph*{PIN configuration}
The pin configuration of the QPDs has been re-adjusted because the AEI electronics is set on a different one. In Fig. there is a scheme of the original configuration; it has been changed to the following:\\
The aim of the collaboration was to test the optical lever prototype in vacuum. We used the South bench of the 10 m prototype at AEI in HAnnover.\\
The device and part of its electronics have been adjusted in order to match the requirements for a measurements using the CDS and facilities at AEI.\\
The pin configuration of the QPDs has been re-adjusted because the AEI electronics is set on a different one. it has been changed to the following:\\
\begin{itemize}
\item Q1: PIN 1 to PIN 1
...
...
@@ -409,25 +454,15 @@ The pin configuration of the QPDs has been re-adjusted because the AEI electroni
\item BIAS: PIN 5 to PIN 4
\end{itemize}
\noindent
Two adaptor cables have been build to connect the UoB boxes to the QPDs with the new pin configurations.
\paragraph*{Device modification}
To isolate the QPD, a small shield of plastic has been added to the QPD mount and the related metal screws have been changed with peek screws. Because of the presence of the new plastic layer, the height of all other components of the platforms has been adjusted.\\
Two adaptor cables have been build to connect the UoB boxes to the QPDs with the new pin configurations.\\
\noindent
Every component has been vacuum-cleaned using an ultra-sonic bath: every mount was dis-mounted and then re-mounted after cleaning.\\
To clean the collimators, it was sufficient to remove all the labels and wipe them with alcohol to remove the residuals of glue.
To isolate the QPD, a small shield of plastic has been added to the QPD mount and the related metal screws have been changed with peek screws. Because of the presence of the new plastic layer, the height of all other components of the platforms has been adjusted.\\
Every component has been vacuum-cleaned using an ultra-sonic bath: every mount was dis-mounted and then re-mounted after cleaning.
\subsection{Installing the device}
After cleaning, we installed the device into the South bench of the 10-m prototype. Due to the availability of the bench, only one fibre could be connected to one collimator; consequently, only one QPD has been connected.\\
\noindent
The lever arm has been set to be 20 cm: with the optical configuration foreseen for this lever arm (see Tech Note) the spot size on the photodiode is w $\simeq$ 1 mm. The power on that point is P = 3.5 mW.\\
%Test images have been taken for a rough beam profiling, using a Wincam camera.
%\caption{Wincam images of the beam: from left to right, the camera has been placed at 6 cm, 14 cm and 34 cm from the output of the collimator.}
%\end{figure}
The lever arm has been set to be 20 cm: with the optical configuration foreseen for this lever arm the spot size on the photodiode is w $\simeq$ 1 mm. The power on that point is P = 3.5 mW.\\
\noindent
Summarizing, the prototype is ready for the test with the following specifications:\\
...
...
@@ -461,12 +496,11 @@ To test if everything was set in the best way, we performed a first measurement
\caption[OpLev test in air]{Preliminary test in air: the traces show the trend of the pitch, yaw and the sum of all QPD quadrants.}
\label{inair}
\end{figure}
\section{Test is vacuum}
\section{Test in vacuum}
We decided to set the vacuum in two steps: this idea allows to have a faster temperature gradient, decreasing the waiting time for temperature (and benches) to stabilize.\\
So for the first step we set the pressure at 30 mbar, the day after we set the pressure at 5 $\times$ 10$^{-3}$ mbar.\\
\noindent
...
...
@@ -482,8 +516,8 @@ Fig. \ref{LVDT} shows the movement of South bench along z axis, that we use as a
\begin{figure}[h!]
\centering
\includegraphics[scale=0.3]{images/LVDT_Z.PNG}
\label{LVDT}
\caption[30 mbar LVDT test]{Motion along z axis of the South bench during vacuum pump to 30 mbar.}
\label{LVDT}
\end{figure}
\noindent
Fig. \ref{QPD} shows the measurements taken with the QPD. The 4 quadrants and the Pitch, Yaw and Sum values show an expected behaviour.\\
...
...
@@ -493,55 +527,53 @@ Some peaks at lower frequencies may be due to bench motion: if the assumption is
The pressure has been set at 5 $\times$ 10$^{-3}$ mbar. What we expect is to find no variations in terms of the peaks we think are due to power fluctuations. Variations in LVDT trend due to temperature stabilization and related variations of Pitch and Yaw due to the more stable bench conditions.\\
\noindent
We took measurements both with AEI and UOB electronics. We have two UOB pre-amps (serial numbers ending with 0604 ans 0519). Firstly, we decided to modify UOB box no. 0604, to better adapt to AEI electronics and avoid saturation. We substituted the R334 Dx non inverting gain (originally of 9.1 k$\Omega$) with DH 0.1\% "Metalschicht" of 6.8 k$\Omega$. Unfortunately, no measurements have been possible, because we saturated. So we used the other box, not modified.
The pressure has been set at 5 $\times$ 10$^{-3}$ mbar. What we expect is to find no variations in terms of the peaks we think are due to power fluctuations. Variations in LVDT trend can be due to temperature stabilization and related variations of Pitch and Yaw are then due to the more stable bench conditions.\\
\begin{figure}[h!]
\begin{figure}[H]
\centering
\includegraphics[scale=0.3]{images/LVDT.PNG}\\
\includegraphics[scale=0.3]{images/ULVDT.PNG}
\label{LVDT_FIN}
\caption[Different pre-amps test: bench LVDT motion]{Bench motion long z axis at 5 $\times$ 10$^{-3}$ mbar pressure conditions. Blue curve is the trend measured by AEI pre-amp, red curve is measured with UOB pre-amp.}
\label{LVDT_FIN}
\end{figure}
\noindent
In this conditions, also the signals from the L4C seismometers and accelerometers on Central bench have been measured.
\caption[Different pre-amps test: bench motion]{Motion of Central bench measured by L4Cs and accelerometers, with AEI and UOB pre-amps.}
\label{central}
\end{figure}
\noindent
QPD performance are shown in the following pictures. With AEI boxes we had expected results: no variations in the power fluctuation peaks and expected behaviour of Pitch and Yaw with single quadrants.\\
However, with UOB box (no. 0519) the measurements do not seem consistent with what we expected: despite the behaviour of each quadrant seems to follow the expected trend (even if differently from AEI trend), the curves of Pitch and Yaw do not match with the quadrants trend. We think that some non-linearities in UOB box could be the cause of the problem: this is still under investigation at UOB.\\
However, with UOB box (no. 0519) the measurements do not seem consistent with what we expected: despite the behaviour of each quadrant seems to follow the expected trend (even if differently from AEI trend), the curves of Pitch and Yaw do not match with the quadrants trend. We think that some non-linearities in UOB box could be the cause of the problem: this is still under investigation at UOB.
\caption[In vacuum QPD test]{QPD performance, with AEI and UOB pre-amps.}
\label{qpd_fin}
\end{figure}
\subsubsection{Noise measurements}
\paragraph*{Electronic noise}
Noise measurements of CDS and unplugged electronics have been taken, to check if there could be issues related to it. However, they do not show any unexpected behaviour.
\begin{figure}[h!]
\begin{figure}[H]
\centering
\includegraphics[scale=0.3]{images/EL_NOISE.PNG}
\label{noise}
\caption[Electronic noise]{Electronic noise measurements of CDS and AEI unplugged electronics.}
\label{noise}
\end{figure}
\section{UOB and AEI comparison}
\paragraph*{UOB and AEI comparison}
The shot noise can be computed as:
\begin{equation}
...
...
@@ -556,3 +588,6 @@ where P$_0$ = 3,5 mW is the output power, $\lambda$ = 1064 nm is the laser wavel
