\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}
...
...
@@ -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.
\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 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 allowed the return to the laboratory, to July 2021.
In this chapter I will introduce the 6D device, a new technology for inertial isolation. This project was presented to the scientific community at the 10th ET Symposium in 2019 \cite{poster}. My contribution to the development of this technique focused 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.\\
The experiment was built and tested in-depth: the laser stabilisation results were limited by excess noise in the sensors and many tests were made to identify and reduce noise. During these tests, it was determined that one of the devices was intrinsically noisier than the other.\\
This work was done entirely at UoB: the design of the project was conducted in 2020, while the experiment was built and tested from September 2020, when the University allowed the return to the laboratory, to July 2021.
\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 below 0.1 Hz \cite{6d}. We have already seen the importance for this frequency window to be opened (chap 2): this sensor can be installed on 3rd generation 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 below 10 Hz \cite{6d}. We have already seen the importance for this frequency window to be opened (chap 2): this sensor could be installed on 2nd generation Earth-based interferometers, with major upgrades, on or under ground, allowing the different instruments to easily use the same device.\\
As the name reminds, the 6D investigates the motion of a reference mass in all 6 degrees of freedom, using 6 interferometers. In Fig. \ref{6d} it is shown a sketch of the design of the facility. \\
\begin{figure}[h!]
...
...
@@ -31,8 +32,8 @@ As the name reminds, the 6D investigates the motion of a reference mass in all 6
\end{figure}
\noindent
All six degrees of freedom are simultaneously low-noise, reducing the cross-coupling affecting low force-noise measurements.\\
The reference mass, suspended from a single, thin, fused-silica fibre and a metal spring, provides supports in the vertical (Z) degree of freedom. An interferometric readout provides control in the X and Y tilting degrees of freedom.\\
The interesting advantage is that this system provides isolation in all the degrees of freedom with the use of only one device Currently aLIGO is seismically isolated by three seismometers and twelve geophones \cite{lisa}: the use of the 6D would replace three seismometers and six geophones on Stage 1 of the chambers.\\
The reference mass, suspended from a single, thin, fused-silica fibre, provides supports in the vertical (Z) degree of freedom. An interferometric readout and control are used in all 6 degrees of freedom.\\
The major advantages is that this system can improve sensitivity, thermal noise, and tilt-to-translation coupling, providing isolation in all the degrees of freedom with the use of only one device. Currently aLIGO is seismically isolated by three seismometers and twelve geophones \cite{lisa}: the use of the 6D would replace three seismometers and six geophones on Stage 1 of the chambers.\\
\noindent
What we expect from 6D is isolation at low frequencies and reduction of fundamental noises: the thermal noise of the suspension is suppressed by the quasi-monolithic, fused-silica fibre; temperature gradients are kept under control thanks to the vacuum enclosure.\\
The expected performance is shown in Fig. \ref{6dsens}: the 6D isolator provides an improvement of the performance of more than two orders of magnitude with respect to what is possible with state of the art seismometers \cite{6d}.\\
...
...
@@ -56,13 +57,13 @@ HoQI is a compact, fibre-coupled interferometer with high sensitivity and large
\label{hoqi1}
\end{figure}
\noindent
This device has been designed to sense motion at low frequency, with a sensitivity of 2 $\times$ 10$^{-14}$ m/ $\sqrt(Hz)$ at 70 Hz and 7 $\times$ 10$^{-11}$ m/ $\sqrt(Hz)$ at 10 mHz \cite{hoqi}. This is a result obtained when combining HoQI devices to inertial sensors to create an "interferometric inertial sensor" \cite{sam}. In the frame of compact devices, HoQIs are designed to be very small in size, so they can easily be attached to sensors. They are then ideal for 6D purposes, not only for their high sensitivity, but also for their small size.
This device has been designed to sense motion at low frequency, with a sensitivity of 2 $\times$ 10$^{-14}$ m/ $\sqrt(Hz)$ at 70 Hz and 7 $\times$ 10$^{-11}$ m/ $\sqrt(Hz)$ at 10 mHz \cite{hoqi}. In the frame of compact devices, HoQIs are designed to be very small in size, so they can easily be attached to sensors: good results were obtained when combining HoQI devices to inertial sensors to create an "interferometric inertial sensor" \cite{sam}. They are then ideal for 6D purposes, not only for their high sensitivity, but also for their small size.
\noindent
The six HoQIs used for the 6D device need to be fed by a laser source that is sent into the vacuum chamber: my project focussed on this source, specifically how to stabilize it in frequency.
The six HoQIs used for the 6D device need to be fed by a laser source that is sent into the vacuum chamber: my project focused on this source, specifically how to stabilize it in frequency.
\section{Laser stabilization: requirements and key technology}
The laser chosen as source for 6D is a 1064 nm RIO ORION Laser Module (see Fig. \ref{rio}). This has been chosen for its low frequency noise, inexpensiveness and small size, relatively to other options. The key point in the stabilization of the frequency noise of this source is that the technology will be based on HoQIs: the same devices used by the 6D are sensitive enough to be installed also to stabilize the laser source. This solution is very convenient in terms of costs and presents practical advantages: the HoQIs are known devices, compact in size and, as we will see, they allow the setup to be moved easily (in vacuum or in air), according to the main 6D requirements.
The laser chosen as source for 6D is a 1064 nm RIO ORION Laser Module (see Fig. \ref{rio}). This has been chosen for its low frequency noise, inexpensiveness and small size, relatively to other options. The key point in the stabilization of the frequency noise of this source is that the technology will be based on HoQIs: the same devices used by the 6D, but with a longer arm-length mismatch, are sensitive enough to be installed also to stabilize the laser source. This solution is very convenient in terms of costs and presents practical advantages: the HoQIs are known devices, compact in size and, as we will see, they allow the setup to be moved easily (in vacuum or in air), according to the main 6D requirements.
\begin{figure}[h!]
\centering
...
...
@@ -93,7 +94,7 @@ We can then apply the same relation of. eq. \ref{df} to the arm length of the Ho
\delta f_{stab} = f \times\frac{H}{L_{stab}}\simeq 16 \frac{Hz}{\surd{Hz}},
\end{equation}\\
which is still much lower than the threshold of 5000 Hz/$\surd{Hz}$.\\
Since we want the setup to be as much compact as possible, we need to find the lowest possible L$_{stab}$ which gives an interesting $\delta f_{stab}$, compared to the current performances of RIO Orion and the best products available.\\
Since we want the setup to be as much compact as possible, we need to find the lowest possible L$_{stab}$ which gives an interesting $\delta f_{stab}$, compared to the current performance of RIO Orion and the best products available.\\
In the plot in Fig. \ref{perf} there is the analysis and comparison with two of the best products available.
\begin{figure}[h!]
...
...
@@ -114,8 +115,8 @@ Fig. \ref{free} shows a plot of the measured frequency noise of the Rio Orion la
\end{figure}
\section{Experiment design}
The optical set up for the stabilization of the laser source is built on a 800 mm $\times$ 650 mm breadboard: this choice allows to adjust the position of the laser source easier when the light is sent to the 6D vacuum chamber. The setup includes two RIO Orion lasers with output power 12 mW each, and a double-check of the light signal through an optical heterodyne detection: the beat frequency is monitored to assure that the two light frequencies are as much similar as possible.\\
The frequency of the lasers is tunable via temperature and input modulation: in the first case, the Thermo-Electric-Controller (TEC) is driven by a software provided by the manufacturer, while in the second case the module can be integrated to any software code to apply a modulation voltage between +4 V and - 4 V: the frequency tuning efficiency with this method is around 80 MHz/V, when a sinusoidal modulation is applied at 10 kHz. From specifications, the range of tuning spans between 50 and 100 MHz/V.\\
The optical set up for the stabilization of the laser source is built on a 800 mm $\times$ 650 mm breadboard: this choice allows to adjust the position of the laser source easier when the light is sent to the 6D vacuum chamber. The setup includes two RIO Orion lasers with output power 12 mW each, and a double-check of the light signal through an optical heterodyne detection: the beat frequency is monitored to assure that the two light frequencies are as much similar as possible. The frequency noise from the heterodyne detection is measured by a frequency counter: we chose this device because it is the least ambiguous, lowest noise, lowest systematic-error, best calibrated measure of the relative fluctuations of the two lasers available for our purposes.\\
The frequency of the lasers is tunable via temperature and input modulation: in the first case, the Thermo-Electric-Controller (TEC) is driven by a software provided by the manufacturer, while in the second case the module can be integrated to any software code to produce an output voltage between +4 V and - 4 V, that is sent to the lasers: the frequency tuning efficiency with this method is around 80 MHz/V, when a sinusoidal modulation is applied at 10 kHz. From specifications, the range of tuning spans between 50 and 100 MHz/V.\\
To minimise airflows, the optical setup has been enclosed into a box made of foam.
\paragraph*{Opto-mechanical design}
...
...
@@ -131,13 +132,13 @@ The whole optical setup lies on the bradboard and it is relatively easy to align
\end{figure}
\paragraph*{HoQI design}
HoQIs for 6D laser stabilization have been built to fit the requirements, as shown previously: the adjustable arm mismatch is of 10 cm; the photodiodes have a bigger active area with respect to the one of the HoQIs inside the 6D vacuum chamber, because the laser spot size is larger than the one travelling into the 6D device. Moreover, this type of HoQI is independent from any inertial sensor, so both the arms end with mirrors on steering mounts, instead of corner cubes use to for the 6D sensing technique. Table \ref{hoqi} shows other small details that have been adapted for this experiment.\\
HoQIs for 6D laser stabilization have been built to fit the requirements, as shown previously: the adjustable arm-length mismatch is of 10 cm; the photodiodes have a bigger active area with respect to the one of the HoQIs inside the 6D vacuum chamber, because the laser spot size is larger than the one travelling into the 6D device. Moreover, this type of HoQI is independent from any inertial sensor, so both the arms end with mirrors on steering mounts, and instead of corner cubes use to for the A+ devices. Table \ref{hoqi} shows other small details that have been adapted for this experiment.\\
\begin{table}[h!]
\centering
\begin{tabular}{|c|| c| c| }
\hline
&\textbf{Original}&\textbf{HoQI stab}\\
&\textbf{A+ HoQIs}&\textbf{Laserstab HoQIs}\\
\hline
Platform thickness & 6 mm & 1 cm\\
\hline
...
...
@@ -180,7 +181,7 @@ To acquire our data, we need to connect the HoQIs and the beat-note receiver to
\begin{figure}[h!]
\centering
\includegraphics[scale=0.8]{images/signals.png}
\caption[Scheme of the signals]{Basic scheme of the row signals for laser frequency control: the power detected by the photodiodes is converted into $\mu$A and sent to a pre-amp, one for each HoQI; the pre-amps convert the signal in double-ended voltage to be sent to an ADC. The output of the CDS is a $\pm$ 10 V double-ended signal out of the DAC: since the lasers require a $\pm$ 4 V single-ended input, the double-ended signal is converted into single with a custom differential- to single-ended amplifier of gain 2.5. The calibration of the CDS gives 0.00061 V/counts, which allows to digit into the CDS the right counts divided by the gain of the differential- to single-ended amplifier to obtain the desired voltage to send to the lasers.}
\caption[Scheme of the signals]{Basic scheme of the row signals for laser frequency control: the power detected by the photodiodes is converted into $\mu$A and sent to a pre-amp, one for each HoQI; the pre-amps convert the signal into double-ended voltage to be sent to an ADC. The output of the CDS is a $\pm$ 10 V double-ended signal out of the DAC: since the lasers require a $\pm$ 4 V single-ended input, the double-ended signal is converted into single with a custom differential- to single-ended amplifier of gain 2.5.}
\label{signals}
\end{figure}
\noindent
...
...
@@ -193,19 +194,18 @@ The beat-note receiver is 15 V powered and connected to a frequency counter, and
\label{cables}
\end{figure}
\noindent
The sensing and control system of the experiment is based on the HoQIs: the software code manipulating the HOQIs signal and driving the input modulation is written with Matlab Simulink and controlled by the CDS. The controller filter has been built taking into account all the features of the loop and implemented into the CDS: since we want a frequency modulation below 0.1 Hz and a robust and stable loop at the same time, we built the filter in three different steps. The first step is to run an AC filter with a pole at 0.01 Hz, at unity gain frequency 228 Hz, to AC-couple the feedback of the lasers. Then other two filters are added at lower and higher frequencies to stabilize the gain in the bandwidth we are interested in and make the whole loop stable. The filters are shown in Fig. \ref{filter}: their performance have been tested looking at the stability of the beat-note peak when each filter is switched on, and when they are on together as the full controller filter.\\
The lasers can then be controlled via input modulation through the feedback control loop built, where HoQIs act both as the sensors and feedback devices for the frequency stability.
The sensing and control system of the experiment is based on the HoQIs: the software code manipulating the HOQIs signal and driving the input modulation is written with Matlab Simulink and controlled by the CDS. The controller filter has been built taking into account all the features of the loop and implemented into the CDS: the filters are shown in Fig. \ref{filter}: their performance have been tested looking at the stability of the beat-note peak when each filter is switched on, and when they are on together as the full controller filter.\\
The lasers can then be controlled via input modulation through the feedback control loop built via the CDS, where HoQIs act as the sensors.
\begin{figure}[h!]
\centering
\includegraphics[scale=0.3]{images/filter.png}
\caption[Controller filter]{Bode plot of the three-step controller filter installed into the CDS (green) and of the closed-loop expected gain when this filter is applied (magenta). This design should push the gain from below 0.1 Hz, assuring stability when applied at lower frequencies.}
\caption[Controller filter]{Bode plot of the controller filter installed into the CDS (green) and of the closed-loop expected gain when this filter is applied (magenta). This design should push the gain from below 0.1 Hz, assuring stability when applied at lower frequencies.}
\label{filter}
\end{figure}
\section{Noise hunting}
%\paragraph*{Noise budget}
The performances of the setup depend strongly on the HoQIs because they are the sensing and feedback devices of the setup: the noise budget in Fig. \ref{noiseb} shows that the measured HoQI readut follows the free running frequency noise of the lasers detected by the frequency counter at low frequencies, and then it sits on the ADC noise (estimated as in \cite{hoqi}) at frequencies above 100 Hz. What we are interested in is getting the lowest possible frequency noise from the lasers, reducing the noises affecting the HoQIs. The improvement of HoQIs sensitivity is crucial to obtain the best performances in sensing frequency fluctuations, in order to provide the correct and stable control and feedback to the setup.
The performance of the setup depend strongly on the HoQIs because they are the sensing and feedback devices of the setup: the noise budget in Fig. \ref{noiseb} shows that the measured HoQI readut follows the free running frequency noise of the lasers detected by the frequency counter at low frequencies, and then it sits on the ADC noise (estimated as in \cite{hoqi}) at frequencies above 100 Hz. What we are interested in is getting the lowest possible frequency noise from the lasers, reducing the noises affecting the HoQIs. The improvement of HoQIs sensitivity is crucial to obtain the best performance in sensing frequency fluctuations, in order to provide the correct and stable control and feedback to the setup.
\begin{figure}[h!]
\centering
...
...
@@ -216,7 +216,19 @@ The performances of the setup depend strongly on the HoQIs because they are the
\subsection{Tested noise sources}
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{Offset/gain parameter matching}
When aligning HoQIs, a significant contribution to the results was given by the fact that the fringes needed to be adjusted to match in gain and offsets: this was done via CDS and it provided one of the most important issues (and noise sources) during the tests. Imperfect fringe-visibility and parameter matching dominate the coupling from intensity to measured-phase, and we saw substantial improvements when these values were optimised. We had insufficient diagnostics to fix the issue, mainly because, to create fringes, the HoQIs needed to be mechanically shaken and tuned, and this could be done only manually and in dark room (to avoid room light to affect the offsets), providing imprecise results. We had no proper way to adjust the laser frequency without changing the intensity, and we had no way to adjust the intensity without changing the frequency. This made the offsets and gains change and pollute the measurements. The plots in Fig. \ref{parameters} show the difference between the row fringes generate while shaking HoQI1 and the ones generated after optimizing the parameters. We observed variations in the settings, which needed to be double-checked and readjusted before every measurement. This was an important issue that slowed the tests down and that it is worthy of further tests and data analysis.
\begin{figure}[h!]
\centering
\includegraphics[scale=0.4]{images/row.jpg}\\
\includegraphics[scale=0.4]{images/opt.jpg}
\caption[Row and optimized fringe parameters for HoQI1]{Row and optimized fringe parameters for HoQI1. This fringes has been generated by changing the temperature of laser1: this induced a frequency variation, detected by HoQI. The plots show the row fringes from each photodiode (upper) and the ones after the optimization of the gains and offsets for each photodiode, with respect to a reference one (lower).}
\label{parameters}
\end{figure}
\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.\\
\begin{figure}[h!]
...
...
@@ -226,18 +238,18 @@ The test in Fig. \ref{sound} shows that the setup is sensitive to acoustic noise
\label{sound}
\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 MHz, 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: temperature change induces differential expansion. In a rigid structure this creates stress and some of the bolted connections slide, introducing noise. This issue has been reduced by inserting rubber rings between the connections: this allows the baseplate to expand differentially without creating stress.\\
\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.}
\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 AC creates air currents, and it is also responsible for changes in the temperature of the room and of the lasers, and it can induce dust in the OPL. All these contributions 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 performance}
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.
...
...
@@ -248,7 +260,7 @@ Another reason of concern about HoQI2 behaviour is that it is not consistent wit
\label{hoqi2}
\end{figure}
\paragraph*{Power variations}
\paragraph{Power variations}
The datasheets of the laser modules put in evidence that the power modulation is different for the two lasers: in particular, for laser2 it is higher than laser1 (compare the datasheets reports in \cite{rio} and \cite{riologbook}). This might in part explain why HoQI2 is in general noisier than HoQI1. We monitored the power of both in-loop lasers: according to the datasheet, the power modulation of laser 2 is about 1.56 times higher than for laser 1. In Fig. \ref{power}, the power variation measured at HoQIs input during frequency modulation for the two lasers is consistent with the datasheet. The higher power variations of laser 2 might induce higher intensity fluctuations, which could affect HoQI2.
\begin{figure}[h!]
...
...
@@ -258,26 +270,25 @@ The datasheets of the laser modules put in evidence that the power modulation is
\label{power}
\end{figure}
\subsection{Loop performances}
\subsection{Loop performance}
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 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.
\begin{figure}[h!]
\centering
\includegraphics[scale=0.3]{images/hoqisOLCL.png}
\caption[In-loop test of HoQIs performances]{In-loop test of HoQIs performances. The out-of-loop traces (cyan and purple) are following the free running frequency noise trace (blue) as expected, while when the loop is closed the HoQI outputs (green and red) show that the controllers are pushing the expected gain (orange). There is an evident un-match with the orange trace below 0.4 Hz and this is likely due to loop leakage.}
\caption[In-loop test of HoQIs performance]{In-loop test of HoQIs performance. The out-of-loop traces (cyan and purple) are following the free running frequency noise trace (blue) as expected, while when the loop is closed the HoQI outputs (green and red) show that the controllers are pushing the expected gain (orange). There is an evident un-match with the orange trace below 0.4 Hz and this is likely due to spectral leakage.}
\label{looptest}
\end{figure}
\section{Laser stabilization: tests and results}
The tests have been performed measuring the stability of the beat-note peak around the 60 Hz setpoint: the frequency counter used for this measurements is a Keysight 53230A 350 MHz - 20 ps. The output of the fast photoreceiver is DC-coupled and can be directly connected to the counter. The measurements has been recorded on a USB drive: the data provided by the counter are in frequency (Hz).\\
Several tests have been taken in different conditions for noise hunting along the frequency range of interest, the best measurements are shown in Fig. \ref{test}. The tests with the heterodyne detection revealed that the system is very sensitive to the external noise sources described above and that the two HoQIs are nor robust and stable enough to assure the stability of the loop, despite the robustness of the controller filter. The solution for reducing these noises might be placing the HoQIs in vacuum.\\
A picture of the experiment is in Fig. \ref{expsetup}.
\begin{figure}[h!]
\centering
\includegraphics[scale=0.3]{images/result.png}
\caption[Results of frequency stabilization tests]{Results of frequency stabilization with respect to the free running frequency noise: the in-loop red trace shows the frequency stabilized lasers as detected by the frequency counter, monitoring the beat-note between the two lasers in the lower frequency range. This trace is the best measurement we obtained below 1 Hz, where we reached 1.67 $\times$ 10$^4$ Hz/$\sqrt{Hz}$ at 0.05 Hz; The green trace is a test taken with the counter set to a higher frequency range: this test shows a result of 3.6 $\times$ 10$^3$ Hz/$\sqrt{Hz}$ at 1 Hz. This is also the test which showed the quietest results above 10 Hz, demonstrating that the HoQIs can reach a good level of stability in air. The black trace is the expected gain activated by the controllers, which is set to maximise the stabilization below 1 Hz.}
\caption[Results of frequency stabilization tests]{Results of frequency stabilization with respect to the free running frequency noise: the in-loop red trace shows the frequency stabilized lasers as detected by the frequency counter, monitoring the beat-note between the two lasers in the lower frequency range. This trace is the best measurement we obtained below 1 Hz, where we reached 1.67 $\times$ 10$^4$ Hz/$\sqrt{Hz}$ at 0.05 Hz; The green trace is a test taken with the counter set to a higher frequency range: this test shows a result of 3.6 $\times$ 10$^3$ Hz/$\sqrt{Hz}$ at 1 Hz. This is also the test which showed the quietest results above 10 Hz, demonstrating that the HoQIs can reach a good level of stability in air. The black trace is the expected gain activated by the controllers, which is set to maximise the stabilization below 1 Hz: when it is lower than the green and red curves, we are not limited by loop gain. When it is below the dashed-black curve, there is sufficient loop gain to meet our noise target.}
\label{test}
\end{figure}
...
...
@@ -289,8 +300,8 @@ A picture of the experiment is in Fig. \ref{expsetup}.
\end{figure}
\section{An alternative test}
An alternative test has been made to make sure that the input modulation is effectively reducing the frequency noise of the lasers. Since we found that HoQI2 is noisier than HoQI1 and that laser2 has larger power fluctuations, we decided to use laser1 to feed both lasers. The new concept is to let only HoQI1 be the in-loop sensor, while HoQI2 will act as the out-of-loop sensor.\\
Results are shown in Fig. \ref{alt} and are encouraging: the frequency noise of the out-of-loop sensor is lowered by about one order of magnitude when the controller on laser1 is active. This means that the control loop works well and that there are still more external noise sources that are reducing the performances of the HoQIs, impacting also on the measurement through the heterodyne detection.\\
An alternative test has been made to make sure that the input modulation is effectively reducing the frequency noise of the lasers. Since we found that HoQI2 is noisier than HoQI1 and that laser2 has larger power fluctuations, we decided to use laser1 to feed both HoQIs. The new concept is to let only HoQI1 be the in-loop sensor, while HoQI2 will act as the out-of-loop sensor.\\
Results are shown in Fig. \ref{alt} and are encouraging: the frequency noise of the out-of-loop sensor is lowered by about one order of magnitude when the controller on laser1 is active. This means that the control loop works well and that there are still more external noise sources that are reducing the performance of the HoQIs, impacting also on the measurement through the heterodyne detection.\\
This test confirmed also that HoQI2 is still noisier than HoQI1, especially in closed loop, despite the use of the laser with less power fluctuations: this clarifies that HoQI2 noise arises from other external sources that HoQI1 is non-sensitive to or imperfections of the setup. A test in vacuum could solve the doubts about the external sources and HoQI2 assembly.
\begin{figure}[h!]
...
...
@@ -308,13 +319,8 @@ The results of this experiment showed that it is possible to stabilize the frequ
interferometers of the same type that are used inside the 6D sensor. With this technology, we managed to reach a
frequency stabilization of 3.6 $\times$ 10$^3$ Hz/$\sqrt{Hz}$ at 1 Hz, without the need of installing the
prototype in vacuum.\\
This is already a promising result, but not yet sufficient for the requirements of 6D, especially below 1 Hz.
The HoQIs are, at present times, too sensitive to external noise sources and hence it might
be required putting the setup in vacuum. The alternative test showed an indirect measurement of laser
stabilization, because the out-of-loop HoQI improved its output signal of about one order of magnitude when the
laser was modulated by the controller filter. This test highlighted that the frequency stabilization through the
heterodyne detection depends on the stability and robustness of the HoQIs.\\
This test has already been considered as a possibility during the experiment design: the HoQI setups can be placed inside the 6D vacuum tank independently from the rest of the components of the stabilization breadboard and electronics. Further tests in these conditions, once the vacuum tank for the 6D will be ready, are scheduled at UoB.
This is already a promising result, but not yet sufficient for the requirements of 6D, especially below 1 Hz. The results showed that we are not limited by loop gain, that acoustic noise and vibration are importnt noise sources, that intensity noise and frequency-to-intensity coupling limit performance, and that both HoQIs show different coupling to these effects. The alternative test showed an indirect measurement of laser stabilization, because the out-of-loop HoQI improved its output signal of about one order of magnitude when the laser was modulated by the controller filter. This test highlighted that the frequency stabilization through the heterodyne detection depends on the stability and robustness of the HoQIs. A concrete plan for next tests is to place the setup in vacuum: this will suppress all the external noises and will possibly highlight the intrinsic issues of HoQIs.\\
This test has already been considered as a possibility during the experiment design: the HoQI setups can be placed inside the 6D vacuum tank independently from the rest of the components of the stabilization breadboard and electronics.
@@ -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}.
...
...
@@ -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}
...
...
@@ -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}
...
...
@@ -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!]
...
...
@@ -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.
