updates to laserstab chapter

main/GW.tex

@@ -18,7 +18,7 @@ The challenging goal of detecting gravitational waves opened a research field de
 This research is important, because detecting gravitational waves means looking at the sources which produced them. There is still a gap in the knowledge of many astrophysical objects, such as Black Holes (BH), Neutron Stars (NS), Supernova events: this new-born branch of astrophysics will help to fill the gap and increase our knowledge of the Universe.\\
 
 \noindent
-The detectors currently in use are sensitive to events from sources emitting at frequencies above $\sim$ 10 Hz, but there is still a broad range of frequencies to which the detectors are blind. Looking at different frequencies of emission means looking at different objects emitting gravitational waves. This would broaden the catalogue of observed objects and the changes to better understand their nature.\\
+The detectors currently in use are sensitive to events from sources emitting at frequencies above $\sim$ 10 Hz, but there is still a broad range of frequencies to which the detectors are blind. Looking at different frequencies of emission means looking at different objects emitting gravitational waves. This would broaden the catalogue of observed objects and the chances to better understand their nature.\\
 
 \noindent
 The work carried on during my PhD studies and exposed in this thesis has been dedicated to the improvement of the sensitivity of the detectors at frequencies below 10 Hz, by the development of new ideas and technologies to reduce noise sources affecting the low-frequency bandwidth, in particular the seismic motion.

main/laserstab.tex

@@ -17,7 +17,7 @@
 %\begin{document}
 \chapter{Laser stabilization for 6D isolation system device}
 In this chapter I will introduce the 6D device, a new technology for inertial isolation. This project has been presented to the scientific community at the 10th ET Symposium in 2019 \cite{poster}. My contribution to the development of this technique focussed on the sensing side: a laser will be injected into the device and will need to be stabilized in frequency. To do it, we propose a new technique based on compact interferometry.\\
-This work has been done entirely at UoB: the design of the project has been conducted in 2020, while the experiment has been built and tested from September 2020, when the University accorded me the permission to return to the lab.
+This work has been done entirely at UoB: the design of the project has been conducted in 2020, while the experiment has been built and tested from September 2020, when the University accorded me the permission to return to the lab, to July 2021.
 
 \section{6D inertial isolation system overview}
 The 6D inertial isolation system is a device based on a new technology under development at University of Birmingham and at Vrije Univestiteit in Amsterdam, which could enable detection of gravitational waves at 10 Hz and below \cite{6d}. We have already seen the importance for this frequency window to be opened (chap 2): this facility can be installed on Earth-based interferometers of every type, on or under ground, allowing the different instruments to easily use the same device.\\
@@ -203,7 +203,7 @@ Models of our system are designed with Symulink and the controller filters is de
 \end{figure}
 
 \section{Noise hunting}
-\paragraph*{Noise budget}
+%\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.
 
 \begin{figure}[h!]
@@ -213,10 +213,10 @@ The performances of the setup depend strongly on the HoQIs because they are the
 \label{noiseb}
 \end{figure}
 
-\paragraph*{Tested noise sources}
+\subsection{Tested noise sources}
 There are several noise sources to take into account: air currents and vibrations from electronics and cables have been reduced placing the optical setup into a foam box and moving the electronic devices suitably. Cables have been isolated from the table and the breadboard by rubber feet.\\
-\noindent
-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 detected 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.\\
+\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!]
 \centering
@@ -225,14 +225,26 @@ The test in Fig. \ref{sound} shows that the setup is sensitive to acoustic noise
 \label{sound}
 \end{figure}
 
-\noindent
+\paragraph*{The role of the temperature}
 Temperature changes affected dramatically the measurements: the two lasers can be driven also via temperature modulation. This method has been used to move the beat-note peak along the frequencies and set it around 60 Hz, being this the setpoint we decided for it. However, both laser modules are sensitive to changes of the room temperature, which make the peak move out from the setpoint on large time scales (~hours): this affects long time measurements. The stabilization of the room temperature requires the use of the air conditioning, which in turn creates air currents visible by the setup below 1 Hz.\\
-Temperature changes 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, 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 reduce by inserting rubber rings between the junctions where different metals are mounted.\\
+Temperature changes are also responsible for deformations of metals; this induces noises into the HoQI platforms because of the different materials they are built of: platform, screws and post holders expand in different ways with temperature changes, and this produces deformations and friction between the metals, which translate into displacement noise visible by the HoQIs. This issue has been reduced by inserting rubber rings between the junctions where different metals are mounted.\\
 
-\noindent
-Another noise we noticed from the datasheets of the laser modules is the intensity fluctuations: in particular, laser2 is more affected by this noise than laser1 (compare the datasheets reports in Fig. ...). This in part explains why HoQI2 is in general noisier than HoQI1, as shown in the following sections.
+\paragraph*{HoQIs performances}
+When monitoring the output of the HoQIs, we noticed that HoQI2 is much noisier than HoQI1: Fig. \ref{hoqi2} shows an out of loop measurement of the output of both HoQIs. This discrepancy has been investigated: possible reasons for that could arise from the laser source of HoQI2, alignment and clipping on the optics, fringe visibility, spurious light, mechanical defects in HoQI2 setup. The laser source has been changed to be the same as HoQI1 and further tests showed that HoQI2 is performing the same way. This relieved the laser of any responsibility, since now the same source is feeding the two HoQIs in the same way. The alignment and the clipping on the optics have been carefully checked and possible sources of stray lights have been meticulously covered. The fringe visibility has been double-checked: the test is still showing more noise from HoQI2 output. What remains to inspect is the possibility of mechanical defects in the optics or in the setup of HoQI2, the latter being a concrete possibility due to errors in manufacturing.\\
+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. 
+
+\begin{figure}[h!]
+\centering
+\includegraphics[scale=0.3]{images/hoqi2_issue.png}
+\caption[Difference between HoQI1 and HoQI2 outputs]{Difference between HoQI1 and HoQI2 outputs in an open-loop test.}
+\label{hoqi2}
+\end{figure}
 
-\paragraph*{Loop performances}
+\paragraph*{Intensity fluctuations}
+%Another noise we noticed from the datasheets of the laser modules is the intensity fluctuations: in particular, laser2 is more affected by this noise than laser1 (compare the datasheets reports in Fig. ...). This in part explains why HoQI2 is in general noisier than HoQI1, as shown in the following sections.
+We also monitored the power of the lasers
+
+\subsection{Loop performances}
 The behaviour of the two HoQIs has been tested in loop and out of loop, to check if they are detecting and responding correctly to the injection of the controller filters through the input modulation of the laser modules. The expectation is that the HoQIs output in out-of-loop mode should show the injection of the gain: Fig. \ref{looptest} shows that the expectations are satisfied.\\
 This test shows that HoQI2 is in general noisier than HoQI1, especially above 1 Hz: this affects laser stabilization measurement and loop stability, thus it has been deeply investigated. The higher intensity fluctuations of laser2 can partially explain the reason of HoQI2 noise.
 
@@ -254,10 +266,10 @@ Several tests have been taken in different conditions for noise hunting along th
 \label{test}
 \end{figure}
 
-\section{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 intensity 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.\\
+\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.\\
-This test highlights also that HoQI2 is still noisier than HoQI1, despite the use of the laser with less intensity fluctuations: this clarify that HoQI2 noise arises from other external sources that HoQI1 is non-sensitive to or imperfections into the optics. A test in vacuum could solve the doubts about the external sources.
+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!]
 \centering
@@ -267,7 +279,22 @@ This test highlights also that HoQI2 is still noisier than HoQI1, despite the us
 \end{figure}
 
 \section*{Conclusions}
-%installation into 6D
+The results of this experiment showed that it is possible to stabilize the frequency of the laser source of the 
+6D device using the technology presented: a compact, easy to handle setup which makes use of small
+interferometers of the same type that are used inside the 6D sensor. With this technology, we managed to reach a
+frequency stabilization of 8 $\times$ 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.
+
+
+
+
 
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@@ -289,3 +316,4 @@ This test highlights also that HoQI2 is still noisier than HoQI1, despite the us
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+