@@ -20,7 +20,7 @@ In this chapter I will introduce the 6D device, a new technology for inertial is
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.
\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 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.\\
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.\\
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!]
...
...
@@ -62,7 +62,7 @@ This device has been designed to sense motion at low frequency, with a sensitivi
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.
\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 tank 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 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
...
...
@@ -71,7 +71,7 @@ The laser chosen as source for 6D is a 1064 nm RIO ORION Laser Module (see Fig.
\label{rio}
\end{figure}
\noindent
What we want from this source is a low-noise readout for the HoQIs inside the 6D tank, and thus the laser source needs to be as low noise in frequency fluctuations as possible at frequencies arond 100 mHz, because this is the range of frequencies where the 6D isolator is aimed to detect and control seismic noise: we are going to use two Rio Orion laser modules to obtain a frequency stabilization suitable for 6D requirements. Constraints to these requirements are mainly given by the HoQIs. For 6D readout, HoQIs are built in such a way that the arm length mismatch is as small as practically possible, e.g. L$_{6D}$$<$ 3 mm. Limitations to this number are given by BOSEM size ($\pm$ 2 mm) and the ability to adjust it, once the devices are in vacuum. Another parameter to take into account is the noise of HoQIs, which is H = 6 $\times$ 10$^{-14}$ m/$\surd{Hz}$ at about 1 Hz \cite{hoqi}. Frequency fluctuations depend on both these parameters and we want it to meet the following requirement:
What we want from this source is a low-noise readout for the HoQIs inside the 6D tank, and thus the laser source needs to be as low noise in frequency fluctuations as possible at frequencies below 0.5 Hz, because this is the range of frequencies where the 6D isolator is aimed to detect and control seismic noise: we are going to use two Rio Orion laser modules to obtain a frequency stabilization suitable for 6D requirements. Constraints to these requirements are mainly given by the HoQIs. For 6D readout, HoQIs are built in such a way that the arm length mismatch is as small as practically possible, e.g. L$_{6D}$$<$ 3 mm. Limitations to this number are given by BOSEM size ($\pm$ 2 mm) and the ability to adjust it, once the devices are in vacuum. Another parameter to take into account is the noise of HoQIs, which is H = 6 $\times$ 10$^{-14}$ m/$\surd{Hz}$ at about 1 Hz \cite{hoqi}. Frequency fluctuations depend on both these parameters and we want it to meet the following requirement:
\begin{equation}
\centering
...
...
@@ -120,7 +120,7 @@ To minimise airflows, the optical setup has been enclosed into a box made of foa
\paragraph*{Opto-mechanical design}
The optical layout is shown in Fig. \ref{las}: the two lasers have a twin optical layout. There is a Faraday Isolator (FI) at each output and then a 1 to 4 fibre beam splitter (BS) which separates the beam into 4 outputs of equal power: 3 outputs go into the vacuum chamber (for a total of 6 laser inputs, one for each 6D HoQI into the vacuum chamber). The remaining output is sent through a fibre coupler to a Schafter-Kirchhoff collimator and gives an output of about 1.2 mW for each laser; this proceeds freely on the breadboard towards a 1 inch, 10/90 (R/T) beam splitter: 10$\%$ of the light is sent to a fast DC coupled 125-MHz photoreceiver (PD) acted to sense the beat-note of the two lasers; two 1 inch mirrors deviate one of the two laser beams towards the transmitting surface of another 1 inch beam splitter, which combines the light from both lasers towards the photoreceiver; the other 90$\%$ of it is sent to the HoQIs, one for each laser. The optical path lengths (OPL) have been set to be equal, to assure the same beam size from both lasers at the photoreceiver.\\
The photoreceiver has strict constraints about the beam size and the input power: a focussing lens in front of the active area assures that the beam size is suitable to fit the 0.3 mm active area. Damping filters are added along the OPL, because the maximum input power of the device is 55 $\mu$W.\\
The photoreceiver has strict constraints about the beam size and the input power: a focussing lens in front of the active area assures that the beam size is suitable to fit the 0.3 mm active area. Neutral density damping filters are added along the OPL, because the maximum input power of the device is 55 $\mu$W.\\
The whole optical setup lies on the bradboard and it is relatively easy to align because all the optomechanical components have been manufactured to make the beams out of the collimator to travel at the same height as HoQI components and the photoreceiver, so that there is no need of pitch tuning.
\begin{figure}[h!]
...
...
@@ -189,7 +189,7 @@ The beat-note receiver is 15 V powered and connected to a frequency counter, and
\begin{figure}[h!]
\centering
\includegraphics[scale=0.7]{images/cables.png}
\caption[Scheme of the electronics]{Detailed scheme of the electronics designed for this experiment. The different colors of the arrows represents different types of cables. Bacardi and Peapsy are the CDS and the computer controlling it, as we named them at UoB. Differential to single ended converters (Diff2SE) are needed because the CDS supports differential outputs while the pre-amps are single ended. The Mokulab is the device used as an oscilloscope and/or as a spectrum analyser, connected to the beat-note fast photoreceiver. Green arrows indicate power supplies. The frequency counter can be connected to a computer to acquire data or a USB drive can be inserted to save data directly from the device. The temperature modulation requires the use of a software provided by the manufacturer and installed on computers. Each laser needs its own software connection. }
\caption[Scheme of the electronics]{Detailed scheme of the electronics designed for this experiment. The different colors of the arrows represents different types of cables. Bacardi and Peapsy are the front-end and the workstation controlling it, as we named them at UoB. Differential to single ended converters (Diff2SE) are needed because the CDS supports differential outputs while the pre-amps are single ended. The Mokulab is the device used as an oscilloscope and/or as a spectrum analyser, connected to the beat-note fast photoreceiver. Green arrows indicate power supplies. The frequency counter can be connected to a computer to acquire data or a USB drive can be inserted to save data directly from the device. The temperature modulation requires the use of a software provided by the manufacturer and installed on computers. Each laser needs its own software connection. }
\label{cables}
\end{figure}
\noindent
...
...
@@ -210,7 +210,7 @@ The performances of the setup depend strongly on the HoQIs because they are the
\caption[Noise budget]{Noise budget of the laser stabilization setup. The paper which provided the HoQI noise and the ADC noise is given by \cite{hoqi}.}
\caption[Noise budget]{Noise budget of the laser stabilization setup, in free air with the controller turned off. The paper which provided the HoQI noise and the ADC noise is given by \cite{hoqi}.}
\label{noiseb}
\end{figure}
...
...
@@ -227,7 +227,7 @@ The test in Fig. \ref{sound} shows that the setup is sensitive to acoustic noise
\end{figure}
\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 10 Hz (Fig. \ref{ACtest} shows the difference between two tests taken with and without AC).\\
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.\\
