GW-Instrumentation
Context
The direct detection of gravitational waves (GWs) is a breakthrough discovery of recent years. The several GW detections by the Advanced LIGO and Virgo detectors, since they were first discovered in 2015, have opened up a new window to the observable universe. The current “second generation” detectors are Michelson interferometers with km-scale arms. The principle behind the detector is based on the fact that when a GW passes through, the arms of the interferometer are stretched and squeezed in opposite ways in the two directions.
Effect of a gravitational wave (propagating in the direction perpendicular to the plane of the paper) on the arms of a Michelson interferometer.
Current GW detectors like LIGO and Virgo are Michelson interferometers with km-scale arms and a laser source of 1064 nm. However, to reduce dominant noise sources, third-generation GW detectors are planned to be built in the next decades. One of these detectors is the Einstein Telescope (ET), which is planned to be built in Europe in the 2030s. Unlike current detectors, ET will be composed of two interferometers, one optimised at low frequencies (ET-LF) and one at high frequencies (ET-HF). In order to improve the low frequency sensitivity, ET-LF will introduce, amongst other things, the use of a longer wavelength for the laser source (1550 nm or 2000 nm). ET-HF, on the other side, will still work with a laser source at 1064 nm, because the improvement of the high frequency sensitivity does not require a change in the wavelength.
The Ghent Gravity Group recently set up an optical laboratory with a 2µm laser source for the development of optical devices for next-generation GW detectors. On the other side, R&D with a 1064 nm laser source is still ongoing and necessary Further upgrades in the Advanced Virgo detectors are planned for the next decade with the purpose to continue the observations until the beginning of the ET operations. These upgrades will be essential for the improvement of the sensitivity and the robustness of the Virgo detector, but also for the development and test of systems that can be used in future detectors.
Motivation
The student would get involved with the technology and challenges of interferometric detectors in the active field of gravitational-wave science.
GW 1: Second harmonic generators for 2µm wavelength
Supervisors: Daniela Pascucci, Archisman Ghosh
CONTACT | TOPIC 44521 ON PLATO
Second harmonic generator (SHG), also known as frequency doubling, is a mechanism happening when a light beam passes through a material with non‐linear dielectric coefficient.
Using non-linear crystals as SHG to create green light from an infrared light source has been widely used in the past in a wide range of applications, like display technology, biomedicine, etc. Also in GW instrumentation this technique is not new. In fact, it is used in Advanced Virgo to generate from a 1064nm input source the green light of auxiliary lasers needed for the cavity control system and the squeezing system. However, it has never been used for 2 μm wavelength. Furthermore, since one of the biggest challenges for next generation detectors is the development of high‐efficiency photodetectors, like cameras and photodiodes, and this study can be a first step to develop a method can be used to overcome the problem, changing the wavelength of the laser beam before it reaches the photodetector. Currently the most used materials (from IR to green) are the Periodically-poled Lithium Niobate (LiNbO3 or PPLN) and the Periodically Poled-Potassium Titanyl Phosphate (KTiOPO4 or PPKTP). However, other materials will be studied and taken into account.
The main objective of the proposed research is to study possible SHG materials for 2 μm wavelength, analysing if they are suitable for GW detectors.
Schematic representation of how a non-linear optical medium acts as a second harmonic generator.
Pictures of two possible materials suitable for 2µm wavelength. Left panel: Periodically-poled Lithium Niobate (LiNbO3 or PPLN). Right panel: Periodically-poled Potassium Titanyl Phosphate (LiNbO3 or PPKTP).
GW 2: Calibrating GW detectors using scattered light
Supervisors: Daniela Pascucci, Archisman Ghosh
CONTACT | TOPIC 44514 ON PLATO
With the increase of the sensitivity, gravitational-wave detectors will require calibration with better accuracy and precision. Currently, gravitational-wave detectors are calibrated using two independent methods, in order to be able to cross-check the results. The two methods are the Photon Calibrator (PCal) and Newtonian calibration method (NCal) and for both of them the calibration is done inducing a displacement on the mirrors.
A new independent technique which can use scattered light as a signal for the detector calibration was recently proposed [1]. The scattered light is injected in the interferometer from the back of the end mirrors using a scattering element that can be modulated in amplitude. Unlike the other two, this technique does not involve the movement of the mirrors and thus it can be very useful to cross-check the results and be sure that the mirror suspensions do not add any error in the model.
The Ghent Gravity Group, in collaboration with the University of Antwerp, is planning to develop for the Advanced Virgo detector this new calibration technique. This is a very innovative work, since this technique has not yet been used in any other GW detector so far.
Layout of Advanced Virgo. The scattered light used for the calibration would come from the Suspended West and North End Benches (SWEB and SNEB) behind the West End (WE) and North End (NE) mirrors.
References[1] M Wąs et al 2021 Class. Quantum Grav.38 075020
GW 3: Modelling the effect of Newtonian noise for the Einstein Telescope
Supervisors: Archisman Ghosh
CONTACT | TOPIC 44797 ON PLATO
- A schematic of Newtonian-noise coupling to the test-mass. (b) Contribution of different fundamental noises to ET’s sensitivity with Newtonian noise dominating the low-frequency contributions
Newtonian noise or gravity gradient noise is an effect of the change of the Newtonian gravitational potential due to fluctuations of displacements on the surface of the Earth. Although it is a small effect for current GW detectors, it can become a major nuisance for future detectors such as the ET. Newtonian noise couples to a test mass in a manner very similar to GWs, and it is not possible, even in principle, to remove this noise source (except via active subtraction). It is therefore important to understand and quantify the impact of Newtonian noise. While estimation of Newtonian noise from homogeneous media has been well explored [1], a lot remains to be known in cases of complex, heterogeneous media.
In this project, you will analyze the generation mechanism of Newtonian noise and simulate scenarios that will best represent the contribution of Newtonian noise to ET’s low-frequency sensitivity. Simulations of elastic waves in a heterogeneous medium will be performed using a spectral finite element solver SPECFEM3D. A software library will be developed to compute the gravitational acceleration on the interferometer’s test-masses, based on the simulated displacement of the medium’s elements. In particular, focus will be given to the understanding of contributions from different parts of the medium like interfaces, and cavern walls that host the vertices of ET.
You will work in collaboration with researchers in the University of Liège, including Dr. Soumen Koley, who is one of the leading experts in modeling Newtonian noise for GW detectors.
Your results may have a significant impact in bringing the ET to Belgium!
GW 4: Optical layout of the ETpathfinder
Supervisors: Daniela Pascucci, Archisman Ghosh
CONTACT | TOPIC 44520 ON PLATO
Ghent University has been involved in the ETpathfinder project since its beginning. The facility in Maastricht is currently still under construction, with the aim to have the interferometer up and running by the end of this year.
Although the layout of ETpathfinder is broadly defined, optical simulations are needed to define position and specifications of all auxiliary optics used to have the proper beam parameters at each step of the path. You will be actively involved in the development of ETpathfinder, working on optical simulations to define the final layout.
You will be a member of the Einstein Telescope Collaboration. You will get involved with the technology and challenges of interferometric detectors in the active field of gravitational-wave science. The project will be done in close coordination with the ETpathfinder community. Travel to Maastricht, where the facility is located, may be required.
GW 5: Optical characterization of 2μm photodetectors
Supervisors: Daniela Pascucci, Archisman Ghosh
CONTACT | TOPIC 44519 ON PLATO
Next generation GW observatories such as the Einstein Telescope (ET) will use silicon mirrors as test masses to detect low frequency GW with unprecedented sensitivities. This implies the use of new laser wavelengths at 1550nm or 2090nm. The performance of such optics will be first tested at the ETpathfinder facility in Maastricht. To detect the main interferometer signal, coming from the GW strain, photodiodes with a high quantum efficiency (HQE) and a large dynamic range for these new wavelengths are needed. At 1550nm HQE InGaAs sensors exist, however at 2090nm more development is needed and custom made extended InGaAs photodiodes with optimized quantum efficiency, especially for 2090 nm, will need to be acquired and tested. In this project we will contribute to the development of such novel 2090nm HQE photodiodes by performing optical characterizations of available photodiodes.
UGent has recently set up an optical lab for characterization of optical components for 2090nm. You will set up a measurement system at UGent optical lab with existing readout electronics (available in UAntwerpen) to test the sensor for e.g. dark noise, efficiency, and optical power levels. The work involved will be hands-on instrumentational, with focus on optics and mechanics. The project will be under joint supervision with Prof. Hans Van Haevermaet (UAntwerpen). You will also get a chance to work at the ETpathfinder facility in Maastricht and be a part of the exciting activity in the Belgian-Dutch region centred around the Einstein Telescope.
