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Study of scattered light in the Advanced Virgo+ gravitational wave detector.

LAPP offers a Master 2 internship in order to participate in the characterization of the light scattered on the suspended optical benches of the Virgo interferometer. To this purpose, the intern student will be asked to identify the possible sources of light diffused on these optical benches and to simulate the propagation of stray light.

Summary of the work required :

The Virgo gravitational wave detector, installed in Pisa, works on the principle of a laser interferometer with 3 km long arms. In 2017, the first joint observation of gravitational waves, detected by the LIGO and Virgo detectors, from the coalescence of a binary neutron stars system, and a gamma-ray burst, followed by the observation of an optical signal, opened a whole new chapter in multi-messenger astronomy. A period of joint observation of LIGO (in the United States) and Virgo detectors followed from April 2019 to March 2020, allowing the weekly detection of coalescences of compact binary systems (notably black holes). Since 2020, the Virgo detector has entered a phase of modifications and improvements (upgrade Advanced Virgo +) with the goal of entering a new observation period in the second half of 2022. Several optical benches, suspended and placed under vacuum, are used to extract the laser beam in different locations (at the output of the interferometer, but also in transmission of the kilometer arms, for example). These benches house optical elements and light sensors (photodiodes, quadrant photodiodes, cameras, etc.) which have the role of extracting the signals useful for the detection of gravitational waves and for the control of the position of the interferometer mirrors. These elements can diffuse part of the incident light. This unwelcome diffused light can recombine with the main beam, and by interference generate variations in the laser power measured on the main photodiodes, variations which are interpreted as the passage of a gravitational wave. In addition, this scattered light also couples by modulating the radiation pressure of the laser beam on the interferometer mirrors, generating a real variation in differential length of the interferometer arms. By these various couplings, this scattered light therefore limits the sensitivity of the detector. In the context of the Advanced Vigo + upgrade, a review of the optical benches is undertaken in order to identify mitigation actions to reduce the scattered light and thus improve the sensitivity of the detector. LAPP offers a Master 2 internship in order to participate in the characterization of the light scattered on the suspended optical benches of the Virgo interferometer. To this purpose, the intern student will be asked to identify the possible sources of light diffused on these optical benches and to simulate the propagation of stray light. During this study, the intern student will rely on a small experiment installed at LAPP which makes it possible to measure the light back scattered by various optical elements. This experiment uses a simple Michelson interferometer, with a suspended diffusing element at the end of one of the arms. Hence, the results obtained with the optical simulation of stray light will be compared to experimental measurements. The proposed study should make it possible to refine the projections of scattered light noise on the Virgo sensitivity curve, and to define possible countermeasures to reduce this noise.

Optional information on a possible PhD subject :

This internship can lead to a PhD subject encompassing the problem of scattered light in the detection system, the study of its effect on the Virgo sensitivity and, more generally, the study and improvement of the sensitivity of the Virgo detector during the next data-taking run O4 (2022-2023), as well as the preparation and the implementation of the upgrades that will improve further the Virgo sensitivity for the future data-taking run O5 (2025).

o Level : Master 2 internship, can be followed by a PhD

o Supervisor : Romain Gouaty

o Phone : 04 50 09 55 19

o e-mail : gouaty@lapp.in2p3.fr