Scattered Light in Virgo F Fidecaro for A

Scattered Light in Virgo F. Fidecaro for A. Chiummo, A. Allocca and before: B. Canuel, R. Day, E. Genin, J. Marque, G. Vajente VIR-0533 A-19

GW Detectors - Noise EGO - VIRGO q Not only fundamental noises. . . Thermal noise (coating + suspension) Radiation pressure fluctuation Stray light Residual gas (phase noise) Residual laser noise . . . a lot of painful “bolts and nuts” work was done on the detectors to decrease not-so-fancy noises 2 Stray light EM field quantum noise GWADW 2019 • Seismic vibration • Newtonian noise

First studies in Virgo After pioneering studies by K. Thorne (1989 unpublished) Use of Monte Carlo methods for photon tracking within GW community Studies in arm pipes 1. 2 m f 80 baffles radius 0. 5 m 3 GWADW 2019 EGO - VIRGO

Results EGO - VIRGO Studies in arm pipes 1. 2 m f 80 baffles with radius 0. 5 m W 0 2 cm 1 ppm scattering on mirror No baffles M / / Forward M / / Backward 4 1 mm 2 reflecting patch / / / / M M 40 Reflection of baffle edges 5 Diffraction of baffle edges 4 40 % steel baffle absorption 15 50 % steel baffle absorption 14 60 % steel baffle absorption 7 40 % steel baffle absorption 9 50 % steel baffle absorption 6 60 % steel baffle absorption 3 GWADW 2019

Downstream the arms EGO - VIRGO q Higher Finesse: more coupling into higher order modes q Third generation: angles are smaller, seismically active walls nearer to the beam mirror Payload baffle Cryotrap baffle L= 3 km 5 GWADW 2019

Possible paths to troubles q. Wide angle scattering from the core mirrors (m-roughness) EGO - VIRGO q. Low angle scattering from the core mirrors (figure defects) This path requires at least three scattering events to recombine 6 GWADW 2019

Possible paths to troubles: motion + q. Stray-light on/from the benches (backscattering, back-reflections, clipping) EGO - VIRGO q. Ghost beams (multiple reflections) This path may require just one scattering event to recombine 7 GWADW 2019

Stray Light noise: 3 -scatter process EGO - VIRGO From R. Day, G 1300532 8 GWADW 2019

Stray Light noise: 3 -scatter process EGO - VIRGO From R. Day, G 1300532 9 GWADW 2019

Stray Light noise: 3 -scatter process EGO - VIRGO At the ETM we get a rather finely structured speckle pattern From R. Day, G 1300532 10 GWADW 2019

Stray Light noise: 3 -scatter process EGO - VIRGO From R. Day, G 1300532 11 GWADW 2019

Stray Light noise: 3 -scatter process EGO - VIRGO From R. Day, G 1300532 12 GWADW 2019

Stray Light simulations q Non-linear coupling (fringe-wrapping) Regardless of the amplitude of motion of the scatterer, scattered light phase modulation cannot exceed 2 pi. For large amplitude we must use an effective displacement: 13 GWADW 2019 EGO - VIRGO

A case study from the past EGO - VIRGO Limiting the possible displacement amplitude is a good practice Swinkels et al, VIR-0560 A-11 14 GWADW 2019

Stray Light noise measurement q Case study: the B 4 ghost beam § A pick-off plate (POP) samples the beam circulating in the com branch of the short Michelson § The pick-off beam is called B 4 15 GWADW 2019 EGO - VIRGO

Stray Light noise measurement q Case study: the B 4 ghost beam § A pick-off plate (POP) samples the beam circulating in the com branch of the short Michelson § The pick-off beam is called B 4 § B 4 is sent to a bench (SPRB) § the spurious reflection (B 4 ghost) goes to a blind flange § B 4 ghost was not properly dumped during O 2 16 GWADW 2019 EGO - VIRGO

Stray Light noise measurement EGO - VIRGO q Possible because of single excitation point From VIR-0730 A-17 But B 4 is used for SSFS and MICH control. Stray light enters the control signals of ancillary dofs 17 GWADW 2019

Stray light noise from benches EGO - VIRGO A different class of problem: Differently from the scattering off the core mirrors, where a 3 scattering process is needed to recombine with the main beam, here a single-scattering process (backscattering off tilted components) or even simple specular reflection could also play a role. Very difficult to simulate, and therefore to control by design Both analytical and monte-carlo approaches employed to simulate the benches, but a full coherent simulation of the scattered field is still missing 18 GWADW 2019

Stray light noise from benches EGO - VIRGO q. Analytical approach: Displacement noise from back scattering and specular reflection of input optics in advanced gravitational wave detectors B. Canuel, E. Genin, G. Vajente, and J. Marque 6 May 2013 | Vol. 21, No. 9 | DOI: 10. 1364/OE. 21. 010546 | OPTICS EXPRESS 10546 19 GWADW 2019

Full geometry EGO - VIRGO � In principle Autocad/Solidworks/… description files are available � This is more likely to happen in the future as there will a significant engineering effort for a 1 GEuro class project � More computing power and software techniques are available � More culture (within younger physicists) � Other fields: GEANT 4 Montecarlo descriptions include support materials and screws � Track from the very high energy to the very soft photons and electrons � Now used for medical treatments � 20 GWADW 2019

Stray light noise from benches Optocad Ray tracing. . . Work started in a within a ph. D thesis: JM Gonzalez Castro Something similar in LIGO 21 GWADW 2019 EGO - VIRGO track motion

Conclusion � EGO - VIRGO Stray-light is an old enemy of GW interferometric antennas � � It comes from a diversity of possible defects It can probe seismically excited mechanical structures and recombine with the ITF main beam burying GW signals It is difficult to simulate It is ineherently non-linear Despite huge efforts, it threatens the achievement of design sensitivity, in particular at low frequency � Further advances in prediction, mitigation and monitoring are needed � New materials with lower BRDF are to be explored for 3 rd gen, and possibly chose a site that is. . . � 22 GWADW 2019

Future prospects: what is needed. . . A quiet place Displacement spectrum and RMS have to be low 23 GWADW 2019 EGO - VIRGO