The Mesa Beam Update Juri Agresti1, Erika D’Ambrosio1, Riccardo DeSalvo1, Danièle Forest2, Patrick Ganau2, Bernard Lagrange2, Jean-Marie Mackowski2, Christophe Michel2, John Miller1,3, Jean-Luc Montorio2, Nazario Morgado2, Laurent Pinard2, Alban Remillieux2, Barbara Simoni1, Marco Tarallo1, Phil Willems1 1. Caltech/ LIGO 2. LMA Lyon/ EGO 3. University of Glasgow

Overview Introduction Results Tilt sensitivty HOM Fundamental mode LIGO R&D

Introduction Detectors limited by fundamental thermal noise Spectral density scales as 1/wn (different n for each type of noise) Diffraction prevent dramatically increasing beam size LIGO R&D

Introduction Sensed Avg Gaussian beams sample a small portion of mirror’s surface Sensed Avg LIGO R&D

Mesa Beam Optimisation produces the mesa beam Higher peak power Steep rim Slow exponential fall Aspheric profile Steeper fall LIGO R&D

Cavity 7.32 m folded cavity Rigid structure Suspended in custom vacuum tank Flat folding mirror Flat input mirror MH mirror 2x 3.5 m INVAR rod Vacuum pipe LIGO R&D

Results TEM10 We have been able to lock to higher order modes These modes exhibit good agreement with theory TEM10 LIGO R&D

Results - HOM Diffraction around beam baffle eliminated LIGO R&D

Gaussian? MH10 fit Modes resemble HG and LG sets LG10 fit LIGO R&D

The Fundamental? 4mrad tilt Alignment is taxing Long periods were spent aligning input optics and cavity mirrors 4mrad tilt LIGO R&D

Improving Alignment The reference during alignment was changed from the intensity profile to the transverse mode spectrum LIGO R&D

The First Mesa Beam LIGO R&D

Non-Linear Fit X LIGO R&D

Non-Linear Fit Y LIGO R&D

How Was This Achieved? We have reinforced flexible mirrors with aluminium rings Thicker substrates have been ordered LIGO R&D

How Was This Achieved? Improved atmospheric isolation Better stability ‘in lock’ LIGO R&D

How Was This Achieved? Improved spectrum More power in the fundamental mode Improved spectrum LIGO R&D

Alignment LIGO R&D

Best Mesa Beam Rsq = 0.996 Rsq = 0.992 LIGO R&D

Jagged top due to imperfect mirrors Best Mesa Beam Jagged top due to imperfect mirrors LIGO R&D

Tilts of Spherical Mirrors Tilts of spherical mirrors translate optical axis LIGO R&D

Tilt Sensitvity Controllability of beam is key Decided to first investigate tilt sensitivity Tilt MH mirror about a known axis LIGO R&D

Profiles Profiles along tilt axis 2 mrad simulated 3.85 mrad experiment 2.57 mrad experiment LIGO R&D

Excuses Lack of temporal stability Stiction PZTs are bad vacuum? LIGO R&D

Summary We are able to produce acceptable flat-topped beams with imperfect optics We have begun to make a quantitative analysis of mesa beam Beam size appears correct Tilt sensitivity shows correct trends LIGO R&D

Further Work With This Set Up Improve profile using new flat mirrors Repeatability/ stability – vacuum operations Complete tilt sensitivity measurements Test other two MH mirrors – mirror figure error tolerances Long term – design and build half of a nearly concentric MH Cavity LIGO R&D

Grazie Riccardo DeSalvo Phil Willems Mike Smith GariLynn Billingsley Marco Tarallo Juri Agresti Chiara Vanni LIGO R&D