1. 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

2. Overview Introduction Results Tilt sensitivty HOM Fundamental mode
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3. 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
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4. Introduction Sensed Avg
Gaussian beams sample a small portion of mirror’s surface
Sensed
Avg
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5. Mesa Beam Optimisation produces the mesa beam Higher peak power
Steep rim
Slow exponential fall
Aspheric profile
Steeper fall
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6. 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
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7. Results TEM10 We have been able to lock to higher order modes
These modes exhibit good agreement with theory
TEM10
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8. Results - HOM
Diffraction around beam baffle eliminated
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9. Gaussian?
MH10 fit
Modes resemble HG and LG sets
LG10 fit
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10. The Fundamental? 4mrad tilt Alignment is taxing
Long periods were spent aligning input optics and cavity mirrors
4mrad tilt
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11. Improving Alignment
The reference during alignment was changed from the intensity profile to the transverse mode spectrum
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12. The First Mesa Beam
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13. Non-Linear Fit X
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14. Non-Linear Fit Y
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15. How Was This Achieved?
We have reinforced flexible mirrors with aluminium rings
Thicker substrates have been ordered
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16. How Was This Achieved? Improved atmospheric isolation
Better stability ‘in lock’
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17. How Was This Achieved? Improved spectrum
More power in the fundamental mode
Improved spectrum
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18. Alignment
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19. Best Mesa Beam
Rsq = 0.996
Rsq = 0.992
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20. Jagged top due to imperfect mirrors
Best Mesa Beam
Jagged top due to imperfect mirrors
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21. Tilts of Spherical Mirrors
Tilts of spherical mirrors translate optical axis
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22. Tilt Sensitvity Controllability of beam is key
Decided to first investigate tilt sensitivity
Tilt MH mirror about a known axis
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23. Profiles Profiles along tilt axis 2 mrad simulated
3.85 mrad experiment
2.57 mrad experiment
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24. Excuses Lack of temporal stability Stiction PZTs are bad vacuum?
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25. 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
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26. 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
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27. Grazie Riccardo DeSalvo Phil Willems Mike Smith GariLynn Billingsley
Marco Tarallo
Juri Agresti
Chiara Vanni
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