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Advanced Virgo: Wedges vs Etalon

Advanced Virgo: Wedges vs Etalon. Stefan Hild, Andreas Freise University of Birmingham Roland Schilling, Jerome Degallaix AEI Hannover January 2008, Virgo week, Pisa. Motivation: Input mirror without wedge. Initial Virgo has no wedges in the input mirrors

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Advanced Virgo: Wedges vs Etalon

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  1. Advanced Virgo:Wedges vs Etalon Stefan Hild, Andreas Freise University of Birmingham Roland Schilling, Jerome Degallaix AEI Hannover January 2008, Virgo week, Pisa

  2. Motivation: Input mirror without wedge • Initial Virgo has no wedges in the input mirrors • The etalon effect could be used for adjusting the cavity finesse (compensating for differential losses) • If etalon effect is not controlled it might cause problems Virgo week January 2008

  3. Motivation: Input mirror featuring a wedge • Used by initial LIGO • Reflected beams from AR coating can be separated from main beam => pick off beams provide additional ports for generation of control signals. • No etalon effect available. Virgo week January 2008

  4. What to use for Advanced VIRGO?Etalon or Wedges ?? • For AdV possibility to adjust cavity finesse gets more important (higher cavity finesse, DC-readout). • For AdV possibility to create more and better control signals seem desirable. Is there a possibilty to have both for Advanced Virgo ?? Fortunately: YES ! Virgo week January 2008

  5. Advanced Virgo: waist inside the cavity • Increase beam size at mirrors => reduce thermal noise contribution of the test masses. • Move beam waist away from input test mass Is there still an etalon effect in the (flat/curved) input mirror ? Virgo week January 2008

  6. Curved/flat etalon: • Mismatch of wavefront curvature Etalon effect:flat/flat vs curved/flat • Flat/flat etalon: • Perfect overlap of wavefronts • Fortunately mirror curvature of a few km is not so far from “flat”. • Simulations show: a reduced etalon effect in curved/flat input mirror is still present Virgo week January 2008

  7. Etalon effect:flat/flat vs curved/flat • Flat/flat etalon: • Perfect overlap of wavefronts Still we have to choose: either wegde in input mirror (Pick-off beams available) or no wedge in input mirror (Etalon effect available) • Curved/flat etalon: • Mismatch of wavefront curvature • Fortunately mirror curvature of a few km are not so far “flat”. • Simulations show: a reduced etalon effect in curved/flat input mirror is still present Virgo week January 2008

  8. IDEA: Wedges at input mirrors and etalon effect at end mirrors • Wedge at input mirrors: • Allows for additional pick off beams • (Concentrate on compensating thermal lensing in input mirror) • Use etalon effect at end test mass • Replace AR-coating by a coating of about 10% reflectivity. • Ideally use a curved back surface (same curvature as front). • End mirror behaves similar to flat/flat etalon. Virgo week January 2008

  9. Now let’s have a lookat numbers for Advanced Virgo Virgo week January 2008

  10. Starting with a single AdV arm cavity • Using a single AdV arm cavity (no IFO). • Figure of merrit = intra cavity power. • Parameters used: • IM trans = 0.007 • IM loss = 50 ppm • EM trans = 50 ppm • EM loss = 50 ppm • AR coatings = 0ppm • IM curvature = 1910m • EM curvature = 1910m • Input = 1W Parameters taken from these 2 documents: Virgo week January 2008

  11. What are the expected differential losses of AdV ? 5ppm? 50ppm? • A differential loss of 15ppm corresponds to a change of 2W intra cavity power in this example. • GOAL: Compensate 2W using etalon effect in end mirror. Influence of losses inside the cavity • Imperfection of optics (surface + coatings) might cause different losses in the arm cavities := differential losses. Virgo week January 2008

  12. End mirror as curved etalon (optimal solution) • Simulation done with finesse. • Back surface of end mirror curved (1910m). • AR coating replaced by coating of 10% or 20% reflectivity. • R=0.1 allows adjustment range of 10W (65ppm) • R=0.2 allows adjustment range of 16W (95ppm) Virgo week January 2008

  13. Etalon changes optical phase • When changing the etalon tuning the optical-phase changes as well. (noise!) • The two etalon surfaces build a compound mirror, whose position depends on the etalon tuning. • A single FSR of the etalon corresponds to about 3pm. Virgo week January 2008

  14. Everything fine as long Etalon matches the specs…… but what if not ??=> need to check !! Virgo week January 2008

  15. Analyzing “worst case” scenario: curved/flat etalon Optical design: Check system integrity for deviations from specs • A deviation in the refelctivity of the etalon coating: • Only changes tuning range (no problem) • A deviation in the curvature of the etalon surface: • Imperfect wave front overlap… • Reduces tuning range … • Beam shape distortions … Virgo week January 2008

  16. FFT-simulation of a flat/curved etalon • Using R. Schilling’s WaveProp, (http://www.rzg.mpg.de/~ros/WaveProp/) • Parameters: • Field: 128x128 • Computing 3000 roundtrips • End mirror front: • 50ppm transmission • R_c = 1910m • End mirror back: • 10, 20, 50% reflectivity • R_c = flat Virgo week January 2008

  17. WaveProp simulation • R=0.1 allows adjustment range of 3W (20ppm) • R=0.2 allows adjustment range of 5W (35ppm) • R=0.5 allows adjustment range of 9W (60ppm) Virgo week January 2008

  18. Comparison of flat/flat and curved/flat etalon For a curved/flat etalon the tuning range (etalon effect) is reduced by about a factor of 3. Virgo week January 2008

  19. Comparison of WaveProp and finesse simulations Waveprop and finesse are in excellent agreement. WaveProp finesse Virgo week January 2008

  20. Simulated beam inside arm cavity • Simulated beam shape inside the arm cavity (using finesse) • Simulation done with etalon on resonance (worst case scenario) • By eye: no change … Curved/curved etalon Curved/flat etalon Virgo week January 2008

  21. Changes the beam shape inside the arm cavity ?? • Subtracted beams indicate the change in beam shape. • Residual light field is a factor of 60000 weaker than the intra cavity beam. Virgo week January 2008

  22. Summary • Advanced Virgo CAN feature wedges in the input mirrors AND use the etalon effect at the end mirrors. • Proposed concept allows us to build ‘arm cavities with adjustable losses’. • A curved/curved etalon would be ideal (a curved/flat etalon should work as well, but with reduced tuning range). Virgo week January 2008

  23. Outlook Potential issues to be investigated: • How does misalignment of the etalon influence the alignment control signals: • For curved/curved etalon: probably fine (to 1st order) • For curved/flat etalon: needs simulation • Check that optical-phase noise from fluctuations is no problem. • Need a control system for etalon tuning (error signal + actuator). • Need a value for the expected differential losses in AdV in order to choose the reflectivity of the etalon Virgo week January 2008

  24. E N D Virgo week January 2008

  25. It is important to compensate the differential losses A differential loss of 70ppm causes already 100mW of waste light at the dark port. • increased shot noise !! AdV simulation (dummy parameters) Virgo week January 2008

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