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

Stefan HildVirgo week January 2008Slide 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

Stefan HildVirgo week January 2008Slide 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.

Stefan HildVirgo week January 2008Slide 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 ??

Stefan HildVirgo week January 2008Slide 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 ?

Stefan HildVirgo week January 2008Slide 6 Etalon effect: flat/flat vs curved/flat Flat/flat etalon: Perfect overlap of wavefronts Curved/flat etalon: Mismatch of wavefront curvature 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

Stefan HildVirgo week January 2008Slide 7 Etalon effect: flat/flat vs curved/flat Flat/flat etalon: Perfect overlap of wavefronts 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 Still we have to choose: either wegde in input mirror (Pick-off beams available) or no wedge in input mirror (Etalon effect available)

Stefan HildVirgo week January 2008Slide 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.

Stefan HildVirgo week January 2008Slide 9 Now lets have a look at numbers for Advanced Virgo

Stefan HildVirgo week January 2008Slide 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 = 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:

Stefan HildVirgo week January 2008Slide 11 Influence of losses inside the cavity Imperfection of optics (surface + coatings) might cause different losses in the arm cavities := differential losses. 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.

Stefan HildVirgo week January 2008Slide 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)

Stefan HildVirgo week January 2008Slide 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.

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

Stefan HildVirgo week January 2008Slide 15 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 … Analyzing worst case scenario: curved/flat etalon

Stefan HildVirgo week January 2008Slide 16 FFT-simulation of a flat/curved etalon Using R. Schillings 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

Stefan HildVirgo week January 2008Slide 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)

Stefan HildVirgo week January 2008Slide 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.

Stefan HildVirgo week January 2008Slide 19 Comparison of WaveProp and finesse simulations Waveprop and finesse are in excellent agreement. WavePropfinesse

Stefan HildVirgo week January 2008Slide 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

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

Stefan HildVirgo week January 2008Slide 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).

Stefan HildVirgo week January 2008Slide 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

Stefan HildVirgo week January 2008Slide 24 E N D

Stefan HildVirgo week January 2008Slide 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)