Advanced Virgo Wedges vs Etalon Stefan Hild Andreas

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

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 Hild Virgo week January 2008 2

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 Hild Virgo week January 2008 3

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

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 Hild Virgo week January 2008 5

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 Hild Virgo week January 2008 6

Etalon effect: flat/flat vs curved/flat ) e l b la i a av ble) Perfect overlap of wavefronts s m aila a e b t av f f o fec k f c i e P : ( n e o Curved/flat etalon: r l s o o r (Eta r o i h m ocurvature cof wavefront r Mismatch t o u r t r p mi e n i v ha e in mirror ut curvature of a Fortunately p e n so far “flat”. d aren not i w g l few km l e Sti er w dge i show: a reduced e w eith Simulations o netalon effect in curved/flat input r o Flat/flat etalon: mirror is still present Stefan Hild Virgo week January 2008 7

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 Hild Virgo week January 2008 8

k o lo a e v a h r s o ’ f t e s l r e w b o o m g N r u i n V t d a e c n a Adv Stefan Hild Virgo week January 2008 9

Starting with a single Ad. V arm cavity Using a single Ad. V 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 = 0 ppm IM curvature = 1910 m EM curvature = 1910 m Input = 1 W Stefan Hild Parameters taken from these 2 documents: Virgo week January 2008 10

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 Ad. V ? 5 ppm? 50 ppm? A differential loss of 15 ppm corresponds to a change of 2 W intra cavity power in this example. GOAL: Compensate 2 W using etalon effect in end mirror. Stefan Hild Virgo week January 2008 11

End mirror as curved etalon (optimal solution) Simulation done with finesse. Back surface of end mirror curved (1910 m). AR coating replaced by coating of 10% or 20% reflectivity. R=0. 1 allows adjustment range of 10 W ( 65 ppm) R=0. 2 allows adjustment range of 16 W ( 95 ppm) Stefan Hild Virgo week January 2008 12

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 3 pm. Stefan Hild Virgo week January 2008 13

g n o l s a e n i e f h t g n s i e h h t c y t r a e m Ev n o … l s a c e Et p s ? ? t o n f i t a h w ut b !! k c … e o ch => Stefan Hild t d e ne Virgo week January 2008 14

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: Analyzing “worst case” scenario: curved/flat etalon Imperfect wave front overlap… Reduces tuning range … Beam shape distortions … Stefan Hild Virgo week January 2008 15

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

Wave. Prop simulation R=0. 1 allows adjustment range of 3 W ( 20 ppm) R=0. 2 allows adjustment range of 5 W ( 35 ppm) R=0. 5 allows adjustment range of 9 W ( 60 ppm) Stefan Hild Virgo week January 2008 17

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 Hild Virgo week January 2008 18

Comparison of Wave. Prop and finesse simulations Wave. Prop finesse Waveprop and finesse are in excellent agreement. Stefan Hild Virgo week January 2008 19

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

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. Stefan Hild Virgo week January 2008 21

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 Hild Virgo week January 2008 22

Outlook Potential issues to be investigated: How does misalignment of the etalon influence the alignment control signals: § For curved/curved etalon: probably fine (to 1 st 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 Ad. V in order to choose the reflectivity of the etalon Stefan Hild Virgo week January 2008 23

END Stefan Hild Virgo week January 2008 24

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