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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

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
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

motivation input mirror featuring a wedge
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

what to use for advanced virgo etalon or wedges
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

advanced virgo waist inside the cavity
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

etalon effect flat flat vs curved flat

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

etalon effect flat flat vs curved flat1
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

idea wedges at input mirrors and etalon effect at end mirrors
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

starting with a single adv arm cavity
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

influence of losses inside the cavity

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

end mirror as curved etalon optimal solution
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

etalon changes optical phase
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

everything fine as long etalon matches the specs but what if not need to check
Everything fine as long Etalon matches the specs…… but what if not ??=> need to check !!

Virgo week January 2008

optical design check system integrity for deviations from specs

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

fft simulation of a flat curved etalon
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

waveprop simulation
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

comparison of flat flat and curved flat etalon
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

comparison of waveprop and finesse simulations
Comparison of WaveProp and finesse simulations

Waveprop and finesse are in excellent agreement.

WaveProp

finesse

Virgo week January 2008

simulated beam inside arm cavity
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

changes the beam shape inside the arm cavity
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

summary
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

outlook
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

slide24

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Virgo week January 2008

it is important to compensate the differential losses
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