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Update on the Advanced Virgo Arm Cavity Design. Stefan Hild , Andreas Freise, Simon Chelkowski University of Birmingham Roland Schilling, Jerome Degallaix AEI Hannover Maddalena Mantovani EGO, Cascina April 2008, Virgo R&D Review. See Maddalenas talks. Overview. - PowerPoint PPT Presentation
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Stefan Hild, Andreas Freise, Simon ChelkowskiUniversity of Birmingham
Roland Schilling, Jerome DegallaixAEI Hannover
Maddalena MantovaniEGO, Cascina
April 2008, Virgo R&D Review
Update on the Advanced Virgo Arm Cavity Design
Stefan Hild VIRGO R&D review, April 2008 Slide 2
Overview
At January’s Virgo week we presented a new concept for arm cavity design of advanced Virgo (www.sr.bham.ac.uk/~hild/presentations/etalon_vs_wedges.ppt)
The new concept combines advantages of wedges and etalon effect.
What is new since last talk? Numerical simulations and Analytical
approximations Quantitative evaluation of etalon
imperfection Temperature stability requirement Influence onto alignment signals Higher order mode buildup See Maddalenas talks
Stefan Hild VIRGO R&D review, April 2008 Slide 3
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 R&D review, April 2008 Slide 4
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 R&D review, April 2008 Slide 5
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 similarly to flat/flat etalon.
Stefan Hild VIRGO R&D review, April 2008 Slide 6
What can we gain by using the proposed arm cavity design?
Experience from current detectors: Reflectivities of coatings accurate, but unexplained losses.
We concentrate on the differential losses => Optimal solution: adjusting end mirror transmittance. (Changing the input mirror would also change the amount of directly reflected light)
Several technical noises (such as laser frequency and laser intensity noise) couple proportional to the asymmetry of the arms.
Illustrating example: 30 ppm differential losses Using the etalon effect it should be possible to reduce the differential
losses to 1 ppm Reduce the noise coupling by a factor of 30 !!
Stefan Hild VIRGO R&D review, April 2008 Slide 7
Starting with a single AdV arm cavity
Using a single AdV arm cavity (no IFO).
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
Figure of merrits = intra cavity power or loss compensation or cavity finesse or transmittance of EM.
Parameters taken from these 2 documents:
Stefan Hild VIRGO R&D review, April 2008 Slide 8
Optimal solution: curved Etalon
Examples of figures of merrit: Transmittance of end mirror (etalon) Finesse of arm cavity
Stefan Hild VIRGO R&D review, April 2008 Slide 9
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 apparent position depends on the etalon tuning.
Stefan Hild VIRGO R&D review, April 2008 Slide 10
Requirement for temperature stability of etalon substrate
Can calculate require-ment for temperature stability for Advanced Virgo etalon
Using ‘worst case’: 1.22pm/deg
dn/dT = 1.09e-5/K Substrate thickness =
10cm
Example @100Hz: 4e-11K/sqrt(Hz)
This requirement is still 2 orders of magnitude above (safer) than temperature stability required from dL/dT of the substrates.
Stefan Hild VIRGO R&D review, April 2008 Slide 11
Everything fine as long
Etalon matches the
specs…
… but what if not ??
=> need to check !!
Stefan Hild VIRGO R&D review, April 2008 Slide 12
Optical design: Check system integrity for deviations from specs
A deviation in the reflectivity of the etalon coating: Only changes tuning range
(no problem)
A deviation in the relative misalignment (parallelism) and relative curvature of the two etalon surfaces: Imperfect wave front overlap… Reduces tuning range … Beam shape distortions …
Stefan Hild VIRGO R&D review, April 2008 Slide 13
FFT-simulation of a non-perfect etalon
Using R. Schilling’s WaveProp, (http://www.rzg.mpg.de/~ros/WaveProp/)
Cross checking with DarkF.
Parameters: Field: 256x256 Computing 3000 roundtrips End mirror front:
50ppm transmission R_c = 1910m
End mirror back: Varying three parameters Reflectance Misalignment (parallelism) Curvature
Stefan Hild VIRGO R&D review, April 2008 Slide 14
Analytic Approximations using Higher-Order Modes
For small misalignments the coupling coefficients knmnm can be approximated. The amount of light which remains in a TEM00 mode is given by:
(q is the Gaussian beam parameter of the light at the mirror)
Reflection at a (slightly) misaligned component can be characterised by scattering into higher order TEM modes
This model is valid for misalignments below half the diffraction angle (paraxial approximation)
The amplitude in the outgoing fields is given by coupling coefficients knmnm
Stefan Hild VIRGO R&D review, April 2008 Slide 15
Misalignment of etalon back surface
Strong influence of relative alignment of etalon surfaces.
Question: What accuracy can state of the art manufacturing provide?
Example: Initial Virgo input mirrors (flat/flat) = 1urad
Stefan Hild VIRGO R&D review, April 2008 Slide 16
Curvature deviation of etalon back surface
Curvature mismatch has only moderate influence to tuning range of the etalon.
Stefan Hild VIRGO R&D review, April 2008 Slide 17
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.
Evaluated and quantified the influence of etalon imperfections using numerical simulations and analytical approximations. Investigations of influence onto alignment signals and higher order mode buildup: See Maddalena’s talk.
Stefan Hild VIRGO R&D review, April 2008 Slide 18
OutlookPotential issues to be investigated:
Need a control system for etalon tuning (error signal + actuator).
Need a value for the expected differential losses in Advanced Virgo in order to choose the reflectivity of the etalon.
More details can soon be found in …
Stefan Hild VIRGO R&D review, April 2008 Slide 19
E N D
Stefan Hild VIRGO R&D review, April 2008 Slide 20
Common Mode Rejection Factor
Finesse and losses are coupled. Probably the differential losses will be the driving element.
Flaminio et al, VIR-NOT-LAP-1390-313
Finesse assymetry Differential losses