1 / 20

Update on the Advanced Virgo Arm Cavity Design

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.

heller
Download Presentation

Update on the Advanced Virgo Arm Cavity Design

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. 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

  2. See Maddalenas talks 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 VIRGO R&D review, April 2008

  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 VIRGO R&D review, April 2008

  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. VIRGO R&D review, April 2008

  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. VIRGO R&D review, April 2008

  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 !! VIRGO R&D review, April 2008

  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: VIRGO R&D review, April 2008

  8. Optimal solution: curved Etalon • Examples of figures of merrit: • Transmittance of end mirror (etalon) • Finesse of arm cavity VIRGO R&D review, April 2008

  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. VIRGO R&D review, April 2008

  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. VIRGO R&D review, April 2008

  11. Everything fine as long Etalon matches the specs…… but what if not ??=> need to check !! VIRGO R&D review, April 2008

  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 … VIRGO R&D review, April 2008

  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 VIRGO R&D review, April 2008

  14. Analytic Approximations using Higher-Order Modes • 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 • 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) VIRGO R&D review, April 2008

  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 VIRGO R&D review, April 2008

  16. Curvature deviation of etalon back surface • Curvature mismatch has only moderate influence to tuning range of the etalon. VIRGO R&D review, April 2008

  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. VIRGO R&D review, April 2008

  18. More details can soon be found in … Outlook Potential 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. VIRGO R&D review, April 2008

  19. E N D VIRGO R&D review, April 2008

  20. Common Mode Rejection Factor • Finesse and losses are coupled. • Probably the differential losses will be the driving element. Finesse assymetry Differential losses Flaminio et al, VIR-NOT-LAP-1390-313 VIRGO R&D review, April 2008

More Related