1. Optical Configuration 03.11.2008 Advanced Virgo Review Andreas Freise for the OSD subsystem

2. A. Freise Advanced Virgo Review 03.11.2008 Slide 2 Optical Simulation and Design  Scope of the OSD subsystem:  Optical design of the core interferometer (Michelson interferometer and Recycling cavities)  Core tasks:  Arm cavity geometry  Arm cavity finesse  Geometry of mirror and beam splitter substrates  Power Recycling cavity  Signal Recycling cavity  Auxiliary beams and scattered light  Optical simulations

3. A. Freise Advanced Virgo Review 03.11.2008 Slide 3 Design Strategy  Design drivers and constraints:  Interferometer design must be improved so that the sensitivity increase by a factor 10 can be achieved  The positions and size of the vacuum tanks must be largely kept unchanged  The envisaged joint operation with Advanced LIGO determines the timing  Strategy  Take existing Advanced LIGO design as start configuration  Adapt parameters within the given constraints  Provide always a consistent set of optical parameters to other subsystems  Automate design tools so that parameter evolution can be done with low effort and high accuracy

4. A. Freise Advanced Virgo Review 03.11.2008 Slide 4 Optical Layout Near-symmetric 3 km long arm cavities (power 760 kW, finesse 900) Non-degenerate Power Recycling cavity (build up factor >20, power on BS 2.7 kW) Non-degenerate Signal Recycling cavity (SRM transmission 11%) 125W laser power injected to interferometer Main detection port: high power photodiode in vacuum Possible optical readout ports for control

5. A. Freise Advanced Virgo Review 03.11.2008 Slide 5 Arm cavity geometry  Science driver: Coating Brownian noise beam radius on mirror Coating Brownian noise of one mirror: Absolute beam size should be as large as possible.

6. A. Freise Advanced Virgo Review 03.11.2008 Slide 6 Arm cavity geometry  Technical constraint: Mirror radius of curvature In order to increase the beam the radii of curvature must be pushed close to the point of an instable cavity. `Corrective coatings' could be employed to achieve precise values (the accuracy of the metrology at LMA for radii of curvatures is ~0.1%). We have chosen the absolute value to be 2% away from cavity instability. In a symmetric 3km cavity this would mean Rc > 1530. mode non-degeneracy

7. A. Freise Advanced Virgo Review 03.11.2008 Slide 7 Arm cavity geometry  Technical constraint: Mirror size Beam radius should be five to six times smaller than the coating diameter to keep the clipping losses below 1ppm (including a safety margin). Coating size is limited by manufacturing process. Mirror size: 35cm Beam size: 5 to 7cm

8. A. Freise Advanced Virgo Review 03.11.2008 Slide 8 Arm cavity geometry  Method: Near symmetric mirror curvatures coating thickness Beam size is determined by mirror radii of curvature: 3  m6  m Asymmetric coating thickness leads so a slight asymmetry for the radii of curvatures:

9. A. Freise Advanced Virgo Review 03.11.2008 Slide 9 Non-degenerate Recycling cavities  Science driver: Signal loss due to scattering into higher-order modes Thermal effects or misalignments scatter light into higher-order modes so that optical signal is lost. Non- degenerate cavities can reduce this effect. Commissioning experience shows that degenerate cavities cause problems for control signals. Y. Pan showed in 2006 that also GW signal is lost. Degenerate cavity First design options

10. A. Freise Advanced Virgo Review 03.11.2008 Slide 10 Non-degenerate Recycling cavities  Technical constraint: Size of vacuum system limits length of Recycling cavities PRM2 in PR tower No change for BS, IMX or IMY No change for BS, IMX or IMY PRM1 and PRM3 in injection tower PRM1 and PRM3 in injection tower Non-degenerate cavities must be longer than the Rayleigh range of the beam. The arm cavity beam has a Rayleigh range of 200m.

11. A. Freise Advanced Virgo Review 03.11.2008 Slide 11 Non-degenerate Recycling cavities  Method: Focusing elements inside the Recycling cavities Use beam propagation calculus to optimize radii of curvatures of the optical elements inside the recycling cavities for a given total length and single-trip Gouy phase. Design procedure has been established, first draft design for Power Recycling cavity: IMX Length total: 24 to 25 m limx= 10 cm lx = 5.5 m lprm3= 10 m lprm2 = 4.5 m lprm1= 4.5 m componentPRM1PRM2PRM3BSIMX (AR) Rc [m]2.1-1.83010.5 w [mm]1.82.216.538.550

12. A. Freise Advanced Virgo Review 03.11.2008 Slide 12 Signal Recycling  Science driver: Sensitivity optimisation (quantum noise reduction)  The Signal Recycling (SR) mirror in the interferometer output makes the detector resonant for a certain signal frequency in order to reduce quantum noise. SR allows us to tune the sensitivity curve - also during operation.  SR is a mature optical technique developed over 10 years in the GEO collaboration and currently planned for all second-generation detectors. Optimize parameters of SR mirror:  microscopic position (tuning)  power transmission

13. A. Freise Advanced Virgo Review 03.11.2008 Slide 13 Signal Recycling  Technical constraint: Fundamental noise sources  A vacuum tank for the Signal Recycling mirror is already present in the Virgo vacuum system.  The potential of the detector optimisation is limited by thermal noise above ~ 30 Hz and ultimately gravity gradient noise at lower frequencies.

14. A. Freise Advanced Virgo Review 03.11.2008 Slide 14 Signal Recycling  Method: Optimize NS-NS inspiral range  The SNR for a NS-NS inspiral is given by the integral with a known frequency dependence:  The SR parameters are tuned in order to maximise the accumulated signal. SNR= SR tuning: 0.15 rad SR transmission: 11%

15. A. Freise Advanced Virgo Review 03.11.2008 Slide 15 Summary Current baseline:  arm cavity geometry fixed  mirror substrate geometry fixed  beam splitter geometry currently being adapted to new beam size  recycling cavity design chosen, parameters to be evaluated  arm cavity finesse to be optimized

16. A. Freise Advanced Virgo Review 03.11.2008 Slide 16 Next steps Geometry of core optics:  Preliminary design has been proposed and will now be checked in detail for consistency  Optical parameters will be refined in an iterative process, based on a study of their impact on other subsystems Arm cavity finesse:  The finesse of the arm cavities is one of the dominant scaling factors for the influence of noise from the small interferometer (BS, IM, PRM, SRM)  A trade-off study for the arm cavity finesse is strongly linked to the R+D program concerning mirror surface losses

17. A. Freise Advanced Virgo Review 03.11.2008 Slide 17...end

18. A. Freise Advanced Virgo Review 03.11.2008 Slide 18 Example of Gouy phase and beam size

19. A. Freise Advanced Virgo Review 03.11.2008 Slide 19 Searching the parameter space R C,IMX =10m Gouy Phase Beam Size

20. A. Freise Advanced Virgo Review 03.11.2008 Slide 20 Searching the parameter space R C,IMX =flat Gouy Phase Beam Size