Advanced Virgo: Optical Simulation and Design Advanced Virgo review Andreas Freise for the OSD Subsystem

A. Freise Advanced Virgo review Slide 2 Optical Simulation and Design  Scope of the OSD subsystem:  Optical design of 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  Sensitivity optimisations  Optical simulations

A. Freise Advanced Virgo review Slide 3 Simple Design Guidelines  If we do things right, the Advanced Virgo sensitivity should be limited by coating thermal noise and quantum noise  Thus OSD should:  propose a robust optical layout (do things right)  which minimizes coating thermal noise (arm cavity geometry)  and optimises quantum noise (SR tuning)

A. Freise Advanced Virgo review Slide 4 Progress Summary 1  Presented the optical configuration to the ERC on the 03/11/08  'Next Steps': 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  Comment from the ERC: do not use the Advanced LIGO design as a reason for choosing yours but compare different options  Activity during the last 4 months was dedicated to validate the preliminary design by evaluating alternatives and by performing more detailed analyses of the baseline design.

A. Freise Advanced Virgo review Slide 5 Progress Summary 2  Very active 4 months for OSD:  OSD workshop 'optical layout' to bring all subsystems together, defining constrains for the optical layout for non-degenerate recycling cavities  Weekly OSD teleconferences  Internal OSD review process, from to  Several new technical notes, mostly regarding non-degenerate recycling cavities (NDRCs):  VIR-007A-09: astigmatism analysis for NDRCs  VIR-005A-09: thermal noise analysis for NDRCs  VIR-004B-09: losses in arm cavities  VIR-087B-08: arm cavity geometry  VIR-101A-08: sensitivity optimisation

A. Freise Advanced Virgo review Slide 6 Progress Summary 3  Preliminary design has been largely confirmed, in particular:  arm cavity mirror sizes  arm cavity geometry (required precision on ROCs to be verified)  arm cavity finesse  choice of SR  choice of NDRCs  New results:  new layout for NDRCs 03/11/2008

A. Freise Advanced Virgo review Slide 7 Executive summary: Beam Geometry  Advanced Virgo needs to have a sensitivity competitive with Advanced LIGO in order to contribute to any network analysis.  This requires very large beam sizes (close to instability).  Trade off decision taking into account:  Sensitivity  Mode non-degeneracy  Mirror size / clipping losses  The current design features:  Beam sizes of 5.5 cm (IM) and 6.5 cm (EM).  The corresponding ROCs are 2% off instability.  The resulting sensitivity is about 30% worse than Advanced LIGO (which is the direct result of the shorter arm length)

A. Freise Advanced Virgo review Slide 8 Executive summary: Arm Cavity Finesse  Current value for the Advanced Virgo arm cavity finesse is 880.  Advanced LIGO will use about 400 (original aimed at 1200)  LGCT plans to use  At the moment there is no strong argument to change this value.  However, in case new or updated information appears, we can perform a new trade-off decision.  The main arguments considered in such trade-off process are:  Signal loss inside the signal recycling cavity  Suppression of noise from the central interferometer  Thermal load of the central interferometer  Lock acquistion (currently not)

A. Freise Advanced Virgo review 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 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

A. Freise Advanced Virgo review Slide 10 A possible layout  One of 4 evaluated schemes: no lenses, folded arm has the first large mirror in the recycling tanks, the second small mirror next to the BS and a third small mirror on the suspended benches (INJ/DET) or inside the recycling tanks.

A. Freise Advanced Virgo review Slide 11 Next steps  Finalise design:  Use detailed Finesse model of the interferometer  Determine tolerances and losses due to astigmatism and thermal deformations  Create a 3D model of the optical layout to verify the design and to design the complete optical layout including all secondary beams  Derive mirror surface specifications from the arm cavity geometry design  Develop simulation tool required for final stages of design process (preparing for detector commissioning)  Currently no-one seems capable of analysing the interferometer performance including higher order modes and radiation pressure effects. A possible solution is to extend Finesse to include radiation pressure effects (TBD)

A. Freise Advanced Virgo review Slide 12...end