Advanced Virgo: Optical Simulation and Design

24.03.2009 Advanced Virgo review

Andreas Freise for the OSD Subsystem

A. Freise Advanced Virgo review 24.03.2009 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 24.03.2009 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 24.03.2009 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 24.03.2009 Slide 5

Progress Summary 2 Very active 4 months for OSD:

OSD workshop 'optical layout' 29.01.2009 to bring all subsystems together, defining constrains for the optical layout for non-degenerate recycling cavities

Weekly OSD teleconferences Internal OSD review process, from 03.02.2009 to 13.03.2009 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 24.03.2009 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 24.03.2009 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 24.03.2009 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 1600.

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 24.03.2009 Slide 9

Non-degenerate Recycling cavities Science driver: Signal loss due to scattering into higher-order

modes

Thermal effects or misalignmentsscatter 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 24.03.2009 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 24.03.2009 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 24.03.2009 Slide 12

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