1. Advanced Virgo Optical Configuration 09.10.2007 ILIAS-GW, Tübingen Andreas Freise - Conceptual Design -

2. 09.10.2007A. Freise2 The Conceptual Optical Configuration  Interferometer layout  Recycling techniques  Beam size and shape  Mirror (optics) specifications  Thermal compensation  Interferometer sensing and control

3. 09.10.2007A. Freise3 The Interferometer Layout  Mirror positions as in Virgo  Arm cavities symmetric  Add a Signal Recycling mirror 3 km ~ 6 m

4. 09.10.2007A. Freise4 Signal Recycling  Signal Recycling can significantly improve the sensitivity over a finite bandwidth  the detector response can be optimized on-line (compatible with Advanced LIGO, GEO HF, etc.)  The base of the vacuum tank for the Signal Recycling mirror is already installed  a new mirror has to be installed and controlled  resonance conditions for RF sidebands become more complex; this typically yields stronger noise couplings Advantages: Disadvantages:

5. 09.10.2007A. Freise5 Signal Recycling

6. 09.10.2007A. Freise6 … the Baseline  Wide-band Resonant Sideband Extraction (RSE)  Arm cavity Finesse ~ 900  Signal Recycling Mirror (for example) T=4%  Signal Recycling Cavity length similar to Power Recycling Cavity

7. 09.10.2007A. Freise7 Mirror Specification  A larger mass reduces the radiation pressure noise  Larger diameters (and thus larger beam sizes) reduce the thermal noise  High-finesse arm cavities require low losses (lower absorption, improved flatness)  Mirror size is limited by technical issues: size of coating and metrology equipment, maximum load of suspension systems  R+D underway to improve quality of substrates and coatings Motivation: State of the art:

8. 09.10.2007A. Freise8 … the Baseline  Mirror diameter: 35 cm  Mirror thickness: 20 cm  Mirror weight: 42 kg  Beam splitter diameter: 55 cm  Beam splitter thickness: 10 cm (very preliminary)

9. 09.10.2007A. Freise9 Wedges  Baseline: wedges on all mirrors  This provides extra beams (optical read-out ports)  Removes the `etalon effect'

10. 09.10.2007A. Freise10 Beam Size and Shape  Maximize spot size (beam radius) on all arm cavity mirrors Limited by the size of the coating on the beam splitter, e.g. for 30 cm coating:

11. 09.10.2007A. Freise11 … the Baseline  TEM 00 mode  Spot size ~ 3.5 cm to 6 cm on all core optics (BS, arm cavity mirrors, recycling mirrors)  Beam waist at center of arm cavity

12. 09.10.2007A. Freise12 … the Alternative  Higher order Laguerre-Gauss mode (LG33 / LG55) Thermal noise scales as:

13. 09.10.2007A. Freise13 … the Alternative  Higher order Laguerre-Gauss mode (LG33 / LG55)  Can reduce thermal noise significantly  Simplifies the requirements for thermal compensation  Can be used with the standard optics (spherical mirrors, lenses, etc.)  First investigations show that control signals are similar to those using TEM00 beams  R+D programme underway to investigate the efficient production of these modes and the noise couplings

14. 09.10.2007A. Freise14 Transverse Mode Stability  Thermal effects due to absorption are already seen in current detectors and will increase due to larger power in Advanced detectors  Overlap of carrier light with RF sidebands is affected by thermal effects and alignment  Thermal compensation must be installed to mitigate the absorption effects  Passive stability of the optical cavities can be improved to increase the mode stability

15. 09.10.2007A. Freise15 … the Baseline  Degenerate Recycling cavities  Thermal compensation on arm cavity input mirrors or compensation plates, following the R+D for and experience from Virgo+

16. 09.10.2007A. Freise16 … the Alternative  Non-degenerate recycling cavities  For example via telescopes inside the cavities  See R+D by LIGO

17. 09.10.2007A. Freise17 Interferometer Sensing+Control  No conceptual design yet  Probably use DC detection for GW signal  Dedicated R+D programme (CALVA) for the use of auxiliary lasers for the lock acquisition

18. 09.10.2007A. Freise18 Next steps  Start or continue dedicated R+D in new or improved optical techniques  Start trade-off analysis based on R+D results and experience from the commissioning of first generation detectors  Set final specification for long lead items  Engage in a system design of the entire (optical) instrument

19. 09.10.2007A. Freise19 … end.