ISC scope and activities Optical spring Outline AdV ISC subsystem ISC scope and activities Optical spring Lock acquisition of arm cavities Full Interferometer Lock Acquisition Parametric instabilities NDRCs Management For each physical topic, address: Design requirements: what are the needed design studies, what are the possible showstoppers Conceptual design: solution envisaged, roughly described Preliminary Design: detail study of the solutions, providing specifications Final Design: every technical detail is decribed Here is a list of selected topics, complete list available in DRD Most important one is the optical spring due to DC offset. François BONDU for ISC group

ISC scope and activities Bring the interferometer to its operating point and keep it here reliably Lock acquisition F. Cavalier (LAL-Orsay) Steady state length control F. Bondu (ARTEMIS – IPR Rennes 1) Angular control M. Mantovani (U. of Siena / INFN Pisa) Parametric instabilities P.-F. Cohadon (LKB - Paris)

ISC scope and activities Scope details, task list, interactions with other systems: VIR-085A-08 Design Requirement Document: VIR-024A-09 Work Breakdown Structure: see ISC wiki page

2. Optical spring Design Requirement DC detection  detuned cavity  Optical spring: Arm power (W) xdet xp (pole) Note that the spring is negative when the cavity is shorter The optical spring is 3 orders of magnitude larger than the pendulum spring; for a beam with w=3 cm, the stiffness associated to the light is 600 GPa, equivalent to the hardest materials on earth -> Mirror pendulums are largely coupled -> The sensitivity of the cavity to displacement is strongly reduced at low frequency detuning (m)

2. Optical spring Design Requirement Effective driving element: K1 Mirror suspension unstable when optical power such that DK = 1 (P=380 W) Actuation flips it sign when power in the arm cavity is higher

2. Optical spring mitigation Design Requirement AdV arms predicted to be unstable? NO: Actually a mitigation based on a Passuello’s like technique for mirror control (next slide) Note that the AdVirgo locking point is right INSIDE the unstable domain O. Arcizet, P.-F. Cohadon, etal, ArXiv:quant-ph/0607205 (2006) // Nature 444 71 (2006)

2. Optical spring mitigation Conceptual Design Solution! driving sensing xseism+xTN h L/2 vmeas xdet F D S K dig is such that it matched the product S x K + + + + K Kdig Digital spring

2. Optical spring mitigation Conceptual Design Angular spring: very similar issue Similar solution! Sidles and Sigg, Phys. Let. A 354 (2006) 167-172 aseism+aTN adet vmeas D Torque S + + Longitudinal variables replaced with angular variables. Same problem, same solution as longitudinal control. K Effective driving: K: torsion spring

3. Arm cavity lock acquisition Design requirement Feedback response << meantime through resonance “Adiabatic” response: cavity response time << meantime through resonance Poirson et al. JOSA B 14 (1997) 2811 Arm power (W) Ex. vmir = 1 mm/s Initial virgo (F=50): Tres / Tsto = 10-2 / 2 10-4 >> 1 AdV (F=880): Tres / Tsto = 5 10-4 / 3 10-2 << 1

3. Arm cavity lock acquisition Conceptual design Enlarge cavity response time: an auxiliary laser with a low finesse wavelength Linewidth specification: 5 Hz on 100 ms Baseline: stab. with reference cavity Alternative solutions: 1) Use pick-off of the main 1.06 laser + 3 km fibers + freq. doubling + phase lock + noise compensation 2) recent breakthrough, fiber stab: Kéfélian et al. Opt. Lett. 34 (2009) 914 detuning (m)

3. Arm cavity lock acquisition Conceptual design CALVA experiment at LAL Test lock acquisition with auxiliary laser, and transfer to main laser Test DC detuning and optical spring mitigation

4. Full interferometer lock acquisition Conceptual design D.O.F. Double demod. DARM AP_DC CARM SP_SB1_P PRCL SP_3+1_P MICH SP_2-1_Q SREC displacement of towers Carrier SB1 SB2 SB3 VIR-068B-08

4. Full interferometer lock acquisition Conceptual design Signal recycling cavity: Control entanglement added  complexity Initial Virgo: lx –ly = 80 cm AdV: lx –ly = 4 cm  decoupling of error signals IMPACT ON VAC / IME

4. Full interferometer lock acquisition Conceptual design Fully deterministic lock acq. procedure: Arm cavity lock acquisition aux. laser Michelson grey fringe 3. Power Recycling Cavity variable finesse 3 cm tower asymmetry Signal Recycling Cavity

5. Parametric instabilities (U.W.A. student, author of main papers on the subject) to come to Paris

5. Parametric instabilities (U.W.A. student, author of main papers on the subject) to come to Paris

5. Parametric instabilities Design requirement PI: stimulated vibration of the mirror, amplified by the light high order modes Determine number of PI modes for Virgo arm cavity design, optical power thresholds, power conversion (optical  mechanical) (mirror damage?) Experimental setup started to better understand how a PI develops in an AdV like configuration LKB applied to join the Virgo Collaboration in April 2009 S. Gras in Paris in summer 2009 (U.W.A. student, author of main papers on the subject) to come to Paris

6. NDRCs Design requirement Conceptual design Control of multi-mirror NDRCs 4 mirror folded cavity: one cavity eigenmode axis one incoming light beam  2 extra mirrors for angular and longitudinal control (good: redundant control handles!)  Mirrors under local controls

Final Design and specifications 7. Management AdV Project start ISC Final Design Validation ISC finished One hour lock Final Design and specifications Pre-com. (simulation) Integ. Commissioning 01/06/09 01/06/11 01/01/13 01/06/13 Fin 2014 One hour lock: no TCS, not full power, not final “sensitivity” (functional test) 2 PhD / post-docs missing hardware 200 keuros: auxiliary lasers with stabilization

CONCLUSION More complex optical design  3 sideband pairs (1 for initial Virgo) + tower moving  control more diagonal than in initial Virgo Parametric instabilities New group to help on this, work with AdV parameters DC detection:  less SB technical noise in dark fringe  optical spring mitigation