1. 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

2. 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)

3. 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

4. 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)

5. 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

6. 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/ (2006) // Nature (2006)

7. 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

8. 2. Optical spring mitigation Conceptual Design
Angular spring: very similar issue
Similar solution!
Sidles and Sigg, Phys. Let. A 354 (2006)
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

9. 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 / >> 1
AdV (F=880): Tres / Tsto = / << 1

10. 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)

11. 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

12. 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

13. 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

14. 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

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

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

17. 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

18. 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

19. 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

20. 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