1. The status of VIRGO Edwige Tournefier (LAPP-Annecy ) for the VIRGO Collaboration HEP2005, 21st- 27th July 2005
The VIRGO experiment and detection of gravitational waves
The commissioning of VIRGO
Conclusions

2. VIRGO French-italian collaboration (CNRS – INFN)
Annecy (LAPP), Firenze, Frascati, Lyon (LMA), Napoli, Nice (OCA), Paris (ESPCI), Perugia, Pisa, Roma, Orsay (LAL)
Virgo site : Cascina close to Pisa
Virgo goal: detection of gravitational waves

3. How to detect gravitational waves?
Effect of a gravitational wave on free masses:
A Michelson interferometer is suitable:
suspend mirror with pendulum => ‘free falling masses’
Gravitational wave => phase shift
Measure: h = L/L
L = length difference between the 2 arms
L = arm length
Suspended
mirror
Beam splitter
LASER ()
Light
Detection

4. The shot noise and the VIRGO optical design
Limitation of a Michelson interferometer due to photon shot noise:
the minimum measurable relative displacement is
=> Can reach h ~ with L=100km and P=1kW
How to achieve that?
Gravitational wave signal
1/ Fabry-Perot cavities to increase the effective length:
( F = finesse )
=> L’ = 100km for L=3km and F=50
2/ Recycling mirror to increase the effective power:
P’ = R P (R = recycling gain)
=> P’ = 1kW with P=20W and R=50

5. Noise sources in interferometers
Laser Noises
Shot
Noise
Detection
Index fluctuation
Seismic
Acoustic
Thermal

6. Noise sources: seismic noise
1014
Seismic noise spectrum for f few Hz:
a ~
 shot noise !
Need a very large attenuation!
Solution:
suspend the mirrors to a chain of pendulums
Transfert function
With a chain of 6 pendulums:
attenuation of the seismic noise by ~1014 at 10 Hz !

7. Suspensions and control of the interferometer
All mirrors are suspended to a cascade of pendulums:
Large attenuation in the detection band ( > 10 Hz)
Large residual motion at low frequencies: < ~1mm
Need active controls to:
maintain the interferometer’s alignment
maintain the required interference conditions
The control is done in 2 steps:
1/ Local control of the suspensions:
Residual motion ~2 m/sec
Obtain interference fringes
2/ To keep the interferometer at interference conditions:
Need to control the length of the cavities to m
Need to keep the interferometer aligned
Use the interferometer signals: photodiodes

8. VIRGO design sensitivity
Main sources of noise limiting the VIRGO design sensitivity
Shot noise 1
Seismic noise
Thermal noise
Shot noise

9. Gravitationnal wave sources and VIRGO design sensitivity
Coalescing binaries (1.4 Mo)
Pulsars: upper limit (1 year)
Supernovae at 15Mpc
Distance to the Virgo cluster = 10Mpc

10. The commissioning of VIRGO
Technical runs (3 to 5 days) at each step
C1(Nov 2003),…, C5(Dec 2004)
Lock stability
Sensitivity/noise studies
Data taking on ‘long’ period
End of construction: 2003
The steps of the VIRGO commissioning:
- recombined (Michelson) ITF: Feb 2004
control of the north FP cavity: Oct 2003
- control of the west FP cavity: Dec 2003
- recycled (full VIRGO) ITF: Oct 2004
output mode cleaner
input mode cleaner
laser
recycling
mirror
beam
splitter
L=3km
l=150m
l=6m
Fabry-Perot cavities
West arm
North arm
Gravitational wave signal

11. The lock of the full VIRGO
Lock of the recycled interferometer (full VIRGO):
Need to control 4 degrees of freedom (3 cavities + Michelson on dark fringe)
The lock is acquired in several steps (‘variable Finesse’ strategy):
Start without recycling
Slowly increase the recycling gain and move to the dark fringe
Laser + -
Lock acquisition
Power stored in the recycling cavity (Watts)
With recycling
Without recycling
Recycling gain ~ 30

12. Sensitivity summary Single arm, P=7 W Recombined, P=7 W
Recycled, P=0.7 W
P = 10W
h ~ /Hz

13. Typical unforeseen difficulties
Injection bench:
A small fraction (bigger than expected) of the light reflected by the
interferometer is retro-diffused by the input mode cleaner mirror
spurious interferences
Temporary solutions:
- tried to rotate the mode cleaner mirror
- reduce the incident light (/10)
We are now working with only Pin = 0.7 Watts
Final solution: install a Faraday isolator
A new input bench will be installed in September 2005
Frequency noise
Recycling mirror:
- aligned
- not aligned

14. Present sensitivity and perspectives
P=0.7 W
- local angular controls
P = 10W
Improvements since C5:
- longitudinal controls
- low noise actuators
Shot noise for P=0.7 W
Futur: the VIRGO sensitivity will significantly improve with
full power (new input bench)
the automatic alignment of the interferometer (global angular control)
the improvement of the longitudinal controls
lower noise actuators …

15. Data analysis: some examples
- Injected events
Test of the data analysis on real data
from the technical runs:
Test the full chain of data analysis
Learn how to put vetoes
Inject events in the real data:
software and hardware injections
-> measure efficiencies, false alarm rate,…
Start collaboration with LIGO:
Coincident analysis will help the detection of GW
=>decrease false alarm rate
(rare events in a non gaussian noise)
Combined data analysis is necessary to extract the source parameters
Event amplitude
Quiet period
Event amplitude

16. Conclusion The recycled (full VIRGO) interferometer is working
Next engineering run (C6), 29/07-12/08:
2 weeks of data taking with the best sensitivity
The sensitivity will make big progress with
New input bench (-> full input power)
Automatic alignment of the mirrors
The data analysis is been prepared and tested on real data
Collaboration with LIGO is starting
First scientific run in 2006/7?

17. Noise studies
Sensitivity measured during C4 run and identified sources of noise
Noise hunting:
1/ Identify the sources of noise which limit the sensitivity
2/ Perform the necessary improvements / implement new controls

18. Comparison with LIGO first science run (S1)
Virgo May 2005

19. Example of lock acquisition
Example of the lock acquisition of a Fabry-Perot cavity
Photodiode used for
lock acquisition
Power stored inside the Fabry-Perot cavity
Error signal of the cavity
Correction sent to the actuators of the mirror
/2
Lock acquisition:
Apply force on the mirror to keep
the error signal at zero
4 seconds
Coil
Mirror
Magnet

21. The commissioning of the CITF
Commissioning of the central interferometer: 09/2001 -> 07/2002
CITF = Recycled Michelson interferometer (no Fabry-Perot cavities)
a lot of common points with VIRGO
The evolution: configuration and
sensitivity: 4 runs of 3 days each
- E0/E1: Michelson
- E2: Recycled Michelson
- E3: + automatic angular alignment
- E4: + final injection system
Results:
Viability of the controls
Sensitivity curve understood
And gain experience for the
VIRGO commissioning
- Improvements triggered by the CITF experience
unit = meters!

22. The mirrors Fused silica mirrors
Coated in a class 1 clean room at SMA-Lyon
(unique in the world).
Low scattering and absorption:
< few ppm
Good uniformity on large dimension:
< 10-3  400 mm
Large mirrors (FP cavities):
 35 cm, 10 cm thick
20 kg

23. The injection and detection systems
Laser: powerful and stable
20W
Power stability: 10-8
Frequency stability: Hz
The input and output mode cleaners:
optical filter => improve signal to noise ratio
Signal detection:
- InGaAs photodiodes, high efficiency

24. Future: how to improve the sensitivity?
The first generation of detectors might not be able to see gravitational waves
Need to push the sensitivity further down:
Seismic noise:
The VIRGO suspensions already meet the requirements for next generation interferometers
The main limit: thermal noise
Monolitic suspensions (silica)
Better mirrors (material,
geometry, coating)
Shot noise
More powerful lasers
Signal recycling technique
And the technical noises
Better sensors
Better electronics
Better control systems
Shot noise

25. Recombined interferometer
keep the two Fabry-Perot cavities on resonance + the Michelson on the dark fringe
Power ‘stored’ inside the FP cavities
Power at the interferometer output
Lock on the dark fringe
Example of lock acquisition