The Virgo Experiment
Michele Punturo
INFN Perugia
3rd VESF School on Gravitational Waves
Build up the interferometer
h
2
h  10 21
L   L0
“Kilometric” detector
L0  103 m  L  10 18 m
NS/NS collapse
@ Virgo cluster
Quadrupolar nature of the
gravitational wave:
•A Michelson interferometer
seems a very appropriate detector
Eout t   E1 t   E2 t  

E1
Ein
E2
Ein
cost  k L1 t   cost  k L2 t  
2
Interference term
Eout
L2 t   L1 t  
 L2 t   L1 t  


cos

t

k
2



2
2
Ein  cos - k
3rd VESF school - Michele Punturo

Virgo
… Build up the interferometer
• Let suppose that TGW>>2L/c:
 2 n
 P 
 2 n

Pout t   Pin  cos 2 
 L  ht   in 1  cos 2
 L  ht  
 laser
 2 
 laser

Pout t  
 2 nt 

Pin t  


1

cos
2

L

h
t





2 

t
 laser

•
The largest high vacuum system in
Europe:
– About 7000 m3
– 1.2 m diameter pipe @ 10-7mbar
(H2 partial pressure)
• Reduction of light fluctuation
given by air flux
– 7 long towers (9m long) with
differential vacuum:
• Usual 10-7 mbar vacuum in the
upper part
• 10-9 mbar in the lower part
3rd VESF school - Michele Punturo Virgo
3
Why a Fabry-Perot?
•
In a Michelson the sensitivity to an arm length difference L=h·L is given by the slope,
in the gray fringe, of
 Mich t  
2L
 rt 
c
4

Lopt 
4

1
L  h   Laser  rt  ht 
Resonant cavity: I t   I 0  e
 s   rt

t
s
P( x )
0.5
 r1r2
F
, F
2
1  r1r2
0
t12 r2 e 2ikL
rFP  r1 
1  r1r2 e  2ikL
0
/26
5 10
 6
4
2
 [rad]
 FP
8 F 2 F  Mich



Lopt

 Lopt
laser
1
00 x
6
/2
5 10
FP100
FP25
Michelson
0
-2
-4
-0.10
3rd VESF school - Michele Punturo Virgo
-0.05
0.00
L/
0.05
0.10
4
 6
Why power recycled?
• The gray fringe working point is not the right choice:
– The ITF is not a “Null Instrument”, that is the output is not null when the
input is null: large DC
– We want to operate in the dark fringe: no DC if zero input
– What to do with the light wasted in the input port?
Recycle it!
Shot noise reduced by the recycling factor, but
how to extract the GW signal if we work at the
dark fringe, where
laser
Pout
0
L
3rd VESF school - Michele Punturo Virgo
5
Modulation-Demodulation
• To operate in the dark fringe, but converting the FP in a linear
instrument we need to adopt a modulation-demodulation scheme:
– Pound-Drever technique
EOM
PBS
/4
Ein t   E0 cosct   cos mod t 
 E0 cosct cos cos mod t   sin ct sin  cos mod t 
 E0 cosct   sin ct   cos mod t 
PHD
LO
Out
E
 E

 E0 cos c t    0 sin  c   mod t   0 sin  c   mod t 
 2
3rd VESF school - Michele Punturo Virgo
carrier

2
sidebands
6
…modulation-demodulation
• The carrier is resonant in the cavity, but not the sidebands ( shift). Hence, the
reflected beam is
Erefl t   E0 cos c t    sin  c t cos mod t 
• Let suppose that there is a GW signal that modulates the phase of the incoming
field. Its effect is present only in the carrier, because it is resonant in the cavity
Erefl t   E0 cos c t   GW t    sin  c t cos mod t  
 E0 cos ct cos GW t   sin ct sin  GW t  E0 sin ct cos mod t  
 E0 cos ct cos GW t   E0  cos mod t   sin  GW t  sin ct 
• At the output of the interferometer, the photodiode reads the power, averaged
over c, hence we must evaluate the square of
• The mixed product term gives:
 sin GW t cos mod t     GW t  cos mod t 
• Demodulating the mod disappears and the output is proportional to the
gravitational signal:
– We build a linear null instrument
3rd VESF school - Michele Punturo Virgo
7
GW interferometer as a
Double-Superheterodyne
Receiver
GW
h(gw)
Antenna
ITF mechanical part
Laser
E(laser)
Parametric
L(gw) transducer
E(lasergw)
i(mod  gw)
Sig.
Photodiode
ITF optical part
E(lasermod)
Preamp
RF
Sig.
Mixer
L.O.
V(gw)
Out
L.O.
V(mod)
L(mod)
Pockel
cell
V(mod  gw)
V(mod)
R.F.
Oscillator
mod
3rd VESF school - Michele Punturo Virgo
Sig. = Signal
L.O. = Local Oscillator
8
Virgo simplified Optical Scheme
140m
3km
3rd VESF school - Michele Punturo Virgo
9
The injection system: The Laser
20 W, Nd:YVO4 laser, two pumping diodes
Injection locked to a 0.7 W Nd:YAG laser
Required power stability: P/P~10-8 Hz-1/2
Required frequency stability: 10-6 Hz1/2
Mode Cleaner L = 143 m
•
•
•
•
Diode pump
Slave Nd:YVO4 Laser
Injection bench
1W master laser
Telescope
ITF
Nd:YAG =1.064 mm
ULE
monolithic
22 W slave laser
Reference
cavity
10
Gaussian beams
• Until now we considered, for the light, the plane wave approximation
• But the beam, coming from a laser, shows a finite size and a approximately
gaussian shape
• In effect the propagation law of an electromagnetic field in an homogeneous
medium gives:
 2 y   2 z  i m n 1
w0
 n 
e
Emn x, y, z, t   E0
 m 
e



wx   wx    wx  
ik
• Where n are the Hermite-gaussian functions:  n    H n   e

y2  z2
2 Rx 
2
2
H 0    1 H1    
• w(x) represents the beam size:
•
•
   x 2 
  x 2 
   w02  1   2  
w2 x   w02  1  
2 
   w0  
  x0  
x 2  x02
R(x) represents the curvature radius of the beam: Rx  
x
x
and  is defined by: tan  
x0
3rd VESF school - Michele Punturo Virgo
e i kx t 
H 2    4 2  2 ...
w0
Minimum
beam
waist
11
Transverse Modes 00 and 01
3rd VESF school - Michele Punturo Virgo
12
Transverse Modes 11 and 22
3rd VESF school - Michele Punturo Virgo
13
Fabry Perot as mode cleaner
• The Fabry-Perot cavity is a resonator that can be tuned to select
the desired resonant frequency
• In fact, the resonance condition is defined by the request that the
complete round trip phase delay of the light inside the cavity is a
integer multiple of 2.
L
• Let suppose for simplicity: xM   xM  2
2
1
• The resonance conditions becomes:
k
 L  
 L 
L
L
   k  m  n  1 tan 1 
  p ,
 m  n  1 tan 1 
2
2
 2 x0  
 2 x0 
mnp

m  n  1 1  L 

 2L p  2
tan 

 2 x0 

3rd VESF school - Michele Punturo Virgo
p  1,2,...
1
14
… FP as mode cleaner
mnp

m  n  1 1  L 

 2L p  2
tan 

 2 x0 

1
• If we want to select the gaussian mode 00, we choose the
length of the cavity in such a way exists a p1 index satisfying
the previous resonance condition: 00p1 resonant
• The mode 00 is then transmitted by the cavity
3rd VESF school - Michele Punturo Virgo
15
Input Mode Cleaner
• Mode cleaner cavity: filters laser noise,
selects TEM00 mode
Input beam
inbeam
Transm. beam
Input mode-cleaner: curved mirror
Refl. beam
• Mode cleaner cavity:
filters laser noise, select
TEM00 mode
outbeam
refbeam
Input mode-cleaner: dihedron
16
Output Optics
• Light filtering: output mode cleaner, 3.6
cm long monolithic cavity
• Light detection: InGaAs photodiodes, 3
mm diameter, 90% quantum efficiency
• Suppression of TEM01 by a factor of 10
• Length control via temperature (Peltier
cell)
3rd VESF school - Michele Punturo Virgo
Detection bench
17
Output Mode-Cleaner
OMC filtering effect
before OMC
after OMC
3rd VESF school - Michele Punturo Virgo
18
Seismic Noise
• The correct and usual way to realize an interferometer in an
University Lab is to rigidly clamp the optics to the table
• We cannot adopt this solution, mainly, because of the seismic
f  0.1Hz
A
noise:
x (f)
m Hz ,
seism
A  10 7
f2
xseism ( f )  xGW  f   f  105 Hz
x
• The simplest seismic filter is an harmonic oscillator, for
frequencies larger than the resonant one:
x   x  xseism   0  ~
x   
2
0
 02 ~
xseism  
 02   2
xseism
• A pendulum is an harmonic oscillator of natural frequency:
f0 
1
2
g
L
• A cascade of N pendulums is a multistage filter whose transfer
2N
function is:
~
 f0 
x  
  
19
~
xseism    f 
Virgo
“Superattenuators”
XYZ pendulum chains
to reduce seismic motion by
a factor 1014 above 10 Hz
Magnetic anti-springs
Blades
20
Passive Isolation performance
•
Expected seismic displacement
of the mirror (TF measured
stage by stage):
•
Thermal noise is dominant
above 4 Hz
Isolation sufficient also for
“advanced” interferometers
•


2


~
f0
x f 


2
~
xseism  f   f 2  f 2  i f 0 


0
Q


~
x  f0 
N

Q
~
xseism  f 0 
N
Residual motion too high at the
resonances (tens of microns): could be
a problem for the ITF operation
3rd VESF school - Michele Punturo  need of damping!
Virgo
21
Working conditions
• A FP is mainly a non-linear device. It can be used
only at resonance where it is sensitive and linear:
Pr_B8_DC
-5
1.8
x 10
1.6
1.4
Power [W]
1.2
1
0.8
0.6
• Keep the main cavity locked to
enhance the phase response
0.4
0.2
0
0
1
2
3
4
5
Samples (@20kHz)
6
7
8
4
4 seconds
x 10
3rd VESF school - Michele Punturo Virgo
22
… working conditions 2
• The power recycling cavity must be kept locked to reduce
the shot noise
• Keep the ITF in the dark fringe to reduce the dependence on
the power fluctuation
3rd VESF school - Michele Punturo Virgo
23
Interferometer Control
• To push the ITF in the working conditions we need to know the
status of the cavities
• Mainly, we need to know 4 length and the angles of the mirrors
respect to the beams
•
•
Photodiodes Bx
provide the error
signals to control the
4 independent
length of the
interferometer
Quadrant
photodiodes provide
the error signals to
control the angular
positions of the
mirrors
3rd VESF school - Michele Punturo Virgo
24
Locking a cavity
NI
NE
BS
correction
B7
B1p
error
NE correction
B7 (Cavitypower)
Locking trials
Locked
3rd VESF school - Michele Punturo Virgo
25
Locking the ITF
B8_phase/B8_DC
West arm
Michelson
B5
B2
B1_quad
North arm
B5_phase/B7_DC
3rd VESF school - Michele Punturo Virgo
26
How the correction is applied?
• Three application points
– Top of the inverted pendulum (Filter 0)
– Marionette
– Mirror
DC-0.01 Hz
• Locking requirement:
L  10-12 m
• Tidal strain over 3 km:
L  10-4 m
• Resonant motions of the mirrors
0.01-8 Hz
L  10-4 m
8-50 Hz
Need to control the mirror position in a large range of frequency and displacement:
•Need of hierarchical control
3rd VESF school - Michele Punturo Virgo
27
Mirror Actuators
3rd VESF school - Michele Punturo Virgo
28
Tides
• Tides stretch Virgo arms up to 200 mm in 6 hours
• The coils at the mirror level can support up to 10V
corresponding to 100 mm displacement
– The tide displacement causes a saturation of the coil voltage and a
consequent delock of the ITF
Tide prediction
cavity power
Coil voltage
Tide effect
saturation
Loss of lock
29
Tide compensation
• Example of hierarchical strategy:
– The mirror level coil drivers haven’t enough dynamical range to
compensate the tidal effect
– This effect is very low frequency
– Moving the inverted pendulum (IP) is easy and soft
– Use the low frequency part of the interferometer signal as error signal
3rd VESF school - Michele Punturo Virgo
30
Designing the detector sensitivity




~
f 02
x  



~
xseism    f 2  f 2  i f 02 

0
Q 

• Best seismic damping with low Q suspension
• But not all the stages can be highly dissipative because of the
suspension thermal noise
• The ITF mirrors are suspended by an oscillator (the pendulum)
that vibrates (Brownian motion) because of its finite temperature
• The mirror mass itself is a system of oscillators (internal modes)
that oscillate because T>0
• How to evaluate the thermal noise contribution?
• You surely know the Nyquist theorem that defines the voltage
noise at the end of a resistor of impedance R: V  4kBT  R
• Translating the electrical impedance into
mechanical one we have the fluctuationdissipation 2theorem:
Ftherm    4k BT  Z  
31
N
Fluctuation-Dissipation theorem
• The previous formulation of the FD theorem is equivalent to:
2
  
xtherm
4 k BT
2
Y   where Y    1
Z  
• Introducing the transfer function of a mechanical system H():
x 

4k T
F   
2

 xtherm
   B H  

F
F  


Z    
x i x 
H   
• For an harmonic oscillator:
2
therm
x
4k BT 02
  
  
m  02   2 2  02     2

 

4k BT 02

  
5
  0
m 
;
Q
 0 
• Thermal noise issues require mechanical Q as high as possible
3rd VESF school - Michele Punturo Virgo
1
32
L3
Special feature of a Pendulum
• A pendulum is an harmonic oscillator where the
restoring force is mainly given by the gravitation
(lossless force). The dissipation is, instead, due to
the elastic force of the suspension wire.
k P  k P 1  i P 

y
k P  k grav  kel  k grav  kel 1  iw 
L2
L1
 P 
Qy
Qz
kelw
k
 el w 
kP
k grav
1
P 
2 Lw
EI
w
n  mg
mg
EI
n
2 L2w

1 mg w
n Lw
I

4
•Very thin (and strong) suspension wires
•Low loss steel wires
Qx
x
z
33
r4
Contribution to the pendulum Q
• The loss angle that enters in the thermal noise formula is not only
due to the “intrinsic” material loss, but other excess losses must
be taken in account:
– Clamping losses
– Frictional losses / eddy currents
– Residual gas losses
3rd VESF school - Michele Punturo Virgo
34
Mirror thermal Noise
• In the modal expansion approximation we can consider the Virgo mirrors as
composed by an infinite number of oscillators.
• In the limit <<1 the thermal noise displacement power spectrum is given by:
    1
x   4k BT

 i 1 M ii2
• A better estimation of that power spectrum can be obtained by a direct
application of the FD theorem (Levin et al.):
  
2
x   8k BT
U wbeam ,...

2
• Where U is the strain energy in the mirror due to a static pressure having the
distribution of the beam profile
• Other dissipation should be taken in account:
–
–
–
–
Coatings
Thermo-elastic effects
Excess losses due to the clamping and control system
3rd VESF school - Michele Punturo …
Virgo
35
Virgo Mirrors
• The Virgo mirrors are the largest (and more expensive) mirrors in
the current GW detectors
• Since the substrate specifications are very stringent a special fused
silica (Suprasil 311 SV) have been realized on purpose for Virgo:
– Low absorption: 0.7 ppm/cm
– Low OH content (<< 50ppm)
– Low birefringence (<5·10-4 rad/cm)
• The coating specifications are still more stringent
• @1064nm the nominal absorption should be about 1 ppm
• The scattering should be lower than 5ppm
• As usual the effective realization is more difficult than expected:
• Excess optical losses in the mounted mirrors (pollution?)
• Thermal effects
3rd VESF school - Michele Punturo Virgo
36
350 mm
100 mm
3rd VESF school - Michele Punturo Virgo
37
Optical read-out noise
• Optical read-out noise is the (incoherent) sum of the shot noise
and radiation pressure
• In the current ITFs only the shot noise plays a relevant role, but it
is instructive to see a formalism (KLMTV) that reports the two
noises in a single expression:
hor   
hSQL  
2
1
  
Shot noise
   
Radiation
pressure
4
PBS
2 FP
   
2
PSQL  FP
  2   2
PSQL
4
M m L2arm FP

4  2  c  
 FP  2 f knee  2
PBS is the power impinging the beam splitter:
PBS  Plaser  Recycling
3rd VESF school - Michele Punturo Virgo
hSQL   
c
4 Larm  F
8
M m 2 L2arm
38
Virgo Nominal Sensitivity
-18
10
(a) Seismic noise
(b) Thermal pendulum
(c) Thermal Mirror
(d) Optical Read-Out
(e) Newtonian noise
(f) Nominal Virgo
(b)
-19
h(f) [1/sqrt(Hz)]
10
-20
10
(a)
(f)
-21
10
(e)
(c)
-22
10
(f)
(d)
-23
10
1
10
100
1000
10000
Frequency [Hz]
3rd VESF school - Michele Punturo Virgo
39
The real life!
• The design of a GW interferometric detector is
an hard job, but the attainment of the design
sensitivity is even harder
• In fact, a GW detector is a complex machine
that needs a deep tuning of many parameters
– Methods and technologies are completely new
– 5 years of commissioning needed in LIGO
– Similar time spent in Virgo
3rd VESF school - Michele Punturo Virgo
40
Commissioning evolution
Phase A: Commissioning of interferometer arms
• Test all aspects of control systems with a simple optical configuration
- locking, automatic alignment, second stage of frequency stabilization
and
suspension hierarchical control (tidal and marionette)
• First shake of the sub-systems
3rd VESF school - Michele Punturo Virgo
41
Commissioning evolution
Phase A: Commissioning of interferometer arms
• Test all aspects of control systems with a simple optical configuration
- locking, automatic alignment, second stage of frequency stabilization
and
suspension hierarchical control (tidal and marionette)
• First shake of the sub-systems
3rd VESF school - Michele Punturo Virgo
42
Commissioning evolution
Phase A: Commissioning of interferometer arms
• Test all aspects of control systems with a simple optical configuration
- locking, automatic alignment, second stage of frequency stabilization
and
suspension hierarchical control (tidal and marionette)
• First shake of the sub-systems
Phase B: Commissioning of interferometer in ‘recombined mode’
• Useful intermediate step towards full interferometer lock
• Verify functioning of BS longitudinal control
• Re-run all aspects of control system in a more complex configuration
• Start noise investigations
3rd VESF school - Michele Punturo Virgo
43
Commissioning evolution
Phase A: Commissioning of interferometer arms
• Test all aspects of control systems with a simple optical configuration
- locking, automatic alignment, second stage of frequency stabilization
and
suspension hierarchical control (tidal and marionette)
• First shake of the sub-systems
Phase B: Commissioning of interferometer in ‘recombined mode’
• Useful intermediate step towards full interferometer lock
• Verify functioning of BS longitudinal control
• Re-run all aspects of control system in a more complex configuration
• Start noise investigations
Phase C: Commissioning of Recycled Fabry-Perot interferometer
• Run full locking acquisition process
• Verify functioning of PR mirror longitudinal control
• Re-run SSFS, tidal control and marionette control
• Implement complete wave-front sensing control
• Continue noise investigations
3rd VESF school - Michele Punturo Virgo
44
Commissioning evolution
Phase A: Commissioning of interferometer arms
• Test all aspects of control systems with a simple optical configuration
- locking, automatic alignment, second stage of frequency stabilization
and
suspension hierarchical control (tidal and marionette)
• First shake of the sub-systems
Phase B: Commissioning of interferometer in ‘recombined mode’
• Useful intermediate step towards full interferometer lock
• Verify functioning of BS longitudinal control
• Re-run all aspects of control system in a more complex configuration
• Start noise investigations
Phase C: Commissioning of Recycled Fabry-Perot interferometer
• Run full locking acquisition process
• Verify functioning of PR mirror longitudinal control
• Re-run SSFS, tidal control and marionette control
• Implement complete wave-front sensing control
• Continue noise investigations
Phase D: Noise hunting
3rd VESF school - Michele Punturo Virgo
45
Sensitivity Improvement
3rd VESF school - Michele Punturo Virgo
46
Sensitivity of the global network
NSNS detection distance
3rd VESF school - Michele Punturo ~6Mpc
Virgo
47
Noise Budget
Unclear excess noise
sources
Residual light
scattering
Shot noise dominated
48
Scientific case:
Thermal effects in the Virgo mirrors
• The 3km Virgo arms are a long Fabry Perot cavity:
LM
LA
AR
HR
~10-3 – 10-4
• Hence, actually, each arm is a double FP cavity:
2
rHRe 2ikL
• Etalon effect r  r  t AR
M
M
AR
1  rAR rHRe 2ikLM
3rd VESF school - Michele Punturo Virgo
tM2 rend e 2ikLA
rFP   rM 
1  rM rend e 2ikLA
49
Etalon Effect
• Hence, the Finesse of the cavity and all the fundamental parameters
of the ITF are affected by the input mirror optical thickness
variation
• But, why the mirror optical thickness fluctuates?
• Temperature!!
2
d
2  dn
dx 
  kx 

n  x   
dT
T 
 
xn
T
  dT
dT 
2  dn

x
 (n  1)    T
  dT

• Hence, knowing the mirror temperature it is possible to predict
some of the ITF performances
• OK, but how to measure the mirror temperature?
3rd VESF school - Michele Punturo Virgo
50
Resonant mode technique
• Obviously the resonant frequencies of a body depend on
the temperature of the body
• For a Virgo mirror we evaluated this dependence with a
ANSYS based FEM
Frequency Variation [Hz]
5
4
3
2
1
Drum mode:
0.61 Hz/K
ButterxM
ButtertM
DrumM
0
-1
-2
-3
Butterfly mode(s): 0.28 Hz/K
-4
282 284 286 288 290 292 294 296 298 300 302
Temperature [K]
51
Calibration crosscheck
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
-0,1
-0,2
-0,3
-0,4
-0,5
-0,6
-0,7
-0,8
-0,9
-1,0
-1,1
-1,2
-1,3
dTWIn
WIoven
Scientific run
30 days
23,8
23,7
23,6
23,5
23,4
23,3
23,2
23,1
23,0
22,9
22,8
22,7
22,6
22,5
22,4
22,3
22,2
22,1
22,0
21,9
T [C]
T resonant mode [K]
• To crosscheck the FEM calibration we compared the prediction
with the temperature measured just outside the towers
000
000
000
000
000
000
728
320
912
504
096
688
5
8
0
3
6
8
6
6
7
7
7
7
8
8
8
8
8
8
Time [GPStime]
52
Evidence of the Etalon effect
• The confidence in the resonant mode is critical in this evaluation.
– Could we find an independent confirmation?
• Etalon effect in the input mirrors
AR
HR
HR
B7/B8 phd
LB
r2
dn
dT
r3
0
0
AR coating
LA
0.02
P/P
r1
varPot ( x)
0.04
0.06
0.067846 0.08
0
0
0.2
0.4
0.6
x
T
0.8
53
1
1
VSR1 BNS horizon
• During the VSR1 the horizon grown from 3.7 to 4 Mpc thanks to
the limited commissioning activity
– Now we are at about 6Mpc
• The fluctuation in the horizon was caused by
– Bad weather
– Alignment fluctuation
– Etalon effect combined with control issues
3rd VESF school - Michele Punturo Virgo
54
The dark side of the experiment!
• The photodiode that contains the GW signal is “just one”.
• We are sensible up to few kHz:
– A sampling rate of 20kHz is “correctly oversized”
– 20kHz  8Bytes = 160kB/s of expected data recording
• Instead we write about 7-8MB/s…… why?
• Many control channels:
– Secondary beams
– Actuation and error signals
– Environmental monitoring
N
I
W
I
B
S
S
R
D
B
P
R
Temperature probes, 88, 1 Hz
Episensors, 6, 1 kHz
Humidity probes, 4, 1 Hz
Accelerometers, 9, 10 kHz
Pressure probes, 4, 1 Hz
Magnetometers, 3, 20 kHz
Microphones, 3, 10 kHz
I
B
B
55
The online architecture
Front-End Data Collection
Fast Digitization , Environment
Local Servo-Loop
Monitoring
Fast Digitization
Locking and Alignment Servo-Loops
Photodiodes
Readout
Global
Control
Damping and
Control
suspensions
Calibration
Frame Builder,
Local Control
Slow
Monitoring
Station
GPS
Timing
Crate
Fast Frame
Builder
Main Frame Builder
Detector Monitoring
Timing System
Slow Frame
Builder
Frame Building
Frame Processing
Data
GPS signals
Archiving
Timing Information
Servo-Loop DOL (Optical Fibers)
DOL (Optical Fibers)
SMS format (Ethernet)
Frame format (Ethernet)
H
Reconstruction
3rd VESF school - Michele Punturo Virgo
Data
Quality
On-line
Processing
Offline data
analysis
56
The evolution
• Virgo concluded its first long run (VSR1) in parallel with the
LIGO-S5 run the 1st of October 2007
• During the winter we had an intense commissioning and upgrade
activity that reduced the noise level mainly at low frequency and
improved our sensitivity and detector understanding
• In the next summer a long shutdown is scheduled to install a
powerful laser (reduction of the shot noise), an improved injection
optical system and new control and DAQ electronics
Virgo+ project
3rd VESF school - Michele Punturo Virgo
57
Detection Probability
• The Initial LIGO and Virgo detectors are often called 1st
generation ITF detectors
• What is the detection probability with them?
– It depends by the typical merger rate in a galaxy times the number of
galaxies “visible” by our detector
– The merger rate is quite uncertain (CBC group)
Source
NS-NS (MWEG-1 Myr-1)
Rmin
Rre
Rpl
Rup
0.1
100
1000
2000
Hence the detection rate is not well defined (but low!):
Source
NS-NS initial LIGO (yr-1)
Rmin
Rre
Rpl
Rup
1.5×10-5
0.015
0.15
0.3
Abbreviation
Rate statement
Physical significance
Up
Upper limit
Rates should be no higher than …
Pl
Plausible optimistic estimate
Rates could be as high as
Re
Realistic estimate
Min
Rates are likely to be
3rd VESF school - Michele
Punturo Expected minimal estimate Virgo
Rates should be at least
58
Let suppose that the
distribution of the galaxies in
the universe is uniform:
Basic Idea
R
3
Nevents Vdetectionsphere  Rdetection
Increasing the detection
distance by 2 (10), a factor 8
(1000) in event rate could be
expected
In effect the galaxies density
is not constant:
Credit: R.Powell
3rd VESF school - Michele Punturo Virgo
59
Detector Evolution program
• Current detectors:
– 1st generation: Virgo & LIGO
• <15MPc
• Enhanced detectors:
– 1.5 generation:
• Virgo+ & enhanced LIGO
– 20-40 MPc
• Advanced detectors:
– 2nd generation:
• Advanced Virgo &
Advanced LIGO
LIGO - Virgo
LIGO+ - Virgo+
Credit: R.Powell
– ~150 MPc
– 3rd generation:
• E.T. FP7 Design Study
– ??? MPc
3rd VESF school - Michele Punturo Virgo
AdvLIGO - AdvVirgo
60
Detector evolution timeline
´06 ´07 ´08 ´09 ´10 ´11 ´12 ´13 ´14 ´15 ´16 ´17 ´18 ´19 ´20 ´21 ´22
Virgo+
Virgo
GEO HF
GEO
LIGO
Advanced Virgo
Hanford
Livingston
LIGO+
Advanced LIGO
Launch
LISA
E.T.
DS
PCP
Construction
3rd VESF school - Michele Punturo Virgo
Transfer
data
Commissioning data
61
Key points of the advanced detectors
• Reduction of the shot noise:
–
–
–
–
Higher laser power (160W laser)
Higher cavity finesse
DC detection
Signal recycling
• Reduction of the mirror thermal noise
–
–
–
–
Improved materials for the substrates
Heavier substrates
Improved coatings materials and geometries
Improved beam geometries (Virgo)
• Reduction of the suspension thermal noise
– Monolithic fused silica suspensions
• Reduction of seismic noise
– Active suspensions (LIGO)
3rd VESF school - Michele Punturo Virgo
62
Sensitivity evolution
10
-18
h(f) [1/sqrt(Hz)]
st
10
-19
10
-20
10
-21
10
-22
10
-23
10
-24
1 generation:
Virgo
LIGO
1.5 generation:
Virgo+
eLIGO
nd
2 generation
advVirgo
advLIGO
10
100
1000
10000
Frequency [Hz]
3rd VESF school - Michele Punturo Virgo
63
rd
3
generation: ET
• Advanced detectors are an evolution “in situ” of the current
detectors
• To gain a further factor 10 in sensitivity the current
infrastructures are insufficient
• We need:
– Underground sites
• Reduction of the seismic and Newtonian noise
– Cryogenic infrastructures
• Reduction of the thermal noise and of the optics lensing
• GEO and Virgo collaborators started, with the European
Commission support, a design study of a 3rd generation GW
observatory: ET
3rd VESF school - Michele Punturo Virgo
64
3rd VESF school - Michele Punturo Virgo
65