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The VIRGO detection system
Paolo La Penna
European Gravitational Observatory
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
VIRGO interferometer
3 km
INPUT
MODE
CLEANER
WE
Rcurv= 3500 m
R= 0.99995
Modulator
single frequency
6.26 Mhz
WE
Rcurv= 3500 m
R= 0.99995
3 km
INPUT
MODE
CLEANER
(144 m)
Nd:YAG
= 1064 nm
20 W
Modulator
single frequency
6.26 Mhz
WI
plane
R= 0.88
5.6 m
Nd:YAG
= 1064 nm
20 W
WI
plane
R= 0.88
3 km
6m
6.4 m
BS
NI
plane
R= 0.88
PR
plane
R= 0.925
NE
Rcurv= 3500
R= 0.99995
OMC
DETECTION
SYSTEM
5.6 m
(144 m)
3 km
6m
6.4 m
BS
NI
plane
R= 0.88
PR
plane
R= 0.925
NE
Rcurv= 3500
R= 0.99995
OMC
DETECTION
SYSTEM
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
DETECTION SYSTEM
Aim:
Detecting the gravitational wave signal by measuring accurately the
output power of the interferometer.
Principle:
Dark fringe (improved shot noise S/N ratio)
Schnupp technique (partially transmitted sidebands (6.26 MHz) beating
with the carrier)
Spatial filtering (isolate the TEM00 mode with the Output Mode
Cleaner):
Much smaller photodiode dynamics (1 W, 16 photodiodes)
Increase the contrast of the interferometer to reach the optimal
sensitivity.
Additional functions:
•Interferometer locking
•Automatic alignment
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
WHY AN OUTPUT MODE CLEANER



Interferometer on the dark fringe to improve the shot noise S/N
But: contrast C < 1 (mirror imperfections, different Rcurv, misalignments, …)
If contrast C < 1  modulation index m has to become larger to contain S/N losses
d min 

2h
1
G (1  C ) 2
2
J 0 m   J1 m 
GP J 0 m J1 m 
4T
m larger  more transmitted power (more sidebands T )
J 0 mJ1 m
S
 1
 
G1  C  2
 N  LOSS
2
J 0 m  J1 m
4T


In order to keep the losses (S/N)LOSS <10%, with C 10-2 , G Rec.Gain=50  m 1.1  1.4
 PTRANS  25%  75% PIN  2.5  7.5 W
Limit on PMAX on photodiodes: too many photodiodes are necessary
The contrast C has to be improved  Output Mode Cleaner (OMC)
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
Output Mode Cleaner
3 km
INPUT
MODE
CLEANER
WE
Rc urv= 3500 m
R= 0.99995
(144 m)
Modulator
single frequency
6.26 Mhz
5.6 m
Nd:YAG
= 1064 nm
20 W
WI
plane
R= 0.88
3 km
6m
6.4 m
BS
NI
plane
R= 0.88
PR
plane
R= 0.925
NE
Rc urv= 3500
R= 0.99995
OMC
DETECTION
SYSTEM
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
VIRGO OMC requirements
OMC:
•FP cavity for spatial filtering: C improvement of a factor 100
 F  50
•Triangular (no feedback back into the ITF)
•Transmit TEM00 for carrier and sidebands (6.26 MHz)
either very long (two contiguous Airy peaks)
or very short cavity (same Airy peak)
•Chosen a short cavity (about 5 cm): more compact,monolithic
(less alignment problems)
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
VIRGO OMC TOWER
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
VIRGO BEAM SCHEME
Four optical benches:
• The suspended detection
bench
B8
Q81
• The external detection
bench,
Q82
IMC
• The north external bench,
LASER
L= 3 km
MC
• The west external bench.
L= 5,6 m
B2
Q21
Q22
Six beams:
L= 6 m
L= 6,4m
IMC_D1T
Q72 Q71
RFC_DT
DT
B7
L= 3 km
RFC
OMC
• B1 (OMC transmitted beam),
• B1p (before entering the
OMC),
• B1s (OMC reflected beam),
• B5 (beam reflected by the
BS second face),
Q12
Q11
CalTech, January 18th 2005
B5
B1p
B1s
• B7, B8 (beam transmitted by
NE and WE mirrors).
B1
The VIRGO detection system
Paolo La Penna (EGO)
Suspended and External Detection Benches
Air
On ground
Vacuum
10-6 mbar
Suspended
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
Suspended Bench Local Control
The OMC suspended bench is controlled locally:




Four leds send light to four mirrors on the bench from four
different directions
The mirrors send the reflections to a CCD camera
The CCD software selects the four zones of the spots and
reconstructs the bench movements
Corrections are sent to the bench coils
The leds are leaning on the tower frame (on ground)
The CCD is leaning on ground
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
Suspended and External Benches Layout
B1: OMC transmission (DF signal)
B1
B1s: OMC reflection
B1s
B1p
B5
B1p: DF before OMC (lock acq)
B5: PR cavity power
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
Suspended Bench Layout (OMC bench)
•Telescope (L1+L2+M1+M2+prism):
separation of the 2 input beams (B1 and
B5), adaptation of the spot size (from 2 cm
to 1 mm), bench alignment
•Quadrant photodiodes (DQ1, DQ2): bench
alignment with the input beam B5
•Lens L3: adaptation of the spot size B1
before entering into the OMC (from 1 mm to
140 μm)
•OMC: Output Mode Cleaner
•Faraday: prevents back reflections
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
OMC Characteristics
B1s
B1p
B1
Material: fused silica
Length  2 cm
Optical path  7.5 cm
Finesse: 50
FSR  2 GHz
 40 MHz
RCURV = 30 cm
Waist = 140 mm
Obtained matching  94%
Losses  1%
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
OMC Characteristics
The cavity is monolithic:
The input beam has to
be elliptical to match the
cavity.
Matching prisms are
used to match the cavity
waist
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
OMC Locking
OMC locking on TEM00:
Via temperature control.
Three signals are used to reach
and keep a stable lock:
• The spot shape (a nongaussian shape means that
other modes are
transmitted),
• The power transmitted by
the OMC (the power is
maximum when the cavity is
resonant for the TEM00),
• The Pound-Drever signal
(this signal is achieved
thanks to a piezo excitation
of the cavity at 28 kHz, on
the upper face).
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
OMC assembling
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
OMC Locking
TEM00
•OMC temperature is scanned (2°=1 FSR in 1000 sec)
•Peak detection: a CCD compares the image with the expected
TEM00
•2 test is performed (10 times/sec)
•When 2 is below a threshold, starts the linear temperature feedback
(less than 0.1 Hz bandwidth)
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
OMC alignment
CalTech, January 18th 2005
The VIRGO detection system

Alignment and
matching is
performed once

Q1 and Q2
quadrants are used
as a reference with
B5 beam

Alignment is
performed
automatically when
Q1 and Q2
asymmetry exceeds
a threshold
Paolo La Penna (EGO)
The External Detection Bench
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
Final External Detection Bench
16 photodiodes, 1 CCD camera for B1
2 quadrant photodiodes for
automatic alignment
2 photodiodes, 1 CCD
camera for B1p/B1s/B5
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
Present EDB Setup
Beams with recycled interferometer
Input power  700 mW (10% attenuation of Pin)
B5 3.18 mW
B1p 11.9 mW
B1 2.7 mW
B1s 0.18 mW
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
OMC and ITF control
Locking procedure:
OMC
is locked after the ITF locking:
B1p (before OMC , 1% of Dark Fringe) is used for locking acq
After ITF locking, the OMC is locked on the TEM00 (beam B1)
After OMC locking, the ITF control is moved to B1 (OMC transmission)
•OMC has been locked with
The Second Stage of Frequency Stabilization (ITF common mode
feedback to laser frequency)
PR locking
Full hierarchical control
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
Performances
The detection system is very reliable and robust:
 The
lock is acquired within 10 minutes (usually a few minutes)
 Switch
to B1 control does not cause any problem
 Automatic
procedures have been implemented for realignment and
relocking when golden state condition is lost or close to be lost
 Lock
is robust: no OMC unlock happens
 Length
precision: about /60,000
CalTech, January 18th 2005
(10-11 m, rms on 1 h ;
requirements is /3,000)
The VIRGO detection system
Paolo La Penna (EGO)
Difference with lock on OMC
VIRGO is not limited by shot noise yet:
some HF difference is due to the fact that B1p 0.5% of the dark fringe
(more amplification, more electronic noise)
Sensitivity with switch on B1 (Feb 21 2004)
Sensitivity with switch on B1p (Feb 21 2004)
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
Low frequency
B1 and B1p spectrum are different at low frequencies for several reasons:

B1p contains higher order modes

The low frequency angular motions couple
to B1 through the OMC and are reinjected
by the control as length noise L :
This length noise slightly cancels
the effect of angular noise on B1
This length noise is measured by
B1p
B1p_ACp
B1_ACp
B1_ACp might not measure the real L:


B1p can be better estimate of L
The real L must be between B1 and B1p
estimate
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)
Summary

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





The OMC of VIRGO is a small triangular monolithic cavity;
The cavity is mounted in vacuum on a suspended bench;
The bench is locally aligned
The OMC is locked with thermal actuators;
The OMC has worked satisfactory in the CITF and in VIRGO;
The recycled VIRGO has been locked using the OMC transmission;
No major problem with the OMC has been detected yet: the system is
robust and reliable, and it is operated routinely with automatic
procedures;
Since VIRGO is still limited essentially by control noise and not by shot
noise, no significant improvement in using the OMC locking is visible
yet;
However, the signal and the beam are clearly cleaner and the cleaning
effect of the OMC is evident.
CalTech, January 18th 2005
The VIRGO detection system
Paolo La Penna (EGO)