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The New VIRGO Injection Bench
Paolo La Penna
European Gravitational Observatory
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Recombined configuration: first half of 2004
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
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Recycled interferometer: July 2004
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
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
PR REFLECTION INSIDE IMC: FREQUENCY NOISE
Black:
PR
misaligned
Red:
PR aligned
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
PR reflection scattering inside IMC
MC scattering (10 ppm)
IMC
Cavity effect
(10% fringes)
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
PR
Paolo La Penna
The PR feedback inside the IMC (Summer 2004 - now)
• Simulations on a lock acquisition technique developed
following the LIGO experience
• Locking trials with this baseline technique failed
(first half of July)
• Attenuator installed (summer)
• Restart of the locking trials with the baseline technique
(21st September)
• Establishement of theVariable Finesse lock acquisition
technique (October)
• PR cavity locking (end of October)
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Temporary solution: Input Beam Attenuation
IMC
10% M6
From the laser
To the interferometer
700 mW (10% of the
full power)
Reference Cavity
Final solution: Insert a Faraday Isolator
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
OLD M6 vs NEW M6
OLD M6
Hz
(Same y-scale)
NEW M6
Hz
PR LOCKING ACQUIRED AFTER ONE MONTH
CONFIRMATION OF THE BACKSCATTERING PROBLEM
NEED OF AN ISOLATOR (FARADAY)
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Beam diameter  10 mm
Beam waist  5mm
If a Faraday is needed it will have a large aperture
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
EOTec Faraday Isolator

TGG (Terbium Gallium Garnet,) vacuum compatible;

20 mm maximum clear aperture : (25 and 50 mm would be “glass based”
rotators, not reliable for 10 W power);

Weight: 7 kg, TGG rod length: 20 mm, isolator length: 25 cm

Extinction measured in Nice 41 db;

Absorption: less than 0.0025 cm-1;

EOTec uses vacuum compatible thin film polarizers at Brewsters angle:
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BK7
Pulsed damage threshold >5J/cm2 10nsec pulse
Extinction >1000:1 @1064 nm
Transmission: 95-97% @1064 nm (97% maximum)
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Faraday isolator: present bench
670 mm
250 mm
Not enough space on the
present bench
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
IB design with a Faraday Isolator
Even with a 20 mm clear aperture Faraday:

The IMC waist is 5 mm: the beam has to be reduced in size
before entering in the Faraday

A telescope for reducing the beam to 2-3 mm in the Faraday has
to be taken into account and designed

Thermal lensing: no plan to correct it (using FK51 for example)
with present input power (about 10 W): the induced focal length
should be of the order of 50-100 m

Induced (stationary) thermal lensing effects will be corrected
acting on the matching telescope.
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Faraday Isolator
Brewster dielectric polarizers
Rotator
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Thermal focal length measurement
20W Pump
Faraday
2f
2f
ShackHartman
HeNe
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Shack-Hartmann measurement
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Plane Power Recycling mirror




The present PR is made by two parts: a curved one inside an external
cylindrical glass mass (t’s a lens, part of the telescope for collimating inside
the ITF)
The curved part is fixed to the cylindrical by means of a steel ring and
pression screws;
There is evidence of mechanical resonances: problems in locking acquisition
and in the future Frequency Stabilization: need of a monolitic mirror
Decided to make it plane (get rid of transverse movements of the beam
induced by the suspension displacements)
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Plane PR: (new) 6 parabolic off-axis telescope
With plane PR: a big magnification is needed,
the telescope has to be short (about 700 mm)
Parabolic mirrors are needed
The computed curvature radius are not on-shelf: custom mirrors
IMC: waist 4.9 mm
Condensing telescope
FI: waist 2.65 mm
M5: f= 75 mm
M6: f = 600 mm
Ø  10 cm
N5-WP1 4TH MEETING, Hannover, 7/04/2005
to PR: waist=20 mm
d= 675 mm
The New Virgo Injection Bench
Paolo La Penna
Plane PR: Beam simulation
Input mirrors
N5-WP1 4TH MEETING, Hannover, 7/04/2005
End mirrors
The New Virgo Injection Bench
Paolo La Penna
Telescope tolerances

A tolerance study for the whole telescope
(Faraday collimating and off-axis parabolic
telescope) has been performed using Zeemax in
the case of plane PR

The global tolerancies for plane PR seem to be
accettable
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
IB layout
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Optical layout - 2
Detector table
IB input window(s)
The New Virgo Injection Bench
N5-WP1 4TH MEETING, Hannover, 7/04/2005
Paolo La Penna
Beam drift control
MC
-- ALIGNMENT SERVO
PICOMOTORS
PIEZOS
LC
ABP_U
M6
Laser
M3
M5
ABP_D
M2
PZT
IMC autoalignment
NF
h
FF
NF
Ref. cav.
v
FF
MATRIX
N5-WP1 4TH MEETING, Hannover, 7/04/2005
RFC autoalignment
The New Virgo Injection Bench
Paolo La Penna
IMC and RFC alignment
10%
ABP OFF
ABP
dismounted
RFC is more stable than IMC:
RFC is fully automatically alignmed
IMC is partially automatically aligned
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
ISYS alignment: modifications to the IB

Automatic alignment of the IMC on the beam: acting on
the IB through the IB marionette;

Separate automatic alignment of the Reference Cavity on
the beam:
 Actuators on the bench are necessary
 Replacement of the present RC fixed steering mirrors with other
actuators (piezos)
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Beam drift control
MC
-- ALIGNMENT SERVO
PICOMOTORS
PIEZOS
ABP_U
M6
Laser
LC
M3
M5
ABP_D
M2
PZT
v
IMC autoalignment
NF
h
MATRIX
FF
NF
Ref. cav.
h
v
Beam prealignment
FF
MATRIX
N5-WP1 4TH MEETING, Hannover, 7/04/2005
RFC autoalignment
The New Virgo Injection Bench
Paolo La Penna
Input beam monitoring
FF telescope: locate quadrant at focus (tilts)
NF telescope: conjugate ABP_D mount on NF_Quadrant (shifts)
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Suspended Bench
Marionette
Reference
Cavity
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Reference Cavity alignment
The design of the RFC layout should not change
M11 and M12 will be
mounted on piezos
for RFC separate
alignment
(Closed loop Physik
Instrumente piezos)
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
VIRGO OPTICAL SCHEME
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
Frontal modulation
N5-WP1 4TH MEETING, Hannover, 7/04/2005
DETECTION
SYSTEM
The New Virgo Injection Bench
Paolo La Penna
Sidebands resonant in the IMC

EOM for the 6.26… MHz is now before the IMC:
sidebands and carrier have to be resonant at the same
time inside the IMC to get into the ITF
lIMC (a.u.)
JN5-WP1 4TH MEETING, Hannover, 7/04/2005
J0
The New Virgo Injection Bench
J+
Paolo La Penna
Sidebands resonant in the IMC

If the modulation frequency is not exactly a multiple of the
IMC FSR there can be an amplitude change in the
sidebands and a signal on the photodiodes signals (in
particular on the ITF reflection)
JN5-WP1 4TH MEETING, Hannover, 7/04/2005
J0
The New Virgo Injection Bench
J+
Paolo La Penna
Modulation frequency tuning
B2_ACq signal: f = 6.264080 MHz 
N5-WP1 4TH MEETING, Hannover, 7/04/2005
f = 6.264150 MHz (tuned)
The New Virgo Injection Bench
Paolo La Penna
Pockels Cell on the Input Bench after the IMC
The IMC length is apparently drifting. A feedback is
needed to keep the modulation frequency matched with
the IMC length (or correct the IMC length drift).

This problem could be also solved placing the EOM after
the IMC (on the IB, in vacuum, on the main beam going
into the ITF).

In this case the first basic question is : where could an
EOM be placed? That is, how could this EOM look like,
and where is there space on the bench?
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Pockels cell on the Input Bench after the IMC

Contacted several companies
One (Leysop) gave more detailed answers (but still not final one)

After discussion, a possible proposed EOM is:

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LiNbO4 (possible occurrence of self focusing and astigmatism)
15 mm aperture (beam waist=2.65 mm)
About 50mm40mm  40mm external dimensions
Half wavelength voltage: 1295 V
Price: about € 4000
Delivery time: no answer yet
For the moment we are trying to design the layout
reserving space for possible accomodation of an EOM
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Space reserved on the new IB
EOM
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Problems

Is it possible to place the EOM after the IMC?
Is it a good idea?

There are many questions:

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thermal focussing effects,
astigmatism,
distortions
high voltage for modulation,
etc.
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna
Other GW experiments
We have no experience on these problems
Which is the experience in the other GW experiments?
As far as we know:

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TAMA: no EOM in vacuum (not on the main beam)
LIGO: no EOM in vacuum (not on the main beam)
GEO:
 there are several EOM in vacuum and at least one EOM
after the last IMC on the main beam
 GEO experience could be very useful to understand the
opportunity of putting this EOM after the IMC
N5-WP1 4TH MEETING, Hannover, 7/04/2005
The New Virgo Injection Bench
Paolo La Penna