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Status of VIRGO
A. Freise
For the Virgo Collaboration
European Gravitational Observatory
SPIE Glasgow 2004
24. June 2004
Andreas Freise
The VIRGO Collaboration
VIRGO is an Italian-French collaboration for
Gravitational Wave research with a ground-based
interferometer.
ITALY - INFN
Firenze-Urbino
Frascati
Napoli
Perugia
Pisa
Roma
FRANCE - CNRS
ESPCI – Paris
IPN – Lyon
LAL – Orsay
LAPP – Annecy
OCA - Nice
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The European Gravitational Observatory
Last mirror installed
June 2003
Inauguration
August 2003
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Expected Sensitivity
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The VIRGO Interferometer
Input Mode Cleaner
144 m long
High quality optics are:
located
in vacuum with
Michelson
Interferometer
suspended
from cavities
3 km long
Fabry-Perot
multi-stage
pendulums
in the arms
and Power
Recycling
Laser 20 W
Output Mode Cleaner
4 cm long
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Main Optics
High quality fused silica mirrors
35 cm diameter, 10 cm thickness
Substrate losses
1 ppm
Coating losses
<5 ppm
Surface deformation l/100
(rms on 150mm)
DR:
<10-4
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Vacuum System
Two tubes: 3 km long, 1.2 m in diameter, installed and
tested, in vacuum since June 2003, 10-6 mbar
All tower (6 long, 2 short) pumping systems: installed,
tested and put in operation, 10-9 mbar
Towers
Tubes
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The Suspension System
The Superattenuator (SA) is designed
to isolate the optical components from
seismic activities (local disturbances).
It is based on the working principle of
a multistage pendulum.
Expected attenuation @10 Hz: 1014
Residual mirror motion (rms)
rotation
<1 mrad
longitudinal
<1 mm
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The Top Stage
Top of an inverted pendulum:
- Inertial damping (70 mHz to 5 Hz)
- Possibility to move the suspension
point with small forces
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Passive Filters
Five seismic filters:
Suspended by steel wires
Vertical isolation by a combination
of cantilever springs and magnetic
anti-springs
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The Local Control
Marionette control:
CCD camera, optical levers and four
coil-magnet actuators: <2 Hz
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Fast Control (Global)
Reference mass and mirror,
four coil-magnet actuators: >2 Hz
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Laser
Located on an optical table outside the vacuum
Nd:YAG master commercial CW single mode (700 mW) @1064 nm
Phase locked to a Nd:YVO4 slave (monolithic ring cavity)
Pumped by two laser diodes at 806 nm (40 W power)
Output power: 20 W
Master
Slave
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Input Mode Cleaner
Triangular cavity, 144 m long, Finesse=1000
Input optics and two flat mirrors are located on a suspended
optical bench
End mirror suspended with a reference mass
for actuation
Transmission 50%
Injection Bench
Mode Cleaner Mirror
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Detection System
Output Mode-Cleaner
Suspended bench in vacuum with optics for beam
adjustments and the output mode cleaner (OMC)
Detection, amplification and demodulation
on external bench
Suspended bench
External bench
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Output Mode Cleaner
4 cm long ring cavity
Suppression of TEM01 by a
factor of 10
Length control via temperature
(Peltier element)
Lock acquisition takes 10 min,
lock accuracy is l/60000
Detection Bench
Output Mode-Cleaner
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Photo Diodes
Output Mode-Cleaner
16 InGaAs diodes for the main beam (dark port)
6 additional photo diodes (and 8 split photo diodes) for
control purposes
External bench
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Control
Fully digital control, local and global
Feedback is send with 20-bit DACs @ 10kHz to the
suspensions
The suspension control is performed by decicated
DSPs (one per suspension)
Interferometer signals are acquired with 16-bit ADCs
@ 20 kHz. The data is transferred via optical links
to Global Control, a dedicated hard and software that
computes correction signals and sends them to the
mirror DSPs
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Computer Control
Monitoring and control of the detector by scientists and
operators using:
- 10 workstations
- a digital video system
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Current Status
Commissioning of the central interferometer and the
injection system from 2001 to 2003
Since September 2003 commissioning of the two arms
and the full interferometer
Interferometer
At the end of this year the detectorCentral
is planned
to be(CITF)
operated in its final configuration
Scientific data taking starts 2005
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Andreas Freise
Commissioning of VIRGO
The commissioning of the full detector is divided into three
phases:
Phase A: the 3 km long arm cavities separatly
Phase B: recombined Michelson interferometer
Phase C: Michelson interferometer with Power Recycling
Currently we have completed phase B.
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Commissioning Runs
Continuous data taking periods are scheduled every
second month:
C1: 14.-17. November 2003
North arm cavity longitudinally controlled
C2: 20.-23. February 2004
North arm cavity with longitudinal and angular control
C3: 23.-27. April 2004
Recombined interferometer
North arm with second stage of frequency stabilisation
C4: 24.-29. June 2004
Recombined interferometer with angular control and
second stage of frequency stabilisation
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Sensing and Control
Modulation-demodulation
scheme with only one
modulation frequency
(6 MHz) to control:
4 lengths
10 angles
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North Arm Cavity
Phase A: the two arm
cavities are used separatly,
starting with the north arm;
control systems are
to be installed and tested.
West arm misaligned
PR misaligned
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North Arm Cavity
In October 2003 the North arm cavity was locked on first trial
using a control algorithm that was tested before with SIESTA,
a time domain interferometer simulation
The West arm cavity was locked in December 2003
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North Arm Sensitivity
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North Arm Sensitivity
Sensitivity limited by
frequency noise between
7 Hz and 4 kHz (C1)
By reducing the bandwidth
of the IMC control loops the
frequency noise could be
improved
The noise below 4 kHz (C3)
still originates in the
injection system but is not
fully understood
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Recombined Interferometer
Phase B:
Recombined Interferometer
- B2 (P) used to control common
mode (L1+L2)
- B2 (Q) used to control beam splitter
- B1/B1’ used to control differential
mode (L1-L2)
Recombined locked
in February 2004
PR misaligned
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Automatic Alignment
Anderson technique:
- Modulation frequency coincident with cavity TEM01 mode
- Two split photo diodes in transmission of the cavity
(at two different Guoy phases)
- Four signals to control the 2x2 mirror angular positions (NI, NE)
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Automatic Alignment
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•
Alignment control allows to
switch off local controls
Power inside the cavities
becomes more stable
Installed and tested for the
recombined interferometer
Bandwidth ~3 Hz
Residual fluctuations ~0.5
urad rms (1 nrad @ 10 Hz)
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Frequency stabilisation
The `second stage´ of
frequency stabilisation
The laser frequency is stabilised to the
common lengths of the
arm cavities (bandwidth ~17 kHz)
The arm cavities are stabilised to the
reference cavity (bandwidth ~2 Hz)
Gains of the frequency control loops
are increased
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Frequency stabilisation
The pre-stabilisation of the laser frequency was
succesfully tested in the central interferometer
The frequency noise of the pre-stabilised laser
limits the sensitivity over a wide range
The second stage required for the designed stability
was completed in June 2004
The performance will be tested in the commissioning
run (C4) starting today
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Present Sensitivity (preliminary)
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Conclusions
The construction of VIRGO is finished, the
commissioning of the full instrument is underway
Both arm cavities were put into operation, longitudinal
and angular control were implemented
The injection system and the detection system with
both mode cleaners are used routinely during the
commissioning
The detector is now used in the recombined mode with
automatic mirror alignment and the second stage of
frequency stabilisation
Starting next week the final configuration including
Power Recycling will be implemented
The first scientific data runs are planned for next year
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End
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Data Acquisition and Storage
16-bit ADCs, up to 20 kHz sampling
frequency
Data is transferred via optical links
to workstations and then written
to disk in Frame format:
- full signal
- down-sampled to 50 Hz
- down-sampled to 1 Hz (trend data)
Frames available for data monitoring
with 3 s delay
Current rate: 7 Mbytes/s (compressed)
On-site storage on spinning media:
- 5 TB for online analysis
- 70 TB for offline analysis
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Injection System
20 W Nd:YAG master-slave laser
IMC
RFC
Input mode cleaner: 144 m long
ring cavity
Reference cavity: 30 cm long ring
cavity, mirrors contacted to a rigid
ULE spacer
Both cavities: Finesse=1000
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Recombined Interferometer
Sensitivity below 300 Hz limited by
electronic noise from a temporary
sensor setup
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Automatic Alignment
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