Status of Virgo experiment

Matteo Barsuglia, LAL/CNRS Orsay

On behalf of the Virgo Collaboration

TAMA symposium, Feb 18th 2005

The VIRGO Collaboration

VIRGO is an Italian-French collaboration

ITALY - INFN

Firenze-UrbinoFrascatiNapoli PerugiaPisaRoma

FRANCE - CNRS

ESPCI – ParisIPN – LyonLAL – OrsayLAPP – AnnecyOCA - Nice

Virgo site (20km from Pisa, Italy)at

European Gravitational Obseervatory (EGO)

Expected Sensitivity

Seismic wall at ~ 4 Hz

The VIRGO Interferometer

Laser 20 W

Output Mode Cleaner4 cm long

Michelson Interferometer with3 km long Fabry-Perot cavitiesin the arms and Power Recycling

Input Mode Cleaner144 m long

High quality optics are:

located in vacuum suspended from

multi-stage pendulums

Vacuum System

Two tubes: 3 km long, 1.2 m in diameter, in vacuum since June 2003, 400 modules6 long and 3 short superattenuators towers

Tubes

TowersCentral area

The Suspension System

The Superattenuator (SA) is designed to isolate the optical components from seismic activities (local disturbances).

Working principle multistage pendulum

Expected attenuation @10 Hz: 1014

Residual mirror motion (rms)

rotation <1 rad longitudinal <1 m

The Top Stage

Top of an inverted pendulum: - Inertial damping (70 mHz to 5 Hz) - Possibility to move the suspension point with small forces

Passive Filters

Five seismic filters:

Suspended by steel wires Vertical isolation by a combination

of cantilever springs and magnetic anti-springs

The Local Controls

Marionette control:CCD camera, optical levers and fourcoil-magnet actuators: <2 Hz

Fast actuators

Reference mass and mirror,four coil-magnet actuators

Injection System

IMC

RFC

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

Slave

Master

Input Mode Cleaner

Triangular cavity, 144 m long, Finesse=1000Input optics and two flat mirrors are located on a suspended optical benchEnd mirror suspended with a reference massfor actuationTransmission 50%

Injection Bench

Mode Cleaner Mirror

Detection System

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

Output Mode-Cleaner

Output Mode Cleaner

Output Mode-Cleaner

Detection Bench

4 cm long ring cavityContrast improvement ~ 10Length control via temperature (Peltier element)Lock acquisition takes 10 min

Photodiodes

16 InGaAs diodes for the main beam (dark port), in air and not suspended6 additional photo diodes for control purposes

External bench

Output Mode-Cleaner

Main beam

Digital Controls

Fully digital control, local and global

Feedback is send with 20-bit DACs @ 10kHz to thesuspensions

The suspension control is performed by dedicated DSPs (one per suspension)

Interferometer signals are acquired with 16-bit ADCs @ 20 kHz. The data is transferred via optical linksto Global Control (dedicated hardware and software that computes correction signals and sends them to the mirror DSPs)

Sensing and Control

Modulation-demodulationscheme with only one modulation frequency (6 MHz) to control:

4 lengths 10 angles

Data Acquisition and Storage

16-bit ADCs, up to 20 kHz sampling frequency

Data in Frame format:- full signal (20 kHz)- down-sampled to 50 Hz- down-sampled to 1 Hz (trend data)

Frames available for data monitoringwith ~few sec delay

Current rate: 7 Mbytes/s (compressed)

Raw data buffer ~ 2.5 s

Control room

Current Status

2001-2002 Commissioning of the central interferometer and the injection system

2002-2003 Tubes commissioning and final mirror installation Since September 2003 commissioning of the full interferometer

Central Interferometer (CITF)

Commissioning of VIRGO

The commissioning of the full detector has been divided into

three phases:

Phase A (sept 2003 - feb2004): the 3 km long arm cavities

separatly

Phase B (feb 2004 – summer 2004): recombined Michelson

interferometer

Phase C (from summer 2004) : Michelson interferometer with

Power Recycling (full detector)

Commissioning of the arms

• Test the cavity locking, and digital control chain • Test the automatic alignment (with the Anderson

technique)• Test the frequency stabilization • Test the locking of the output mode-cleaner

North Arm Cavity commissioning

PR misaligned

West arm misaligned

Phase A: the two arm cavities are used separatly, starting with the north arm;

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 simulationThe West arm cavity was locked in December 2003

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)

PR misaligned

Recombined locked in February 2004

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)

Automatic Alignment

• 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)

Frequency stabilization

The `second stage´ of frequency stabilization

1. The laser frequency is stabilised to the common lengths of thearm cavities (bandwidth ~17 kHz)

2. The arm cavities are stabilised to the reference cavity (bandwidth ~2 Hz)

C4 run (june 2004)• Both cavities automatically aligned• BS alignment with local control• Michelson (l1-l2) controlled with ref_quad• Laser frequency stabilized to cavities common mode• Cavities common mode locked to reference cavity• Output mode-cleaner locked to dark fringe• Arms differential mode controlled with OMC transmission• Tide control on both arms

C4-june 2004 (recombined)

• Configuration: recombined ITF with 90% complete control system:- automatic alignment of input beam and beam splitter missing

• Duration: 5 days • Test periods at the beginning and at the end of the run (~ 0.5 day)• 9 losses of lock during quiet periods (all understood, one due to an earthquake in Alaska !)• Longest locked period: ~ 28 h, relatively stable noise level

C4 (recombined) sensitivity

2.5 · 10-20

DAC noise

10-1

100

101

102

103

10-20

10-15

10-10

10-5

Actuators noise: current status

Frequency (Hz)

m/H

z1/2

Reference Mass - Mirror Actuators NoiseFilter #7 - Marionetta Actuators NoiseVIRGO Sentivity

103

Transfer of the low frequency component of the locking feedback force to the marionette

DC-0.01 Hz

0.01-1.5 Hz

1.5-50 Hz

Suspension hierarchical control - I

• Main difficulty:Driving of tilt modes when pushing on the marionette need for a good diagonalization of the driving

• Transfer of locking force to marionette tested

• Crossing frequency ~ 0.5 Hz 1.5 Hz 8 Hz !

Force applied to the mirror (via reference mass coils) decreased by ~10

Force applied to the mirror (a.u.) Force applied to marionette (a.u.)

Suspension hierarchical control - II

• Almost all the controls running • Noise quite understood• C4/C5 data used for data analysis purposes

in july 2004 lock acquisition trial for the recycled ITF started

After C4, recombined ITF

Input laser beam

ITF reflected beam

• Solutions:

- Short term: insert attenuator between the IMC and the ITF

- Mean term: insert Faraday isolator (input bench upgrade)

Laser Frequency (Hz)

Light backscattered by the mode-cleaner

Power divided by 10

“A recycled ITF with a low recycling factor is similar to recombined interferometer “

Lock the 4 degrees of freedom of the ITF on the half or white fringe

Bring the interferometer slowly on the dark fringe

Lock acquisition of the recycled ITF

The variable finesse lock acquisition

North arm

West arm

- Lock of the long arms indipendently with the

end photodiodes

- Lock of the recombined on the HALF FRINGE

With PR misaligned of 10 urad some light goes in the reflected

beam -> Used to lock PR

ASY DC POWERASY DC POWER

MICH ERROR SIGNALMICH ERROR SIGNAL

Step 1: lock on the half fringe

Ref_3f phase

Asy_ DC

West transmission_phase

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-0.5

0

0.5

1

North transmission_phase

Step 2: align the power recycling mirror

Coming from TAMA experience

Pick_off_Phase

Ref_3f phase

LASER

West transmission_phase diff arm mode

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-0.5

0

0.5

1

Asy_ DC

Step 3: common mode laser frequency

MICH ERROR SIGNALMICH ERROR SIGNAL

ASY DC POWERASY DC POWER

Offset : 0.5 0.2

Step 4: reducing the offset

Ref_3f phase

LASER

West transmission_phase

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-0.5

0

0.5

1

Step 5: change the error signal for the michelson control DCAC

Pick_off_PhasePick_off_Quad

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-0.5

0

0.5

1

Recycling gain big increase: intermediate steps to arrive to the Recycling gain big increase: intermediate steps to arrive to the dark fringedark fringe

ITF on the operating point

Step 6: going to the dark fringe

Power on beam splitter during lock acquisition

Recombined interferometer

Recycling gain ~ 25-30

300 times

Ref_3f phase prcl

LASER

Asy_Quad diff arm mode

Switch to a “detection” mode

Pick_off_Phase common arm mode

Pick_off_Quad mich

2.5 hours lock Stored power

C5 run – december 2005

• Recycled ITF

• Second stage of frequency stabilization (common mode servo)

• output mode-cleaner

Control noises

BS and PR

Oscillator phase noise (?)?

Recyled sensitivity

Recyled interferomter high frequency sensitivity

During the lock of the recycled ITF the high frequency noise is more than ~ 40 times higher than the expected electronic noiseThe level of this noise fluctuates with the quality of the alignemnt

C5 recycled sensitivity

Electronic noise (with closed shutter)

Shot noise

Phase noise ( model with = 0.45 rad/(Hz) )

• Run the automatic alignment

• Complete the alignmnet/locking automation

• Prepare known noises reduction:

• control noises (< 100 Hz)

• phase noise (>500 Hz)

Plans from now to april

• Install new injection bench with Faraday isolator (2-4 weeks stop)

• Run the interferometer with 10 times more power

Plans after april

• Noise hunting

New injection bench Faraday isolator

Sensitivity evolution

Summary

• 1.5 years of commissioning (from september 2003 to february 2005)

• Recombined interferometer major step:

• almost all the control running

• noise hunting and data analysis studies

• Recycled interferometer locked with a “variable finesse” lock acquisition

• Power at PR ~ 0.7 Watts (because of backscattered light by IMC)

• Sensitivity 500 Hz

• high frequency: phase noise (oscillator/demod board)

• low frequency: control noise

• Next step: automatic alignment

• Injection bench replacement ~ april

• Next summer 10 times more power

8 · 10-21 @