1 GWADW Elba 30.05.2006

The commissioningof the Virgo interferometer

H. Heitmann

Observatoire de la Côte d’Azur, Nice

for the VIRGO collaboration

• Evolution of commissioning

• Upgrades

• Recent commissioning activities

• Outlook

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Status of commissioning before shutdown

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Commissioning sensitivity evolution

Run C7duty cycle 64%14 h lock

Run C6duty cycle 90%40 h lock

Single armSingle armSingle armRecombinedRecycledRecycledRecycled

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Problems with configuration up to C7

Fringes due to backscattering in mode cleaner=> Operate with reduced power (10%)

Non-monolithic, curved power recycling mirrorResonances => Control problemsShift sensitivity => alignment drifts

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Recent upgrades

• New power recycling mirror

• New injection bench

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VIRGO layout

Injection bench (IB)

Power recycling mirror

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Power recycling mirror

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120 mm

R=4100 mm

350 mm

Translations induce misalignment and jitter noiseflat mirror

Old power recycling mirror

PR mirror is • non-monolithic• curved (part of beam matching telescope)

Incident Beam

PR mirror transfer function

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New power recycling mirror

Monolithic mirrorNo more internal resonances in the control band !!

Flat surfaces=> no more lens effect (no more part of input telescope)=> larger beam coming out of injection bench => parabolic telescope needed on IB

PR mirror transfer function

OldNew

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Injection bench

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Laser Frequency Noise

Power Recyclingmirror misaligned

Power Recyclingmirror aligned

Hz

Time

Fringes in the interferometer

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Origin of the fringes

Backscattering in the mode cleaner

for enabling power recycling (C7 configuration)

Faraday Isolator needed on theinjection bench for full power operation

Injected power lowered to 700 mW

Laser lightBackscattered light

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10% reflectivitymirror

Old injection bench

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New injection bench

Main requirements:• Faraday isolator• Parabolic output telescope• Thinner suspension wires

more suitable resoance frequencies

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New injection bench (OptoCad design)

Design and realizationEGO optics group

Parabolic outputtelescope

Faradayisolator

From laser

To interferometer

Mode cleaner

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Referencecavity

New injection bench (below)

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New injection bench status

Bench isinstalledalignedcontrolled

Beamoutput power 7 W (10x C7)matching: coupling into arm cavities 95-97%

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Some recent commissioning activities

• The variable finesse locking technique

• Mode cleaner mirror radiation pressure

• Suspension improvements

• New injection system autoalignment

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The variable finesse locking technique

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Locking: Step 1

WE

NENI

WI

BSPR

• Power recycling mirror misaligned

=> no signal mixing

• Each cavity locked independently

• ITF output at « grey fringe » (50%)

Recombined interferometer

North arm

West

arm

50%

ControlsNE North cav. transmission PWE West cavity transmission PBS Dark fringe power DC

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WE

NENI

WI

BSPR

• Laser locked on ITF common mode very high bandwidth loop

=> get rid of dominating

common mode signal

Recombined interferometer

North arm

West

arm

40%

ITF common mode

ControlsNE-WE North cav. transmission PBS Dark fringe power DClaser Recycling cavity pick-off PPR ITF reflection (3) P

Locking: Step 2

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WE

NENI

WI

BSPR

• Power recycling mirror aligned

Recycled interferometer

North arm

West

arm

ITF common mode

40%

ControlsNE-WE North cav. transmission PBS Dark fringe power DClaser Recycling cavity pick-off PPR ITF reflection (3) P

Locking: Step 3

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WE

NENI

WI

BSPR

• Stepwise going down to dark fringe

Recycled interferometer

North arm

West

arm

ITF common mode

5%

ControlsNE-WE North cav. transmission PBS Recy.cav. pick-off Q laser Recy.cav. pick-off PPR ITF reflection (3) P

Locking: Step 4

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WE

NENI

WI

BSPR

0%

North arm

West

arm

ITF common mode

• Dark fringe reached

Recycled interferometer

ControlsNE-WE Dark fringe PBS Recy.cav. pick-off Q laser Recy.cav. pick-off PPR ITF reflection (3) P

Locking: Step 5

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Mode cleaner mirror: radiation pressure

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Mode cleaner mirror: radiation pressure effect

Frequency increase

y

MC locked full power MC locked 60% powerMC unlocked

x: 2.13 -> 1.97 Hz

y: 1.27 -> 2.0 Hz

mirror dimensions: 30x80 mm360 grams

Mode position changes with mirror alignment extra torque resonance frequency change

Problem:• Autoalignment loops became unstable• Corrector adaptation was needed

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Recent mirror suspension improvements

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Suspension: recent problems and solutions

30 mHz

acce

lero

met

er

ground-based

sensorsGround-independent control above 30 Hz

Mirror excitation with bad weather less stable lock

Changed suspension top stage control seismic

acceleration

noisy daycalm day

Micro-seismic peak(sea waves)

DAC noise on mirror actuation coils

Hierarchical control Rearrange forces going to the suspensionSwitch to low-noise coil drivers

Contr

ibuti

on t

o c

ontr

ol

(See talk by G. Losurdo)

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New injection system autoalignment

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Injection system autoalignment

Old systemIdea: if IB is rigid, no misalignments

(turned out not to be true)

IB under local control

New systemBeam aligned on fixed mechanical reference (external bench)Mode cleaner fully aligned on beamAutocentering of beam onto MC end mirror

Future enhancementBeam aligned on 3 km target

10%

2004

Mode cleaner transmitted power fluctuations

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MATRIX

Old injection system autoalignment layout

Ref. cav.

MCmirror

Laser

M5

M6

Ref. cav. autoalignment

MATRIX

Injection bench

MC mirror autoalignment--Inj.B. local control--Beam autoalignment--

IB Coils

MCAA

Picomotors

Piezos

Wavefront sensors

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MATRIX

New injection system autoalignment layout

Ref. cav.

MCmirror

Laser

M5

M6

MATRIX

RFC AA

Beam prealignment

MATRIX

MC IBAA

IB Coils

Ref.cav. autoalignment

Picomotors

Piezos

--

Injection bench

MC mirror autoalignment--Inj.B. autoalignment--Beam autoalignment--

DC position sensors

Wavefront sensors

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Outlook

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Present status

Full locking of interferometer1-2 hours locking periodsor 5 minutes …

Thermal effectsAffect recycling cavity stability(modulation sidebands drop)

30-50 Hz burst events (skeephs)Locking stability problems

Misalignment sensitivity?Hasten full autoalignment (5/10 d.o.f. OK)

2h0 1000 2000 3000 4000 5000 6000 7000

0.00

0.01

0.02

0.03

0.04

0.05

0.06

-2.0x10-5

0.0

2.0x10-5

4.0x10-5

6.0x10-5

8.0x10-5

1.0x10-4

1.2x10-4

1.4x10-4

1.6x10-4

1.8x10-4

2.0x10-4

2.2x10-4

2.4x10-4

Pr_

B5_D

C

time [s]

Pr_B5_DC_mean

Pr_B5_2f_ACq_mean

Carrier power

Sideband power

Time 0

Thermal effects in rec. cavity

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Next steps

Improve sensitivityLow frequency

control noise (alignment)modulation frequency tuning (servo)

Medium frequencyscattered lightacoustic shielding in laser lab

High frequencyincreased powernew low-noise HF modulation generator

New MC mirrorbetter surface qualitylower losseslarger, heavier?

avoid radiation pressure problemsfacilitate control

...

End

End

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Old injection bench (OptoCad design)

10% reflectivitymirror for power reduction

From laser

To interferometer

Mode cleaner

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14 hours locked

C7 (5 days Sept. 2005)

40 hours locked

Duty Cycle of last two commissioning runs

C6 (14 days Aug. 2005)

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1. Mirror excitation in windy conditions=> more frequent unlocks when weather is bad

2. DAC noise on mirror actuation coilsHigh force needed for lock acquisition=> bad DAC dynamics in steady conditions (low force)

Micro-seismic peak(sea waves)

Suspension: recent problems

seismic acceleration

wind speed

susp displacement

noisy daycalm day

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Suspension: inertial damping modification

L(s)H(s)

L+H = 1

accelerometer

LVDT

Inertial dampingInverted pendulum top platform is immobilized byHF accelerometers (inertial sensors)LF LVDT’s (ground based) => introduce seismic noise

SolutionReduced HF/LF cross-over frequency to 30 mHzNot so simple ... (see G. Losurdo’s talk)

LVDT = linear variable differential transformer

30 mHz

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Suspension: hierarchical control

DC-0.01 HzTide control

0.01-5 Hz

5-50 Hz

After lock acquisition: reduction of mirror actuator gain=> reduction of DAC noise

Initial condition:-lock acquisition-all force goes to mirror

Static condition:-interferometer locked-force split according to frequency bands

DAC noise

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1.4 Mo coalescence detection range

M31 (Andromeda) 700 kpc

C7 (19 Sept 05)

C6 (12 Ago 05)

Environmental disturbance re-injection through the control system

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C7 noise budget

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Mirror centering

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Mirror centering

Needed because of astigmatism and suspected beam clipping

Advantage: learned techniques for mirror centering Necessary for reducing alignment noise which limits us at low frequencies

Two techniques usedVisual centering

Where possible…

Mirror shaking at natural resonanceFind frequency in longitudinal motion (locking error signal)

x

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Beam splitter visual centering

Before centering After centering

cm

cm

Mirror

Coil

Local control camera imageobservation of diffused beam spot while moving mirror