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Locking in Virgo Matteo Barsuglia

ILIAS, Cascina, July 7th 2004

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• Introduction: optical scheme, terms, etc…• Actors: Hardware, software, simulation • Results:

• Experience with a single arm (cavity locking, frequency stabilization, ouput-mode cleaner locking)• The recombined interferometer (almost all the controls working)• Full detector lock acquisition (preparation and first results)

Outline

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Virgo optical scheme

West cavityF=50

North cavityF=50

Recycling cavityG=50

Dark fringe

Fabry-Perot Michelson

Power recycling

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Standard base• MICH = ln-lw• PRC= lrec+(lN+ lw)/2• CARM= LN+LW

• DARM= LN-LW

LN

Lw

lw

lN

lrec

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Intro: photodiode names

• 3 signals for each photodiode: DC, ACp, Acq

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8

2 1

5 and 5_2f

In- phasequadrature

reflection Antisymetric port

Transmission north

Transmission west

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Detection system

InGaAs photodiodes 6.26 MHz (only 1 modulation)

16 bits ADC

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Last stage

Digital controls

Control signals(DOL’s)

Photodiodes signals(DOL’s)

Alignment Locking

3 for each suspension

trigger, signal processing, filtering

• 10 kHz sampling• VME based, homemade• software in C

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Mirror actuators

Reference mass

4 coils

40 mN

Beam splitter

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• Real time simulation package of the Virgo experiment• Written in C, configuration cards • produces frames • Can be interfaced to the real time control system (global control)

Siesta

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SIESTA

Control signalsPhotodiodes signals

Algorithms running in the global control

Siesta

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Control signalsPhotodiodes signals

Algorithms running in the global control

VIRGO

Siesta

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North cavity locking

north arm

Test: Locking

Autoalignment Frequency

stabilization tidal control

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Linearized error signal

No Linearized error signal

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Lock acquisition speed threshold ~ 10 m/s

Signals and linearization

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Signals and simulation

Correction Transmitted power

Time domain Simulation (Siesta)

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Cavity first locking • Locking at the first trial• first lock ~ 1 hour• frequency noise

Transmitted power

Frequency noisereduction

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Lock acquisition statistics 24 locking events collected locking and delocking the cavity 23 lock acquisition at the first attempt, only 1 failed locking attempt

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Relative velocity between the mirrors computed for each locking attempt

8 m/s: maximum velocity for the lock acquisition success

12.5 m/s: velocity of the failed event

Failed locking attempt

v ~ 12.5

sμm

sμm

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2.5 m/s: mean value of the velocity

Lock acquisition statistics

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Cavity locking accuracy

3 picometers RMS

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Output mode-cleaner locking

1. Cavity locked with ~ 1% of the light

2. Mode-cleaner locked

3. Control transferred to this phd ~ 99% of the light

After OMC Before OMC

Sensitivity (m/Hz)

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Output mode-cleaner locking Transmitted P Reflected P 2

State TemperatureError signal

2 min

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Frequency stabilization- North cavity error signal sent to the input mode-cleaner (below 200 Hz) and to the laser (above 200 Hz)- Reference cavity error signal used to control cavity length at DC

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Recombined ITF

north arm

west arm

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Recombined ITF

10 W~ 1W

• Sensitivity ~ 201500PBS ~

(500 W)

• 3 d.o.f. decoupled • fields are not mixed • lock acquisition easy• no “dynamical effects”

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Lock Acquisition – OverviewB8_phase/B8_DC

B5_phase/B7_DCB2_quad

NE

WE

BS

Lock of the two cavities (independently) Corrections sent to NE and WE Lock of the michelson length Corrections sent to BS

3 Steps lock acquisition:

North arm

West arm

Michelson length

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Linear Locking - Overview

⊗B1p_phase

B2_quad

North arm

West arm

B2_phase

Michelson length

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Complete scheme

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Run C4

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reach the Virgo sensitivity: recycling

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• Technique choosen: use the LIGO one (developed by Evans et al.)• VIRGO and LIGO have the same optical scheme, and similar optical parameters.• VIRGO and LIGO have similar control systems (digital, quite similar sampling frequencies,…).• VIRGO and LIGO have similar simulation packages (real time, etc…)• pragmatic point of view …the LIGO approach works

• Few differences between LIGO and VIRGO • Pick-off signal different • Arm finesse in LIGO = 200 (Virgo =50) • Suspension and local controls system simpler in LIGO

• Reproduce the LIGO technique with SIESTA • only optics (TEMO00, no saturation, no superattenuators) • Include fine effects (saturations, etc…)

Full Virgo lock acq approach

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Full virgo lock acq approach very difficult to have the 4 d.o.f. satisfied, in a linear regime

• Sequencial & Statistical (the states are not stable)• used in LIGO, works well in 3 itf’s• lot of signal processing (linearization, dynamical matrix inversion)• simulation crucial

central cavity locked

central cavity + first arm all ITF locked

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Optical characherization

• parameters of the optical matix (~ 10 ) determined by simulation• very important to reproduce in simulation the optical behaviour of the interferometer • During the CITF the locking parameters (2) what dermined in this way• optical characterization is a strategic item

micdiffmichcommon

armdiffarmcommon

Assss

acq

acqacqacqacq

__

__

4321

Each element = K P(measured throughB7/B8/B5_2f)

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Simulation

West cavity Trans power

North cavityTrans power

Power inside the rec cavity

Sidebands power inside the rec cavity

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“step 3” locking • Lock of the central cavity (CITF) on the sidebands + lock of the north arm (on the carrier)

B2_Q

B2_PB1_Q

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Lock acquisition state3

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• Experience with a single cavity and recombined very interesting to understand the hardware/software/signals/simulations• Real time simulation crucial tool (understand signals, test algos, save commissioning time)• Hardware and software tested and performant (algos in C++, parameter in a database, etc…)

• This summer: try to lock the recycled interferometer and prepare linear lock and final frequency stabilization.

Conclusions and next steps