1 la general training session, cascina 20.02.2006 virgo alignment - overview - nonlinear alignment =...

1 LA general training session, Cascina 20.02.2006

Virgo alignment- overview -

Nonlinear alignment = prealignment

Linear alignment = autoalignment


Linear alignment


Virgo alignment

N

W

EOM Injection Bench

Recycling mirror

Input mirrors

West end mirror

North end mirror

Correct mirror alignment necessary for •keeping arm cavities resonant•keeping recycling cavity resonant•keeping interference on dark fringe


Local control system of ITF mirrors

C C D

35 o

(z) beam axis

dif fusive markers

ha logen

illuminat or

XY

Err(xy)

Err(xy)

PSD focal p lane

PSDErr(z)

red laser diode

X Y

Err(xy)

PSD f oca l plane

red laser diode

30 o

ac t uat o r

marionette

mirror

Local control system uses •diode lasers, •CCD cameras •Position Sensitive Devices (PSD)

Residual motions:1 μm / 1 μrad


Mirror alignment requirements

100 nradrms

20 nradrms

3 nradrms

3 nradrms20 nradrms

Local control: 1 µrad=> mirror motion is 10 . . . 300 times too high+ slow relative drifts of mirrors

Autoalignment systemUses light coming out of cavities for understanding relative mirror misalignment ("global" control system)


Gaussian beams

Laser

near field(waist)

far field

flat wavefront

curved wavefront

cavity


Simple cavity misalignment (end mirror)

We use a differential wave front sensing technique. (Anderson technique); at each beam, we have two quadrant diodes

"Near field"+ 0° 0°

TEM0

0

TEM0

1

"Far field"


Simple cavity misalignment (input mirror)

+ 0° 90°

TEM0

0

TEM0

1

"Far field"

"Near field"

We use a differential wave front sensing technique. (Anderson technique); at each beam, we have two quadrant diodes


Quadrant photodiode

From each QD we get:2 DC signals

simple difference between elements* horizontal/verticalQD centering information

4 AC signalsdemodulated difference signal* horizontal/vertical* in phase/in quadrature

Warning: AC/DC not in the electronic sense!

Diff.

Diff. horiz.

vert.


Longitudinal control:1 DC signal2 demodulated signals

Alignment control:4 DC signals8 demodulated signals

Detection


Anderson technique:

uses the light transmitted by the arm cavities (no pick-off beams needed)

requires a specially tuned RF modulation frequency

strongly coupled alignment degrees of freedom: each mirror rotation is seen at each output port

Anderson-Giordano technique:

two quadrant diodes are used in the transmitted beams (near-field, far field) [G. Giordano, Frascati]

The Anderson-Giordano Technique


Reconstruction

Matrix

Control: error signal acquisition


Control: correction signal distribution

Reconstruction

Matrix


The behaviour of the alignment sensing system is measured by sending a sinus perturbation (line) on each mirror, and measuring the effect of each mirror's line on each QD signal. This measurement gives the optical matrix.

The inversion of the optical matrix gives the reconstruction matrix, which allows to calculate the misalignment of each mirror from the QD signals.

Reconstruction

Matrix

Optical

Matrix

Reconstructed Angular Positions

Line on mirror Lines from all mirrors QD signal

The angle reconstruction


16x6 optical matrix (x2)

after the shutdown: matrix also includes DC signals and IB =>

30x7 matrix

The optical matrix (before C6)

B8_q2_ACq

B8_q2_ACp

B8_q1_ACq

B8_q1_ACp

B7_q2_ACq

B7_q2_ACp

B7_q1_ACq

B7_q1_ACp

B5_q2_ACq

B5_q2_ACp

B5_q1_ACq

B5_q1_ACp

B2_q2_ACq

B2_q2_ACp

B2_q1_ACq

B2_q1_ACp

WEWINENIPR + BS


Control modes

Linear alignment modeMirror angles are entirely controlled by reconstructed LA error signalsFast control (bandwidth 3 Hz)Low noise

Drift control modeMirror angles are controlled by local controlLA error signals are added as offsets=> drift control bandwidth 10 mHzLocal control noiseAdvantage: no loop stability problems due to bad reconstruction &c


Basic alignment strategy

Cavity alignment : angular motion of 5 mirrors to be controlled (DC – 4 Hz)

Beam drifts : Input beam and Beam Splitter to control (DC- 0.01 Hz)

Main interferometer:


C6 configuration


Drift control power stability

← dark fringe (B1p) improvement

Recycling power (B5) improvement→


C7 configuration (tx)


Effect of autoalignment (N cavity)

AA turned ON

AA Off

A. FreiseM. Loupias


Arm cavity common/differential mode control

One DC signal in present control scheme

Possible schemeControl NE-WE with fast loop (AC)

diff. mode and mirror resonances

Control NE+WE with slow loop (DC)drifts

WE

NEDC

AC


Prealignment steps


Cam7p

Cam8p

WE

WI

NI NEBS

PR

Direct beam alignment

The direct beams are centered on the cameras

M6 picomotors on IB

Input mirrors misaligned


Nonlinear alignment: coarse

Cam7p

B7

B8

Cam8p

WE

WI

NI NEBS

PR

Maximise the resonance flashes on the photodiodes by

moving the cavity mirrors

M6 picomotors on IB

Mirrors aligned, cavities not locked


B7_q1

Cam7p

B7_q2

B7

B8_q2

B8

Cam8p

B8_q1

M6 picomotors on IB

WE

WI

NI NEBS

PR

Zero the QD error signal, moving both mirrors of a cavity

Nonlinear alignment: fine

Mirrors aligned, cavities locked


B7_q1

Cam7p

B7_q2

B7

B8_q2

B8

Cam8p

B8_q1

M6 picomotors on IB

WE

WI

NI NEBS

PR

LA remains on for some time=> position memories keepmirrors in aligned position

Close "cavities alignment" loop

Cavities locked, independent LA loops running for N & W cavity


Quadrant diode centering

Movement of beam on quadrant diodeUnits: normalized asymmetry (x2-x1)/(x2+x1)0.5 means: ¾ of beam on one half of QD

QD1

QD2

B2 B5 B7 B8

-0.5 0.5

Quadrant autocentering activeTranslation stagesRecentering every 5 sec


Some details on control strategy


Beam splitter linear alignment

Original scheme: BS under "slow" control (WE quadrant centering)Present scheme: BS under LA, WI under local control

Reason: BS local control noisier than other mirrorsAdvantage: no control hierarchy needed for WI control with WE quadrant

But: WI control to be tested Noise ?

Drift controlLinear alignment

BS

DC

WI

DC

Original scheme Presently foreseen scheme


C6 alignment matrices

PR NI NE WI WE 0 0 0 0 1 B2_d1_DC13 0 0 0 0 B2_d1_ACp

0 0 1 0 0 B1p_d2_ACp

0 1 0 0 0 B7_d1_ACq 0 1 0 0 0 B7_d2_ACq

0 0 0 1 0 B8_d1_ACp 0 0 0 -1 0 B8_d1_ACq 0 0 0 -1 0 B8_d2_ACq

PR NI NE WI WE 0 0 0 0 1 B2_d1_DC

0 0 1 0 0 B1p_d2_ACq

0.45 0.2 0 0.5 0 B7_d1_ACp 1 0.2 0 0 0 B7_d2_ACp-0.53 -0.2 0 -1 0 B7_d2_ACq -0.56 -0.2 0 1 0 B8_d1_ACp

ThX

ThY

Drift controlLinear alignment


C7 alignment matrices

ThX

ThY

PR NI NE BS WE2.5 -6.4 -4.5 15 B2_1_DC-24 17.9 9.4 5.38 B2_1_p0.052 0.037 1 -0.108 0.073 B1p_1_p0.042 0.37 -0.035 -0.027 B7_1_q0.042 0.37 -0.035 -0.027 B7_2_q-0.25 0.052 0.65 0.07 B8_1_p-0.25 -0.052 -0.65 -0.07 B8_1_q

PR NI NE BS WE1 B2_1_DC

1 B1p_1_q0.33 0.36 B7_1_p0.68 0.36 B7_2_p-0.36 -0.36 B7_2_q-0.46 -0.36 -1 B8_1_p

1 B8_2_p


Details on control strategy

tx

ty

Lines for matrix measurementat frequencies of high gain

Sequential closing of loopsClose easiest degrees of freedomInject lines on non-controlled mirrors=> matrix simplification=> elimination of dominant modes

e.g. differential arm mode



Switch B5 → B1pReason: main losses of recycled power through dark fringe misalignmentIdea: measure misalignment where it is apparent => dark fringe

Although, with Anderson technique not so obvious . . .

End mirror differential mode control with 1 diodecontrol on NE mirror

Before After



Deviation from pure matrix inversion strategyOne-by-one identification of suitable signals

End mirror diff. mode (B1p)End mirror common mode (B2_DC)CITF mirror thetaX

Matrix inversion on sub-matrixCITF mirror thetaY

Drift control as preliminary step to LAAlignment stability for C6Helps understanding of loop stabilityBasis for LA matrix


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)


Main IFO and Input Beam

linear alignment : 10 degrees of freedom in main IFO

4 degrees of freedom for incoming beam


Combined Degrees of Freedom

WE rotation by NE rotation by -

Same motion inside PR cavity


"Cavities alignment" configuration

• North and West cavities: independently aligned on their transmitted beams

Suspended bench External bench

Output Mode-Cleaner

B7

B8


Milestones

Commissioning run C6 29/07 – 12/08/2005 (2 weeks)

tx PR BS NI NE WI WE ty PR BS NI NE WI WE 40 hours continuous lock

Minirun M9 25/08/2005 (1 day)

tx PR BS NI NE WI WE ty PR BS NI NE WI WE1 night continuous lock

Commissioning run C7 14/09 – 19/09/2005 (5 days)

tx PR BS NI NE WI WE ty PR BS NI NE WI WE 14 hours continuous lock (max. 28 hours in this configuration)

XX Linear alignmentXX Drift controlXX Local controlXX DC error signal


Prealignment steps

1 la general training session, cascina 20.02.2006 virgo alignment - overview - nonlinear alignment =...

Documents

la general training

field slide

autoalignment slide

rad slide

bs slide

loupias slide

local control la error

reconstruction matrix