1 la general training session, cascina 20.02.2006 virgo alignment - overview - nonlinear alignment =...
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1 LA general training session, Cascina 20.02.2006
Virgo alignment- overview -
Nonlinear alignment = prealignment
Linear alignment = autoalignment
2 LA general training session, Cascina 20.02.2006
Linear alignment
3 LA general training session, Cascina 20.02.2006
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
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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
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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)
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Gaussian beams
Laser
near field(waist)
far field
flat wavefront
curved wavefront
cavity
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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"
8 LA general training session, Cascina 20.02.2006
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
9 LA general training session, Cascina 20.02.2006
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.
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Longitudinal control:1 DC signal2 demodulated signals
Alignment control:4 DC signals8 demodulated signals
Detection
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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
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Reconstruction
Matrix
Control: error signal acquisition
13 LA general training session, Cascina 20.02.2006
Control: correction signal distribution
Reconstruction
Matrix
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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
15 LA general training session, Cascina 20.02.2006
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
16 LA general training session, Cascina 20.02.2006
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
17 LA general training session, Cascina 20.02.2006
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:
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C6 configuration
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Drift control power stability
← dark fringe (B1p) improvement
Recycling power (B5) improvement→
20 LA general training session, Cascina 20.02.2006
C7 configuration (tx)
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Effect of autoalignment (N cavity)
AA turned ON
AA Off
A. FreiseM. Loupias
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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
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Prealignment steps
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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
25 LA general training session, Cascina 20.02.2006
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
26 LA general training session, Cascina 20.02.2006
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
27 LA general training session, Cascina 20.02.2006
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
End
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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
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Some details on control strategy
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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
32 LA general training session, Cascina 20.02.2006
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
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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
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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
35 LA general training session, Cascina 20.02.2006
Details on control strategy
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
36 LA general training session, Cascina 20.02.2006
Details on control strategy
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
37 LA general training session, Cascina 20.02.2006
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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Main IFO and Input Beam
linear alignment : 10 degrees of freedom in main IFO
4 degrees of freedom for incoming beam
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Combined Degrees of Freedom
WE rotation by NE rotation by -
Same motion inside PR cavity
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"Cavities alignment" configuration
• North and West cavities: independently aligned on their transmitted beams
Suspended bench External bench
Output Mode-Cleaner
B7
B8
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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
42 LA general training session, Cascina 20.02.2006
Prealignment steps