Transcript Output Mode

Automatic Alignment using the
Anderson Technique
A. Freise
European Gravitational Observatory
Roma 21.10.2004
19. October 2004
A. Freise
Overview
Linear alignment
Output Mode-Cleaner
Drift control
Non-linear alignment
Simulation
Procedure/Documentation
Automation
Suspended bench
External bench
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Linear Alignment: Status
Output Mode-Cleaner
B8
Linear alignment implemented for
North arm, West arm and
the recombined Michelson, using B7
and B8
Performs well for full power or
reduced power (10%)
B7
Suspended bench
External bench
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Autoalignment: Why ?
Superimpose beam axes
Maximize light power
Stabilze optical gain
Center beam spots on mirrors
Minimize angular to longitudinal noise
coupling
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Autoalignment: How ?
Differrential wavefront sensing
(analog feedback for 14 DOF in GEO)
Spot position sensing
(digital feedback for 20 DOF in GEO)
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The VIRGO Interferometer
W
N
2 Perot Fabry cavities
EOM
Injection
Bench
Recycling
mirror
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‚Linear Alignment‘ for VIRGO
linear alignment : angular
motion of 5 mirrors to be
controlled (DC – 4 Hz)
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Modulation-Demodulation
For obtaining control signals
a modulation-demodulation
technique is used. Only one
modulation
frequency
is
applied to generate all
signals for longitudinal and
angular control of the main
interferometer.
6.26 MHz
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Resonance Condition
Upper Sideband
TEM00
Carrier
Lower Sideband
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Resonance Condition
Upper Sideband
TEM01
Carrier
Lower Sideband
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Cavity Alignment
The Anderson technique uses signals in transmission
of a cavity. The detectors are positioned in :
Near field
Far field
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Cavity Alignment
The Anderson technique uses signals in transmission
of a cavity. The detectors are positioned in :
Near field
Sensitive to translation of the mode
Far field
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Cavity Alignment
The Anderson technique uses signals in transmission
of a cavity. The detectors are positioned in :
Sensitive to tilt of the mode
Near field
Far field
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Detection
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Detection
In each of four outport ports
we can set:
 two Gouy phases
 two (four) demodulation
phases
to get 4x4 output signals for
each direction
(horizontal/vertical)
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Detection
For tuning the telescopes one can move
L2, L3, L4a and L4b. The most critical
adjustment is required for L2.
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Tuning Telescopes
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Control Matrix
In total: 8 Gouy phases have to be tuned, 16
demodulation phases to be set.
This yields 32 signals to control 10 degrees of freedom
(5 horizontal, 5 vertical).
Control topology (phases+control matrix) has been
designed by G. Giordano.
The optical matrix has to be measured to generate two
5x16 control matrices using a 2 reconstruction method.
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Example Matrix (16x5)
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Signal Amplitudes
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Alignment Control
DC: beam positions are defined by reference
marks, spot position control, below 0.1 Hz
around the resonance frequencies of the
suspension pendulums the beam follows the
input beam from the laser bench, differential
wave-front sensing, 0.1 Hz to 10 Hz
no active control at the expected signal
frequencies, the two mode cleaners suppress
geometry fluctuations by ~106
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The GEO 600 Detector
4 degrees of freedom
for MC 1
+4 for MC 2
+4 for MI common mode
+2 for MI differential mode
+2 for signal recycling
16 +
32 = 48
differential wave-front sensing
spot position control
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Signal Amplitudes in 2D
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Zero Crossings
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Angular Fluctuation
Residual fluctuations:
~ 1 nrad @ 10 Hz
~ <1urad RMS
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Filter design
open loop transfer function for NI/NE tx.
unity gain
3.2 Hz
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The Suspension Control
Main mirrors are suspended for seismic
isolation. Active control is necessary to
keep the mirrors at their operating
point:
•
•
•
inertial damping
local damping
local control, i.e. steering of the
mirrors
~5 Hz,
of the
Good performance Bandwidth
for operating
thepositioning
interferometer
mirror to
~1necessary
mrad and <1
but more precise controls
are
to mm
reach
the expected sensitivity of the instrument.
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Feedback
Feedback is applied to the Marionette via
the four coil-magnet actuators used also
for the local control.
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Current Status
Output Mode-Cleaner
Interferometer currently used
in recombined
mode (Recycling mirror is misaligned)
North and West arm cavities are
automatically aligned (to the beam) since:
North arm: December 2003
West arm: May 2004
Longest continuous lock >32h
Beam drift correction not yet implemented
Suspended bench
External bench
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Cavity Power
AA Off
AA turned ON
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Angular Fluctuation
AA ON
AA OFF
From Local to Global control
Bandwidth ~4 Hz
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Angular Fluctuation
Residual fluctuations:
~ 1 nrad @ 10 Hz
~ <1urad RMS
Limited by:
input beam jitter
resonance peaks of the main
suspensions (e.g. 0.6 Hz)
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Conclusion
Output Mode-Cleaner
First implementation of the Anderson technique on a large
scale interferometer
Both arms of the interferometer are automatically aligned:
Local controls can be switched OFF
The angular mirror motions are reduced and the power
fluctuations of the arm cavities minimized
Facilitate the recombined lock acquisition
Unity gain frequency around 4Hz
32 hours continuous lock of the interferometer with
automatic alignment control
Next steps
Beam drifts correction
Recycling mirror automatic alignment
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End
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Global Control
8 quadrant diodes yield 32 signals
Output Mode-Cleaner
Signals are linearised by the DC power on the quadrant
A static matrix is used to create 10 signals for angular
control of the mirrors
Unity gain bandwidths is 3 – 5 Hz
Automatic alignment allows switch off the Local controls
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