M. Mantovani ILIAS Cascina March 2008
Automatic Alignment system
Improvements after the VSR1
M. Mantovani for the Alignment team
2 M. Mantovani ILIAS Cascina March 2008
Summary
Installation of the galvo system for the terminal bench quadrant diodes (actually we are using only for one WE bench quadrant)
Angular control system noise performances Optimization of the control filters Improving the sensor electronic noise
Input mirrors angular control Rearranged the end benches optical components and retuned the Gouy phase for the end bench quadrant diodes
M. Mantovani ILIAS Cascina March 2008
Why are the galvo useful?
The miscentering of the beam on the diode spoils the error signal
Translation stages: too noisy (centering
rate ~4sec)
Galvo:
bandwidth of tens of Hz
M. Mantovani ILIAS Cascina March 2008
Q81 galvo installation
M. Mantovani ILIAS Cascina March 2008
Asymmetry fluctuations at step 12
Q81
Beam fluctuations on WE bench
Q81
Galvo off Galvo on
Loop unity gain frequency ~ 20Hz
M. Mantovani ILIAS Cascina March 2008
Control Filters optimization
Sensitivity improvement
improving the high frequency cut off
M. Mantovani ILIAS Cascina March 2008
Control Filters optimizationimproving the low frequency stability
improving the low frequency gain of the differential mode improves the accuracy of the alignment, visible also on the dark fringe
M. Mantovani ILIAS Cascina March 2008
Sensor electronic noise reduction
Q1pACQ1pAC
to control the to control the differential end differential end
modemodeQ21DCQ21DC
to control the common to control the common end modeend mode
Q81ACQ81AC
to control the BSto control the BS
M. Mantovani ILIAS Cascina March 2008
Sensor electronic noise reduction
Starting fro 10 Hz most of the sensors are limited by electronic noise
thanks to the new electronics the electronic noise can be reduced
M. Mantovani ILIAS Cascina March 2008
End bench optics rearrangement
the quadrant diodes have been placed in the optimum Gouy phase to detect the input mirrors
M. Mantovani ILIAS Cascina March 2008
Input mirrors control
The error signals are able to detect the input mirror angular displacement, but these are strongly affected by the air currents inside the acoustic enclosure. Thus: the air currents have to be reduced by improving the insulation of the optical bench (not trivial)
the error signals have to be made insensible to the air flow and to the bench motions by retuning slightly the Gouy phases (not clear if it is feasible)
Air flow
Benchmovements
M. Mantovani ILIAS Cascina March 2008
Conclusions and next steps
• The galvos have been installed on the terminal benches, for the moment we are actually using only one of them
• The angular control system noise can be reduced below the design sensitivity by acting on the control filter performances, on the electronics and on the mirror/beam centring.
• The input mirrors can be controlled by using the terminal quadrants if the low frequency noise on the sensors can be reduced
• Improve the beam/mirror centering by using a permanent frequency line excitation on the terminal mirrors