m. mantovani ilias cascina march 2008 automatic alignment system improvements after the vsr1 m....

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


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


Q81 galvo installation


Asymmetry fluctuations at step 12

Q81

Beam fluctuations on WE bench

Q81

Galvo off Galvo on

Loop unity gain frequency ~ 20Hz


Control Filters optimization

Sensitivity improvement

improving the high frequency cut off


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


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


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


End bench optics rearrangement

the quadrant diodes have been placed in the optimum Gouy phase to detect the input mirrors


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


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

m. mantovani ilias cascina march 2008 automatic alignment system improvements after the vsr1 m....

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