04. november 2004 a. freise a. freise, m. loupias collaboration meeting november 04, 2004 alignment...

04. November 2004 A. Freise

A. Freise, M. Loupias

Collaboration Meeting

November 04, 2004

Alignment Status


Overview

Suspended bench External bench

Output Mode-Cleaner

Pre-alignment

Non-linear alignment

Linear alignment

Drift control

Simulation

Hardware


Pre-Alignment


Output Mode-Cleaner

Interferometer pre-aligned since more than one year

Nevertheless, some pre-alignment is always necessary:

re-alignment of a single component after an intervention: Procedure as in the beginning. Difficult with low light power (10% input light) but it was already successfully done for the input beam alignment (including automatic beam matching)

manual re-alignment after a long period without (global) control: procedures exist for the recombined IFO and are currently being developed by the detector operation / locking groups


Non-Linear Alignment


Output Mode-CleanerNon-linear alignment: every kind of computer algorithm performing alignment of the main optics or the main beam

The current software from the Napoli group canautomatically align the North and West arm cavity using a progressive accuracy (from several 10 urads to less than 1 urad)

Automatically align PR to 2 urad (to be improved)

The software is now regularly used in the detector operation

The next version of the software is planned to also align the input beam and the BS to center the beam on the end mirrors (before the cavity alignment)

Provide a new algorithm for the alignment of full VIRGO which is fully automatic and has a better resolution for the PR


Linear Alignment

Output Mode-CleanerVIRGO is the only project to employ the Anderson technique

First time this technique is tried for interferometers with suspended mirrors

Expected advantages and disadvantages Signals in transmission of the cavity are comparably `clean’

The different degrees of freedom can be detected with comparable strengths in all output ports

Possibly a larger coupling of noise into the gravitational wave channel


Frequency Range of 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 5 Hz

no active control at the expected signal frequencies


Quadrant Sensors

Linear Alignment: Differential wave-front sensing

Drift Control: Spot position sensing


Linear Alignment: Current Status


Output Mode-CleanerInterferometer until recently 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

Linear alignment ON is the standard condition, no known problems

Longest continuous lock >32h


Angular Fluctuations

Residual fluctuations:~ 1 nrad @ 10 Hz~ <1urad RMSLimited by:

input beam jitterresonance peaks of the main suspensions (e.g. 0.6 Hz)


Problems and Solutions (I)


Output Mode-CleanerLarge transient when switched from Local Control to Linear Alignment:

problem originates in the low frequency components of the control filters

could be solved by re-arranging the switches and filters in the DSP

Difference between vertical and horizontal readout signals made it impossible to optimize the control matrix:

believed to originate in the readout optics (not completely understood)

can be solved completely by using independant control matrix for Tx and Ty. This requires one extra phase shifter (one electronic board) per detector. Currently a simple phase shifter (cable) is used. A new set of phase shifters are being made by Frascati, they will be ready in two weeks


Problems and Solutions (II)


Output Mode-CleanerBeam position on terminal benches was moving

Supports of terminal benches and of the first big lens after the end mirrors have been changed

Beam profile on terminal benches could not be measured because of strong spherical aberration

Two big lenses were replaced by doublets


Problems and Solutions (III)


Output Mode-Cleaner

Several problems and changes with respect to the Global Control (see F. Cavalier’s talk tomorrow)

A few minor problems with the Quadrant centering (QaServer), to be fixed by upgrading the software soon

Most common problems were related to detector operation only:

excited suspensions (300 mHz, 600 mHz)

detuned demodulation phases for B7/B8 (1-5 Hz)


Problems and Solutions (…)


Output Mode-Cleaner…


From Recombined to Recycled (I)


Output Mode-CleanerWe only have one mirror more to control BUT the entire control scheme changes when the PR mirror is aligned:

We CAN NOT use the control for the arm cavities any more because for the new alignment readout the `cavities’ as such do not exist any more

In the recycling state: either we can control all mirrors or we can control none

There are two new readouts to be implemented2 quadrant diodes in the detection lab and 2 quadrant diodes in the laser lab


Optical Readout

Quadrant readout on B2, B5, B7 and B8

The optical readout has been moved from B1 to B5 for the first implementation

The telescope setup in the detection lab is different from the design due to lack of space, need to design a new telescope

All quadrant diodes are on translation stages: all tested, one broken, can be replaced next week

Gains have to be tuned for the full power


Linear Alignment Matrix (I)

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.

(Without BS control and drift control)


Linear Alignment Matrix (II)


Output Mode-Cleaner

We want to measure the 32x10 matrix in a few minutes:

Requires injecting high frequency lines via the reference mass coils, currently tested by P. Ruggi, requires all coils to be connected to DACs

It might be possible to inject such lines continuously (as calibration lines) to measure the optical matrix online, to be discussed

Matlab software has been written to automatically extract the matrix amplitude coefficients from frame data, currently extended to the full matrix


From Recombined to Recycled (II)


Output Mode-CleanerAs soon as the recycling lock has become more `natural’ we can implement Linear Alignment for full VIRGO

The implementation will consist of an very experimental period:

During a recycling lock the optical matrix has to be measured

The misalignment angles can be reconstructed online using a chi^2 algorithm with this (static) matrix inside the Global Control

With these one tries to close Linear Alignment control loops

A stable control should be found by understanding the behavior of the control loops during locking tries and by doing an iterative series of matrix measurements and locking tries


Global Control Algorithm


Output Mode-CleanerThe current Alignment algorithm inside the Global Control makes use of all the available features:

The configuration is performed via database objects

Derived channels (triggers, error states, reconstructed angles) are send to the DAQ system

On-the-fly switches allow to change between algorithms by clicking a button

An extended algorithm has to be written for full VIRGO control:

The new channels (error signals, feedback) have to be tested

In addition, the chi^2 reconstruction method must be integrated


Beamsplitter Control


Output Mode-CleanerIn the final configuration also the local controls of the Beamsplitter should be switched off

The Beamsplitter motion is associated to the same degree of freedom as the West input mirror, therefore it cannot be controlled independently

Instead a `common’ feedback to BS and WI can be implemented, this requires an additional control loop for the DC position (drift control)

In preparation of this work we have already demonstrated

Sending WI feedback only to BS

Sending `common’ feedback

The drift control of the BS


Drift Control

beam drifts : injection bench and beam splitter to control 2 axes (DC- 0.1 Hz)

DC

DC


Conclusion

Output Mode-CleanerFirst implementation of the Anderson technique on a large scale interferometer

…


End


Example VIRGO Tilt Matrix (16x5)

04. november 2004 a. freise a. freise, m. loupias collaboration meeting november 04, 2004 alignment...

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