Linear Alignment System for the VIRGO Interferometer

1. Linear Alignment System for the VIRGO Interferometer M. Mantovani, A. Freise, J. Marque, G. Vajente

2. The Virgo Detector Layout W N Michelson interferometer with 3km long Fabry-Perot cavities Mode Cleaner Input beam Main output port

3. The Virgo Mirror Suspension • 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 Angular Fluctuation ~ 1mradRMS

4. The Virgo Mirror Suspension • 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 Angular Fluctuation ~ 1mradRMS • Alignment precision requests: • 10-7radRMS for the recycling mirror • 2·10-8radRMSfor the cavity input mirrors • 3·10-9radRMS for the cavity end mirrors Shot noise: 10-13 rad/sqrt(Hz) @10Hz

5. The Virgo Mirror Suspension • 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 A more precise alignment system is needed

6. Linear Alignment System The purpose of the linear alignment system is to keep the beams and mirrors at their set position, in order to: • allow a stable interferometer operation over long periods, i.e. perform a control for low frequencies, where the SA does not suppress motions. • minimise the coupling of noise into the dark fringe signal In other words: the automatic alignment control should not actively suppress motion in the measurement band (>10Hz) the linear alignment should allow to switch of „noisy“ local controls.

7. Linear Alignment System Overview

8. Linear Alignment System Recombined Mode 4 Quadrant Photodiodes (→ 8 signals for each degree of freedom tx or ty)

9. Linear Alignment System Recombined Mode 4 Quadrant Photodiodes (→ 8 signals for each degree of freedom tx or ty) 4 Mirrors to control

10. Quadrant Photo-Detector • Specification • photodiode sensitivity = 0.45 A/W • maximum DC power = 3 mW • transmittivity = 2 kW • Bias voltage = 180 V Shot Noise ~ 4 nV/sqrt(Hz)

11. Linear Alignment System Recombined Mode

12. Feedback Feedback is applied to the Marionette via the four coil-magnet actuators used alsofor the local control.

13. Linear Alignment System Recycled Mode 8 Quadrant Photodiodes (→ 16 signals for each degree of freedom tx or ty )

14. Linear Alignment System Recycled Mode 8 Quadrant Photodiodes (→ 16 signals for each degree of freedom tx or ty ) 5 Mirrors to control

15. Linear Alignment System Recycled Mode

16. Linear Alignment System Recycled Mode

17. Reconstruction Algorithm The reconstructed signals are computed by using a χ2 algorithm starting from the optical matrix. The optical matrix is measured by injecting frequency lines, at the level of the reference mass of the mirrors or at the level of the marionette, and then it is computed by calculating the transfer function, at the lines frequencies, and the quadrants signals (Matlab script)

18. Reconstruction Algorithm function [Matrix]=Mmeasure(TxTy,GPSb,GPSe,fres,ave,filename,lines,checkpast) • Loads the ffl file starting from the GPS time • Computes the fft of the mirror signals and quadrant signals • Searches the lines frequencies by using the nominal frequency values • Computes the transfer functions between the mirror signals and the quadrant signals at the line frequencies • Makes a coherence analysis in order to estimate the measurement noise • Prints the signal to noise ratio matrix in order to control the amplitude of the frequency lines • Prints the optical matrix

19. Reconstruction Algorithm Optical Matrix

20. Matrix Measurements Method • Measure the matrix coefficients evolution as a function of the demodulation phases of the quadrants in order to: • Understand the behavior of the matrix • Tune the demodulation phase Each measurement point takes 3 minutes => 18 minutes for the whole evolution measurement (180 sec for 15 FFT averages)

21. Matrix Measurements Method • Measure the matrix coefficients evolution as a function of the demodulation phases of the quadrants in order to: • Understand the behavior of the matrix • Tune the demodulation phase

22. Demodulation Phases Tuning for the Recombined Mode In this situation we have decided to minimize one signal respect to the other Fine tuning for the demodulation phases

23. Experimental Progress

24. Characterising the Optical System • Studied the evolution behavior for the matrix coefficients depending on the fringe offset • Checked the repeatability of the phase tuning measurement the repeatability of the matrix measurement • Tried to work at 0.2 offset from the dark fringe in order to benefit from the higher stability of the lock in this state • Discovered some, not understood, problems at 20% of the dark fringe which obliged us to work at 0.08 fringe offset • Found some anomalies of the optical matrix measured in the recycled mode by using a set of frequency lines injected at the level of the reference mass of the mirrors • Measured the optical matrix by injecting the frequency lines at the level of the marionette

25. Optical Matrix measurement for different fringe offsets 0.1 fringe offset dark fringe The amplitudes of the matrix coefficients are very different for the 0.1 fringe offset with respect to the dark fringe We can not measure the optical matrix at 0.1 fringe offset

26. Optical Matrix measurement for different fringe offsets dark fringe 0.05 fringe offset The matrix coefficients at the 0.05 fringe offset and at the dark fringe match well

27. Repeatability of the Matrix Measurement The Matrix measurement done at the dark fringe in successfully repeatable

28. Matrix Measurement at 0.2 Fringe offset We do not understand the reason of this behavior, we decided to work at 0.08 of fringe offset.

29. Reference Mass Line Injecting Point We have measured the optical matrix of the system by injecting high frequency lines (from 20 to 50 Hz) at the level of the reference mass in the Recombined Mode (in which we did not have any problem) and in Recycled Mode. In the Recycled configuration we observed a strange behavior of the measured optical matrix (even if the sine behavior of the matrix as a function of the demodulation phase was verified)

30. Reference Mass Line Injecting Point Excessively high coefficient values

31. Reference Mass Line Injecting Point Sign Flips

32. Marionette line injecting point We decided to measure the optical matrix by injecting the lines at the level of the marionette (going to low frequency 5 to 9 Hz) • The strangely high amplitude of the coefficients is disappeared • There are not sign flip anymore • The matrix measurements seem to be nicely repeatable

33. Offline Validation of the Linear Alignment Loops

34. Reconstructing Matrix Optical Matrix Offline Data Analysis The error signals are constructed, in an offline analysis, starting from the measured quadrant signals and then applying the reconstruction matrix Reconstructed Correction Signals In this way we can easily check the quality of our reconstruction taking the decoupling of the injected signals as a measure

35. Offline Data Analysis PR NI NE WI WE

36. Further Matrix Quality Analysis Optical matrix computed with SIESTA simulation (G.Giordano)

37. Further Matrix Quality Analysis Angle between column vectors: Minimum angle: 6 deg (matrix subset: 30 deg) Conditioning of the system: 300 c2noise distribution: Suspended bench External bench

38. Latest Results

39. Closing the Tx Loops

40. Conclusions and Next Steps • Closed the Tx loops in a stable state for 5 min • We will continue following the same strategy • We have to analyze the data to understand the different behavior from the theory • But before we want to close the Ty loops to have more precise data

42. Offline Data Analysis In order to have an evaluation of the goodness of the algorithm, used to reconstruct the mirror angular positions, we have injected lines on the mirrors and measured the quadrant signals