1 LA general training session, Cascina Virgo alignment - overview - Nonlinear alignment = prealignment Linear alignment = autoalignment

2 LA general training session, Cascina Linear alignment

3 LA general training session, Cascina Virgo alignment N W EOM Injection Bench Recycling mirror Input mirrors West end mirror North end mirror Correct mirror alignment necessary for keeping arm cavities resonant keeping recycling cavity resonant keeping interference on dark fringe

4 LA general training session, Cascina Local control system of ITF mirrors C C D 35 o ( z ) beama x i s d i f fu s i v ema r ke r s ha l o g e n i ll um i nato r X Y E rr(  x  y ) E rr(  x  y ) PS Dfoca l p l a ne P SD E rr ( z ) r ed l aser d i od e X Y E r r (  x  y ) P S D f o c a l p l an e r ed l as e r d i ode 3 0 o a c t u ato r marionette mirror Local control system uses diode lasers, CCD cameras Position Sensitive Devices (PSD) Residual motions: 1 μm / 1 μrad

5 LA general training session, Cascina Mirror alignment requirements 100 nrad rms 20 nrad rms 3 nrad rms 20 nrad rms Local control: 1 µrad => mirror motion is times too high + slow relative drifts of mirrors Autoalignment system Uses light coming out of cavities for understanding relative mirror misalignment ("global" control system)

6 LA general training session, Cascina Gaussian beams Laser near field (waist) far field flat wavefront curved wavefront cavity

7 LA general training session, Cascina Simple cavity misalignment (end mirror) We use a differential wave front sensing technique. (Anderson technique); at each beam, we have two quadrant diodes "Near field" + 0° TEM 0 0 TEM 0 1 "Far field"

8 LA general training session, Cascina Simple cavity misalignment (input mirror) + 0° 90° TEM 0 0 TEM 0 1 "Far field" "Near field" We use a differential wave front sensing technique. (Anderson technique); at each beam, we have two quadrant diodes

9 LA general training session, Cascina Quadrant photodiode From each QD we get: 2 DC signals simple difference between elements * horizontal/vertical QD centering information 4 AC signals demodulated difference signal * horizontal/vertical * in phase/in quadrature Warning: AC/DC not in the electronic sense! Diff. horiz. vert.

10 LA general training session, Cascina Longitudinal control: 1 DC signal 2 demodulated signals Alignment control: 4 DC signals 8 demodulated signals Detection

11 LA general training session, Cascina Anderson technique: uses the light transmitted by the arm cavities (no pick-off beams needed) requires a specially tuned RF modulation frequency strongly coupled alignment degrees of freedom: each mirror rotation is seen at each output port Anderson-Giordano technique: two quadrant diodes are used in the transmitted beams (near-field, far field) [G. Giordano, Frascati] The Anderson-Giordano Technique

12 LA general training session, Cascina Reconstruction Matrix Control: error signal acquisition

13 LA general training session, Cascina Control: correction signal distribution Reconstruction Matrix

14 LA general training session, Cascina The behaviour of the alignment sensing system is measured by sending a sinus perturbation (line) on each mirror, and measuring the effect of each mirror's line on each QD signal. This measurement gives the optical matrix. The inversion of the optical matrix gives the reconstruction matrix, which allows to calculate the misalignment of each mirror from the QD signals. Reconstruction Matrix Optical Matrix Reconstructed Angular Positions Line on mirror Lines from all mirrors QD signal The angle reconstruction

15 LA general training session, Cascina x6 optical matrix (x2) after the shutdown: matrix also includes DC signals and IB => 30x7 matrix The optical matrix (before C6) + BS

16 LA general training session, Cascina Control modes Linear alignment mode Mirror angles are entirely controlled by reconstructed LA error signals Fast control (bandwidth 3 Hz) Low noise Drift control mode Mirror angles are controlled by local control LA error signals are added as offsets => drift control bandwidth 10 mHz Local control noise Advantage: no loop stability problems due to bad reconstruction &c

17 LA general training session, Cascina Basic alignment strategy Cavity alignment : angular motion of 5 mirrors to be controlled (DC – 4 Hz) Beam drifts : Input beam and Beam Splitter to control (DC Hz) Main interferometer :

18 LA general training session, Cascina C6 configuration

19 LA general training session, Cascina Drift control power stability ← dark fringe (B1p) improvement Recycling power (B5) improvement→

20 LA general training session, Cascina C7 configuration (tx)

21 LA general training session, Cascina Effect of autoalignment (N cavity) AA turned ON AA Off A. Freise M. Loupias

22 LA general training session, Cascina Arm cavity common/differential mode control One DC signal in present control scheme Possible scheme Control NE-WE with fast loop (AC) diff. mode and mirror resonances Control NE+WE with slow loop (DC) drifts WE NE DC AC

23 LA general training session, Cascina Prealignment steps

24 LA general training session, Cascina Cam7p Cam8p WE WI NI NE BS PR Direct beam alignment The direct beams are centered on the cameras M6 picomotors on IB Input mirrors misaligned

25 LA general training session, Cascina Nonlinear alignment: coarse Cam7p B7 B8 Cam8p WE WI NI NE BS PR Maximise the resonance flashes on the photodiodes by moving the cavity mirrors M6 picomotors on IB Mirrors aligned, cavities not locked

26 LA general training session, Cascina B7_q1 Cam7p B7_q2 B7 B8_q2 B8 Cam8p B8_q1 M6 picomotors on IB WE WI NI NE BS PR Zero the QD error signal, moving both mirrors of a cavity Nonlinear alignment: fine Mirrors aligned, cavities locked

27 LA general training session, Cascina B7_q1 Cam7p B7_q2 B7 B8_q2 B8 Cam8p B8_q1 M6 picomotors on IB WE WI NI NE BS PR LA remains on for some time => position memories keep mirrors in aligned position Close "cavities alignment" loop Cavities locked, independent LA loops running for N & W cavity

End

29 LA general training session, Cascina Quadrant diode centering Movement of beam on quadrant diode Units: normalized asymmetry (x2-x1)/(x2+x1) 0.5 means: ¾ of beam on one half of QD QD1 QD2 B2 B5 B7 B Quadrant autocentering active Translation stages Recentering every 5 sec

30 LA general training session, Cascina Some details on control strategy

31 LA general training session, Cascina Beam splitter linear alignment Original scheme: BS under "slow" control (WE quadrant centering) Present scheme:BS under LA, WI under local control Reason: BS local control noisier than other mirrors Advantage: no control hierarchy needed for WI control with WE quadrant But: WI control to be tested Noise ? Drift control Linear alignment BS DC WI DC Original scheme Presently foreseen scheme

32 LA general training session, Cascina C6 alignment matrices PR NI NE WI WE B2_d1_DC B2_d1_ACp B1p_d2_ACp B7_d1_ACq B7_d2_ACq B8_d1_ACp B8_d1_ACq B8_d2_ACq PR NI NE WI WE B2_d1_DC B1p_d2_ACq B7_d1_ACp B7_d2_ACp B7_d2_ACq B8_d1_ACp ThX ThY Drift control Linear alignment

33 LA general training session, Cascina C7 alignment matrices ThX ThY PRNINEBSWE B2_1_DC B2_1_p B1p_1_p B7_1_q B7_2_q B8_1_p B8_1_q PRNINEBSWE 1B2_1_DC 1B1p_1_q B7_1_p B7_2_p B7_2_q B8_1_p 1B8_2_p

34 LA general training session, Cascina Details on control strategy tx ty Lines for matrix measurement at frequencies of high gain Sequential closing of loops Close easiest degrees of freedom Inject lines on non-controlled mirrors => matrix simplification => elimination of dominant modes e.g. differential arm mode

35 LA general training session, Cascina Details on control strategy Switch B5 → B1p Reason: main losses of recycled power through dark fringe misalignment Idea: measure misalignment where it is apparent => dark fringe Although, with Anderson technique not so obvious... End mirror differential mode control with 1 diode control on NE mirror Before After

36 LA general training session, Cascina Details on control strategy Deviation from pure matrix inversion strategy One-by-one identification of suitable signals End mirror diff. mode (B1p) End mirror common mode (B2_DC) CITF mirror thetaX Matrix inversion on sub-matrix CITF mirror thetaY Drift control as preliminary step to LA Alignment stability for C6 Helps understanding of loop stability Basis for LA matrix

37 LA general training session, Cascina Automatic Alignment Anderson technique: - Modulation frequency coincident with cavity TEM01 mode - Two split photo diodes in transmission of the cavity (at two different Guoy phases) - Four signals to control the 2x2 mirror angular positions (NI, NE)

38 LA general training session, Cascina Main IFO and Input Beam linear alignment : 10 degrees of freedom in main IFO 4 degrees of freedom for incoming beam

39 LA general training session, Cascina Combined Degrees of Freedom WE rotation by  NE rotation by - Same motion inside PR cavity

40 LA general training session, Cascina "Cavities alignment" configuration North and West cavities: independently aligned on their transmitted beams Suspended bench External bench Output Mode-Cleaner B7 B8

41 LA general training session, Cascina Milestones Commissioning run C6 29/07 – 12/08/2005 (2 weeks) txPR BSNINE WI WE tyPRBSNI NE WI WE 40 hours continuous lock Minirun M9 25/08/2005 (1 day) txPRBSNINEWIWE tyPRBSNI NEWI WE 1 night continuous lock Commissioning run C7 14/09 – 19/09/2005 (5 days) txPRBSNINEWIWE tyPRBSNI NEWI WE 14 hours continuous lock (max. 28 hours in this configuration) XXLinear alignment XXDrift control XXLocal control XXDC error signal

42 LA general training session, Cascina Prealignment steps