1. Lisa Barsotti - University and INFN Pisa – on behalf of the Virgo Collaboration CASCINA - January 24 th, 2005 ILIAS  Locking of Full Virgo Status of VIRGO

2. VIRGO Optical Scheme BS NI WI PR 3-km Fabry Perot cavities in the arms

3. Commissioning Plan Steps of increasing complexity: Sept 2003 – Feb 2004 Sept 2003 – Feb 2004 A SINGLE FABRY-PEROT CAVITY PR misaligned North Cavity

4. Commissioning Plan Steps of increasing complexity: Sept 2003 – Feb 2004 Sept 2003 – Feb 2004 A SINGLE FABRY-PEROT CAVITY PR misaligned West Cavity Check of the performances of the sub-systems Check of the control systems in a simple configuration

5. Commissioning Plan Steps of increasing complexity: Feb 2004 – Dec 2004 Feb 2004 – Dec 2004 A FABRY-PEROT MICHELSON ITF “RECOMBINED” MODE PR misaligned North Cavity West Cavity Intermediate step towards full Virgo Start of noise analysis

6. Commissioning Plan Steps of increasing complexity: Since Sept 2004 Since Sept 2004 A POWER RECYCLED MICHELSON ITF  Final configuration PR aligned North Cavity West Cavity “RECYCLED” MODE

7. Commissioning of a Single Fabry-Perot Cavity - I WE NE NI WI BS PR WE NE NI WI BS PR T=8% Transmitted Power laser freq noise & mirror angular motion Power Fluctuations Demodulated osymmetric beam Control Scheme Lock at the first trial 28 th Oct 2003

8. Commissioning of a Single Fabry-Perot Cavity - II  C1 (14-17/11/2003) - North cavity and OMC locked Three Commissioning runs in a single cavity configuration:  C2 (20-23/02/2004) - C1 + Automatic alignment - West arm locked  C3 (23-27/04/2004) - C2 + Laser freq stabilization IMC control noise reduced Transmitted Power

9. Commissioning of a Single Fabry-Perot Cavity – III  Sensitivity Progress C1 C2 C3  C1 (14-17/11/2003) - North cavity and OMC locked Three Commissioning runs in a single cavity configuration:  C2 (20-23/02/2004) - C1 + Automatic alignment - West arm locked  C3 (23-27/04/2004) - C2 + Laser freq stabilization IMC control noise reduced

10. Commissioning of the Recombined ITF NE NI WI BS WE NE NI BS PR

11. Commissioning of the Recombined ITF NE NI WI BS WE NE NI BS ~ 1 W P BS 10 W P0P0 PR P BS expected in recycled mode ~ 500 W Sensitivity ~ Start of some noise characterization ( 500 W)

12. Recombined ITF Optical Scheme 1 5 7 8 2 WE NE NI WI PR WE NE NI WI PR T=8% BS Reflected beam Asymmetric beam West Transmitted beam North Transmitted beam Pick-off beam

13. Recombined ITF Optical Scheme 1 5 7 8 2 WE NE NI WI PR WE NE NI WI PR T=8% North Cavity West Cavity Simple Michelson BS  3 d.o.f. ‘ s to be controlled: Lengths of the kilometric arms: L1 and L2 Michelson asymmetric length: l1 – l2  fields not mixed L1L1 L2L2 l1l1 l2l2

14. Recombined ITF – Lock Acquisition North arm West arm Michelson length Lock of the two arms indipendently with the end photodiodes Corrections sent to NE and WE Lock of the michelson with the asymmetric port signal Corrections sent to BS 8_demod 7_demod 1_demod 2_quad

15. Recombined ITF - Linear Locking 2_quad North arm West arm Michelson 2_phase1_demod End photodiodes very usuful for lock acquisition but too noisy Cavities controlled with the reflected and the asymmetric beams Common mode of the cavities Differential mode of the cavities

16. Commissioning Run C4 - June 2004 ITF controlled with the reflected and the asymmetric beams Automatic alignment of the cavities Laser frequency stabilized to cavities common mode Cavities common mode locked to reference cavity Output Mode Cleaner locked on the dark fringe Tidal control on both arms  Recombined Data Taking Mode

17. Commissioning Run C4 - June 2004  5 days of run  Longest lock ~ 28 h  Lock losses understood  h reconstruction on line

18. Commissioning Run C4 : Noise Characterization Coupling of IB resonances into the michelson controller signal due to a mismatch between modulation frequency and input mode-cleaner length C4 After frequency modulation tuning Michelson controller signal see Flaminio’ s talk

19. After C4  July – August Upgrade of the terminal benches -> Re-tuning and improvement of the linear automatic alignment Suspension full hierarchical control started Commissioning of the Recycled ITF started Effect of the backscattered light in the IMC -> attenuator installed between the IMC and the ITF  Mid September: Re-Start  October – November: -> Recombined ITF locked with the full hierarchical control of the end suspensions -> ITF locked in recycled mode

20. Suspension Hierarchical Control  10 3  Locking acquired and maintained acting at the level of the mirror z z x y marionette reference mass mirror  Reduce the strength of the mirror actuators by a few 10 3 to reach Virgo design sensitivity

21. Suspension Hierarchical Control DC-0.01 Hz 0.01-8 Hz 8-50 Hz Corrections sent to the marionette Corrections sent to the mirror Force on the mirror reduced of a factor 20  Switch to low noise coil drivers TIDAL CONTROL RE-ALLOCATION OF THE FORCE

22. Suspension Hierarchical Control  Single arm locked with the hierarchical control for the first time in July -> controllability of the superattenuator demonstrated  Last main result: hierarchical control of the recombined ITF in the C4 configuration, with automatic alignment and frequency servo engaged  Stable lock -> tested in the last commissioning run (C5, 2-6 December 2004)  SUMMARY

23. Lock Acquisition of full VIRGO  Simulations on a lock acquisition technique developed following the LIGO experience  Locking trials with this baseline technique (first half of July)  Attenuator installed (summer)  Restart of the locking trials with the baseline technique (21st September)  Debugging of the sub-systems  Establishement of theVariable Finesse lock acquisition technique (October)  Chronology

24. Recycled ITF: Base and Photodiodes 5 8 WE NE NI WI BS PR WE NE NI WI BS PR LWLWLWLW LNLNLNLN MICH = l n -l w PRCL= l rec +(l N + l w )/2 CARM= L N +L W DARM= L N -L W lWlWlWlW lNlNlNlN l rec 4 lengths to be controlled: 7 2 Reflected beam Asymmetric beam West Transmetted beam North Transmetted beam 1

25. Baseline Technique Based on the LIGO technique Multi–states approach Dynamical inversion of the sensing matrix

26. Experimental Activity: Lock of Stable States - I Sidebands locked in the recycling cavity 2_quad Reflected f-demod signals to control MICH and PRCL STABLE STATE 2 2_phase

27. Experimental Activity: Lock of Stable States – II Sidebands locked in the recycling cavity, carrier locked in the FP Reflected f-demod signals to control MICH and PRCL STABLE STATE 3 2_quad 2_phase

28. From f-demod to 3f–demod signal  CARM contamination in the PRCL reconstruction  Frequency Response of the f-demod signal very sensitive to the ITF losses State 4 Simulated Sensing Matrix

29. PRCL Frequency Response - I B2_f_phase Input FP Mirrors Losses 1% o Non - Minimum Phase SIMULATION

30. PRCL Frequency Response - II Input FP Mirrors Losses 1% o B2_3f_phase Minimum Phase SIMULATION

31. VIRGO Lock Acquisition Scheme 1_phase 5_phase REF BEAM phase REF BEAM quad 3f - Demod Signals Good decoupling MICH / PRCL Less CARM contamination in the PRCL signal Almost Diagonal Sensing Matrix

32. First Locking Trials NORTH and WEST Power Recycling Cavity Power BS – PR CorrectionsNE – WE Corrections

33. Drawbacks of the Baseline Technique: PR Transfer Function  PR transfer function The lock acquisition technique is “statistical”.  transients, ringing MARCH OCTOBER Compensation of the PR Resonances: critical, high Q

34.  The optical design of the ITF makes the response of the reflected 2_f signal very depending by the losses Use of the 2_3f signal in the lock acquisition phase  The CARM contamination is anyway critical : use of SSFS is possible only in a steady state regime Drawbacks of the Baseline Technique: the CARM contamination

35. A new strategy: theVariable Finesse Lock Acquisition

36. The Variable Finesse Locking Strategy “A recycled ITF with a low recycling factor is similar to a recombined ITF “  End photodiodes  Lock immediately the 4 degrees of freedom of the ITF on the half/white fringe (low recycling factor) lock of PR prevents ringing and transient effects lock of the cavities prevents CARM contamination  Bring the interferometer adiabatically from the half to the dark fringe increasing the recycling factor

37. The Variable Finesse Locking Strategy NINE WE WI BS PR Low Recycling Factor  Lock immediately the 4 degrees of freedom of the ITF on the half fringe:  end photodiodes to acquire the lock of the long cavities  simple michelson locked on the half fringe with the asymmetric DC signal  3f demodulated reflected signal to control the recycling cavity length Half Fringe

38. The Variable Finesse Locking Strategy End photodiodes start to see both the cavities: We can not continue to control the arms indipendently

39. The Variable Finesse Locking Strategy NINE WE WI BS PR Low Recycling Factor  Laser frequency stabilization engaged  One of the end photodiodes used to control the differential mode of the cavities Half Fringe  Laser stabilized on the common mode of the cavities  PR realigned  Offset in the mich DC error signal reduced approaching the dark fringe

40. 5_ph 2_3f_ph LASER WEST TRANSM BEAM 5_q The Variable Finesse Locking Strategy  From the DC to a demod signal to control the michelson length

41. The Variable Finesse Locking Strategy  Final Step :  Final Step : To the Dark Fringe ITF on the operating point

42. 5_ph 2_3f ph LASER 5_q ASY BEAM 1_demod RUNNING MODE: Switch to the main GW signal to control the DARM mode: end photodiode very noisy The Variable Finesse Locking Strategy

43. POWER IN THE RECYCLING CAVITY ITF not locked The Variable Finesse Locking Strategy Lock Acquisition ITF locked on the dark fringe “Variable Finesse” of the recycling cavity

44. POWER IN THE RECYCLING CAVITY Recombined interferometer (~ 60 mW) Recycled interferometer (~ 17 W) T PR=8% -> Recycling factor ~ 25 The Variable Finesse Locking Strategy

45. Recycling Cavity Power Lock duration limited by the natural misalignment of the mirrors Longest Lock: 2h30 Need of the linear automatic alignment ( Usually about 30-40 minutes )

46. The Variable Finesse Locking Strategy  First lock of the recycled ITF on the end of last October  Stable lock of the recycled interferometer ~ 40-50 minutes  no linear automatic alignment yet  next step  Locking procedure tested several times  lock acquired in few minutes  New original lock acquisition procedure established, combining end photodiodes, frequency servo, 3f-demod signal, slightly misalignement of PR mirror, and lock on the half fringe  1 day and half of test in the last commissioning run C5  SUMMARY

47. Commissioning Run C5 - December 2004 C5 configurations: - RECOMBINED ITF as in C4 ( automatic alignment, laser frequency stabilization servo, OMC locked) + suspension hierarchical control -> end of the commissioning of the recombined ITF - RECYCLED ITF (1 day and half)

48. Commissioning Run C5 - December 2004 Best VIRGO Sensitivity RECOMBINED RECYCLED

49. Noise hunting: C5 sensitivity Short michelson control Recycling cavity controlLaser freq control COHERENCES with the GW signal  Sensitivity limited by control noise Longitudinal locking control signal BS tx local control Local control signal

50. Noise hunting: C5 sensitivity What about the noise at high frequency ? ? Observation: noise level change with time i.e. with alignment

51. Noise hunting: C5 sensitivity 2 minutes of C5 data Power on dark fringe Main ITF output Other quadratureAveraged noise spectrum

52. Noise hunting: C5 sensitivity Noise variation at high frequency vs alignment

53. Noise hunting: C5 sensitivity At low frequency (< 100-300 Hz) - Switch OFF local controls (possible when automatic alignment will be used) - Use of less noisy error signals to control the ITF (2_3f -> 2_f) - Use of more complex controller filters - Reduce sensitivity to IMC length noise (  tune IMC length and Fmod) At high frequency (> 100-300 Hz) - Implement ITF automatic alignment - Have a better look into noise when alignment is/will be better Next Steps:

54. Something not undertood yet: lock losses in C5 data RECYCLING STORED POWER Lock acquired, but not stable Stable Lock  Any evident difference in the two periods (analysis in progress)

55. Something not understood yet : the “JUMPS”  “Jumps” in the powers observed with the recycled locked Recycling Cavity Power Maximum power Jumps very big -> less than half power They can unlock the ITF More frequent in these last weeks Some days it was impossible to work Not always present: any evident difference observed in the ITF status when jumps appeared with respect to the quite situation

56. First idea: jumps connected with the alignment of the ITF Aligned position Misaligned positions  Some experimental tests: NI misaligned of few urad Jumps start to appear when the mirror is misaligned of 2-3 urad Same results obtained misaligning the PR mirror …but jumps are seen also with the “ well aligned” ITF (maximum stored power observed)

57.  Some experimental tests: change of the PRCL error signal (2_3f) demodulation phase with respect to the alignment of the PR Recycled Stored Power PR - ty 2_3f demod phase Aligned position Aligned position: no jumps for a scan of several tens of degrees of the demodulation phase  More PR is misaligned and more the demod phase is critical

58. ITF locked in a “bad” way?  Sometimes the ITF works better - higher power, more stable - when it is still present an offset in the michelson error signal (5% out from the dark fringe)  A constant offset is present in the out loop reflected signal when the ITF is locked. The 2_f signal is planned to be used to control PRCL (switch 2_3f -> 2_f needed for noise reduction) IN LOOP OUT LOOP Offset equivalent to 5 nm PR displacement

59. ITF locked in a “bad” way? IN LOOP OUT LOOPIN LOOP OUT LOOP When the switch 2_3f -> 2_f is done the stored power decreases of the 50 % Switch to 2_f Stored Power Refl 2_3f_phase signal Refl 2_f_phase signal The offset in the 2_f signal is independent from the alignment conditions An offset in the 2_3f signal ?

60. Something not understood yet : offset in the end signal IN LOOP Offset ITF LOCKED Dark Fringe Power ITF on the dark fringe Stored Power GW signalDARM error signal MICH error signal  As soon as the switch from the end to the GW signal to control DARM is done, an offset appears on the end signal  The dark fringe is “ darker” if the ITF is locked with the GW signal

61. An offset in the laser frequency servo? The error signal used to control DARM is one of the end signals It sees not only DARM, but also CARM An offset in the laser frequency servo error signal could keep the ITF bad locked in the CARM d.o.f  the end signal sees the CARM offset, which is transferred to the DARM d.o.f and which is visible on the dark fringe power  the GW signal sees only DARM, so it does not see the offset  could it explain also the offset in the reflected signal?

62. An offset in the laser frequency servo? DARM error signal Ref 2_f_phase OUT LOOP same offset  ITF locked with the GW signal, offset added to the frequency servo error signal Offset Stored Power

63. Conclusions  1 year of commissioning 28 th Oct 2003 First lock of the north cavity 26 th Oct 2004 First lock of the recycled ITF

65. Next Steps  Improvement of the recycling locking robustness and understanding of jumps and offsets: - real time simulation under development + dedicated shifts  Automation - pre-alignment (in progress) and locking procedures (done)  Linear automatic alignment of the full ITF - work started, other 3-4 weeks planned  Laser frequency stabilization optimization - preliminary measurements done