1. Interferometer Control Matt Evans …talk mostly taken from…

2. The control of the VIRGO interferometer for gravitational wave detection Pisa, 20 th April 2006 Lisa Barsotti Università degli Studi di Pisa Scuola di Dottorato Galileo Galilei Ph.D. in Applied Physics

3. 3 km European Gravitational Observatory (EGO) (Cascina-Pisa)

4. Things to Know -> If you have a question, raise your hand and wave, or take some other action to draw my attention. This may be difficult because I haven’t slept much this week, so don’t be shy. ->The topic of this talk is “Interferometer Control”, but after years of work in this field I still don’t really know what I’m doing, so I tried to make this talk easy to understand. I probably failed… ask questions. -> I am American, and I suffer from the common misconception that Americans speak English. This means that I talk fast (because I think I speak English well), but I am difficult to understand (because I’m not really speaking English). ->This talk will require about 15 minutes if I talk fast and nobody stops me. ->I like talking to myself, but not in public…

5. The Virgo Interferometer Injection System -> Input Mode Cleaner -> Laser Beam 20 W -> It provides the beam entering the ITF with the required power and frequency stability -> Beam RF modulation -> High quality optics with low absorption, suspended in vacuum -> Output Mode-Cleaner to improve the contrast -> Detection, amplification and demodulation Detection System EOM

6. Virgo Design Sensitivity Seismic wall @ 4 Hz

7. Operating Point The ITF has its nominal sensitivity only at its operating point  resonant light inside the cavities to increase the phase sensitivity  L < 5x10 -9 m RMS (integrated DC-10 kHz)  anti-symmetric port on the dark fringe in order to prevent intensity noise from dominating over shot noise Constraints on the tolerable fluctuations of the relative position of the mirrors  L < 10 -12 m RMS

8.  The Superattenuator is a multi-stage pendulum, with passive attenuation: 10 @ 10 Hz Suspension System At lower frequencies the noise is instead totally transferred to the mirror, even amplified by the pendulum resonances Residual longitudinal motion of the mirror L ~ 10 -6 m RMS 14 Local active control of the Superattenuator reduces mirror motion below a few Hz 10 14

9. Length Control: Why Intensity noise based requirement L < 10 -12 m RMS Residual longitudinal motion of the mirror L ~10 -6 m RMS A global control system is needed to hold the ITF on its operating point by controlling relative mirror positions

10. Length Control: Why Transmitted Power

11. Filtering  Error signals are filtered to compute correction signals Hz Gain Length Control: What  Pound-Drever-Hall error signals giving the deviation from the operating point are extracted at the output ports of the ITF Length Sensing  Correction signals are sent to the optics by means of coil-magnet actuators Actuation CA SB

12. Filtering  Error signals are filtered to compute correction signals (brain)  Different mechanical systems require different filters Hz Gain Control Example: Filtering  Error signal giving the deviation from the operating point are extracted from our volunteer (eyes) Length Sensing  Correction signals are sent to the optics by means of a bio-actuator (hand) Actuation

13. The Length Control Chain Signals are acquired with 16-bit ADCs @ 20 kHz Data are transferred via optical links to the Global Control which computes correction signals Corrections signals are sent to the DSPs of the involved suspension, passed through DACs and applied to the mirror Global Control

14. Filtering  Error signals are filtered to compute correction signals (brain 1? brain 2?) Hz Gain Control Example: Delay  Error signal giving the deviation from the operating point are extracted from volunteer 1 (eyes) Length Sensing  Correction signals are sent to the optics by means of a bio-actuator (hand) Actuation

15. Length Control: Data

16. The Lock Acquisition Problem Correction signal Transmitted power  Only 1 degree of freedom  Correction signal sent to the mirror at a resonance crossing Error signals are available only when the ITF is around resonance  no signals available far from resonance

17. Recombined ITF: lock acquisition Lock of the two arms indipendently Lock of the michelson  More complex optical scheme (3 degrees of freedom: the two cavity lengths and the Michelson length)  Lock acquisition can be made similar to the single cavity by using the end photodiodes

18. Recombined ITF: linear lock  Once the ITF is locked on its operating point, the longitudinal control scheme is optimized in order to reduce the control noise: * use of less noisy error signals * use of more aggressive filters linear lock control scheme

19. Recycled ITF: after locking  Frequency Servo used for common arms  GW signal used to control differential arms  BS controlled to keep anti-symmetric port dark  PR controlled to keep power level high

20. Angular Problem  The end mirrors are 3km away  The beam travels this distance many times  Small angles (1 micro-radian) cause big problems

21. Angular Sensors P Y P Y P Y

22. Recycled ITF: angles  There are 6 mirrors to control, each with 2 degrees of freedom  The input beam has 4 DOFs  16 total DOFs

23. Angular Matrix P Y

24. Other Loops…  Laser System  Beam position  Intensity  Modulation Frequency  Infrastructure  Building temperature  Vacuum pressure  Suspension Systems  Inertial damping  Local Control

25. Conclusions  Control loops should be avoided  Coupled systems should be placed firmly in the rubbish bin without hesitation or remorse  Interferometers are evil  Sleep is good