Interferometer Control

Matt Evans…talk mostly taken from…

The control of the VIRGO interferometer for gravitational

wave detection

Pisa, 20th April 2006

Lisa Barsotti

Università degli Studi di PisaScuola di Dottorato Galileo Galilei

Ph.D. in Applied Physics

3 km

European Gravitational Observatory (EGO)

(Cascina-Pisa)

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…

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

Virgo Design Sensitivity

Seismic wall @ 4 Hz

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

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

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

Length Control: Why

Transmitted Power

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

CASB SB

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

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

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

Length Control: Data

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

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

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

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

Angular Problem

The end mirrors are 3km away The beam travels this distance

many times Small angles (1 micro-radian)

cause big problems

Angular Sensors

P

Y

P

Y

P

Y

Recycled ITF: angles

There are 6 mirrors to control, each with 2 degrees of freedom

The input beam has 4 DOFs 16 total DOFs

Angular Matrix

P

Y

Other Loops…

Laser SystemBeam positionIntensityModulation Frequency

InfrastructureBuilding temperatureVacuum pressure

Suspension SystemsInertial dampingLocal Control

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