Damping and Tuning of Damping and Tuning of Violin Modes in Violin Modes in Monolithic Silica Monolithic Silica

SuspensionsSuspensions

Stefan Goßlerfor the

GEO 600 Team

MotivationMotivation

Example: GEO 600 monolithic silica suspensionsmade in December ´02

But: High Q violinmodes may interfere with the interferometer length-control servo

To supress the influence of thermal noise, high Q suspensions are a key element of the advanced interferometric detectors

Damping of the violinmodeswithout degrading the pendulum Q

GEO Suspensions ...GEO Suspensions ...

Cantilever Springs

StackIsolator

Coilholder forLocal Controlat the Upper mass

Fibers

Upper Mass

IntermediateMass

Mirror

5.6 kg180 mm

... with Feedback... with Feedback

3 mm

Cantilever Springs

StackIsolator

Mirror

Upper Masses

Coilholder

Fibers

IntermediateMass

Cantilever springs

ElectrostaticActuator(10 – 100 Hz)

Magnet-CoilActuator(0 – 10 Hz)

ViolinmodesViolinmodes

Q ~ Resonant Enhancement

Q = fres , = Decay Time for Factor 1/e

Am

plit

ude

1

Frequency [Hz]1 500

f0f1

Example: 3 108/(500 Hz) = approx. 2,3 Days

P. Willems et al.: Q = 5,2 108

How to keep the servos happy?How to keep the servos happy?

• f0: Q < 3*106

• f1: Q < 2*106

• Frequencies (each eight for f0 and f1) within 10%

• Frequencies as high as possible (650 Hz)

Frequency [Hz]100 1000

Mag

nitu

de

10-2

1

102

f0

Damping & TuningDamping & Tuning......

f1By coating sections of the fibers with lossy material(i.e, amorphous Teflon dissolved in fluorinertin our case)

...violin modes without degrading the pendulum Q:

Damping & TuningDamping & Tuning......

f0 f1

Damping scales stronger with coating diameter than with coating length

Frequency shift scales with mass added

...violin modes without degrading the pendulum Q:

By coating sections of the fibers with lossy material(i.e, amorphous Teflon dissolved in fluorinertin our case)

Measuring Violinmode Measuring Violinmode QQss50 kg

Isolation Fiber(Fused Silica) Fused Silica Fiber

with GEO parameters:280 mm Length220 µm Diameter

1,4 kg

Lock-in Ampl.

Spectrum An.

Laser Split-Photodiode

Elektrostatic ActuatorFused Silica Plate

Fused Quarz Block(400 g)

Fused Quarz Block (withSteel Clamp)

Vacuum: 10-5 mbarQmax = 3*107 ,

Measuring Violinmode Measuring Violinmode QQss

1,4

kg

Elektrostatic Actuator

Vacuum: 10-5 mbarQmax = 3*107 ,

Measuring Violinmode Measuring Violinmode QQss50 kg

Isolation Fiber(Fused Silica) Fused Silica Fiber

with GEO parameters:280 mm Length220 µm Diameter

1,4 kg

Lock-in Ampl.

Spectrum An.

Laser Split-Photodiode

Elektrostatic ActuatorFused Silica Plate

Fused Quarz Block(400 g)

Fused Quarz Block (withSteel Clamp)

Vacuum: 10-5 mbarQmax = 3*107 ,

„„Reference Fiber“Reference Fiber“

A fiber with:Length: 284 mmDiameter: 210 – 240 µm

f0 = 664 Hz

Q0 = 2 106

Coating length: 26 mm@ 1/2 of the fiber lengthFiber diam.: 240 µmAfter coating: 300-325 µm

f0

Bounce mode: 14.9 Hz

Q1 = 5 105

Coating length: 31 mm @ 1/4 of the fiber lengthFiber diam.: 210 µmAfter coating: 340-370 µm

f1 = 1313 Hz

f1

f0: 743 Hzf1: 1451 Hz

Selective Tuning...Selective Tuning...

f0 [Hz]

f1 [Hz]

... of the violin modes:

741

1415

660

1306

UncoatedAfterInitial coating

691

1325

After 1st corrective coating

678

1316

After 2nd

corrective coating

665

1316

724

1490

676

1380

662

1353

f0 [Hz]

f1 [Hz]

Target: 664 Hz 1313 Hz

Fiber Preparation for Fiber Preparation for GEOGEO

20 of 30 coated Fibers within:

Before coating 3,5%

f0 = 663 Hz 0,48%Before coating 4,2%

f1 = 1314 Hz 0,86%

But: Welding and suspending increases the spread by not yet known amount

Target: 664 Hz 1313 Hz

TTununing after weldinging after welding ((in situin situ))

Tuning after welding (Tuning after welding (in situin situ))

After welding:

f0 = 669,5 Hzf1 = 1344 Hz

After correctivecoating:

f0 = 654 Hzf1 = 1324 Hz

Two of eight fibers were treated with a corrective coating after welding

Example:

ResultsResults: : FrequenciesFrequencies & & QQss

Am

plit

ude

[a.u

.]

Frequenz [Hz]620 640 660630 650 670

f0 = 645,5 Hz 3,86%

Q0= 6 105 – 2,8 106

Q1= 5,4 105 –1 106

f1 = 1287 Hz 4,86%

Target: Q < 2 106 .

Target: Q < 3 106 .

ConclusionConclusion

It is possible to tune the violin mode frequencies (downwards) almost independently to the desired values with great accuracy

In December ´02 we successfully suspended two GEO mirrors with damped and tuned violin modes (And the beam splitter only with slightly damped violin modes)

It is possible to bring the Qs to the desired levels, BUT it is NOT possible to measure them prior to final installation