GEO‘s experience with Signal Recycling

Harald LückPerugia, 22.9.2005

~600mMPR

MSR

Dual Recycling in GEO600

Output Mode Cleaner

MichelsonInterferometer

14.904875 MHz

9.017375 MHz

Differential arm length:(gravitational wave signal)

heterodyne detection Schnupp modulation

Signal-Recycling control:

separate modulation frequency

reflected beam from beam splitter AR coating

Michelson Length Control

SR / PR cavity data

f-mod

MHz

# of FSRs FSR

kHz

L

m

PR 14.904875 119 125.251050 1196.766

SR 9.017375

- tuning

72 125.241319 1196.859

ΔL = 93 mm

Output Mode Cleaner

MichelsonInterferometer

14.904875 MHz

9.017375 MHz

Problems with SR Error-Signal: Small catching range large influence of MI-deviation from dark fringe.

Michelson Length Control

Acquisition signal for SRM

SRM tuning [nm]

Sig

nal A

mpl

itud

e [A

rb.]

2.5 kHz0 kHz 5 kHz

0

-1

-2

2

1

Output Mode Cleaner

MichelsonInterferometer

14.904875 MHz

9.017375 MHz

Michelson Length Control

~~

-C

Tunable Optical Gain (200-5000Hz)A

mpl

itude

[V

/m]

Tuning of the SR cavity

Tuning is automated using a Labview programme

SR tuning parameters:

• SR modulation frequency

• SR demodulation phase

• SR servo gain

• MI demodulation phase

• MI servo gain

• MI AA servo gain

Resonance conditions of Control SBs in SR cavity

MI SB

SR SB

119 FSRSR

119 FSRPR

72 FSRSR

72 FSRPR

72 FSRSR

72 FSRPR

119 FSRSR

119 FSRPR

119 FSRSR

119 FSRPR

72 FSRSR

72 FSRPR

72 FSRSR

72 FSRPR

119 FSRSR

119 FSRPR

Enhanced peak gain @ 1kHz tuningA

mpl

itude

[V

/m]

Dark port contrast / Mode healing

~ 1 % < 0.001 %

Power rec. MI.without therm.compensation

Power rec. MIwith therm.compensation

Dual rec. MIwith therm.compensation

-Ratio of carrier light power at dark port / power incident on beamsplitter

~ 0.05%(SB-dominated, 2% MSR)

Power inside PR cavity increases from PR to DR mode by 40 %.

Dual Recycled Performance

• Stable locks at desired tuning frequency with durations of up to 121h.

• Tuning frequencies 200 – 5000 Hz.

• High duty cycle in extended data taking periods ( ~97% during S4, i.e. 4 weeks)

Output Mode Cleaner

MichelsonInterferometer

14.904875 MHz

9.017375 MHz

Differential arm length:(gravitational wave signal)

heterodyne detection Schnupp modulation

Signal-Recycling control:

separate modulation frequency

reflected beam from beam splitter AR coating

2 EP Quadratures

Q

P

90°

Uncalibrated Michelson Error Points

P

Q

On-line optical TF measurements

actuator optical

CAL P and Q

Optical&Electronic Gain

Calibration

Calibrated EP Quadrature Signalsh

[1/s

qrt(

Hz)

]

Combining hP(t) and hQ(t)

• Create filters from noise floor estimates

h(t) = Pfilter{hP(t)} + Qfilter{hQ(t)}h(t) = Pfilter{hP(t)} + Qfilter{hQ(t)}

PP

QQ

PQ

Combining hP(t) and hQ(t) – results

h [1/sqrt(Hz)]

Get the best of hP and hQ plus a little extra!

Get the best of hP and hQ plus a little extra!

Optical Spring

Radiation pressure causes a tuning-dependant force onto cavity mirrors in detuned cavities.

From LIGO-P030052-00-R

The dashed curve shows the radiation pressure on the cavity mirror as a function of the detuning from resonance. The solid curve shows the derivative of the optical force, i.e. the optical spring constant. Positive displacement corresponds to increasing the cavity length. The circulating power on resonance is 60 W.The cavity finesse was 380. The oscillator had a mass of approximately 1.2 g, a measured resonance frequency of 303 Hz, a Q of order 3000 (limited by gas damping), and an inferred mechanical spring constant of 2800 Nm−1.

longershorter

Optical spring in GEO600 for different intra-cavity powers

… Leistungen

Optical spring for GEO600 for different MSR positions

… MSR Positionen

P=10kW

Summary

• tuneable response• mode healing• GW info in both quadratures• more complex SB throughput / noise TFs• optical Spring needs to be taken into account• detailed numeric simulations + understanding required for advanced detectors

?Questions?

GEO600 layout (S4 values)

1.5 kW

T=900ppm

Mode healing or SB enhancement?

• Check which part of the intra cavity power enhancement comes from the control sidebands becoming resonant inside the SR cavity

• We get an enhancement of intracav power between PR and SR350Hz of 3.4/2.4 a.u.

• Sideband power inside SR? Behind MSR say 50mW -> 2W in front, so the resonant sidebands do not contribute to the power enhancement

Theoretical Noise budget

102

103

10-24

10-23

10-22

10-21

10-20

10-19

10-18

10-17

Freq. [Hz]

ASD

[h/ H

z]GEO600 Theoretical Noise Budget

SeismicSuspension TNSubstrate TNCoating TNThermorefractiveShot 350HzTotalShot Sept 1 h(t) Sept 1