1. Measuring Gravitational Waves with GEO600
Martin Hewitson, AEI Hannover for GEO600

2. GEO600 optical layout MC2 MPR MC1 MCn MFn MSR BS MCe MFe
DPG, March 2005

3. Overview GEO 1 GEO h(t) v(t) [V] Noise e.g., seismic, laser calibrate
h(t) + noise
DPG, March 2005

4. + Inside the GEO box Optical cavity filter P(t) [V] Q(t) [V] h(t)
h(t) detected
filter
P(t) [V]
Q(t) [V]
h(t)
Optical
cavity +
v(t) [V]
Seismic
noise
Keep detector at its operating point (dark fringe)
DPG, March 2005

5. Calibration of GEO600 Time-domain calibration method
Optimisation routine is used to quasi-continuously estimate parameters of optical response
Update rate is 1Hz
DPG, March 2005

6. Calibration procedure
Michelson servo model
Correct for MI loop and Optical transfer function(s) x =
Optical transfer functions
DPG, March 2005
Calibration functions

7. On-line optical TF measurements
P and Q
optical
actuator
DPG, March 2005

8. Calibration overview
calibration
DPG, March 2005

9. Combining hP(t) and hQ(t) - I
Idea is that
So we can form
Do this in the time-domain to fit in with the current calibration method
hP(t) = h(t) + nP(t)
hQ(t) = h(t) + nQ(t)
hopt(t) = a(t,f).hP(t)+b(t,f).hQ(t)
DPG, March 2005

10. Combining hP(t) and hQ(t) – II
Form optimal frequency dependent weighting of hP(t) and hQ(t) by considering variance of noise components
Convert to time-domain filters
DPG, March 2005

11. Combining hP(t) and hQ(t) – III
Simplification: consider only magnitude
Create filters from noise floor estimates
sQQ
sPP
sPQ
h(t) = Pfilter{hP(t)} + Qfilter{hQ(t)}
DPG, March 2005

12. Combining hP(t) and hQ(t) – results
Get the best of hP and hQ plus a little extra!
DPG, March 2005

13. Summary and future work
Online time-domain calibration of GEO600 output(s) to strain sensitivity
Both output demodulated output quadratures are calibrated and combined to give (almost) optimal h(t)
Extend this to allow for time variation of combining filters and to include non-linear phase correlations in the output quadratures
DPG, March 2005