1. Commissioning progress Stefan Hild Ilias WG1 meeting, Sep 2005
Title
GEO 600
Commissioning progress
Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut)
Stefan Hild Ilias WG1 meeting, Sep 2005

2. GEO 600 layout
0.09
1.5-2 kW

3. Tuning signal recycling to 300 Hz
Better knowledge of IFO parameters:
more accurate prediction
from simulations
downtuning to 200 Hz
realized
Fixed instability of MI auto-alignment by setting up a new telescope for DWS.
We will stay at 330 Hz !

4. GEO 600 design sensitivity

5. Calibration and demod phase @ 300 Hz
Using signal recycling
GW signal is split in
P and Q quadrature
P-response independant of demod phase
Q-response only for larger than 20 to 30 deg demod phase compatible with current calibration
(zero + complex pole)
Decided to use 40 deg for time being.
Drawback: ‚P‘ used for MI servo is not really only ‚P‘

6. High RF on HPD / Q-compensation
Q-comp OFF
Q-comp ON
Observations:
Found huge RF level after MI diode (1 Volt)
Large DC level in Q after mixing.
Q-compensation:
Injected RF directly into resonance circuit of the HPD to make Q DC level zero.
Improved sensitivity above 700 Hz

7. RF setup OLD setup NEW setup Old setup:
Dominated by phase noise in the LO path (phaseshifter and splitter(0/90°)
Due to this noise from Q was mixed into P quadrature.
That is why Q-compensation worked.
OLD setup
New setup:
Using a less noisy splitter(0/90°)
Using a cable as phaseshifter
NEW setup

8. Shot noise
Green line is the shot noise level calculated from the DC current of the photodiode.
We are within a factor sqrt(2) shot noise limited above 1kHz !

9. Proposed by Rana and others
Future RF setup
Proposed by Rana and others
GOALS:
Using only high quality hardware
Guaranteeing high RF levels in all components

10. Noise projections 21st of july

11. Noise in the SR-long loop
PROBLEM
Noise in the detection band is entirely limited by front-end-noise (shot noise from the detector).
Limits sensitivity from 100 to 500 Hz
Large locking range
OLG of 10^6 at 0.1 Hz
No feedback noise above100 Hz
Requirements
Servo has a locking mode and a running mode
Lowering UGF to get less gain in detection band
Using lowpass filter with very steep roll off above UGF (implemeted using a dSPACE system)
Solutions

12. Analog SR electronics Red = acq amp, green = acq pha
Dark blue = run amp, brown = run phase
Light blue = run2 amp, pink = run2 pha

13. Signal Recycling digital
Hz = -147 deg

14. SR feedback: analog vs. digital

15. SR loop filter future design
Four complex integrators:
2.25 (2x),1.3, 0.56 Hz
Filter not used at the moment. Can be implemented when
more gain is needed around pendulum resonances

16. Sensitivity improvement July to August
High frequency improvements: reduction of RF phase noise
Low frequency improvements: reduction of Signal recycling feedback noise

17. PR-bench Historically grown layout of PR-bench not optimal !
beam clipping
too many transmissive optical components
too many polarizing components
acoustic coupling
Goal:
Shorten and simplify the HPD path

18. Sensitivity after PR bench work
Removed resonance structures between 100 and 200 Hz.

19. Noise projections 21st of july

20. Michelson length control
< 0.1Hz
main problem before coincidence run was range of ES drive
however this was solved

21. Michelson length control
< 0.1Hz
< 10 Hz
Reaction Pendulum:
3 coil-magnet actuators at intermediate mass, range ~ 100µm
main problem before coincidence run was range of ES drive
however this was solved

22. Michelson length control
< 0.1Hz
< 10 Hz
Reaction Pendulum:
3 coil-magnet actuators at intermediate mass, range ~ 100µm
Electrostatic actuation on test mass bias 630V, range 0-900V= 3.5µm
> 10 Hz
main problem before coincidence run was range of ES drive
however this was solved

23. Using ESD as actuator
Force is proportional to square of applied voltage.
high voltages are needed
for bipolar acting a bias voltage needs to be applied
Noise is introduced:
loop electronics
sqrt circuits
intrinsic HV amplifier noise

24. Sqrt circuits in MI loop
ESD: F  U^2
Sqrt circuits are necessary to give full linear force range for acquisition.
Drawback: sqrt circuits are noisy 1µV/sqrt(Hz)
ESD)

25. Sqrt circuits in MI loop
ESD: F  U^2
Sqrt circuits are necessary to give full linear force range for acquisition.
Drawback: sqrt circuits are noisy 1µV/sqrt(Hz)
ESD)
Bypassing sqrt circuits after lock is acquired.

26. Noise in MI loop HVA noise = 100nV/sqrt(Hz) (=10µV/sqrt(Hz) @ ESD)
HV-amplifier noise can be reduced by decreasing bias voltage or active noise suppression.
Suppressing noise introduced by loop electronics needs whitening

27. MI loop whitening / dewhitening
Whitening right after mixer:
zero 3.5 Hz
pole 35 Hz
Dewhitening for both split passes
Passive dewhit-ening done in HV path (0-1kV)
dewhiten
dewhiten
dewhiten
Whiten

28. Sensitivity after fixing noise from MI loop

29. Michelson servo design

30. MI loop gain problem Strange observations:
We are not able to increase the low frequency loop gain, even though it should work from loop model (oscillation around 8 Hz)
OLG measurements show that there is nearly no gain around 8 to 10 Hz.
We needed to turn down the MI crossover gain continiously for the last few months
Findings:
Influence of MI AA gain on maximum cross over frequency (AA-tilt ,  cross over possible.
 phase margin of crossover, can  crossover frequency, but adding integrator still causes 8Hz oscillation
 IM gain ( crossover frequency),  less noise in servo 4 to 8 Hz.
 IM gain, still low gain around 8 to 10 Hz (low gain can‘t be caused by IM)

31. Injecting LF noise in MI loop
Injected noise from 5 to 11 Hz
Between 6 and 7 Hz the noise is not suppressed !

32. Measurement of open loop gain (PRMI)
Resonance structure
is clearly visible
Very low gain
around 9 Hz
Design gain:
7 @ 15 Hz
20 @ 10 Hz
80 @ 6 Hz

33. Mi servo design detail

34. TF: Alignment to longitudinal
Tilt couples a factor of 10 stronger than 10 Hz.
(IM-FF tilt2long)
Why are the two rot-TF so different?

35. How to go on with the MI loop gain problem
Measure MI loop gain for various conditions to decouple crossover and AA-tilt.
Implementing digital AA control to be more flexible (nearly done).
Measure tilt2long for single suspensions, with the goal of setting up a FF system.
Investigate possible advantages of using the ESDs for angular alignment.
....

36. Segmented ESD for alignment

37. Sensitivity improvement of GEO

38. Latest noise projections

39. Current sensitivity vs. Design sensitivity

40. Discussion Michelson loop gain problem:
What experience exists in VIRGO with angular to longitudinal and longitudinal to angular couplings ?
Is somewhere gain lost ?
...
Digital filtering:
What kind of filters are used within VIRGO for steep roll off?

41. E n d