Formulation of Instrument Noise Analysis Techniques and Their Use in the Commissioning of the Gravit

1
Formulation of Instrument Noise Analysis
Techniques and Their Use in the Commissioning of
the Gravitational Wave Observatory, GEO 600

• Joshua Smith PhD Defense 28.07.06

2
Gravitational ( gravity) waves
Gravity Waves Disturbances in atmosphere caused
by air parcels raised by some force (eg.,
buoyancy storm updraft, mountain) and restored by
gravity.
Gravitational Waves Disturbances in space-time
that travel at speed of light. Waves created by
(likely) astrophysical sources cause relative
length changes between (free) objects on earth,
(strain hlt10-21)
3
Gravitational wave detection network
4
GEO 600 simplified optical layout
600m
Optics suspended as double or triple pendulums
5
Prototype for future detectors suspensions

• Triple pendulum suspensions
• provide f-6 filtering of seismic noise above
  resonances.
• Quasi-monolithic fused silica
• Removed interfaces where possible
• Expect very low thermal noise due to high quality
  factor materials

6
Prototype for future detectors configuration
7
Theoretical sensitivity limit
8
Theoretical limit vs. detuning
9
Sensitivity development 02-06
3000x better _at_ 500Hz in 4 years
10
Gravitational-wave observatory

• Current data-taking run (S5)
• Full time operation since May 1st
• High(est) instrumental duty-cycle
• Peak RMS strain sensitivity hRMS 2.510-22

June 96 duty cycle
11
Laser-IFO network sensitivity
(as of May 1) Significant
improvement expected
12
Technical noise in GEO600
Q Why is the instrument not at its theoretical
sensitivity limit? A Technical noise coupling to
the detector output causes a larger apparent
strain signal.

• Myriad of noise sources and paths for them to
  couple
• Hundreds of analog and digital control loops
• External noises eg wind, humans

A method to systematically measure the influence
of these noise sources on the detector output is
needed!!
13
Detector output I
transfer function
detector output
noise source
detector output
14
Detector output II
transfer function
detector output
noise source
detector output
15
Detector output III
transfer function
detector output
noise source
detector output
16
Noise channels
noise channel
transfer function
detector output
noise source
detector output
17
Noise projections
noise channel
transfer function
detector output
noise source
detector output
noise projections
noise budget
18
Simplified procedure I
Causality key

• Notes
• Causality key
• Simultaneous DFTs allow comparison
• Other variants exist, eg open-loop which requires
  CLTF

19
Simplified procedure II
Causality key
Normal operation
20
Simplified procedure III
Causality key
Normal operation
21
Limitations loop gain
Single-path loop
Simulated results
Workarounds exist eg., for high loop gain
22
Noise projection alignment FB
? feedback
noise projection
transfer function
23
Noise projection laser frequency noise
? Transfer function calculation would require
very sophisticated model
Noise projections well-suited for very
complicated systems
24
Noise removal, eg., frequency noise
Most important role of NPs in commissioning
Identify limiting noise sources team removes
them
25
Noise budget for S5
26
Automation
Goal Noise projections (full budget) at any time.

• Transfer functions
• Measure occasionally and
• If dynamic, must be tracked
• 2. Fourier transforms of Nj and H
• Signals recorded continuously anyway

27
Calibration lines
28
Tracking transfer functions

• At calibration line, projection and H should have
  same amplitude, phase.
• Ratio and difference indicate TF accuracy.
• For 10 day test, about 10 10o

29
Vetoes of false gravitational waves

• Vetoes crucial for burst search
• Noise channels contain almost no GW info
• Coincident (within windows) events are not GWs !

Example injected burst-like waveform
Further research and application continues within
AEI Hannover
30
Noise subtraction alignment FB
Error at a given freq. vs. filter accuracy
H
H
Results of a time-domain subtraction
31
Summary

• The sensitivity of GEO 600 and other GW detectors
  is partly limited by technical noise.
• The Noise Projections technique systematically
  determines level of contribution of noise sources
  to detector output.
• Can be used for many other instruments!
• Application to GEO 600 has been very important
  for commissioning.
• Extensions such as automation, vetoes and noise
  subtraction have been tested, and will continue
  to develop.

32
The End
33
Limitations II
Single-path loop
Split-path loop
Workarounds exist eg., for high loop gain
34
Noise projection (de)modulation phase
Works well for very complicated systems
35
H vs signal recycling FB