Report from UWA

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Report from UWA
UWA Optics Team Ju Li, Chunnong Zhao, Sascha
Schediwy, Pablo Barriga, Slavek Gras, Jerome
Degallaix, Yan Zewu, Fan Yaohui, S Sunil,
Jean-Charles Dumas, David Blair

• The Big Lift
• First full scale isolator is lifted out of
  assembly tent and successfully installed in ITM
  tank 16 Aug 2006

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Light Scattering in LIGO Fused Silica Sample

• As arranged in last telecon Helena sent a sample
  of fused silica for UWA to measure light
  scattering in the bulk.
• We have measured one plane using our ASM
  machine.(Automatic scattering measuring
  machinetomography using single beams.)
• Result very high scatteringsee next
  slide74ppm/cm

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Parametric Instability Studies at UWA

• Quantifying PI taking into account as many
  uncertainties as possible.
• Suppressing PI using ring damping
• Suppressing PI using active feedback
• Experimental studies at Gingin

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Three new results on Parametric Instability

1. Tilts and Spot Size Both tilt fluctuations and
   spot size variations can tune PI. Tilt induced
   degeneracy breaking between HG and LG modes can
   introduce further unstable modes.
2. Ring damper suppression of PI. Preliminary
   results for AdvLIGO with sapphire test masses
   (easiest case, fewer modes) show stable operating
   windows with 5 increased thermal noise, 2.3
   degraded ns-ns inspiral performance.
3. Enhanced optical spring effects allow short
   tranquiliser cavities for parametric instability
   control to use lower power and lower finesse.

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Higher Order Modes
Pablo Barriga
Perfectly aligned cylindrically symmetric cavity
with finite size mirrors it can only sustain LG
modes. HG modes can be expressed as linear
combinations of LG modes, but the frequencies are
degenerate no contribution to PI Tilt
fluctuations break the degeneracy additional
modes potentially contribute to PI. We are
quantifying this effect.
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Spot Size Tuning
Pablo Barriga
Thermal lens induced changes in spot size will
change the mode gap and the resonant conditions
for Parametric Instabilities.(Edge effects) A
change of 1 in the spot/mirror ratio produces
typically 30Hz change of frequency. Sufficient
to cause significant parametric gain modulation..
Advanced LIGO
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Frequency Gap Tilt Effects
The frequency change will depend on the
orientation of the mode profile in relation to
the tilt. For PI only the frequency gap between
TEM00 and HOM is significant. Therefore the
frequency gap between fundamental and higher
order modes will also depend on this relative
orientation.
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Frequency Shift for LG Modes of Order 4
Transverse mode frequency variation with mirror
tilt. Frequency difference between fundamental
mode and modes of order 4.
HG40
LG20
LG12
HG31
HG22
LG04
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Conclusion Tilts and Spot Size

• Effects are small and are unimportant assuming
  spot size is well controlled and tilts are within
  AdvLIGO specs10nrad
• However spot size is important in obtaining
  accurate estimates of PI

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Ring Damper

• We have done the ring damper modelling for
  sapphire because it involves fewer modes and we
  can get results in less time.
• Fused silica now underway.
• Choose to use a standard optical coating for the
  ring damper as it is a well understood vacuum
  compatible material.

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Ring damper model

• various positions,
• different dimensions
• thickness
• width
• different properties
• -loss angle (thermal noise)

-tantala/silica layers -loss freq. dependent
Test mass radius r 0.157m Thickness
d 0.13m
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• PI and Ring Damper calculation proceedure
• High resolution FEM model (half-plane symmetry
  model) with coatings calc freq and mode shape of
  all acoustic modes to 160kHz (coating has small
  effect on mode shape, frequency and mode
  effective mass. (some non-symmetric modes
  omitted)
• Low resolution FEM(full cylinder model with
  inertia relief) to calculate thermal noise as
  seen by Gaussian beam due to substrate and
  coatings losses. (confirmed no resolution induced
  errors)
• Use Matlab code eigenvalue method to calculate
  diffraction losses for different ROC
  configurations.
• Calculate optical mode shapes analytically
  corresponding to infinite mirror approximation
• Run Matlab Parametric Instability Code using
  optical modes to 9th order, to calculate overlap
  parameter as function of radius of curvature.
• Create histogram of number of unstable modes,
  R-values etc.
• Repeat for different ring dampers. Each ring
  damper is designed so that it increases the
  brownian thermal noise by a given percentage at
  100Hz referred to the Brownian thermal noise for
  ROC of 2076km beam spot radius 6cm.
• Use bench code to estimate binary inspiral range.
  (Switch off residual gas and suspension TN and
  gravity gradient noise). Photothermal noise and
  quantum noise terms at default values in Bench
  Code.

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Power recycling mirror /coating
Comparison of R gain for different RoC, with PRM
(red) and without PRM (blue)
Effect of the coating losses on parametric gain
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Parametric Instability control

• Ring damper 3cm x 20microns, 5 x 10-3 loss angle
  sufficient to create stable windows (worst case
  model, no suppression of instability by arm
  imbalance).
• Brownian thermal noise degraded 5, inspiral
  range degraded 1.5..see next slide

stable windows
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ALS, DTN5 About 30 0f phase space is
instability free
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NS/NS binary inspiral range vs. ETM RoC (ITM
fixed)
AdvLIGO
Spot sizes
6.5,6.3 6.37,6.27 6.24,6.18 6.12,6.09
6.01,6.01 5.90,5.93
It seems that the NS/NS range degradation due to
the ring damper remains constant for different
RoC_ETM.
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ALS with a ring vs AdvLIGO _at_ 2076 m
ALS
AdvLIGO
212.03Mpc (with strip)
178.17 Mpc (no strip)
We are surprised that sapphire with ring damper
is better than the Adv LIGO number we get from
default settings of bench. It would be very
useful have independent verification of these
results!
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Enhanced Optical SpringParametric Instability
Sascha Schediwy - schediwy_at_physics.uwa.edu.au
Frad
Stable Unstable
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Enhanced Optical SpringParametric Instability
Sascha Schediwy - schediwy_at_physics.uwa.edu.au

• Background information for next slide
• --Optical spring Q change arrises due to the
  phase lag introduced by a non-zero
  cavity storage time.
• Next slide shows Q-1 vs. cavity offset for 10cm
  Nb cavity.
• Cavity is locked with PDH feedback servo.
• Servo uses analogue f-1 filter with corner
  frequency fc.
• Servo electronics introduces phase lag.
• Optical spring instability or damping enhanced
  20 times.

Closed Loop TFs
Niobium resonator fmech 750Hz
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Enhanced Optical SpringParametric Instability
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Enhanced Optical SpringParametric Instability
Sascha Schediwy - schediwy_at_physics.uwa.edu.au

• Formulae to calculate the change in the quality
  factor Qnew for the enhanced optical spring from
  kopt and the servo introduced phase lag ?

where
and
with
and

• Above spring constant formulae derived from 1st
  principles and also from

Braginsky, V.B. and S.P. Vyatchanin,. Physical
Letters A, 2002. 293 p. 228-234. Sheard, S.
Gray, M. Mow-Lowry, C. and McClelland, D.
Physical Review A, 2004 69 051801(R).
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Conclusion

1. Short Optical Cavity Tranquiliser looks much more
   practical as strong optical damping can be
   achieved for relatively low finesse.
2. Need to confirm noise performance
3. Ring damper looks very promising but difficult to
   completely elliminate instability without serious
   noise price.
4. Thermal tuning spot size effects not a problem
5. Need to repeat analysis for fused silica.

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AL - IFO modeled
IFO Sapphire ETM/ITM r 0.16m
d 0.13m Spot size w
6.0cm arm length L4km
diffraction loss (per bounce _at_
2076m) 3.81ppm Unstable
modes (Rgt1) 380
Ring damper model (all TMs) Strip width 3
cm Strip thickness 20µm Loss angle chosen in
such way that the ifo Brownain thermal noise _at_
2076 would increase by 5. F 3.5909e-3
Brownian Noise at 2076 m (FEM result )
ITM ETM Coating
2.7242e-21 4.1503e-21 Substrate 1.4448e-21
1.4448e-21 Ring 1.2063e-21
1.2063e-21 Total 7.5304e-21
Coating info ITM T5 -gt 8 layers of Ta2O5 8
layers SiO2 ETM T5ppm -gt 18 layers of Ta2O5 18
layers SiO2
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Notes

• -100 , 90 and 75 corresponds to the coating
  TN. I assumed that future coating may be better
  than the present ones by these factors used in
  the Bench code.
• If Total Brownian TN _at_ 2076 m is equal 7.5304e-21
  (without a strip) than by adding a strip with
  loss 3.5909e-3 it causes 5 Brownian thermal
  noise degradation .
• If Coating TN was reduced to 90 for both ITM and
  ETM mirrors than the strip with the same loss
  value as above would cause DT14.5. If
  coating TN was 75 smaller than DT increases to
  32.3.
• According to NS/NS results (slide 3), inspiral
  range does not follow this pattern. This range
  would be only decrease by 1.7 for all cases
  (0,10,25). Im not sure why it is so maybe
  its due to the high thermoelastic noise? As seen
  in the slide 5 this noise is much higher than the
  strip one. If this is true, than silica test
  mass with much lower thermoelastic noise would
  be more susceptible to the strip noise (I think)
• Slide 4 shows only NS/NS range for not reduced
  coating TN (100).