Mirrors for Advanced Interferometer substrate and coating requirements

1
Mirrors for Advanced Interferometer substrate
and coating requirements

• S.Rowan
• ESF workshop
• Perugia
• 20-23rd September 2005

2
Reminder of motivation

• Analyse the recent developments in technologies
  foreseen for Advanced detectors to explore the
  path needed for a European 3rd generation
  gravitational wave detector

• Consider here technology status of some aspects
  of the detector mirrors and coatings
• Thermal noise from mirrors and coatings forms an
  important limit to design sensitivities at most
  sensitive point in mid-frequency band

Coated fused silica mirror 18cm diameter
3
Timescales

• VIRGO/GEO/LIGO all plan Advanced upgrades
• VIRGO (Benoit, yesterday)
• 2008/9 VIRGO
• 2011 (?) Advanced VIRGO
• LIGO
• 2008/9 (?) staged improvements
• 2010-13 Advanced LIGO
• GEO
• 2008 ? GEO-HF staged improvements
• 3rd European detector (20??)
• Common theme for Advanced detectors is higher
  laser power (Benno) and new mirrors
• What is the status of technologies related to
  low-thermal-noise mirrors? (Gregg will talk re
  thermal loading effects)

4
Current mirrors

• All detectors currently use fused silica
  substrates with coatings formed from SiO2/Ta2O5
• Optics in the detectors were installed several
  years ago

• Design curves for GEO, LIGO, VIRGO which we use
  were based on best models for thermal noise at
  that time
• The same optics are still installed but our
  models for the thermal noise have changed a lot

LIGO fused silica mirror (10kg) in suspension
cradle
5
Three significant changes

• Levin for mirrors with inhomogeneous loss we
  should not simply add incoherently the noise
  from the thermally excited modes of a mirror
  loss from a volume close to the laser beam
  dominates
• Penn et al loss in silica may be modelled as sum
  of surface, thermoelastic, and frequency
  dependent bulk losses the latter improving
  towards low frequency
• Levin, (Nakagawa, Crooks, Harry et al)
• Dissipation from dielectric mirror coatings is
  at a significant level

6
Substrates - Fused silica

• Two big vendors used Corning (LIGO) ,
• Heraeus (LIGO, VIRGO, GEO)
• Each vendor makes a number of different optical
  grades
• Empirical measurements suggest
• Heraeus fused silica has lower mechanical loss
  than Corning
• The various Heraeus Suprasil grades have
  different loss from one another

7
Substrates fused silica

• Semi-empirical model developed by Penn et al
  (Phys Rev Lett, Submitted) arXivegr-qc/0507097

Mechanical loss in fused silica

• C1, C2, C3, C4 are constants fitted to existing
  loss measurements, and dependent of the exact
  grade of silica used

8
Substrates fused silica

• Penn et al point out
• The internal friction of very pure fused
  silica is associated with strained Si-O-Si bonds,
  where the energy of the bond has minima at two
  different bond angles, forming an asymmetric
  double-well potential. Redistribution of the bond
  angles in response to an applied strain leads to
  mechanical dissipation
• Empirically we deduce that the manufacturing and
  processing of the different grades of silica is
  affecting the distribution of bond angles

9
Bulk loss

• Empirically it seems that Suprasil 311, 312 are
  the grades of silica with the lowest loss (SV not
  as low ??)
• Good! -We tend to choose these for our optical
  needs
• However we dont yet understand in detail what
  processing (annealing/cooling/ temps/rates
  geometry etc) optimises the mechanical loss (eg
  why is Corning silica not as good as Heraeus..?)
• (Penn et al actively researching this area)
• Understanding this would perhaps allow us to
  lower loss even further

10
Surface loss

• Empirically, measurements are consistent with the
  existence of a surface loss limit
• Annealing samples allows them to approach this,
  but dissipation then reaches a lower limit
• The source(s) of dissipation for this surface
  layer are not unambiguously determined
  (microcracks, polishing damage what about flame
  annealled samples??)

11
Substrates fused silica

• Status of current models and experiments suggest
  substrate thermal noise could be 10 times lower
  (or more?) than old design sensitivities - good
  news!!
• Maybe we can lower it even further however.
• Coatings now are a dominant source of thermal
  noise

12
Consider an Advanced LIGO-Like design
Penn et al

• Coating thermal noise is expected to be the
  dominant noise source at mid frequencies for
  advanced interferometer designs

13
Coating studies

• Thermal noise from the dielectric mirror coatings
  applied to test masses is -essentially
  acceptable- for Adv. LIGO, (Adv. VIRGO ?)
• However, reduction in coating noise translates
  directly to interferometer sensitivity
• Unacceptable for any future detectors beyond Adv.
  LIGO
• Studies carried out with coatings from number of
  vendors
• (MLD, Waveprecision, REO, LMA Lyon)
• to study the mechanical dissipation of
  ion-beam-sputtered dielectric
• coatings via loss measurements
• Focussed initially on SiO2/Ta2O5 coatings

14
Mechanical loss of multi-layer SiO2/Ta2O5
coatings with varying proportions of SiO2 and
Ta2O5
15
Silica and tantala mechanical loss results
5.E-04
y 1.78E-09x 3.17E-04
5.E-04
Assume for each material fresidual f0 fff
4.E-04
4.E-04
3.E-04
Loss
3.E-04
2.E-04
Silica residual loss
2.E-04
Tantala residual loss
y 1.32E-09x 1.16E-04
1.E-04
5.E-05
0.E00
0
10
20
30
40
50
60
70
80
Frequency kHz
For tantala fresidual (3.2 0.1) x
10-4 f(1.8 0.4) x 10-9
For silica fresidual (1.2 0.2) x
10-4 f(1.3 0.5) x 10-9
16
Status

• Measured losses are dominated by intrinsic loss
  of the materials involved
• Ta2O5 is mechanically lossier than SiO2
• Studies carried out of loss of Ta2O5 doped with
  TiO2 - suggestion by LMA

17
Doping of Ta2O5 with TiO2
Loss Angle of SiO2 /TiO2 doped Ta2O5 at 100 Hz

• Clear improvement with addition of titania
• Appears no strong correlation with amount of TiO2
• However exact concentrations of TiO2 not known
• Results from Ian MacLaren in Glasgow now
  available

-4
x 10
3
Small Coater
Large Coater
2.5
Loss Angle
2
1.5
0
10
20
30
40
50
60
Relative Concentration
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Doping of Ta2O5 with TiO2

• Mechanism by which TiO2 reduces dissipation not
  yet known
• (Helping prevent movement of oxygen
  vacancies..??)
• Recent measurements by Black et al (Caltech)
  confirm reduction in thermal noise from doped
  coatings

Loss Angle of SiO2 /TiO2 doped Ta2O5 at 100 Hz
-4
x 10
3
Small Coater
Large Coater
2.5
Loss Angle
2
1.5
0
10
20
30
40
50
60
Relative Concentration
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Importance of material properties

• NB to get previous loss results needed to know
  the Youngs modulus of the individual coating
  materials
• Previous results use best estimates of
  properties ( these are typically not well known
  for ion-beam-sputtered coatings)
• I. Wygant et al (Stanford) measured the acoustic
  impedance of witness multi-layer samples using an
  ultrasonic reflection technique
• If coating density is known then this allows
  Youngs modulus to be found
• However it has proved difficult to extract
  precise properties of the individual materials
  from measurements of multi-layers

20
Material properties next steps

• Studies of some single layers of materials would
  be very valuable
• Study loss, Youngs modulus and density (may have
  to study as a function of thickness)
• These would then help inform our analysis of
  multi-layer coatings
• Necessary both to quantify our loss measurements
  and thermal noise calculations

21
Other approaches

• Pinto et al studying algorithms to vary
  thickness and periodicity of coating layers
• Optimise for desired reflectivity whilst
  minimising amount of Ta2O5 present
• Use flat-topped laser beams to more efficiently
  average coating and substrate thermal noise?

22
Conclusions

• 2nd generation of detectors will use fused silica
  optics
• Coatings will be the limiting source of thermal
  noise in these advanced detector test masses
• To go to 3rd generation detectors we need better
  coatings or maybe to cool??
• Results from Yamamoto et al suggest coating loss
  angle does not decrease significantly with
  lowering T but still gain in reducing thermal
  noise

23
Where does this leave us for 3rd generation
detectors?

• Limited by coating thermal noise/optical noise
• Possibly considering cooling to reduce the
  coating noise
• Thermal noise is not the only issue for substrate
  and coating developments
• Other substrate and coating issues
• Thermal loading effects can be significant see
  talk by Gregg
• The low thermal conductivity of silica may prove
  to make it unattractive for higher power
  operation
• Necessitate switch to sapphire/silicon some other
  material??

24
Challenges for future detectors

• Future detectors may require higher levels of
  laser power
• In addition, further reductions in test mass and
  suspension thermal noise are required
• Possible materials meeting these requirements are
  sapphire or silicon are there others???

• Mirror substrates must sustain high thermal loads
  and maintain optical figure
• Deformation of mirror surface is proportional to
  a/kth Winkler et al., 1991.
• a substrate expansion coefficient
• kth substrate thermal conductivity
• Would like a substrate material for which a/kth
  is minimised

25
Mechanical dissipation - silicon

• Silicon
• Both thermoelastic and intrinsic thermal noise
  may be reduced by cooling

• Thermoelastic noise is proportional to a and
  should vanish at T 120 K and 18 K where a tends
  to zero
• Intrinsic thermal noise exhibits two peaks at
  similar temperatures
• Silicon may allow significant thermal noise
  improvements at low temperatures but material
  properties need further study

Calculated intrinsic thermal and thermoelastic
noise _at_ 10 Hz in a single silicon test mass,
sensed with a laser beam of radius 6 cm
26
Mechanical dissipation - sapphire

• Sapphire
• studied in the US as part of Ad LIGO substrate
  downselect
• studied by colleagues in Japan for LCGT
• Likely to have levels of intrinsic and
  thermoelastic dissipation similar to silicon
  (slightly lower) but without the nulls in
  expansion coefficient
• Could be interesting, particularly at higher
  frequencies

Sapphire piece used in spot polishing
compensation demonstration 25cm diameter sample
(photo courtesy Goodrich).
27
Mechanical dissipation from coatings

• Potential sources of loss
• Dissipation intrinsic to the coating materials
  (defects, vacancies etc?)
• Thermoelastic damping (see Fejer et al, Phys Rev
  D, Braginsky,PLA) resulting from the different
  thermal and elastic properties of the coating and
  substrate

• In both cases resulting thermal noise level
  depends on relative thermal and elastic
  properties of coating and substrate
• It follows that the optimum coating for a fused
  silica or sapphire mass may not be the ideal
  choice for a silicon mass

28
Mechanical dissipation in coatings (contd)

• Diffractive coatings
• To use silicon as a diffractive optic, either
• a diffraction grating can be etched on to the
  surface of the test mass onto which a coating is
  applied
• (Institute for Applied Optics, University of
  Jena)
• or
• the test mass can be coated, and a diffraction
  grating etched into the coating surface
• (Lawrence Livermore National Laboratories).
• The mechanical dissipation associated with such
  coatings (room and cryo) needs investigated

29
3rd generation detectors - a problem of size

• Test masses of gt50 kg are desirable
• Silicon ingots of 450kg have been manufactured,
  but aspect ratio is not optimal
• Sapphire is available up to only 40kg
• Use composite test masses??,

Silicon ingot in growth furnace
Cradle ? Segmented design?
Bonded interfaces
Pic. from D. Coyne
Separate mass segments
30
Conclusions cont

• Analyse the recent developments in technologies
  foreseen for Advanced detectors to explore the
  path needed for a European 3rd generation
  gravitational wave detector
• Status of substrate/coating technology for
  Advanced Detectors is in pretty good shape
  (silica doped coatings)
• Limited by coating thermal noise but various
  approaches discussed here may help us
• For 3rd generation detectors cooling and/or a
  change of substrate material is likely to be
  needed really need to work hard on how to beat
  coating thermal noise