Transcript Developments towards an Interferometric Readout System for

Mirrors for Advanced Interferometer –
substrate and coating requirements
S.Rowan
ESF workshop
Perugia
20-23rd September 2005
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
Timescales
VIRGO/GEO/LIGO all plan ‘Advanced’ upgrades:
 VIRGO (Benoit, yesterday)
 2008/9
 2011
VIRGO +
(?) 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)
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
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
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
Substrates – fused silica
 Semi-empirical model developed by Penn et al (Phys Rev Lett,
Submitted) arXive:gr-qc/0507097
 V
  f ,   bulk  surface  thermoelastic
 S
Mechanical loss in
fused silica
 C2  f 
C3
1
V 
 C1    C4
S
 C1, C2, C3, C4 are constants fitted to existing loss
measurements, and dependent of the exact grade of silica used
Substrates – fused silica
 V
  f ,   bulk  surface  thermoelastic
 S
1
 C2  f 
C3
V 
 C1    C4
S
 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
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 don’t 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
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??)
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
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
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
Mechanical loss of multi-layer SiO2/Ta2O5 coatings
with varying proportions of SiO2 and Ta2O5
5.0E-04
4.5E-04
4.0E-04
3.5E-04
Loss
3.0E-04
2.5E-04
2.0E-04
1.5E-04
lambda/4 silica, lambda/4 tantala
lambda/8 tantala, 3lambda/8 silica
3lambda/8 tantala, lambda/8 silica
1.0E-04
5.0E-05
0.0E+00
0
10
20
30
40
50
Frequency [kHz]
60
70
80
Silica and tantala mechanical loss results
5.E-04
y = 1.78E-09x + 3.17E-04
5.E-04
Assume for each material:
residual = 0 + ff
4.E-04
4.E-04
Loss
3.E-04
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.E+00
0
10
20
For tantala:
For silica:
30
40
50
Frequency [kHz]
60
70
80
residual = (3.2 ± 0.1) x 10-4 + f(1.8 ± 0.4) x 10-9
residual = (1.2± 0.2) x 10-4 + f(1.3 ± 0.5) x 10-9
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
Doping of Ta2O5 with TiO2
•
•
•
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
Loss Angle of SiO2 /TiO2 doped Ta2O5 at 100 Hz
-4
3
x 10
Small Coater
Large Coater
2.5
Loss Angle
•
2
1.5
0
10
20
30
40
Relative Concentration
50
60
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
3
x 10
Small Coater
Large Coater
2.5
Loss Angle
•
2
1.5
0
10
20
30
40
Relative Concentration
50
60
Importance of material properties
 NB to get previous loss results needed to know the Young’s 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 Young’s modulus
to be found
 However it has proved difficult to extract precise properties of the
individual materials from measurements of multi-layers
Material properties – next steps
 Studies of some single layers of materials would be very
valuable
 Study loss, Young’s 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
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?
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
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??
Challenges for future detectors
 Future detectors may require higher levels of laser power


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
 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???
Mechanical dissipation - silicon
 Silicon
 Both thermoelastic and intrinsic thermal noise
may be reduced by cooling:

1.E-18
Dis pla ce me nt (mHz
-0.5
)
Te mpe rature (K)
0
50
100
150
200
250
300
1.E-19

1.E-20
1.E-21
1.E-22
Intrins ic the rma l
nois e
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
The rmoe la s tic nois e
1.E-23
Calculated intrinsic thermal and thermoelastic
noise @ 10 Hz in a single silicon test mass, sensed
with a laser beam of radius ~ 6 cm

Silicon may allow
significant thermal noise
improvements at low
temperatures but
material properties need
further study
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).
Mechanical dissipation from coatings
Coating
 Potential sources of loss :
y
x
z
 Dissipation intrinsic to the coating materials
(defects, vacancies etc?)
af
as
 Thermoelastic damping (see Fejer et al, Phys Rev D,
Braginsky,PLA) resulting from the different thermal
and elastic properties of the coating and
substrate
l 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
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
3rd generation detectors - a problem of size
 Test masses of >50 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??,
Cradle ?
Segmented design?
Pic. from D. Coyne
Silicon ingot in growth furnace
Bonded interfaces
Separate mass
segments
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