1. Overview of Advanced LIGO Coating Status
Sheila Rowan
University of Glasgow
for the Core Optics Working Group
Plenary technical session
LSC meeting, Livingston, LA
17th March 2004

2. Coating Development Specifications for Test Masses
Parameter
Sapphire goal
Sapphire requirement
Fused Silica goal
Fused Silica requirement
Mechanical loss
2 x 10-5
6 x 10-5
1 x 10-5
3 x 10-5
Optical Absorption
0.5 ppm
1 ppm
0.2 ppm
Thermal expansion
5 x 10-6/K
< 2 x 10-5/K
>1 x 10-6/K
5 x 10-7/K
< 2 x 10-6/K
>1 x 10-7/K
Birefringence
1 x 10-4 rad
2 x 10-4 rad -
Scatter5
2 ppm
Thickness
uniformity
10-3 (over 21.5 cm diameter)
10-2 (over 33.0 cm diameter)
10-2 (over 30.0 cm diameter)
ITM HR transmission
5 x 10-3
±2.5 x 10-4
ETM HR transmission
5 ppm
10 ppm
Test Mass HR matching
1 x 10-2
AR reflectivity
200 ±20 ppm

3. Adv LIGO Coating Requirements
Mechanical loss
Fused silica : f < 3 x 10-5 (goal 1 x 10-5)
Sapphire: f < 6 x 10-5 ( goal 2 x 10-5 )
These numbers are guides, thermal noise will depend on other parameters also.
Source of requirements on all parameters influencing thermal noise:
Brownian thermal noise (Nakagawa/Gretarsson)
Thermoelastic noise (Braginsky/Fejer)
AdvLIGO sensitivity modeling with BENCH
Optical absorption
Fused silica: 0.5 ppm (goal 0.2 ppm)
Sapphire: 1 ppm (goal 0.5 ppm)

4. Results for Initial LIGO Silica/Tantala Coating
Silica substrates were coated with layers of alternating silica and tantala, similar to the initial LIGO coating
Q factors were measured by exciting resonances in the samples and recording the subsequent decay
Fused silica substrates of 2 different aspect ratios used:
‘Thick’ samples (3” dia. x 1” thick):
c=(2.8 ± 0.7) x 10-4
‘Thin’ samples (3” dia. x 0.100” thick):
butterfly = 2.7 x 10-4
drumhead = 3.1 x 10-4

5. Reminder of Previous Results
Measurements on several SiO2/Ta2O5 coatings with different amounts of layers (2 to 60) and with various layer thickness in different combinations (λ/4 - λ/4; λ/8 - 3λ/8; 3λ/8 - λ/8)
Concluded that:
Substrate / coating interface is not a significant source of loss.
Coating layer interfaces are not a dominant source of loss
Ta2O5 has a higher mechanical loss than SiO2
fsilica = (4±3)x f (2.7±0.9)x10-9
ftantala = (4.2±0.4)x f (4±9)x10-10

6. Status of experiments
Measured mechanical loss (and optical absorption) of a number of alternate coatings:
eg: Al203/Ta2O5 SiO2/Al203 Nb2O5/ SiO2
Mechanical loss results are broadly similar to SiO2/Ta2O5 – no significant benefits from using coatings listed above.
Most promising results to date:
Ti-Doped (3%) Ta2O5 / SiO2
Reduction in coating loss:
c= (1.8 +/- 0.1) x 10-4
c= (1.5 +/- 0.7) x 10-4

7. Program Overview Driver is development of low mechanical loss coating
Optical and thermo-mechanical properties will be studied during development, (vendors, Caltech, Stanford), but will not drive program until mechanical loss is better understood and/or a low mechanical loss coating is developed
Coating vendors selected:
LMA/Virgo in Lyon, France
CSIRO in Sydney, Australia
Next phase of coating development has begun

8. Current results and immediate coating development plans
CSIRO:
First ‘calibration’ coating studies of SiO2/Ta2O5 coating from CSIRO:
fc = (4.4 +/- 0.6) x 10-4
fbutterfly = (4.1 +/- 0.1) x 10-4
fdrumhead = (5.0 +/- 0.1) x 10-4
‘secondary ion-bombardment’ used in deposition process
(Nb: coating appeared to show variations in thickness across surface)
LMA/VIRGO:
Currently working on:
a) depositing single layers of coating materials for study of mechanical loss and thermo-mechanical properties
b) depositing doped SiO2/Ta2O5 coating on sapphire substrate

9. Coating development plans
Dopants
Continue with studies of effects of doping on mechanical loss
SiO2/Ta2O5 doped with 3% Ti showed a reduction in mechanical loss without sacrificing n, Y, or optical absorption.
Alternate high index materials
HfO2/SiO2 sample produced – mechanical loss to be measured shortly
Annealing
Systematic studies of effects of annealing on coating mechanical loss
Ion bombardment of substrate during coating
Controls density and stress of coatings – investigate effect on mechanical loss

10. R & D Milestones Start Coating Development January 2004
Substrate Material Downselect June 2004
Coating Material Downselect June 2005
LASTI’s ETM Delivered January 2006