1. LIGO Core Optics: a decade of development and experience
W. Kells
LIGO Laboratory, Caltech
With acknowledgement of entire LIGO team for its optical development
GWADW
June 1, 2006

2. LIGO “Core” Optics
6 (4 test masses; splitter; recycling mirror) large f optics which form high power cavities.
11Kgm (f 25cm, h 10cm)
Low loss, low distortion fused silica
Designed (epoch ’94-97) to achieve science requirements:
with 6Watt input
Extensive simulations
Protracted “pathfinder” fabrication test pieces
Transition from 535 to 1064nm
Valuable lessons learned from Caltech 40m
prototype interferometer
GWADW
June 1, 2006

3. Major early concerns Fabrication tolerances: match of optical modes
D ROC of mirrors arm imbalance: excessive “contrast defect” to dark port
D reflectivity, loss
Coating stability and uniformity
Thermal lensing: effect on recycling cavity “point design”
Long term contamination build up on HR surfaces
Uncertain residual Silica bulk absorption.
Static charging on suspended dielectric TMs
Inherent unstable recycling cavity design
Hypersensitivity to polish, coating, homogeneity errors
Effective loss of long cavities with figure distortions
Essential target of “FFT” studies
Coupled with coating reflectivity tolerance: rifo >/< 0 (point design recycling)
GWADW
June 1, 2006

4. Optical Loss Expectations
Goal: based on older polish/coating information
Pathfinder development & fabrication proved much better:
Micro roughness srms <0.28 nm prompt loss ~(4 p srms/l)2 <10 ppm
Super polished substrate 2 - 3x lower srms
Simulation (FFT) with Fab. Data:
Figure= modal distortion
Roughness= loss
Low absorption= cold “start up”
Witness sample reflectivity
Simulated G (at least: CR field not affected
by degenerate recycling) far exceeds goals
Consistent with Advanced ligo requirements
localized roughness
Global surface metrology
FFT mirror map
H1 ETMy polished surface PSD
GWADW
June 1, 2006

5. Scatterometer studies
Observed interferometer gains lower than Sim. predictions.
Consistent with 50-70ppm avg. additional loss per TM.
Consistent with “visibilities” (resonant reflectivity defect) of individual arms
In situ studies: Some HR surfaces 3 angles:
Angular dependence more isotropic,
“point like” than metrology prediction
In situ observed scatter ~70 ppm mirror
~same level, character for every TM
independent of history/cleaning.
Scatterometer port Sr
ITM
Main arm beam
FFT map
representation
H1 ETMy
roughness
k-1
k-2
GWADW
June 1, 2006

6. In Situ Optics Performance
~41, which is:
Consistent with measured arm visibilities
Consistent with total arm loss
dominated by prompt scatter.
Scatterometer data extrapolated
to absolute loss
Consistent with lower than anticipated
contrast defect ( and small FFT dependence on maps)
Replaced ITM
CAVITY V
TITM
TWITNESS
Scatter
2k X
.0222
.0277
.0283
0.85
2k Y
.0211
.0272
.0281 7
4k X
.0241
.0279
.0275
7.5
4k Y
.0214
.0263
.028
8.8
GWADW
June 1, 2006

7. Homogeneous roughness ?
Expect isotropic glow from “homogeneous” polish roughness
Find: “point” defect scatter dominates
Bench scans (1064nm) also show excess
Is it just dust ??
Resonant arm, Gaussian illuminated ETM
Reference calibration:
known cavity loss
GWADW
June 1, 2006

8. Analysis of the “Globular Cluster”
Cleanest point scatter image: 2k ETMy:
Grab video stills for
detailed analysis:
This point defect background
~same for all optics.
Diffuse (micro roughness) background
contributes < 1/3 of total scatter.
Other blemishes don’t dominate total (?)
Puzzle: Why these point defects
missed in Lab. QA?
Defocused
Focused
GWADW
June 1, 2006

9. Coatings sensitive to handling
For several years Hanford 2k performed poorly
X arm visibility (resonant reflectivity) poor
Ugly recycling cavity “mode” pattern
Excess dark port contrast
More dramatic: unlocked arm cavity
Found: AR coating anomaly
Hypothesis: extended harsh cleaning of
surfaces had etched coating layers.
Lesson: coating sensitivity to thickness
change (confirmed by model).
Bench scan of removed ITM
GWADW
June 1, 2006

10. Contamination & thermal lensing
~7 years of installed Core optics
No evidence of accumulating contamination (scattering or absorbing)
Routine full lock only ~5 yrs. High power only 1-2 yrs.
Some optics >6 yrs hanging have no evidence of HR absorption >1ppm (design)
Net scatter loss seems independent of TM installation epoch (though high !)
Residual absorption has been found consistent with materials/Fab. expected.
As anticipated by simulations, this level essentially only affects SB fields
Bulk silica absorption not controlled sufficiently for “point” thermal design.
“TCS” system required for compensating residual variations.
This typical experience: extrapolates well to Adv. LIGO !
Outstanding discrepancy: installed TM scatter loss far too high
Assumed either treatable “dust” issue; or adjustment of coating process
However also contamination accidents
High power operation revealed >10x residual coating absorption
Unique to pair of ITMs: no evidence in other Hanford optics. When ??
GWADW
June 1, 2006

11. Contamination in LIGO I TMs
Goal: corroborate in situ performance with bench tests
Many LIGO COC optics studied
Comparisons establish
“typical” from anomalous
Absolute calibration to various
reference mirrors.
Components of “loss test” cavity
Example: What is anomalous contaminant
on H1 ITMs?
Absorption is lumpy but not point like
Scatter also anomalous and correlates
well spatially with absorption
Easily removed by surface cleansing
Fine absorbing dust, sucked in during vent?
Normalized
Correlation = 0.5
Mean Abs.= 11.8ppm
GWADW
June 1, 2006

12. Conclusions, Direction
LIGO I optical performance meets design.
“As built” expectations far exceeded design.
Can be of significant concern for Advanced LIGO, which
has initially assumed at least duplicating “as built” performance
Design OTF tests to understand anomalous scatter:
“Frozen” in the coatings ?
Surface contamination (~ common to all installed optics !)
Also apparent cleaning streaks/defects: significant in terms of loss?
GWADW
June 1, 2006

13. Expectations vs Performance
Expectations: FFT simulations.
Design era (c ’ ’98). Remarkable agreement with current operations.
“As built” simulations based on bench measurements of actual fabricated optics (c summer ’03)
In Situ measurements (here, fullest story: H1 interferometer)
Scatterometer sampling of in lock beam scatter from HR surfaces.
Arm visibilities (~’ culminating 11/02)
Operational performance (recycling gains, contrast defect) (to present).
Comparison with super polished H2 ETMs.
Detailed study of HR surface (Image analysis) “beam spots” (10/03)
GWADW
June 1, 2006

14. “As built” FFT Simulation
FFT simulation of H1 with no free parameters:
“Cold” state: no thermal lens (little effect on CR light)
~ 92 (observed ~ 41)
FFT uses measured distortion maps,all HR interfaces
minor effect on FFT
~13% for full as built simulation. Negligible for loss matched case.
Consistent with very good ifo contrast defect
for H1
for L1
Other in situ observations (e.g H1 arm visibilities) are consistent with arm loss needed to “match” observed
GWADW
June 1, 2006