Transcript LIGO Continuing Operations

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 2006
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June 1, 2006
LIGO “Core” Optics
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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
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June 1, 2006
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
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Thermal lensing: effect on recycling cavity “point design”
» Long term contamination build up on HR surfaces
» Uncertain residual Silica bulk absorption.
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Static charging on suspended dielectric TMs
Inherent unstable recycling cavity design
» Hypersensitivity to polish, coating, homogeneity errors
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Effective loss of long cavities with figure distortions
» Essential target of “FFT” studies
» Coupled with coating reflectivity tolerance: rifo >/< 0 (point design recycling)
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June 1, 2006
Optical Loss Expectations
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CR
 30 based on older polish/coating information
Goal: GRC
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
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Simulation (FFT) with Fab. Data:
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Global surface metrology
Figure= modal distortion
Roughness= loss
Low absorption= cold “start up”
Witness sample reflectivity
FFT mirror map
Simulated G (at least: CR field not affected
by degenerate recycling) far exceeds goals

localized roughness
Consistent with Advanced ligo requirements
H1 ETMy polished surface PSD
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June 1, 2006
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 viewable @ 3 angles:
Scatterometer port 5.5 10-8 Sr
Main arm beam
ITM
H1 ETMy
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Angular dependence more isotropic,
“point like” than metrology prediction
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In situ observed scatter ~70 ppm mirror
~same level, character for every TM
independent of history/cleaning.
FFT map
representation
k-1
roughness
k-2
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June 1, 2006
In Situ Optics Performance
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CR
GRC
~41, which is:
» Consistent with measured arm visibilities
» Consistent with total arm loss
CAVITY
dominated by prompt scatter.
2k X
» Scatterometer data extrapolated
2k Y
to absolute loss
4k X
4k Y
Replaced ITM
V
TITM
TWITNESS Scatter
.0222
.0277
.0283
0.85
.0211
.0272
.0281
7
.0241
.0279
.0275
7.5
.0214
.0263
.028
8.8
» Consistent with lower than anticipated
contrast defect ( and small FFT dependence on maps)
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June 1, 2006
Homogeneous roughness ?

Expect isotropic glow from “homogeneous” polish roughness
» Find: “point” defect scatter dominates
» Bench scans (1064nm) also show excess
Is it just dust ??
Reference calibration:
known cavity loss
Resonant arm, Gaussian illuminated ETM
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June 1, 2006
Analysis of the “Globular
Cluster”
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Cleanest point scatter image: 2k ETMy:
» Grab video stills for
detailed analysis:
Defocused
Focused
 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?
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June 1, 2006
Coatings sensitive to handling
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For several years Hanford 2k performed poorly
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X arm visibility (resonant reflectivity) poor
Ugly recycling cavity “mode” pattern
Excess dark port contrast
More dramatic: unlocked arm cavity r  1
Found: AR coating anomaly
» Hypothesis: extended harsh cleaning of
surfaces had etched coating layers.
Lesson: coating sensitivity to thickness
change (confirmed by model).
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Bench scan of removed ITM
June 1, 2006
Contamination & thermal lensing
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~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.
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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
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However also contamination accidents
– High power operation revealed >10x residual coating absorption
– Unique to pair of ITMs: no evidence in other Hanford optics. When ??
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June 1, 2006
Contamination in LIGO I TMs
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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?
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Mean Abs.= 11.8ppm
Normalized
Correlation = 0.5
June 1, 2006
Conclusions, Direction
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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?
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June 1, 2006
Expectations vs Performance
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Expectations: FFT simulations.
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In Situ measurements (here, fullest story: H1 interferometer)
» Design era (c ’96 ’98). Remarkable agreement with current operations.
» “As built” simulations based on bench measurements of actual fabricated
optics (c summer ’03)
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Scatterometer sampling of in lock beam scatter from HR surfaces.
Arm visibilities (~’00 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)
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June 1, 2006
“As built” FFT Simulation
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FFT simulation of H1 with no free parameters:
» “Cold” state: no thermal lens (little effect on CR light)
CR
» GRC ~ 92 (observed ~ 41)
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FFT uses measured distortion maps,all HR interfaces
CR
» minor effect on FFT GRC
» ~13% for full as built simulation. Negligible for loss matched case.
» Consistent with very good ifo contrast defect
– 6 10-4 for H1
– 3 10-5 for L1
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Other in situ observations (e.g H1 arm visibilities) are
CR
consistent with arm loss needed to “match” observed GRC
.
GWADW 2006
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June 1, 2006