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 June 1, 2006

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 2006 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 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 2006 June 1, 2006

Optical Loss Expectations Goal: 30 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 2006 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: 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 5.5 10-8 Sr ITM Main arm beam FFT map representation H1 ETMy roughness k-1 k-2 GWADW 2006 June 1, 2006

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 2006 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 ?? Resonant arm, Gaussian illuminated ETM Reference calibration: known cavity loss GWADW 2006 June 1, 2006

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 2006 June 1, 2006

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 2006 June 1, 2006

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 2006 June 1, 2006

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 2006 June 1, 2006

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 2006 June 1, 2006

Expectations vs Performance Expectations: FFT simulations. Design era (c ’96 ’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 (~’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) GWADW 2006 June 1, 2006

“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 6 10-4 for H1 3 10-5 for L1 Other in situ observations (e.g H1 arm visibilities) are consistent with arm loss needed to “match” observed . GWADW 2006 June 1, 2006