1. Study of scattering points on LIGO mirrors
Lamar Glover (Cal State LA)
Riccardo DeSalvo (Cal State LA)
Innocenzo Pinto (Unisannio)
LVC meeting 03/15/16 LIGO-G
Related doc: LIGO-G

2. Brief Summary
At Cal State LA, we are analyzing scattering points detected in beam illuminated photos of Advanced LIGO mirrors
In LIGO mirrors, A large number of scatterers were observed where none would be expected, through the depths of the coating layer stack
Perhaps the problem of scatterers can now be better understood
LVC meeting 03/15/16 LIGO-G

3. Method
Individual scatterers were identified using astronomical algorithms for stars in galaxies
Extract apparent amplitude and position of each scatterer
Fit the stored beam position and profile to determine the illumination power on each scatterer.
LVC meeting 03/15/16 LIGO-G

4. Daophot mechanism: Point Spread Templates
“daophot” creates a point spread template from selected sources within the image.
The template is used to search scatterers
Magnitude data is derived for each identified source.
Point Spread Template for an exposure time of 1.25 x 10-4 s
LVC meeting 03/15/16 LIGO-G

5. Identifying/subtracting scatterers
Original Photo
Subtracted Photo
ignored
256
100
Dark pixels
pixel scale is expanded
256
Black dots mark the place where bright scatterers have been excised
“dirt” does not fit template and is ignored by daophot and is less bright
LVC meeting 03/15/16 LIGO-G

6. Daophot effectiveness and resolution
Pixel amplitude histogram of original Image
100
200
In a good image Daophot is very effective in extracting all scatterers
Width of residuals (2-3 pixels FWHM) illustrates Daophot’s good resolution
Residual pixel amplitude histogram after scatterer extraction
Note for you, the central pixel does not count because it is just the black background that daophot ignores, but this is log scale, ignoring the central pixel you get 2-3 pixel FWHM
-50 50
LVC meeting 03/15/16 LIGO-G

7. Scatterer light intensity distribution @ Exposure Time = 0.0125 sec70
The number of reconstructed scatterers grows rapidly at low amplitude 40
5000
20000
LVC meeting 03/15/16 LIGO-G

8. Number of Scatterers vs. Exposure Time
Exposure scatterers
@ ms
@ 1 ms exposure 11,806
@ 400 ms exposure 363,789
Number increases almost linearly with exposure
Exploring smaller scatterers
But also deeper in the dielectric coating layers ! !
LVC meeting 03/15/16 LIGO-G

9. The apparent size of scatterer
correctable in photos:
Local illumination level
not correctable:
Physical scatterer size inside host layer
Depth of host layer inside dielectric coating stack
Beam
profile
Just put the image there and state that it is correctable, do not tell if you corrected or not.
One image is worth a thousand words
(Note: all scatterers are nanometer scale, diffraction limited)
LVC meeting 03/15/16 LIGO-G

10. 1: Crystallite Formation in a single evaporated layer
Growth direction
Side View
Top View
Growth Direction
As mirror layers are formed, crystallites appear at different depths.
Those that start earlier in the depositing process grow more and create larger scatterers
LVC meeting 03/15/16 LIGO-G

11. Scattered light detected by CCD
2: Depth in stack effect
There are two depth attenuation components:
Attenuation of impinging light due to reflections through depth of coating layers
Attenuation of Scattered light from back-reflection on outer layers
Scattered light detected by CCD
Complex effect to simulate
LVC meeting 03/15/16 LIGO-G

12. Discussion of crystallite number and distribution intensity
How can so many scatterers exist, and be so uniformly distributed?
Cannot be only “dirt” !
Only a thermodynamical source can explain the observed large number
Probable dirt smear
Unresolved by daophot
LVC meeting 03/15/16 LIGO-G

13. What’s Next Two steps proposed:
Take more images at Hanford/Livingston and study more photos
Study spare ad-LIGO mirrors with Caltech’s setup to determine scatterer’s depth distribution through layers and size distribution
How to?
LVC meeting 03/15/16 LIGO-G

14. AdLIGO Multi-Layered Mirror
X, Y Axis micropositioning
CCD Camera 1 3 2
White light LED
50% Reflector
@ 45°
Z Axis piezo focusing
Microscope
lens
AdLIGO Multi-Layered Mirror
LVC meeting 03/15/16 LIGO-G

15. X,Y Grid scan of AdLIGO Mirror… Xn
Raster scan X-Y of microscope frames on mirror surface to identify rough scatterer position Y1 Y2
Defocused
Scatterers … Yn
LVC meeting 03/15/16 LIGO-G

16. Depth determination by Z- focussing
No scatterer present:
No reflected light
Scatterer present out of f-plane
Low illumination density
Defocused scatterer image
Z-zoom with piezo-movement to bring focal plane on scatterer:
Brighter illumination
Focused airy disk image
Defocused
Scatterer
Focused
Scatterer
Defocused
Scatterer
Maximum Luminosity +Z -Z
LVC meeting 03/15/16 LIGO-G

17. Depth determination by Z- focussing fits
Depth of field µm / n
Sub-wavelength
Z-position determination from Z-feedback fit
For your information, n is the refraction index of the material, in Ta2O5 or TiO2 it is between 2.5 and 3
Defocused
Scatterer
Focused
Scatterer
Defocused
Scatterer
Maximum Luminosity
Nikon lens calculator +Z -Z
LVC meeting 03/15/16 LIGO-G

18. X-Y determination by profile fitting
Sub-wavelength X-Y position determination from Airy disk profile fitting
D = 0.61λ/NA
LVC meeting 03/15/16 LIGO-G

19. Scatterer strength Within Mirror Layers
In photo images, the signal amplitude depends on scatterer depth, due to reflectivity on layers
White light and large angle focusing mitigate the effect
Complex but probably soluble or mappable function 1 2
LVC meeting 03/15/16 LIGO-G

20. Scatterers strength Within Mirror Layer1
Independent of illumination/trans-reflection function:
at same depth, information of scatterer’s size is extracted from amount of light collected 2
LVC meeting 03/15/16 LIGO-G

21. Conclusions
Further measurements on actual mirrors should nail down the observation that millions of scatterers are present throughout the layers of the Ad-LIGO dielectric coatings
Such large number of scatterers require a thermodynamic origin (see next presentation)
LVC meeting 03/15/16 LIGO-G