Low scatter and ultra-low reflectivity measured in a fused silicawindow

Cinthia Padilla,1 Fabian Magana-Sandoval,1 Erik Muniz,1

Joshua R. Smith,1, ∗ Peter Fritschel,2 and Liyuan Zhang3

1Gravitational Wave Physics and Astronomy Center,

California State University Fullerton, Fullerton, CA 92831, USA2Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

3California Institute of Technology LIGO Project MS 18-34 1200 E. California Blvd. Pasadena, CA 91125

compiled: September 28, 2018

We investigate the reflectivity and optical scattering characteristics at 1064 nm of an antireflectioncoated fused silica window of the type being used in the Advanced LIGO gravitational-wave detec-tors. Reflectivity is measured in the ultra-low range of 5-10 ppm (by vendor) and 14-30 ppm (by us).Using an angle-resolved scatterometer we measure the sample’s Bidirectional Scattering DistributionFunction (BSDF) and use this to estimate its transmitted and reflected scatter at roughly 20-40 ppmand 1 ppm, respectively, over the range of angles measured. We further inspect the sample’s lowbackscatter using an imaging scatterometer, measuring an angle resolved BSDF below 10−6 sr−1 forlarge angles (10–80 from incidence in the plane of the beam). We use the associated images to(partially) isolate scatter from different regions of the sample and find that scattering from the bulkfused silica is on par with backscatter from the antireflection coated optical surfaces. To confirm thatthe bulk scattering is caused by Rayleigh scattering, we perform a separate experiment, measuring thescattering intensity versus input polarization angle. We estimate that 0.9–1.3 ppm of the backscattercan be accounted for by Rayleigh scattering of the bulk fused silica. These results indicate that mod-ern antireflection coatings have low enough scatter to not limit the total backscattering of thick fusedsilica optics.

OCIS codes: (290.1483) BSDF, BRDF, and BTDF; (120.5820) Scattering measurements;(310.1210) Antireflection coatings; (110.0110) Imaging systems.

http://dx.doi.org/10.1364/XX.99.099999

1. Introduction

Low-scatter optics are important for many scien-tific applications, notably ring-laser gyroscopes [1,2], high-power laser systems, and interferometricgravitational-wave detectors [3, 4]. Advances inion-beam sputtering techniques to deposit dielec-tric multilayer coatings onto super polished sub-strates [5–7], such as improved thickness control,now allow for the production of very low scatteroptics and more accurate antireflection coatings.Total scatter loss of 10 ppm or less at 1064 nmhas become standard for ion-beam-sputtered coat-ings [4, 8, 9].

∗ Corresponding author: josmith@fullerton.edu

Low-scatter, low reflectivity antireflection coat-ings have important applications including correc-tive lenses [10], photography, solar cells [11, 12],laser crystals and non-linear crystals [13], and op-tical viewports [14]. The context of this work isinterferometric gravitational-wave detection, whereantireflection coatings are used to minimize reflec-tions and scatter from the non-reflective secondarysurfaces of the interferometer optics and from bothsurfaces of viewports used to transmit laser beamsinto and out of the vacuum system.

A worldwide network of second-generationgravitational-wave detectors, including AdvancedLIGO [15], Advanced VIRGO [16], KAGRA [17],and GEO-HF [18] is currently under construction.These interferometers require exquisite displace-ment sensitivity, of order 1×10−20m/

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100 Hz, in order to directly measure the weak effectsthat gravitational waves from astrophysical systemshave on test masses on earth.

Scattered light can decrease the sensitivity ofgravitational-wave detectors in several ways. Eachdetector’s primary optics are made of fused silicasubstrates with ion-beam-sputtered dielectric coat-ings [3] to produce highly reflective, anti-reflective,or beam-splitting optical surfaces. Optical lossfrom light scattered by the highly reflective primaryoptics used in optical cavities can reduce the laserpower build-up in the interferometers and decreasetheir shot-noise-limited sensitivity. The use of non-classical light, such as squeezed light, to improvethe quantum-noise limited sensitivity of the inter-ferometers can be severely degraded by light scat-tering losses from the optics used to prepare andinject the squeezed states [9, 19, 20]. Finally, scat-tered light from the primary and auxiliary optics,including viewport windows that are used to passbeams into and out of the vacuum system, can cou-ple back into the interferometer adding non-linearnoise [21].

This paper presents a characterization of the lightscattering properties of an ion-beam-sputtered anti-reflective coated viewport. This optic is found tohave very low scatter and therefore is suitable foruse in Advanced LIGO.

2. Sample and preparation

A room light image of the sample investigated here,Research Electro-Optics, serial number ESW03, isshown in Figure 1. The substrate is made ofhigh-quality fused silica (Corning 7980, 0A) witha six inch diameter and a thickness of one inch.Both flat optical surfaces are super polished with10/5 scratch-dig surface quality and have ion-beam-sputtered antireflection coatings that were specifiedto provide very low reflectivity for 1064 nm (goal of10 ppm).

Prior to measurement, the sample surfaces werecleaned of dust and other contaminants by drag-wiping with optic tissues and ultra-pure methanol.To further reduce contamination from dust, thescatter measurements were conducted in a clean-room environment.

3. Measurements

The sample was measured to have ultra-low reflec-tivity at 1064 nm for small angles of incidence. Re-flectivity measurements performed by the vendor,Research Electro Optics, using a 5.7 W Nd:YAGlaser and calibrated photodetector, yielded valuesof 8 ppm and 6 ppm for the front (arrowed), and

Fig. 1. Fused silica viewport ESW03 shown in roomlightmounted on the rotation stage of the Fullerton ImagingScatterometer. A laser beam is passing through bothsurfaces, but is not visible in the photo. The arrow indi-cates the forward surface. Both surfaces have identicalantireflective coatings.

back surfaces, respectively. Later measurements atCaltech, using a 1 mm beam at 1064 nm and an an-gle of incidence < 1 found reflectivities of 30 ppmfor the front side and 14 ppm for the back side. It isnot clear why these two measurements differed, butit may have to do with nano-layers of contaminantsthat have been observed on other optics [22].

Two types of Angle Resolved Scatter (ARS) mea-surements were performed on the sample. Trans-mitted and reflected scatter were measured witha photodiode-based commercial scatterometer andfollowup measurements of the low backscatter weremade with a CCD-camera based imaging scat-terometer.

For both experiments, the laser source was1064 nm and the scatter was quantified accordingto the standard Bidirectional Scattering Distribu-tion Function [23],

BSDF (θs) =Ps

PiΩ cos θs, (1)

where Pi is the laser power incident on the sampleand Ps is the scattered light power collected by a de-tector subtending solid angle Ω at polar scatteringangle θs in the plane of the laser beam. The BSDFis also referred to as BRDF and BTDF in the fol-lowing to explicitly denote scattered light measuredin reflection and transmission, respectively.

From these BSDF measurements, the Total In-tegrated Scatter (TIS) of the sample for the wave-length and incidence angle used were estimated byintegrating the cosine-corrected BSDF assuming in-dependence of scatter on the azimuthal angle, fol-

3

Fig. 2. Setup for the Caltech Angle Resolved Scattermeasurements.

10-6

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φbeam = 1 mm

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TIS (1° ≤ |θs - θi| ≤ 85°)

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DF

Fig. 3. Scatter measurements of the ESW-03 view-port from the transmitted (BTDF, above) and reflected(BRDF, below) sides, made using the modified CASIscatterometer at Caltech. Total integrated scatter esti-mates are also indicated in the legend. The calibrationprecision for these BRDF values is estimated at betterthan 5%.

lowing the steps described in reference [9]. How-ever, it should be noted that although this tech-nique is widely used, it is not strictly correct, sincea smooth sample illuminated normally by linearlypolarized light will not exhibit constant scatter forall azimuthal angles at a given polar angle [23].

3.A. Photodiode ARS measurements

Angle resolved scatter measurements for the view-port sample were carried out at Caltech using amodified version of the commercial Complete An-gle Scatter Instrument (CASI), manufactured bySchmitt Measurement Systems, Inc., and shown in

Figure 2. The CASI system originally had a He-Ne laser installed in its source box. To test scat-ter at 1064 nm, this source was replaced with aNd:YAG laser (CrystaLaser CL1064-100, 100 mWCW), along with a half-wave plate to allow changesof polarization. The beam diameter at the sampleis about 1 mm. This system is capable of measur-ing scattered light over all polar angles, −90 <θs < 90 from normal to the sample surface, inthe plane of incidence, by rotating a photodetec-tor around the sample. A map of scatter over thesample surface can also be obtained by fixing theangle of the detector while scanning the beam po-sition on the sample. To avoid biasing the resultsfor samples with low scatter, both transmitted andreflected beams have to be carefully trapped withbeam dumps.

Figure 3 shows the ARS results for a representa-tive area on the ESW03 window measured on thetransmission (BTDF) and reflection (BRDF) sides.For the BTDF measurement, three scans were takenby aligning the front surface (facing laser source),middle of the window (bulk) and back surface atthe rotation axis respectively. However, additionaltests revealed that the scatterometer is not able todistinguish scatter from the front, back, and bulksurfaces at small angles (where the three curvesmatch closely).

Also shown in Figure 3 is the calculated TIS byintegrating cosine corrected and background (sig-nature) subtracted BTDF and BRDF within 1 <|θs| < 85 and 5 < |θs| < 25 for transmission andreflection sides respectively. The two sides, θs > θiand θs < θi are averaged in the calculation. Theseresults indicate that most of the scatter (20-40 ppmtaking into account the overlapped BTDF at smallangles) is in transmission, while only about 1 ppmis backscatter.

3.B. Imaging ARS measurements

The Fullerton Imaging Scatterometer (FIS) isshown in Figure 4 and described in detail in ref-erence [9]. Here the setup is briefly recounted andand differences from the previous setup are high-lighted. The light source is a 1064 nm linearly po-larized Nd:YAG laser (Innolight Mephisto INN401).This is coupled into a 90:10 fiber beamplitter. Thefiber’s low-power output is connected to a powermonitor and its high-power output is connected toa reflective collimator that collimates the beam to8 mm radius. This light passes through a thin-film linear polarizer set to pass horizontal polar-ization. The beam is then spatially truncated by

4

Power monitor

Rotation stage

Iris

Iris

Linear polarizerReflective collimator

Beam splitter

Lens

CCD camera

1064 laserComputer

automation

θs

LIGO Viewport

Tube

Camera

Fig. 4. Setup of the Fullerton imaging scatterometerthat is described further in the text and in [9].

an iris with an opening of 4 mm, and is incidenton the sample at near normal incidence. Becausethe viewport is antireflection coated, the reflectedbeams were extremely weak and were directed backto the iris to dump their power. Care was taken todump the transmitted beam power using a multi-reflection trap made of black welder’s glass. Thefiber launch, collimator, polarizer, iris, sample, andbeam dumps are all mounted on a motorized ro-tation stage so that when the sample is rotated afixed angle of incidence is maintained and all beamsremain dumped. A single converging lens and irisare used to form an image of a 1.3” x 1.3” regionof the sample on the 1024 x 1024 pixel CCD cam-era (Apogee Alta U6 low-noise cooled astronomicalcamera). Much care is taken with optical bandpassfilters, tubes, and beam blocks to minimize othersources of light that could reduce the sensitivity ofthe CCD images to scatter from the sample.

The scatter measurements were supervised bya LabView automation Virtual Instrument. Thismoves the rotation stage to the desired scatteringangle, measures the laser light at the power mon-itor, and exposes the CCD chip for imaging. Us-ing this procedure, images and incident power mea-surements were collected at fixed scattering angles0 ≤ θs ≤ 90, in one degree increments of θs. Ex-posure times were set to be as long as possible such

Fig. 5. Upper left: Cartoon of the viewport from theperspective of the CCD camera for a scattering angle of30 degrees. The CCD images a square 1.3” region thatcontains the front surface scattering, the back surfacescattering, and the bulk scattering. Other panels: CCDimages for three separate scattering angles, 20, 30, and60 degrees, showing the scattering spots on the front andback surfaces, and the bulk scattering from the illumi-nated volume.

that no pixels saturated in the region of interest (seeFigure 6), typically between 30 and 50 seconds. Af-ter the 90 images are collected, the same procedureis followed, but with the laser off. These “dark im-ages” are subtracted from the scattering images toreduce noise and hot pixels. The subtracted imagesare then analyzed using a custom Matlab script thatcalibrates the images (see below) and calculates thescattered light power in the regions of interest bysumming all of the pixel values.

Figure 5 shows a cartoon of the viewport sam-ple from the perspective of the CCD camera andindicates the imaged region. As shown, the bulkscattering and back surface scattering are imagedthrough the front surface. Also shown are full 1024x 1024 pixel CCD images for three different scatter-ing angles, each using the same black/white scaling.In these images, scattering from the front and backsurfaces is visible as a constellation of points andbulk scattering from the illuminated volume is seenas a uniform “glow” with roughly the same bright-ness. At small angles, of 20 or less, the imagesare bright, the front and back scattering spots spa-

5

Fig. 6. Zoomed-in images showing the regions of interestused to capture the total scattered light (R1) and isolatescatter from the near surface (R2) and far surface (R3).For small viewing angles, such as θs = 20, shown on theleft, the near and far surface scatter is highly overlappedwith the bulk scatter. For wider angles, such as θs = 60,shown on the right, the near and far surface scatter isspatially separated.

tially overlap, and the bulk scattering is difficultto distinguish from the surface scattering. For an-gles above 30 the front and back scattering spotsare spatially separated and the bulk scattering canbe clearly seen, appearing roughly as bright as thesurface scattering.

Figure 6 shows a zoom of the CCD images, andthe regions of interest used to estimate the scatterfrom the front, back, and bulk scattering. RegionR1 encompasses all scatter from the sample bulkand surface within the radius of the laser beam.Regions R2 and R3 are centered on the front andback spot, respectively, with their starting positionset by hand. Their centers follow the beam spot mo-tion as the sample rotates, and decrease in widthwith the cosine of the scattering angle. AlthoughR2 and R3 are centered on the front and back sur-face spots, they capture a significant amount of bulkscatter at all angles, and overlap spatially for scat-tering angles below 30 degrees. Still, a lower limitestimate of the bulk scatter can be obtained by sub-tracting the measured scattered power in R2 and R3from the total scattered power in R1 and convert-ing this to BSDF. Research is ongoing to implementelliptical regions of interest in future analyses.

The calibration technique used to convert CCDcounts into physical units (BRDF) is the same asthat described in [9]. A diffuse scattering target(Spectralon Diffusion Material, 100 0.01200 disk,SM-00875- 200) is illuminated with the 1064 nmlaser at normal incidence. The resulting diffusescatter is measured by a calibrated power meterand the BRDF is calculated following Equation 1

using the measured incident power, the scatteredlight power at several scattering angles, and thesolid angle subtended the power meter, giving avalue close to 1/π sr−1 for 0 ≤ θs ≤ 90. Thenimages are taken of the same diffusing target at thesame scattering angles using the CCD camera (andan additional T = 1/256 neutral density filter toreduce the light power). Relating the BRDF mea-surements to the images gives a calibration factor.For these measurements, a value of F = 3.20×10−14

W sec Counts−1 sr−1 was used.

The major problem with this method is that itrelies on the linearity of the CCD camera, the shut-ter timing, and dark noise subtraction in extrapo-lating from measured light intensities proportionalto BRDFSpectralon/Tfilter = 1/(256π) ≈ 10−3 fromthe Spectralon sample (with filter) to much smallerintensities proportional to BRDF = 10−7 fromthe viewport sample. Measuring linearity over thisrange is challenging because of limitations to cal-ibrated power meters. Taking the known factorsinto account gives a calibration error of up to 50%.

The results and references presented below indi-cate that the bulk scattering from fused silica pro-duces a BRDF that is of the same order of magni-tude as the BRDF expected from the optical sur-faces of low scattering samples. Research is nowunderway to calibrate the FIS CCD camera by di-rectly measuring the Rayleigh scattering in fusedsilica and comparing it with theoretical and mea-sured values available in the literature. The firststeps toward this are presented in Appendix A.

Figure 7 shows the measured BRDF for the view-port sample. The total backscatter (from regionR1) is very low, 2–9×10−6sr−1 over the angles mea-sured, but an order of magnitude above the instru-ment signature. This BRDF is comparable to high-quality highly-reflecting ion beam deposited coat-ings on superpolished substrates [6, 9]. The regionsR2 and R3, centered on the near and far surfacescattering, respectively, have nearly equal BRDF,but these values are not a clean indication of thescatter from the separate surfaces because the as-sociated regions spatially overlap for small angles,and contain a significant amount of bulk scatteringfor all angles.

Also shown in Figure 7 is a lower limit on bulkscattering, estimated by subtracting the BRDFfrom the near and far surface scattering from the to-tal BRDF. This has a value of roughly 10−7sr−1 forangles above 40, and for larger angles, approachesthe expected BSDF for Rayleigh scattering (dot-ted lines) converted from the intensity ratio values

6

10 20 30 40 50 60 70

10−8

10−7

10−6

10−5

BR

DF

[1/s

tr]

Scattering Angle θs [degs]

R1 (Total Scatter)

R2 (Centered on Near Spot)

R3 (Centered on Far Spot)

R1−(R2+R3) (Lower limit on Bulk Scatter)

Imaged BRDF Limit

Rayleigh BRDF SiO2 (Theory)

Rayleigh BRDF SiO2 (Chen et al)

Fig. 7. BRDF versus scattering angle for the viewportsample. Total scatter from region R1 (squares), whichincludes the near surface, far surface, and bulk scatter, isin the range 2–9×10−6sr−1. Regions R2 (right-pointingtriangle) and R3 (left pointing triangle), are centeredon scatter from the near spot and far spot, respectively,and a lower limit on the BRDF from bulk scatter isestimated by subtracting R2 and R3 from R1 (circles).Dotted lines indicate the BSDF expected from Rayleighscattering according to [24]. The instrument signatureBRDF (stars) is a few ten times lower than the totalscatter. The calibration precision for these BRDF valuesis better than 50%.

in [24]. The bulk scattering lower limit does notmatch the calculated Rayleigh scattering BRDF forsmaller angles because there the near and far sur-face regions spatially overlap in the images. Atlarge angles, the lower limit is close to the BRDFexpected from Rayleigh scattering even though, asseen in Figure 6, R1 and R2 contain nearly as muchbulk scatter as region in between. If this were cor-rected for, the measured data would thus be abouta factor of two above the dashed lines. This dis-crepancy could be due to one of several small an-gles present in the setup. The input light polariza-tion was only accurate to a few degrees since it wasaligned by hand using the polarization axis mark-ings on a half-inch polarizer. Also, as can be seenfrom Figure 6, the incident beam was not entirelynormal to the sample, instead it had a vertical angleof incidence of 2.2. To ensure that the bulk scat-ter was caused by Rayleigh scattering, additionalmeasurements were made to test the scattering in-tensity dependence on input polarization. Theseare presented in Appendix A.

Table 1 shows integrated scatter estimates forthe scattering regions described above, with an as-

Table 1. Integrated scatter estimates for regions of theanalyzed images (total, front spot, back spot, and sub-tracted regions to give lower limit on Rayleigh scatter-ing). These values are compared to TIS for Rayleighscattering in fused silica calculated from theoretical andmeasured scattering coefficients from Reference [24] mul-tiplied by the viewport sample thickness (TIS = αsct).

Region θs Range Ω (sr) TIS (ppm)

Total (R1) 10− 80 deg 1.62π 1.26

R2 (front spot) 10− 80 deg 1.62π 0.62

R3 (back spot) 10− 80 deg 1.62π 0.56

R1−(R2+R3) 10− 80 deg 1.62π 0.21

Rayleigh [24] (meas.) 0− 180 deg 4π 1.8

Rayleigh [24] (theo.) 0− 180 deg 4π 2.5

sumed independence of scatter on the azimuthal an-gle, calculated as described in [9]. Also shown arethe TIS estimated by Chen et. al. based on theirmeasured and theoretical values [24]. A significantfraction of the total backscatter can be attributedto Rayleigh scattering in the fused silica bulk ma-terial.

4. Discussion

The viewport measured here exhibits low forwardscatter, very low backscatter, and ultra low reflec-tivity. For this sample, Rayleigh scattering fromthe fused silica substrate is on par with scatterfrom the two optical surfaces. This means thation-sputtered antireflective coatings have nearlyreached the limit where scattering from the bulkwill dominate backscattering from the surface andcoatings, for thick optics (viewports, compensationplates, etc). The scattered measured here was re-stricted to angles greater than one or a few degrees,however very small angle scattering from viewportsis also of interest for gravitational-wave detector op-tics, and should be measured.

Acknowledgments

This work is supported by National Science Foun-dation Awards PHY-0970147 and PHY-1255650and by the Research Corporation for Science Ad-vancement Cottrell College Science Award #19839.E. Muniz is supported by (STEM)2, fundedby the US Department of Education (Grant#P031C110116-12). The authors thank K. Wanserand G. Childers (CSU Fullerton) for useful dis-cussions about Rayleigh scattering. We thank ourcolleagues in the LIGO Scientific Collaboration for

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fruitful discussions about this research and for re-view of this manuscript.

References

[1] N. L. Thomas, “Low-scatter, low-loss mirrors forlaser gyros,” in Laser Inertial Rotation Sensors, S.Ezekiel and G. E. Knausenberger, eds., Proc. Soc.Photo-Opt. Instrum. Eng. 157, 41-48 (1978).

[2] S. Chao, W. L. Lim and J. A. Hammond ”Lock-ingrowth in a ring laser gyro”, Physics of Optical RingGyros (SPIE), vol. 487, pp. 50 -57 1984.

[3] G. M. Harry, T. P. Bodiya, and R. DeSalvo, eds.,Optical coatings and thermal noise in precision mea-surement (Cambridge University Press, 2012).

[4] The VIRGO Collaboration, et. al., “The VIRGOlarge mirrors: a challenge for low loss coatings,”Class. Quantum Grav. 21 S935 (2004).

[5] D.T. Wei, “Ion beam interference coating for ul-tralow optical loss,” Applied Optics, Vol. 28, Issue14, pp. 2813-2816 (1989).

[6] S.E. Watkins, J.P. Black, and B.J. Pond, “Opticalscatter characteristics of high-reflectance dielectriccoatings and fused-silica substrates,” Applied Op-tics, Vol. 32, Issue 28, pp. 5511-5518 (1993).

[7] B. Cimma et. al., “Ion beam sputtering coatings onlarge substrates: toward an improvement of the me-chanical and optical performances,” Applied Optics,Vol. 45, Issue 7, pp. 1436-1439 (2006).

[8] G. Rempe, R. J. Thompson, H. J. Kimble, and R.Lalezari, “Measurement of ultralow losses in an op-tical interferometer,” Optics Letters, Vol. 17, Issue5, pp. 363-365 (1992).

[9] F. Magana-Sandoval, R. X. Adhikari, V. Frolov,J. Harms, J. Lee, S. Sankar, P. R. Saulson, andJ. R. Smith,“Large-angle scattered light measure-ments for quantum-noise filter cavity design stud-ies,” JOSA A, Vol. 29, Issue 8, pp. 1722-1727 (2012).

[10] T.R. Reynolds, “Ion beam assisted deposition ofophthalmic lens coatings,” United States PatentApplication Publication US 2011/0229659 A1, Sep.22, 2011.

[11] D.J. Aiken, “Antireflection coating design for seriesinterconnected multi-junction solar cells,” Progressin Photovoltaics: Research and Applications, Vol-ume 8, Issue 6, pages 563570, (2000).

[12] M. Victoria, C. Dominguez, I. Anton, and G. Sala,“Antireflective coatings for multijunction solar cellsunder wide-angle ray bundles,” Optics Express, Vol.20, Issue 7, pp. 8136-8147 (2012).

[13] M. Stefszky, C.M. Mow-Lowry, K. McKenzie, SsChua, B.C. Buchler, T. Symul, D.E. McClellandand P.K. Lam, “An investigation of doubly-resonantoptical parametric oscillators and nonlinear crystalsfor squeezing,” J. Phys. B: At. Mol. Opt. Phys. 44015502 (2011).

[14] D. C. Massey, “Spacelab Optical Viewport GlassAssembly Optical Test Program For The StarlabMission,” Proc. SPIE 1164, Surface Characteriza-tion and Testing II, 236 (December 20, 1989).

[15] G. M. Harry (for the LIGO Scientific Collab-oration), “Advanced LIGO: the next generationof gravitational wave detectors,” Class. QuantumGrav. 27 084006 (2010).

[16] Advanced Virgo Baseline Design, The Virgo Col-laboration, note VIR027A09 May 16, 2009https://tds.ego-gw.it/itf/tds/file.php?

callFile=VIR-0027A-09.pdf

[17] K. Somiya (for the KAGRA collaboration), “De-tector configuration of KAGRAthe Japanese cryo-genic gravitational-wave detector,” Class. QuantumGrav., 29, 124007 (2012).

[18] B. Willke et al., “The GEO-HF project,” Class.Quantum Grav. 23, S207S214 (2006).

[19] J. Aasi, et al. (The LIGO Scientific Collaboration),“Enhanced sensitivity of the LIGO gravitationalwave detector by using squeezed states of light,”Nature Photonics 7, 613619 (2013).

[20] J. Abadie, et al. (The LIGO Scientific Collabora-tion), “A gravitational wave observatory operat-ing beyond the quantum shot-noise limit,” NaturePhysics 7, 962965 (2011).

[21] D. Ottaway, P. Fritschel, and S. Waldman, “Impactof upconverted scattered light on advanced interfer-ometric gravitational wave detectors,” Opt. Express20, 8329-8336 (2012).

[22] Harald Luck, personal communication.[23] J. C. Stover, Optical Scattering, third edition (SPIE

Press, 2012).[24] X. Chen, L. Ju, R. Flaminio, H. Luck, Zhao C,

D.G. Blair, “Rayleigh scattering in fused silicasamples for gravitational wave detectors,” OpticsComm, 284, 4732-473 (2011).

[25] E. Brinkmeyer and W. Eickhoff, ”Ultimate Limit ofPolarization Holding in Single-Mode Fibres,” Elec-tronics Letters, Vol. 19 No. 23 pp. 996-997 (1983).

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Appendix A: Confirming Rayleigh scattering ascause of imaged bulk scatteringAn additional experiment was performed to confirmthat the bulk scattering from the viewport mea-sured by the Fullerton Imaging Scatterometer wasdominated by Rayleigh scattering of the fused sil-ica material. A diagram of the experimental setup,which follows that used in [24], is shown in Figure 8.The input beam is normally incident on the view-port and the transmitted beam is dumped. TheCCD camera observes the bulk of the optic throughits barrel, perpendicular to the input beam and par-allel to the table. Beam blocks are used to blockthe light from the beveled edges of the optic barrel,and only the central 1cm of the optic is imaged.The linear polarization angle of the input beam isadjusted by rotating a linear polarizer on a gradedrotation mount, and power is kept high by rotat-ing the input light polarization to match the axisof the polarizer. Images of the bulk scattering forthree different input polarization angles are shownin Figure 9.

Sample

β

Polarization Angle

x

z

y

θ

observation direction

Fig. 8. Diagram of the setup used to check for thefunctional dependence of Rayleigh scattering intensityviewed through the side of the viewport versus inputpolarization angle β.

The measured intensity ratio, based on the CCDcalibration above, is shown in Figure 10. Alsoshown is a measurement of the background noisein each image. This background is likely due to ad-ditional stray light in the setup that is not presentin the dark images, and does not spatially overlap

Fig. 9. Imaged scattered light intensity through the sideof the viewport for vertically, 20 degrees, 45 degrees, andhorizontally polarized input light. The scatter RoI is thewide rectangle around the scattering and the backgroundRoI is the small box at the bottom of each image.

0 20 40 60 800

1

2

3

4

5x 10

−8

β [degs.]

I s/(I iΩ

) [c

m−

1sr

−1]

Measured ScatterPredicted Rayleigh Scatter [Ju et al]

Image BackgroundPredicted Rayleigh + Image Background

Fig. 10. Squares represent the scattering intensity ratiomeasured through the side of the viewport versus vary-ing input beam polarization angle β. The dashed line isthe expected intensity function for Rayleigh scatteringbased on measurements in Ju et. al. [24] and the cos(β)2

dependency. For each image, an estimate of the back-ground noise, produced by taking the intensity ratio in a50x50 pixels region (vertically aligned with the scatter-ing, but far below the beam in the image) and scaling itby the ratio of pixels contained in the scattering RoI tothose contained in the background RoI, is marked withan x. The measured scatter matches well the sum of theexpected Rayleigh scattering and the image backgroundnoise.

9

with the beam, as would be expected for e.g., fromadditional Rayleigh scattering due to depolariza-tion in fused silica [25]. Also shown is the expectedintensity ratio for Rayleigh scattering based on theI ∝ cos(β)2 functional dependence and the mea-

sured maximum scattering (at β = 0) from [24].The measured scatter matches well the sum of theexpected Rayleigh scattering ratio and image back-ground noise, confirming that it is Rayleigh scat-tering.