Low scatter and ultra-low reflectivity measured in a fused silica window
We investigate the reflectivity and optical scattering characteristics at 1064 nm of an antireflection coated fused silica window of the type being used in the Advanced LIGO gravitational-wave detectors. 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 distribution function (BSDF) and use this to estimate its transmitted and reflected scatter at roughly 20–40 and 1 ppm, respectively, over the range of angles measured. We further inspect the sample's low backscatter using an imaging scatterometer, measuring an angle resolved BSDF below 10^(−6) sr^(−1) for large 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 bulk fused silica is on par with backscatter from the antireflection coated optical surfaces. To confirm that the bulk scattering is caused by Rayleigh scattering, we perform a separate experiment measuring the scattering intensity versus input polarization angle. We estimate that 0.9–1.3 ppm of the backscatter can be accounted for by Rayleigh scattering of the bulk fused silica. These results indicate that modern antireflection coatings have low enough scatter to not limit the total backscattering of thick fused silica optics.
© 2014 Optical Society of America. Received 5 December 2013; accepted 8 January 2014; posted 24 January 2014 (Doc. ID 202514); published 24 February 2014. This work is supported by National Science Foundation Awards PHY-0970147, PHY-1255650, and PHY-0107417 and by the Research Corporation for Science Advancement Cottrell College Science Award #19839. E. Muniz is supported by (STEM)2, funded by the U.S. Department of Education (Grant# P031C110116-12). The authors thank K.Wanser and G. Childers (CSU Fullerton) for useful discussions about Rayleigh scattering. We thank our colleagues in the LIGO Scientific Collaboration for fruitful discussions about this research and for review of this manuscript.
Submitted - 1312.1569v1.pdf
Published - ao-53-7-1315.pdf