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23 September 2015 Anti-reflective coating corrrection for ultra-low-reflectance large optics
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In order to enhance the final performances of complex optical systems it is required to limit the overall wasted reflected light coming from all the different surfaces involved. Ultra-low-reflectance coating becomes even a crucial point for high sensitivity experiments such as gravitational wave detectors where surfaces must have a reflectance lower than 100 ppm. Some tenths of percent is a common value for Anti-Reflective (AR) coating but reflectance below 100 ppm is trickier to achieve. The coating design sensitivity with respect to thickness errors or refractive index error can lead quickly to noncompliant reflectance.

When an AR coating has failed it is very difficult to recover the low reflectance. In theory adding one or two layers could correct the reflectance but it requires knowing exactly the actual coated stack. For large optics (diameter up to 500 mm), we developed a new technique based on reflectance measurements with different polarizations and incidence angles at one wavelength. The measurements were performed in s-polarized and p-polarized light to discriminate between several solutions. Then a correction based on one or two layers is computed in order to decrease the reflectance. The efficiency of this method is demonstrated in the case of a four-layer AR coating designed for zero reflectance at 1064nm and coated onto a 350mm diameter and 200mm thick substrate. The reflectance has decreased from 500ppm to 26ppm thanks to a correcting bilayer.
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B. Sassolas, J. Teillon, J. Degallaix, C. Michel, L. Pinard, and G. Cagnoli "Anti-reflective coating corrrection for ultra-low-reflectance large optics", Proc. SPIE 9627, Optical Systems Design 2015: Advances in Optical Thin Films V, 96271V (23 September 2015);

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