Low-loss mirrors fabricated by ion-beam-sputtering machines for possible application in an interfere-metric gravitational wave antenna were evaluated by use of Nd:YAG laser light (λ = 1064 nm) with two distinct measurements: a tabletop experiment that used a short cavity with a small beam with a beam waist of approximately 2w_(0) = 0.82 mm, and an optical test that used a 20-m prototypical gravitational-wave detector with a large beam with a beam waist of approximately 2W_(0) = 4.4 mm. A multilayer coating comprised 29 layers of SiO_(2)/Ta_(2)O_(5) and one protective coating SiO_(2). The best values obtained as a result of these measurements were 16 ppm (parts in 10~(6)) and 30 ppm in total loss, respectively. Also, a two-dimensional loss map generated by use of a small beam successfully revealed the existence of a loss structure within the coating surface. These results imply that a high-reflectance multilayer coating has some inhomogeneities and a loss distribution with a typical scale of a few millimeters and that the total measured losses depend on the beam spot size.
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