Loss factors of mirrors for a gravitational wave antenna

Low-loss mirrors fabricated by ion-beam-sputtering machines for possible ap plication in an interferometric gravitational wave antenna were evaluated b y use of Nd:YAC laser light (lambda = 1064 nm) with two distinct measuremen ts: 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 b eam waist of approximately 2w(0) = 4.4 mm. A multilayer coating comprised 2 9 layers of SiO2/Ta2O5 and one protective coating of SiO2. The best values obtained as a result of these measurements were 16 ppm (Darts in 10(6)) and 30 ppm in total loss, respectively. Also, a two-dimensional loss map gener ated by use of a small beam successfully revealed the existence of a loss s tructure within the coating surface. These results imply that a high-reflec tance multilayer coating has some inhomogeneities and a loss distribution w ith a typical scale of a few millimeters and that the total measured losses depend on the beam spot size. (C) 1999 Optical Society of America OCIS cod es: 230.4040, 230.4170, 310.1620, 290.5820, 240.0310.