  Interferometric gravitational wave detectors use mirrors whose substrates are
formed from materials of low intrinsic mechanical dissipation. The two most
likely choices for the test masses in future advanced detectors are fused
silica or sapphire. These test masses must be coated to form mirrors, highly
reflecting at 1064nm. We have measured the excess mechanical losses associated
with adding dielectric coatings to substrates of fused silica and calculate the
effect of the excess loss on the thermal noise in an advanced interferometer.
