Although the reflection surfaces an

Although the reflection surfaces and substrates of the mirrors are carefully designed and manufactured, part of the laser beam injected to the mirrors scatters in various directions due to imperfection or inhomogeneity. Light scattered from the mirrors interacts with the surfaces of surrounding mechanical components, which are highly vibrating with seismic motions, and then is recombined to the main laser beam resulting in additional phase noise of the interferometer. Therefore in interferometric gravitational wave detectors, scattered light has to be retained as much as possible by placing black-coated optical baffles in appropriate positions to avoid scattered light noise.