The cleanliness of optical surfaces is of great concern as the Advanced Laser Interferometer Gravitational-Wave
Observatory (LIGO) project transitions from installation to operation at full power. More particulates than expected were
observed on and near the core optics as a result of assembly and installation work, prompting a re-evaluation of longheld
contamination control practices. Even low particulate levels can potentially damage the fused silica optics and
reduce overall interferometer sensitivity. These risks are mitigated from a combination of the following approaches:
quantifying the extent of the contamination, identifying its sources, improving practices to reduce the generation of
particulates, introducing a non-contact in-situ cleaning technique for suspended optics in air, qualifying cleanliness levels
against induced damage, and developing methods for remotely measuring and cleaning suspended optics under vacuum.
While significant progress has been made in understanding and mitigating contamination, and thus, protecting the optics
from losses and damage, there is still more work to be done to reach ultimate performance requirements.