This research award will enable the continuation of investigations important to the Advanced Laser Interferometer Gravitational-wave Observatory (Advanced LIGO). First, the quality of the mirror surfaces is crucial to the performance of Advanced LIGO. A method is under development to monitor mirror surface contaminants during the operation of Advanced LIGO using a nonlinear optical technique known as surface second harmonic generation (SSHG). Theoretical investigation of SSHG generation from multi-layer dielectric coatings and experimental investigation of SSHG, specifically investigations of controlled contamination on superpolished mirrors used in Advanced LIGO, will be conducted. Second, optical isolation of high power laser beams is important for Advanced LIGO, and this research addresses issues with the development of optical (Faraday) isolators capable of handling higher laser powers. Activities to be undertaken are characterization of thermal depolarization, isolation ratio, and thermal lensing for a large aperture Faraday isolator design and testing of Advanced LIGO Faraday isolators in vacuum.
Advanced LIGO will open a new astronomical window --- the ability to probe some of the most cataclysmic events in universe through the emission of gravitational waves, a mechanism proposed by Albert Einstein almost 100 years ago, but yet to be directly observed. These investigations will both aid in the operation of Advanced LIGO over long time scales and potentially improve its performance once it is in operation. They will also impact many applications which use high power lasers. Finally, this research will also foster international collaborations.
This project is a collaboration of the Institute of Applied Physics (IAP) of the Russian Academy of Science and the University of Florida (UF) . The IAP is a large (1500 person) laboratory in Nizhny Novgorod, Russia. Since 1994, the IAP has been collaborating with the UF LIGO group and others at Florida on development of optical components for gravitational-wave detectors. Under this project, we studied the theory of second harmonic generation from interfaces of dielectric surfaces and designed a scheme for measuring this signal. We proved that the presence of a multilayer dielectric layers (such as are used in high-reflectivity laser mirrors) strongly influences the intensity of second harmonic generation. A range of angles of incidence for the radiation exist where the second harmonic generation strongly decreases, making it suitable to detect reliably a signal from contaminated surfaces. A second area of research was the design, construction, and testing of high-performance Faraday isolators. We improved functionality of permanent magnets in the Faraday isolators, allowing us to increase significantly the light aperture and the strength of the magnetic field. Using these designs, we developed a wide-aperture Faraday isolator that be used for lasers with kilowatt average power. An isolator having 15 mm clear aperture was constructed and its performance evaluated. The device provided an isolation ratio of 30 dB at 650 W average power, making it suitable for many applications.
