Pathfinder: Low-loss 2 UM Faraday Isolators for 3G Detectors

In September 2015 the twin Advanced LIGO detectors directly observed for the first time the ripples from the merger of two black holes that travelled through space for more than one billion years. More than 50 other detections followed, in coincidence with the Virgo detector, from a variety of sources, including the collision of two neutron stars, an event observed by the broad astronomical community in both gravitational-waves and light, marking the beginning of the new era of multi-messenger astronomy. A new generation of detectors with much increased sensitivity will observe gravitational waves from sources across the cosmic time of the universe. They will make more frequent detections, and from a wider variety of sources, with extraordinary sky localization accuracy. This will help us expand our understanding of the dynamic of the universe and of the warped nature of the spacetime and will surely astonish us with new and unexpected discoveries. This new generation of detectors is envisioned with much larger mirrors, cryogenically cooled to remove thermal vibrations and lower thermal noise that will likely operate at longer laser wavelength. Low coating absorption for cryogenically cooled silicon seems to favor a nominal wavelength of 2 microns. Optical components known as Faraday isolators are key components in determining the sensitivity of gravitational wave detectors by rejecting the scattered light from building up and resonating in the interferometer. Their performance is strongly affected by the operating wavelength, while high-power operation presents additional challenges. This work will address the development of low-loss Faraday isolators at 2 microns, including high-power characterization as well as research on low-absorption materials for improved isolator performance. The proposed project will continue to provide research training and mentoring to undergraduate students at Montclair State University in experimental gravitational physics. It will help the students develop practical scientific skills in areas of optics and lasers, spectroscopy, vacuum systems and cryogenics. In addition, it will give them opportunities to interact with other researchers and engineers in the field.

This award supports the development of low-loss Faraday isolators for 3G detectors, including high power studies for near-future upgrades of the LIGO detectors. Parasitic interferences due to backscatter from moving surfaces are limiting factors on the sensitivity of gravitational wave detectors, and improvements in the performance of Faraday isolators directly boost the sensitivity range. Developing and designing low-loss Faraday isolators at 2 micron is important in several areas, including the input optics, output and squeezer path, and the pre-stabilized lasers. This award also supports broadband and cryogenic studies of low-absorption magneto-optical materials for 3G gravitational wave detectors.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.