Thermal Coating Noise at Low Temperatures and Low Loss Faraday Isolators

The LIGO observatories were built to detect gravitational waves from many different astrophysical sources such as neutron star and black hole binary systems or supernova explosions. In the wake of the currently ongoing upgrade, the detectors will be limited by coating thermal noise and, at higher frequencies, by shot noise, the statistical properties of classical fields. Coating thermal noise is caused by the Brownian motion of the atoms which comprise the dielectric coatings on the surfaces of the mirrors. We will measure coating thermal noise of the Advanced LIGO coatings, other dielectric coatings, and of newly developed crystalline coatings as a function of beam size and temperature over the entire Advanced LIGO frequency band in a fast turn-around testbed to characterize these coating and verify the theoretical models used to describe them. It is known for almost 30 years that non-classical light fields, squeezed light, can in principle be used to reduce shot noise. The improvements are limited by optical losses between the source of the non-classical field, the squeezer, and the main interferometer. One of the dominating loss mechanisms are optical losses in the Faraday isolator; an optical directional coupler for the injected laser light. We will develop low loss Faraday isolators to reduce the losses and improve the range of Advanced LIGO and other interferometric gravitational wave detectors. These improvements, together with improvements made by others in the LIGO Science Collaboration, will help to increase the detection rate and fidelity of the Advanced LIGO signals to further improve the understanding of these astrophysics signals.

Advanced LIGO is one of the most complex experiments ever built. It continues to push many technologies to its fundamental limits and meeting these limits gives US companies the needed edge to maintain their lead and compete in the world markets. The optical industry in the US was pushed by LIGO to continuously improve their coatings and the leading commercial coating companies are all in the US. In addition, the development of high quality optical coatings with very low thermal coating noise has spin-offs in other areas like time and frequency standards which enable potential improvements in GPS and in precision metrology. Precision metrology leads to improvements in precision manufacturing and increased shares of world markets. Low loss optical components such as the low loss Faraday isolator we will develop have commercial potential and are required in many future scientific experiments in optical information technology, quantum computing, laser physics, and other areas. The potential improvements in the sensitivity of Advanced LIGO will increase the number of detectable gravitational wave sources by at least an order of magnitude, making visible the most energetic events in the Universe and attracting new minds to astronomy and astrophysics and to science in general.