Low-loss Faraday isolators at 2 μm – progress and challenges

Low-loss Faraday isolators are used in different areas of the current gravitational-wave detectors to prevent stray light from building inside the resonant cavities. Their requirements are stringent and varied. In the input optics path the Faraday isolator prevents phase noise in the input mode cleaner, and must withstand high average laser powers while maintaining performance over a broad power range. The isolators in the output (squeezer) path are critical to throughput losses, particularly scattering, as losses drastically offset benefits the quantum vacuum squeezing. Low-loss isolators have been recently built for the output and squeezer paths, and operate in the current LIGO detectors. For longer wavelengths however, as considered for next generation of gravitational-wave detectors (e.g. LIGO-Voyager), the options are more limited. In this talk we give an update on the development of 2 um low-loss isolators as pathfinders for Voyager, highlight promises and challenges, and discuss suitability for the Voyager design. We also review available promising magneto-optical materials at this wavelength, and discuss their potential for satisfying exigencies in the input and squeezer paths.