Advancing the development of 2 μm low-loss Faraday isolators for future gravitational-wave detectors

Low-loss Faraday isolators are needed in various areas of the interferometer in current and future Gravitational-wave (GW) detectors, as they directly limit the amount of scatter from creating phase noise. Future upgrades of the GW detectors, such as LIGO-Voyager, consider an operating wavelength around 2 um, to benefit of the much improved coating noise of the test masses around this wavelength. This in turn requires a redesign of the Faraday isolators, as their performance is strongly determined by the operating wavelength. Particularly in the output area of the interferometer, the performance of Faraday isolators, especially losses, pose severe limitations to the performance and effectiveness of the frequency-dependent squeezing. In this talk, we will give an update on the development of 2 um low-loss isolators, will present a characterization of a custom vacuum-compatible design, and will discuss further potential improvements and challenges for the Voyager design. We will also review promising magneto-optical materials at this wavelength.