Characterization of a commercial Faraday isolator at 2 micron wavelength

With the goal of further increasing the detection sensitivity, future upgrades of the gravitational-wave (GW) detectors consider a change in the operating wavelength from 1 um to 2 um, which is more promising for reducing thermal noise in the coatings of the mirrors that act as test masses. One key device that directly influences the sensitivity of the GW detectors is a Faraday isolator. This device (a combination of optics and a magnet) uses the polarization of the laser beam to reject scatter or other undesired laser light, in order to avoid them reaching the detection ports, thus contaminating the GW signal. Commercial Faraday isolators are available, however their performance might not meet the stringent requirements needed in the gravitational-wave detectors. We are characterizing a commercial Faraday isolator, with the goal of understanding their overall performance, ways of optimization, and limitations for operating in future upgrades of the gravitational-wave detectors.