Zero-Area-Loop Sagnac Interferometer Xia et. al. Modified Sagnac interferometer for high-sensitivity magneto-optic measurements at cryogenic temperatures, Appl. Phys. Lett. 89, 062508 (2006)

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Kerr rotation maps of few-layer Cr₂Ge₂Te₆
Gong, C. et al. Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals. Nature 546, 265–269 (2017).
Speed
Beam 1 — in-plane pol., fast axis Beam 2 — out-of-plane pol., slow axis oscillating bar = linear pol. · spinning ring = circular pol.