Modified Sagnac interferometer for high-sensitivity magneto-optic measurements at cryogenic temperatures

The authors describe a geometry for a Sagnac interferometer with a zero-area Sagnac loop for measuring the magneto-optic Kerr effect at cryogenic temperatures. The apparatus is capable of measuring absolute Kerr rotation without any modulation of the magnetic state of the sample and is intrinsically immune to reciprocal effects such as linear birefringence and thermal fluctuations. A single strand of polarization-maintaining fiber is fed into a He-3 cryostat, eliminating the need for optical viewports. With an optical power of only 10μW, they demonstrate static Kerr measurements with a shot-noise-limited sensitivity of 1×10−7rad∕Hz down to 0.5K.