  We describe a geometry for a Sagnac interferometer with a zero-area Sagnac
loop for measuring magneto-optic Kerr effect (MOKE) at cryogenic temperatures.
The apparatus is capable of measuring absolute polar Kerr rotation at 1550 nm
wavelength without any modulation of the magnetic state of the sample, and is
intrinsically immune to reciprocal effects such as linear birefringence and
thermal fluctuation. A single strand of polarization-maintaining (PM) fiber is
fed into a liquid helium probe, eliminating the need for optical viewports.
This configuration makes it possible to conduct MOKE measurements at much lower
temperatures than before. With an optical power of only 10 $\mu$W, we
demonstrate static Kerr measurements with a shot-noise limited sensitivity of
$1\times 10^{-7}$ rad/$\sqrt{\rm Hz}$ from room temperature down to 2K. Typical
bias drift was measured to be $3\times 10^{-7}$ rad/hour.
