  The Sagnac Interferometer has historically been used for detecting
non-reciprocal phenomena, such as rotation. We demonstrate an apparatus in
which this technique is employed for high resolution measurements of the
Magneto-Optical Polar Kerr effect--a direct indicator of magnetism. Previous
designs have incorporated free-space components which are bulky and difficult
to align. We improve upon this technique by using all fiber-optic coupled
components and demonstrate operation at a new wavelength, 820 nm, with which we
can achieve better than 1 $\mu$rad resolution. Mounting the system on a
piezo-electric scanner allows us to acquire diffraction limited images with 1.5
$\mu$m spatial resolution. We also provide extensive discussion on the details
and of the Sagnac Interferometer's construction.
