  Sagnac interferometry can provide a significant improvement in
signal-to-noise ratio compared to conventional magnetic imaging based on the
magneto-optical Kerr effect (MOKE). We show that this improvement is sufficient
to allow quantitative measurements of current-induced magnetic deflections due
to spin-orbit torque even in thin-film magnetic samples with perpendicular
magnetic anisotropy for which the Kerr rotation is second-order in the magnetic
deflection. Sagnac interfermometry can also be applied beneficially for samples
with in-plane anisotropy, for which the Kerr rotation is first order in the
deflection angle. Optical measurements based on Sagnac interferometry can
therefore provide a cross-check on electrical techniques for measuring
spin-orbit torque. Different electrical techniques commonly give quantitatively
inconsistent results, so that Sagnac interferometry can help to identify which
techniques are affected by unidentified artifacts.
