Polarization measurement has gained broad applications in many research topics, including magnetic anisotropy, spin dynamics[2,3] in magnetic material, birefringence in chiral media, and electro-optic sampling technique. In magnetism, one convenient and popular analytical tool is based on the magneto-optical effect, which alters the polarization of the reflected (Kerr effect) and the transmitted light (Faraday effect) through the asymmetric dielectric tensor induced by magnetization. Since its first application to surface magnetism, the magneto-optical Kerr effect (MOKE) has been developed as a non-intrusive and versatile probe for remote measurements on static or dynamic properties of spin systems with very high sensitivity, e.g., spin Hall effect[7,8], ultrafast spin dynamics, imaging magnetic domain and nanostructure[10,11], as well as magneto-optic information storage. However, because the polarization of light is very sensitive to a large variety of noise sources, it is difficult to achieve a sensitivity of in MOKE measurement, especially in the DC detection scheme[1,8,13–15]. This hampers the application of MOKE in many emerging subjects, such as spin Hall effect, time-reversal-symmetry-breaking (TRSB) states in a superconductor[16,17], where a sensitivity of is urgently needed.