An intensity-stabilised He-Ne laser for measuring small magneto-optic Kerr rotations from thin ferromagnetic films

The change in intensity of light transmitted through an analysing polariser after reflection from a ferromagnetic surface can be related to the magneto-optic Kerr rotation. The construction of an inexpensive apparatus for measuring small rotations based on the use of an external intensity stabiliser for a laboratory He-Ne laser is discussed. We examine the limitations on the signal-to-noise ratio as consequence of the detector noise, shot noise and technical noise present in the system. An intensity stability of 0.05% has been achieved and has enabled us to obtain hysteresis loops from a 5 nm thick iron film with a signal to noise ratio of 40:1 for a data collection time of 100 s.