Differential phase contrast and magneto-optic edge detection

Magneto-optic (MO) edge detection is an alternative readout method for MO data storage systems. I show the equivalence of MO edge detection and the standard phase-detection methods of scanning optical microscopy but with the detected quantity depending primarily on spatial variations in the ellipticity rather than the phase of the reflected light. In particular, the equivalence of MO edge detection with a split detector and differential-phase-contrast detection is proven by a general theory of the optical imaging process and the symmetries in the system. The MO edge-detection signal results from variations in the phase of the light along the detection branches, these phase variations depending primarily on the ellipticity of the reflected light. This method of analysis gives not only a simple explanation of the detection techniques but allows the experience gained in scanning optical microscopy to be applied to MO edge detection.