Magneto-optical high resolution measurement of small displacements between light beams of different color

The magneto-optical Kerr-effect of a 40 nm thin iron film is used to modulate light beams of different color (e.g., red and green) up to 100 kHz. Yielding a resolution of 35 nm at a total light power of 5 μW, the displacement between the two focal points of these beams is measured by the time difference of the corresponding photo diode signals. The relation between the time and a locally defined magnetization reversal of the film is achieved by a ferrite C-yoke with gradient pole shoes and the rectangular hysteresis of the iron film. Besides various optical measurement applications, the method is foremost useful for studying the atmospheric refraction, where the dispersion of the refractive index leads to small angles (≈1 μrad) between light beams of different wavelengths