Magneto-optical position-resolving sensor for displacement measurements between light beams

Abstract

The dispersion of the refractive index of the atmosphere leads to small angles (δβ ≈ 1 μrad) between light beams of different wavelengths (e.g., red and green) and therefore the images of these beams in the focal plane of a telescope are displaced by a few micrometres. According to this fact, geodetical refraction measurements can be accomplished by a position-resolving sensor. The magneto-optical transversal Kerr effect of a 40 nm thin iron film is used to modulate the light beams up to 100 kHz. Separating the beams by a colour-sensitive beamsplitter, the displacement between the two focal points is measured by the time difference of the corresponding photodiode 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. Simulating the conditions of refraction measurements in the atmosphere at a total light power of 5 μW on the front end of the telescope (focal length 1.5 m), a resolution of 35 nm is achieved, which corresponds to an angular resolution for δβ of 23 nrad.