Atomic Force Microscope Using an Optical Fiber Heterodyne Interferometer Free from External Disturbances

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Abstract
This paper describes an atomic force microscope (AFM) using an optical fiber heterodyne interferometer free from external disturbances in the optical paths of two fibers used for measuring the small displacements of a cantilever. For the elimination of disturbances, the phases of beat signals for two points are differentiated. A special plano-convex lens with double optical axes is used for obtaining two beams at two points. The noise level of this interferometer is 4.5×10-3 nm/√Hz and is limited by the phase noise of the phase meter used. It is confirmed, by measuring the electric force between the tip and the sample, that the interferometer can be used for measuring the displacement of the cantilever. By inspecting the surface of an echellete grating, it is demonstrated that this AFM is a powerful tool for measurement of the surface on the nanometer order.