Atomic force microscope detection system using an optical fiber heterodyne interferometer free from external disturbances

Abstract

This paper describes an optical fiber heterodyne interferometer which is free from external disturbances, in the optical paths of two fibers used for measuring small displacements of the cantilever of an atomic force microscope. For eliminating the disturbances, the phases of the beat signals for the two points are differentiated. By the use of a special plano-convex lens with double optical axes, the beam is divided into two beams, and one of the beams is focused on a cantilever and the other on the reference point at a holder of the cantilever. The noise level in the frequency region over 100 Hz is very low (1.5×10 -2 nm/√Hz). It is confirmed, by measuring the electric force between a tip and a sample, that this interferometer is a useful tool for measuring the displacement of the cantilever.