2-Frequency Laser Surface Prof Ilometry

The use of a 2-frequency laser allows precise interferometric measurements to be made by observing the phase of the detected beat frequency, rather than the intensity of a conventional interference pattern. This allows both improved resolution and insensitivity to low-frequency source intensity fluctuations. This paper describes two different implementations of 2-frequency interferometric micro-profilometry: the first is based on a conventional Michelson interferometer configuration, and the second is a common-path technique which senses the spatial derivative of the surface profile and which is largely insensitive to vertical displacements of the surface relative to the optical components. These instruments can be used to measure either profiles of reflective surfaces, or optical thickness profiles of transparent objects placed on a plane mirror.