Visual introduction to Gaussian beams using a single lens as an interferometer

Weak surface reflections from a simple lens can be used to observe interferences between collimated and focusing laser beams. The superimposed beams ͑one resulting from a double reflection within the lens͒ can be made to have similar intensities near the focus of the weaker beam by choosing the lens prescription and the divergence of the incident laser. An advantage to this inexpensive setup is its stability against vibrations. The relative phase between the two beams can be easily controlled through small adjustments to the lens alignment. Students using this setup with a CCD camera can explore the amplitude and phase properties of a Gaussian laser beam in the vicinity of the focus. This article also includes a brief review of Gaussian beams in the context of the Fresnel diffraction.