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

Weak surface reflections from a simple lens can be used to observe interfer ences between collimated and focusing laser beams. The superimposed beams ( one resulting from a double reflection within the lens) 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 thi s inexpensive setup is its stability against vibrations. The relative phase between the two beams can be easily controlled through small adjustments t o the lens alignment. Students using this setup with a CCD camera can explo re the amplitude and phase properties of a Gaussian laser beam in the vicin ity of the focus. This article also includes a brief review of Gaussian bea ms in the context of the Fresnel diffraction. (C) 2001 American Association of Physics Teachers.