A simple device was developed for measuring surface tension of liquids ~typically water solutions! based on the mechanical generation of capillary waves on a sample surface. The detection of the phase shift among two laser beams reflected by the liquid surface excited with a sinusoidal mechanical perturbation allows the calculation of the sample surface tension. Preliminary experimentation using a prototype of the instrument with water and water plus surfactant has been successfully performed. Some theoretical considerations are also pointed out to show the main design issues for proper operation mode. The proposed prototype appears innovative and suitable for online or field operation.